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Title: Hawkins Electrical Guide v. 7 (of 10) - Questions, Answers, & Illustrations, A progressive course - of study for engineers, electricians, students and those - desiring to acquire a working knowledge of electricity and - its applications
Author: Hawkins, Walter
Language: English
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*** Start of this LibraryBlog Digital Book "Hawkins Electrical Guide v. 7 (of 10) - Questions, Answers, & Illustrations, A progressive course - of study for engineers, electricians, students and those - desiring to acquire a working knowledge of electricity and - its applications" ***
THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
[Illustration:
HAWKINS
ELECTRICAL GUIDE
NUMBER
SEVEN
QUESTIONS
ANSWERS
&
ILLUSTRATIONS]
A PROGRESSIVE COURSE OF STUDY
FOR ENGINEERS, ELECTRICIANS, STUDENTS
AND THOSE DESIRING TO ACQUIRE A
WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
[Illustration: THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK.]
COPYRIGHTED, 1915,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS
GUIDE NO. 7.
~ALTERNATING CURRENT SYSTEMS~ 1,531 to 1,586
Advantages of the alternating current--~classification
of systems~--vector summation; examples--~forms of
circuit~: series, parallel, parallel series, series
parallel--~transformer systems~: individual transformers;
transformation at distribution centers--~single phase
system~; two wire transmission and three wire distribution;
objections to single phase systems; advantages--~monocyclic
system~--~two phase systems~: adaptation; ordinary voltages
used; two phase three wire system; two phase five wire
system--~three phase systems~: six wire; four wire; three
wire; connections: star, delta, star delta, delta star;
evolution of three wire system; pressure and current relations;
connection of transformers; open delta connection--~change
of frequency~--Schaghticoke-Schenectady transmission
line--~transformation of phases~: three to one, three to
two, two to six, and three to six phase--~Scott connection~
for transforming from three to two phase--three to two phase
with three star connected transformers--~economy of a.c.
systems~--~relative weights of copper required for polyphase
systems~--aermotor towers of Southern Power Co.--~choice of
voltage~--~usual transmission voltages~--~diagram of three
phase distribution~--~mixed current systems~; usual d.c.
pressure on traction lines; use of mixed systems.
~AUXILIARY APPARATUS~ 1,587 to 1,588
~Classification of auxiliary devices~: switching
devices, types--current or pressure limiting devices,
types--lightning protection devices, types--regulating devices,
types--synchronous condensers, types--indicating devices.
~SWITCHING DEVICES~ 1,589 to 1,612
Definition of a switch--behaviour of the current when the
circuit is broken--points on design--installation of single
throw and double throw switches--plug switches--~forms of
break~: open, enclosed, fuse, horn, oil--disconnecting
switches--pole top switches--~horn break switches~--motor
starting switch--~oil switches~; nature of an oil
break--~remote control oil switches~--~motor operated
switches~--rupturing capacity of oil switches--~float switches~.
~CURRENT AND PRESSURE LIMITING DEVICES~ 1,613 to 1,676
Necessity for these devices; steam analogy--~fuses~: advantages
and disadvantages; types: plug, cut out, expulsion, no arc,
magnetic blow out, quick break fuse, etc.--metal used--~current
limiting inductances~: construction, location--~circuit
breakers~: progressive breaking of the circuit; carbon
contacts--~automatic features~: overload trip, underload trip,
low voltage trip, auxiliary circuit trip--~relays~: adaptation;
classification: protective, regulative, communicative, a.c. and
d.c., circuit opening, circuit closing, primary, secondary,
overload, underload, over voltage, low voltage, reverse energy,
reverse phase, instantaneous, time limit, inverse time limit,
differential--~how to select relays~.
~LIGHTNING PROTECTION DEVICES~ 1,677 to 1,714
~Essential parts~: air gaps, resistances, inductances,
arc suppressing devices--requirements--~air gap
arresters~--~multi-gap arresters~; difference between spark
and arc; distribution of stress; sparking at the gaps; how
the arc is extinguished; effect of frequency; graded shunt
resistances; the cumulative or breaking back effect--arresters
for grounded ~Y~ and non-grounded neutral systems--multiplex
connection--~horn gap arresters~: operation; objection to
the horn gap--~electrolytic arresters~: critical voltage,
temporary and permanent; determination of number of cell;
putting cell in commission; nature of the film; horn gaps on
electrolytic arresters; charging of electrolytic arresters;
charging arresters for non-grounded circuits--~grounded and
non-grounded neutral circuits~--~ground connections~--~choke
coils~: principal objects; principal electrical conditions to
be avoided; why choke coils are made in the form of an hour
glass; cooling--~static interrupters~; how to connect condenser
and choke coil; effect of condenser.
~REGULATING DEVICES~ 1,715 to 1,762
~Regulation of alternators~--~a.c. feeder
regulation~--~application of induction type regulators~;
types: induction, and variable ratio transformer regulators;
operation of induction regulators; neutral position;
regulator capacity--~polyphase induction regulators~:
construction, operation; _automatic control_; why two relays
are used; difficulties encountered in operation of relays;
vibration or chattering of the contacts; poor contact
of primary relay--~variable ratio transformer voltage
regulators~: types: drum, and dial; dial type for high
voltage--~small feeder voltage regulators~: construction and
operation; adjustment--~automatic voltage regulators for
alternators~: method of regulation--~line drop compensators~:
essential parts; connections; construction and operation;
diagram of automatic voltage regulator using a line drop
compensator--~starting compensators~: necessity for;
construction and operation--~star delta switches~.
~SYNCHRONOUS CONDENSERS~ 1,763 to 1,776
Characteristics--effect of fully loaded and lightly
loaded induction motors on the power factor--synchronous
motor used as condenser--~effects of low lagging power
factors~; example--~cost of synchronous condenser vs. cost
of copper~--location of condenser--synchronous condenser
calculations and diagram for same.
~INDICATING DEVICES~ 1,777 to 1,838
Virtual value of an alternating current or pressure--the
word ~effective~ _erroneously_ used for ~virtual~: steam
engine analogy illustrating this error--classification of
instruments: electromagnetic or moving wire, hot wire,
induction, dynamometer--~electromagnetic or moving iron
instruments~: types: plunger, inclined coil, magnetic vane;
character of scale; objections and precautions--~inclined
coil instruments~--~magnetic vane instruments~--~hot wire
instruments~--~induction instruments~: types: shielded pole,
rotary field; operation of both types--~dynamometers~:
construction and operation; how arranged to measure
watts--~watthour meters~: types: commutator, induction,
Faraday disc; essential parts; object of the motor; object
of generator; objection to commutator meter--~principles
of induction watthour meters~: essential parts; strength
of rotating field; moving element; retarding element;
registering element; frame and bearings; friction compensator;
power factor adjustment; frequency adjustment--~Faraday
disc, or mercury motor ampere hour meter~: construction
and operation--~frequency indicators~: types: synchronous
motor, resonance, induction; synchronous motor as
frequency indicator--~resonance frequency indicators~:
adaptation--~induction frequency meter~: construction
and operation--~synchronism indicators~: types: lamp or
voltmeter, resonance or vibrating reed, rotating field--~power
factor indicators~: wattmeter type; disc, or rotating field
type--~ground detectors~.
CHAPTER LV
ALTERNATING CURRENT SYSTEMS
The facility with which alternating current can be transformed from
one voltage to another, thus permitting high pressure transmission of
electric energy to long distances through small wires, and low pressure
distribution for the operation of lighting systems and motors, gives a
far greater variety of systems of transmission and distribution than is
possible with direct current.
Furthermore, when the fact that two phase current can be readily
transformed into three phase current, and these converted into direct
current, and vice versa, by means of rotary converters and rectifiers,
is added to the advantages derived by the use of high tension
systems, it is apparent that the opportunity for elaboration becomes
almost unlimited. These conditions have naturally tended toward the
development of a great variety of systems, employing more or less
complicated circuits and apparatus, and although alternating current
practice is still much less definite than direct current work, certain
polyphase systems are now being generally accepted as representing the
highest standards of power generation, transmission and distribution.
A classification of the various alternating current systems, to be
comprehensive, should be made according to several points of view, as
follows:
1. With respect to the arrangement of the circuit, as
_a._ Series;
_b._ Parallel;
_c._ Series parallel;
_d._ Parallel series.
2. With respect to transformation, as
Transformer;
3. With respect to the mode of transmitting the energy, as
_a._ Constant pressure;
_b._ Constant current.
4. With respect to the kind of current, as
_a._ Single phase { two wire;
{ three wire;
_b._ Monocyclic
{ four wire;
_c._ Two phase { three wire;
{ five wire;
{ six wire;
{ three wire;
{ four wire;
_d._ Three phase { star connection;
{ delta connection;
{ star delta connection;
{ delta star connection;
_e._ Multi-phase { of more than
{ three phases;
5. With respect to transmission and distribution, as
_a._ Frequency changing;
_b._ Phase changing;
_c._ Converter;
_d._ Rectifier.
In order to comprehend the relative advantages of the various
alternating current systems, it is first necessary to understand the
principle of _vector summation_.
~Vector Summation.~--This is a simple geometrical process for
ascertaining the pressure at the free terminals of alternating current
circuits. The following laws should be carefully noted:
_1. If two alternating pressures which agree in phase are connected
together in series, the voltage at the free terminals of the circuit
will be equal to their arithmetical sum, as in the case of direct
currents._
[Illustration: FIG. 2,123.--_Vectors._ ~A vector is defined as~: _a
line, conceived to have both a fixed length and a fixed direction in
space, but no fixed position_. Thus A and B are lines, each having a
fixed length, but no fixed direction. ~By adding an arrow head~ the
direction is fixed and the line becomes a vector, as for example vector
C. The fixed length is usually taken to represent a definite force,
thus the fixed length of vector C is 4.7 which may be used to represent
4.7 lbs., 4.7 tons, etc., as may be arbitrarily assumed.]
When there is phase difference between the two alternating pressures,
connected in series, the following relation holds:
2. _The value of the terminal voltage will differ from their
arithmetical sum, depending on the amount of their phase difference._
When there is phase difference, the value of the resultant is
conveniently obtained as explained below.
~Ques. How are vector diagrams constructed for obtaining resultant
electric pressure?~
Ans. On the principle of the _parallelogram of forces_.
~Ques. What is understood by the parallelogram of forces?~
Ans. It is a graphical method of finding the resultant of two forces,
according to the following law: _If two forces acting on a point
be represented in direction and intensity by adjacent sides of a
parallelogram, their resultant will be represented by the diagonal of
the parallelogram which passes through the point._
[Illustration: FIG. 2,124.--Parallelogram of forces. OC is the
resultant of the two forces OA and OB. The length and direction of the
lines represent the intensity and direction of the respective forces,
the construction being explained in the accompanying text.]
Thus in fig. 2,124, let OA and OB represent the intensity
and direction of two forces acting at the point O, Draw AC
and BC, respectively parallel to OB and OA, completing the
parallelogram, then will OC, the diagonal from the point at
which the forces act, represent the intensity and direction of
the resultant, that is, of a force equivalent to the combined
action of the forces OA and OB, these forces being called the
_components_ of the force OC.
~Ques. Upon what does the magnitude of the resultant of two forces
depend?~
Ans. Upon the difference in directions in which they act, as shown in
figs 2,125 to 2,128.
~Ques. Is the parallelogram of forces applied when the difference in
direction or "phase difference" of two forces is 90 degrees?~
Ans. It is sometimes more conveniently done by calculation according to
the law of the right angle triangle.
[Illustration: FIGS. 2,125 to 2,128.--Parallelograms of forces
showing increase in magnitude of the resultant of two forces, as
their difference of direction, or electrically speaking, their _phase
difference_ is diminished. The diagrams show the growth of the
resultant of the two equal forces OA and OB as the phase difference is
reduced from 165° successively to 120, 60, and 15 degrees.]
According to this principle, if two alternating pressures
have a phase difference of 90 degrees they may be represented
in magnitude and direction by the two sides of a right angle
triangle as OA and OB in fig. 2,129; then will the hypotenuse
AB represent the magnitude and direction of the resultant
pressure. That is to say, the resultant pressure
AB = √((OA)² + (OB)²) (1)
EXAMPLE.--A two phase alternator is wound for 300 volts on one
phase and 200 volts on the other phase, the phase difference
being 90°. If one end of each winding were joined so as to
form a single winding around the armature, what would be the
resultant pressure?
By calculation, substituting the given values in equation (1),
Resultant pressure = √(300² + 200²) = √(130,000) = 360.6 volts.
This is easily done graphically as in fig. 2,129 by taking a
scale, say, 1" = 100 volts and laying off OA = 3" = 300 volts,
and at right angles OB = 2" = 200 volts, then by measurement
AB = 3.606" = 360.6 volts.
[Illustration: FIG. 2,129.--Method of obtaining the resultant of two
component pressures acting at right angles by solution of right angle
triangle. The equation of the right angle triangle is explained at
length in Guide No. 5, page 1,070.]
~Ques. When the two pressures are equal and the phase difference is
90°, is it necessary to use equation (1) to obtain the resultant?~
Ans. No. The resultant is obtained by simply multiplying one of the
pressures by 1.41.
This is evident from fig. 2,130. Here the two pressures OA
and OB are equal as indicated by the dotted arc. Since they
act at right angles, OB is drawn at 90° to OA. According
to the equation of the right angle triangle, the resultant
AB = √(1² + 1²) = √2̅ = 1.4142 which ordinarily is taken as
1.41.
_This value will always represent the ratio between the
magnitude of the resultant and the two component forces,
when the latter are equal, and have a phase difference of 90
degrees._
~Forms of Circuit.~--Alternating current systems of distribution may
be classed, with respect to the kind of circuit used, in a manner
similar to direct current systems, that is, they may be called series,
parallel, series parallel, or parallel series systems, as shown in
figs. 2,131 to 2,134.
[Illustration: FIG. 2,130.--Diagram for obtaining the resultant of two
equal component pressures acting at right angles.]
~Series Circuits.~--These are used in arc lighting, and series
incandescent lighting, a constant current being maintained; also for
constant current motors and generators supplying secondary circuits.
[Illustration: FIGS. 2,131 to 2,134.--Various forms of circuit. These
well known forms of circuit are used in both alternating and direct
current systems. The simple series circuit, fig. 2,131, is suitable
for constant current arc lighting. Fig. 2,132, shows the parallel
constant pressure circuit; this form of circuit is largely used but
is seldom connected direct to the alternator terminals, but to a step
down transformer, on account of the low pressure generally required.
Fig. 2,133 illustrates a parallel series circuit, and 2,134, a series
parallel circuit.]
Several forms of constant current alternator, analogous to the
Thompson-Houston and Brush series arc dynamos, have been introduced.
In the design of such alternators self-induction and armature reaction
are purposely exaggerated; so that the current does not increase very
much, even when the machine is short circuited. With this provision, no
regulating device is required.
[Illustration: FIG. 2,135.--Typical American overhead 6,600 volt single
phase interurban trolley line, Baltimore and Annapolis short line,
Annapolis, Md.]
An objectionable feature is that the voltage of a constant current
alternator will rise very high if the circuit be opened, because it is
then relieved of inductance drop and armature reaction.
To guard against a dangerous rise of voltage, a film cut out or
equivalent device is connected to the terminal of each machine so that
it will short circuit the latter if the voltage rise too high.
~Ques. What advantage have constant current alternators over constant
current dynamos?~
Ans. The high pressure current is delivered to the external circuit
without a commutator, hence there is no sparking difficulty.
The above relates to the revolving field type of alternator.
There are, however, alternators in which the armature revolves,
the current being delivered to the external circuit through
collector rings and brushes. This type of alternator, it should
be noted, is for moderate pressures, and moreover there is
no interruption to the flow of the current such as would be
occasioned by a tangential brush on a dynamo in passing from
one commutator segment to the next.
In the revolving field machine, though the armature current be
of very high pressure, the field current which passes through
the brushes and slip rings is of low pressure and accordingly
presents no transmission difficulties.
[Illustration: FIG. 2,136.--Diagram of ~parallel circuit~. _It is a
constant pressure circuit_ and is very widely used for lighting and
power. If each lamp takes say ½ ampere, the current flowing in the
circuit will vary with the number of lamps in operation; in the above
circuit with all lamps on, the current is ½ × 5 = 2½ amperes.]
~Ques. State a disadvantage.~
Ans. Some source of direct current for field excitation is required.
~Ques. In a constant current series system, upon what does the voltage
at the alternator depend?~
Ans. The number of devices connected in the circuit, the volts required
for each, and the line drop.
~Parallel Circuits.~--These are used for constant pressure operation.
Such arrangement provides a separate circuit for each unit making them
independent so that they may vary in size and each one can be started
or stopped without interfering with the others. Parallel circuits are
largely used for incandescent lighting, and since low pressure current
is commonly used on such circuits they are usually connected to step
down transformers, instead of direct to the alternators.
[Illustration: FIG. 2,137.--Diagram of ~parallel series circuit~,
_showing fall of pressure between units_. ~This system~ _is very rarely
used_; it has the disadvantage that if a lamp filament breaks, the
resistance of the circuit is altered and the strength of the current
changed. The voltmeter shows the fall of pressure along the line.
~It should be noted~ that, although the meter across AB is shown as
registering zero pressure, there is, strictly speaking, a slight
pressure across AB, in amount, being that required to overcome the
resistance of the conductor between A and B.]
~Parallel Series Circuits.~--Fig. 2,137 shows the arrangement of a
parallel series circuit and the pressure conditions in same. Such
a circuit consists of groups of two or more lamps or other devices
connected in parallel and these groups connected in series.
Such a circuit, when used for lighting, obviously has the disadvantage
that if a lamp filament breaks, the resistance of the group is
increased, thus reducing the current and decreasing the brilliancy of
the lamps. This arrangement accordingly does not admit of turning off
any of the lights.
~Series Parallel Circuits.~--The arrangement of circuits of this kind
is shown in fig. 2,134; they are used to economize in copper since by
joining groups of low pressure lamps in series they may be supplied by
current at correspondingly higher pressure.
Thus, if in fig. 2,134, 110 volt, ½ ampere lamps be used, the
pressure on the mains, that is, between any two points as A
and B would be 110 × 3 = 330 volts. Each group would require ½
ampere and the five groups ½ × 5 = 2½ amperes.
[Illustration: FIG. 2,138.--44,000 volt lines entering the Gastonia
sub-station of the Southern Power Co. The poles used are of the twin
circuit two arm type, built of structural steel, their height varying
from 45 to 80 feet, the latter weighing 9,000 pounds each. These poles
have their bases weighted with concrete.]
~Transformer Systems.~--Nearly all alternating current systems are
transformer systems, since the chief feature of alternating current
is the ease with which it may be transformed from one pressure to
another. Accordingly, considerable economy in copper may be effected by
transmitting the current at high pressure, especially if the distance
be great, and, by means of step down transformers, reducing the voltage
at points where the current is used or distributed.
Ordinarily and for lines of moderate length, current is sent out
direct from the alternator to the line and transformed by step down
transformers at the points of application.
With respect to the step down transformers, there are two arrangements:
1. Individual transformers;
2. One transformer for several customers.
[Illustration: FIG. 2,139.--Diagram of transformer system with
individual transformers. The efficiency is low, but such method of
distribution is necessary in sparsely settled or rural districts.]
Individual transformers, that is, a separate transformer for each
customer is necessary in rural districts where the intervening
distances are great as shown in fig. 2,139.
~Ques. What are the objections to this method of distribution?~
Ans. It requires the use of small transformers which are necessarily
less efficient and more expensive per kilowatt than large transformers.
The transformer must be built to carry, within its overload capacity,
all the lamps installed by the customer since all may be used
occasionally.
Usually, however, only a small part of the lamps are in use,
and those only for a small part of the day, so that the average
load on the transformer is a very small part of its capacity.
Since the core loss continues whether the transformer be loaded
or not, but is not paid for by the customer, the economy of the
arrangement is very low.
In the second case, where one large transformer may be placed
at a distribution center, to supply several customers, as in
fig. 2,140, the efficiency of the system is improved.
~Ques. Why is this arrangement more efficient than when individual
transformers are used?~
[Illustration: FIG. 2,140.--Diagram of transformer system with
one transformer located at a distribution center and supplying
several customers as A, B, and C. Such arrangement is considerably
more efficient than that shown in fig. 2,139, as explained in the
accompanying text.]
Ans. Less transformer capacity is required than with individual
transformers.
~Ques. Why is this?~
Ans. With several customers supplied from one transformer it is
extremely improbable that all the customers will burn all their lamps
at the same time. It is therefore unnecessary to install a transformer
capable of operating the full load, as is necessary with individual
transformers.
~Ques. Does the difference in transformer capacity represent all the
saving?~
Ans. No; one large transformer is more efficient than a number of small
transformers.
~Ques. Why?~
Ans. The core loss is less.
For instance, if four customers having 20 lamps each were
supplied from a single transformer, the average load would
be about 8 lamps, and at most not over 10 or 15 lamps, and a
transformer carrying 30 to 35 lamps at over load would probably
be sufficient. A 1,500 watt transformer would therefore be
larger than necessary. At 3 per cent. core loss, this gives a
constant loss of 45 watts, while the average load of 8 lamps
for 3 hours per day gives a useful output of 60 watts, or an
all year efficiency of nearly 60 per cent., while a 1,000 watt
transformer would give an all year efficiency of 67 per cent.
For long distance transmission lines, the voltage at the
alternator is increased by passing the current through a _step
up_ transformer, thus transmitting it at very high pressure,
and reducing the voltage at the points of distribution by step
down transformers as in fig. 2,141.
[Illustration: FIG. 2,141.--Diagram illustrating the use of _step up_
and _step down_ transformers on long distance transmission lines. The
saving in copper is considerable by employing extra high voltages on
lines of moderate or great length as indicated by the relative sizes of
wire.]
~Ques. In practice, would such a system as shown in fig. 2,141 be used?~
Ans. If the greatest economy in copper were aimed at, a three phase
system would be used.
The purpose of fig. 2,141 is to show the importance of the
transformer in giving a flexibility of voltage, by which the
cost of the line is reduced to a minimum.
~Ques. Does the saving indicated in fig. 2,141 represent a net gain?~
Ans. No. The reduction in cost of the transmission is partly offset by
the cost of the transformers as well as by transformer losses and the
higher insulation requirements.
[Illustration: FIG. 2,142.--Single and twin circuit poles (Southern
Power Co.). The twin circuit pole at the right is used for 11,000 volt
circuits, while the single circuit poles at the left carry 44,000 volt
conductors, being used on another division for 100,000 volt line.]
Every case of electric transmission presents its own problem,
and needs thorough engineering study to intelligently choose
the system best adapted for the particular case.
~Single Phase Systems.~--There are various arrangements for
transmission and distribution classed as single phase systems. Thus,
single phase current may be conveyed to the various receiving units
by the well known circuit arrangements known as series, parallel,
series parallel, parallel series, connections previously described and
illustrated in figs. 2,131 to 2,134.
Again single phase current may be transmitted by two wires and
distributed by three wires. This is done in several ways, the simplest
being shown in fig. 2,143.
[Illustration: FIG. 2,143.--Diagram illustrating single phase two wire
transmission and three wire distribution. The simplified three wire
arrangement at A, is not permissible except in cases of very little
_unbalancing_. Where the difference between loads on each side of the
neutral may be great some form of balancing as an auto-transformer or
equivalent should be used, as at B.]
~Ques. Under what conditions is the arrangement shown in fig. 2,143
desirable?~
Ans. This method of treating the neutral wire is only permissible where
there is very little unbalancing, that is, where the load is kept
practically the same on both sides of the neutral.
~Ques. What advantage is obtained by three wire distribution?~
Ans. The pressure at the alternator can be doubled, which means,
for a given number of lamps, that the current is reduced to half,
the permissible drop may be doubled, the resistance of the wires
quadrupled, and their cost reduced nearly 75 per cent.
[Illustration: FIG. 2,144.--100,000 volt "Milliken" towers with one
circuit strung (Southern Power Co.). These towers are mounted on metal
stubs sunk 6 feet in the ground. Where the angle of the line is over
15 degrees, however, these stubs are weighted with rock and concrete,
and where an angle of over 30 degrees occurs, two and sometimes three
towers are used for making the turn. The weight of the standard
"Milliken" tower is 3,080 lbs., and its height from the ground to peak
is 51 feet. The towers are spaced to average eight to a mile and a
strain tower weighing 4,250 lbs. is used every mile. For particularly
long spans a special heavy tower weighing 6,000 lbs. is used. The
circuits are transposed every 30 miles. Multiple disc insulators are
used, four discs being used to suspend each conductor from standard
towers and ten discs to each conductor on strain towers. The standard
span is 600 feet, sag 11 ft at 50° Fahr.]
~Ques. What modification of circuit A (fig. 2,143), should be made to
allow for unbalancing in the three wire circuit?~
Ans. An auto-transformer or "balance coil" as it is sometimes called
should be used as at B.
This is a very desirable method of balancing when the ratio of
transformation is not too large.
~Ques. For what service would the system shown in fig. 2,143 be
suitable?~
Ans. For short distance transmission, as for instance, in the case of
an isolated plant because of the low pressure at which the current is
generated.
The standard voltages of low pressure alternators are 400, 480,
and 600 volts.
[Illustration: FIG. 2,145.--View of a typical isolated plant. The
illustration represents an electric lighting plant on a farm showing
the lighting of the dwelling, barn, tool house and pump house. The
installation consists of a low voltage dynamo with gas engine drive and
storage battery together with the necessary auxiliary apparatus.]
~Ques. In practice are single phase alternators used as indicated in
fig. 2,143?~
Ans. Alternators are wound for one, two or three phases. Three phase
machines are more commonly supplied and in many cases it will pay to
install them in preference to single phase, even if they be operated
single phase temporarily.
For a given output, three phase machines are smaller than
single phase and the single phase load can usually be
approximately balanced between the three phases. Moreover, if
a three phase machine be installed, polyphase current will be
available in case it may be necessary to operate polyphase
motors at some future time.
Standard three phase alternators will carry about 70 per cent.
of their rated kilowatt output when operated single phase, with
the same temperature rise.
~Ques. How are three phase alternators used for single phase circuits?~
Ans. The single phase circuit is connected to any two of the three
phase terminal leads.
[Illustration: FIG. 2,146.--Diagram showing ~arrangement of single
phase system~ for two wire transmission and three wire distribution,
_where the transmission distance_ ~is considerable~. In order ~to
reduce the cost~ of the transmission line, _the current must be
transmitted at high pressure_; this necessitates the use of a step down
transformer at the distributing center as shown in the illustration.]
~Ques. What form of single phase system should be used where the
transmission distance is considerable?~
Ans. The current should be transmitted at high pressure, a step down
transformer being placed at each distribution center to reduce the
pressure to the proper voltage to suit the service requirements as
shown in fig. 2,146.
Thus, if 110 volt lamps be used on the three wire circuit, the
pressure between the two outer wires would be 220 volts. A
transformation ratio of say 10:1 would give 2,220 volts for the
primary circuit. The current required for the primary with this
ratio being only .1 that used in the secondary, a considerable
saving is effected in the cost of the transmission line as must
be evident.
With the high pressure alternator only one transformation of
the current is needed, as shown at the distribution end.
In place of the high pressure alternator, a low pressure
alternator could be used in connection with a step up
transformer as shown in fig. 2,147, but there would be an
extra loss due to the additional transformer, rendering the
system less efficient than the one shown in fig. 2,146. Such an
arrangement as shown in the fig. 2,147 might be justified in
the case of a station having a low pressure alternator already
in use and it should be desired to transmit a portion of the
energy a considerable distance.
~Ques. How could the system shown in fig. 2,147 be made more efficient
than that of fig. 2,146?~
Ans. By using a high pressure alternator in order to considerably
increase the transmission voltage.
Thus, a 2,200 volt alternator and 1:10 step up transformer
would give a line pressure of 22,000 volts, which at the
distribution end could be reduced, to 220 volts for the three
wire circuit, using a 100:1 step down transformation.
[Illustration: FIG. 2,147.--Diagram illustrating how electricity can
be economically transmitted a considerable distance with low pressure
alternator already in use.]
~Ques. Would this be the best arrangement?~
Ans. No.
~Ques. What system would be used in practice for maximum economy?~
Ans. Three phase four wire.
~Ques. What are the objections to single phase generation and
transmission?~
Ans. It does not permit of the use of synchronous converters,
self-starting synchronous motors, or induction motor starting under
load. It is poorly adapted to general power distribution, hence it is
open to grave objections of a commercial nature where there exists any
possibility of selling power or in any way utilizing it for general
converter and motor work.
[Illustration: FIG. 2,148.--Angle tower showing General Electric strain
insulators. The tower being subject to great torsional strains is
erected on a massive concrete foundation. The construction is similar
to the standard tower but of heavier material, and having the same
vertical dimensions but with bases 20 ft. square.]
~Ques. For what service is it desirable?~
Ans. For alternating current railway operation.
There are advantages of simplicity in the entire generating,
primary, and secondary distribution systems for single phase
roads. These advantages are so great that they justify
considerable expense, looked at from the railway point of view
only, the single phase system throughout may be considered as
offering the most advantage.
~Ques. What are the objectionable features of single phase alternators?~
Ans. This type of alternator has an unbalanced armature reaction which
is the cause of considerable flux variation in the field pole tips and
in fact throughout the field structure.
In order to minimize eddy currents, such alternators must accordingly
be built with thinner laminations and frequently poorer mechanical
construction, resulting in increased cost of the machine. The large
armature reaction results in a much poorer regulation than that
obtained with three phase alternators, and an increased amount of field
copper is required, also larger exciting units. These items augment the
cost so that the single phase machine is considerably more expensive
than the three phase, of the same output and heating.
[Illustration: FIG. 2,149.--Elementary alternator developing one volt
at frequencies of 60 and 25, ~showing the effect of reducing the
frequency~. Since for the same number of pole, the R.P.M. have to
be decreased to decrease the frequency, increased flux is required
to develop the same voltage. ~Hence in construction~, low frequency
machines require _larger magnets, increased number of turns in series
on the armature coils, larger exciting units_ as compared with machines
built for higher frequency.]
~Ques. What factor increases the difficulties of single phase
alternator construction?~
Ans. The difficulties appear to increase with a _decrease_ in frequency.
The adoption of any lower frequency than 25 cycles may result
in serious difficulties in construction for a complete line
of machine, especially those of the two or four pole turbine
driven type where the field flux is very large per pole.
~Monocyclic System.~--In this system, which is due to Steinmetz, the
alternator is of a special type. In construction, there is a ~main~
single phase winding an auxiliary or ~teaser~ winding connected to the
central point of the main winding in quadrature therewith.
The teaser coil generates a voltage equal to about 25 per cent. of that
of the main coil so that the pressure between the terminals of the main
coil and the free end of the teaser is the resultant of the pressure of
the two coils.
[Illustration: FIG. 2,150.--Diagram of monocyclic system, showing
lighting and power circuits.]
By various transformer connections it is possible to obtain a
practically correct three phase relationship so that polyphase motors
may be employed.
In this system, two wires leading from the ends of the single phase
winding in the alternator supply single phase current to the lighting
load, a third wire connected to the end of the teaser being run to
points where the polyphase motors are installed as shown in fig. 2,150.
The monocyclic system is described at length in the chapter on
alternators, Guide No. 5, pages, 1,156 to 1,159.
~Two Phase Systems.~--A two phase circuit is equivalent to two
single phase circuits. Either four or three wire may be employed in
transmitting two phase current, and even in the latter instance the
conditions are practically the same as for single phase transmission,
excepting the unequal current distribution in the three wires. Fig.
2,151 shows a two phase four wire system.
[Illustration: FIG. 2,151.--Diagram of two phase four wire system.
It is desirable for supplying current for lighting and power. The
arrangement here shown should be used only for lines of short or
moderate length, because of the low voltage. Motors should be connected
to a circuit separate from the lighting circuit to avoid drop on the
latter while starting a motor.]
~Ques. For what service is the system shown in fig. 2,151 desirable?~
Ans. It is adapted to supplying current for lighting and power at
moderate or short distances.
Either 110 or 220 volts are ordinarily used which is suitable
for incandescent lighting and for constant pressure arc lamps,
the lamps being connected singly or two in pairs.
~Ques. Where current for both power and light are obtained from the
same source how should the circuits be arranged?~
Ans. A separate circuit should be employed for each, in order to avoid
the objectionable drop and consequent dimming of the lights due to the
sudden rush of current during the starting of a motor.
[Illustration: FIG. 2,152.--Diagram of two phase three wire system. A
wire is connected to one end of each phase winding as at A and B, and a
third wire C, to the other end of both phases as shown.]
Disagreeable fluctuation of the lights are always met with
when motors are connected to a lighting circuit and the effect
is more marked with alternating current than with direct
current, because most types of alternating current motor
require a heavy current usually lagging considerably when
starting. This not only causes a large drop on the line, but
also reacts injuriously upon the regulation of transformers and
alternators, their voltage falling much more than with an equal
non-inductive load.
~Ques. What voltages are ordinarily used on two phase lines of more
than moderate length?~
Ans. For transmission distances of more than two or three miles,
pressures of from 1,000 to 2,000 volts or more are employed to
economize in copper. For long distance transmission of over fifty
miles, from 30,000 to 100,000 volts and over are used.
~Ques. For long distance transmission at 30,000 to 40,000 volts, what
additional apparatus is necessary?~
Ans. Step up and step down transformers.
[Illustration: FIG. 2,153.--Diagram illustrating two phase three wire
transmission. The third wire C is attached to the connector between one
end of phase A, and phase B windings.]
~Ques. Explain the method of transmitting two phase current with three
wires.~
Ans. The connections at the alternator are very simple as shown in fig.
2,152. One end of each phase winding is connected by the brushes _a_
and _b'_, to one of the circuit wires, that is to A and B respectively.
The other end of each phase winding is connected by a lead across
brushes _a'_ and _b_, to which the third wire C is joined.
The current and pressure conditions of this system are
represented diagrammatically in fig. 2,153. The letters
correspond to those in fig. 2,152, with which it should be
compared.
As shown in the figure each coil is carrying 100 amperes at
1,000 volts pressure. Since the phase difference between the
two coils is 90°, the voltage between A and B is √2̅ = 1.414
times that between either A or B and the common return wire C.
The current in C is √2̅ = 1.414 times that in either outside
wire A or B, as indicated.
~Ques. How should the load on the two phase three wire system be
distributed?~
Ans. The load on the two phases must be carefully balanced.
[Illustration: FIG. 2,154.--Diagram of two phase three wire system and
connections for motors and lighting circuits.]
~Ques. Why should the power factor be kept high?~
Ans. A high power factor should be maintained in order to keep the
voltage on the phases nearly the same at the receiving ends.
~Ques. How should single phase motors be connected and what precaution
should be taken?~
Ans. Single phase motors may be connected to either or both phases,
but in such cases, no load should be connected between the outer wires
otherwise the voltages on the different phases will be badly unbalanced.
Fig. 2,154 shows a two phase three wire system, with two
wire and three wire distribution circuits, illustrating the
connection for lighting and for one and two phase motors.
[Illustration: FIG. 2,155.--Diagram of two phase system with four wire
transmission and three wire distribution. In the three wire circuits
the relative pressures between conductors are as indicated; that is,
the pressure between the two outer wires A and B is 141 volts, when the
pressure between each outer wire and the central is 100 volts.]
~Ques. Describe another method of transmission and distribution with
two phase current.~
Ans. The current may be transmitted on a four wire circuit and
distributed on three wire circuit as in fig. 2,155.
The four wire transmission circuit is evidently equivalent to
two independent single phase circuits.
In changing from four to three wires, it is just as well to
connect the two outside wires A and B together (fig. 2,152), as
it is to connect _a´_ and _b_. It makes no difference which two
secondary wires are joined together, so long as the other wires
of each transformer are connected to the outside wires of the
secondary system.
~Ques. For what service is the two phase three wire system adapted?~
Ans. It is desirable for supplying current of minimum pressure to
apparatus in the vicinity of transformers. It is more frequently
used in connection with motors operating from the secondaries of the
transformers.
~Ques. How should the third or common return wire be proportioned?~
Ans. Since the current in the common return wire is 41.4 per
cent. higher than that in either of the other wires it must be of
correspondingly larger cross section, to keep the loss equal.
[Illustration: FIGS. 2,156 and 2,157.--Conventional diagrams
illustrating star and delta connected three phase alternator armatures.]
~Ques. What is the effect of an inductive load on the two phase three
wire system and why?~
Ans. It causes an unbalancing of both sides of the system even though
the energy load be equally divided. The self-induction pressure in one
side of the system is in phase with the virtual pressure in the other
side, thus distorting the current distribution in both circuits.
~Ques. Describe the two phase five wire system.~
Ans. A two phase circuit may be changed from four to five wires by
arranging the transformer connections as in fig. 2,158.
As shown, the secondaries of the transformers are joined in
series and leads brought out from the middle point of each
secondary winding and at the connection of the two windings,
giving five wires.
With 1,000 volts in the primary windings and a step down ratio
of 10:1, the pressure between A and C and C and E will be 100
volts and between the points and the connections B or D at the
middle of the secondary coils, 50 volts.
The pressure across the two outer wires A and E is, as in the
three wire system, √2̅ or 1.41 times that from either outer
wire to the middle wire C, that is 141 volts.
The pressure across the two wires connected to the middle of
the coils, that is, across B and D, is 50 × √2̅ = 70.5 volts.
[Illustration: Fig. 2,158.--Two phase four wire transmission and five
wire distribution system. The relative pressures between the various
conductors are indicated in the diagram.]
~Three Phase Systems.~--There are various ways of arranging the circuit
for three phase current giving numerous three phase systems.
1. With respect to the number of wires used they may be classified as
_a._ Six wire;
_b._ Four wire;
_c._ Three wire.
[Illustration: FIG. 2,159.--Line connections of three phase three
wire long distance transmission, and distribution system. The three
phase alternator A, is driven by the water wheel B, and furnishes
current at say 2,200 volts plus sufficient pressure to compensate
for line drop. With 1:10 step up transformers C, this would give a
transmission pressure of 22,000 volts plus line drop. _It is_ ~this
transformation~ _that secures the_ ~copper economy~ _of the system_.
At the distribution end are the step down transformers; one set
reducing the voltage down to 2,200 volts, and supplying current direct
to the synchronous motor, and through another set of other step down
transformers, as L and K, to lighting and power circuits at 220 volts.
Another set of step down transformers M reduce the pressure directly to
120 volts for power and lighting, the pressure being regulated by the
regulators G. Arc lamps with individual transformers further reducing
the pressure to 50 volts are connected to this circuit as shown.]
2. With respect to the connections, as
_a._ Star;
_b._ Delta;
_c._ Star delta;
_d._ Delta star.
The six wire system is shown in fig. 2,160. It is equivalent to three
independent single phase circuits. Such arrangement would only be used
in very rare instances.
[Illustration: FIG. 2,160.--Three phase six wire system. It is
equivalent to three independent single phase circuits and would be used
only in very rare cases.]
~Ques. How can three phase current be transmitted by three conductors?~
Ans. The arrangement shown in fig. 2,160 may be resolved into three
single circuits with a common or grounded return.
When the circuits are balanced the sum of the current being
zero no current will flow in the return conductor, and it
may be dispensed with, thus giving the ordinary star or ~Y~
connected three wire circuit, as shown in fig. 2,163. The
transformation from six to three wires being shown in figs.
2,161 to 2,163.
[Illustration: FIGS. 2,161 to 2,163.--Evolution of the three phase
three wire system. Fig. 2,161 is a conventional diagram of the three
phase six wire system shown in fig. 2,160. A wire is connected to both
ends of each phase winding, giving six conductors, or three independent
two wire circuits. In place of the wires running from A, B, and C,
they may be removed and each circuit provided with a _ground_ return
as shown in fig. 2,162. The sum of the three currents being zero, or
nearly zero, according to the degree of unbalancing, the ground return
may be eliminated and the ends A, B, and C of the three phase winding
connected, as in fig. 2,163, giving the so called _star point_.]
Fig. 2,166 is a view of an elementary three phase three wire
star connected alternator.
~Ques. What are the pressure and current relations of the star
connected three wire system?~
[Illustration: FIGS. 2,164 and 2,165.--Three phase ~four wire~ _star
connected_ alternator and conventional diagram showing pressure and
current relations.]
[Illustration: FIGS. 2,166 and 2,167.--Three phase ~star connected~
alternator, and conventional diagram showing pressure relations.]
Ans. These are shown in the diagram, fig. 2,166 and 2,167.
Assuming 100 amperes and 1,000 volts in each phase winding, the
pressure between any two conductors is equal to the pressure in
one winding multiplied by √3̅, that is 1,000 × 1.732 = 1,732
volts.
The current in each conductor is equal to the current in the
winding, or 100 amperes.
~Ques. Describe the delta connection.~
Ans. In the delta connection, the three phase coils are connected
together forming an endless winding, leads being brought out from these
points.
Fig. 2,168 shows a delta connected three phase alternator, the
pressure and current relation being given in fig. 2,169.
[Illustration: FIGS. 2,168 and 2,169.--Three phase ~delta connected~
alternator and conventional diagram showing pressure and current
relations.]
~Ques. What are the pressure and current relations of the delta
connected three wire system?~
Ans. They are as shown in fig. 2,169.
Assuming 100 amperes and 1,000 volts in each phase winding,
the pressure between any two conductors is the same as the
pressure in the winding, and the current in any conductor is
equal to the current in the winding multiplied by √3̅, that
is 100 × 1.732 = 173.2 amperes, that is, disregarding the
fraction, 173 amperes.
~Ques. What are the relative merits of the star and delta connections?~
Ans. The power output of each is the same, but the star connection
gives a higher line voltage, hence smaller conductors may be used.
[Illustration: FIG. 2,170.--~T~ connection of transformers in which
three phase current is transformed with two transformers. The
connections are clearly shown in the illustration. The voltage across
one transformer is only 86.6% of that across the other, so that if each
transformer be designed especially for its work one will have a rating
of .866 EI and the other EI. The combined rates will then be 1.866 as
compared with 1.732 EI for three single phase transformers connected
either star or delta.]
When it is remembered that the cost of copper conductors is
inversely as the square of the voltage, the advantage of the
~Y~ connected system can be seen at once.
Assuming that three transformers are used for a three phase
system of given voltage, each transformer, star connected,
would be wound for 1 ÷ √3̅ = 58% of the given voltage, and for
full current.
For delta connection, the winding of each transformer is for
58% of the current. Accordingly the turns required for star
connection are only 58% of those required for delta connection.
~Ques. What is the objection to the star connection for three phase
work?~
Ans. It requires the use of three transformers, and if anything happen
to one, the entire set is disabled.
~Ques. Does this defect exist with the delta connection?~
Ans. No.
One transformer may be cut out and the other two operated at
full capacity, that is at ⅔ the capacity of the three.
~Ques. Describe the ~T~ connection.~
Ans. In this method two transformers are used for transforming three
phase current. It consists in connecting one end of both windings of
one transformer to the middle point of like windings of the other
transformer as in fig. 2,170.
[Illustration: FIG. 2,171.--Open delta connection or method of
connecting two transformers in delta for three phase transformation. It
is used when one of the three single phase delta connected transformers
becomes disabled.]
~Ques. What is the open delta connection?~
Ans. It is a method of arranging the connections of a bank of three
delta connected transformers when one becomes disabled as in fig. 2,171.
~Change of Frequency.~--There are numerous instances where it is
desirable to change from one frequency to another, as for instance to
join two systems of different frequency which may supply the same or
adjacent territory, or, in the case of a low frequency installation,
in order to operate incandescent lights satisfactorily it would be
desirable to increase the frequency for such circuits. This is done
by motor generator sets, the motor taking its current from the low
frequency circuit.
Synchronous motors are generally used for such service as the frequency
is not disturbed by load changes; it also makes it possible to use the
set in the reverse order, that is, taking power from the high frequency
mains and delivering energy at low frequency.
[Illustration: FIG. 2,172.--Course of the Schaghticoke-Schenectady
transmission line of the Schenectady Power Co. This transmission line
carries practically the entire output of the Schaghticoke power house
to Schenectady, N. Y., a distance of approximately 21 miles. The line
consists of two separate three phase, 40 cycle, 32,000 volt circuits,
each of 6,000 kw. normal capacity. These circuits start from opposite
ends of the power house, and, after crossing the Hoosic River, are
transferred by means of two terminal towers, fig. 2,173, to a single
line of transmission towers. The two circuits are carried on these on
opposite ends of the cross arms, the three phases being superimposed.
The power house ends of the line are held by six short quadrangular
steel lattice work anchor poles with their bases firmly embedded in
concrete, the cables being dead ended by General Electric disc strain
insulators. This equipment, together with the lightning arrester horn
gaps and the heavy line outlet insulators mounted on the roof of the
power house, is shown in fig. 2,174. While each circuit carries only
6,000 kw. under normal conditions, either is capable of carrying the
entire output of the station; in this case, however, the line losses
are necessarily augmented. This feature prevents any interruption
of the service from the failure of one of the circuits. There are
altogether 197 transmission towers, comprising several distinct types.]
~Ques. In the parallel operations of frequency changing sets what is
necessary to secure equal division of the load?~
Ans. The relative angular position of the rotating elements of motor
and generator must be the same respectively in each set.
[Illustration: FIG. 2,173.--Beginning of Schaghticoke-Schenectady
transmission line; view showing transfer towers with power house in
background.]
~Ques. How is this obtained?~
Ans. Because of the mechanical difficulty of accurately locating the
parts, the equivalent result is secured by arranging the stationary
element in one of the two machines so that it can be given a small
angular shift.
~Transformation of Phases.~--In alternating current circuits it is
frequently desirable to change from one number of phases to another.
For instance, in the case of a converter, it is less expensive and more
efficient to use one built for six phases than for either two or three
phases.
[Illustration: FIG. 2,174.--View from roof of power house of the
Schaghticoke-Schenectady transmission line, showing anchor poles,
strain insulators, lightning arrester horn gap and line entrance
bushings.]
The numerous conditions met with necessitate various phase
transformations, as
1. Three phase to one phase;
2. Three phase to two phase;
3. Two phase to six phase;
4. Three phase to six phase.
These transformations are accomplished by the numerous arrangements and
combinations of the transformers.
[Illustration: FIG. 2,175.--Three phase to one phase transformation
with two transformers. The diagram shows the necessary connections and
the relative pressures obtained.]
~Three Phase to One Phase.~--This transformation may be accomplished
by the use of two transformers connected as in fig. 2,175 in which one
end of one primary winding is connected to the middle of the other
primary winding and the second end of the first primary winding at a
point giving 86.6 per cent. of that winding as shown. The two secondary
windings are joined in series.
~Three Phase to Two Phase.~--The three phase system is universally used
for long distance transmission, because it requires less copper than
either the single or two phase systems. For distribution, however,
the two phase system presents certain advantages, thus, it becomes
desirable at the distribution centers to change from three phase to two
phase. This may be done in several ways.
~Ques. Describe the Scott connection.~
[Illustration: FIG. 2,176.--The Scott connection for transforming from
three phase to two phase. In this method one of the primary wires B
of the .866 ratio transformer is connected to the middle of the other
primary as at C, the ends of which are connected to two of the three
phase wires. The other phase wire is connected at D, the point giving
the .866 ratio. The secondary wires are connected as shown.]
Ans. Two transformers are used, one having a 10:1 ratio, and the other,
a ½√3̅:1, that is, an 8.66:1 ratio. The connections are arranged as in
fig. 2,176.
It is customary to employ standard transformers having the
ratios 10:1, and 9:1.
~Ques. What names are given to the two transformers?~
Ans. The one having the 10:1 ratio is called the ~main~ transformer,
and the other with the 8.66:1 ratio, the ~teaser~ transformer.
In construction, the transformers may be made exactly alike so
that either may be used as main or teaser.
In order that the connections may be properly and conveniently
made, the primary windings should be provided with 50% and
86.6% taps.
[Illustration: FIG. 2,177.--Three phase to two phase transformation
with three star connected transformers. Two of the secondary windings
are tapped at points corresponding to 57.7% of full voltage; these two
windings are connected in series to form one secondary phase of voltage
equal to that obtained by the other full secondary winding.]
~Ques. Describe another way of transforming from three to two phases.~
Ans. The transformation may be made by three star connected
transformers, proportioning the windings as in fig. 2,177, from which
it will be seen that two of the secondary windings are tapped at points
corresponding to 57.7 per cent. of full voltage.
~Three Phase to Six Phase.~--This transformation is usually made for
use with rotary converters and may be accomplished in several ways. As
these methods have been illustrated in the chapter on Converters (page
1,462), it is unnecessary to again discuss them here. Fig. 2,178, below
shows the _diametrical_ connection for transforming three phase to six
phase.
[Illustration: FIG. 2,178.--Diagram of ~diametrical connection~,
_three phase to six phase_. ~It is obtained~ _by bringing both ends
of each secondary winding to opposite points on the rotary converter
winding_, utilizing the converter winding to give the six phases. This
transformation of phases may also be obtained with transformers having
two secondary windings.]
~Alternating Current Systems.~--The saving in the cost of transmission
obtained by using alternating instead of direct current is not due
to any difference in the characteristics of the currents themselves,
but to the fact that in the case of alternating current very high
pressures may be employed, thus permitting a given amount of energy to
be transmitted with a relatively small current.
In the case of direct current systems, commutator troubles limit the
transmission pressure to about 1,000 volts, whereas with alternating
current it may be commercially generated at pressures up to about
13,000 and by means of step up transformers, transmitted at 110,000
volts or more.
[Illustration: FIG. 2,179.--End of Schaghticoke-Schenectady
transmission line at Schenectady; view showing entrance bushings and
lightning arrester horn gaps.]
~Relative Weights of Copper Required by Polyphase Systems.~--A
comparison between the weights of copper required by the different
alternating current systems is rendered quite difficult by the fact
that the voltage ordinarily measured is not the maximum voltage, and as
the insulation has to withstand the strain of the maximum voltage, the
relative value of copper obtained by calculation depends upon the basis
of comparison adopted.
As a general rule, the highest voltage practicable is used for long
distance transmission, and a lower voltage for local distribution.
Furthermore, some polyphase systems give a multiplicity of voltages,
and the question arises as to which of these voltages shall be
considered the transmission voltage.
If the transmission voltage be taken to represent that of the
distribution circuit, and the polyphase system has as many independent
circuits as there are phases, the system would represent a group of
several single phase systems, and there would be no saving of copper.
Under these conditions, if the voltage at the distant end be taken as
the transmission voltage, and the copper required by a single phase
two wire system as shown in fig. 2,180, be taken as the basis of
comparison, the relative weights of copper required by the various
polyphase systems is given in figs. 2,181 to 2,188.
[Illustration: FIG. 2,180.--Single phase line, used as basis of
comparison in obtaining the relative weights of copper required by
polyphase systems, as indicated in figs. 2,181 to 2,188.]
In the case represented in fig. 2,180, if the total drop on the line
be 100 volts, the generated voltage must be 1,100 volts, and the
resistance of each line must be 50 ÷ 1,000 = .05 ohms. Calculated on
this basis, a two phase four wire system is equivalent to two single
phase systems and gives no economy of copper in power transmission over
the ordinary single phase two wire system. This is the case also with
any of the other two phase systems, except the two phase three wire
system.
[Illustration: FIGS. 2,181 to 2,188.--Circuit diagrams showing relative
copper economy of various alternating current systems.]
In this system two of the four wires of the four wire two phase
system are replaced by one of full cross section.
The amount of copper required, when compared with the single
phase system, will differ considerably according as the
comparison is based on the highest voltage permissible for any
given distribution, or on the minimum voltage for low pressure
service.
If E be the greatest voltage that can be used on account of
the insulation strain, or for any other reason, the pressure
between the other conductors of the two phase three wire system
must be reduced to E ÷ √2̅.
The weight of copper required under this condition is 145.7%
that of the single phase copper.
On the basis of minimum voltage, the relative amount of copper
required is 72.9% that of the single phase system.
[Illustration: FIG. 2,189.--Twin circuit "aermotor" towers carrying
44,000 volt conductors (Southern Power Co.). These towers vary in
height from 35 to 50 feet, and the circuits are transposed every 10
miles. The towers are assembled on the ground and erected by means of
gin poles. They are normally spaced 500 feet apart with a sag of 5
feet 8 inches. The minimum distance between towers is 300 feet and the
maximum 700 feet.]
Figs. 2,187 and 2,188 are two examples of three phase four wire
systems. The relative amount of copper required as compared
with the single phase system depends on the cross section of
the fourth wire. The arrangement shown in fig. 2,188, where
the fourth wire is only half size, is used only for secondary
distribution systems.
[Illustration: FIG. 2,190.--General Electric standard tower for high
tension three phase transmission line.]
[Illustration: FIG. 2,191.--General Electric transposition tower for
high tension three phase transmission line.]
~Choice of Voltage.~--In order to properly determine the voltage for
a transmission system there are a number of conditions which must be
considered in order that the economy of the entire installation shall
be a maximum.
The nature of the diversely various factors which affect the problem
makes a mathematical expression difficult and unsatisfactory.
~Ques. What is the relation between the cross sectional area of the
conductors and the voltage?~
Ans. For a given circuit, the cross sectional area of the conductors,
or weight varies inversely as the voltage.
[Illustration: FIG. 2,192.--General Electric standard tower under
construction.]
~Ques. Would the highest possible voltage then be used for a
transmission line?~
Ans. The most economical voltage depends on the length of the line and
the cost of apparatus.
For instance, alternators, transformers, insulation and circuit
control and lightning protection devices become expensive when
manufactured for very high pressures. Hence if a very high
pressure were used, it would involve that the transmission
distance be great enough so that the extra cost of the high
pressure apparatus would be offset by the saving in copper
effected by using the high pressure.
In the case of the longest lines, from about 100 miles up,
the saving in copper with the highest practicable voltage
is so great that the increase in other expenses is rendered
comparatively small.
In the shorter lines as those ranging in length from about
one mile to 50 or 75 miles, the most suitable voltage must be
determined in each individual case by a careful consideration
of all the conditions involved. No fixed rule can be
established for proper voltage based on the length, but the
following table will serve as a guide:
[Illustration: FIG. 2,193.--Line of the Schenectady Power Company
crossing the tracks of the Boston and Maine Railroad near Schaghticoke.]
Usual Transmission Voltages
+----------------------------------------+
| Length of line | Voltage |
| in miles | |
+------------------+---------------------+
| 1 | 500 to 1,000 |
| 1 to 2 | 1,000 to 2,300 |
| 2 to 3 | 2,300 to 6,600 |
| 3 to 10 | 6,600 to 13,200 |
| 10 to 15 | 13,200 to 22,000 |
| 15 to 20 | 22,000 to 44,000 |
| 20 to 40 | 44,000 to 66,000 |
| 40 to 60 | 66,000 to 88,000 |
| 60 to 100 | 88,000 to 110,000 |
+------------------+---------------------+
~Ques. What are the standard voltages for alternating current
transmission circuits?~
Ans. 6,600, 11,000, 22,000, 33,000, 44,000, 66,000, 88,000.
The amount of power to be transmitted determines, in a
measure, the limit of line voltage. If the most economical
voltage considered from the point of view of the line alone,
be somewhere in excess of 13,200, step up transformers must
be employed, since the highest voltage for which standard
alternators are manufactured is 13,200. In a given case, the
saving in conductor by using the higher voltage may be more
than offset by the increased cost of transformers, and the
question must be determined for each case.
[Illustration: FIG. 2,194.--View of a three phase, 2,300 volt, 60 cycle
line at Chazy, N. Y. The current is transmitted at the alternator
voltage 2¾ miles over the single circuit pole line. The poles are of
cedar with fir cross arms, and are fitted with pin insulators. They are
from 35 to 40 feet high and are spaced at an average of about 120 feet.
The conductors are bare copper wire No. 00 B. & S. The alternators
consist of one 50 kw., and one 100 kw. General Electric machines.]
~Ques. What are the standard transformer ratios?~
Ans. Multiples of 5 or 10.
[Illustration: FIGS. 2,195 to 2,197.--Diagram showing electric railway
system. Three phase current is generated at the main station where it
passes to ~step up~ transformers to increase the pressure a suitable
amount for economical transmission. At various points along the railway
line are _sub-stations_, where the three phase current is reduced in
pressure to 500 or 600 volts by ~step down~ transformers, and converted
into direct current by rotary converters. The relatively low pressure
direct current is then conveyed by "feeders" to the rails, this
resulting in a considerable saving in copper.]
~Mixed Current Systems.~--It is often desirable to transmit electrical
energy in the form of alternating current, and distribute it as direct
current or vice versa.
Such systems may be classed as mixed current systems. The usual
conversion is from alternating current to direct current because of
the saving in copper secured by the use of alternating current in
transmission, especially in the case of long distance lines. Such
conversion involves the use of a rotary converter, motor generator set,
or rectifier, according to the conditions of service.
[Illustration: FIG. 2,198.--Example of converter sub-station, showing
the Brooklyn Edison Co. Madison sub-station. The transformers are
seen on the left, the converter shown at the right is a Westinghouse
synchronous booster rotary converter, consisting of a standard rotary
converter in combination with a revolving armature alternator mounted
on the same shaft with the converter and having the same number of
poles. _The function of the machine is to_ ~convert~ and ~regulate~
_the pressure_. By varying the field excitation of the alternator, the
A. C. voltage impressed on the rotary converter proper can be increased
or decreased as desired. Thus, the D. C. voltage delivered by the
converter is varied accordingly. This type of converter is well adapted
for any application for which a relatively wide variation, either
automatic or non-automatic, in direct current voltage is necessary.
Also especially for serving incandescent lighting systems where
considerable voltage variation is required for the compensation of drop
in long feeders, for operation in parallel with storage batteries and
for electrolytic work where extreme variations in voltage are required
by changes in the resistance of the electrolytic cells.]
The suburban trolley forms a good example of a mixed system, in
which alternating current is generated at the _central station_ and
transmitted to _sub-stations_, where it is transformed to low pressure,
and converted into direct current for use on the line. Fig. 2,195 shows
the interior of a sub-station of this kind.
~Ques. What direct current pressure is usually employed on traction
lines?~
Ans. 500 volts.
~Ques. Mention another important service performed by a mixed system.~
Ans. If the generator furnish alternating current it must be converted
into direct current in order to charge storage batteries.
CHAPTER LVI
AUXILIARY APPARATUS
For the proper control of the alternating current in any of the
numerous systems described in the previous chapter, various devices,
which might be classed as "auxiliary apparatus," are required. These
may be grouped into several divisions, according to the nature of the
duty which they perform, as
1. Switching devices;
_a._ Ordinary switches;
_b._ Oil break switches;
_c._ Remote control switches.
2. Current or pressure limiting devices;
_a._ Fuses;
_b._ Reactances;
_c._ Circuit breakers;
_d._ Relays.
3. Lightning protection devices;
_a._ Air gap arresters;
_b._ Multi-gap arresters;
_c._ Horn gap arresters;
_d._ Electrolytic arresters;
_e._ Vacuum tube arresters;
_f._ Choke coils;
_g._ "Static" interrupters.
4. Regulating devices;
_a._ Induction voltage regulators;
_b._ Variable ratio transformer regulators { drum type;
{ dial type;
_c._ Compensation shunts;
_d._ Pole type regulators;
_e._ Small feeder voltage regulators;
_f._ Automatic voltage regulators;
_g._ Line drop compensators;
_h._ Starting compensators;
_i._ Star delta switches.
5. Power factor regulating devices;
_a._ Condensers;
_b._ Synchronous condensers.
6. Indicating devices;
{ plunger type;
_a._ Moving iron instruments { inclined coil type;
{ magnetic vane type;
_b._ Hot wire instruments;
_c._ Induction instruments { shielded pole type;
{ repulsion type;
_d._ Dynamometers;
_e._ Instrument transformers;
{ commutator type;
_f._ Watthour meters { induction type;
{ Faraday disc type;
{ synchronous motor type;
_g._ Frequency indicators { resonance type;
{ induction type;
{ lamp type;
_h._ Synchronism indicators { voltmeter type;
{ resonance type;
{ rotating field type;
_i._ Power factor indicators { wattmeter type;
{ rotating field type;
_j._ Ground detectors;
_k._ Earth leakage cut outs;
_l._ Oscillographs.
CHAPTER LVII
SWITCHING DEVICES
A switch is _a piece of apparatus for making, breaking, or changing the
connections in an electric circuit_.
The particular form and construction of any switch is governed by the
electrical conditions under which it must operate.
_Since the electric current cannot be_ ~stopped instantly~ _when the
circuit in which it is flowing, is broken_, ~an arc is formed~ _as the
switch contacts separate_; this tends to burn the contacts, and to
short circuit, the severity of such action depending on the voltage and
the proximity of the switch terminals. Accordingly in switch design,
provision must be made to counteract these tendencies. Thus,
1. The contacts should separate along their entire length, rather than
at a point;
2. The terminals should be far enough apart and properly protected to
prevent short circuiting of the arcs;
3. The break should be quick;
4. The gap should be surrounded by the proper medium (air or oil) to
meet the requirements of the electrical conditions.
A great variety of switches have been introduced to suit the different
requirements. Knife switches are used for low pressure service, the
multiple break form being used where it is desired to reduce the arcing
distance.
~Ques. How should single throw switches be installed?~
Ans. They should open downward so gravity will keep them open.
[Illustration: FIGS. 2,199 and 2,200.--General Electric triple pole
solenoid operated, single throw remote control switch, and push button
switch for operating same. Switch is a self-contained unit with two
sets of contacts, main laminated copper brushes, and carbon auxiliary
contacts to take the arc on breaking the circuit. The main brushes are
so made that each lamination makes an end on contact with the switch
blade without any tendency to force the laminations apart. A wiping
effect, given to the contacts every time the switch is closed, keeps
the contact surfaces clean and insures good contact at all times.
The carbon auxiliary contacts are made of blocks of carbon fastened
without screws. ~In operation~, the switch is actuated by a double coil
solenoid, one coil for closing and one for opening, controlled by the
single pole double throw push button switch shown in fig. 2,200, which
is normally in the open position and remains closed only when held by
the operator. One of these switches is furnished with each control
switch and must always be used, as the solenoid coils are not intended
for continuous service. The power required to operate the remote
control switch is small, being approximately 1.6 amperes at 110 volts,
0.81 amperes at 220 volts direct current, and 10 amperes at 110 volts,
and 6 amperes at 220 volts alternating current 60 cycles. The main
switch can be closed and opened by hand, and the push button located at
any point.]
~Ques. How should double throw switches be installed?~
Ans. Horizontally.
[Illustration: FIGS. 2,201 and 2,202.--Palmer service switch and
fuse box, for either plug, cartridge or open link fuses. Fig. 2,201
illustrates the box in open position for the inspection of fuses, etc.
The cover is held open by a simple lock so that the switch cannot
fall closed by gravity, the box may be mounted so that the service
wires lead directly into a sealed terminal chamber from any direction,
and all current carrying parts made accessible by the opening of the
switch are dead. Fig. 2,202 illustrates the device with side of box
and cover cut away to show interior and the normally sealed cover
of terminal chamber removed. The switch contacts do not enter their
contact clips until the flanged cover of the box has closed the switch
opening, no current connections being made to line or load until the
box is completely closed, and in consequence there is no opportunity
to make improper connections to any live parts of switch, when conduit
connections are used to the service and meter wires.]
~Ques. What is a plug switch?~
Ans. A switch in which the current is ruptured in a tube enclosed at
one end, thereby confining the arc and limiting the supply of air.
They are used on high pressure circuits of from 10,000 to
20,000 volts, for transferring live circuits and for voltmeter
and synchronizing circuits where there is very little energy.
The usual current capacity is from 4 to 7½ amperes.
[Illustration: FIG. 2,203 and 2,204.--~Bus transfer plug switch.~ The
method of supporting the contact farthest from the panel consists
of a porcelain pillar of the same height as the receptacle, clamped
to a brass connecting or bus bar which in turn is fastened to the
receptacle.]
[Illustration: FIG. 2,205.--~Ammeter jack.~ This plug switch is
insulated for high pressure and ~consists of two parts~: _the ammeter
jack, and the ammeter jack plug, cable, and bushing_. The receptacle,
which is simple in construction, consists of a brass bushing well
insulated from the panel and protected on the front of the panel by a
porcelain bushing. ~On the end of this tube~ and insulated from it,
is a phosphor bronze spring which, when the plug is out, rests on the
brass tube and keeps the circuit closed. ~The plug~ consists of a
brass rod well insulated and set in a brass tube, both being fastened
in a handle which is stained black and polished. Inside the handle
is run a twin conductor cable, one side being soldered in the brass
tube and the other to the brass rod. The other end of the cable is run
through a bushing set in the panel and thence to the ammeter or current
transformer. Where it is desired to remove the plug and cable from the
board, or to plug both ends of the cable in different receptacles, a
plug instead of a bushing should be used. In this case a cable should
be provided with a plug on each end.]
~Forms of Break.~--On high pressure circuits there are several types of
switch: they are classified with respect to the break, that is to say,
according as the break takes place,
1. In open air;
2. In an enclosed air space;
3. Aided by a metal fuse;
4. Aided by a horn;
5. In oil.
[Illustration: FIG. 2,206.--Westinghouse fused starting switch for
squirrel cage motors. It is arranged for National Electric Code fuses
on one end only and has springs on the other end to open the switch
automatically if left closed at this end. The corresponding terminals
at both ends of the switch are connected in grooves in the back of the
slate base so that the wiring need be connected to one set of these
terminals only, thus decreasing the number of connections necessary,
as shown in fig. 2,207. In starting an induction motor, the switch is
thrown to the end that is not fused and held there until the motor is
up to running speed; then it is quickly thrown to the fused position,
thus protecting the circuit under running conditions.]
[Illustration: FIG. 2,207.--Diagram of connections of Westinghouse
fused starting switch for squirrel cage motors. The starting current of
induction motors is several times the normal running current and, when
the controlling switch is fused to carry the running load only, the
fuses are apt to blow when the motor is started. The fuses must be of a
capacity to prevent overloads under running conditions. These switches
are designed to meet this difficulty and are used without auto-starters
to control motors up to 5 horse power rating.]
~Ques. What is the objection to open air break?~
Ans. The relatively long gap required to extinguish the arc, limiting
this form of switch to low or moderate pressure circuits.
The open air arc may cause very high voltage oscillations when
the circuit contains inductance and capacity unless the break
occur at zero value.
[Illustration: FIG. 2,208.--Westinghouse single pole disconnecting
switch. Disconnecting switches are used primarily for isolating
apparatus from the circuit for purposes of inspection and repair; also
for sectionalizing feeders. They are not designed for opening under
load, and therefore no attempt should be made to open them with current
in the circuit. In connection with lightning arrester installations,
disconnecting switches are particularly useful, providing a simple
and effective means for isolating the arresters while cleaning and
inspecting. The switch is opened and closed with a hook on the end of a
wooden pole, which hook engages in a hole provided in the switch blade.
This type of disconnecting switch is intended for wall mounting. The
live parts are mounted on porcelain insulators carried on a cast iron
yoke or base, forming a simple and substantial construction.]
~Ques. What are disconnecting switches?~
Ans. Knife switches in series with other switches so that the apparatus
controlled by the latter may be repaired in safety by entirely
disconnecting it from the bus bars or live circuit.
Such switches are not intended to rupture the load current.
[Illustration: FIGS. 2,209 and 2,210.--Westinghouse disconnecting
switches for pressures over 3,300 volts.]
[Illustration: FIGS. 2,211 and 2,212.--Westinghouse selector type
disconnecting switch. Fig. 2,211, view showing both sides closed; fig.
2,212, view with one side open. The selector type of disconnecting
switch is a transfer switch which does not require the circuit to be
interrupted while making the change. It can also be used to connect two
independent circuits in parallel. In construction, it is in effect two
single throw, single pole disconnecting switches with the hinge jaws
connected together and mounted on the same insulator. The hinge jaw is
also provided with dummy jaws to hold either blade of the switch in
the open position. Except for these differences in the hinge jaws, the
construction is similar to the switch shown in fig. 2,209. It should
not be used to open the circuit when loaded.]
[Illustration: FIG. 2,213.--Hook stick for operating a disconnecting
switch.]
~Ques. What are the features of the enclosed air break?~
Ans. The switch is more compact than the open air break type, but
pressure oscillations are caused on opening the circuit the same as
with the open air break, and it is not desirable for heavy current.
[Illustration: FIG. 2,214.--Baum 35,000 volt, 200 ampere, double break
pole type switch. While designed for disconnecting purposes only, it
can break considerable amperage. The levers and couplings are fastened
with tape pins. The control shaft coupling is adjustable to any
angle, and the switch can be locked in the open or closed position. A
removable wooden handle is supplied and the switch can be handled in
any weather. The arms can be extended to hold fuse fittings, or dead
end insulators in the event of a heavy strain, but it is preferable
to have fuses on another structure as a precaution against coming
in contact with the energized portion of the switch, and it is also
preferable to take the strain of the line on a pole a few feet from the
switch, rather than on the switch structure, particularly in the larger
sizes. An insulating wood section in the control shaft separates the
control handle from the remainder of the switch. Discharging horns can
be fitted to this type of switch and when so equipped they have been
found capable of breaking considerable loads.]
~Ques. How is the fuse arranged in the metal fuse break type of switch?~
Ans. It is placed in a tube fitted with powdered carbonate of lime or
some other insulating powder.
[Illustration: FIG. 2,215.--Pacific swivel type blade for Baum pole
top switches. The ~twist type~ _of blade_, here shown, is especially
adapted to switches operating in freezing or sleety weather. ~It
will be seen~ that the first few degrees through which the rotating
insulator is moved _have the effect of twisting the blade between the
shoes of the contact_, which breaks any seal through freezing, or
corrosion.]
[Illustration: FIG. 2,216.--Pacific 22,000 volt, 100 ampere, ~pole
top switch~ equipped with fuse tubes; designed to meet the need for a
small group controlled disconnecting switch, having several features
making it suitable for use with service transformer installations and
line branches. ~The switch is made~ with clamped pipe arms permitting
adjustment. It is equipped with fuse tubes and fittings, but should
the fuses be not desired, the arm may be shortened. Provision is made
for fitting insulator pins to the top of the arms, when the switch is
mounted vertically, which will hold insulators at right angles to the
switch, making it possible to end a line on the top of these arms and
then drop down through the switch to the bank of transformers. _The
switch is so constructed that_ ~gravity~ _tends to hold it in either
the open or the closed position_. Provision can be made for locking.]
[Illustration: FIG. 2,217.--Horn break switch. ~In operation~, the
arc formed at break, will travel toward the extremities of the horns
because of the fact that a circuit will tend to move so as to embrace
the largest possible number of lines of force set up by it. Hence, the
arc that starts between the horns where they are near together rises
between them until it becomes so attenuated that it is extinguished.]
[Illustration: FIG. 2,218.--Westinghouse rear connected motor starting
switch, for pressures up to 600 volts. It is used for starting rotary
converters and direct current motors of large capacity having starting
torque small enough to permit cutting out the starting resistance in
few steps. The clips can be connected to any type of resistor, the
steps of which are successively short circuited as the switch closes;
the amount of resistance in the armature circuit is thus gradually
reduced. A pause should be made after each step of resistance is thrown
in to allow the motor speed to accelerate. If the starting switch do
not have to carry the full load current and can be short circuited by
another switch, a starting switch of smaller capacity equivalent to 50
per cent of running current of the machine can be used. The switch is
of the single pole, single throw, rear connected, four point, knife
blade type.]
~Ques. Describe its operation.~
Ans. The moving arm of the switch draws the fuse through the tube, thus
opening the circuit without much disturbance.
[Illustration: FIG. 2,219.--Baum disconnecting switch with horns and
auxiliary contacts (Pacific Mfg. Co.). This switch is for use on
systems operating at 100,000 volts or over. It has a spacing of five
feet between outer insulators, is equipped with auxiliary shoes that
break the circuit between the horns, diverting it from the current
carrying contacts so that they are not attacked by the arc.]
~Ques. What is the objection to the metal fuse switch?~
Ans. The powder is set flying by the explosion of the arc, which, as
it settles, gets into the bearings of any machine that may be in the
vicinity.
~Ques. What is a horn break switch?~
Ans. One provided with horn shaped extensions to the contacts, as shown
in fig. 2,219.
The arc formed on breaking the circuit, as it travels toward
the extremities of the horns, becomes attenuated and is finally
ruptured.
[Illustration: FIG. 2,220.--Kelman switching mechanism. The pantograph
arrangement of the contact blades gives a double horizontal break deep
down in the oil. This gives over the break a heavy head of oil which
immediately closes in around the thin blades as they leave the contacts
in opening, thus effectually extinguishing the arc. The opening spring
acts within the pantograph itself without any intervening mechanism,
and the light weight of the few moving parts enables the spring to
accelerate the blades rapidly, thus obtaining a quick break. The
contacts are of the return bend type, which makes a flexible contact,
to obtain alignment with the blades at all times. The pantograph and
contacts are supported on corrugated porcelain insulators on a hardwood
base or insulator board. The insulators are fitted with iron ends for
securing the different parts. At each end of the insulator board is an
upright or lifting board which serves to lift the switching mechanism
out of the tank. The leads are heavily insulated.]
~Ques. What are the objections to this type of switch?~
Ans. The considerable space required for the horns and arcs, and the
line surges caused by the arc.
Horn switches were used extensively for high pressure
alternating current circuits before the introduction of oil
switches.
[Illustration: FIG. 2,221.--Sectional view of Pacific weatherproof oil
switch for use in places exposed to the weather. All moving and contact
parts are supported from the cast iron top and are readily removable
for inspection or repair.]
~Oil Switches.~--The extensive use of high pressure currents and
alternating current motors and other devices introducing inductance
make it necessary to use switches radically different from the ordinary
air break types.
[Illustration: FIG. 2,222.--General Electric central station triple
pole single throw oil switch; view of switch in tank. This type is for
pressures up to 110,000 volts, being adapted for stations employing
open wiring, since the connections are made at the top of the switch
and its construction obviates the need for isolating it in a cell. One
tank with two breaks in series are used for each phase.]
The opening of circuits of considerable current value with inductive
loads is not possible with old style switches which were quite
adequate for the service for which they were designed. These circuits
are controlled with ease and certainty by the oil switch.
[Illustration: FIGS. 2,223 to 2,226.--Westinghouse indoor, two pole
double throw oil switch for pressures not over 6,600 volts. Fig. 2,223,
open position; fig. 2,225, closed position. This type of switch is
suited for a wide range of application, being made in both switchboard
and wall mounting styles; also for remote mechanical control by the
use of bell cranks and connecting rods. The wall mounting style is
adaptable to motor installations on account of the facility with which
it may be mounted on any support, convenient to the motor operator. The
lever and handle extend outward over the oil tank, so that the switch
may readily be mounted against a wall, post or any vertical support.
The characteristic features of this type of switch are: knife blade
contacts submerged in oil; live parts carried on a porcelain base
affording a permanent insulation between adjacent poles, and between
the frame and live parts; compactness and accessibility; enclosure of
all live metal parts; and low first cost. Each contact jaw has attached
to it an arcing piece which takes the final break, thus preventing any
burning of the jaws. These arcing pieces are inexpensive and readily
replaced when worn or burnt away. The contact making parts are enclosed
in a sheet metal oil tank which has an insulating lining. The leads are
brought out at the top. Connections to the outside circuit are made
inside the switch and a porcelain insulator is slipped over the joint,
thus providing a straight continuous connection from the line with
maximum insulation. On the 6,600 volt switch, insulation is obtained
by the use of porcelain bases for supporting the live parts. In the
3,300 volt switch specially treated wooden bases are used, suitable
barriers being provided between the poles where necessary to prevent
arcs communicating.]
~Ques. What is an oil switch?~
Ans. One in which the contact is broken under oil.
This type of switch is the one almost universally used on high
pressure alternating current circuits, because of the fact that
the oil tends to cause the current to break when at its zero
value, thus preventing the heavy arcing which would occur with
an air break switch, and the consequent surges in the line
which are so often the cause of breakdown of the insulation of
the system.
[Illustration: FIG. 2,227.--Kelman electric control unit for oil
switch. It consists of an iron frame which contains the opening and
closing coils and the bearings for the operating bell crank. A small
switch on the frame automatically opens the coil circuit at the end of
the stroke in either direction and operates signal lamps to indicate
the open or closed position. The automatic overload release opens the
switch by closing the opening coil circuit. This electrical operating
unit gives satisfactory service through a wide variation of voltage.
It requires a momentary expenditure of energy of from 1,500 to 4,000
watts, depending on the size.]
~Ques. What is the nature of an oil break?~
Ans. It is not a quick break.
Oscillograph records show that the effect of the oil is to
allow the arc to continue during several cycles and then to
break the current, usually at the zero point of the wave.
~Remote Control Oil Switches.~--It is desirable in the case of switches
on high pressure circuits to locate the parts which carry the high
pressure current at some distance from the switchboard in order that
they may be operated with safety.
With respect to the manner in which the switches are operated they may
be classed, as
1. Hand operated;
2. Power operated.
[Illustration: FIGS. 2,228 and 2,229.--Views showing mechanism of hand
operated remote control switches. Fig. 2,228, straight mechanism; fig.
2,229, angular mechanism.]
~Ques. What kind of power is used?~
Ans. Electricity is used in most cases; in some installations, switches
are operated by compressed air.
~Ques. For what pressures should remote control switches be used?~
Ans. For pressures above 1,100 volts.
~Ques. Describe the operating mechanism of a remote control, hand, and
electrically operated switch.~
Ans. For hand operation, the mechanism between the operating lever and
switch proper, consists simply of a system of links and bell cranks.
Various shapes of bell crank are used, to permit change in direction or
position of the force applied to operate the switch.
[Illustration: FIG. 2,230.--Pacific ~oil switch with solenoid control~,
designed for 60,000 and 70,000 volt installations; it is capable of
handling a 25,000 kw. generating station. The break is horizontal,
made by the rotation of a flat member edgewise through the oil. The
solenoid, at its extreme outer position, has a free start before
commencing to move the control parts of the switch. As it approaches
the extreme inner position, where the opening spring and the contacts
begin to offer the greatest resistance, the magnetic action is, of
course, most powerful, and the leverage by which it is applied moves
to an increasing radius, by means of rollers working in the curved
slots of the control shaft levers. These curved slots and rollers have
the additional advantage of making the opening action very free and
smooth. The tripping coil does not act on the latch directly, but gives
a hammer blow that is positive. The latch proper is a roller having a
powerful hold and easy release. Current can not be left on either the
closing or opening coils, as they are automatically cut out by the
movement of the switch.]
~Ques. Name two classes of electrically operated remote control switch.~
Ans. Those operated by solenoids, and those operated by motors.
The solenoid type are closed by the action of a plunger
solenoid, and opened either by another solenoid called a
"tripping coil" or by gravity. Some examples of remote control
are shown in the accompanying illustrations.
~Ques. What indicating devices are used with electrically operated
switches?~
Ans. Red and green lamps; ~red~ for _closed_ and ~green~ for _open_ as
shown in fig. 2,231.
[Illustration: FIG. 2,231.--Diagram of connections of motor operated
remote control switch. The motor which operates the switch is
controlled by a small lever generally mounted on the panel with the
instruments which are in the circuit controlled by the switch. The
standard pressure for operating the motors is 125 volts.]
~Ques. For what service are motor operated switches used?~
Ans. For exceptionally heavy work where the kilowatt rupturing capacity
is greater than that for which the other types are suitable.
[Illustration: FIG. 2,232.--General Electric motor operated three
phase oil switch. The operation of the oil switch is accomplished by
a small hand controlling switch, generally mounted on the panel, with
the instruments which are in the circuit controlled by the oil switch.
The standard pressure for the operating motor is 125 volts. The switch
has six breaks, each break being a separate tank. In addition to this
isolation of the breaks, each phase is enclosed in a fireproof brick
compartment, making it impossible for trouble in one phase to be
communicated to another. The cells are constructed of brick with top
and bottom slabs of slate. The capacities of such switches, range from
2,500 to 60,000 volts, and from 100 to 1,000 amperes.]
~Rupturing Capacity of Oil Switches.~--While an oil switch may be
designed for a given pressure and to carry a definite amount of
current, it should not be understood that the switch will necessarily
rupture the amount of normal energy equivalent to its volt ampere
rating.
[Illustration: FIGS. 2,233 to 2,235.--Diagrams showing connections for
General Electric single, double, and triple pole, solenoid operated
remote control switches. The operating coils are shown connected to
main switch circuit, but may be connected to an entirely separate
control circuit. Connections are the same for either alternating or
direct current.]
Oil switches are often used on systems with generator capacity of
many thousand kilowatts. It is therefore essential that the switches
shall be able to break not only their normal current, but also greatly
increased current that would flow if a short circuit or partial short
circuit occur.
[Illustration: FIG. 2,236.--Westinghouse three pole hand operated
remote control oil switch, adapted for the control of alternating
current circuits of small and moderate capacities, the pressures of
which do not exceed 25,000 volts. Each unit is installed in a separate
masonry compartment. The open position of contacts is maintained by
gravity. Up to and including the 600 ampere capacity, the contacts
are cone shaped with an arcing tip, as shown for capacities in excess
of 600 amperes, brush contacts are furnished with auxiliary arcing
contacts of the butt type. Each pole has two sets of contacts, thus
providing a double break in each line. With both types of contact, the
final break of the arc is taken and the main contacts protected by
auxiliary arcing contacts which are inexpensive and readily renewable.
The upper or stationary contacts are mounted on porcelain insulators
secured in the soapstone base. The lower or movable contacts are
carried by a wooden rod connected to and moved vertically by the
operating mechanism. The operating mechanism of the hand operated
breaker consists of a simple system of levers, bell cranks, and rods.
The necessary energy for making a positive contact is small owing to
the use of a toggle mechanism. The leads are brought out of the top
of the breaker through heavy porcelain insulators. On breakers above
3,500 volts, the connections to the line wires are made by means of
a union which can be tightened with a socket wrench fitting inside
the insulator. As the leads coming into the switch are necessarily
insulated wire or cable, this arrangement eliminates all exposed live
parts and is well adapted to making connections readily to bus bars
located above or in the rear of the circuit breakers.]
[Illustration: FIG. 2,237.--Cutler-Hammer ~enclosed float switch~,
designed for the automatic control of alternating current motors
operating pumps used to fill or empty tanks, sumps or other reservoirs.
~The switch is operated~ _by the rise and fall of a copper float which
is connected to the switch lever by a brass rod or copper chain_. As
the water level rises and falls, the float moves up and down. This
movement is transmitted to the switch lever and the switch (if the
movement be sufficient) is tripped to make or break the motor circuit.
~To insure the best operation~ it is necessary that the float rod be
provided with a guide so that the float will move up or down in a
vertical line, as shown. The minimum difference in water level at which
the switch will operate is approximately 10 to 12 inches. When the
float is placed in a closed tank, the minimum height inside from the
bottom of the tank to the top should be at least 6 inches greater than
the difference in water level to provide sufficient clearance for the
float. ~When this type switch is used as a tank switch~, the contacts
are closed when the water level is low, putting the motor, driving the
pump, in motion. When the water in the tank reaches a predetermined
high level the float arm opens the switch contacts, and the motor is
disconnected from the line. ~For sump pump purposes~, the contacts open
on low level and close on high level, the lever being reversed for this
purpose. Two pole, three pole and four pole switches of this type are
made, all arranged to completely disconnect single phase, two phase and
three phase motors from their circuits. When used with small motors
which may be thrown across the line to start, the switch may be used
without a self starter if desired.]
Under short circuit conditions alternators develop instantaneously many
times their normal load current, while the sustained short circuit
current is approximately two and a half to three times normal, or even
higher with turbine alternators. Hence, circuit breakers of the so
called instantaneous type must be capable of rupturing the circuit
when the current is at a maximum, whereas, non-automatic switches, or
circuit breakers with time limit relays will be required to interrupt
only the sustained short current circuit. The reason is evident, since
the delay in opening the switch allows the current to approach the
sustained short circuit conditions.
CHAPTER LVIII
CURRENT AND PRESSURE LIMITING DEVICES
In any electric installation there must be provided a number of
automatic devices to secure proper control. The great multiplicity
of devices designed for this purpose may be divided into two general
classes, as
1. Current limiting;
2. Pressure limiting.
Because of the heating effect of the current which increases in
proportion to the square of the strength of the current, it is
necessary to protect circuits with devices which do not allow the
current to exceed a predetermined value.
Accordingly fuses, circuit breakers, reactances, etc., are used, each
possessing certain characteristics, which render it suitable for
particular conditions of service.
For instance, just as in analogy, steam boilers must be
protected against abnormal pressures by safety valves, electric
circuits must be guarded against excessive voltages by pressure
limiting devices, otherwise much damage would occur, such as
the burning out of incandescent lamps, grounding of cables, etc.
The control of steam is simple as compared to the electric
current, the latter being the more difficult to manage because
of its peculiar behaviour in certain respects, especially in
the case of alternating current which necessitates numerous
devices of more or less delicate construction for safety both
to the apparatus and the operator.
~Fuses.~--A fuse is "an electrical safety valve", or more specifically,
_the actual wire or strip of metal in a cut out, which may be fused by
an excessive current_, that is to say, by a current which exceeds a
predetermined value. A fuse, thus serves to protect a circuit from any
harm resulting from an undue overload.
Fuses have been treated at such length in Guide No. 2, Chapter
XXV, that very little can be said here, without repetition.
[Illustration: FIG. 2,238.--Sectional view of Noark 250 volt, 400
ampere ~enclosed fuse~. The ~fusible element~ _is divided into strips_
A, B, C, and D. This parallel link construction results, upon the
operation of the fuse, in the formation of a number of small arcs, thus
facilitating the absorption of the metal vapor formed when the fuse
blows. ~The fusible strips~, of which there are two or four in number,
according to the ampere capacity of the fuse, are entirely surrounded
by a granular material which is chemically inactive with respect to the
fusible link and whose function is to absorb the metallic vapor formed
upon the blowing of the fuse. ~The contact blades~ T and L are made of
round edge copper, the round edges facilitating the insertion of the
fuses in the circuit terminals. R and S are the end ferrules, attached
to cover E, by the pin M.]
~Ques. What effect have the terminals on a fuse?~
Ans. The current at which a fuse melts may be greatly changed by the
size and shape of the terminals.
If near together and large, they may conduct considerable heat
from the fuse thus increasing the current required to blow the
fuse.
~Ques. What is the objection to large fuses?~
Ans. The discharge of molten metal when the fuse blows is a source of
danger.
~Ques. What should be used in place of large fuses?~
Ans. Circuit breakers.
~Ques. What are the objections to fuses in general?~
Ans. The uncertainty as to the current required to blow them; the
constant expansion and contraction is liable to loosen the terminal
screws when screws are used.
~Ques. What is the advantage of fuses?~
Ans. They form an inexpensive means of protecting small circuits.
[Illustration: FIG. 2,239.--Cross section through plug fuse. With this
type of fuse it is impossible to place any except the correct size of
plug in the socket.]
~Ques. Describe a plug fuse.~
Ans. It is constructed as shown in fig. 2,239, the fuse wire being
visible and stretching between the two metal portions of the plug.
~Ques. What is a cut out fuse?~
Ans. One similar to a simple fuse, but provided with clip contacts as
used for knife switch contacts.
The fuse wire is usually contained in a china or porcelain
tube, which also serves the purpose of a handle for withdrawing
the fuse.
~Ques. What is an expulsion fuse?~
Ans. One in which the fuse is placed in an enclosed chamber with a vent
hole.
In operation, when the fuse blows, the hot air and molten metal
are expelled through the vent.
~Ques. What is a no arc fuse?~
Ans. A cartridge type fuse, in which the space surrounding the fuse
wire is filled with powdered material.
[Illustration: FIG. 2,240.--Inside view of end ferrule of Noark
enclosed fuse. Two prongs O and V, which are a part of the knife blade
K, pass through the square holes in the ends of the ferrule R, and
are riveted to the anchor plate T. ~The object of this plate~ _is to
stiffen the structure and to increase the current carrying capacity
of the metal between the holes, also to permit of proper alignment
of the plates_. In each ferrule is placed a ~vent screen~, composed
of reticulate material, such as cheese cloth. The fuzz between the
threads of the cheese cloth prevents the escape of the granular
material through the vent holes A, but when the fuse operates, allows
free egress of the air, thereby permitting the vapor formed upon the
operation of the fusible element to quickly and freely pass through the
interstices of the filling material and become cooled, eliminating any
possibility of flame issuing from the ends of the tube.]
The object of the powdered material is to assist in
extinguishing the arc formed when the fuse blows.
~Ques. What is a magnetic blow out fuse?~
Ans. An enclosed fuse which is subject to the action of a magnetic
field produced by the current, the magnetic field tending to blow out
the arc when fusing occurs.
~Ques. What is a quick break fuse?~
Ans. One having a weight suspended from its center, or springs attached
to its ends so that the arc formed at fusing is quickly attenuated and
extinguished.
~Ques. What is the disadvantage of a fuse as compared to an oil switch
circuit breaker?~
Ans. When a fuse blows, the arc causes oscillations in the line, which
cause excessive rise of pressure under certain capacity conditions,
whereas this disturbance is reduced to a minimum with an oil switch.
[Illustration: FIG. 2,241.--~Quick break fuse.~ The fuse wire is
connected between the fixed terminal A and the movable arm B, and is
held under tension by the spring which exerts pressure on the movable
arm in a direction tending to separate A and B. ~In operation~, when
the fuse blows, the movable arm quickly moves to the position B´, thus
attenuating the arc and accelerating its extinguishment.]
~Ques. What metal is used for fuse wires?~
Ans. Various metals. Ordinary fuse wire is made of lead or an alloy of
lead and tin.
~Ques. What is the objection to aluminum?~
Ans. It becomes coated with oxide or sulphide, which acts as a tube
tending to retain the metal inside and prevent rupture.
~Ques. What is the objection to copper?~
Ans. Its high fusing point.
~Current Limiting Inductances.~--The great increase in capacity of
power stations, for supplying the demands of densely populated centers
and large manufacturing districts, together with the decrease in the
reactance of modern alternators and transformers due to improvement
in design to obtain better regulation, has presented a problem
in apparatus protection not contemplated in the earlier days of
alternating current distribution. This problem is entirely separate and
distinct from that of eliminating the tendency toward short circuit,
incident to the high voltages now common in transmission lines. It
accepts that all short circuits must occasionally occur and considers
only the protection of the connected apparatus against the mechanical
forces due to the magnetic stresses of such enormous currents.
[Illustration: FIG. 2,242.--~Notched end fuse.~ This is a simple form
of fuse ~consisting of~ _a strip of metal (or wire) fixed between
two end pieces to fit around the terminals_. This type is often
proportioned so that it is only possible to place the correct size of
fuse in the terminals. Sometimes, in place of the end pieces as shown,
the fuse metal is fixed between two clamping screws.]
~Ques. What means are employed to limit the value of a short circuit
current?~
Ans. A current limiting inductance coil (called a _reactance_) is
placed in series with the alternators or transformers.
[Illustration: FIG. 2,243.--General Electric current limiting
reactance; view showing details of construction. The core consists
of a hollow concrete cylinder, alloy anchor plates or sockets being
embedded in the core near the ends to receive the radial brass bolts.
An extension at each end of the core provides for clamping and bracing
the reactance in installation. The supports for the winding are made of
resin treated maple and are located upon the core by radial brass studs
screwed into the alloy sockets, and insulated by mica tubes. The nuts
by which the structure is tightened, rest upon heavy fibre washers.
Wooden barriers fitted and shellacked into the supports add to the
creepage surface between layers of the winding and between the winding
and the core. The supports of the layer next to the core are separated
from the core by strips of treated pressboard. The coil consists of
bare stranded cable in several layers, usually three in number. It is
wound into grooves in the treated wood supports, which are protected
from contact with the cable by heat shields of asbestos shellacked
into the grooves. The winding is usually in the form of two back turn
sections, thereby allowing the terminals of the coil to be brought out
at the ends of the outside layer. This assures accessibility and ease
of connection, and the removal of the leads from proximity to the core.
Two turns at each end of the winding are given extra spacing for the
purpose of additional insulation. The final turn at each end of the
coil is securely held in place by alloy clamps bolted to the supports.
The wood is protected from contact with the clamps by shields of
asbestos. The ends of the cable between the two sections are welded by
the oxyacetylene process.]
~Ques. What are its essential features of construction?~
Ans. It consists of bare stranded cable wound around a concrete core
and held in place by wooden supports as shown in fig. 2,243.
In order to avoid the prohibitive expense of high voltage
insulation, the reactance coil is designed for the low tension
circuit. This requirement prohibits the use of a magnetic core
which, if economically designed for normal operation, would
become saturated at higher densities, or, if designed large
enough to avoid saturation at short circuit conditions, would
become prohibitive in cost and dimensions.
The elimination of all magnetic material from the construction
of the concrete core reactance permits of no saturation, and
assures a straight line voltage characteristic at all current
loads.
[Illustration: FIG. 2,244.--Westinghouse ~magnetic blow out circuit
breaker~, designed for the protection of street railway and electric
locomotive equipments; it serves the combined purpose of fuse block
and canopy switch. ~The contact tips~ _are surrounded by a moulded
arc chute which confines and directs the arc until the magnetic blow
out extinguishes it_. The current carrying contacts consist of copper
strips separated by air spaces. An auxiliary contact or "arcing tip"
at the end of the switch lever takes the burning of the arc when the
breaker opens, and thus confines the burning to a very small piece
which can be easily removed and replaced at small cost. The hand
tripping lever and the resetting lever have insulated handles, so that
they can be safely handled, even in the dark.]
~Ques. Where is the proper location for a current limiting reactance?~
Ans. As near the alternator as possible.
~Ques. Why?~
Ans. To lessen the possibility of a short circuit occurring between the
reactance and the alternator.
~Ques. Beside limiting the current, what other service is performed by
the reactance?~
Ans. It protects the alternator from high frequency surges coming in
from the outside, and limits the current from other machines on the
same bus.
[Illustration: FIG. 2,245.--General Electric magnetic blow out circuit
breaker. This type may be used in air or water tight boxes and is
peculiarly adapted for service where the arc must be confined.]
~Circuit Breakers.~--The importance of circuit protective devices,
commonly called circuit breakers, is fully recognized. The duty
of a circuit breaker is to protect the apparatus in an electrical
circuit from undesirable effects arising from abnormal conditions,
by automatically breaking the circuit. Accordingly a circuit breaker
must comprise a switch in combination with electrical control devices
designed to act under abnormal conditions in the circuit.
A circuit breaker is a _device which_ ~automatically~ _opens the
circuit in event of abnormal conditions, in the circuit_.
[Illustration: FIG. 2,246.--Magnetic blow out circuit breaker. This is
a direct current breaker in which the final break occurs in a magnetic
field. ~It is a principle in electromagnetics~ _that a conductor
carrying a current in a magnetic field will tend to move in a direction
at right angles to the field_. The arc set up on breaking a circuit
constitutes a conductor, and in magnetic blow out circuit breakers, as
generally manufactured, there is an electromagnet, energized by the
current to be broken, which produces a field in the neighborhood of
the arc, with the result that the arc moves outward, and so becomes
attenuated and is finally extinguished. The form shown in the figure is
used on cars equipped with heavy motors. When so used, it is in many
cases mounted in a box with the handle H projecting at one end. A and K
are the terminals of the breaker and B is the tripping coil, which also
serves to set up the magnetic field necessary for blowing out the arc.
X is the armature of coil B and is pulled down against the action of
the spring S whenever the current exceeds that for which the breaker is
set. The tripping current is adjusted by means of nut T. The iron plate
P and a similar one back of it are magnetized by the current in coil
B, and as the break takes place between these two poles, the arc is
promptly extinguished by the field that exists there. ~In operation~,
A and K are the terminals, D D is a contact that is forced up against
F, F when the breaker is set. The current then takes the path A-B-F-D
D-F-K. ~When the breaker trips~, the contact piece D D flies down
and the tendency is for an arc to form between F, F; the magnetic
field blows the arc upwards, and whatever burning takes place is on
the contacts E, E, which are so constructed that they may be readily
renewed. ~To trip the breaker by hand~, the knob N is pressed.]
In the design of circuit breakers, there are several methods used to
effect the rupturing of the arc between contacts when opened on heavy
overload, such as:
1. Magnetic blow out; 2. Thermal break; 3. Carbon break.
In the magnetic blow out type, the arc is extinguished between
auxiliary contacts confined by a chute in which the arc is rapidly
blown out due to a powerful magnetic field from one or more
electromagnets. This type may be used in air or watertight boxes and is
peculiarly adapted for service where the arc must be confined.
[Illustration: FIG. 2,247.--~Thermal overload circuit breaker.~ ~In
construction~ two contact blocks are fixed rigidly to, but insulated
from, the switch arm. They are connected electrically by two parallel
strips of suitable metal, each fitted with a steel catch piece. ~When
the switch is closed~ the strips are sprung apart over a fixed catch,
and the full rated current does not release the catch. Overload causes
the strips to move apart, and the circuit breaker flies off under the
action of a spring.]
In a carbon break type, the arc is finally ruptured between carbon
break contacts. The breaking of the circuit is accomplished
progressively, that is to say, it is done in three stages, by several
sets of contact, known respectively as
1. The main contacts;
2. The intermediate contacts;
3. The carbon contacts.
_In operation_, as the circuit breaker acts to break the circuit, first
the ~main~ _contacts_, separate, then the ~intermediate~ _contacts_,
and finally the ~carbon~ _contacts_ between which the arc is ruptured.
~Ques. What is the object of the intermediate contacts?~
Ans. To prevent the forming of an arc on the main contacts.
[Illustration: FIG. 2,248.--Carbon break discs of Condit circuit
breaker. The two pairs of similar discs which slide past each other
are so arranged that these surfaces coincide at the instant the
intermediate contacts separate after which, as the contact arm opens
further, they gradually disengage.]
~Ques. What is the object of the carbon contacts?~
Ans. First to protect the intermediate contacts by providing a path
for the current after the intermediate contacts separate, and 2, to
"slow down" the current by means of the considerable resistance of the
carbon, thus reducing to a minimum the arc which is formed when the
carbon contacts separate.
~Ques. How is the automatic operation of a circuit breaker usually
accomplished?~
Ans. Usually through the medium of a solenoid, or electromagnet
energized by current from the circuit controlled by the breaker.
[Illustration: FIG. 2,249.--Mechanically connected insulated latches
used on Condit circuit breakers to produce inter-locking tripping.]
The essential features of construction and operation of a
circuit breaker is shown in the elementary diagrams, figs.
2,250 to 2,253. ~In construction~ as shown in fig. 2,250 it
consists essentially of three sets of contacts, a swinging
contact arm which is set in the closed position by the handle
operating through the toggle joint, the movement of which is
limited in the closing direction by the stop. The latter is
made adjustable by an eccentric pin or equivalent. Connected
to the toggle is the plunger of the solenoid whose winding is
energized by current from the circuit which the circuit breaker
is to control.
[Illustration: FIGS. 2,250 to 2,253.--Elementary diagrams illustrating
the operation of a carbon circuit breaker of the overload type, showing
the progressive opening of such device. Fig. 2,250, closed position;
fig. 2,251, main contacts open; fig. 2,252, intermediate contacts open;
fig. 2,253, carbon contacts open, circuit broken.]
~In operation~, the circuit is closed by hand by turning the
handle downward to the position shown in fig. 2,250, that is as
far as it will go.
Since the toggle has passed the center line the arm will be
held normally in this position because of the spring action of
the contacts. Now, if the current rise above a predetermined
limit, the pull exerted by the solenoid will overbalance the
tendency of the toggle to remain in the closed position, and
pull the two toggle links downward below the center line,
drawing the contact arm back and breaking the circuit.
[Illustration: FIG. 2,254.--I-T-E overload circuit breaker. ~In
operation~: the current from one side of the circuit enters the circuit
breaker at A, passing through the laminated bridge B to contact block
C, thence through coil D and terminal E to the motor. The coil D
surrounds a magnetic core, having pole pieces F and G and armature H.
The effect of the current in the coil is to energize the magnet, thus
tending to lift the armature against the force of gravitation. The
volume of current required to trip the circuit breaker is determined
by the position of the armature, which is subject to ready adjustment,
and is indicated on the calibration plate P. From the opposite side
of the line, the current enters at I, passing downward through the
laminated bridge member J, into terminal K, whence it passes out to the
motor. When the current passing through the circuit breaker attains
sufficient volume, the force generated by the magnetic coil overcomes
the weight of the armature H; and the latter is drawn upward toward
the pole pieces with constantly increasing force, until the insulated
projections L and M strike against the respective restraining latches
N and O, thereby releasing the two switch members, which at once open
in response to the force supplied by the spring of the contact members
and auxiliary springs provided for the purpose. Positiveness in opening
is further assured by the blow of the armature, which is added to the
other opening forces; hence, the heavier the overload, the more violent
the blow and the quicker the circuit breaker opens; or the greater the
current the more promptly it is interrupted. This is the ~I-T-E~ or
~I~nverse ~T~ime ~E~lement principle.]
[Illustration: FIG. 2,255.--Condit 600 volt, 1,200 ampere, single pole,
type K, circuit breaker with pull down handle.]
[Illustration: FIG. 2,256.--Condit 600 volt, 6,000 ampere, single pole,
switch board mounting, circuit breaker, with pull down handle.]
[Illustration: FIG. 2,257.--General Electric triple pole, overload,
circuit breaker, with two overload coils, capacity 300 amperes, 480
volts.]
The progressive action which takes place during this operation
is shown in figs. 2,250 to 2,253 in which the main contacts
separate first, then the intermediate, and finally the carbon
contacts as mentioned before.
~Ques. What name is given to this type of circuit breaker?~
Ans. It is called an overload circuit breaker.
[Illustration: FIG. 2,258.--Parts of General Electric 2,000 ampere 650
volt circuit breaker. A, cover for secondary contact bracket; B, spring
washer for Ea.; C, pin for links and G; D, spring for carbon support;
E, plate for F; F, carbon support; G, secondary contact bracket; H,
contact plate; I, screw for H; J, nut for K and W; K, contact stud,
upper; L, laminated brush, complete with support; M, leather buffer for
L; N, main link; O, pin for Na and La left hand and Cb and Na right and
left hand; P, screw for N and magnet frame shaft; Q, washer for N and
magnet frame shaft; R, screw for S and V; S, index plate; T, plate for
Gb; U, screw for T; V, magnet frame; W, contact stud, lower; X, pin for
Cb, Na and V; Y, washer for X and O; Z, calibrating screw with thumb
nut; Aa, armature with contact plate; Ba, catch lever complete with
catch Ca, button handle for Ba; Da, spring cotter for Ea; Ea, pin for
F and Fa; Fa, operating link for G; Ga, pin for D; Ha, carbon holder
with copper and carbon contacts; Ia, flexible connections for G and F;
Ja, screw for G and flexible connection plate; Ka, screw for Na and Ha;
La, copper secondary contact; Ma, screw for La; Na, secondary contact
lever; Oa, cross bar for Na; Pa, screw for L and M; Qa, secondary
toggle link (left hand); Ra, spring cotter for Wa and O; Sa, brush
lever; Ta, buffer for Cb and Sa; Ua, secondary toggle link (right
hand); Va, washer for Wa; Wa, pin for Cb, Qa, Ua and N; Xa, pin for Sa
and Cv; Ya, spring cotter for all pins, except Wa, catch lever pin and
buffer; Za, secondary contact link; Ab, washer for Fb; Bb, guard for
Fb; Cb, handle lever; Db, catch for Cb; Eb, screw for Db; Fb, handle
with stud; Gb, secondary connection.]
~Automatic Features.~--There are three methods of connecting the
winding of the solenoid, or _trip coil_ as it is called:
[Illustration: FIGS. 2,259 to 2,262.--Elementary diagrams illustrating
the various methods of electromagnetic control for circuit breakers.
Fig. 2,259, overload trip; fig. 2,260, underload trip, fig. 2,261, low
voltage trip; fig. 2,262, control from auxiliary circuit by means of a
"relay."]
1. In series with the main circuit;
2. In shunt with the main circuit;
3. In shunt with an auxiliary circuit.
[Illustration: FIG. 2,263.--Diagram of General Electric low voltage
trip with tripping switch _normally_ ~open~.]
The automatic controls arising from these connections give various
kinds of protection to the circuit and are known as
1. Overload trip;
2. Underload trip;
3. Low voltage trip;
4. Auxiliary circuit trip.
[Illustration: FIG. 2,264.--Diagram of General Electric low voltage
trip, with tripping switch _normally_ ~closed~.]
~Ques. What is the object of the overload trip?~
Ans. It is intended to open the circuit when the current exceeds a
predetermined value.
~Ques. What modifications are made in the mechanism shown in the
elementary diagrams?~
Ans. Sometimes a latch is used in place of the toggle and a magnet in
place of the solenoid as in figs. 2,265 and 2,266.
~Ques. Why is a magnet used in combination with a latch?~
Ans. Because with this arrangement very little movement is required to
trip the breaker, and for such conditions, a magnet is more efficient
than a solenoid.
[Illustration: FIGS. 2,265 and 2,266.--Circuit breaker with automatic
control mechanism consisting of ~magnet and latch~; views showing
breaker in open and closed positions, and essential features. The
toggle is used to obtain sufficient leverage to easily close switch
against the pressure of the brush contacts but not to lock switch,
this being done by the latch as shown, the latter closing by the
action of a spring, there being a roller R at the end which engages
the arm to reduce friction. ~In operation~, when the current exceeds
a predetermined limit the magnet attracts the latch and releases the
contact arm. The brush contacts which are exerting pressure against the
contact arm, rapidly push it away, and assisted by gravity, the arm
flies open to the position shown in fig. 2,266.]
~Ques. How does the latch arrangement work?~
Ans. When the proper current is reached, the magnet pulls open the
latch and the contact arm of the breaker moves by the force of gravity
or other means and opens the circuit.
~Ques. How does the underload trip operate?~
Ans. The same as the overload type except that they operate on a
diminution of current instead of an excess.
[Illustration: FIGS. 2,267 and 2,268.--Positions in circuit of
_current_ and _pressure_ coils of circuit breakers.]
~Ques. Describe the no voltage trip.~
Ans. The energy for the trip of this breaker is derived from a high
resistance or fine wire coil which is arranged to be placed directly
across the line, in operation, when the current flowing through the
circuit falls below a predetermined value, the energy of the coil is
insufficient to counteract the force of a spring, which then trips the
breaker.
[Illustration: FIG. 2,269.--Diagram of General Electric shunt trip
with coil connected beyond breaker and thrown out of circuit after
tripping.]
~Ques. Describe the auxiliary circuit trip.~
Ans. A pressure coil is used which is energized by current from an
auxiliary circuit. The coil is only _momentarily_ energized, by push
button, relay or other control, as distinguished from the preceding
types, in which the coil is _constantly_ energized.
[Illustration: FIG. 2,270.--Diagram of General Electric shunt trip with
auxiliary circuit opening switch to throw coil out of circuit after
tripping.]
[Illustration: FIG. 2,271.--General Electric shunt trip attachment.
The shunt trip attachment has been designed to provide for conditions
under which the low voltage attachment cannot be successfully applied.
It resembles the low voltage attachment in construction, but differs in
that it trips the circuit breaker when energized. The shunt trip should
be allowed to remain only momentarily in circuit; hence it should
be so connected that the opening of the circuit breaker immediately
disconnects it from the circuit. Whenever it is impossible to connect
the shunt trip in this manner, the circuit opening auxiliary switch
should be used in connection with it.]
[Illustration: FIG. 2,272.--General Electric low voltage attachment
for circuit breakers. This low voltage trip is designed to operate the
circuit breaker when the line voltage drops to approximately 50 per
cent or less of the normal voltage. It should be noted that the coil
is always in circuit, as is the case with the overload and underload
coils, and that it operates with the _releasing_ of its armature. It is
always necessary to use a fixed amount of resistance (depending upon
the voltage of the system) in series with the low voltage release.
The low voltage release performs the functions of a shunt trip coil
when used in conjunction with a push button, auxiliary switch or
speed limiting device, and is generally preferred to the shunt trip
attachment.]
[Illustration: FIG. 2,273.--General Electric ~circuit opening~
_auxiliary switch_. This switch opens an auxiliary circuit when the
circuit breaker opens, and is intended to be used in connection with
a shunt trip attachment to insure the immediate disconnection of the
shunt coil from the circuit. It may also be employed to serve other
purposes, such as tripping another circuit breaker having a low voltage
attachment, and permitting another circuit breaker to remain closed
only when the circuit breaker equipped with the auxiliary switch is
open.]
~Ques. What other name is given to the auxiliary circuit trip?~
Ans. It is sometimes called the shunt trip, though ill advisedly so.
[Illustration: FIG. 2,274.--General Electric ~circuit closing~
_auxiliary switch_. This switch closes when the circuit breaker opens,
and may be used to announce the automatic opening of the circuit
breaker through the means of an indicating lamp or an alarm bell. It
is often necessary to arrange one circuit breaker so that, in opening,
it will trip others. This may be accomplished by using a circuit
closing auxiliary switch in connection with a low voltage or shunt trip
attachment on the circuit breakers to be tripped. The construction of
this type of switch is such that it may be opened by hand after the
circuit breaker opens, but it is automatically reset when the circuit
breaker is closed.]
~Relays.~--Oil break switches and carbon break circuit breakers are
commonly used to open electrical circuits at some given overload and
on short circuit. To secure additional protection under a variety of
abnormal conditions or to provide for a certain predetermined operation
or sequence of operations, relays may be employed.
[Illustration: FIG. 2,275.--General Electric type C circuit breaker.
Specially adapted to motor driven machine tool applications. For use in
mills, machine shops, factories, foundries and office buildings. For
general motor work, automobile charging outfits, storage batteries,
rectifier sets, cranes, etc. List of parts: A, calibrating post; B,
laminated contact; C, secondary contact spring; D, contact blade; E,
cotter pin for G; F, toggle link; G, pin for D and F; H, stop for Aa;
I, hinge frame; J, operating lever; K, pin for I and J; L, toggle
link; M, connection; N, screw for M, O and P; O, nut for N and P;
P, terminal; Q, tripping coil; R, calibrating screw; S, laminated
contact; T, calibrating scale; U, calibrating spring; V, connection
post; W, knob; X, washer for Y; Y, handle; Z, buffer; Aa, armature; Ba,
laminated connection; Ca, connection; Da, base.]
A relay is defined as: _A device which_ ~opens~ _or_ ~closes~ _an_
~auxiliary circuit~ _under predetermined electrical conditions in the
main circuit_.
The object of a relay is generally to act as a sort of electrical
multiplier, that is to say, _it enables a comparatively weak current to
bring into operation a much stronger current_.
[Illustration: FIG. 2,276.--Diagram of connections of General Electric
shunt trip coil ~with~ and ~without~ circuit opening auxiliary switch.]
~Ques. For what service are relays largely used?~
Ans. They are employed in connection with high voltage switches
where the small amount of energy derived from an ordinary instrument
transformer is insufficient for tripping.
The connections between relays and circuit opening devices are
usually electrical. Combinations of this nature are extremely
flexible since they permit the use of a number of devices, each
having a different function, with a single circuit breaker or
oil switch as well as with two or more switches, to secure the
desired operation and protection.
~Selection.~--In all electrical installations protection of apparatus
is important, but in some large central stations this is secondary to
continuity of service.
To combine maximum protection without interruptions of service is
not always possible, but these requirements can be approximated very
closely by the use of reliable and simple controlling or protecting
devices if proper care be taken to select the relays suited to the
special conditions of the installation. To do this intelligently, a
knowledge of the various types of relay is necessary.
[Illustration: FIG. 3,073.--Diagram of connections of General Electric
low voltage release coil when used with speed limiting device on rotary
converter.]
There is a multiplicity of types and a classification to be
comprehensive, should, as in numerous other cases, be made from several
points of view. Accordingly relays may be classified:
1. With respect to the nature of the service performed, as
_a._ Protective;
_b._ Regulative;
_c._ Communicative.
2. With respect to the operating current, as
_a._ Alternating current;
_b._ Direct current.
3. With respect to the manner of performing their function, as
_a._ Circuit opening;
_b._ Circuit closing.
4. With respect to the operating current circuit, as
_a._ Primary;
_b._ Secondary.
5. With respect to the abnormal conditions which caused them to
operate, as
_a._ Overload;
_b._ Underload;
_c._ Over voltage;
_d._ Low voltage;
_e._ Reverse energy;
_f._ Reverse phase.
6. With respect to the time consumed in performing their function, as
_a._ Instantaneous (so called);
_b._ Definite time limit;
_c._ Inverse time limit.
7. With respect to the character of its action, as
_a._ Selective;
_b._ Differential.
8. With respect to whether it acts directly or indirectly on the
circuit breaker, as
_a._ Main;
_b._ Auxiliary.
[Illustration: FIG. 2,278.--General Electric overload and low voltage
type C circuit breaker for 600 volts or less. It has one overload, and
one low voltage coil as shown. Screens are provided between contacts.]
~Protective Relays.~--These are used to protect circuits from abnormal
conditions of voltage, or current, which would be undesirable or
dangerous to the circuit and apparatus contained therein.
~Ques. How do protective relays operate?~
Ans. They act in combination with automatic circuit breakers, operating
when their predetermined setting has been reached, energizing the trip
coil of the circuit breaker and opening the circuit.
Fig. 2,279 shows the principles of relay operation. When
the current or pressure in the main circuit reaches the
predetermined value at which the protective system should
operate, the relay magnet attracts the pivoted contact arm and
closes the auxiliary circuit; this permits current to flow from
the current source in that circuit and energize the trip coil
thus opening the main circuit.
[Illustration: FIG. 2,279.--Diagram illustrating the operation of a
~circuit closing relay~. When the predetermined abnormal condition is
reached in the main circuit, the relay closes the auxiliary circuit,
thus energizing the trip coil and opening the breaker.]
~Regulative Relays.~--This class of relay is used to control the
condition of a main circuit through control devices operated by a
secondary circuit.
~Ques. For what service are relays of this class employed?~
Ans. They are used as feeder circuit or generator regulators.
~Ques. How do they differ from protective relays?~
Ans. They have differentially arranged contacts, that is to say,
arranged for contact on either side of a central or normal position.
[Illustration: FIG. 2,280.--Diagram showing a railway synchronous
converter protected by a single pole overload circuit breaker with
low voltage release attachment and bell alarm switch. The low voltage
attachment trips the breaker on failure of direct current voltage also
when speed limit device closes. Internal troubles are taken care of by
the alternating current automatic devices (not shown).]
~Communicative Relays.~--These are used for signalling in a great
variety of ways for indicating the position of switching apparatus or
predetermining the condition of electric circuits.
~A. C. and D. C. Relays.~--As here used, the classification refers to
the kind of current used on the auxiliary circuit. In some cases direct
current is used to energize the trip gear of the circuit breaker or oil
switch, and in others, alternating current.
[Illustration: FIG. 2,281.--Diagram showing three phase motors
protected by triple pole overload circuit breakers, with two overload
coils, also one overload coil and low voltage release coil. The use
of the low voltage release allows the breaker to be tripped from a
distance by means of a short circuiting switch or push button.]
A. C. and D. C. relays are respectively known as _circuit opening_ and
_circuit closing_ relays, being later fully described.
~Circuit Opening Relays.~--The duty of a circuit opening relay is
_to open the_ ~auxiliary circuit~, _usually alternating current, nd
thereby cause the oil switch or circuit breaker to be opened by the use
of a trip coil in the secondary of a current transformer, or by low
voltage release coil_.
The trip coil of the breaker is generally shunted by the relay
contacts and when the moving contact of the relay disengages
from the stationary contact, the current from the transformer
which supplies the relay, flows through the trip coil thus
opening the breaker. These features of operation are shown in
fig. 2,282.
[Illustration: FIG. 2,282.--Diagram illustrating the operation of a
~circuit opening relay~. When the relay contacts are in the normal
closed position, as shown, the coil is short circuited. When the
predetermined abnormal condition is reached in the main circuit, the
relay contacts are opened with a quick break, sending the current
through the trip coil momentarily, and opening the breaker.]
~Ques. Where are circuit opening relays chiefly employed?~
Ans. In places where direct current is not available for energizing the
trip coil.
~Ques. What is the objection to alternating current trip coils?~
Ans. They have relatively high impedance and impose a heavy volt ampere
load on the transformers.
~Circuit Closing Relays.~--The duty of a circuit closing relay is to
close the auxiliary circuit at the time when the predetermined abnormal
condition is reached in the primary circuit. The closing of the
auxiliary circuit energizes the trip coil and opens the breaker.
[Illustration: FIGS. 2,283 to 2,291.--General Electric instantaneous
overload ~circuit opening~ relays, covers removed. Circuit opening
relays are used chiefly in those cases where direct current for the
tripping circuit is not available. Alternating current trip coils
have relatively high impedance and impose a heavy volt ampere load on
the current transformers. To reduce this load during normal operation
the circuit opening relay is frequently used and is usually necessary
where instruments and meters are to be operated on the same current
transformers as the trip coils if the greatest accuracy be required.
The relay contacts in the normal, closed position, short circuits
the trip coil. When the relay operates on overload or other abnormal
condition the contacts are opened with a quick break, sending the
current through the trip coil circuit momentarily and tripping the
switch. With circuit opening relays, the trip coils of the oil switch
must be set to trip somewhat lower than the setting of the relay. ~In
construction~ the relay consists of a solenoid with iron frame forming
the support for the relay; a central plunger or armature of special
construction which is picked up or released by the magnetic action of
the solenoid; a plunger rod which actuates the relay contacts, which
are mounted on an insulated base usually above the solenoid; a tube or
plate for the calibration marking and adjustment; covers of glass or
metal to keep out dust; terminal boards with points corresponding to
tagged leads from relay coils and external wiring diagrams. The relay
contacts are of two kinds, circuit opening, as shown above, and circuit
closing, as shown in figs. 2,292 to 2,300.]
~Ques. What kind of current is generally used for the auxiliary circuit
of a circuit closing relay?~
Ans. Direct current.
~Ques. At what pressure?~
Ans. From 125 to 250 volts.
~Ques. Where is this current usually obtained?~
Ans. From a storage battery, or from the exciter.
~Ques. For what current are the contacts ordinarily designed?~
Ans. About 10 amperes.
[Illustration: FIGS. 2,292 to 2,300.--General Electric alternating
current, instantaneous overload ~circuit closing~ relays, covers
removed. The function of a circuit closing relay is to close an
electrical circuit, usually direct current, through a trip coil on an
oil switch or circuit breaker, or it may short circuit a low voltage
release coil, and thereby open the oil switch or circuit breaker
on occurrence of the condition upon which the relay is designed to
operate. Direct current at 125 or 250 volts taken from exciter bus
bars or storage battery system is generally used for the tripping
circuit. Circuit closing contacts have a cone shaped central element
of carbon or metal which makes contact with flexible contact fingers
symmetrically arranged above the cone. These contacts will make and
break a circuit of 10 amperes at 125 volts without the use of auxiliary
circuit opening switches. Relays are made with two or three contacts
for connecting one side of a direct current circuit through one or
two separate circuits, or trip coils respectively, to the side of
opposite polarity. Usually only two contacts are required. Where two
or more trip coils are used, which may not be connected permanently in
parallel, the three contact relays are selected and in some cases four
contacts furnished.]
~Primary and Secondary Relays.~--Primary relays are sometimes called
series relays as they have the current coils connected directly in
series with the line, both on high and low tension circuits.
Secondary relays receive their current supply from the secondary
circuits of current transformers. Alternating current relays connected
to secondary of pressure transformers and relays with both current and
pressure windings are included in this class.
~Ques. What is the usual winding of the coils?~
Ans. The current coils are usually wound for 5 amperes and the pressure
coils for 110 volts.
[Illustration: FIG. 2,301.--Alternating current low voltage circuit
closing low voltage relay, for 600 volts or less. The contacts are
similar to those of the circuit closing overload type except that they
are inverted. As long as the pressure is normal the contact cone is
held above the contacts. When the pressure falls below one half normal,
the cone and plunger rod drop and close the contact. This relay does
not pick up its own plunger. The plunger rod is pushed up by hand after
the pressure circuit is established. Low voltage relays are generally
used in connection with a low voltage release or shunt trip coil on
an oil switch or a circuit breaker. They are used in connection with
motor booster sets to prevent a disastrous speed of the booster which
might result from the loss of alternating current power. They are also
sometimes used for indicating purposes.]
~Ques. What refinement is made in the design of relays and why?~
Ans. Care is exercised to reduce to a minimum the volt ampere load
imposed by the relay on the current transformer to permit the use of
un-stranded meters and relays upon the same transformer.
The use of circuit opening relays to cut out the trip coil of
an oil switch during normal operation, has been described,
and in the short time that the trip coil is in circuit, it
does not affect the accuracy of the instrument readings. This
practice, however, does not apply in the case of curve drawing
meters, voltage compensators or other devices which have in
themselves sufficient load for separate current transformers.
In this connection it should be noted that to obtain accurate
instrument and meter readings; the current transformers should
not be loaded beyond certain limits which depend upon the volt
ampere load and power factor of each of the connected devices.
[Illustration: FIG. 2,302.--Condit type K circuit breaker with shunt
trip and no voltage attachment. ~The shunt trip~ is usually applied
as an auxiliary to other types of trip. It consists of a fine wire
coil which is mounted as a self-contained part of the breaker and
which when energized, trips the circuit breaker. It is used to open
the breaker from some distant point, and the coil is arranged to be
connected across the line. The coils are so arranged that the circuit
breakers will operate on a voltage 25% above or 25% below normal. The
shunt trip coil is not intended to remain across the line and should
be only momentarily energized. ~The no voltage trip~, receives energy
from a high resistance or fine wire coil which is arranged to be placed
directly across the line, but in contradistinction to the shunt trip
type, in which the coil is momentarily energized to trip the breaker,
the no voltage coil is _constantly_ energized and a _decrease_ or
failure of pressure trips the breaker. It can be used as a remote
control device the same as the shunt trip. Its general use, however, is
to cause the circuit breaker to open when the voltage of the line fails
from any cause. Its use is recommended on all motor circuits, as it
affords an additional protection against accidents, for if the voltage
should fail, the breaker immediately opens, and before the machine can
start again the attendant must close the breaker. It will not work for
the protection of storage batteries or of motor generator sets charging
storage batteries, as, when the voltage of the generator fails, the
voltage of the battery still maintains its full value. The action of
the coil is independent of the direction of flow of current; it simply
allows the breaker to stay closed as long as the voltage is on the line
and opens the breaker when the voltage on the line ceases. No voltage
circuit breakers are normally so adjusted that they will not release
until the voltage approaches 50% of normal.]
So great is the variety of combination used and the variations of these
factors in their several combinations at different loads and settings,
that special consideration of each arrangement is advisable.
[Illustration: FIG. 2,303.--General Electric alternating current high
pressure series overload relays controlling 45,000 volt oil switches.
These relays are connected in series with the line. If current
transformers are to be used on the same circuit for other purposes, and
have sufficient capacity to supply energy for operating relay coils,
then secondary relays would be more economical, otherwise the series
relays are much less expensive. By means of a specially treated wooden
rod, the relay operates a tripping switch, closing a separate tripping
circuit, usually 125 or 250 volts direct current. Relays and switches
are for mounting on flat surfaces. Series relays are essentially the
same as secondary relays except in the coil winding and insulation.
The corrugated horizontal arms which carry the relays, as shown, are
insulated posts, insulating the relays from the ground. The wood rod
from each relay is connected directly to a tripping shaft on the oil
switch which buckles an auxiliary toggle, thereby opening the main
toggle and tripping the oil switch.]
~Overload Relays.~--Series relays are connected directly in series with
the line and are chiefly used with high pressure oil break switches
for overload protection. If current transformers are to be used on the
same circuits for other purposes, and have sufficient capacity to admit
of adding a relay coil, secondary relays would be more economical;
otherwise, the series relays are less expensive.
By means of a specially treated wooden rod, the relay operates a
tripping switch, closing a separate tripping circuit, usually 125 or
250 volts direct current. Series relays are essentially the same as
secondary relays except in the coil winding and insulation.
~Underload Relays.~--These are similar in construction to low voltage
relays but have current instead of pressure windings.
~Over Voltage Relays.~--These are usually of the circuit closing type
and are similar to secondary overload relays, but have pressure instead
of current windings.
~Low Voltage Relays.~--Relays of this class are in most cases used
[Illustration: FIG. 2,304.--Condit 600 volt, 1,500 ampere single pole
back connected type K circuit breaker, ~motor operated~. The mechanical
and electrical features of the circuit breakers are no different than
when hand operated, the only difference being that the motor is used
for the operating means. This motor is so arranged that even should it
over travel, due to an accident to the controlling circuit, it cannot
produce more than a predetermined strain on the circuit breaker. In
other words, after the motor has closed the circuit breaker, further
travel of the motor will not result in putting a strain on the
operating parts. Suitable motors are supplied for this service, the
type of motor varying in accordance with the character of the operating
current supplied. The advantage of this type of electrical operation
is that it puts very little strain on the switch mechanism, takes very
little operating current, allows the use of standard parts, and makes
an extremely substantial and flexible structure. Its disadvantage is
that it closes slowly, and it must not, therefore, be used in places
where quick closing is essential.] for the protection of motors in the
event of a temporary weakening or failure of the pressure. They are
also used in connection with a low voltage release or shunt trip coil
on an oil switch or a circuit breaker.
~Reverse Energy Relays.~--The chief object of this species of relay is
to protect the generator. When so used, the overload adjustment is set
at the maximum value to give overload protection only at the maximum
carrying capacity of the generator and a sensitive reverse protection
to prevent a return of energy from the line.
[Illustration: FIG. 2,305.--General Electric direct current solenoid
control relay. Solenoids for operating large switches, etc., frequently
require comparatively large operating currents in the "closing" coils.
This necessitates the use of relatively heavy leads between the control
switch and the solenoid and is the cause of severe arcing at the
control switch, especially with solenoids of high inductance. These
objectionable features can best be eliminated by the use of a suitable
control relay located near the solenoids. The control relay consists
of a solenoid plunger and switch, the latter insulated from the frame
of the relay. It operates satisfactorily on one-half the rated voltage
and requires only a very small operating current. The terminals of the
switch and the relay coils are independent. The relay can be wound for
operation on 125, 250, or 600 volt circuits.]
~Reverse Phase Relays.~--This type of relay is used chiefly to prevent
damage in case of reversal of leads in reconnecting wiring to two or
three phase motors.
~Time Element.~--It is often inconvenient that a circuit breaker should
be opened immediately on the occurrence of what may prove to be merely
a momentary overload, so that time lag attachments are frequently
provided, particularly with relays. These devices, which may form part
of the relay or may be quite distinct from it, retard its action until
the overload has lasted for a predetermined time--several seconds or
more.
[Illustration: FIG. 2,306.--Alternating current series reverse phase
single pole, circuit closing, two contact relay for 600 volts or less.
This type of relay is used chiefly to open motor circuits for elevators
to prevent damage in case of reversal of leads in reconnecting wiring
to two or three phase motors. The relay is provided with a dust proof
metal cover.]
~Ques. What should preferably govern the time lag?~
Ans. It should depend on the extent to which the overload is reduced as
the time elapses.
~Instantaneous Relays.~--The so called instantaneous relays operate
almost instantly on the occurrence of the abnormal condition that they
are to control.
There is of course a slight time element comparable with that
of an overload circuit breaker, but for practical purposes, the
operation may be considered as instantaneous.
[Illustration: FIG. 2,307.--Electric circuits of Condit type "A" relay.
The construction is described in fig. 2,309. As here shown, the relay
is not in operation, but should the current passing through the coil
be of sufficient value to cause the lower movable half of the magnetic
circuit to approach the upper stationary half of the circuit, _the
relay will be transformed from an ordinary_ ~electromagnet~ _into a_
~repulsion motor~. The contact will short circuit the brushes of the
armature and thus cause it to revolve, the speed of rotation being
dependent on the amount of current flowing to a predetermined point,
and thereafter the speed of rotation of the motor remains constant
irrespective of the current value. ~Time adjustment:~ This is obtained
by varying the distance through which the contact travels, provision
being made whereby adjustment can be made as close as .1 of a second.
Current adjustment: This is obtained by means of a calibrated spring.
Standard relays are calibrated at 6, 8, 10, and 12 amperes, the coils
being designed to carry five amperes continuously, with a temperature
rise not exceeding 86° Fahr. ~Power to operate relay:~ The relay
requires twenty volt amperes for its operation at full load; the
influence of this type of relay on the ratio and phase angle of current
transformers is small.]
~Time Limit Relays.~--Under this classification there are two
sub-divisions.
1. Definite time limit;
2. Inverse time limit.
[Illustration: FIG. 2,308.--Characteristic curves of Condit type A
selective relay. Curves 1, 2, 3, and 4 show the time variation of
this relay with different settings at the various current values. The
relay may be adjusted to trip the switch at any point represented
between curves 1 and 4. This relay is a combination of an _inverse time
limit_ relay and a _definite time limit_ relay. The combination of the
characteristics of the two types are seen in the curve, the first part
of which is ~inverse~, and the latter part ~definite~ from a point of
three or four times full load current. This combination of features
being desirable as, for instance, in transmission work, particularly
where it is necessary to use circuit breakers set selectively, as, due
to the inverse feature of the curve, the relays can be set so that on
a moderate overload, they will require the proper length of time to
operate, and at the same time will operate quickly enough on heavy
short circuits to prevent damage to the distribution system or its
apparatus. Due to the definite feature of the latter part of the curve,
the relays of the varying circuit breakers when once set to operate at
different time values will never operate simultaneously irrespective of
the value of the short circuit current, thus tending toward continuity
of service.]
~Ques. Describe the time mechanism of a definite time limit relay.~
Ans. It consists of an air dash pot, and an air diaphragm or equivalent
retarding device connected to the contact mechanism.
~Ques. How does it operate?~
Ans. In some designs, when the contacts are released, they descend
by gravity against the action of the retarding device thereby making
contact a definite interval after the occurrence of the abnormal
condition.
[Illustration: FIG. 2,309.--Condit type "A" ~selective relay~, designed
for use with circuit breakers where selective or discriminating
action is required. The circuits and connections of this relay are
illustrated in fig. 2,307, and its characteristics in fig. 2,308. ~In
construction~, the relay consists of _a special motor with a short
circuited armature and a split field_. ~Under normal conditions~, the
fields are separated from each other and the motor armature does not
revolve. The force tending to pull the two faces of the field together
is opposed by a spring, the compression of which determines the number
of amperes necessary to cause the relay to begin operation. The motor
structure performs the whole work and the motor itself un-meshes and
meshes the gears without the aid of any external device.]
~Ques. How does the inverse time limit type operate?~
Ans. The actuating and contact mechanism is attached directly to an
air bellows and in operation tends to compress the bellows against the
action of a specially constructed escape valve in the latter.
[Illustration: FIG. 2,310.--Condit type "B" ~time limit attachment~,
designed to give sufficient time to allow an induction motor to start
without opening the circuit breaker, and not have the circuit breaker
trip on the momentary rush of current. ~Its action is inverse~; that
is, _the greater the current the less time it takes to operate_ and
is so arranged that four to five times full load current or a short
circuit will trip the circuit breaker instantly. The time limit
attachment is applied directly to the armature which trips the circuit
breaker and is adapted for the so called primary trip. It consists
of an air vacuum dash pot with a graphite piston, the dash pot being
fastened to the stationary calibrating ring of the trip coil and the
moving outside cylinder is fastened to the armature of the circuit
breaker. When the current reaches a point where it overcomes the weight
of the armature and lifts the same, the magnetic force tending to raise
the armature is opposed by the vacuum created in the interior of the
cylinder. As the magnetic force continues the vacuum is overcome due to
the leakage of air past the plunger and the armature gradually moves up
until it reaches the point where it trips the circuit breaker. If at
any point of the armature travel, the current drop back to normal, the
armature immediately resets itself by means of a ball valve in the top
of the brass cylinder.]
~Ques. Why is the arrangement called _inverse_ time limit?~
Ans. Because the retardation varies inversely with the pressure on the
bellows, and therefore inversely with the magnitude of the abnormal
condition.
~Ques. What other device may be used to retard the operation?~
Ans. A damping magnet is sometimes used which acts on a disc or drum
and which may be adjustable.
[Illustration: FIGS. 2,311 and 2,312.--General Electric alternating
current ~low pressure~ series overload relays. Fig. 2,311,
instantaneous time limit relay; fig. 2,312, inverse time limit relay.
These relays have carbon contacts and will make or break a direct
current circuit of 10 amperes at 125 volts without auxiliary circuit
opening switch. They are used where several circuits are controlled
by one automatic oil break switch or one shunt trip, overload and
shunt trip or low voltage release carbon break circuit breaker. These
relays may be used for signal purposes; they are back connected, the
connections can be seen in the illustrations.]
~Ques. How is the inverse time element introduced by this arrangement?~
Ans. The retardation is due to eddy currents induced by moving the disc
or drum through the magnetic field. The reaction thus induced varies
inversely with the magnitude of the force with which the disc or
drum is urged through the field and hence inversely with the abnormal
condition.
~Ques. What are the ordinary limits of adjustment for inverse time
limit relays?~
Ans. From one-half second to 30 seconds, depending upon the time
setting and magnitude of the overload current.
[Illustration: FIGS. 2,313 to 2,321.--General Electric time limit
overload circuit opening relays with covers removed. The construction
of this relay is similar to that of the inverse time limit relay,
except that it has a compression spring interposed between the plunger
and diaphragm. The plunger compresses the spring and further motion
is prevented by a stop, making the relay practically independent of
the amount of the overload, only the stored energy of the spring, if
the overload continue, applies power, dependent on its own mechanical
strength, to the diaphragm. The time limit therefore becomes
practically a constant for any given setting under ordinary conditions
of overload or short circuit. If, however, the overload come on slowly
so that the spring is not fully compressed at once, the time limit will
vary slightly. If the scheme of selective operation make it necessary
to take care of a creeping load of this character, two relays may
be used and definite time limit positively secured. In this case,
an instantaneous circuit closing, overload relay would be used and
a definite time limit relay, provided with a direct current coil in
circuit with the closing contacts of the first relay. The time limit
relay would be of the circuit closing type and control a direct current
trip coil on the oil switch.]
A setting of from two to six seconds is ordinarily used,
depending upon the requirements. Where selective operation is
desired a minimum setting of two seconds is recommended.
~Differential Relays.~--In this type of relay there are two
electromagnets. In normal working these oppose and neutralize each
other. Should, however, either winding become stronger or weaker than
the other, the balance is upset, the magnet energized, and the relay
comes into operation.
[Illustration: FIG. 2,322.--Differential relay transformer and reverse
current circuit breaker discriminating device. A differential relay is
_one whose electromagnet has two windings_. ~In normal working~ _these
oppose and neutralize one another_. Should however, either winding
become stronger or weaker than the other, the balance is upset, the
magnet is energized, and the relay comes into operation. A modification
of such a relay for alternating current is here shown, from which it
will be seen that when the currents are as indicated, the circuit A has
the larger pressure induced in it, whereas, should the main current
reverse with reference to the shunt current, the circuit B would have
the larger induced pressure.]
A modification of such a relay for alternating current is shown in
fig. 2,322, from which it will be seen that when the currents are
as indicated, the circuit A has the larger pressure induced in it,
whereas, should the main current reverse with reference to the shunt
current, the circuit B would have the larger induced pressure.
[Illustration: FIG. 2,323.--Diagram of modern power house wiring and
busses showing location of relays.]
~[1]How to Select Relays.~--The following general information on
relays, together with reference to the one line diagram, fig. 2,323,
will be of interest and assistance in making a selection from the
various relays previously described to meet the requirements of modern
power house and sub-station layouts.
[1] NOTE.--As suggested by the General Electric Co.
~Single pole relays~ are used on single phase and on balanced
three phase circuits.
~Double pole relays~ are used on ungrounded three phase and on
quarter phase.
[Illustration: FIG. 2,324 to 2,329.--General Electric inverse time
limit overload circuit closing relays. In this type of relay its
mechanism is so designed that a delay or lapse of time in opening the
circuit breaker after a predetermined condition of the circuit has
been reached, depends on the flow of current, that is, if the current
be great, the time will be small, and if the current be of a moderate
value, the time will be correspondingly longer.]
~Triple pole relays~ are used on three phase grounded neutral
and interconnected quarter-phase.
~Circuit closing relays~ are recommended in all cases where a
constant source of direct current is available for operating
trip coils.
The conditions for which relays have been designed for power
circuits may perhaps be best described, by considering a
one line diagram from the generator end to the sub-station
auxiliary machines and feeders.
Considering first alternating current circuits, the
prevailing practice is to make the circuit breakers by
which the alternators are connected to the low tension bus
_non-automatic_, in order to insure minimum interruption of
alternator service. The chance of trouble in this part of the
circuit is remote, but should it occur, the station attendant
could generally open the circuit breaker before the machines
would be injured.
~Reverse current relays~ of instantaneous or time limit types
are often connected to the secondaries of current and of
pressure transformers to indicate by lamp or bell any trouble
that may occur in the generator circuit.
These relays operate with a low current reversal at full
pressure and conversely with a proportionally greater current
at voltages less than normal. At zero pressure, the relay would
act as an overload one, set for high overload. At zero current,
a voltage considerably in excess of normal would be required to
operate it.
[Illustration: FIG. 2,330.--Diagram showing two phase motor or feeder
circuit protected by double pole double coil, overload circuit breaker
(or two single pole breakers interlocked) with bell alarm switch.]
Specifications sometimes call for automatic generator circuit breakers:
in this case _definite time limit overload relays_ are used. They are
connected in the secondaries of current transformers and are designed
to give the same time delay for all trouble conditions; they allow the
defective circuit to be opened, if possible, at a point more remote
from the generator than the generator circuit breaker.
When the total generator capacity exceeds the rated rupturing capacity
of the circuit breakers, one or more sectionalizing circuit breakers
are placed in each bus.
If operating conditions admit, these devices are made
non-automatic and are left disconnected except in case of
emergency; but if it be necessary for them to be continually in
service, they may be made automatic by _means of instantaneous
overload relays_ connected to current transformers in the low
voltage bus; the relays being adjusted to trip the circuit
breaker under short circuit conditions, confining the trouble
to one section and preventing the circuit breakers rupturing
more than their rated capacity.
Installations with but _one bank of power transformers_, and without
high voltage bus, are provided with automatic circuit breakers operated
by an _inverse time limit relay_.
The relay is connected to the secondaries of current
transformers, which in turn are connected in the low voltage
side of the power transformer.
Stations with _more than one bank of power transformers_, a
high voltage bus, and high and low voltage circuit breakers,
may have both circuit breakers arranged to trip at the same
time or one after the other. As in the former case, they are
operated from the inverse time limit relay connected in the low
voltage side.
[Illustration: FIG. 2,331.--Condit 600 volt, 1,500 ampere, single pole
type K circuit breaker ~pneumatically operated~. It is the same as the
electrically operated circuit breaker, except that a pneumatic cylinder
mechanism is supplied in place of either the electromagnet or the
motor. This cylinder mechanism is so arranged that the air pressure is
only on the cylinder at the instant of operation. At all other times
the air pressure is shut off by means of a control valve. The kind of
remote control to be used depends on local conditions. In general, the
hand operated remote control device is preferable where conditions are
such that it can be used, and where it is necessary to use electrically
operated, the motor operated type is recommended if conditions be such
that slow closing is not objectionable.]
[Illustration: FIGS. 2,332 and 2,333.--Diagram showing two phase
four wire no voltage connections for I-T-E circuit breaker. The two
no voltage coils for two phase four wire circuits are connected
respectively to binding posts B, C and A, D on the face of the base. B
and D are connected to lower spring contacts 2 and 1 respectively, of
the small disconnecting switch. (In instruments supplied on individual
bases, these connections are made in the factory, let into channels
in back of base and covered with wax.) Each of the upper contacts _a_
and _b_ of the disconnecting switch is connected respectively through
resistance R2 and R1 to one main in each phase at _aa_ and _bb_. C
and A are respectively connected to the other main in each phase at
3 and 4. Thus each of the no voltage coils operates across one phase
independent of the other. The terminals 3, _aa_, _bb_ and 4, must,
in all cases, be so connected that they will be subject to the full
voltage of the circuit, irrespective of the position of the starting
switch.]
[Illustration: FIGS. 2,334 and 2,335.--Diagram showing two phase
three wire no voltage connections for I-T-E circuit breaker. The two
no voltage coils for two phase, three wire circuits are connected
respectively to binding posts B, C and A D on the face of the base,
and from A and C connections are made to lower contacts 2 and 1
respectively of the disconnecting switch. Binding posts B and D are
connected together on the back of the board. (In instruments supplied
on individual bases, these connections are made in the factory, let
into channels in back of base and covered with wax.) Each of the upper
contacts _a_ and _b_ is connected respectively through resistance R2
and R1 to one of the mains at _aa_ and _bb_ as shown. D is connected
through resistance R3 to the common wire of both phases at 3 B and D
being connected as aforesaid, thus forming a common connection for
both no voltage coils. Terminals _aa_ and _bb_ of the resistances must
be connected to the outside main across the two phases, terminal 3 to
the main common to both phases, the connections being so made that
these terminals will be subject to the full voltage of the circuit
irrespective of the position of the starting switch.]
[Illustration: FIG. 2,336 and 2,337.--Diagram showing three phase no
voltage connections for I-T-E circuit breaker. The no voltage coils
for three phase circuits are connected in Δ by means of binding posts
A, B, C and D on the face of the base, and from the A and B of the no
voltage coils, connections are made respectively to spring contacts
1 and 2 of the small disconnecting switch. Each of the contacts
_a_ and _b_ of the disconnecting switch is connected respectively
through resistance R2 and R1 to one of the mains at _aa_ and _bb_.
The terminal C is connected through resistance R3 on the back of
the board to the middle main as shown at point 3. The terminal D is
linked on the back of the board to terminal B to complete the [Greek:
D] connection. The terminals _aa_, _bb_ and 3 of the circuit breaker
must, in all cases, be so connected that they will be _subject to full
voltage of the circuit_ irrespective of the position of the starting
switch. Each no voltage coil is supplied with two terminal wires, one
covered with green and one with black insulation. In replacing these
coils particular care should be taken to see that the terminal wires
connected to any one binding post are of unlike color.]
In plants in which two or more banks of transformers are operated in
parallel between high and low voltage busses, it is desirable to have
for each transformer bank, an automatic circuit breaker equipment which
will act selectively and disconnect only the bank in which trouble
may occur. With a circuit breaker on each side of transformer bank,
selective action may be secured in two ways as follows:
1. By means of an instantaneous differential relay connected in the
secondaries of current transformers installed on both the high and low
voltage sides of each transformer bank.
The relay operates on a low current, reversal on either side of
the bank.
2. By means of one inverse time limit, secondary or series relay
installed on that side of the transformer bank which is opposite the
source of power, the relay being arranged to trip both the high and low
voltage circuit breakers.
_The first method_ has the disadvantage of high first cost
due to the high voltage current transformers required, but is
more positive than the second method and is independent of the
number of transformer banks in parallel.
_The second method_ is the less expensive of the two and
protects against overloads as well as short circuits in the
transformers, but it is less positive and introduces delay
in the disconnection of the transformer when trouble occurs.
Furthermore, it is not selective when less than three banks are
operating in parallel.
The automatic circuit breakers in the outgoing line may be operated
from inverse time limit relays connected in the secondaries of current
transformers; or in case transformers are not necessary for use with
instruments, series high voltage inverse time limit relays connected
directly in the line may be used.
Whether to select current transformers with relays insulated for low
voltage, or to choose series relays, is a question of first cost and
adaptability to service conditions. Below 33,000 volts, the commercial
advantages in favor of the series relay are slight, and since it
is somewhat difficult to design this device for the large current
capacities met with at the lower voltage, it is generally the practice
to use the relay with current transformer, because of its operating
advantage. This practice, however, is not entirely followed, since some
service conditions (described later) make the use of series relays very
desirable and practical.
[Illustration: FIGS. 2,338 and 2,339.--General Electric instantaneous
direct current reverse current or ~"discriminating" relays~. Fig.
2,238, for 500 amperes; fig. 2,339 for 2,000 amperes. These relays are
designed for mounting directly on circuit breaker studs. ~These relays
consist~ _of a horseshoe magnet with a shunt wound armature pivoted
between its poles_. ~The magnet~ is mounted on the current carrying
stud of the circuit breaker between the back of the panel and the
first contact or supporting nut, and is placed in a vertical position.
~The contacts~ are insulated from the magnet permitting the use of an
auxiliary circuit for the tripping device, independent of the circuit
controlled by the circuit breaker. The magnet is excited by the current
flowing through the stud, and the armature is connected across the
line in series with suitable resistance. Rotation of the armature in
the normal direction is prevented by a stop. ~Reversal of the current~
flowing through the stud _changes the direction in which the armature
tends to rotate_, causing it to move away from the stop and close the
circuit through an auxiliary trip coil and trip the circuit breaker.
~These relays are used~ to protect dynamos, storage batteries, or main
station busses from damage on reversal of current due to short circuit,
or from the grounding of machines or connection. Relay contacts must
not be used to open the shunt trip coil circuits. An auxiliary switch
should be provided for this purpose in all cases where the opening of
the circuit breaker does not disconnect the trip coil from the source
of supply.]
_Inverse time limit_ relays are satisfactory for one, or more than two
outgoing lines in parallel as they act selectively to disconnect the
defective line only, but installations with only two outgoing lines
in parallel have the same load conditions in both lines and selective
tripping of the circuit breakers in the defective line is obtained by
means of a selective relay acting instantaneously under short circuit
conditions only.
[Illustration: FIG. 2,340.--General Electric ~direct current, reverse
current relay,~ _used to protect dynamos, storage batteries, or main
station busses from damage on reversal of current due to short circuits
or from the grounding of machine or connections._ It is mounted on
vertical bus bars as in the case of cables, on the side wall, or other
flat surface, and the cables threaded through the frame. ~When used
to trip a circuit breaker,~ the breaker is provided with a shunt trip
connected across the circuit, the tripping circuit being closed through
the relay contacts on the occurrence of sufficient reverse current to
lift the relay armature. The relay is either instantaneous or time
limit as desired. ~In the time limit relay,~ the time interval is
obtained by the leather bellows shown in the illustration. The time
setting can be varied within certain limits by means of a valve on the
bellows outlet. ~The operation of the relay~ _depends on the relative
value and direction of magnetic flux set up by a pressure coil, shown
in the illustration, and the current in the vertical bars._ Under
normal conditions these fluxes are in the same direction and circulate
around a closed magnetic circuit. ~When the current in the bars
reverses,~ _the two fluxes oppose each other and force flux through
the normally open leg of the magnetic circuit._ When the reversal of
current is of predetermined value, the relay armature is lifted and the
purpose of the relay accomplished.]
The relay design and action is similar to the reverse current
relay previously mentioned, and is connected to the secondaries
of current transformers in each high voltage line and pressure
transformers in the low voltage bus.
In the sub-station, the conditions are the reverse of those in
the main station, the incoming lines becoming the source of
power.
If there be only one incoming line and no high voltage bus,
the line circuit breaker is generally non-automatic. With
one incoming line and high voltage bus, the circuits from the
service side of the bus are equipped with automatic circuit
breakers and relays. These relays and those used for other
arrangements of two or more incoming lines in parallel, as
well as high and low voltage circuit breakers, are of the same
design and are applied in the same manner as for the generating
station.
Regarding the relay equipments for auxiliary machines, the same
practice is recommended with the generator end of alternating
current motor generator sets as with the main generators, the
outgoing feeder circuit breakers being tripped from inverse
time limit or instantaneous relays.
[Illustration: FIG. 2,341.--General Electric ~direct current
differential relay~ for balancer set; instantaneous, 500 (or less) volt
type for mounting on panel. In many power plants direct current, three
wire, power service is furnished by "high voltage" two wire dynamos
operating in connection with balancer sets consisting of two "low
voltage" machines on a common shaft. With this combination of machine,
a short circuit or heavy overload on one side of the system will shift
the neutral considerably, and the lamps on the opposite side may "burn
out". To protect the lamps, a differential relay operating on 15 volts
unbalancing, is commonly used; it is connected to trip either the
dynamo's circuit breakers (or a circuit breaker connected in the bus
between the balancer set and the other dynamos).]
With several synchronous machines in parallel, the relays
are arranged to operate with the least time delay with which
it is possible to get selective action, in order to prevent
the machines being thrown out of step in event of trouble
conditions causing a decrease of voltage.
The various types of _induction motor_ and various conditions
under which they are employed, have brought about the
development of several types of relay to protect the motors and
the apparatus with which they are used.
It is desirable to disconnect a _large motor_ in case of voltage
failure, and with conditions requiring either a motor operated, or
a solenoid operated circuit breaker, a _low voltage relay_ is used
to close the tripping circuit whenever the voltage decreases to,
approximately, 50 per cent. below normal.
[Illustration: FIG. 2,342.--Condit ~time limit relay~, designed
primarily for use in connection with feeder circuits, where close
selection or discrimination of circuit breakers is not required.
~It may be used~ satisfactorily on lighting and power circuits and
also where there are sudden, momentary fluctuations of current. This
relay is used in connection with series transformers. The contact
arrangements are provided so that the relays may be used as circuit
closing or circuit opening relays. ~The delayed action~ is produced
by an air vacuum dash pot with a graphite piston. The piston of the
dash pot is connected to an arm arranged to be moved by the armature.
When the current reaches a point where it overcomes the weight of the
armature and lifts the same, the magnetic force tending to lift the
armature is opposed by the pull of the vacuum created in the interior
of the shell into which fits the graphite piston. As the magnetic pull
continues the vacuum is overcome due to the leakage of air past the
piston, and the armature gradually moves until it reaches a point where
it causes the circuit breaker to trip, either by closing the contacts
in the circuit closing type, or by opening the contacts in the circuit
opening type. If, at any portion of its travel, the current drop to
normal, the armature immediately resets. ~The time adjustment~ consists
of an arrangement whereby the distance through which the armature moves
before tripping the breaker, may be changed, thus altering the time of
tripping. ~The current adjustment~ is made _by changing the effective
turns of the actuating coil_, the travel of the armature and the force
exerted by it being the same for all current adjustment. The winding is
designed to carry 5 amperes continuously with a temperature rise not
exceeding 68° Fahr. standard calibration is provided so that the relay
will start to operate at 5, 6, 8 and 12 amperes.]
Up to 550 volts, these relays may be connected across the line,
but for higher voltages they are connected to secondaries
of pressure transformers. _Smaller motors_ with which hand
operated circuit breakers are used, are generally provided with
low voltage release attachments that perform the same function
as the relay.
Induction motors are sometimes subjected to _high voltage conditions_
and to protect them from injury, high or excess voltage relays are
employed to trip the automatic circuit breaker. These relays are of
similar design and wired in the same manner as the low voltage relays.
[Illustration: FIG. 2,343.--Characteristic curves of Condit time limit
relay as illustrated in fig. 2,342. Settings: curve A, 5 amperes; curve
B, 6 amperes; curve C, 8 amperes; curve D, 12 amperes.]
_Reverse phase relays_ have been developed for operating conditions
under which a _reversal of phase_ would cause trouble, as for example,
in the case of _elevator motors_.
These are so designed that any phase reversal that would
reverse an induction motor, would operate the relay and
disconnect the automatic circuit breaker.
The design is based on the principle of the induction motor,
and in the case of low voltage motors of limited capacity,
the relay may be connected in series in the motor leads. If
the voltage or capacity of the motor make this arrangement
inexpedient, the relay may be placed in the secondaries of
current or pressure transformers connected in the motor leads.
_Underload relays_ are often used to trip the automatic circuit breaker
that is placed in the primaries of _arc lighting circuits_ to prevent
an abnormal rise of secondary voltage in case of a break in the
secondary circuit.
[Illustration: FIG. 2,344.--Diagram showing _storage battery and
charging dynamo protected by_ ~double pole single coil underload
circuit breaker. In operation,~ _the circuit breaker disconnects the
battery when fully charged, and protects the dynamo from reverse
current_.]
The underload relay is similar in design to the low voltage
relay excepting that it acts on a decrease of current.
The problem of _protecting induction motors_, from injury, that may
result from running on single phase, or from an overload, and at the
same time permit the motor to be started with the necessarily high
starting current that may be greatly in excess of the overload current,
has caused the development of the _series relay_.
[Illustration: FIG. 2,345.--Diagram showing direct current motors
protected by overload circuit breakers with bell alarm switches: _a_,
double pole single coil breaker no switch required. Low voltage device
is on the starting rheostat; _b_, single pole breaker in series with
lever switch. Low voltage attachment on the breaker.]
[Illustration: FIG. 2,346.--Diagram showing two wire dynamo, protected
by a single pole overload circuit breaker with bell alarm switch.
Breakers must be on opposite side from the series field.]
[Illustration: FIG. 2,347.--Diagram showing dynamo protected by a
single pole overload circuit breaker with reverse current relay and
combined circuit opening and bell alarm switch.]
This device may be connected in series with the motor leads for
voltages up to 2,500; it is designed with an inverse time limit
device which may be adjusted to give the desired protection.
The field for relays is more extensive for alternating current than
for direct current power circuits, the latter being generally confined
to much smaller and simpler systems and areas of distribution, and
generally sufficient selective action can be obtained by the use of
fuses or circuit breakers arranged with instantaneous trip.
[Illustration: FIG. 2,348.--Diagram showing three wire dynamo protected
by double pole double coil overload circuit breaker (or two single pole
breakers with interlock) with bell alarm switches. Complete protection
is secured as breaker is connected between armature and series field.]
Operating conditions sometimes make it advisable for the generator
circuit breakers to open only after the auxiliary and feeder circuit
breakers have failed to isolate the trouble.
This is accomplished by using direct current _series inverse
time limit relays_ to trip the generator circuit breakers.
_Instantaneous reverse current relays_ are used to trip the machine
circuit breaker of battery charging sets, rotaries and motor generator
sets to prevent their running as a motor on the charging or direct
current end. These relays can act only in case of current reversal.
To prevent serious unbalancing of voltages in Edison three-wire systems
causing trouble, _differential balance relays_ are used to trip the
circuit breakers on a small percentage of unbalancing.
CHAPTER LIX
LIGHTNING PROTECTION DEVICES
Lightning protection devices, or lightning _arresters_, are devices
for providing a path by which lightning disturbances or other static
discharges may pass to the earth.
Lightning arresters, designed for the protection of transmission lines,
must perform this function with a minimum impairment of the insulation
of the lines.
In general the construction of lightning arresters comprise
1. Air gaps;
2. Resistances;
3. Inductances;
4. Arc suppressing devices.
~Ques. What are the causes of static charges?~
Ans. They may be caused by sandstorms in dry climates, or may be due to
grounds on the high pressure side of a system.
~Ques. What causes high frequency oscillations?~
Ans. They are usually due to lightning discharges in the vicinity of
the line.
~Ques. What are the requirements of lightning protection devices?~
Ans. They must prevent excessive pressure differences between line and
ground, line and line, and between conductor turns in the electrical
apparatus.
~Air Gap Arresters.~--method of relieving any abnormal pressure
condition is to connect a discharge air gap between some point on an
electric conductor and the ground. The resistance thus interposed
between the ground and the conductor is such that any voltage very
much in excess of the maximum normal will cause a discharge to ground,
whereas at other times the conductor is ungrounded because of the air
gap. This forms the principle of air gap arresters.
[Illustration: FIG. 2,349.--Non-arcing multi-gap arrester. Based on
the principle of employing for the terminals across which the arc is
formed, such metals as are least capable of maintaining an alternating
arc between them. This so called non-arcing property of certain metals
was discovered by Alexander Wurtz. The action is such that the "line
current" which follows the lightning discharge follows as an arc, but
is stopped at the end of one alternation because of the property of the
non-arcing metals to carry an arc in one direction, but requiring an
extremely high voltage to start a reverse arc. The non-arcing metals
ordinarily employed are alloys of zinc and copper. Plain multi-gap
arresters as here shown operate satisfactorily with the smaller
machines and on circuits of limited power, but for large machines
of close regulation, and therefore of very large momentary overload
capacity, especially when a number of such are operated in parallel,
such arresters were found insufficient, the line current following the
lightning discharge frequently was so enormous that the circuit did
not open at the end of the half wave, that is the arrester held the
arc and was destroyed. The introduction of synchronous motors made it
necessary that the arc should be extinguished immediately, otherwise
the synchronous motors and converters would drop out of step, and the
system would in this way be shut down. To insure the breaking of the
arc, resistance was introduced in the arrester, the modified device
being known as the low equivalent arrester as shown in fig. 2,350.]
The single gap while adequate for telegraph line protection, was found
insufficient for electric light and power circuits, because since the
current in such circuits is considerable and usually at high pressure
it would follow the lightning discharge across the gap. Thus the
problem arose to devise means for short circuiting the line current
resulting in various modifications of gap arrester.
~Multi-gap Arresters.~--The essential elements of an arrester of this
type are a number of cylinders spaced with a small air gap between them
and placed between the line to be protected and the ground, or between
line and line.
_In operation_, the multi-gap arrester discharges at a much lower
voltage than would a single gap having a length equal to the sum of the
small gaps. In explaining the action of multi-gaps, there are three
things to consider:
[Illustration: FIG. 2,350.--Low equivalent arrester. This is a
modification of the multi-gap arrester shown in fig. 2,349. About half
of the total number of gaps are shunted by a resistance, and another
resistance inserted between the cylinders and the earth. With this
arrangement the middle point is at ground pressure, and there are
between line and ground only one half of the total number of gaps.
This is sufficient to prevent a bridging of the gaps under normal
conditions.]
1. The transmission of the static stress along the line of the
cylinders; 2. The sparking at the gaps; 3. The action and duration of
the current which follows the spark, and the extinguishment of the arc.
~Ques. What is a spark?~
Ans. The conduction of electricity by air.
~Ques. What is an arc.~
Ans. The conduction of electricity by vapor of the electrode.
~Distribution of Static Stress.~--The cylinders of the
multi-gap arrester act like plates of condensers in series.
This condenser function is the essential feature of its
operation.
When a static stress is applied to a series of cylinders
between line and ground, the stress is immediately carried from
end to end.
If the top cylinder be positive it will attract a negative
charge on the face of the adjacent cylinder and repel an equal
positive charge to the opposite face and so on down the entire
row.
The second cylinder has a definite capacity relative to the
third cylinder and also to the ground; consequently the charge
induced on the third cylinder will be less than on the second
cylinder, due to the fact that only part of the positive
charge on the second cylinder induces negative electricity
on the third, while the rest of the charge induces negative
electricity to the ground. Each successive cylinder, counting
from the top of the arrester, will have a slightly smaller
charge of electricity than the preceding one.
[Illustration: FIG. 2,351.--General Electric 2,200 volt multi-gap
arrester for station installation. It consists of fourteen ⅝" knurled
cylinders and two shunt resistance rods mounted on a porcelain base.
One of these rods has a low resistance, and shunts nine gaps; the
other rod has a high resistance, and shunts eleven gaps. The effect
of the shunt resistance in extinguishing the line current arc is the
same, therefore, as that of an equal series resistance but is without
the objectionable features of the latter. Series resistance limits
the discharge current to such an extent that an arrester with series
resistance fails to protect against destructive rises of voltage when
the conditions are severest. Graded shunt resistance responds to all
frequencies and opens a discharge path for excessive voltage when the
frequencies are high as well as when they are low. Its further effect
in withholding the line current from the gaps after the relieving
discharge has occurred, is to aid the non-arcing quality of the metal
cylinders in quickly suppressing the arc that follows a discharge. The
arc is extinguished at the end of the half cycle of line current in
which the discharge takes place.]
~Sparking at the Gaps.~--The quantity of electricity induced
on the second cylinder is greater than on any lower cylinder
and its gap has a greater pressure strain across it as shown in
fig. 2,357. When the voltage across the first gap is sufficient
to spark, the second cylinder is charged to line voltage and
the second gap receives the static strain and breaks down. The
successive action is similar to overturning a row of ten-pins
by pushing the first pin against the second. This phenomenon
explains why a given length of air gap concentrated in one gap
requires more voltage to spark across it than the same total
length made up of a row of multi-gaps.
[Illustration: FIG. 2,352.--General Electric 2,200 volt arrester in the
act of discharging, and shunting the line current. The figure shows
an actual discharge taking place. It will be seen that the heavy line
current passes across only four of the gaps, and then goes through the
resistance rods; while the static discharge passes straight across
the entire series of thirteen gaps. When the gaps of an arrester are
shunted by even a low resistance, discharges of very high frequency
find it relatively difficult to pass through the resistance rods, owing
to the impedance of the rods, but comparatively easy to pass across
all the gaps, owing to the capacity effect in breaking down the gaps.
The higher the frequency, the more pronounced is this effect, hence
the discharges select different paths through gaps and resistances
depending upon the frequency. By frequency is meant, not the frequency
of the line current but the lightning frequency, which may run into
hundreds of thousands, or into millions of cycles. The equivalent
needle gap for this arrester is shown by tests to be nearly the same
for all frequencies and quantities of discharge; that is, the arrester
is equally responsive to all frequencies.]
[Illustration: FIGS. 2,353 to 2,355.--Oscillograph record of the
phenomena that take place in the different circuits or selective paths
of a multi-gap arrester during a discharge such as shown in fig.
2,352.]
As the spark crosses each successive gap, the voltage gradient
along the remainder readjusts itself.
~How the Arc is Extinguished.~--When the sparks extend across
all the gaps the line current will follow if, at that instant,
the line pressure be sufficient. On account of the relatively
greater line current, the distribution of pressure along the
gaps becomes equal, and has the value necessary to maintain the
line current arc on a gap.
The line current continues to flow until the voltage of the
generator passes through zero to the next half cycle, when the
arc extinguishing quality of the metal cylinders comes into
action.
[Illustration: FIGS. 2,356 and 2,357.--Diagram showing condenser action
of cylinders and pressure gradient for static stress.]
The alloy contains a metal of low boiling point which prevents
the reversal of the line current. It is a rectifying effect,
and before the pressure again reverses, the arc vapor in the
gaps has cooled to a non-conducting state.
~Effect of Frequency.~--The higher the frequency of the
lightning oscillation, the more readily will the multi-gap
respond to the pressure.
Briefly stated, the problem is to properly limit the line
current so that the arc may be extinguished; to arrange a shunt
circuit so that the series resistance will be automatically cut
out if safety demand it on account of a heavy lightning stroke
and, while retaining these properties, to make the arrester
sensitive to a wide range of frequency.
It should be noted that series resistance limits the rate of discharge
of the lightning as well as of the line current. The greater the
value of the line current, the greater the number of gaps required to
extinguish the arcs.
~Graded Shunt Resistances.~--Any arc is unstable and can be
extinguished by placing a properly proportioned resistance in parallel
with it. All the minor discharges then pass over the resistances and
the unshunted spark gaps, the resistance assisting in opening the line
current after the discharge.
Very heavy discharges pass over all the spark gaps, as a path without
resistance, but those spark gaps which are shunted by the resistance,
open after the discharge.
The line current, after the first discharge is accordingly deflected
over the resistances, and limited thereby, the circuit being finally
opened by the unshunted spark gaps. The arrangement of shunted
resistances is shown in fig. 2,358.
[Illustration: FIG. 2,358.--Arrangement of graded resistances on
multi-gap arrester.]
~The Cumulative or "Breaking Back" Effect.~--The graded shunt
resistance gives a valuable effect, where the arrester is considered as
four separate arresters. This is the "cumulative" or "breaking back"
action.
When a lightning strain between line and ground takes place, the
pressure is carried down the high resistance H (figs. 2,365 and 2,366),
to the series gaps GS, and the series gaps spark over.
Although it may require several thousand volts to spark across an
air gap, it requires relatively only a few volts to maintain the arc
which follows the spark. In consequence, when the gaps GS spark over,
the lower end of the high resistance is reduced practically to ground
pressure.
If the high resistance can carry the discharge current without giving
an ohmic drop sufficient to break down the shunted gaps GH, nothing
further occurs--the arc goes out.
If, on the contrary, the lightning stroke be too heavy for this, the
pressure strain is thrown across the shunted gaps, GH, equal in number
to the previous set. In other words, the same voltage breaks down
both of the groups of gaps, GS and GH, in succession. The lightning
discharge current is now limited only by the medium resistance M, and
the pressure is concentrated across the gaps, GM.
If the medium resistance cannot discharge the lightning, the gap GM
spark, and the discharge is limited only by the low resistance.
The low resistance should take care of most cases but with
extraordinarily heavy strokes and high frequencies, the discharge can
~break back~ far enough to cut out all resistance.
In the last steps, the resistance is relatively low in proportion to
the number of shunt gaps, GL, and is designed to cut out the line
current immediately from the gap, GL. This "breaking back" effect is
valuable in discharging lightning of low frequency.
[Illustration: FIGS. 2,359 to 2,364.--Westinghouse safety spark gaps.
Fig. 2,359, indoor type; figs. 2,360 to 2,364, outdoor type. It is
well known that with transformers, operating on high voltage lines and
having large ratios of transformation, there may occur, on the low
tension side, momentary voltages to ground greatly in excess of the
normal. These momentary increases in voltage between the low tension
circuits and ground are commonly called ~"static disturbances."~ In
general they are the result of a change in the static balance of
the high tension side and its connecting circuits. Unless certain
precautions are taken, such a static disturbance on the low tension
side may cause serious stresses in the secondary insulation of a
transformer with a high ratio of transformation. This induced static
voltage is independent of the ratio of transformation. The static
stresses are more serious in a high ratio transformer simply because
the insulation of its secondary is less able to withstand them. _A
method of relieving this disturbance_ is to connect a discharge spark
gap between some point of the low tension side of the transformer to
be protected (a middle or neutral point, if one be available) and the
ground. The spark gap opening is such that any voltage very much in
excess of the maximum normal will cause a discharge to ground, and
thus the low tension side is practically tied to ground during such
disturbance, while at other times it is ungrounded. The Underwriters
recommend the grounding of the neutral point of low tension circuits
when the conditions are such that the maximum normal voltage between
the point connected and ground will not exceed 250 volts. The rule
allows one side of a 250 volt circuit or the middle point of a 550 volt
circuit to be grounded. ~The spark gaps shown above~ are designed for
use on transformer secondary circuits and for protecting individual
series arc lamps. These spark gaps are single pole, and consist of
two cylinders of non-arcing metal with an air gap between. One of the
cylinders is connected to the ground, the other to the line.]
[Illustration: FIGS. 2,365 and 2,366.--Graded shunt resistance arrester
connections. Fig. 2,365, connections for 33,000 volt ~Y~ system with
grounded neutral; fig. 2,366, connections for 33,000 volt delta or
ungrounded ~Y~ systems. The type of arrester shown above may be
considered as ~four arresters in one~. ~First,~ for small discharges
there are a few gaps in series with a high shunt resistance. This
part of the arrester will safely discharge accumulated static and
also all disruptive discharges of small ampere capacity. This path
is shown through H (resistance) and GS (gaps). ~Second,~ there are a
number of gaps in series with a medium shunt resistance which will
discharge disruptive strokes of medium ampere capacity. This path is
shown through M (resistance) and GH plus GS (gaps). ~Third,~ there are
a greater number of gaps in series with a low shunt resistance which
will discharge heavy disruptive strokes. This path is shown through L
(resistance) and GM plus GH plus GS (gaps). ~Fourth,~ the total number
of gaps has no series resistance, thus enabling the arrester to freely
discharge the heaviest induced strokes. This path is shown through zero
resistance and GH plus GM plus GH plus GS (gaps). In each of the above
circuits the number of gaps and the resistance are so proportioned as
to extinguish the line arc at the end of the half cycle in which the
lightning discharge takes place.]
[Illustration: FIG. 2,367.--Installation of a General Electric 12,500
volt, three phase, multi-gap lightning arrester in the Garfield Park
sub-station of the West Chicago park common. The "V" unit multi-gap
arrester, which is plainly seen in the illustration, is made up of
"V" units consisting of gaps between knurled cylinders and connected
together at their ends by short metal strips. The base is of porcelain,
which thoroughly insulates each cylinder, and insures the proper
functioning of the multi-gaps. The cylinders are made of an alloy
that contains metal of low boiling point which gives the rectifying
effect, and metals of high boiling point which cannot vaporize in the
presence of the one of low boiling point. The cylinders are heavily
knurled. As the arc plays on the point of a knurl it gradually burns
back and when the metal of low boiling temperature is used up, the gap
is increased at that point. The knurling, thus, insures longer life to
the cylinder by forcing successive arcs to shift to a new point. When
worn along the entire face, the cylinder should be slightly turned.
The low resistance section of the graded shunt is composed of rods of
a metallic alloy. These rods have large current carrying capacity, and
practically zero temperature coefficient up to red heat. The medium and
high resistance rods are of the same standard composition previously
used. The contacts are metal caps shrunk on the ends; the resistances
are permanent in value and the inductance is reduced to a minimum. The
rods are glazed to prevent absorption of moisture and surface arcing.]
After the spark passes, the arcs are extinguished in the reversed
order. The low resistance, L, is proportioned so as to draw the arcs
immediately from the gaps, GL. The line current continues in the next
group of gaps, GM, until the end of the half cycle of the generator
wave.
[Illustration: FIGS. 2,368 to 2,370.--Multi-gap or low equivalent
lightning arrester. It consists of: 1, a number of gap units in series;
2, a number of gap units in shunt with a resistance; and 3, a series
resistance. ~All resistances~ are wire wound and the series resistance
is non-inductive. The shunt resistance and gap units are mounted on
marble. ~When a discharge occurs,~ the series gaps are broken down,
and if the discharge be heavy enough, it will meet opposition in the
shunt resistance and pass over the shunted gaps, through the series
resistance to the ground. ~The arc~ which tends to follow the discharge
is then withdrawn from the shunted gaps by the shunt resistance, and
aided by both resistances is suppressed by the series gaps. ~The
pressure of discharge~ is determined by the number of series gaps as
sufficient number is used to withstand the normal voltage and yet give
a proper factor of safety for the severest service.]
At this instant the medium resistance, M, aids the rectifying quality
of the gaps, GM, by shunting out the low frequency current of the
alternator.
On account of this shunting effect the current dies out sooner in the
gaps, GM, than it otherwise would.
In the same manner, but to a less degree, the high resistance, H, draws
the line current from the gaps, GH.
This current now being limited by the high resistance, the arc is
easily extinguished at the end of the first one-half cycle of the
alternator wave.
~Ques. What is the difference between arrester for grounded Y and
non-grounded neutral systems?~
Ans. The connections are shown in figs. 2,365 and 2,366. The difference
in design lies in the use of a fourth arrester leg between the
multiplex connection and ground or ungrounded system.
~Ques. Why is the fourth leg introduced?~
Ans. The arrester is designed to have two legs between line and line.
If one line become accidentally grounded, the full line voltage would
be thrown across one leg if the fourth or ground leg were not present.
[Illustration: FIG. 2,371.--Westinghouse three pole or four pole
arrester in weather proof wooden case which protects the arrester units
from rain and snow when they are installed in exposed locations, as on
poles or buildings.]
On a ~Y~ system with a grounded neutral, the accidentally
grounded phase causes a short circuit of the phase and the
arrester is relieved of the strain by the tripping of the
circuit breaker. Briefly stated, the fourth or ground leg of
the arrester is used when, for any reason, the system could be
operated, even for a short time with one phase grounded.
~Ques. Describe the multiplex connection.~
Ans. It consists of a common connection between the phase legs of the
arrester above the earth connection and provides an arrester better
adapted to relieve high pressure surges between lines than would
otherwise be possible.
Its use also economizes in space and material for delta and
partially grounded or non-grounded ~Y~ systems.
[Illustration: FIGS. 2,372 and 2,373.--Westinghouse multi-gap
lightning arrester and views showing parts. ~In construction,~ a
series of gaps, between non-arcing metal cylinders arranged in a row,
is connected between line and ground in series with a composition
stick resistor having a resistance of something between 80 and 120
ohms. ~In operation,~ if an excessive pressure be developed on a
line, electric discharge arcs form between the metal cylinders, and
the charge of electricity flows to ground, relieving the excessive
stress. The resistance of the stick resistor limits the flow so that
an excessive power current cannot pass through the arrester. The
tendency for a destructive power arc to follow the discharge arc is
thus counteracted. ~The composition resistors~ and the gap cylinders
are mounted in pairs on a porcelain base, and complete units are
arranged within weather proof wooden boxes as indicated. ~For two pole
arresters,~ one unit is mounted on the back of the box. ~For three pole
and four pole arresters,~ two units are used; one is secured on each
side of the box.] [Illustration: FIGS. 2,374 to 2,376.--Connections
for Westinghouse multi-gap (type G) arresters. These arresters may be
installed outside on poles or buildings, or indoors on station walls.
The weather proof wooden case (as shown in fig. 2,371) protects the
arrester units from rain and snow when installed in exposed places.
Fig. 2,374 shows single phase installation, fig. 2,375, two phase
installation, and fig. 2,376, three phase installation. ~On a two
pole circuit~ one line wire is connected to the top of each of the
composition resistors of each arrester unit, as shown in fig. 2,374,
and the ground wire is connected to the middle point of the gap series.
~On four pole circuits,~ fig. 2,375, the same scheme of connections is
used, but two arrester units are necessary and the connections of both
are the same. ~On three pole circuits,~ two arrester units are used,
with the same connections as for four pole circuits, except that there
are but three line connections instead of four as in fig. 2,376.]
~Horn Gap Arresters.~--A horn gap arrester consists essentially of two
horn shaped terminals forming an air gap of variable length, one horn
being connected to the line to be protected and the other to the ground
usually through series resistance as shown in fig. 2,378.
~Ques. How does the horn gap arrester operate?~
Ans. The arc due to the line current which follows a discharge, rises
between the diverging horn and becoming more and more attenuated is
finally extinguished.
[Illustration: FIG. 2,377.--Horn gap arrester, diagram showing arrester
and connections between line and ground. The horn type arrester
was invented by Oelschlaeger for the Allgemeine Electricitaets
Gesellschaft, and like the Thomson arc circuit arrester, ~its
operation~ is based on the fact that _a short circuit once started at
the base, the heat generated by the arc will cause it to travel upward
until it becomes so attenuated that it is ruptured_. On circuits of
high voltage this rupture sometimes takes a second or two, but seems
to act with little disturbance of the line. ~Sometimes~ _a water
resistance_ is used, a _choke coil being_ inserted in the circuit in
series. ~In one installation~ for a 40,000 volt line, the horns were
made of No. 0,000 copper wire with gap knees 2¼ to 3 or 3¼ inches. The
capacity of the water resistance receptacle was 15 gallons. ~Users
differ~ as to whether the water should contain salt. ~The choke coil~
can be made of about 18 turns of iron wire wound on a 6 inch cylinder.]
~Ques. What is the objection to the horn gap on alternating current
circuits?~
Ans. The arc lasts too long for synchronous apparatus to remain in step.
~Ques. What provision was made to shorten the duration of the arc?~
Ans. A series resistance was inserted in the arrester circuit as shown
in fig. 2,377.
~Ques. What difficulty was caused by the series resistance?~
Ans. With sufficient series resistance to prevent loss of synchronism,
the arrester failed to protect the system under severe conditions.
~Ques. With these objections what use was found for the horn gap
arrester?~
Ans. It is used as an emergency arrester on some overhead lines, to
operate only when a shut down is unavoidable, also for series lighting
circuits.
[Illustration: FIG. 2,378.--General Electric horn gap with charging
resistance for cable system. Arresters for cable systems differ from
arresters for overhead circuits only in the construction of the horn
gaps. The necessity for this difference is due to the fact that a
cable system has a very much higher electrostatic capacity and much
less inductance than an overhead system. In consequence, the currents
which flow into the arrester during charging are somewhat higher. It
is desirable to avoid these heavier currents, especially during the
time of breaking the arc at the horn gap. This is accomplished by
using a special horn gap and resistance. This consists of an auxiliary
horn mounted above and insulated from the regular horn in such a
manner as to intercept the arc if it rise on the regular horns. Enough
resistance is connected in series with this auxiliary horn so that the
current flow and arc across this gap are always limited to a moderate
value. Such a device has several advantages. Since the mechanism is so
arranged that the charging is always done through the auxiliary horn
the current rush is limited during the charging and thus troubles from
carelessness or ignorance are avoided. It also gives a nearer uniform
charging current. In the use of this auxiliary horn gap and resistance
there are three successive stages, as follows: 1, light discharges
will pass across the smaller gaps to the auxiliary horn and through
the series resistance to the cells; 2, if the discharge be heavy, the
resistance offers sufficient impedance to cause the spark to pass to
the main horn. This is accomplished with only a slight increase in
pressure because the gap is already ionized. If the cells be in normal
condition, the spark at the gap is immediately extinguished, without
any flow of line current; 3, if the cells be in poor form, the line
current may follow the discharge across the main gap and the arc will
rise to the safety horn and be extinguished through a resistance. For
mixed overhead and cable systems the choice of arrester will be a
matter of judgment. If there be a comparatively short length of cable,
the usual practice for overhead systems may be adopted. For direct
connection to busbars, arresters with charging resistance should be
used.]
[Illustration: FIGS. 2,379 and 2,380.--Diagram showing connections of
horn type lightning arresters on series circuits.]
The necessity of service requires that series lightning systems
be fully equipped against damage by lightning and similar
trouble. The most common disturbances occurring on series
circuits are the surges set up by the sudden opening of the
loaded circuit. These disturbances are especially severe where
circuits are accidentally grounded, due to contact of the wires
where they pass through other circuits.
~Ques. How are the spark gaps adjusted?~
Ans. They are set to give a low spark pressure relative to the voltage
of the line.
[Illustration: FIG. 2,381.--General Electric horn type arrester,
mounted for 15 light series arc circuit. The horn type arrester
consists of a horn gap with series resistance between each line and
ground. The resistances and horn gaps are mounted on porcelain bases
and the latter on insulating wooden supports. The supports have
asbestos barriers (except for lowest voltages), and backs to eliminate
liability of damage from the arc which forms in the horn gap at the
time of the discharge. The spark gaps are adjusted to give a low spark
pressure relative to the voltage of the circuit. The number and ohmic
value of the resistance rods used in the various arresters depend upon
the voltage and current of the circuit.]
~Ques. Why are horn arresters well suited to protect series lighting
circuits against surges?~
Ans. Because the surges are damped out before the arc which forms
across the horn gaps is interrupted.
These arcs last for several cycles, since the length of the
time of action of the arrester depends upon the lengthening of
the arc between the horn gaps, limited by the series resistance.
Since practically all disturbances on lighting circuits are
of low frequency, the series resistance can be used with good
results; it aids the horn in extinguishing the arc, limits the
size of the arc and prevents short circuits occurring during
the period of discharge.
[Illustration: FIG. 2,382.--General Electric horn arrester for pole
installation. Quite frequently series circuits are run underground
in cables for some distance from the generating station. In order to
protect the cables it is advisable to place horn arresters at the
points where the cable joins the overhead wires. The resistance units
are mounted in the wooden box. This design is used to economize space,
since if the horn gaps be placed in the box the latter would have to
be made very large to accommodate the asbestos barriers and backs. In
installing this type of arrester it is advisable to place it as near as
possible to the top of the pole so that the arc may rise unobstructed
and thus avoid the likelihood of live wires coming in contact with the
horns which, during the operation of the series current, are alive.]
~Electrolytic Arresters.~--Arresters of this class are sometimes
called aluminum arresters because of the property of aluminum on which
their action depends; that is, _it depends on the phenomenon that a
non-conducting film is formed on the surface of aluminum when immersed
in certain electrolytes_.
If however, the film be exposed to a higher pressure, it may be
punctured by many minute holes, thus so reducing its resistance that a
large current may pass. When the pressure is again reduced the holes
become resealed and the film again effective.
[Illustration: FIGS. 2,383 and 2,384.--Elevation and plan of General
Electric horn gaps and operating stand for high voltage arresters.]
In construction, the aluminum arrester consists essentially of a system
of nested aluminum cup shaped trays, supported on porcelain and secured
in frames of heated wood, arranged in a steel tank.
The system of trays is connected between the line and ground, and
between line and line, a horn gap being inserted in the arrester
circuit which prevents the arrester being subjected to the line voltage
except when in action.
The electrolyte is poured into the cones and partly fills the space
between the adjacent ones. The stack of cones with the electrolyte
between them is then immersed in a tank of oil. The electrolyte between
adjacent cones forms an insulation. The oil improves this insulation
and prevents the evaporation of the solution.
[Illustration: FIG. 2,385.--Cross section of General Electric aluminum
(electrolytic) lightning arrester.]
A cylinder of insulating material concentric with the cone stack is
placed between the latter and the steel tank, the object being to
improve the circulation of the oil and increase the insulation between
the tank and the cone stack. The arrester, as just described consists
of a number of cells connected in series.
~Ques. Of what does a single cell consist and what are its
characteristics?~
Ans. It consists of two of the cone shaped aluminium trays or plates
and an electrolyte, which forms a condenser that will stand about 350
volts before breaking down. When this voltage is exceeded the cell
becomes a fairly good conductor of electricity, but as soon as the
voltage drops its resistance again resumes a very high value.
~Ques. What is the critical voltage?~
Ans. The voltage at which the current begins to flow freely.
[Illustration: FIG. 2,386 to 2,390.--Parts of General Electric 15,000
volt aluminum lightning arrester, not including horn gaps, etc.]
Up to a certain voltage the cell allows an exceedingly low
current to flow, but at a higher voltage the current flow is
limited only by the internal resistance of the cell, which is
very low. A close analogy to this action is found in the well
known safety valve of the steam boiler, by which the steam is
confined until the pressure rises above a given value, when
it is released. On the aluminum plates there are myriads of
minute safety valves, so that, if the electric pressure rise
above the critical voltage, the discharge takes place equally
over the entire surface. It is important to distinguish between
the valve action of this hydroxide film and the failure of any
dielectric substance.
~Ques. When a cell is connected permanently to the circuit what two
conditions are involved?~
Ans. The _temporary_ critical voltage and the permanent _critical
voltage_.
For instance, if the cell have 300 volts applied to it constantly,
and the pressure be suddenly increased to, say 325 volts, there will
be a considerable rush of current until the film thickness has been
increased to withstand the extra 25 volts; this usually requires
several seconds. In this case 325 volts is _the_ ~temporary~ _critical
voltage of the cell_.
Similar action will occur at any pressure up to about the ~permanent~
critical voltage, or _the voltage at which the film cannot further
thicken_, and therefore allows a free flow of current.
If the voltage be again reduced to 300 the excess thickness of film
will be gradually dissolved, and if it vary periodically between two
values, each of which is less than the permanent critical value, the
temporary critical voltage will be the higher value. This feature is of
great importance as it provides a means of discharging abnormal surges,
the instant the pressure rises above the impressed value.
[Illustration: FIG. 2,391.--Volt ampere characteristic curve of a
General Electric aluminum (electrolytic) cell on alternating current.
The permanent critical voltage is between 335 and 360 volts. With
alternating current, the cell acts as a fairly good condenser, and
there is not only the leakage through the film, but also a capacity
current flowing into the cell. The phase of this current, then, is
nearly 90 degrees ahead of the pressure and represents a very low
energy factor.]
~Ques. How is the number of cells required for a given circuit
determined?~
Ans. The number required for a given operating voltage is determined by
allowing about 250 to 300 volts per cell.
~Ques. In putting cells in commission how is the electrolyte
introduced?~
Ans. It is poured into the aluminum trays and the overflow drawn off at
the bottom of the tank.
~Ques. Describe the further operations in putting cells in commission.~
Ans. After putting in the electrolyte it is allowed to stand for a few
days until part has evaporated, then the oil is poured over the surface
to prevent further evaporation.
[Illustration: FIG. 2,392.--Westinghouse electrolytic lightning
arrester, for three phase ungrounded neutral service, 25,000 maximum
voltage. These arresters are designed for the protection of alternating
current circuits from all kinds of static disturbances. They have been
standardized for installation on three phase circuits of voltages of
2,200 to 110,000. They cannot be used for voltages of less than 13,500.
For voltages below this the horn gaps cannot, with safety, be set
close enough together, out of doors, to take advantage of the freedom
of discharge of the electrolytic element. If the horn gaps be set too
close together they may be short circuited by rain. A shelter should be
built for arresters of 13,500 volts and below for their protection when
installed outside.]
~Ques. What action takes place when the trays stand in the electrolyte
and cell is disconnected from the circuit?~
Ans. Part of the film deteriorates.
~Ques. What is the nature of the film?~
Ans. The film is composed of two parts, one of which is hard and
insoluble, and apparently acts as a skeleton to hold the more soluble
part. The action of the cell seems to indicate that the soluble part of
the film is composed of gases in a liquid form.
~Ques. What action takes place when a cell which has stood for some
time disconnected, is reconnected to the circuit?~
[Illustration: FIGS. 2,393 and 2,394.--Aluminum trays for Westinghouse
electrolytic lightning arresters.]
Ans. There is a momentary rush of current which reforms the part of the
film which has dissolved.
This current rush will have increasing values as the intervals
of rest of the cell are made greater.
Many electrolytes have been studied, but none has been found
which does not show this dissolution effect to a greater or
lesser extent.
If the cell has stood disconnected from the circuit for some
time, especially in a warm climate, there is a possibility that
the initial current rush will be sufficient to open the circuit
breakers or oil switches. This current rush also raises the
temperature of the cell, and if the temperature rise be great,
it is objectionable.
When the cells do not stand for more than a day, however, the
film dissolution and initial current rush are negligible.
~Ques. What is the object of using horn gaps on electrolytic arresters?~
Ans. The use is threefold: 1, it prevents the arrester being subjected
continually to the line voltage; 2, acts as a disconnecting switch to
disconnect the arrester from the line for repairs, etc., and 3, acts as
a connecting switch for charging.
[Illustration: FIG. 2,395.--Horn gaps and transfer device of General
Electric aluminium lightning arrester for 12,500 volt non-grounded
neutral circuit. The object of the transfer device is to provide a
means for interchanging the ground stacks with one of the line stacks
of cones during the charging operation so that the films of all the
cells will be formed to the same value. The transfer device consists of
a rotating switch which may be turned 180 degrees, thus interchanging
the connections of the ground stack and one of the line stacks.
For arresters up to 27,000 volts the device is mounted with three
insulators on the pipe frame work, and is operated by a hand wheel; for
arresters of higher voltage, the transfer device is mounted directly
over the tanks and is operated by bevel gears and hand wheel.]
~Charging of Electrolytic Arresters.~--In electrolytic arresters all
electrolytes dissolve the film when the arrester is on open circuit,
the extent of the dissolution depending upon the length of time the
film is in the electrolyte, and upon its temperature. It is therefore
necessary to _charge_ the cells from time to time and thus prevent the
dissolution and consequent rush of current which would otherwise occur
when the arrester discharges.
~Ques. Describe the charging operation for arresters with grounded
circuits.~
Ans. It consists in simply closing simultaneously the three horn gaps
so that the full pressure across the cells causes a small charging
current to flow and form the films to their normal condition.
[Illustration: FIG. 2,396.--Sectional view of General Electric ~vacuum
tube arrester~ for railway signal circuits. ~The arrester~ _is
essentially a gap in a vacuum_. ~In construction~, the gap is formed
between the inner wall of a drawn metal shell and a disc electrode
mounted concentric with it. ~The electrode~ is supported on a brass rod
which serves as the lead in connection, and has ample current carrying
capacity. The electrode system is insulated from the tube and rigidly
supported in position by a bushing made of vitreous material. The
bushing does not form the vacuum seal, that being made by a special
compound. ~The open end of the tube~ is finally closed by a porcelain
bushing. The tube is exhausted in a special machine which solders
a small hole in the end after the vacuum has been established. The
possibility of solder entering the active part of the vacuum space
is prevented by a diaphragm punching, and both the electrode and the
lining of the tube are of non-arcing metal. ~The arrester has a spark
pressure of~ from 350 to 600 volts direct current, and an equivalent
needle gap of about .005 inch. The arrester will not stand a continuous
flow of current due to excessive heating, hence if there be a
possibility of this due to high pressure crosses, fuses should be used.
R.R.S.A. standard terminals are used.]
~Ques. Describe the charging operation for arresters for non-grounded
circuits. ~ Ans. First, the horn gaps are closed for five seconds and
opened again to normal position, thus charging the cells of the three
line stacks. Second, with the horn gaps still in normal position, the
position of the transfer device is reversed and the horn gaps are again
closed for five seconds and returned to the normal position.
The complete charging operation takes but a few moments and
should be performed daily. The operation is valuable, not only
in keeping the films in good condition, but also in giving the
operator some idea of the condition of the arrester by enabling
him to observe the size and color of the charging spark.
[Illustration: FIG. 2,397.--Highland Park sub-station, Charlotte, N.C.,
showing old lightning arrester tower on the left and General Electric
aluminum (electrolytic) cell lightning arrester and horn gaps in
foreground.]
~Grounded and Non-grounded Neutral Circuits.~--It is important to avoid
the mistake of choosing an arrester for a thoroughly grounded neutral
when the neutral is only partially grounded, that is, grounded through
an appreciable resistance. Careful consideration of this condition will
make the above statement clear.
In an arrester for a grounded neutral circuit, each stack of cones
normally receives the neutral pressure when the arrester discharges,
but if a phase become accidentally grounded, the line voltage is thrown
across each of the other stacks of cones until the circuit breaker
opens the circuit. The line voltage is 173 per cent. of the neutral or
normal operating voltage of the cells and therefore about 150 per cent.
of the permanent critical voltage of each cell. This means that when
a grounded phase occurs, this 50 per cent. excess pressure is short
circuited through the cells until the circuit breaker opens.
[Illustration: FIG. 2,398.--Westinghouse ~electrolytic station
lightning arrester~ for direct current up to 1,500 volts consists of a
tank of oil in which are placed, on properly insulated supports, a nest
of cup shaped aluminum trays. The spaces between the trays are filled
with electrolyte, a sufficient quantity for one charge being furnished
with each arrester. The top tray is connected with the line through a
60 ampere fuse, and the bottom tray is connected to the tank which is
thoroughly grounded by means of a lug. The fuse is of the enclosed type
and mounted on the cover of the arrester. ~A small charging current~
flows through the trays continuously and keeps the films on the trays
built up, so that no charging is required. This charging current is
not, however, of sufficient value to raise the temperature appreciably.
The immersed area of each tray is 100 square inches. The shape and
the arrangement of the trays is such that any gases generated by the
discharge can pass out readily without disturbing the electrolyte
between the trays.]
The amount of energy to be dissipated in the arrester depends upon the
kilowatt capacity of the generator, the internal resistance of the
cells, and the time required to operate the circuit breakers. It is
evident that the greater the amount of resistance in the neutral, the
longer will be the time required for the circuit breakers to operate.
Therefore, in cases where the earthing resistance in the neutral
is great enough to prevent the automatic circuit breakers opening
practically instantaneously, an arrester for a non-grounded neutral
system should be installed.
[Illustration: FIGS. 2,399 to 2,401.--Westinghouse ground fittings.
Fig. 2,399, ground plate; fig. 2,400, ground point; fig. 2,401,
cap. The ground plate consists of a circular piece of cast iron,
12 inches in diameter, 1⅜ inches thick with a ¾ inch pipe tap in
center to connection to arrester. The surface is increased by means
of corrugations, as shown in the accompanying illustrations, to 461
square inches, affording ample contact with the earth and enabling it
to take care of all discharges through the arrester. The plate should
preferably be buried at the foot of the pole so that the ground wire
runs to it in a straight line from the arrester. Care should, of
course, be taken to see that the earth in which the plate is buried
is damp. If the ground wire be placed within the pipe leading to the
ground plate it should be soldered to a cap at the top of the pipe
to eliminate the inductive effect due to the wire being surrounded
by iron. A simple and effective method of securing a good ground is
by means of an iron pipe with a malleable iron point having a dipped
galvanized finish, and a brass cap with a lug for soldering the ground
wire. The pipe may be driven into the earth, or if it be too hard to
permit driving, a hole may be dug and the pipe placed therein. It
should extend from eight to ten feet above and below the earth to
secure, respectively, a good ground and prevent any tampering with the
ground wire. Should it be desired to make use of a longer pipe which
would be inconvenient to drive into the earth, two pieces can be used
and connected together by a coupling. The brass cap and malleable iron
point are tapped for use with ¾ inch pipe.]
~Ground Connections.~--In all lightning arrester installations it is
of the utmost importance to make proper ground connections, as many
lightning arrester troubles can be traced to bad grounds. It has been
customary to ground a lightning arrester by means of a large metal
plate buried in a bed of charcoal at a depth of six or eight feet in
the earth.
A more satisfactory method of making a ground is to drive a number of
one inch iron pipes six or eight feet into the earth surrounding the
station, connecting all these pipes together by means of a copper wire
or, preferably, by a thin copper strip. A quantity of salt should be
placed around each pipe at the surface of the ground and the ground
should be thoroughly moistened with water. It is advisable to connect
these pipes to the iron framework of the station, and also to any water
mains, metal flumes, or trolley rails which are available.
[Illustration: FIGS. 2,402 to 2,404.--General Electric ~magnetic blow
out arrester~ for use on railways. ~It consists of~ _an adjustable
spark gap in series with a resistance_. ~Part of the resistance~ is in
shunt with a blow out coil, between the poles of which is the spark
gap. The parts are mounted in a strong, porcelain box, which, for car
and pole use, is in turn mounted in a substantial asbestos lined,
wooden box. ~In operation~, when the lighting pressure comes on the
line, it causes the spark gap to break down and a discharge occurs
through the gap and the resistance rod to ground. Part of the current
shunts through the blow out coil producing a strong magnetic field
across the spark gap. The magnetic field blows out the discharge arc
and restores normal conditions. The resistance is only 60 ohms (for 500
volt rating work), and the spark gap only one-fortieth of an inch (.025
in.).]
The following suggestions are made for the usual size station.
1. Place three pipes equally spaced near each outside wall,
making twelve altogether, and place three extra pipes spaced
about six feet apart at a point nearest the arrester.
2. Where plates are placed in streams of running water, they
should be buried in the mud along the bank in preference to
being laid in the stream. Streams with rocky bottoms are to be
avoided.
3. Whenever plates are placed at any distance from the
arrester, it is necessary also to drive a pipe into the
earth directly beneath the arrester, thus making the ground
connection as short as possible. Earth plates at a distance
cannot be depended upon. Long ground wires in a station cannot
be depended upon unless a lead is carried to the parallel
grounding pipes installed as described above.
4. As it is advisable occasionally to examine the underground
connections to see that they are in proper condition, it is
well to keep on file exact plans of the location of ground
plates, ground wires and pipes, with a brief description, so
that the data can be readily referred to.
[Illustration: FIG. 2,405.--General Electric ~magnetic blow out
arrester~ for line use. ~It consists~ essentially of _a small spark
gap which is in series with a resistance, and between the poles of a
magnet_. ~The operation~ is similar to that of the arrester shown in
figs. 2,402 to 2,404, but the magnet is a permanent magnet instead
of an electromagnet. The spark gap and the magnet are mounted within
porcelain blocks in such a way that the discharge arc is blown by the
magnet through an arc chute and a cooling grid which is also held by
the porcelain. ~The cooling grid~ in the arc chute materially assists
the magnet in extinguishing the discharge arc, giving the arrester a
high arc rupturing quality. ~The series rod~ is carborundum and is
connected externally to the other portion of the arrester. The arrester
is self-contained.]
5. From time to time the resistance of these ground connections
should be measured to determine their condition. The resistance
of a single pipe ground in good condition has an average value
of about 15 ohms. A simple and satisfactory method of keeping
account of the condition of the earth connections is to divide
the grounding pipes into two groups and connect each group to
the 110 volt lighting circuit with an ammeter in series.
~Choke Coils.~--A lightning discharge is of an oscillatory character
and possesses the property of self-induction, accordingly it passes
with difficulty through coils of wire. Moreover, the frequency of
oscillation of a lightning discharge being much greater than that of
commercial alternating currents, a coil can readily be constructed
which will offer a relatively high resistance to the passage of
lightning and at the same time allow free passage to all ordinary
electric currents.
Opinions on the design of choke coils for use with lightning
arresters vary considerably. Some engineers recommend the
use of very large choke coils, but while large choke coils
of high inductance do choke back the high frequency currents
better than smaller coils of less inductance, they cost more,
and under many conditions they are a menace to the insulation
unless the lightning arresters be installed on both sides of
them.
[Illustration: FIG. 2,406.--Westinghouse line suspension choke coil.
It is so designed that it can be inserted directly in the transmission
line wire or in the station wiring and held in position therein by
the tension of the line or station wires. Because of the fact that no
insulators are required, solely to support this choke coil, and that
it can be installed in either a vertical or a horizontal position it
can often be utilized effectively in power and sub-station layouts.
Terminals, each having a ½ inch round hole, to accommodate the
conductors are provided at each end of the coil. Three square headed
binding screws are supplied which clamp the conductors in position.
The coil is provided with a strain insulator, so arranged within the
coil at its axis, that it assumes any mechanical tension transmitted
from the conductors. No mechanical tension reaches the turns of the
choke coil proper. In construction, the choke coil is made in but one
size having a current carrying capacity of 200 amperes and is suitable
for a voltage of 2,000 to 22,000. For higher voltages than 22,000,
several choke coils are connected in series. One coil is used for each
22,000 volts or fraction thereof, of the pressure between the wires of
the circuit. ~Application~: This type of choke coil may be used for
alternating current service for the entire range from 22,000 to 110,000
volts. It may be used on transformers, but is not recommended for the
protection of generators.]
Part of the functions of the choke coil are performed by the
end turns of a transformer and extra insulation is invariably
installed in all power transformers built in recent years.
The choice of choke coils must be influenced by the condition
of insulation in the transformers as well as by the cost,
pressure regulation, and nature of the lightning protection
required.
~Ques. What are the primary objects of a choke coil?~
Ans. To hold back the lighting disturbance from the circuit apparatus
during discharge, and to lower the frequency of the oscillation so that
whatever charge gets through the choke coil will be of a frequency too
low to cause serious pressure drop around the first turns of the end
coil in either alternator or transformer.
[Illustration: FIGS. 2,407 to 2,409.--General Electric choke coils.
Fig. 2,407, hour glass choke coil, 45,000 volts; fig. 2,408, low
voltage choke coil, 6,600 volts; fig. 2,409, low voltage choke coil,
4,600 volts.]
If there be no arrester, the choke coil cannot perform the
first function, accordingly a choke coil is best considered as
an auxiliary to an arrester.
~Ques. What is the principal electrical condition to be avoided with a
choke coil?~
Ans. Resonance. The coil should be so arranged that if continual
surges be set up in the circuit, a resonant voltage due to the presence
of the choke coil cannot build up at the transformer or generator
terminals. This factor is a menace to the insulation. Another way of
stating the condition is as follows: So arrange the choke coil as
not to prevent surges, originating in a transformer, passing to the
arrester.
~Ques. What is another electrical condition to be avoided and why?~
Ans. Internal static capacity between adjacent turns
[Illustration: FIG. 2,410.--Westinghouse choke coil for high pressure
transmission circuits, 2,200 to 25,000 volts. Choke coils of this type
are wound without iron cores on circular or elliptical center blocks.
They have a large number of layers and few turns per layer (except
those made for small currents, they usually have one turn per layer),
which give the best condition for insulating and cooling. They are air
cooled, heavily insulated and have a line lead at the top, as shown.
Choke coils are designed to prevent the short circuits sometimes caused
by the local concentration of pressure such as may be produced by a
lightning discharge. They limit, to some extent, an abnormal rise of
pressure on the apparatus by delaying the advance of a static wave from
the line and thus give the arrester more time to act. The disturbance
caused by a lightning flash passes along the line in the form of a
surge or "tidal wave." If this wave pass a choke coil, it is flattened
out, and if the coil be of sufficient power, becomes practically
harmless. It is evident, however, that the choke coil receives the
full force of the wave, and that, consequently, it must be heavily
insulated; moreover, the choke coil must not overheat under load, nor
introduce into the circuit excessive inductive resistance.] of the
choke coil, because this lowers the effectiveness of the coil.
~Ques. What is the object of making choke coils in the form of an hour
glass?~
Ans. To prevent sagging between the supports.
[Illustration: FIG. 2,411.--Westinghouse ~air cooled choke coil~
particularly suitable for outdoor use. ~The method of mounting~ is
such that insulation for any desired voltage is readily obtained with
the same type of porcelain, and mounting in any position is possible.
~The coil~ is a helix of aluminum rod, about 15 inches in diameter
and containing about 30 turns. Bracing clamps are provided to give
mechanical strength to the helix, and the rod used is of sufficient
diameter to carry 200 amperes. The coil is supported on two insulating
columns made up of porcelain insulators, which, except for the end
pieces, are interchangeable. The number of insulators used in the
columns depends on the voltage of the circuit in which the coil is to
be used. The apparatus can be mounted in any position convenient for
the wiring, on floor, wall, or ceiling. ~It is intended principally~
_for the protection of transformers_. Where greater reactance than is
afforded by a single coil is desired on the higher voltage circuits, it
is recommended that two or more coils be connected in series, one coil
being used for each 22,000 line voltage. This coil should not be used
for generators. The insulating columns are supported on substantial
cast iron blocks on wooden bases.]
~Ques. How are choke coils cooled?~
Ans. By air, or by oil.
~Ques. For what service are oil cooled choke coils used?~
Ans. On circuits of pressures above 25,000 volts, choke coils immersed
in oil, as are transformer coils, have advantages in that the coil is
amply insulated not only from the ground but against side flash, and
that copper of comparatively small section may be used without undue
heating.
[Illustration: FIG. 2,412.--Westinghouse ~air cooled choke coil,~
for voltages of from 2,200 to 110,000. ~In construction,~ the coils
are made of aluminum rod wound into a helix of about 15 inches in
diameter and having 20 turns. The helix is supported on two insulators.
For mechanical reasons it is necessary to have the aluminum rod of
sufficient size to secure rigidity, consequently every coil has a
capacity of 200 amperes and may be used on any circuit up to that
capacity. ~The coils~ are insulated according to the standard practice
for disconnecting switches, the insulators being mounted on wooden
pins supported by a wooden base. This apparatus can be mounted in
any position. The wiring of a station or sub-station is facilitated
because the protection may be placed so as to simply form part of the
wiring. The coils are symmetrical so that it is immaterial which end is
connected to the line or to the apparatus.]
~"Static" Interrupters.~--A static interrupter is a _combination of a
choke coil and a condenser_, the two being mounted together and placed
in a tank and oil insulated.
It is used on high pressure circuits and its function is to so
delay the erroneously called "static" wave in its entry into
the transformer coil, that a considerable portion of the latter
will become charged before the terminal will have reached full
pressure.
A choke coil alone sufficiently powerful to accomplish this would be
too large and costly on very high pressure and would interfere with the
operation of the system.
~Ques. How is the condenser and choke coil connected?~
Ans. The condenser is connected between the line and ground behind the
choke coil near the apparatus to be protected as shown in fig. 2,413.
[Illustration: FIG. 2,413.--Diagram showing connections of static
interrupter for protecting a transformer.]
~Ques. What is the effect of the condenser?~
Ans. The condenser, which has a very small electrostatic capacity, has
no appreciable effect upon the normal operation, but a very powerful
effect upon the static wave on account of its extremely high frequency.
CHAPTER LX
REGULATING DEVICES
~Regulation of Alternators.~--Practically all the methods employed
for regulating the voltage of direct current dynamos and circuits,
are applicable to alternators and alternating current circuits. For
example: in order that they shall automatically maintain a constant or
rising voltage with increase of load, alternators are provided with
composite winding similar to the compound winding of direct current
dynamos, but since the alternating current cannot be used directly
for exciting the field magnets, an accessory apparatus is required to
rectify it or change it into direct current before it is used for that
purpose.
It is a fact, however, that composite wound alternators do not regulate
properly for inductive as well as non-inductive loads.
In order to overcome this defect compensated field alternators have
been designed which automatically adjust the voltage for all variations
of load and lag. These machines have already been described.
~Alternating Current Feeder Regulation.~--With slight modification, the
various methods of feeder regulation employed with direct current, may
be applied to alternating current distribution circuits. For instance,
if a non-inductive resistance be introduced in any electric circuit,
the consequent drop in voltage will be equal to the current multiplied
by the resistance. Therefore, feeder regulation by means of rheostats
is practically the same in the case of alternating current as in that
of direct current. In the case of the former, however, the effect of
self-induction may also be utilized to produce a drop in voltage. In
practice, this is accomplished by the use of self-induction coils which
are commonly known as reactance coils.
[Illustration: FIG. 2,414.--Diagram _illustrating the principle of_
~induction voltage regulators. The primary coil~ P, _consisting of many
turns of fine wire_, is connected across the main conductors C and D,
coming from the alternator. ~The secondary coil~ S, _consisting of a
few turns of heavy wire_, is connected in series with the conductor
D. The laminated iron core E, mounted within the coils, is capable of
being turned into the position shown by the dotted lines. When the
core is vertical, the magnetic lines of force produced in it by the
primary coil, induces a pressure in the secondary coil which aids the
voltage; when turned to the position indicated by the dotted lines, the
direction of the magnetic lines of force are reversed with respect to
the secondary coil and an opposing pressure will be produced therein.
Thus, by turning the core, the pressure difference between the line
wires G and H, can be varied so as to be higher or lower than that of
the main conductors C and D. Regulators operating on this principle may
be used for _theatre dimmers_, as controllers for series lighting, and
also to adjust the voltage or the branches of unbalanced three wire
single phase and polyphase systems.]
~Application of Induction Type Regulators.~--In supplying lighting
systems, where the load and consequently the pressure drop in the
line increases or decreases, it becomes necessary to raise or lower
the voltage of an alternating current, in order to regulate the
voltage delivered at the distant ends of the system. This is usually
accomplished by means of _alternating current regulators or induction
regulators_. A device of this kind is essentially a transformer, the
primary of which is excited by being connected directly across the
circuit, while the secondary is in series with the circuit as shown in
fig. 2,414. By this method the circuit receives the voltage generated
in the secondary.
[Illustration: FIG. 2,415.--Diagram of induction regulator ~raising the
voltage~ 10%. In the diagram an alternator is supplying 100 amperes
at 2,200 volts. The regulator raises the feeder pressure to 2,420
volts, the current being correspondingly reduced to 91 amperes, the
other 9 amperes flowing from the alternator through the primary of the
regulator, back to the alternator.]
[Illustration: FIG. 2,416.--Diagram of induction regulator ~lowering
the voltage~ 10%. The diagram shows the regulator lowering the feeder
pressure to 1,980 volts with an increase of the secondary current to
111 amperes, the additional 11 amperes flowing from the feeder, through
the primary back to the feeder.]
~Ques. Name two types of pressure regulator.~
Ans. The induction regulator, and the variable ratio transformer
regulator.
~Ques. Of what does an induction regulator consist?~
Ans. It consists of a primary winding or exciting coil, a secondary
winding which carries the entire load current.
[Illustration: FIG. 2,417.--Moving element or ~primary~ of Westinghouse
motor operated single phase induction regulator. It consists of a core
of punchings built up directly on the primary shaft and carrying the
primary winding, which is divided into four coils. The primary coils
are machine wound and the layers of the winding are separated from each
other by heavy insulating material in addition to the cotton covering
of the inductors. The complete coils are insulated and impregnated with
insulating compound before being placed in the slots. The coils are
held in position by fibre wedges.]
The primary is wound for the full transmission voltage, and is
connected across the line, while the secondary is connected in
series with the line.
~Ques. What is its principle of operation?~
Ans. When the primary coil is turned to various positions the magnetic
flux sent through the secondary coil varies in value, thereby causing
corresponding variation in the secondary voltage, the character of
which depends upon the value and direction of the flux.
~Ques. What is the effect of turning the secondary coil to a position
at right angles with the primary coil?~
Ans. The primary will not induce any voltage in the secondary, and
accordingly it has no effect on the feeder voltage.
~Ques. What is this position called?~
Ans. The neutral position.
~Ques. What are the effects of revolving the primary coil from the
neutral position first in one direction then in the other?~
Ans. Turning the primary in one direction increases the voltage induced
in the secondary, thus increasing the feeder voltage like the action of
a booster on a direct current circuit while turning the primary in the
opposite direction from the neutral position, correspondingly decreases
the feeder voltage.
[Illustration: FIG. 2,418.--Moving element or ~primary~ of Westinghouse
motor operated polyphase induction regulator.]
~Ques. It was stated that for neutral position the primary had no
effect on the secondary; does the secondary have any effect on the
feeder voltage?~
Ans. The secondary tends to create a magnetic field of its own
self-induction, and has the effect of a choke coil.
~Ques. How is this tendency overcome?~
Ans. The primary is provided with a short circuited winding, placed at
right angles to the exciting winding. In the neutral position of the
regulator, this short circuited winding acts like the short circuited
secondary of a series transformer, thus preventing a choking effect in
the secondary of the regulator.
~Ques. What would be the effect if the short circuited winding were not
employed?~
[Illustration: FIG. 2,419.--Top end of stationary element or
~secondary~ of Westinghouse polyphase induction regulator; view showing
leads. The secondary is built up in a short skeleton frame with
brackets for the rotor bearings bolted to the frame and the top cover
bolted to the top brackets. In assembling the secondary, the punchings
are stacked loosely in the skeleton frame and an expanding building
mandrel placed inside the punchings and expanded, thereby truing up the
latter before they are finally compressed and the end plates keyed in
position. Then, prior to removing the mandrel a finishing cut is taken
on the surface of the frame to which the bearing brackets are attached,
and as the top cover and brackets are also accurately machined the
alignment of the primary with the secondary is almost perfect, thus
reducing to a minimum the tendency to develop vibration and noise.]
Ans. The voltage required to face the full load current through the
secondary would increase as the primary is turned away from either the
position of maximum or minimum regulation, reaching its highest value
at the neutral position.
The short circuited winding so cuts down this voltage of
self-induction that the voltage necessary to force the full
load current through the secondary when the regulator is in the
neutral position is very little more than that necessary to
overcome the ohmic resistance of the secondary.
~Ques. What effect is noticeable in the operation of a single phase
induction regulator?~
Ans. It has a tendency to vibrate similar to that of a single phase
magnet or transformer.
[Illustration: FIG. 2,420.--Bottom end of stationary element or
~secondary~ of Westinghouse polyphase induction regulator.]
~Ques. Why?~
Ans. It is due to the action of the magnetizing field varying in
strength from zero to maximum value with each alteration of the
exciting current, thus causing a pulsating force to act across the air
gap, which tends to cause vibration when the moving part is not in
perfect alignment.
~Ques. Explain the effect produced by bad alignment?~
Ans. If the bearings of the primary be not in perfect alignment with
the bore of the secondary, thereby making the air gap on one side
smaller than that on the other, the crowding over of the flux to the
smaller air gap will cause an intermittent pull in that direction,
which will develop vibration unless the primary bearings are tight and
the shaft sufficiently stiff to withstand the pull.
~Ques. Upon what does the regulator capacity for any given service
depend?~
[Illustration: FIG. 2,421.--Westinghouse two kw., hand operated, air
cooled induction regulator for testing purposes.]
Ans. It depends upon the range of regulation required and the total
load on the feeder.
~Ques. How is the capacity stated?~
Ans. In percentage of the full load of the feeder.
For instance, on a 100 kilowatt circuit, a 10 kw. regulator
will give 10 per cent. regulation, and a 5 kw. regulator, 5 per
cent. regulation.
~Polyphase Induction Regulators.~--The polyphase induction regulator
is similar to the single phase regulator except that both the primary
and secondary elements are wound with as many sets of coil as there are
phases in the circuit.
In construction these windings are distributed throughout the complete
circumference of the stationary and moving elements and closely
resemble the windings of an induction motor.
[Illustration: FIG. 2,422.--Westinghouse polyphase motor operated
induction regulator showing operating mechanism. The primary shaft is
turned by means of a bronze worm wheel engaging a forged steel worm,
provided with a ball bearing end thrust. This worm gear is housed in a
separate casting bolted to the cover. The casting is made separate in
order to permit close adjustment between the worm wheel and the worm
to aid in counteracting the tendency to vibration. Finished surfaces
on the worm gear casting are provided for mounting the motor and the
brake. On the automatic regulator, the worm shaft is connected to the
motor through a spur gear and pinion, which constitutes a compact
driving device having very little friction. Provision is made for
either alternating current or direct current motor drive. When a
motor driven regulator is operated by hand, the brake must be held in
the release position, otherwise it will be impossible to operate the
regulator. In the hand operated regulator the spur gear is replaced by
a hand wheel and the regulator is driven directly from the worm shaft.]
Polyphase regulators have but little tendency to vibrate because the
field across the air gap is the resultant of two or more single phase
fields and is of a constant value at all times. This field rotates
at a rate depending upon the number of poles and the frequency of
the circuit. This produces a mechanical pull of constant value which
rotates with the magnetic field varying its position from instant to
instant.
It is evident that this pull is of an entirely different character from
that produced by the single phase field and that there is no tendency
to set up the vibration that the mechanical pull of the single phase
regulator tends to establish.
[Illustration: FIG. 2,423.--General Electric ~adjustable compensation
shunt~. ~It is used~ as the compensating shunt for direct current
voltage regulators. ~In operation,~ the shunt may be adjusted so as
to compensate for any desired line drop up to 15 per cent. It is
preferably placed in the principal lighting feeder but may be connected
to the bus bars so that it will take the total current. The latter
method is sometimes undesirable, as large fluctuating power loads on
separate feeders might disturb the regulation of the lighting feeders.
~Adjustment is made~ _by sliding the movable contact_ shown at the
center of the shunt. This contact may be clamped at any desired point
and it determines the pressure across the compensating winding of the
regulator's control magnet. Where pressure wires are run back to the
central station from the center of distribution, they may be connected
directly to the pressure winding of the main control magnet, and it is
unnecessary to use the compensating shunt.]
There is, however, considerable torque developed, and the device for
revolving the moving element must be liberally designed so as to
withstand the excess torque caused by temporary overloads or short
circuits.
~Ques. In what respects do polyphase induction regulators differ in
principle from single phase regulators?~
Ans. The induced voltage in the secondary has a constant value, and
the regulation is effected by varying the phase relation between the
line voltage and the regulator voltage.
~Ques. How is the primary wound?~
Ans. It is wound with as many separate windings as there are phases in
the circuit, and these primary or shunt windings are connected to the
corresponding phases of the feeder.
[Illustration: FIG. 2,424.--General Electric ~direct current~ (form
S) ~voltage regulator. It consists of~ _a main control magnet, relay,
condenser and reversing switch_, as shown in the diagram fig. 2,428.
This regulator cannot be used for compensation of line drop as the
current coil is omitted; it is not a switchboard instrument, but is
designed for inexpensive installations such as for regulating the
voltage of motor generator sets when the current is taken from a
trolley line or some other fluctuating source. ~The regulating outfit
comprises,~ besides the regulator, one or more condenser sections
according to field discharge, set of iron brackets when regulator
cannot be mounted on front of switchboard, one compensating shunt,
when it is desired to compensate for line drop. Field rheostats having
sufficient resistance to reduce the voltage the proper amount must
be used with voltage regulator installations. To prevent undue decay
at the relay contacts, allow one section for each 15 kw. capacity of
dynamo with laminated poles, and one for each 22 kw. capacity with
solid steel poles.]
~Ques. What kind of magnetizing flux is produced by the primary
windings?~
Ans. A practically constant flux which varies in direction.
~Ques. How is the secondary wound?~
Ans. There is a separate winding for each phase.
~Ques. Why is the voltage induced in the secondary constant?~
Ans. Because of the constant magnetizing flux.
~Ques. How is the line voltage varied by a polyphase regulator?~
Ans. When the regulator is in the position of maximum boost, the line
AB, fig. 2,425 represents the normal busbar voltage, BC the regulator
voltage, and AC the resultant feeder voltage. When the regulator
voltage is displaced 180 degrees from this position, the regulator is
in the position to deliver minimum voltage to the feeder, the regulator
voltage being then represented by BD, and the resultant feeder voltage
by AD. When the regulator voltage is displaced angularly in the
direction BF, so that the resultant feeder voltage AF becomes equal to
the normal busbar voltage AB, the regulator is in the neutral position.
Intermediate resultant voltages for compensating the voltage variations
in the feeders may be obtained by rotating the moving element or
primary in either direction from the neutral position. For example, by
rotating the primary through the angle FBE, the resultant voltage may
be made equal to AE or AJ, thereby increasing the feeder voltage by
an amount BJ; or by rotating it in the opposite direction through the
angle FBG, the feeder voltage may be reduced by an amount BH.
[Illustration: FIG. 2,425.--Diagram illustrating operation of polyphase
induction regulator.]
~Ques. How are induction regulators operated?~
Ans. By hand or automatically.
~Ques. How is automatic operation secured?~
Ans. By means of a small motor, controlled by voltage regulating relays.
~Ques. How is the control apparatus arranged?~
Ans. Two relays are employed with each regulator, a primary relay
connected to the feeder circuit and operating under changes of voltage
therein, and a secondary relay connected between the primary relay and
the motor, and operated by the contacts of the former, for starting,
stopping and reversing the motor in accordance with changes in the
feeder voltage, thereby causing the regulator to maintain that voltage
at its predetermined normal value.
[Illustration: FIG. 2,426.--Westinghouse voltage regulating ~primary~
relay; view of mechanism with case removed. This relay is practically a
voltmeter arranged for making and breaking contacts with fluctuations
of voltage. As shown in the figure, it consists essentially of a
solenoid and a balance beam carrying two movable contact points on one
end and attached to the solenoid core at the other. The oscillation of
the core causes the contact carrying end of the beam to move between
two stationary contact points connected to the auxiliary or secondary
relay circuit. The stationary contact points are fitted with adjusting
screws for either increasing or decreasing the distance between them,
to the amount of change in the voltage required for making or breaking
contact; in other words, for varying the sensitiveness of the relay.
Means for varying the normal voltage which it is desired to maintain
are provided in the spring attached to the balance beam and controlled
by the micrometer adjusting screw. Increasing the tension of the spring
results in lowering the normal voltage position. The relay is wound
for a normal voltage of 110 volts, and has a range of adjustment from
90 to 130 volts. The total energy required for its operation is about
50 watts at normal voltage. Voltage transformers having at least 50
watts capacity are, therefore, required. The parts are: A, solenoid;
B, solenoid core; C, end of balance beam; D, pivots, bearings; E,
movable contact arm; F, upper stationary contact point; G, lower
stationary contact point; H, adjusting screw; K, adjusting spring; L,
feeder binding posts; M, auxiliary circuit and secondary relay binding
posts.]
[Illustration: FIG. 2,427.--Westinghouse voltage regulating ~secondary~
relay; view showing relay removed from oil tank. The secondary relay
is practically a motor starting switch of the double pole double throw
type, electrically operated through the contacts of the primary relay.
It is provided with contact points of one-half inch rod. The relay is
suitably connected for starting, stopping and reversing the motor and
for properly operating the motor brake. The parts are: A, solenoid; B,
laminated field; C, movable contact arm; D, stationary contact arms; E,
removable brass contact points; F, terminal block; G, terminals.]
~Ques. Why are two relays used?~
Ans. For the reason that a primary relay, of sufficient accuracy and
freedom from errors due to temperature and frequency variations, could
not be made sufficiently powerful to carry the relatively large current
required for operating the motor.
~Ques. What names are given to the relays?~
Ans. Primary and secondary.
~Ques. What difficulties were encountered in the operation of relays? ~
[Illustration: FIG. 2,428.--Diagram of connections of General Electric
~direct current~ (form S) ~voltage regulator,~ for 125, 250, and 550
volts. ~The range of voltage~ is given in the following table:
-----------------------------------
|Regulator|Range of voltage |
| |------------------------
| |16 |17 |18 |19 |20 |
-----------------------------------
|125 |105 |110 |115 |120 |125|
|250 |210 |220 |230 |240 |250|
|550 |550 | | | | |
-----------------------------------]
Ans. Vibration or chattering at the contacts of both relays and
tendency of the movable contact arm of the primary relay to hug closer
to one of the stationary contact points than to the other, thereby
operating too often.
~Ques. What causes vibration or chattering at the contacts?~
Ans. This is due to the voltage frequently approximating the value
required for closing a contact, thereby causing the contact points to
barely touch and make several poor contacts in succession.
~Ques. What objectionable action is produced by vibration at the
contacts?~
Ans. Arcing, burning and pitting of the contacts, even when alloys of
the rarer metals are used, such as those of the platinum group, having
extreme hardness and high melting points.
[Illustration: FIG. 2,429.--Diagram of connections of automatic
induction regulator and auxiliary apparatus on single phase circuit.]
~Ques. What effect is produced by poor contact of the primary relay?~
Ans. It causes chattering in the secondary relay; which burns out and
wears away its contact points, increasing the heating of the motor,
creating objectionable noise and entailing wear and tear on the whole
outfit.
~Ques. Why does the movable contact arm of the primary relay tend to
remain nearer one of the stationary contact points than the other?~
Ans. This is due to the tendency of the relay to open the contact
whenever the voltage equals that at which the contact closes.
[Illustration: FIG. 2,430.--Diagram of connections of automatic
induction regulator and accessory apparatus on three phase feeder
circuit.]
~Ques. What provision is made in the primary relay to prevent vibration
or chattering?~
Ans. Two auxiliary windings are provided: one in series with each of
the stationary contact points and so arranged as to assist in making
the contact by increasing the pressure on the contact points at the
instant of closure.
The best effect of the compounding action of the auxiliary
coils is obtainable when arranged for ¾ per cent. of the torque
of the main coil.
[Illustration: FIG. 2,431.--Westinghouse drum type variable transformer
voltage regulator. ~It consists of~ a drum and finger type switch. A
preventive resistance is used between the different contacts, making
it unnecessary to open the circuit when moving from one tap of the
regulating transformer to the next tap. A spring actuated, quick
moving, central stopping mechanism is used to prevent burning the
resistances. The regulator is arranged to give 40 points of regulation.
In many cases this large number of points is not absolutely necessary,
but it is desirable to use them because the voltage per step is thus
reduced to a small value, and a corresponding increase in the life
of the contacts results because of the reduced sparking at the lower
voltage. ~Two drums~ are employed. The ~first drum~ has ten contacts
and a corresponding number of fingers, the latter being mounted upon
an insulated bar. These fingers are connected to the floating coils
of the regulating transformer, and as the drum is rotated, the finger
connected to the line is brought into contact successively with each of
the ten taps. The ~second drum~ is of similar construction and consists
of a changing and reversing switch. It connects the two floating coils
to the various taps on the main secondary coil of the regulating
transformer at the proper time, and also reverses the transformer so
that the total winding can be used for either raising or lowering the
voltage. All the points of regulation are obtained by a continuous
motion of the handle, the various connections produced in the manner
are shown in the diagram, fig. 2,433. The top and base of the regulator
are made of cast iron and the top is supported by steel bars, two of
which are insulated, and used to support the metallic bases finger
to which the cable leads are attached. The drums consist of metal
castings mounted upon insulated shafts. The first drum, which is the
only one upon which arcing can take place, is provided with removable
copper contact tips. The main castings are made of aluminum to secure
low inertia of the drum. A sheet iron cover is used to enclose the
regulator, and the leads are brought out through the bottom of the
controller.]
A non-inductive resistance placed in parallel with each coil
of the secondary relay, takes current approximately in phase
with the current in the main coil of the primary relay, and
of proper strength to make the number of ampere turns in the
auxiliary coil three-fourths per cent. of the number in the
main coil. The resistances have the additional effect of
absorbing the "discharge" from the main coils of the secondary
relay when the contacts are broken, thereby obviating sparking
at the primary contact points.
[Illustration: FIG. 2,432.--Diagram showing connections of the
Stillwell regulator.]
[Illustration: FIG. 2,433.--Diagram showing ~position of the floating
coil~ on different steps of Westinghouse ~drum type variable ratio
transformer regulator.~ _The upper half of the diagram_ shows the
connections of the various coils for each position of the regulator
handle. This arrangement applies to a regulator used in connection
with an independent regulating transformer. When regulators are
used in connection with large power transformers, the regulating
transformer can be omitted and auxiliary coils can be placed on the
main transformer to provide the necessary taps for regulating purposes.
_The lower half of the diagram_ shows the connections used when
auxiliary coils are added to a large transformer. ~The diagram shows~
connections for a ~single phase regulator~. Where polyphase regulators
are required, the connections consist essentially of two sets of single
phase connection, and the controller is extended in length so as to
contain double sets of drum and contact.]
~Variable Ratio Transformer Voltage Regulators.~--The principle of
operation of this class of regulator is virtually the same as that
of the induction type regulator; that is to say, both consist of
regulating transformers, but ~in the variable ratio method~ _the
primary or series coil is divided into a number of sections which may
be successively cut in or out of the circuit to be regulated_, instead
of varying the flux through the entire coil, as in the induction type.
There are two distinct mechanical forms of variable ratio regulator:
1. Drum type;
2. Dial type.
~Drum Type Regulators.~--This form of variable ratio transformer
consists essentially of a drum and finger type switch, similar to a
railway controller.
There are many contacts, giving a large number of points of regulation,
obtained by the use of changing switches and floating coils.
The floating coil is a part of the secondary winding of the regulating
transformer which is insulated from the main portion of the winding,
and is sub-divided by taps into a number of equal sections.
The sub-divisions of the main secondary winding are much larger, each
one being equivalent to the whole of the floating coil.
[Illustration: FIG. 2,434.--Diagram of connections of General Electric
~high voltage cut out relay~ (form A) for voltage regulators. ~Its use~
in connection with the regulator protects the system from any sudden
rise in voltage due to some accident to the regulator which might
cause the relay contacts to stick, thus producing _full field_ on the
exciter. ~In construction,~ _the control magnet is connected in series
with the alternating current control magnet on the regulator and the
contacts are connected in series with the rheostat shunt circuit._
Then, ~should the voltage rise beyond a certain value,~ predetermined
by the setting of the thumb screw supporting the plunger of the control
magnet, _the contacts of the relay are tripped open which, by inserting
all the resistance in the exciter field, reduces the exciter voltage
which in turn reduces the alternating current voltage_. This relay has
to be reset by hand.]
~Ques. Describe the operation of a drum regulator.~
Ans. The floating coil and main windings are first connected in series
with each other and with the line to be regulated. The floating coil
is then cut out of the circuit step by step. When entirely cut out
it is transferred to the next lower tap on the main winding, after
which it is again cut out step by step and then transferred again. By
continuing this process a large number of steps are provided with but
comparatively few actual taps on the transformer.
~Ques. How many floating coils are used and why?~
Ans. Two floating coils are included in each regulator so that one can
be transferred while the other is supplying the current to the line.
~Dial Type Regulators.~--This form of variable ratio transformer
regulator consists of a regulating transformer and a dial type switch
as shown in the accompanying illustrations. The regulating transformer
is similar to a standard transformer except that the secondary winding
is provided with a number of taps leading to the contact of the dial
switch as shown in the diagram fig. 2,437.
[Illustration: FIG. 2,435.--Dial of Westinghouse dial type variable
ratio voltage regulator. The dial consists of a marble slab, upon which
the contacts are mounted in a circle as shown. The contact arm is
arranged to move from contact to contact. The alternate small contacts
are dummies, serving to prevent the contact arm springing down between
contacts when moving from one to another. The panel contains a changing
switch which makes it possible to double the range of a regulator,
since the transformer connections can be changed to both raise and
lower to an extent equal to the full range of the transformer. The
total range in voltage from a certain per cent. below to a certain per
cent. above the line voltage can be obtained in a number of steps equal
to twice the number of divisions into which the secondary winding of
the transformer is divided.]
~Ques. What modification is made to adapt dial regulators for heavy
current?~
Ans. A dial with a series transformer, and a shunt or auto-transformer
are employed as shown in fig. 2,436.
~Ques. Why is such modification desirable?~
Ans. Because, the additional cost of a series transformer is small
in comparison with the cost of building a dial with a large current
carrying capacity, and the cost of bringing out a number of heavy leads
from a small transformer.
[Illustration: FIG. 2,436.--Diagram of connections for Westinghouse
~11 point dial, series transformer and auto-transformer~. The
auto-transformer has a number of taps connected across the line, the
series transformer is placed in series with one side of the line, and
connected to a dial, as shown.]
~Ques. How are dial regulators modified for high voltage?~
Ans. Standard dials may be used with series and shunt transformers
similar to the method used for heavy current circuits.
~Ques. Describe the connections.~
Ans. The primary of the shunt transformer is connected across the line
and the secondary has a number of taps which are connected to contacts
on the dial. The primary of the series transformer is connected in
series with the line and two leads from the secondary winding are
connected to the dial.
The connections are similar to those shown in fig. 2,437, except that
shunt transformers are used instead of auto-transformers.
[Illustration: FIG. 2,437.--Diagram of connections for Westinghouse
~dial type variable ratio voltage transformer. In construction~ the
secondary winding of the transformer is divided into 10, 14, or
20 parts giving 11, 15, or 21 taps which are brought out from the
secondary winding and connected to the various points of the dial.
The diagram shows connections for an 11 point dial and regulating
transformer. ~Since there is a difference of voltage~ between adjacent
contacts, _the contact arm must not touch the contact toward which
it is moving until after it has left the contact upon which it was
resting_. Moreover, it is undesirable to open the circuit each time in
moving from one contact to the next. These conflicting requirements
are met by the use of arcing tips which are placed on the contact arm
so that a very close adjustment can be obtained, and so arranged that
the contacts are not short circuited but always have a gap of from
one-sixteenth to one-eighth inch in the circuit during the time of
changing from one contact to the next. The air gaps form a "preventive
resistance." ~A quick moving mechanism~ is used to accelerate the
movement from one contact to the next, a very quick movement being
necessary to avoid undue arcing. ~The capacity of the regulator~ is
200 amperes at 2,200 volts, being arranged to give a maximum increase
in voltage of 400 volts. The maximum pressure between contacts is 25
volts.]
[Illustration: FIG. 2,438.--Diagram of connections of General Electric
pole type regulator. The operation of the regulator is obtained by
means of a small single phase motor which is in continuous operation,
and which by mechanical means may be connected to the regulator shaft.
The control of the mechanism is obtained by means of a voltage relay.
The operating motor, which is of the drawn shell type, is provided
with a starting clutch and will consequently start up with full load.
Under actual operating conditions it will, of course, be comparatively
seldom that the motor will be called upon to start up. ~A non-inductive
resistance~, made up from standard units, is connected in series with
the relay winding and several taps are provided, so that the relay
can be adjusted for any voltage from 10 per cent. below normal. In
order to readily dissipate the heat developed in the resistance, it
has been mounted in a pocket on the back of the tank, openings being
provided for natural air ventilation. ~The relay plunger~ is hinged
to one end of a balance arm, which arm is provided with two trip pins
to control the mechanism. ~An adjustable helical spring~ is attached
to the other end of the arm to assist the magnetic pull of the coil
in balancing the plunger and also for adjustment. The relay is not
provided with series winding for line drop compensation, but it may
be used with a standard line drop compensator, which then has to be
installed outside of the regulator. The voltage relay must be connected
to the feeder side of the regulator, the necessary low voltage to be
obtained from a distributing transformer, or if this should not be
available in the immediate vicinity, a 200 watt step down transformer
will be satisfactory. ~The motor is designed to operate~ in parallel
with the relay, the normal connections being as shown. The speed of
the motor and the ratio of the gearing is such that it requires about
90 seconds to operate the regulator from limit to limit, but, as this
regulator is not intended to take care of sudden voltage fluctuations,
the comparatively long time of operation will not be objectionable.]
[Illustration: FIGS. 2,439 to 2,443.--General Electric ~pole type
regulator~ removed from tank. ~It consists essentially~ _of a primary
and secondary coil, operating motor, and voltage relay mechanism_. The
regulator and mechanism is suspended in a cast iron tank, the lower
part, containing the regulator core and coils, being filled with oil.
The leads for the regulator are brought out at the upper part of the
tank. The outgoing leads are compressed into bushings and connected
to the leads of the regulator by means of terminals, the arrangements
being such that the regulator with mechanism can be removed from the
tank without difficulty. Besides the cover, the tank is also provided
with a hinged door on the front side so as to give access to the
mechanism. The door is provided with a gasket and the construction is
practically rain and dust proof. However as there is always danger of
the door not being clamped down perfectly, thus making it possible for
water to enter the tank, a pocket has been provided inside the tank
and underneath the door to collect the water. Capacity up to 2.3 kw.,
to control 2,300 volts, 60 cycle, 10 ampere feeders, and for a voltage
range of 10 per cent. above or below normal, the operating motor and
relay being designed for 110 or 220 volts. No provision is made for
line drop compensation, although this can be obtained by installing
a current transformer and a line drop compensator externally to the
regulator.]
It will be seen that the circuit comprising the dial, the secondary
of the shunt, transformer and the secondary of the series transformer
form a circuit which is not electrically connected to the main circuit.
It can therefore be grounded without disturbing the main circuit as
a safeguard to render it impossible for the pressure of the dial to
be higher above the ground than the secondary voltage of the shunt
transformer.
[Illustration: FIGS. 2,444 to 2,446.--Sectional views of General
Electric pole type regulator winding and core. The secondary core has
only two slots containing a single coil, while the rotor or primary
core has four slots. Two of these slots are occupied by a single
primary coil, and the two circular slots in quadrature thereto contain
the compensating or short circuit winding. This winding also serves
to hold the primary punchings together, and it consists of two copper
rods riveted to the two cast brass flanges. The secondary coil is
form wound, while the primary coil is wound directly on the core. The
rotor flanges, both top and bottom, are provided with discs which are
turned in alignment with the punchings, and these discs bear against
the top and bottom flanges between which the secondary punchings are
clamped. These secondary flanges are also turned in alignment with the
secondary punchings, so that an even air gap between the primary and
the secondary is assured. The secondary coil is wound with an opening
in the upper horizontal part which affords passage for the operating
shaft of the rotor. A bearing for this shaft is provided in the table
which supports the mechanism and from which the regulator is suspended.
Flexible leads are brought out from the rotor and twisted around the
shaft as in standard regulator practice. The regulator being two pole,
the rotor is turned through an angle of 180 deg. to obtain the full
range of the regulator.]
~Small Feeder Voltage Regulators.~--In some generating stations the
voltage is maintained constant at the busbars and the line drop
compensated by automatically operated regulators connected in the main
feeders. It is possible in this way to obtain constant voltage at all
loads at the various distribution centers, that is, at those points on
the feeders where the lines of the majority of consumers are connected
as shown in fig. 2,447.
[Illustration: FIGS. 2,447 and 2,448.--Systems of distribution
illustrating use of ~small feeder~ or ~pole type voltage regulators~.]
It is evident, however, that, while the voltage at the center of
distribution can be maintained constant, no account can be taken of the
drop in the lines between this center and the consumers. This drop is
generally negligible, except in some particularly long lines, as, for
example, consumer _B_ in fig. 2,447.
In order to obtain perfect regulation at B, it would be necessary
to install a separate regulator in that line, this regulator to be
installed either at the center C or preferably at B.
In a great many cases the power distribution is not as ideal as
indicated in fig. 2,447, but rather as shown in fig. 2,448, that is,
the consumers are connected all along the feeder. In this case there
is no definite center of distribution, and the automatic regulator
installed in the station can be adjusted to give only approximately
constant voltage at an imaginary center of distribution C; that is, the
voltage cannot be held constant at any definite point during changes of
load distribution.
[Illustration: FIG. 2,449.--General Electric pole type regulator in
service; its construction is shown in fig. 2,450.]
The majority of the consumers may, however, obtain sufficiently good
voltage while a few may have reason for criticism. To overcome this
difficulty it is necessary either to increase the copper in the feeder
or else to install small automatic regulators.
There are also cases where a small amount of power is transmitted a
long distance through a feeder direct from the station.
The amount of copper required to reduce the line drop is usually too
great to be considered and the cost of the ordinary automatic regulator
is also comparatively high. In such cases small pole type regulators as
shown in fig. 2,449 are desirable.
~Ques. Describe the operation of the regulator mechanism shown in fig.
2,450.~
Ans. Assuming the voltage to be normal, the balance arm of the relay
will be held horizontal, the trips F will not engage with the triggers
E, and no movement is therefore transmitted to the ratchet wheel C. If
the voltage drops below normal, the left hand trip will descend until
it finally gets in the way of the left hand trigger just before it
reaches the limit of its counterclockwise travel. This trigger will
therefore release the left pawl D, which will engage with the ratchet
wheel and will consequently turn it clockwise until the rocker arm
reaches its right hand limit. Before the rocker arm reaches the left
hand limit, the released pawl must be locked by its trigger, so that
if the voltage has reached its normal value, further movement of the
ratchet wheel will not take place, whereas if the voltage be still too
low, the trigger will again release the pawl by striking the trip of
the relay.
[Illustration: FIG. 2,450.--Mechanism of General Electric pole type
regulator. The operating motor (described in fig. 2,438) is direct
connected to a worm and gear, the shaft of which is provided with a
bell crank. A rod A, connects the crank with the rocker arm B, which
thus may be caused to oscillate over a ratchet wheel C. The rocker arm
is provided with two pawls D, which can engage with the teeth of the
ratchet wheel, so that this wheel can be rotated one way or the other.
The ratchet wheel is mounted on the same shaft as a worm, which engages
with the gear segment carried by the regulator shaft, so that the
movement of the ratchet wheel is directly transmitted to the regulator.
Besides the two large pawls D, the rocker arm also carries two smaller
ones E, called the triggers. These triggers usually hold the pawls
locked in such positions as not to engage with the ratchet wheel, but
the pawls will be released when the triggers strike the trips F of the
relay arm. A limiting device for the movement of the ratchet wheel and
the regulator segment is provided, as shown. This device consists of
two cams K, mounted on a common arm, which can turn on the shaft of the
ratchet wheel. Normally these cams are not within reach of the pawls,
but through a lever arrangement, controlled by the regulator segment,
the arm holding the cams may be rotated so that, if the trigger has
been raised, so as to release the pawl, the tip of the pawl will bear
on the cam of the limiting device, and before the pawl can engage with
the ratchet wheel it has already been locked by its trigger. A further
movement of the ratchet wheel in that particular direction is therefore
impossible, while it is free to be moved the other way. A positive
stop for the gear segment is also provided. The motor is provided with
oil ring bearings, and the gear for the motor worm runs in oil, the
supporting casting forming a well therefor.]
[Illustration: FIG. 2,451.--Diagram of connections of General Electric
~direct current voltage regulator~ (form T) with two dynamos and
one exciter. In cases where several shunt or compound wound direct
current machines are operating in parallel, either on two wire or
three wire systems, a good arrangement for voltage regulation and line
drop compensation is obtained by using this regulator and a separate
exciter. The compensating shunt as well as pressure wires can be used
to maintain a constant pressure at the center of distribution.]
~Ques. How is this automatic locking of the pawl obtained?~
Ans. By having a lip G on the under side of the pawl strike a finger H
fastened to the bearings in front of the ratchet wheel.
The pawl is thus raised just before it reaches the limit of its
clockwise travel sufficient to be locked by its trigger.
[Illustration: FIG. 2,452.--~Condenser sections~ and method of
assembling same with tripod. ~The tripod bolts~ are made of extra
length to accommodate the addition of extra condenser sections if
necessary. The illustration shows three sections in position.]
~Ques. How does the mechanism operate when the voltage rises above
normal?~
Ans. As described above, with the exception that the right hand trip
causes a rotation of the regulator in the opposite direction.
~Ques. How is adjustment made for various voltages?~
Ans. Taps are provided on the resistance in series with the relay, and
finer adjustment can be obtained by means of the helical spring on the
right hand end of the balance arm.
In order to adjust the sensitiveness of regulation, the bearing
for the balance arm can be raised or lowered by means of a stud
J, fig. 2,450, connecting this bearing with the bearing of the
operating shaft, and the regulator can be made to maintain the
voltage within 1 per cent. above or below normal.
[Illustration: FIG. 2,453.--Westinghouse unit switch type pressure
regulator, designed for handling heavy currents where a variable ratio
transformer type of regulator is desired. The regulator consists of
a number of electrically operated switches controlled from a master
switch. These switches are arranged to perform practically the
same cycle of operation as previously described for the drum type
regulators. The transformer windings are divided into sections, and
two floating coils are provided which are connected to various taps
on the main auto-transformer. These floating coils have intermediate
steps, and the successive operation of the switches connects the
floating coils in proper sequence to the main auto-transformer, and
transfers the line connection from one point of the floating coil to
the next. In this way a 23 point regulator with sixteen switches, and
a 71 point regulator with 21 switches may be supplied. The master
switches are arranged with an automatic lock to prevent their being
operated too rapidly. The magnet switches themselves are so interlocked
that the proper sequence of operation is insured. The electrically
operated switches may be of the open type, mounted on a slate or marble
switchboard, when the whole control outfit is placed in a room which
is comparatively free from dust or dirt of any kind, and where there
is no danger of employees coming in contact with the switches. The
other type of switch is entirely enclosed, the main contacts being
oil immersed. The frames of these switches are grounded and the whole
design is arranged to operate under ordinary dirty conditions. All
of these switches, however, should receive the necessary inspection
and attention. The contacts have a long life and are easily renewed.
Regulators of this type are adapted for metallurgical purposes where
the regulation is effected in the primary circuit and the secondary
circuit is of very low voltage but large current capacity and is used
for supplying power to the furnaces. These regulators have been built
in capacities up to 800 amperes at 3,300 volts.]
~Ques. What provision is made for convenient inspection?~
Ans. A snap switch is provided by means of which the power to the motor
and relay can be disconnected.
[Illustration: FIGS. 2,454 and 2,455.--Front and rear views of General
Electric automatic voltage regulator. The regulator has _a_ ~direct
current~ _control magnet_, _an_ ~alternating current~ _control magnet,
and a relay_. The ~direct current control magnet~ is connected to the
exciter bus bars. This magnet has a fixed stop core in the bottom and
a movable core in the top which is attached to a pivoted lever having
at the opposite end a flexible contact pulled downward by four spiral
springs. For clearness, however, only one spring is shown in the
figure. Opposite the direct current control magnet is ~the alternating
current control magnet~ which has a pressure winding connected by means
of a pressure transformer to the alternator or bus bars. ~There is an
adjustable compensating winding~ on the alternating current magnet
connected through a current transformer to the principal lighting
feeder. The object of this winding is to raise the voltage of the
alternating current bus bars as the load increases. The alternating
current control magnet has a movable core and a lever and contacts
similar to those of the direct current control magnet, and the two
combined produce what is known as the "floating main contacts." ~The
relay~ _consists of a_ ~U~ _shaped magnet core having a differential
winding and a pivoted armature controlling the contacts which open and
close the shunt circuit across the exciter field rheostat_. ~One of the
differential windings~ of the relay is permanently connected across the
exciter bus bars and tends to keep the contacts open; the other winding
is connected to the exciter bus bars through the floating main contacts
and when the latter are closed, neutralizes the effect of the first
winding and allows the relay contacts to short circuit the exciter
field rheostat. ~Condensers~ are connected across the relay contacts to
prevent severe arcing and possible injury.]
~Automatic Voltage Regulators for Alternators.~--The accurate
regulation of voltage on any alternating current system is of
importance. The desired voltage may be maintained constant at the
alternator terminals by rapidly opening and closing a shunt circuit
across the exciter field rheostat.
[Illustration: FIG. 2,456.--Diagram of connections of General Electric
~_contact making_ ammeter~ for operating on alternating current
circuits. _The instrument is designed to indicate_ with the aid of
a current transformer, _certain values of current in an alternating
current system_. This value depends upon the setting of the regulating
rheostat in parallel with the pressure coil of the ammeter. It is also
possible with this instrument, together with the necessary control
apparatus, to _hold_ certain values of current. ~By using a different
magnet coil~ this meter may be connected to a shunt instead of a
current transformer and used on a direct current system.]
~Ques. Describe in more detail this method of regulation.~
Ans. The rheostat is first turned in until the exciter voltage is
greatly reduced and the regulator circuit is then closed. This short
circuits the rheostat through contacts in the regulator and the voltage
of the exciter and alternator immediately rise. At a predetermined
point, the regulator contacts are automatically opened and the field
current of the exciter must again pass through the rheostat. The
resulting reduction in voltage is arrested at once by the closing of
the regulator contacts which continue to vibrate in this manner and
keep the generator voltage within the desired limits. The connections
are shown in fig. 2,457.
[Illustration: FIG. 2,457.--Diagram of General Electric automatic
voltage regulator connections with alternator and exciter. ~In
operation~, the circuit shunting the exciter field rheostat through the
relay contacts is opened by means of a single pole switch at the bottom
of the regulator panel and the rheostat turned in until the alternating
current voltage is reduced 65 per cent. below normal. This weakens both
of the control magnets and the floating main contacts are closed. This
closes the relay circuit and demagnetizes the relay magnet, releasing
the relay armature, and the spring closes the relay contacts. The
single pole switch is then closed and as the exciter field rheostat is
short circuited, the exciter voltage will at once rise and bring up
the voltage of the alternator. This will strengthen the alternating
current and direct current control magnets, and at the voltage for
which the counterweight has been previously adjusted, the main contacts
will open. The relay magnet will then attract its armature and by
opening the shunt circuit at the relay contacts will throw the full
resistance into the exciter field circuit tending to lower the exciter
and alternator voltage. The main contacts will then be again closed,
the exciter field rheostat short circuited through the relay contacts
and the cycle repeated. This operation is continued at a high rate of
vibration due to the sensitiveness of the control magnets and maintains
a steady exciter voltage.]
~Line Drop Compensators.~--In order that the actual voltage at a
distant point on a distribution system may be read at the station some
provision must be made to _compensate_ for the line drop, that is
to say, for the difference in voltage between the alternator and the
center of distribution.
In order to do this a device which is known as a "line drop
compensator" is placed in the voltmeter circuit as shown in the
diagram, fig. 2,458.
~Ques. What are the essential parts of a line drop compensator?~
Ans. The elements of a line drop compensator are a variable resistance,
and a variable inductance.
~Ques. Describe the connections.~
Ans. The secondary of a pressure transformer is connected in series
with the compensator inductance and resistance, and the secondary of a
current transformer as shown in the diagram, fig. 2,458.
[Illustration: FIG. 2,458--Diagram showing ~essential parts and
connections for a line drop compensator~. _The compensator corrects
the voltmeter indication at the supply end of a feeder for the ohmic
and inductive drop in pressure between that point and the point of
consumption_, so that the reading of the station voltmeter corresponds
with the actual voltage at the point of consumption, independent of
the power factor and current. ~It is especially useful~ for adjusting
pressure regulators.]
~Ques. How are the inductance and resistance wound?~
Ans. They are wound so that any proportion of the winding of either can
be put in or out of the voltmeter circuit.
[Illustration: FIG. 2,459.--General Electric line drop compensator. It
has two dial switches with many taps to the resistance and reactance in
the box so that it can be adjusted to compensate accurately for line
losses with loads of varying power factor. Dial R changes resistance,
and dial X, reactance.]
[Illustration: FIG. 2,460.--General Electric line drop compensator.
This compensator contains besides resistance and inductance, a current
transformer, the secondary of the transformer being connected in series
with the resistance and inductance; the primary of the contained
current transformer is connected to an external current transformer.
The reactance and resistance are both so wound that any proportion
of the winding can be cut in or out of the voltmeter circuit. Both
elements have 12 points of adjustment of one volt each, giving a total
combined drop at maximum setting of about 17 volts.]
~Ques. How can the voltmeter indicate the pressure at the center of
distribution?~
Ans. If the amount of inductance and resistance be properly adjusted,
there will be produced a local circuit corresponding exactly in all
its characteristics to the main circuit. Hence, any change in the main
circuit produces a corresponding change in the local circuit, and
causes the voltmeter to always indicate the pressure at the end of the
line or center of distribution or at any point for which the adjustment
is made.
[Illustration: FIG. 2,461.--Westinghouse line drop compensator. For
single phase circuits, one compensator and one series transformer,
that is the instrument as listed with transformers, will give correct
indications for a single phase circuit. The same voltage transformer
serves for both voltmeter and compensator. For balanced two phase
circuits one compensator and one transformer connected in one of the
phases is sufficient. Two single phase compensators should be used for
unbalanced two phase circuits. For three phase circuits the compensator
should be connected by means of two series transformers.]
~Ques. How should the adjustment be made?~
Ans. It is advisable to calculate the ohmic drop for full load and
set the resistance arm at the point which will give the required
compensation and then adjust the inductance arm until the voltmeter
reading corresponds to the voltage at the point on the line selected
for normal voltage.
[Illustration: FIG. 2,462.--Diagram of ~automatic voltage regulator~,
using ~line drop compensator~. For ordinary installations the
compensating winding on the alternating current control magnet
is connected to a current transformer in the main feeder. A dial
switch is provided by which the strength of the alternating current
control magnet can be varied and the regulator made to compensate
for any desired line drop up to 15 per cent. according to the line
requirements. Where the power factor of the load has a wide range of
variation, a special line drop compensator, such as shown in fig.
2,459, adapted to the regulator would be desirable. The connections are
readily understood by the diagram. The number of condenser sections
which will prevent undue arcing at the relay contacts depends on
the characteristics of the exciter. They may be roughly estimated
by allowing one section for each 15 kw. capacity for exciters with
laminated poles, and one for each 22 kw. capacity for exciters with
solid steel poles. It is necessary though to have one condenser section
for each pair of relay contacts, and at times it becomes necessary to
apply a double section for each pair of contacts. In the lower part of
the figure the line drop compensation and connections is reproduced in
more detail on a larger scale.[2]]
[2] NOTE.--It is desirable, in any system of distribution, to read the
active voltage at the point of distribution, by means of the voltmeters
in the station. A compensator proper consists of a variable resistance
and a variable inductance, and sometimes a current transformer. In
wiring, the voltmeter, instead of being connected directly across the
secondaries of a pressure transformer, has inserted in series with it,
portions of the resistance and inductance of the compensator. These are
so connected that the drop in pressure across them will be combined
with that of the pressure transformer, so that the voltmeter reading
indicates the pressure at the center of distribution or end of the
line.
~Starting Compensators.~--These are used for starting induction motors
and consist of inductive windings (one for each phase) with a number of
taps connecting with switch contacts as shown in fig. 2,463. A starting
compensator is similar to a rheostat except that inductive windings are
used in place of the resistance grids.
[Illustration: FIG. 2,463.--Diagram of connections of General Electric
two phase starting compensator with no voltage release and fuses.]
~Ques. Describe the inductive windings.~
Ans. The compensator winding consists of an inductive coil in each
phase with each coil placed on a separate leg of a laminated iron core.
Each coil is provided with several taps so located that a number of
sub-voltages may be obtained.
~Ques. Are starting compensators necessary for small motors? Why?~
Ans. No, because the full voltage starting current taken, although
equal to several times the load current, is nevertheless so small,
compared with the capacity of the station alternators or feeders, that
it does not materially affect the regulation of the circuit.
[Illustration: FIG. 2,464.--Diagram of connections of General Electric
three phase starting compensator with low voltage release and fuses.]
[Illustration: FIG. 2,465.--Diagram of connections of General Electric
two phase starting compensator with no voltage release and overload
relays for 1,040 to 2,500 volt circuits.]
Motors larger than about 7 horse power cause an objectionably
heavy rush of current if thrown directly on the line. Starting
compensators obviate such sudden variations of line load and
are accordingly recommended for motors above 7 horse power
except in cases where voltage variations and excessive starting
currents are not objectionable.
[Illustration: FIGS. 2,466 and 2,467.--General Electric three ~phase
hand operated starting compensator~. Fig. 2,466, compensator in case;
fig. 2,467, compensator with case removed. ~The compensator consists
of~ _a core and windings, a cable clamp, and a switch, assembled in
a substantial metal case with external operating handle and release
lever_. ~The windings~ consist of coils wound on separate legs of
a laminated core, and tapped at several points, the connections
terminating at the switch contacts. The shaft of the switch extends
through the sides of the compensator case, and is operated by a lever
at the right, being held in the running position by a lever at the
left. It is provided with wiping contacts. The switch is immersed
in oil, and is intended to be used as a line switch as well as for
starting the motor. ~The lever has three positions:~ "off," "starting,"
and "running." ~In the _off_ position~, _both compensator and motor
windings are disconnected from the line_. ~In the starting position~,
_the switch connects the line to the ends and the motor to the taps
of the compensator winding_ without overload relays or fuses in
circuit. ~In the running position~, _the compensation winding is cut
out and the motor is connected to the line through suitable fuses or
overload relays_ mounted directly above the compensator. To prevent the
attendant throwing the motor directly on the line, and thereby causing
a rush of current which it is the object of the compensator to avoid,
an automatic latch is provided and so arranged that the lever at _off_
position can be thrown only into the _starting position_ (backward);
and can be thrown thence into the _running position_ (forward) only by
a quick throw of the lever, whereby any appreciable drop in speed and
consequent increase in current in passing from the starting into the
running position is avoided.]
~Ques. What should be noted with respect to the compensator winding
taps?~
Ans. The choice of a tap giving so low a voltage as to require over one
minute for starting should be avoided so as to prevent the overheating
to which starting compensators, in common with other motor starting
devices, are liable if left in circuit unnecessarily long, or if the
motor be started several times in rapid succession.
[Illustration: FIG. 2,468.--Diagram of connections of General Electric
three phase starting compensator with no voltage release and overload
relays.]
It should also be noted that the starting current diminishes
rapidly as full speed is approached. It is, therefore,
important that the switch be kept in the starting position
until the motor has finished accelerating to prevent any
unnecessary rush of current when the switch is thrown to the
running position.
[Illustration: FIG. 2,469.--General Electric starting compensator with
low voltage release and overload relays. On the switch shaft there are
mounted two levers, held together with a strong spring which operates
in either direction and prevents the switch being left on the starting
position. On the running side it is held by the external low voltage
release lever until released either by hand or by the action of a low
voltage relay. The low voltage release consists of a cast iron frame
open at the bottom and totally enclosing the coil. A laminated plunger
is used to hold the tripping lever, the latter engaging with the lever
mounted on the switch shaft. The compensator cannot be thrown into the
running position without first going to the starting position and it
cannot be left on the starting position.]
~Ques. What is the usual arrangement of starting compensators for large
motors?~
Ans. Starting compensators may be wound for any voltage or current for
which it is practicable to build motors. For very large motors the
switching device is generally separate from, the compensator itself and
consists of triple and four pole switches for three phase and two phase
motors respectively. One double throw switch or two interlocked single
throw switches are required for the motor and a single throw switch for
energizing the compensator, the running side of the motor circuit being
provided with fuses or automatic circuit breakers, or the switches
provided with low voltage and overload release attachments.
~Star Delta Switches.~--These are starting switches, designed for use
with small three phase squirrel cage motors having their windings so
arranged that they may be connected in star for starting and in delta
for running.
~Ques. Describe the operation of a star delta switch.~
Ans. In starting the motor, the drum lever is thrown in the starting
direction which connects the field windings of the motor ~in star~.
When the motor has accelerated and has come partially up to speed the
starting lever is quickly thrown to the running position in which
position the field windings are connected ~in delta~. The effect of
connecting the field winding in star at starting is to reduce the
voltage applied to each phase winding, while in the running position
each phase of the field winding has full line voltage impressed upon it.
[Illustration: FIGS. 2,470 to 2,474.--General Electric ~time limit
overload relay~ _for starting compensator_. In case of overload, the
relay armature is raised and at the end of its travel, opens the small
switch at the top which in turn opens the circuit of the low voltage
release coil causing the compensator switch to return to the "off"
position. ~The oil dash pot~ provides a certain time element and can
be adjusted to operate immediately upon overload or at any interval up
to five minutes. Each relay has five calibrating points, the lowest
being the normal full load current of the motor, the highest 300 per
cent. full load current. The scale on the calibration tube reads direct
and shows various values of current at which the relay may be set to
operate. ~To change overload setting:~ 1, loosen set screw; 2, turn
relay plunger on piston rod until white mark comes opposite required
value of current; 3, tighten set screw. ~Time element adjustment:~
Removing oil dash pot by turning to the left will expose the cup shaped
piston, made of which are two concentric discs (B and C) held together
by a milled lock nut, A. There is a hole in each disc through which
the oil must pass when the plunger of the relay is raised. The time
element may be varied by changing the size of the opening between these
discs, that is, to have the relays operate in a shorter period of time,
increase the size of the opening and vice-versa. ~To change the time
setting:~ 1, remove the oil dash pot; 2, raise the discs B and C on the
piston rod; 3, loosen the lock nut A; 4, change the opening between
B and C, giving a larger opening for shorter time of operation, and
a smaller opening for longer time; 5, tighten lock nut A; 6, replace
discs in piston D; 7, replace oil dash pot.]
[Illustration: FIGS. 2,475, and 2,476.--Front and side views (oil tank
removed) of Cutler-Hammer ~star delta switch~ _for starting small three
phase squirrel cage motors_. ~In construction~, the switch consists
of one set of stationary fingers and a rotating wooden cylinder,
carrying two sets of contacts. These parts are supported from the
switch frame casting and are enclosed in a steel tank which contains
an insulating oil. Flexible oil proof cable leads are brought out
through insulated bushings in the top of the switch and tagged for
convenience in connecting to the lines and motor. To prevent seepage
of oil, the leads are sealed into the top of the cover with an oil
proof sealing wax. The lever of the star delta switch is arranged
with an interlock which prevents its being thrown directly into the
running position from the off position. It is necessary to throw the
lever first into the starting position and then with an uninterrupted
movement to the running position. The circuit of the motor is broken
only for an instant in changing from star to delta and no heavy
inrush current occurs. ~No voltage release~ protection is provided
by a latching solenoid which holds the spring centered drum cylinder
in the running position. The no voltage release coil is mounted in
the lower part of the starting switch, immersed in the oil tank, and
is protected against mechanical injury and grounding. The coil is in
circuit during the running period only and requires not more than 8
to 15 watts to hold the switch in the running position. The operation
of this protective device is such that on failure of voltage the
star delta switch will immediately be returned to the off position.
~Overload release~ protection consists of two relays on a small slate
panel, which is mounted directly on the side of the star delta switch.
The switch contacts of the overload release are connected in series
with the connections to the no voltage release coil so that when an
overload occurs the overload relay operates to open the circuit to the
no voltage release coil, thus permitting the switch lever to return to
the off position. The overload relays do not afford overload protection
during the starting period, and when such protection is desired
starting fuses should be installed. These fuses, if used, should have a
capacity of 250 to 300 per cent. of the normal full load current of the
motor.]
[Illustration: FIG. 2,477.--Diagram of connections of General Electric
three phase starting compensator with low voltage release and overload
relays for 1,040-2,500 volt circuits.]
CHAPTER LXI
SYNCHRONOUS CONDENSERS
~Synchronous Condensers.~--A synchronous motor when sufficiently
excited will produce a leading current, that is, when over excited
it acts like a great condenser, and when thus operated on circuits
containing induction motors and similar apparatus for the purpose of
improving the power factor it is called a _synchronous condenser_.
Although the motor performs the duty of a condenser it possesses almost
none of the properties of a stationary condenser other than producing
a leading current, and is free from many of the inherent defects of a
stationary condenser.
The relation of power factor to the size and efficiency of prime
movers, generators, conductors, etc., and the value of synchronous
condensers for improving the power factor is generally recognized.
Induction motors and other inductive apparatus take a component of
current which lags behind the line pressure, and thereby lowers the
power factor of the system, while a non-inductive load, such as
incandescent lamps, takes only current in phase with the voltage and
operates at unity power factor.
Since transformers require the magnetizing current, they may seriously
affect the power factor when unloaded or partially loaded, but when
operating at full load their effect is practically negligible.
The relative effect of fully loaded and lightly loaded induction motors
on the power factor is indicated by the diagram, fig. 2,478. The
magnetizing current is nearly constant at all loads and is wattless,
lagging 90 degrees behind the impressed pressure, or at right angles to
the current which is utilized for power.
In the figure, AB is the magnetizing component, which is always
wattless, and CB the power component. The angle ACB gives the
phase relation between voltage and current; the cosine of this
angle CB ÷ AC is the power factor.
[Illustration: FIG. 2,478.--Diagram showing relative effect of fully
loaded and lightly loaded induction motors on power factor.]
It is evident from the diagram that if the load be reduced, the
side CB is shortened, and as AB is practically constant, the
angle of lag ACB is increased. It therefore follows that the
cosine of this angle, or the power factor is reduced.
The figure clearly shows the reason for the low power factor of
induction motors on fractional loads and also shows that since
the magnetizing current is practically constant in value, the
induction motor can never operate at unity power factor.
With no load, the side CB (real power) is just sufficient to
supply the friction and windage. If this be represented by DB,
since AB remains constant, the power factor is reduced to 10
or 15 per cent. and the motor takes from the line about 30 per
cent. of full load current. It therefore follows that a group
of lightly loaded induction motors can take from the system a
large current at exceedingly low power factor.
The synchronous motor when used as a condenser, as before stated,
has the property of altering the phase relation between pressure and
current, the direction and extent of the displacement being dependent
on the field excitation of the condenser.
It can be run at unity power factor and minimum current input, or
it can be over excited and thereby deliver leading current which
compensates for the inductive load on other parts of the system. The
synchronous condenser, therefore, can supply magnetizing current to
the load on a system while the power component is supplied by the
generators.
[Illustration: FIG. 2,479.--General Electric 400 kw., 550 volt, 600
R.P.M., ~synchronous condenser~ with direct connected exciter installed
in sub-station No. 1 of the Colorado Light & Power Co., Cripple Creek,
Colo. The machine is designed for alternating current starting by means
of a compensator. The field is provided with a standard synchronous
motor winding, and, in addition, an amortisseur winding which assists
in starting and serves as a damping device to minimize hunting.]
[Illustration: FIG. 2,480.--Diagram showing relative location of
alternators and synchronous motors in plant of Witherbee Sherman & Co.,
Mineville N. Y. The distribution system of the Company is provided
with three synchronous motors, as shown. The system includes two
hydro-electric, one turbine driven, and one engine driven generator
plants; from three of these, current is transmitted to the fourth,
which is located in Mineville, at the point "A", the current being
distributed to the motor circuits from the points "A" and "B." The
transmission to the central station at Mineville is over three phase
circuits at 6,600 volts. For operating the mine at Cheever, current
is transmitted direct from the generating station at Port Henry. The
distribution from "A" and "B" is all at 3,300 volts, being stepped down
to 440 volts for the operation of the motors, which have a total rated
capacity of 4,762 horse power. Excepting three synchronous motors,
the load is practically all inductive, there being less than 10 kw.
required for lighting. The actual power demand ranges from 60 to 65
per cent. of the rated motor capacity, and prior to the installation
of the synchronous motors, the power factor was approximately 68 per
cent., the condenser effect of these motors making it possible to
maintain an average of about 90 per cent. power factor in spite of
the fact that a considerable portion of the induction motor load is
very widely distributed. The three synchronous motors are partially
loaded, each motor driving an air compressor through belting. The 180
kva. motor at Cheever takes about 150 kw. for the operation of a 1,250
cubic foot compressor, while the two 360 kva. machines take about 300
kw. each, for the operation of two 2,500 cu. ft. sets. The operation
of these compressors affords a method of utilizing a portion of the
motor capacity mechanically, inasmuch as the load on the motors is
practically constant during the time the mines are in operation, and
thereby permit the motors to be run at approximately 80 per cent. power
factor.]
~Effects of Low Lagging Power Factors.~--Transformers are rated in kva.
output; that is, a 100 kva. transformer is supposed to deliver 100 kw.
at unity power factor at normal voltage and at normal temperatures;
but, if the power factor should be, say .6 lagging, the rated energy
output of the transformer would be only 60 kw. and yet the current
and, consequently, the heating would be approximately the same as when
delivering 100 kw. at unity power factor.
[Illustration: FIG. 2,481.--Field of a synchronous condenser. Note
the ~amortisseur winding~, _erroneously_ called ~_squirrel cage_~
~winding~, consisting of two end rings which serve to short circuit
spokes passing through the pole tips as shown. ~The amortisseur
winding~ _assists in_ ~starting~ _and serves also as a damping device
to minimize_ ~hunting~.]
The regulation of transformers is inherently good, being for small
lighting transformers about 1½ to 2 per cent. for a load of unity
power factor, and about 4 to 5 per cent. at .7 power factor. Larger
transformers with a regulation of 1 per cent. or better at a unity
power factor load, would have about 3 per cent. regulation at .7 power
factor.
Alternators also are rated in kva. output, usually at any value of
power factor between unity and .8.
The deleterious effects of low power factor loads on alternators
are even more marked than on transformers. These are, decreased kw.
capacity, the necessity for increased exciter capacity, decreased
efficiency, and impaired regulation.
Assume the case of a 100 kva. .6 power factor, 60 kw. output.
It is probable that normal voltage could be obtained only with
difficulty, unless the alternator was especially designed for
low power factor service. The lagging power factor current
in the armature sets up a flux which opposes the flux set up
by the fields, and in consequence tends to demagnetize them,
resulting in low armature voltage.
It is often impracticable, without the installation of new
exciters, to raise the alternator voltage by a further increase
of the exciting voltage and current. The field losses, and
therefore the field heating of the alternator, when it is
delivering rated voltage and current, are greater at lagging
power factor than at unity. Increased energy input and
decreased energy output both cause a reduction in efficiency.
[Illustration: FIG. 2,482.--Diagram of a section of the Northern
California Power Co.'s transmission system, showing relative location
of alternators and synchronous condenser. The synchronous condenser is
installed at Kennett, which is served by generating stations at Kilarc
and Volta, located respectively 28 and 38 miles from the point at which
the condenser is operated. The local demand amounts to about 6,500
kw., and before the installation of the synchronous condenser, the
power factor was about 79 per cent. and after installing, about 96 per
cent. while the voltage at the point where the synchronous condenser
is installed is raised approximately 10 per cent. during the change
from no load to full load. In order to obtain close voltage regulation,
a regulator is used in connection with the synchronous condenser
and holds the voltage, at the center of distribution, within 2 per
cent. The regulator is mounted on the side of the control panel and
connected in the field of the synchronous condenser to automatically
change the excitation and compensate for voltage variations. A graphic
demonstration of the improvement in voltage regulation, which has been
secured in this case, is given by the curve drawing voltmeter records
reproduced in fig. 2,483.]
The regulation at unity power factor of modern alternators capable of
carrying 25 per cent. overload, is usually about 8 per cent. Their
regulation at .7 power factor lagging is about 25 per cent. The effect
of low power factor on the lines can best be shown by the following
example:
EXAMPLE.--Assuming a distance of five miles and a load of 1,000
kw. and desiring to deliver this load at a pressure of about
6,000 volts, three phase, with an energy loss of 10 per cent.,
each conductor at unity power factor would have to be 79,200
c.m., at .9 power factor, 97,533 c.m., and at .6 power factor,
218,000 c.m. In other words, at the lower power factor of .6,
the investment in copper alone would be 2.8 times as much.
[Illustration: FIG. 2,483.--Curve drawing ~voltmeter records~ at
Kennett, Cal. ~The upper curve~ shows voltage regulation with
synchronous condenser out of service, and ~the lower curve~, with
synchronous condenser in operation.]
If the same size of wire were used at both unity and .6 power
factor lagging, the energy loss would be about 2.8 times
the loss at unity power factor, or about 28 per cent. Low
lagging power factor on a system, therefore, will generally
mean limited output of prime movers; greatly reduced kilowatt
capacity of generator, transformer and line; and increased
energy losses. The regulation of the entire system will also be
poor.
~Cost of Synchronous Condenser vs. Cost of Copper.~--Referring to the
example given in the preceding paragraph, and calculating the necessary
extra investment in copper with the .6 power factor load, and copper
at 17 cents per pound, the result is that 29,292 pounds more copper is
required than with the power factor of .9 which means a total extra
investment in copper alone of $5,000 (29,292 × $.17). A synchronous
condenser of sufficient capacity to accomplish the same result would
cost about the same amount. It would therefore cost less to install the
condenser because at the same time a considerably increased capacity
would be obtained from the alternators, transformers, etc.
[Illustration: FIG. 2,484.--Diagram showing the ~field current taken
by a synchronous motor~ of normal design when operating at normal kva.
input at various power factors. ~It will be noted~ that a ~slight
departure from unity power factor necessitates a considerable change
in field current~. As the field curves increase with the square of the
current, there is a rapid increase in temperature with leading current.
_This action of leading or lagging current serves automatically to keep
the flux constant in the armature with changes in field excitation._
~When the motor is running at unity power factor~, an increase in field
excitation causes a leading current to flow, and at the same time
this leading current demagnetizes the field until the density of the
armature is restored to its normal value. ~If the field be decreased~
a lagging current flows which in turn magnetizes the field bringing
the density back to its original value. Therefore, with a constant
line voltage, the iron losses in a synchronous motor are approximately
constant irrespective of the field excitations with the exception that
the internal voltage will vary slightly due to the armature I R drop,
the density being a trifle lower at full load than at no load.]
~Synchronous Condenser Calculations.~--In figuring on the installation
of a condenser for correcting power factor troubles, a careful survey
of the conditions should be made with a view of determining just what
these troubles are and to what extent they can be remedied by the
presence of a leading current in the system.
[Illustration: FIG. 2,485.--Diagram showing a set of ~phase
characteristic curves~ taken ~from~ a General Electric _synchronous
motor_. ~These curves show~ _the current input to the motor at various
loads with constant voltage and varying field excitation_. There is a
certain field current at each load that causes a minimum current. Any
increase or decrease of field from the value increases the current and
causes it to lead or lag with respect to the line voltage. By referring
to the _minimum input_ curve, it will be noted that if the machine
be running at full load minimum input current and load is taken off,
the current will be leading or vice versa. In each case the phase
characteristic curve was run back on the lagging side to the break down
point. ~At no load~ and one quarter load the motor still ran in step
when the field was reduced to zero and even taken off altogether, and
it was necessary to reverse the field current in order to back down
the motor. ~The motor runs without slip~, as a synchronous motor, in
this condition, obtaining its excitation from the lagging current and
running as a reaction machine. The amount of load a machine will carry
without field varies with the design, the average being about 40% of
full load. ~It will be noted~ from the _limit of stability_ curve that
the lighter the load on the machine when it breaks down from lack of
sufficient excitation, the greater the current input at this point.
~The no load characteristic~ rises sharply on each side with slight
change in field current, while it flattens out with increase in load
until at overload the current input is practically the same throughout
a large range of field current.]
[Illustration: FIG. 2,486.--Comparison of the ~speed current curves~
and ~speed power factor curves~ of a typical synchronous, and induction
motor. It will be noted that the power factor of the synchronous motor
at start is higher than that of the induction motor owing to the higher
resistance of the squirrel cage winding. ~As the machine approaches
synchronism~, however, the magnetizing current of the induction motor
drops to a very much lower value than in the synchronous motor and
the power factor is consequently much higher. The magnetizing current
of the induction motor at full speed is usually 25 per cent. of full
load current while that of the synchronous motor is from 200 to 250
per cent. of full current, or even higher when running full speed
and normal voltage. This of course is due to the large air gap on
the synchronous machine. ~The current at start~ with full voltage
applied is usually higher in an induction motor owing to the fact
that the total impedance of the stator and rotor are less due to the
greater distribution of the windings and the lower resistance of the
squirrel cage. ~The high magnetizing current~ of a synchronous motor
should not be lost sight of as it is a very important consideration
in starting the machine. Even though the motor can be brought
practically to synchronous speed while still on the compensator, if
line voltage be thrown on, there will be a very heavy rush of current.
The obvious thing to do is to get the field on the motor while still
on the compensator, whenever possible, to avoid the high magnetizing
current. This magnetizing current is obviously equal to the circuit
current of the machine at no load field. In some cases additional
torque near synchronism can be obtained by short circuiting the field
winding through the field rheostat. This has the effect of reducing
the resistance of the rotor winding to some extent and causing the
motor to have less slip with a given load. The gain from this source
is small, however, in most cases, as the self-inductance of the field
winding is so high as to allow very little current to flow even if the
field be short circuited so that the total effective resistance of the
rotor winding is not materially reduced. In some cases where the torque
is nearly sufficient, however, enough gain may be obtained to take
care of the conditions. ~If the field be short circuited~ before the
motor is started there will be a reduction in starting torque and an
increase in current from the line, hence if this method be resorted to,
arrangements must be made to short circuit the field after the motor
has come to constant speed.]
It is necessary to possess a thorough knowledge of the system, covering
the generating capacity in energy and kva., average and maximum load,
and power factor on the alternators, average and maximum load, and
power factor on the feeders, system of distribution, etc.
[Illustration: FIG. 2,487.--Curves showing amount of wattless component
required to raise the power factor of a given kw. load to required
higher value. The wattless components are expressed as percentages of
the original kw. load. ~The numbers at the right~ which indicate the
points of tangency of the power factor curves to the 100 per cent.
line, show the amount of wattless component required to raise a given
kw. load of given lagging power factor to unity power factor. Obviously
the addition of further wattless component in a given case would result
in a leading power factor less than unity.]
The desirable location of a condenser is, of course, nearest the
inductive load in order to avoid the transmission of the wattless
current, but it often happens that a system is so interconnected that
one large condenser cannot economically meet the conditions, in which
case it may be better to install two or more smaller ones.
The question of suitable attendance should also be considered and, for
this reason, it may be necessary to compromise on the location. When
the location of the condenser has been decided upon and the load and
power factor within its zone determined, the proper size of condenser
to raise the power factor to a given value can be found as follows:
The method of procedure can best be explained by reference to
a concrete case. Assume a load of 450 kw. at .65 power factor.
It is desired to raise the power factor to .9. What will be the
rating of the condenser?
[Illustration: FIG. 2,488.--Diagram for synchronous condenser
calculations.]
Referring to the diagram, fig. 2,488, it is necessary to start
with 450 kw. At .65 power factor, or 692 kva., this has a
wattless lagging component of √(692² - 450²) = 525 kva. With
the load unchanged and the power factor raised to .9, there
will be 500 apparent kva., which will have a wattless component
of √(500² - 450²) = 218 kva.
It is obvious that the condenser must supply the difference
between 525 kva. and 218 kva., or 307 kva. A 300 kva. condenser
would, therefore, meet the requirements.
If it be desired to drive some energy load with the condenser
and still bring the total power factor to .9, proceed as
indicated in fig. 2,489. Assume a total load of 150 kw. on the
motor. As before, 450 kw. at .65 power factor, or 692 kva.,
with a wattless component of 525 kva.
The energy load will be increased from 450 to 600 kw. as
indicated, and with the power factor raised to .9 there will be
a kva. of 667 with a wattless component of √(667² - 600²) = 291.
There must be supplied 525 - 291 = 234 in leading kva.
The synchronous motor then must supply 150 kw. energy
and 234 kva. wattless, which would give it a rating of
√(150² + 234²) = 278 kva. at .68 power factor.
[Illustration: FIG. 2,489.--Diagram for synchronous condenser
calculation for cases where it is desired to drive some energy load
with the condenser and still bring the total power factor to .9.]
The standard 300 kva. condenser would evidently raise the power
factor slightly above .9 power factor leading.
By reference to the chart, fig. 2,490, the size of the required
condenser can be obtained direct without the use of the above
calculation. The method of using this curve is as follows:
Assume a load of say 3,000 kw. at .7 power factor and that it
be desired to raise the power factor to .9. Run up the vertical
line at 3,000 kw. to the .7 power factor line, and from there
along the horizontal line to the margin and find a wattless
component at this power factor of 3,000 kva., approximately.
Again run up the 3,000 kw. vertical line to the .9 power factor
line and from there along the horizontal line to the margin
and find a wattless component of 1,500 kva. The rating of the
condenser will then be 3,000 kva. - 1,500 kva. = 1,500 kva.
This table of course can be used for hundreds of kilowatts as
well.
[Illustration: FIG. 2,490.--Curve showing the relation of energy load
to apparent load and wattless components at different power factors.]
For determining the rating of a synchronous motor to drive an
energy load this curve is not so valuable, although it can
be used in determining the wattless component direct in all
cases where the energy component and power factor are known.
Knowing this energy component and power factor or wattless
component, the energy load can obviously be found by referring
to the curved lines on the diagrams, the curve that crosses the
junction of the vertical energy line and the power factor or
wattless component line giving the total apparent kva.
CHAPTER LXII
INDICATING DEVICES
Alternating current ammeters or voltmeters indicate the _virtual_
values of the current or pressure respectively, that is to say, they
indicate, the _square root of the mean square of a variable quantity_.
[Illustration: FIG. 2,491.--Line curve of alternating current,
illustrating various current or pressure values. The virtual value,
or .707 × maximum value, is the value indicated by an ammeter or
voltmeter. Thus, if the maximum value of the current be 100 volts, the
virtual value as indicated by an ammeter is 100 × .707 = 70.7 amperes.]
The virtual value of an alternating current or pressure _is equivalent
to that of a direct current or pressure which would produce the same
effect_.
For instance an alternating current of 10 virtual amperes will
produce the same heating effect as 10 amperes direct current.
The relation of the virtual value of an alternating current to the
other values is shown in fig. 2,491. When the current follows the sine
law, the square root of the mean square, value of the sine functions is
obtained by multiplying their maximum value by 1 ÷ √2̅ or .707.
[Illustration: FIG. 2,492.--Wagner tubular aluminum pointer.]
The word ~effective~ is commonly used _erroneously_ for ~virtual~,
even among the best writers and the practice cannot be too strongly
condemned[3].[4] The difference between the two is illustrated in Guide
No. 5, page 1,013, fig. 1,237, the mechanical analogy here given may
make the distinction more marked.
[3] NOTE.--I adhere to the term virtual, as it was in use before the
term efficace which was recommended in 1889 by the Paris Congress
to denote the _square root of mean square value_. The corresponding
English adjective is _efficacious_, but some engineers mistranslate
it with the word _effective_. I adhere to the term virtual mainly
because effective is required in its usual meaning in kinematics to
represent the resolved part of a force which acts obliquely to the line
of motion, the effective force being the whole force multiplied by the
cosine of the angle at which it acts with respect to the direction of
motion.--_S. P. Thompson._
[4] NOTE.--The author adheres to the term _virtual_ because in
mechanics the adjective _effective_ is used to denote the difference of
two opposing forces; for instance, at any instant in the operation of a
steam engine, _effective pressure = forward pressure - back pressure_,
hence, to be consistent in nomenclature, the term effective cannot
be used for the forward or virtual pressure, that is, the pressure
impressed on an electric circuit.
In the operation of a steam engine, there are two pressures
acting on the piston:
1. The _forward_ pressure; 2. The _back_ pressure.
The forward pressure on one side of the piston is that due
to the live steam from the boiler, and the back pressure,
on the other side, that due to the resistance or opposition
encountered by the steam as it exhausts from the cylinder.
In order that the engine may run and do external work, it is
evident that the forward pressure must be greater than the back
pressure, and it follows that the pressure available to run the
engine is the difference between these two pressures, _this
pressure difference being known as the_ ~effective pressure~,
that is to say
_effective pressure = forward pressure - back pressure_
Thus, electrically speaking, the effective voltage is that
voltage which is available for driving electricity around the
circuit, that is,
_effective volts = virtual volts - back volts_
_= virtual pressure - (virtual pressure - drop)_
In the case of the steam engine, the forward pressure absolute,
that is, measured from a perfect vacuum is the virtual pressure
(not considering the source). The back pressure may vary widely
for different conditions of operation as illustrated in figs.
2,493 and 2,494.
[Illustration: FIGS. 2,493 and 2,494.--Steam engine ~indicator
cards~, _illustrating in mechanical analogy_, ~the misuse of the term
effective~ as applied to the pressure of an alternating current. The
card fig. 2,493, represents the performance of a steam engine taking
steam at 60 lbs. (gauge) pressure and exhausting into the atmosphere.
The exhaust line being above the atmospheric line shows that the
friction encountered by the steam in flowing through the exhaust pipe
produces a back pressure of two lbs. Hence at the instant represented
by the ordinate _y_, the ~effective pressure~ is 60 - 2 = 58 lbs.,
or using ~absolute pressures~, 74.7 - 16.7 = 58 lbs., the ~virtual
pressure~ being 60 lbs. gauge, or 74.7 lbs. absolute. Now, the ~back
pressure~ may be considerably reduced by exhausting into a condenser
as represented by the card, fig. 2,494. Here, most of the pressure of
the atmosphere is removed from the exhaust, and at the instant
_y_, the back pressure is only 6 lbs., and the effective pressure
74.7 - 6 = 68.7 lbs. Thus, in the two cases ~for the same virtual
pressure~ of 60 lbs. gauge or 74.7 lbs. absolute, the ~effective
pressures are~ 58 lbs. and 68.7 lbs. respectively.]
In the measurement of alternating current, it is not the average,
or maximum value of the current wave that defines the current
commercially, but the _square root of the mean square_ value, because
this gives the equivalent heating effect referred to direct current.
There are several types of instrument for measuring alternating
current, and they may be classified as
1. Electromagnetic (moving iron);
2. Hot wire;
3. Induction;
4. Dynamometer.
~Electromagnetic or Moving Iron Instruments.~--This type of instrument
depends for its action upon the pull of flux in endeavoring to reduce
the reluctance of its path. This pull is proportional to the product
of the flux and the current, and so long as no part of the magnetic
circuit becomes saturated, the flux is proportional to the current,
hence the pull is proportional to the square of the current to be
measured.
[Illustration: FIG. 2,495.--A ~calibrated~ scale. This means that
printed scales are not employed, but each instrument has its scale
divisions plotted by actual comparison with standards, after which
the division lines are inked in by a draughtsman. There are makes
of direct current instruments employing printed scales in which the
scale deflections are fairly accurate, even though the scales are
printed, but printed scales should not be used on alternating current
instruments.]
~Ques. What are some objections to moving iron instruments?~
Ans. Instruments of this type are not independent of the frequency,
wave form, or temperature and external magnetic fields may affect the
readings temporarily.
[Illustration: FIG. 2,497.--Plunger form of electromagnetic or moving
iron type of ammeter.]
There are several forms of moving iron ammeters, which may be
classified as
1. Plunger;
2. Inclined coil;
3. Magnetic vane.
~Ques. Describe the plunger type.~
Ans. This type of ammeter consists of a series coil and a soft iron
plunger forming a solenoid, the plunger is so suspended that the
magnetic pull due to the current flowing through the coil is balanced
by gravity, as shown in fig. 2,497.
~Ques. How should the plunger be constructed to adapt it to alternating
current, and why?~
Ans. It should be laminated to avoid eddy currents.
[Illustration: FIG. 2,497.--One form of ~plunger instrument~ as made by
Siemens. _It has gravity control, is dead beat, and is shielded from
external magnetic influence._ ~The moving system consists of~ a thin
soft iron pear shaped plate I pivoted on a horizontal spindle S running
in jewelled centers. To this spindle S is also attached a light pointer
P and a light wire W, bent as shown, and carrying a light piston D,
which works in a curved air tube T. This tube T is closed at the end B
but fully open at the other A, and constitutes the air damping device
for making the instrument dead beat.]
~Ques. What is the character of the scale and how should it be
constructed?~
Ans. The scale is not uniform and should be hand made and calibrated
under the conditions which it is to be used.
~Ques. What is the objection to moving iron ammeters?~
Ans. Since the coil carries the entire current they are large and
expensive.
~Ques. What precaution should be taken in installing moving iron
ammeters?~
Ans. Since gravity is the controlling force, the instrument should be
carefully levelled.
~Ques. Describe an inclined coil instrument.~
Ans. It consists of a coil mounted at an angle to a shaft carrying
the vane and pointer, as shown in fig. 2,498. A spring forms the
controlling force and holds the pointer at zero when no current is
flowing.
[Illustration: FIG. 2,498.--Inclined coil form of electromagnetic or
moving iron instrument.]
~Ques. What is the principle of operation of the inclined coil
instrument?~
Ans. When a current is passed through the coil, the iron tends to take
up a position with its longest sides parallel to the lines of force,
which results in the shaft being rotated and the pointer moved on the
dial, the amount of movement depending upon the strength of the current
in the coil.
~Ques. Describe a magnetic vane instrument.~
Ans. It consists of a small piece of soft iron or _vane_ mounted
[Illustration: FIG. 2,499.--Magnetic vane form of electromagnetic or
moving iron instrument.]
[Illustration: FIG. 2,500.--Magnetic vane movement of a Wagner
instrument; it is used both for voltmeters and ammeters. This type
differs from the dynamometer movement in that a vane of very soft iron
replaces the moving coil. The magnetic vane movement makes use of its
controlling spring only for the purpose of resisting the pull on the
vane and the returning of the needle to zero. The spring does not carry
any current.] on a shaft that is pivoted a little off the center of a
coil as shown in fig. 2,499, and carrying a pointer which moves over a
scale.
~Ques. How does it work?~
Ans. Its principle of operation is that _a piece of soft iron placed in
a magnetic field and free to move, will move into such position as to
conduct the maximum number of lines of force_.
The current to be measured is passed around the coil, producing a
magnetic field through the center of the coil. The magnetic field
inside the coil is strongest near the inner edge, hence, the vane will
move against the restraining force of a spring so that the distance
between it and the inner edge of the coil will be as small as possible.
[Illustration: FIG. 2,501.--Solenoid and plunger ~illustrating the
operation~ _of moving iron instruments_. When a current flows through
the coil, a field is set up as indicated by the dotted lines of force.
The current flowing in the direction indicated by the arrow induces
a north pole at N, which in turn induces a south pole in the plunger
at S, thus attracting the plunger. ~The effect of the field upon the
plunger~ may also be stated by saying that _it tends to cause the
plunger to move in a direction so as to conduct the maximum number
of lines of force, that is, toward the solenoid_. Thus if ABCD be
the initial position of the plunger only five lines of force pass
through it: should it move to the position A´B´C´D´, the number of
lines passing through it will then be 9, assuming the field to remain
unchanged.]
The operation of moving iron instruments of the plunger type
may be explained by saying that the current flowing in the
coil produces a pole at its end and induces an unlike pole at
the end of the plunger nearest the coil, thus attracting the
plunger, as illustrated in fig, 2,501 above.
[Illustration: Figs, 2,502 and 2,503.--Wagner series transformers.
Fig. 2,502, wound primary series transformer; fig. 2,503, open primary
transformer. Wagner series transformers are made in three general
types: One for switchboard mounting with wound primary; one for
switchboard mounting with open primary, and one with open primary
suitable for slipping over bus bars or switch stud. These transformers
have 5 ampere secondary winding, and are intended for use in connection
with instrument of scale capacity 0-5, although the scale should be
calibrated to indicate the primary current. The capacities are from 2
watts to 50 watts, being suitable for operation on circuits of 750 to
66,000 volts.]
~Hot Wire Instruments.~--Instruments of this class depend for their
operation on the expansion and contraction of a fine wire carrying
either the current to be measured or a definite proportion of that
current.
The expansion or contraction of the wire is caused by temperature
changes, which in turn are due to the heating effect of the current
flowing through the wire.
Since the variations in the length of the wire are extremely small,
considerable magnification is necessary. Pulleys or levers are
sometimes used to multiply the motion, and sometimes the double sag
arrangement shown in fig. 2,504.
As shown here, A is the active wire carrying the current to
be measured and stretched between the terminals T and T´. It
is pulled taut at its middle point by another wire C, which
carries no current, and is, in its turn, kept tight by a thread
passing round the pulley D attached to the pointer spindle,
the whole system being kept in tension by the spring E.
Hot wire instruments are equally accurate with alternating or
direct current, but have cramped scales (since the deflection
is proportional to the square of the current), and are liable
to creep owing to unequal expansion of the parts. There is also
the danger that they may be burnt out with even comparatively
small overloads. They are not affected by magnetic fields but
consume more current than the other types, these readings are
inaccurate near either end of the scale.
[Illustration: FIG. 2,504.--Diagram illustrating the principle of hot
wire instruments. The essential parts are the active wire A, stretched
between terminals T and T´, tension wire C, thread E, and pulley D to
which is attached the pointer.]
~Induction Instruments.~--These were invented by Ferraris, and are
sometimes called after him. They are for alternating current only, and
there are two forms:
1. Shielded pole type;
2. Rotary field type.
~Ques. Describe the shielded pole type of induction instrument.~
Ans. As shown in figs. 2,505, and 2,506 it consists, essentially of a
disc A, or sometimes a drum and a laminated magnet B. Covering some
two-thirds of the pole faces are two copper plates or shields C, and a
permanent magnet D.
[Illustration: FIGS. 2,505 and 2,506.--Plan and elevation of shielded
pole type of induction instrument.]
~Ques. How does it work?~
Ans. Eddy currents are induced in the two copper plates or shields C,
which attract those in the disc, producing in consequence a torque in
the direction shown by the arrow, against the opposing action of a
spring. Magnet D damps the oscillations.
[Illustration: FIG. 2,507.--Diagram showing construction and operation
of Hoskins instrument. It is of the modified induction type in which
the torque is produced from the direct repulsion between a primary
and a secondary, or induced current. As shown in the diagram, the
instrument embodies the principle of a short circuited transformer,
consisting of a primary or exciting coil A, a secondary or closed coil
B, linked in inductive relation to the primary by a laminated iron
core C, constructed to give a completely closed magnetic circuit,
that is, without air gap. The secondary is so mounted with respect to
the primary as to have a movement under the influence of their mutual
repulsion when the primary is traversed by an alternating current.
This movement of the secondary B is opposed by a spiral spring, so
that the extent of movement will be dependent upon and will indicate
the strength of the primary current. To increase the sensitiveness
of the instrument and also to adjust the contour of the scale, an
adjustable secondary D, which has an attraction effect upon the coil
B, is provided upon the core. The effect of this coil is inversely
proportional to its distance from the end of the swing of the coil B.
The vane, E, which is a part of the stamping B, is adjusted to swing
freely and with a large amount of clearance, between the poles of a
permanent magnet F, which acts as a damper on the oscillation of the
moving element, but does not cause any friction or affect the accuracy
of the calibration. The primary, like that of a transformer, is an
independent electrical circuit and may be highly insulated. This meter
will withstand several hundred per cent. overload for some time because
of the very high value of the self-induction and the fact that the
controlling spring is not in the circuit and therefore cannot burn
off.]
[Illustration: FIGS. 2,508 to 2,511.--Hoskins instruments. Fig. 2,508,
voltmeter, small pattern; fig. 2,509, ammeter, large pattern; fig.
2,510, voltmeter, horizontal edgewise pattern; fig. 2,511, illuminated
dial voltmeter.]
~Ques. Describe the rotary field type of induction instrument.~
Ans. The parts are arranged similar to those of wattmeters, the
necessary split phase being produced by dividing the current into two
circuits, one inductive and the other non-inductive.
[Illustration: FIG. 2,512.--Hoskins instrument with case removed. It
has a very short magnetic circuit which is composed of silicon steel,
permitting low magnetic densities to be used.]
~Dynamometers.~--This type of instrument is used to measure volts,
amperes, or watts, and its operation depends on the reaction between
two coils when the current to be measured is passed through them. One
of the coils is fixed and the other movable.
[Illustration: FIG. 2,513.--Diagram of Siemens' dynamometer. It
consists of two coils on a common axis but set in planes at right
angles to each other in such a way that a torque is produced between
the two coils which measures the product of their currents. This torque
is measured by twisting a spiral spring through a measured angle
of such degree that the coils shall resume their original relative
positions. _When constructed as a_ ~voltmeter~, both coils are wound
with a large number of turns of fine wire, making the instrument
sensitive to small currents. Then by connecting a high resistance in
series with the instrument it can be connected across the terminals
of a circuit whose voltage is to be measured. _When constructed as a_
~wattmeter~, one coil is wound so as to carry the main current and the
other made with many turns of fine wire of high resistance suitable for
connecting across the circuit.]
[Illustration: FIG. 2,514.--Wagner dynamometer movement. In this
type of instrument the deflection is proportional to the square of
the current, producing a constantly decreasing sensitiveness as the
pressure applied is decreased. The dynamometer movement is, for any
indication, more accurate than the magnetic vane, but cannot readily be
employed for the indication of current, as required in ammeters.]
~Ques. Describe the construction of a dynamometer.~
Ans. It consists, as shown in fig. 2,513, of a fixed coil, composed
of a number of turns of wire, and fastened to a vertical support. The
fixed coil is surrounded by a movable coil composed of a few number of
turns or often of only one turn of wire. The movable coil is suspended
by a thread and a spiral spring attached to a tortive head which passes
through the center of a dial. The ends of the movable coil dip into
mercury cups, which act as pivots and electrical contacts, making
connection with one end of the fixed coil and one terminal of the
instrument as shown. The tortion head can be turned so as to place the
planes of the coils at right angles to each other and to apply tortion
to the spring to oppose the deflection of the movable coil for this
position when a current is passed through the coils. A pointer attached
to the movable coil indicates its position on the graduated dial
between the two stops. Another pointer attached to the tortion head
performs a similar function.
[Illustration: FIG. 2,515.--Armature of Wagner dynanometer movement.
Greater accuracy is claimed for this movement than the magnetic vane,
but it cannot readily be employed for the indication of current flow,
as required in ammeters. ~The magnetic vane movement~ is used on the A.
C. ammeter, and can be used also in the A. C. voltmeters; it makes use
of its controlling spring only for the purpose of resisting the pull
on the vane and the returning of the pointer to zero. The dynanometer
movement is recommended for voltmeters.]
[Illustration: FIG. 2,516.--Wagner 25 watt pressure transformer for
use with various alternating current instruments, such as voltmeters,
wattmeters, etc. They are made in capacities 25, 50, 100, and 200
watts, and are built for pressures of 750 to 60,000 volts.]
~Ques. How does the dynamometer operate?~
Ans. When current is passed through both coils, the movable coil is
deflected against one of the stop pins, then the tortion head is turned
to oppose the movement until the deflection has been overcome and the
coil brought back to its original position.
[Illustration: FIG. 2,517.--Moving element of Keystone dynamometer
instrument. The illustration shows the movable coil, pointer, aluminum
air vane for damping the oscillations, controlling springs, and counter
weights.]
[Illustration: FIG. 2,518.--Keystone dynamometer movement. Since the
law governing this type of instrument is the law of current squares, it
follows that in the case of voltmeters, equally divided scales cannot
be obtained. In the case of ~wattmeters~, the scale is approximately
equally divided, due to the fact that the movement of the moving coil
is proportional to the product of the current in the fixed and moving
coils. ~The moving parts~ have been made as light in weight as is
consistent with mechanical strength, and the entire moving system is
supported on jeweled bearings. The motion of the pointer is rendered
aperiodic by the use of an aluminum air vane moving in a partially
enclosed air chamber. This method of damping the oscillations of the
moving parts renders unnecessary the use of mechanical brakes or
other frictional devices, which tend to impair the accuracy of the
instrument. The illustration shows a ~voltmeter~, which, however,
differs but little from a wattmeter. In the case of a wattmeter the
fixed coils are connected in series with the line, either directly
or through a current transformer, while the moving coil is connected
in shunt to the line.] The angle through which the tortion head was
turned, being proportional to the square root of the angle of tortion,
the current strength in amperes is equal to the square root of the
angle of tortion _multiplied by a calculated constant_, furnished by
the maker of the instrument.
~Ques. How is the dynamometer arranged to measure watts?~
Ans. When measuring watts, the instrument should be so arranged that
one coil carries the main current, and the other a small current which
is proportional to the pressure.
[Illustration: FIG. 2,519.--Leeds and Northrup ~electro-dynamometer~.
_It is a reliable instrument for the measurement of alternating
currents of commercial frequencies_. ~When wound with fine wire~ and
used in connection with properly wound resistances, it is equally
useful for measuring alternating pressures, and may thus be employed
to calibrate alternating current voltmeters as well as ammeters. ~To
give accurate results~ the instruments must be carefully constructed
and designed with a view to avoiding the eddy currents always set up
by alternating currents in masses of metal near, or in the circuits.
~The constant of a dynamometer~ may be obtained with a potentiometer,
but this is usually done with precision by the manufacturer and a
certificate giving the value of the constant is furnished with the
instrument. ~The size and cost~ of dynamometers rapidly increase with
the maximum currents which they are designed to carry, and when more
than 500 amperes are to be measured, the use of other instruments and
methods is recommended.]
~Ques. In the construction of a dynamometer what material should not be
used and why?~
Ans. No iron or other magnetic material should be employed because
of the hysteresis losses occasioned thereby. The frame should be of
non-conducting material so as to avoid eddy currents.
[Illustration: FIGS. 2,520 to 2,526.--Various types of Wagner
instruments. Fig. 2,520, small round type; fig. 2,521, horizontal
edgewise type; fig. 2,522, smallest switchboard type; fig. 2,523,
portable type; fig. 2,524, combination voltmeter and ammeter in one
case; fig. 2,525, vertical type; fig. 2,526, polyphase type.]
~Watt Hour Meters.~--A watt hour meter is a watt meter that will
register the watt hours expended during an interval of time. _Watt hour
meters are often erroneously called_ ~recording~ _or_ ~integrating~
_watt meters_.
There are several types of the electromotor form of watt hour meter,
which may be classified as
1. Commutator type;
2. Induction type;
3. Faraday disc type.
[Illustration: FIG. 2,527--Interior Weston single phase wattmeter.
The general appearance of the dynamometer movement and the relative
positions of the various parts are clearly shown. The parts are
assembled on one base, the whole movement being removable by
unfastening two bolts. ~The fixed winding~ is made up of two coils,
which together produce the field of the wattmeter. ~The movable coil~
is wound to gauge with silk covered wire and treated with cement.
While winding, the coil is spread at diametrical points to allow the
insertion of the staff, which is centered by means of two curved plates
cemented to the inside surface of the coil and forming a part thereof.
The coil is held in a definite position by two tiny pins which pass
through the staff and engage with ears on the curved plates.]
[Illustration: FIG. 2,528.--Westinghouse ~single phase induction type
watt hour meter~ removed from case. ~The friction compensation,~ or
light load adjustment, is accomplished by slightly unbalancing the
two legs of the shunt magnetic circuit. To do this a short circuited
loop is placed in each air gap, and means are provided for adjusting
the position of the loops so that one loop will enclose and choke
back more of the flux than the other loop, and thus produce a slight
torque. It will be noted, that this torque depends on voltage alone,
which is practically constant, and is entirely independent of the load.
Adjustment is accomplished by means of either of two screws which makes
micrometer adjustment possible. It is clamped when adjusted by means
of a set screw, which prevents change. This method makes possible
an accuracy of adjustment which effectively prevents creeping. ~The
power factor adjustment~ consists of an adjustable compensating coil
placed around the shunt pole tip. This is adjusted at the factory by
twisting together the leads of the compensating coil, thus altering
its resistance until the desired lagging effect is had. ~Frequency
adjustment.~ 133 cycle meters are first calibrated on 60 cycles and
the leads then untwisted to make them correct on 133 cycles. To change
such a meter for use on 60 cycles it is necessary only to retwist these
leads to the point shown by the condition of the wire.]
~Ques. What are the essential parts of a watt hour meter?~
Ans. A motor, generator, and counting mechanism.
[Illustration: FIG. 2,529.--Pointer and movable system of Weston
wattmeter. ~The coil~ is described in fig. 2,527. ~The pointer~
consists of a triangular truss with tubular members, an index tip of
very thin metal being mounted at its extremity. ~The index tip~ is
reinforced by a rib stamped into the metal. The pointer is permanently
joined to a balance cross, consisting of a flat center web, provided
with two short arms and one long arm, each arm carrying a nut by means
of which the balance of the system may be adjusted. The longest arm,
which is opposite the pointer, carries a balance nut, consisting of a
thin walled sleeve provided with a relatively large flange at its outer
end. The sleeve is tapped with 272 threads to the inch, the internal
diameter of the sleeve being made slightly smaller than the outside
diameter of the screw, and the sleeve is split lengthwise; therefore
when sprung into place and properly adjusted it will remain permanently
in position. A sleeve which is forced over the end of the staff carries
the pointer firmly clamped between a flanged shoulder and a nut. By
perforating the web plate of the balance cross with a hole having two
flat sides that fit snugly over a similarly shaped portion of the
sleeve, the pointer is given a definite and permanently fixed angular
position. ~The air damper~consists of two very light symmetrically
disposed vanes, which are enclosed in chambers made as nearly air
tight as possible. These vanes are formed of very thin metal stiffened
by ribs, stamped into them and by the edges, which are bent over to
conform to the surface of the side walls of the chambers. They are
attached by metal eyelets to a cross bar carried on a sleeve similar in
construction to the one at the upper end of the staff. This cross bar
is held in place by a nut, and is provided at the center with a hole
having two flat sides, being similar in shape to the one in the balance
cross. This hole likewise fits over a sleeve and definitely locates the
vanes with reference to the other parts of the system. The damper box
is cast in one piece to form the base that carries the field coils and
the movable system.]
~Ques. What is the function of the motor?~
Ans. Since the motor runs at a speed proportional to the energy
passing through the circuit, it drives the counting mechanism at the
proper speed to indicate the amount of energy consumed.
~Ques. What is the object of the generator?~
Ans. It furnishes a suitable counter torque or load for the motor.
[Illustration: FIG. 2,530.--Westinghouse polyphase induction type watt
hour meter, covers removed. This type is made for two phase three wire
and four wire, and three phase three wire and four wire circuits.
Meters for circuits of more than 300 amperes or 500 volts require
transformers, but, like the self-contained meters, are calibrated to
read directly in kilowatt hours on the dial, without a multiplying
constant.]
~Ques. Is there any other resistance to be overcome by the motor?~
Ans. It must overcome the friction of all the moving parts.
~Ques. Is the friction constant?~
Ans. No.
[Illustration: FIGS. 2,531 to 2,533.--Diagram of electromagnetic
circuit of Westinghouse induction type watt hour meter, and diagram
showing rotation of field. The dotted lines show the main paths of the
magnetic flux produced by the two windings, the directions, however,
are constantly reversing owing to the alternations of the current in
the coils. Denoting the shunt and series pole tips by the letters as
shown, a clear statement of the relation of the fields for each quarter
period may be given. The signs + and - represent the instantaneous
values of the poles indicated. Thus, ~at one instant~ the shunt pole
tips A, C, and A₁ are maximum +, -, and +, respectively because the
instantaneous value of the current is maximum, while the value of the
series flux is zero. ~At ¼ period later~ the shunt current is zero,
giving zero magnetic pressure at the pole tips, while the series
current has reached a maximum value, giving maximum-and + at the pole
tips B and D. ~At the next ¼~ ~period~ the shunt current is again
maximum, but in a direction opposite to what it was at the beginning,
making the pole tips A, C, and A₁ +, -, and +, respectively, while the
series current again is zero, etc., the values for the complete cycle
being given in fig. 2,533. It will be observed from the table that
both the + and - signs move constantly in the direction from A₁ to A,
indicating a shifting of the field in this direction, the process being
repeated during each cycle.]
~Ques. What provision is made to correct the error due to friction?~
Ans. The meter is compensated by exciting an adjustable auxiliary field
from the shunt or pressure circuit.
~Ques. What is the construction of the generator?~
Ans. In nearly all meters it consists of a copper or aluminum disc
carried on the same shaft with the motor and rotated in a magnetic
field of constant value.
~Ques. How is the counter torque produced?~
Ans. When the disc is rotated in the magnetic field, eddy currents are
induced in the disc in a direction to oppose the motion which produces
them.
~Ques. For what services is the commutator type meter used?~
Ans. It is used on both direct and alternating current circuits.
[Illustration: FIGS. 2,534 and 2,535.--Cross section of bearings of
Westinghouse induction type watt hour meter. The lower bearing consists
of a very highly polished and hardened steel ball resting between two
sapphire cup jewels, one fixed in the end of the bearing screw and the
other mounted in a removable sleeve on the end of the shaft. Owing to
the minute gyrations of the shaft the ball has a rolling action, which
not only makes a lower friction coefficient than the usual rubbing
action, but presents constantly new bearing surfaces and thus produces
long life. The upper bearing is only a guide bearing to keep the shaft
in a vertical position, and is subject to virtually no pressure, and
consequently little friction. It consists of a steel pin fastened to a
removable screw and projecting down into a bushing in a recess drilled
in the shaft. The bottom of this recess is filled with billiard cloth
saturated with watch oil. A film of oil is maintained around the pin by
capillary action.]
~Ques. What is the objection to the commutator meter?~
Ans. The complication of commutator and brushes, and the fact that the
friction of the brushes is likely to affect the accuracy of the meter.
[Illustration: FIG. 2,536.--Diagram of Fort Wayne, induction watt hour
meter. ~It is designed~ to register the energy of alternating current
circuits regardless of the power factor, ~and embodies~ _the usual
induction motor, eddy current generator and registering mechanism_. The
electrical arrangement of the meter consists of ~_a current circuit_~
composed of two coils connected in series with each other and in series
with the line to be measured, and ~_a pressure circuit_~ consisting
of a reactance coil and a pressure coil connected in series with each
other and across the line to be measured. In addition, the pressure
circuit contains a light load coil wound over a laminated sheet steel
member, adjustably arranged in the core of the pressure coil and
connected across a small number of turns of the reactance coil so as
to give a field substantially in phase with the impressed pressure.
The light load winding is further provided with a series adjustable
resistance furnished for the purpose of regulating the current flowing
in the light load winding, thereby providing a means of lagging the
meter on high frequencies, such as 125 or 140 cycle circuits. The
pressure circuit also comprises a lag coil wound over the upper limb
of the core of the pressure circuit and provided with an adjustable
resistance for obtaining a held component in quadrature with the shunt
field.]
~Ques. What are its characteristics?~
Ans. It is independent of power factor, wave form, and frequency when
no iron is used in the motor.
~Ques. What meter is chiefly used on A. C. circuits?~
Ans. The induction meter.
[Illustration: FIG. 2,537.--Fort Wayne multiphase induction watt hour
meter. ~The construction of the mechanism~ _is essentially two single
phase motor elements_, one at the bottom of the meter in a suitable
position, the other inverted and placed at the top of the meter. ~Each
element~ acts on a separate cup, but both cups are mounted on a single
shaft so that the registration is due to the resultant torque of the
two elements. The meter is provided with three supporting lugs, the
one at the top being keyholed and one of the bottom two, slotted to
facilitate leveling. ~The registering mechanism~ is mounted on a cast
iron bracket at the middle of the meter between the two motor elements.
The supporting bracket is attached to the meter base by two screws
and aligned by two dowel pins. The register is of the four dial type,
reading in kilowatt hours. Each division of the right hand circle,
or that passed over by the most rapidly moving pointer, equals one
kilowatt hour in meters without a dial constant. In meters of larger
capacities, dial constants of 10, 100 and 1,000 are used, in which case
it is only necessary to add one, two or three ciphers to the observed
reading.]
~Principles of Induction Watt Hour Meters.~--Every commercial meter
of this type is made up of a number of elements, described below.
Each of these elements and parts has certain functions, and each is
therefore necessary to the successful operation of the meter; moreover,
each element, unless correctly designed, may introduce a source of
inaccuracy. These elements are:
1. The field producing element; 2. The moving element; 3. The retarding
element; 4. The registering element; 5. The mounting frame and
bearings; 6. The friction compensator; 7. The power factor adjustment;
8. Frequency adjustment; 9. The case and cover.
[Illustration: FIGS. 2,533 to 2,541.--Connections of Fort Wayne
multiphase watt hour meters (type k₃--forms MAB and MAK), for 100-625
volt circuits, 5-150 amperes. Fig. 2,538 two and three phase, three
wire circuit, 25-36 cycles; fig. 2,539 two and three phase, 3 wire
circuit, 36 cycles and above; fig. 2,540, two phase 4 wire circuit,
25-36 cycles; fig. 2,541 two phase, 4 wire circuit 36 cycles and
above.]
[Illustration: FIG. 2,542.--Fort Wayne single phase induction watthour
meter with cover removed. The rotating parts consist of an aluminum
disc mounted on a short shaft of small diameter. The lower end has
inserted in it a hardened steel pivot which rests in a cup shaped
jewel bearing. The top of the meter shaft is drilled and provided with
a small washer having the central hole of very small diameter. Into
this hole there extends a steel pin around which the shaft turns.
Two micrometer screws are provided for load adjustment--one for the
full load and the other for the light load adjustment. The adjustment
for accuracy on full load is secured by varying the position of the
permanent magnets, sliding them either in or out from the center of
the rotating disc of the meter depending on whether it is desired to
increase or decrease the speed of the disc. The micrometer screw shown
in the figure serves to vary the position of the permanent magnets,
causing the shoe in which the two magnets are firmly clamped to slide
on the milled magnet support which is cast as an integral part of
the meter frame. When the proper position of the magnets has been
accurately determined by adjustment and test, the shoe which holds
the two magnets is clamped firmly to the milled magnet support by
two screws, one of which is shown in the figure. The adjustment for
accuracy on light load is secured by varying the position of a metal
punching, known as the starting plate, laterally under the pressure
pole in the path of the pressure flux. This lateral movement is
accomplished by means of the micrometer screw. When the proper position
of this punching has been accurately determined by adjustment and test,
it is secured in place by tightening the two brass screws which serve
to clamp it to the meter frame.]
~1. The Field Producing Element.~--This consists of the electromagnetic
circuit and the measuring coils. One of these coils, connected in
series with the circuit to be metered, is wound of few turns and is
therefore of low inductance. The current through it is in phase with
the current in the metered circuit. The other coil, connected across
the circuit, is highly inductive, and therefore the current in it is
nearly 90 degrees out of phase with, and proportional to the voltage of
the metered circuit across its terminals. Therefore, when the current
in the circuit is in phase with the voltage (100 per cent. power
factor) the currents in the meter coils are displaced almost 90 degrees
with respect to each other.
~Ques. How is this angle made exactly 90 degrees?~
Ans. By means of the power factor adjustment.
[Illustration: FIG. 2,543.--Rear view of Fort Wayne single phase
induction watthour meter with back cover plate removed. The pressure
and current coils and their respective cores lie behind the main frame
of the meter. This complete electromagnetic unit can be removed as a
whole from its mounting in the case. The pressure coil is wound from
enameled wire, the number of turns being very high. The current coils
have but few turns each and are wound from cotton covered wire. All
coils are treated with insulating compound before assembling in the
meters. The laminated iron cores placed within these coils are built
up from magnetic steel. The magnetic circuits formed by the cores of
the pressure and current coils are so arranged that they exert a high
torque upon the disc of the rotating element in order that minute
variations in the friction of the moving parts, which are likely to
occur will not cause any appreciable error in the registration of the
meter. The iron case surrounding the electrical elements protect that
part of the meter from the effects of external stray fields, while
the astatic arrangement of the permanent magnets tends to prevent any
influence on the damping system. The fact that the iron frame of the
meter lies between the permanent magnets and the current coils protects
the magnets from the effects of short circuits which create a strong
magnetic field within the meter itself.]
~Ques. How are the coils mounted?~
Ans. They are so mounted on the core that the currents in them produce
a rotating or shifting field in the air gap, in somewhat the same
manner that the currents in the primary windings of an induction motor
produce a rotating field.
[Illustration: FIG. 2,544.--Fort Wayne single phase induction watthour
meter with cover register and permanent magnets removed to show solid
meter frame. A heavy steel back plate held in place by two screws
inserted from the front of the central casting encloses the back part
of the completely assembled meter. A felt gasket lying on a suitable
ledge seals the joint against the entrance of dust or moisture when the
back plate is drawn down firmly by the screws. The cover which encloses
the back part of the meter is a non-magnetic metallic stamping. It
is held in place by wing nuts on the two light brass studs extending
forward from the meter frame. This joint between the main frame and
the cover is also sealed against the entrance of dust and moisture by
the use of a suitable felt gasket. Two glass windows are provided in
this cover, one to permit the reading of the register dials, the other
to permit observation of the disc's rotation. The cover is sealed
in place in the usual way by passing a sealing wire through a hole
drilled in the cover sealing stud and thence through a hole provided
in the wing of the seal nut. The terminal chamber is an extension of
the casting which supports all the inner parts of the meter. The heavy
brass terminals used for connecting the meter in circuit are held
permanently by a non-combustible insulating compound which is moulded
in place around them. This construction gives excellent insulation and
is a safeguard against accidental short circuits across terminals. A
punched terminal cover which fits over the terminal chamber is hinged
at the upper left hand corner so that it will of its own accord swing
out of the way when the terminal cover sealing screw is removed. This
hinged style of cover will be found convenient when installing and
connecting the meter in circuit. When this cover is swung back into
closed position it is fastened in place by passing a seal right through
the seal screw and through a lug provided on the cover.]
~Ques. What is the strength of the rotating field with 90 degrees phase
difference between the currents?~
Ans. It is proportional to the product of the currents in the two
coils and therefore proportional to the product of current and voltage
in the metered circuit.
At any other power factor the field is proportional to this
product multiplied by the sine of the angle of phase difference
between the two meter currents. If the current in the voltage
coil be in quadrature with the voltage of the metered circuit,
at any power factor the sine of the angle of phase difference
between the currents in the meter circuits will be equal to
the cosine of the angular displacement between the current
and voltage in the metered circuit. Under these conditions
therefore the strength of the shifting field is proportional
also to the power factor of the circuit. In other words, the
strength of the rotating field is proportional to the product
of the volts, amperes and power factor and is therefore a
measure of the actual power.
[Illustration: FIG. 2,545.--Sangamo single phase induction watt hour
meter; view with cover removed showing mechanism.]
~Ques. In what part of the meter is energy consumed?~
Ans. In the field producing element.
[Illustration: FIG. 2,546.--Main grid or supporting frame of Sangamo
single phase induction watt hour meter. The grid is of cast iron and
its design is such that the weight of the permanent magnets, series
laminated element and return plate are carried on the main portion,
the smaller projecting brackets carrying no weight except that of the
moving system. The supporting grid is removed by taking out the three
screws locating and holding it in position, to the iron base, also
removing at the same time the screws connecting the leads of the series
coils to the binding posts at the bottom. The meters are all built with
four binding posts so that they may be connected either with two series
leads and a tap for the pressure connection or with both sides of the
circuit carried through the meter. The wire meters employ a 220 volt
shunt coil, connected across the binding posts within the meter, one
series coil being in each of the outer lines of the three wire system.
This renders unnecessary the use of a pressure tap.]
It is upon the design of this element that the losses in
the meter depend. Current is flowing through the shunt coil
continuously, even when no energy is being taken, and the
higher the inductance of this coil, the smaller will be the
energy component of the constant flow. The series coil causes
a loss of energy proportional to the square of the current
flowing. It also causes a drop in voltage, both inductive and
resistive, hence, the resistance and inductance of the series
coil of the meter should be as low as possible.
~Ques. How should the magnetic circuit be designed?~
Ans. The design should be such that the increase of magnetic flux with
high voltage or high current will not have a retarding action but will
act only to increase the torque.
If the retarding effect be not prevented, the meter will,
of course, run slow at overloads. A comparative test of
meters at varying load and at varying voltage will reveal the
characteristics of the magnetic circuit.
~2. The Moving Element.~--This usually consists of a light metal disc
revolving through the air gap in which the rotating field is produced.
[Illustration: FIG. 2,547.--Moving element of Sangamo single phase
induction watt hour meter. It consists of a light aluminum disc mounted
on a hard brass shaft, the entire system weighing 15.6 grams. The disc
is swaged under heavy pressure, to render it stiff. The arrangement of
the disc, shaft, and bearings is shown in fig. 2,548. By unscrewing
the upper and lower bearings the disc and shaft can be removed without
disturbing the magnets or adjustments.]
~Ques. What is the action of the disc?~
Ans. It acts like the squirrel cage armature of an induction motor,
developing the motive torque for the meter.
~Ques. How is this torque counter balanced?~
Ans. By the retarding element so that the speed is proportional to the
torque.
~Ques. How should the disc be made and why?~
Ans. As light as possible to reduce wear on the bearings to a minimum.
[Illustration: FIG. 2,548.--Bearing system of Sangamo ~single phase
_induction_ watthour meter~. ~The upper pivot, or bearing~ is made of
tempered steel wire and of sufficiently small diameter to be quite
flexible in the length between the top of the brass shaft and the guide
ring in which it rotates. The guide ring, made of phosphor bronze, has
the heavy hole lined and burnished. The upper bearing screw, in which
the bronze bushing is carried, is so constructed that a long brass
sleeve closely surrounds the upper pivot of the spindle. Any blow
against the moving system, caused by accident or short circuit, will
slightly deflect the shaft until the steel pivot touches against the
side of the shell, thus preventing danger of breaking off or bending
the upper pivot. At the same time a cushioning or flexible action
between the shaft and the bearing shell is secured, thus eliminating
the effect of vibration in the moving system, which would tend to
produce rattling. ~The lower bearing~ consists of a cup sapphire jewel,
supported in a threaded pillar, the upper end of which is provided with
a sleeve so located that it prevents the moving element dropping out
during shipment. This protecting sleeve is held friction tight on the
shaft and can be removed if it be desired to inspect the jewel.]
~3. The Retarding Element.~--This part acts as a load on the induction
motor and enables the adjustment of its speed to normal limits. In
order that the speed shall be proportional to the driving torque, which
varies with the watts in the circuit, it is necessary that the torque
of the retarding device be proportional to the speed. For this reason
a short circuited constant field generator, consisting of a metal disc
rotating between permanent magnet poles, has been generally adopted.
~Ques. How is the retarding torque produced?~
Ans. Eddy currents are induced in the disc in rotating through the
magnetic field which, according to Lenz's law, oppose the force that
produces them, thus developing a retarding torque.
~Ques. How is the constant field for the retarding disc produced?~
Ans. By permanent magnets.
The retarding disc may be the same disc used for the moving
element, in which case the meter field acts on one edge while
the permanent magnet field acts on the edge diametrically
opposite. This arrangement simplifies the number of parts and
saves space and weight of moving element.
~Ques. What error is likely to be introduced by the retarding element?~
Ans. If the strength of the permanent magnets change from any cause,
the retarding torque will be changed and the calibration of the meter
rendered inaccurate.
~Ques. How may the strength of the permanent magnets be changed?~
Ans. They may become weak with age, or affected by the proximity of
other magnetic fields. The series coil of the meter may, under short
circuit so affect the strength of the permanent magnets as to render
the meter inaccurate.
~Ques. What precautions are taken to keep the strength of the permanent
magnets constant?~
Ans. Weakening with age is prevented by the process of "Aging." The
effect of neighboring fields is overcome by iron shields; this prevents
the electromagnets affecting, through overloads, the strength of the
permanent magnets.
~4. The Registering Element.~--This mechanism comprises the dials,
pointers, and gear train necessary to secure the required reduction in
speed. This gear train is driven directly by the rotor and therefore
its friction should be low and constant. The dials should be easily
read and should register directly in kilowatt hours. If a constant
be used to reduce the reading to kilowatt hours, it should be some
multiple of 10, to avoid errors in multiplication. By means of suitable
gears in the meters this is easily accomplished.
[Illustration: FIG. 2,549.--Register dial of Sangamo single phase
induction watt hour meter (full size). The dial circles read 10, 100,
1,000, and 10,000 kilowatt hours from right to left.]
~5. The Mounting Frame and Bearings.~--These parts have an important
influence on the accuracy of the meter, as it is in the bearings that
most of the friction in the meter occurs. The frame should be rigid
and free from vibration, so that the bearings will be at all times in
perfect alignment.
Initial friction is unavoidable in any meter construction
and can be easily compensated for. A _change_ in the initial
friction, however, due to wear of bearings, makes readjustment
necessary.
In selecting a meter the special attention should therefore
be given, to the construction of the bearings, particularly
the lower, or "step" bearing which supports the weight of the
moving element.
[Illustration: FIG. 2,550.--Canadian dial of Sangamo single phase
induction watt hour meter. It has a small test circle indicating one
kilowatt hour per revolution in all sizes where the first regular
circle indicates 10. This is provided to conform with the requirements
of the Canadian government and it is intended that the hand on the test
circle shall make not less than ½ revolution in one hour with full
load on a meter. In the case of a 10 ampere meter, it will make one
complete revolution in one hour and for a 20 ampere, two revolutions,
and so on. The train or indicating mechanism is carried on a rigidly
formed and swaged brass bracket accurately located by two dowel pins
set in the top face of the main grid, and is held to the grid with two
screws easily accessible when it is desired to remove the train for
any purpose. All indicating trains used on type "H" meters are marked
with symbols on the back of the train and on the compound attachment to
indicate the gear ratio of each combination; this ratio being different
for meters of different capacities in order to obtain a direct reading
in kilowatt hours on the dial.]
~Ques. Describe a good construction for the step bearing.~
Ans. A desirable construction would consist of a very highly polished
and hardened ball with jewel seats.
~6. The Friction Compensator.~--The object of this device is to
overcome the initial friction of the moving parts. It is evident that
if this initial friction were not compensated some of the driving
torque of the meter would be used in overcoming it, and the meter would
therefore not rotate at very light load, and not fast enough at other
loads, thus rendering the registration inaccurate, especially at light
loads.
[Illustration: FIG. 2,551.--Base and shunt coil of Sangamo single
phase induction watt hour meter. Since the shunt or pressure coil
sometimes breaks down or burns out, due to abnormal line conditions
or accident, provision is made for easy replacement. The shunt magnet
with its coil is held to the base by two dowel pins and four screws,
enabling it to be removed as a unit as shown. A new core and coil may
then be substituted without the necessity of removing and replacing
laminations. The shunt coil in 25 cycle meters is wider and contains
more steel than the 60 and 133 cycle coils, the winding also being
correspondingly increased. The return plate and series coil laminations
are also changed in proportion to correspond to the increased width
of the shunt magnet. The laminations forming the core are laced into
the shunt coil, and subjected to enormous hydraulic pressure, the
rivets being set at the same time, to form a compact unit and eliminate
humming. The laminated core of the shunt element has but a single air
gap in which these discs rotate.]
Since meters are usually run at light loads it is important that an
efficient light load adjustment or friction compensator should be
provided.
~Ques. What important point should be considered in the design of the
friction compensator?~
Ans. The compensating torque must not cause the moving element to
rotate or "creep" without current in the series coil.
The rotation of a meter is caused by two distinct torques, the
varying meter torque, dependent on the power in the circuit,
and the constant torque adjusted to compensate the initial
friction.
The friction at all speeds is not exactly the same as the
initial friction, and therefore the friction compensating
torque may be in error a few per cent. at high speeds.
[Illustration: FIGS. 2,552 AND 2,553.--Arrangement of magnetic circuit
of Sangamo single phase induction meter. Fig. 2,552, front view; fig.
2,553, rear and side view. As shown, the gap of the shunt held in which
the disc rotates, projects in between the poles of the series magnet,
the return plate bending around so as to clear the upper leg of the
shunt magnet. This gives the desired proximity of shunt and series
fields with a maximum radius of action for both sets of field. In all
capacities up to and including 60 amperes, 2 wire and 3 wire, round
wire and taped series coils are used, and in capacities of 80 and 100
amperes, strap windings. Meters exceeding 100 ampere capacity have five
ampere coils and are operated from external current transformers having
5 ampere secondaries. The series windings or coils are mounted on a
laminated iron U shaped magnet having a laminated return path above the
disc of the meter, thus forming air gaps in which the disc rotates.
The series coils in all capacities not having strap windings are held
firmly in position on the yoke so that they cannot slip up from the
lowest position. This is accomplished by means of a pair of spring
brass clips slipped through the coils on the rear face of the yoke, the
clips being held by the two screws which fasten the series magnet to
the main grid. As an additional precaution, spring steel lock washers
are put beneath the heads of the holding screws, thus eliminating any
chance of the series magnet loosening and changing position.]
If the compensating torque be small compared with the driving
torque, this small error percentage is negligible in its
effect on accuracy. The smaller it is, the greater will be
the accuracy at all loads, and therefore, as the compensating
torque is adjusted to balance the initial friction, the initial
friction should be small compared with the driving torque.
A high driving torque and low initial friction are therefore
desirable, but any increase in the driving torque which
necessitates an increase in friction, is obviously useless.
The desirable feature of a meter is high ratio of torque to
friction. As the friction is practically proportional, to the
weight of the moving element, in meters having the same form
of bearing, the ratio of torque to weight of rotor gives an
approximation to the ratio of torque to friction, but the
design of bearing should not be overlooked.
A meter having a high torque obtained by using a thick and
consequently heavy disc, often has a lower ratio of torque to
weight than another with lower torque, and is consequently
likely to be less accurate over a given range. Furthermore, the
heavy disc is a distinct disadvantage because it produces more
wear on the bearings and thus reduces the life.
[Illustration: FIGS. 2,554 AND 2,555. Connections of Sangamo ~single
phase _induction_ meter~. Fig. 2,554, 2 wire meter, 5-100 ampere
capacity; fig. 2,555, 3 wire meter, 5-100 ampere capacity.]
~7. The Power Factor Adjustment.~ This adjustment is necessary to make
the phase angle between the shunt and series field components 90° with
unity power factor in the metered circuit. Owing to the resistance and
iron loss in the shunt field circuit, that field is not shifted quite
90° with respect to the voltage. Yet exact quadrature is necessary to
make the strength of the resultant field, and consequently the rotor
speed, proportional to the power factor, as explained in the discussion
of the field producing element.
~Ques. What is the usual construction of the power factor adjustment?~
Ans. It usually consists of a short circuited loop enclosing part or
all of the shunt field flux.
~Ques. How does this loop act?~
Ans. It acts like the secondary of a transformer.
The flux induces a current in it which, acting with the current
in the shunt coil, produces a slightly lagging field. By
shifting the position of the resistance of the short circuited
loop, the lag may be so adjusted that the shunt field flux
is in exact quadrature with the voltage. It should be noted,
however, that this adjustment makes the meter correct at or
near one frequency only. This feature is not objectionable if
reasonable accuracy be maintained within the limits of normal
variation of frequency.
[Illustration: FIGS. 2,556 AND 2,557. Connections of Sangamo ~single
phase _induction_ meter~. Fig. 2,556, 2 wire meter exceeding 100
amperes; fig. 2,557, 3 wire meter exceeding 100 amperes.]
~8. Frequency Adjustment.~--This is often desirable, particularly for
systems operating at 133 cycles. Most makes of meter are provided with
means for changing the adjustment from 133 to 60 cycles in case of
change in the system.
~9. The Case and Cover.~--These parts should be dust and bug proof, to
avoid damage to the bearings, insulation and moving parts, and should
of course be provided with means for sealing.
Terminal chambers so arranged that the cover of the meter element
need not be removed in connecting up, are an important feature,
particularly in meters that require no adjustment at installation, as
they prevent entrance of dust into the main meter chamber.
A window through which the rotation of the disc can be observed in
checking, should be provided for the same reason.
[Illustration: FIG. 2,558.--Faraday disc, or mercury motor ampere hour
meter; view showing electric and magnetic circuits.]
~The Faraday Disc, or Mercury Motor Ampere Hour Meter.~--On this type
of meter the mercury motor consists essentially of a copper disc
floated in mercury between the poles of a magnet and provided with
leads to and from the mercury at diametrically opposite points. The
theoretical relations of the various parts are shown in fig. 2,558.
[Illustration: FIG. 2,559.--Diagram showing relative direction of
current, magnetic flux, and motion of disc in Faraday disc, or mercury
motor ampere hour meter.]
~Ques. Explain its operation.~
Ans. The electric current, as shown in fig. 2,558, enters the contact
C, passes through the comparatively high resistance mercury H to the
edge of the low resistance copper disc D across the disc to the mercury
H and out of contact C'. The magnetic flux cuts across the disc on
each side from N to S, making a complete circuit through M and M'. The
relative directions of the magnetic flux and the current of electricity
as well as the resulting motion are shown in fig. 2,559. According to
the laws of electromagnetic induction, if a current carrying conductor
cut a magnetic field of flux at right angles, a force is exerted upon
the conductor, tending to push it at right angles to both the current
and the flux. When connected to an eddy current damper or generator
which requires a driving force directly proportional to the speed of
rotation, the mercury motor generator becomes a meter. The speed
of such a meter is a measure of the current or rate of flow of the
electricity through the motor element, and each revolution of the motor
corresponds to a given quantity of electricity. Then, by connecting a
revolution counter to this motor generator, a means is provided for
indicating the total quantity of electricity in ampere hours that is
passed through the meter.
[Illustration: FIG. 2,560.--Sectional view of Faraday disc or
mercury motor ampere hour meter as made by Sangamo Electric Co. The
illustration does not show the magnets and indicating mechanism.]
~Ques. How is the flux produced in the alternating current form of
Faraday disc meter?~
Ans. By the secondary current of a series transformer.
~Frequency Indicators.~--A frequency indicator or meter is an
instrument used for determining the frequency, or number of cycles per
second of an alternating current. There are several forms of frequency
indicator, whose principle of operation differs, and according to
which, they may be classed as
1. Synchronous motor type;
2. Resonance type;
3. Induction type.
[Illustration: FIG. 2,561.--Circuit diagram of ~simple shunt~ Sangamo
~ampere hour meter~. It is rated at 10 amperes, larger currents being
measured by using shunts. ~In operation~, the main or line current to
be measured passes through the shunt, while a part proportional to the
drop across the shunt, is shunted through the meter and measured. The
only effect of reversing the current will be to reverse the direction
of rotation of the meter. In battery installations it is never possible
to take the same number of ampere hours from a battery as are put into
it, hence, if the simple shunt ampere hour meter be used for repeated
and successive charges and discharges, it will be necessary to reset
the pointer to zero each time the battery is fully charged. When the
shunt meter is equipped with a charge stopping device, the pointer is
reset while charging, to allow for a predetermined overcharge.]
[Illustration: FIG. 2,562.--Circuit diagram of Sangamo ~differential
shunt type ampere hour meter~ _for use in battery charging_. Since
a battery absorbs more energy on charge than it will give out on
discharge, at its working voltage, it is usually given a certain
amount of overcharge. This makes desirable a meter that automatically
allows for the proper amount of overcharge. Such a meter indicates at
all times the amount of electricity available for useful work without
resetting the pointer every time the battery is charged. In other
words, the battery and the meter would keep in step for considerable
periods of time without readjustment. The Sangamo differential
shunt meter is designed to meet these requirements, and it consists
essentially of a Sangamo meter with two shunts connected as shown. The
relative value of shunt resistance is adjustable by means of slider
G, so that the meter can be made to run slow on charge or fast on
discharge, whichever may be desired. The usual method is to allow the
meter to register less than the true amount on charge and the exact
amount on discharge, the difference representing the loss in the
battery, or the overcharge. If the meter be provided with a charge
stopping device, the battery can be given an amount of overcharge
predetermined by the setting of the slider G. Therefore the amount of
overcharge can be fixed in advance by a skilled man and the actual
charging done by any unskilled person, since all there is to do is to
make the connection.]
~Ques. How is a synchronous motor employed as a frequency indicator?~
Ans. A small synchronous motor is connected in the circuit of the
current whose frequency is to be measured. After determining the
revolutions per minute by using a revolution counter, the frequency is
easily calculated as follows:
_frequency_ = (_revolutions_ ~per second~ × _number of poles_) ÷ _2_.
[Illustration: FIGS. 2,563 and 2,564.--Frahm ~resonance type frequency
meter~. Fig, 2,563, portable meter; fig. 2,564, switchboard meter. The
readings are correct in either the vertical or horizontal position.
~The energy consumption~ at 100 volts is about 1 to 2 volt amperes,
and is approximately proportional for other pressures. The regular
portable meters are arranged for pressures of from 50 to 300 volts,
and for this purpose they are fitted with terminals for 65, 100, 130,
180, and 250 volts. In order to obtain full amplitude at intermediate
pressures, a milled headed screw is provided for adjusting the base
piece mechanically, and thereby permitting of regulating the pressure
range within ±20 per cent; this insures indications of maximum
clearness. Should it be desired to extend the standard pressure range
of 50 to 300 volts, up to 600 volts, two further terminals for 350 and
500 volts are necessary, so that these instruments are provided with
eight fixed terminals in addition to the mechanical regulating device.
Instruments which are intended for connecting to one specific supply
or to the secondary of a pressure transformer, require only a single
pressure range, say 100 volts, with the aforementioned regulating
device. ~The frequency range~ is from 7.5 to 600 cycles per second. In
order to obtain easily readable indications, one reed is provided for
every quarter period for frequencies below 30, for every half period
for frequencies between 30 and 80, and for every whole period for
frequencies between 80 and 140. The use of a smaller number of reed,
that is to say, of larger intervals between the periods of vibration of
adjoining reeds, is only recommended for circuits having very variable
frequencies, as otherwise no reed might respond to the vibrations
caused by intermediate frequencies. The arrangement of the separate
reeds on a common base piece, permits supplying any combination of
interval that may be required. It is often desirable to secure two
ranges with one set of reed. To do this a second electromagnet is
supplied. It is polarized, and operates on the same base plate. In the
case of alternating current when the unpolarized magnet is used the
reeds receive two impulses during each cycle, while with the polarized
magnet they receive but one impulse per cycle. A commutator is provided
to easily make the change from one range to the other. If there be
two sets of reed, the one commutator may be connected to change both.
This device is only applicable when alternating current is measured.
Instruments with unpolarized magnets are made with frequencies of 15 to
300 cycles per second.]
~Ques. Describe the resonance method of obtaining the frequency.~
Ans. In construction, the apparatus consists of a pendulum, or reed,
of given length, which responds to periodic forces having the same
natural period as itself. The instrument comprises a number of reeds of
different lengths, mounted in a row, and all simultaneously subjected
to the oscillatory attraction of an electromagnet excited by the supply
current that is being measured. The reed, which has the same natural
time period as the current will vibrate, while the others will remain
practically at rest.
[Illustration: FIGS. 2,565 and 2,566.--Side and end views of Frahm
resonance type frequency meter reeds. Owing to the principle employed
in the meter it is evident that the indications are independent of the
voltage, change of wave form, and external magnetic fields.]
The construction and operation of the instrument may be better
understood from figs. 2,565 and 2,566, which illustrates the
indicating part of the Frahm meter. This consists of one or
more rows of tuned reeds rigidly mounted side by side on a
common and slightly flexible base.
The reeds are made of spring steel, 3 or 7 mm. wide, with a
small portion of their free ends bent over at right angles as
shown in fig. 2,566 and enameled white so that when viewed end
on they will be easily visible. The reeds are of adjustable
length, and are weighted at the end.
A piece of soft iron, rigidly fastened on the base plate which
supports the reeds, forms the armature of a magnet.
When the magnet is excited by alternating current, or
interrupted direct current, the armature is set in vibration,
and that gives a slight movement to the base plate at right
angles to its axis, thereby affecting all the reeds, especially
those which are almost in tune with its vibrations.
The reed which is in tune will vibrate through an arc of
considerable amplitude, and so indicate the frequency of the
exciting current.
~Ques. For what use is the resonance type of frequency meter most
desirable?~
Ans. For laboratory use.
[Illustration: FIG. 2,567.--Westinghouse induction type frequency
meter. The normal frequency is usually at the top of the scale to
facilitate reading. The damping disc moves in a magnetic field, thus
damping by the method of eddy currents. The standard meters are
designed for circuits of 100 volts nominal and can be used for voltages
up to 125 volts. For higher voltages, transformers with nominal 100
volt secondary should be used.]
~Ques. Describe the induction type of frequency meter.~
Ans. It consists of two voltmeter electromagnets acting in opposition
on a disc attached to the pointer shaft. One of the magnets is in
series with an inductance, and the other with a resistance, so that any
change in the frequency will unbalance the forces acting on the shaft
and cause the pointer to assume a new position, when the forces are
again balanced. The aluminum disc is so arranged that when the shaft
turns in one direction the torque of the magnet tending to rotate it
decreases, while the torque of the other magnet increases. The pointer
therefore comes to rest where the torques of the two magnets are equal,
the pointer indicating the frequency on the scale.
[Illustration: FIG. 2,568.--Langsdorf and Begole frequency meter. The
operation of this meter is based on the fact that if an alternating
pressure of E Volts be impressed on a condenser of capacity C, in
farads, the current in amperes will be equal to 2π ~ EC, provided the
pressure be constant. ~In construction~, the scale is mounted on the
same axis as the pressure coil, across the mains so as to render the
instrument independent of variation of voltage. For a discussion of
this meter, see Electrical Review, vol. LVIII, page 114.]
[Illustration: FIG. 2,569.--General Electric horizontal edgewise,
induction type frequency indicator. It is provided with an external
inductance and resistance placed in a ventilated cage for mounting
on the back of the switchboard. Means are provided for adjusting the
instrument for the characteristics of the circuit on which it is
installed. Standard instruments are wound for 100 to 125 volt circuits
only, but can be wound for circuits up to and including 650 volts.
Instruments for use on circuits in excess of 650 volts are always
provided with pressure transformers. The normal operating point is
marked at approximately the center of the scale, thus giving the
advantage of very open divisions. The standard frequencies are 25, 40,
60,125 and 133.]
~Ques. What is the object of the aluminum disc?~
Ans. Its function is to damp the oscillations of the pointer.
[Illustration: FIG. 2,570.--Westinghouse rotary type of synchroscope or
synchronism indicator. The indication is by means of a pointer which
assumes at every instant a position corresponding to the phase angle
between the pressures of the busbars and the incoming machine, and
therefore rotates when the incoming machine is not in synchronism. The
direction of rotation indicates whether the machine be fast or slow,
and the speed of rotation depends on the difference in frequency. The
pointer is continuously visible, during both the dark and light periods
of the synchronizing lamps.]
~Synchronism Indicators.~--These devices, sometimes called
synchroscopes, or synchronizers _indicate the exact difference in phase
angle at every instant_, and the difference in frequency, between an
incoming machine and the system to which it is to be connected, so that
the coupling switch can be closed at the proper instant. There are
several types of synchronizer, such as
1. Lamp or voltmeter;
2. Resonance or vibrating reed;
3. Rotating field.
The simplest arrangement consists of a lamp or preferably a voltmeter
connected across one pole of a two pole switch connecting the incoming
machine to the busbars, the other pole of the switch being already
closed.
If the machines be out of step, the lamps will fluctuate in brightness,
or the voltmeter pointer will oscillate, the pulsation becoming
less and less as the incoming machine approaches synchronous speed.
Synchronism is shown by the lamp remaining out, or the voltmeter at
zero.
[Illustration: FIG. 2,571.--General Electric synchronism indicator.
~The synchronism indicator~ _is a motor whose field is supplied with
single phase current from one of the machines to be synchronized, and
its armature from the other_. The armature carries two inductance
coils placed at a large angle, one supplied through a resistance, the
other through an inductance. This arrangement generates a rotating
field in the armature, while the stationary field is alternating. The
armature tends to assume a position where the two fields coincide when
the alternating field passes through its maximum; hence, the armature
and pointer move forward or backward at a rate corresponding to the
difference of frequency, and the position when stationary depends
on the phase relation. When the machines are running at the same
frequency and in phase the pointer is stationary at the marked point.
~In construction~, it is like a small, two phase, bipolar synchronous
motor, the field being supplied with alternating instead of direct
current. _The armature_ is mounted in ball bearings in order to make
it sufficiently sensitive and smooth in operating. The armature coils
are not exactly 90 degrees apart, since it is not possible to get the
current in the two coils exactly in quadrature without introducing
condensers on other complicated construction. _Standard ratings_
are for 110 and 220 volt circuits. _Synchronism indicators should
be ordered for the frequency of the circuit on which they are to be
operated_, although the instruments may be used on circuits varying
10 per cent to 15 per cent from the normal. The words "~Fast~" and
"~Slow~" on the dial indicate that the frequency on binding posts E
and F is respectively higher or lower than that on A and B; or, in
other words, clockwise rotation of the pointer means that the incoming
machine is running at too high speed, counter clockwise rotation
indicating too low speed.]
~Ques. How does the resonance type of synchronism indicator operate?~
Ans. On the same principle as the resonance type of frequency
indicator, already described.
~Ques. What is the principle of the rotating field type of synchronism
indicator?~
Ans. Its operation depends on the production of a rotating field by
the currents of the metered circuits in angularly placed coils, one
for each phase in the case of a polyphase indicator. In this field is
provided a movable iron vane or armature, magnetized by a stationary
coil whose current is in phase with the voltage of one phase of the
circuit. As the iron vane is attracted or repelled by the rotating
field, it takes up a position where the zero of the rotating field
occurs at the same instant as the zero of its own field. In the single
phase meter the positions of voltage and current coils are interchanged
and the rotating field is produced by means of a split phase winding
connected to the voltage circuit.
[Illustration: FIG. 2,572.--General Electric external resistance and
inductance for 110 volt synchronism indicator. Both the resistance and
inductance are intended to be placed behind the switchboard.]
[Illustration: FIGS. 2,573 to 2,576.--Connections of General Electric
synchronism indicator. Fig. 2,573, connections with grounded
secondaries on pressure transformers; fig. 2,574, connections
with ungrounded secondaries on pressure transformers; fig. 2,575,
connections for 200 to 240 volt circuits, with six point receptacles;
fig. 2,576, connections for checking location of needle. ~The various
letters referred to~ in the diagrams will be found marked on the ends
of the instrument studs and back of reactance coil box. It is important
that the instrument be connected in circuit in the proper manner so
that the needle will come to the mark on the upper part of the scale
when synchronism is obtained. In case the pointer become moved or a
change in its position be necessary, it is advisable to make a check
on the indication before relocating the needle. ~This test can be made
as follows~: Connect together (fig. 2,573) studs marked B and E and
connect stud A to terminal F on the external reactance box. When these
connections are made, the instrument can be connected to a single phase
circuit of normal voltage and if the instrument be correct, the pointer
will stand vertically at the point of synchronism. If it do not, the
needle can be moved and should be fastened in the correct position. The
synchronizing lamps when connected as illustrated in the diagrams, show
dark when synchronism is reached. This is the only connection possible
when grounded secondaries are used, as in fig. 2,573, and for the high
voltage indicators when used as in fig. 2,575, but with ungrounded
secondaries (fig. 2,574) the lamps may be connected as indicated, when
they will show bright at the moment of synchronism. The connections to
the synchronism indicator remain the same as before.]
~Power Factor Indicators.~--Meters of this class indicate the phase
relationship between pressure and current, and are therefore sometimes
called _phase indicators_. There are two types:
1. Wattmeter type;
2. Disc, or rotating field type.
[Illustration: FIG. 2577.--General Electric synchronizing receptacle
and plug for use with synchronism indicator.]
[Illustration: FIG. 2,578.--Westinghouse rotating field type power
factor meter. The rotating field is produced by the currents of the
metered circuits in angularly placed coils, one for each phase of the
system, in the case of polyphase meters. In the three phase meter the
rotating field is produced by three coils spaced 60° apart; in the
two phase meter by two coils spaced 90°; in the single phase meter
the positions of voltage and current coils are interchanged and the
rotating field is produced by means of a split phase winding, connected
to the voltage circuit. There are no movable coils or flexible
connections. Single phase meters indicate the power factor of a single
phase circuit, or of one branch of any polyphase circuit. Special
calibration is necessary in order to use a single phase instrument on a
three phase circuit unless the voltage coil be connected from one line
to the neutral. Polyphase meters indicate the average angle between the
currents and voltages and are superior for polyphase service to meters
having only one current coil.]
In the wattmeter type, the phase relation between the pressure
and the current fluxes is such that on a non-inductive load the
torque is zero.
For instance, in a dynamometer wattmeter, the pressure circuit
is made highly inductive and the instrument then indicates
_volts × amperes × sin φ_ instead of _volts × amperes × cos φ_,
that is to say, it will indicate the wattless component of the
power. A dynamometer of this type is sometimes called an idle
current wattmeter.
[Illustration: FIG. 2,579.--Single phase power factor meter of the
rotating field or disc type.]
~Ques. Describe a single phase power factor meter of the disc or
rotating field type.~
Ans. It consists of two pressure coils, as shown in fig. 2,579,
placed at right angles to each other, one being connected through a
resistance, and the other through an inductance so as to "split" the
phase and get the equivalent of a rotating magnetic field.
The coils are placed about a common axis, along which is
pivoted an iron disc or vane. The magnetizing coils FG are in
series with the load. If the load be very inductive, the coil M
experiences very little torque and the system will set itself
as shown in the figure. As the load becomes less inductive, the
torque on S decreases and on M increases so that the system
takes up a particular position for every angle of lag or lead.
~Ground Detectors.~--Instruments of this name are used for detecting
(and sometimes measuring) the leakage to earth or the insulation of a
line or network and are sometimes called _ground or earth indicators,
or leakage detectors_.
For systems not permanently earthed anywhere, these instruments
are nearly all based on a measurement of the pressure
difference between each pole and earth, two measurements being
required for two wire systems, and three for three wire,
whether direct current single phase, or polyphase alternating
current. ~In the case of direct current~ systems, the
insulation, both of the network and of the individual lines,
can be calculated from the readings, ~but with alternating
current~, the disturbance due to capacity effects is usually
too great. ~In any case~, however, the main showing the
smallest pressure difference to earth must be taken as being
the worst insulated.
For low tension systems ~moving coil~ (_for alternating
current_) or ~moving iron~ instruments (_for direct current_)
are the most used, while for high tension systems electrostatic
voltmeters are to be preferred. ~On systems having some point
permanently earthed~ at the station, as for instance the
_neutral wire_ of direct current system, or the neutral point
of a three phase system, an ammeter connected in the _earth
wire_ will serve as a rough guide. It should indicate no
current so long as the insulation is in a satisfactory state,
but on the occurrence of an earth it will at once show a
deflection. The indications are, however, often misleading, and
serve more as a warning than anything else.
[Illustration: FIG. 2,580.--Westinghouse single phase electrostatic
ground detector.]
[Illustration: FIG. 2,581.--Westinghouse three phase electrostatic
ground detector.]
[Illustration: FIG. 2,582.--Wallis-Jones ~automatic earth leakage cut
out~. It is _an instrument which so protects a direct current circuit
that the circuit is broken whenever a leak occurs from either main to
earth, and so that the circuit cannot be permanently re-established
until the leak has been removed_. The instrument and its connections
may be explained by the aid of the accompanying diagram, in which
T₁ and T₂ represent the points of connection from the mains, and T₃
and T₄, the points of connection to the circuit to be protected. So
S₂, and S₃ will preferably be ordinary tumbler switches, but they
are diagrammatically represented as plain bar switches, their fixed
contacts being diagrammatically represented by dotted circles. When the
three switches S₁, S₂, and S₃ are closed, the current passes from T₁ to
T₃ through the small resistance R₁, through circuit L to T₄, and back
through the resistance R₂ to T₂. In shunt with R₁ and R₂, are the two
moving coils C and C₂, working in the magnetic field of the magnets NS,
NS, and rigidly fixed on one spindle, which is broken electrically by
an ebonite block E. The points of connection to the shunts are adjusted
so that when the same current passes out through one and back through
the other, the effect on the two coils is equal and opposite, and there
is thus no movement. Should, however, any minute portion of the current
through R₁ leak to earth instead of returning via R₂, the balance is
disturbed, C becomes stronger than C₂, the system is deflected, and a
contact is made by the arm A at B, no matter in which direction the
coils deflect. The system is similarly deflected for a leak on the
other lead. In the diagram these contacts are shown at right angles to
their normal plane. As soon as the contact is made, the electromagnet
M is energized, the arm of S₁ is released and the spring at once pulls
it off its contact, at the same time breaking S₂. The positions of the
blades when the switches are open are shown dotted. The only means
the user has of closing the circuit is by putting on S₃ by the handle
H, which is outside the locked box. The first effect of putting on S₃
is to break its circuit; it then by means of the slotted bar P begins
to pull on S₂ and S₁, which can thus be closed again, and held closed
by the trigger as before. The circuit, is, however, still broken till
S₂ is pushed back. Then if the leak be still on, the slot in P allows
S₁ and S₂ to open at once as before. It is therefore impossible to
keep the circuit closed while the leak exists. The working condition
of the instrument can be tested at any time by switching a lamp on in
the circuit and depressing one of the keys K₁, K₂. This short circuits
R₁ or R₂, throws the coils out of balance, and the switch opens. The
contact arm is closed in an inner dust tight case, and it will be noted
that it _makes_ contact only; the _break_ occurs at the switches, thus
avoiding any sparking. Since the two coils work in the two gaps of
one and the same field, changes in the strength of the magnets have
no effect, the apparatus is enclosed in a locked metallic box, and
the only part to which the user has access is the handle H, and, if
desired, the testing keys K₁ K₂.]
HAWKINS PRACTICAL LIBRARY OF
ELECTRICITY
IN HANDY POCKET FORM PRICE $1 EACH
* * * * *
_They are not only the best, but the cheapest work published on
Electricity. Each number being complete in itself. Separate numbers
sent postpaid to any address on receipt of price. They are guaranteed
in every way or your money will be returned. Complete catalog of series
will be mailed free on request._
~ELECTRICAL GUIDE, NO. 1~
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.
~ELECTRICAL GUIDE, NO. 2~
The construction of Dynamos, Motors, Armatures, Armature
Windings, Installing of Dynamos.
~ELECTRICAL GUIDE, NO. 3~
Electrical Instruments, Testing, Practical Management of
Dynamos and Motors.
~ELECTRICAL GUIDE, NO. 4~
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,
Storage Batteries.
~ELECTRICAL GUIDE, NO. 5~
Principles of Alternating Currents and Alternators.
~ELECTRICAL GUIDE, NO. 6~
Alternating Current Motors, Transformers, Converters,
Rectifiers.
~ELECTRICAL GUIDE, NO. 7~
Alternating Current Systems, Circuit Breakers, Measuring
Instruments.
~ELECTRICAL GUIDE, NO. 8~
Alternating Current Switch Boards, Wiring, Power Stations,
Installation and Operation.
~ELECTRICAL GUIDE, NO. 9~
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
~ELECTRICAL GUIDE, NO. 10~
Modern Practical Applications of Electricity and Ready
Reference Index of the 10 Numbers.
~Theo. Audel & Co., Publishers. 72 FIFTH AVENUE, NEW YORK~
TRANSCRIBER'S NOTES
Silently corrected simple spelling, grammar, and typographical
errors.
Retained anachronistic and non-standard spellings as printed.
Enclosed italics markup in _underscores_.
Enclosed bold markup in ~tildes~.
*** End of this LibraryBlog Digital Book "Hawkins Electrical Guide v. 7 (of 10) - Questions, Answers, & Illustrations, A progressive course - of study for engineers, electricians, students and those - desiring to acquire a working knowledge of electricity and - its applications" ***
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