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Title: Hawkins Electrical Guide v. 6 (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. 6 (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
SIX
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, 1914,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS
GUIDE NO. 6.
~ALTERNATING CURRENT MOTORS~ 1,267 to 1,376
Classification--~synchronous motors~--essential
parts--_synchronous motor principles:_ condition for
starting; effective pressure; dead centers; speed; limit
of lag; effect of load changes--effect of altering the
field strength--disadvantages of synchronous motors;
advantages--~the "V" curve~--adaptation--efficiency--~hunting
of synchronous motors;~ mechanical analogy--use as
condenser--surging--characteristics of synchronous motors:
starting; running; stopping; effect upon circuit; power
factor; auxiliary apparatus; adaptation--~induction
(asynchronous) motors~--essential parts--types--_oscillating
magnetic field--rotating magnetic field_--operation of
single phase motor; why not self starting; provision for
starting--operation of polyphase induction motor; why called
asynchronous--speed; classification according to speed--the
terms _primary_ and _secondary_--why polyphase induction
motors are explained before single phase--~polyphase induction
motors~--features--essential parts--_principles_--production
of rotating field--Tesla's rotating field--method of
obtaining resultant flux of Tesla's field--_Arago's
rotations;_ explanation--Faraday's experiment--production
of two phase rotating field; resultant poles--six and
eight pole two phase rotating fields--_physical conception
of two phase rotating field_--production of three phase
rotating field; with ring winding--_physical conception
of three phase rotating field_--three phase six pole
winding--~slip~--_copper cylinder illustrating principle of
operation of induction motor_--calculation of slip--table
of synchronous speeds--variation of slip; why so small;
variation with load; table of variation--sector method of
measuring slip--_evolution of the squirrel cage armature;_
construction--~the field magnets;~ parts; construction--~field
windings for induction motors~--calculation for
revolutions of rotating field; objection to high speed of
field--difficulty with low frequency currents--general
character of field winding--formation of poles--grouping of
coils--_starting of induction motors:_ external resistance,
auto-transformer, internal resistance methods--~internal
resistance induction motors;~ adaptation--how resistance
is cut out--why not desirable for large sizes--~external
resistance or slip ring motors~--operation--armature
connections--~single phase induction motors~--service
suitable for--disadvantage--parts--why not self-starting--how
started--~phase splitting; production of rotating field
from oscillating field~--methods--starting coils--~shading
coils~--character of the starting torque--modification of
armature for starting with heavy load--clutch type of single
phase induction motor; its action in starting--~commutator
motors~--classification--_action of closed coil rotating
in alternating field_--the transformer pressure--generated
pressure--self-induction pressure--local armature currents;
reason for sparking; how reduced--high resistance
connectors--effect of low power factor--effect of
frequency--~series motor~--features--adaptation--~neutralized
series motor~--conductive method--inductive method--~shunt
motors--repulsion motors~--difficulty with early motors--means
employed to stop sparking--~essentials of single repulsion
motors~--the term repulsion induction motor--compensated
repulsion motor--~power factor of induction motors~--its
importance--false ideas in regard to power factor--~speed and
torque of motors~.
~TRANSFORMERS~ 1,377 to 1,456
Their use--essential parts--~basic principles~--the primary
winding--the secondary winding--_magnetic leakage_--~the
induced voltage~--no load current--magnetizing current--action
of transformer with load--classification--~step up
transformers~--use--construction--copper economy--~step
down transformers~--use--construction--~core
transformers~--construction--advantages--~shell
transformers~--comparison of core and shell
types--choice--~combined core and shell transformers~--economy
of construction--~single and polyphase transformers~--features
of each type--choice of types for polyphase
currents--operation of three phase transformer with one phase
damaged--~transformer losses~--~hysteresis~--what governs
the loss--how reduced--~eddy currents~--lamination--thickness
of laminæ--importance of iron losses--how to reduce
iron losses--~copper losses~--how caused--effect
on power factor--effect of resistance--~cooling of
transformers~--cooling mediums employed--heating of
transformers--objection to heating--~dry transformers~--~air
cooled transformers~--natural draught type--forced
draught or air blast type--construction of coils for air
cooling--requirements with respect to air supply--quantity
of air used--~oil cooled transformers~--circulation of
the oil--action of the oil--objection to oil--kind of
oil used--oil requirements--moisture in oil--~water
cooled transformers~--internal coil type--external coil
type--_thermo-circulation_--quantity of circulating
water required--~transformer insulation~--the
~"major"~ and ~"minor" insulation~--mica--outdoor
transformers for irrigation service--oil insulated
transformers--efficiency of transformers--efficiency
curve--~all day efficiency of transformers~--transformer fuse
blocks--auto-transformers--~constant current transformers
for series arc lighting~; elementary diagram illustrating
principles--regulation--transformer connections--single phase
connections--combining transformers--precautions--operating
secondaries in parallel--connections for different
voltages--precautions--two phase connections--~three phase
connections:~ delta, star, delta star, star-delta--comparison
of star and delta connections--three phase
transformers--comparison of air blast, water cooled, and oil
cooled transformers--standard transformer connections--~how
to test transformers~--transformer operation with grounded
secondary--transformer capacity for motors--transformer
connections for motors--arc lamp transformer--transformer
installed on pole--static booster or regulating transformer.
~CONVERTERS~ 1,457 to 1,494
Where used--kinds of converter--A.I.E.E.
classification--~rotary
converters~--operation--speed--principles--relation
between input and output pressures--single and polyphase
types--advantage of polyphase converters--armature
connections of polyphase converter--pressure
relation--voltage variation--advantage of unity power
factor--effect of field too strong--~compounding of
rotary converters~--~ratio of conversion~--~voltage
regulation~--split pole method--regulating pole method--best
location of regulating poles--~reactance method~--~multi-tap
transformer method~--~synchronous booster method~--winding
connections--field connections--adaptation--~motor generator
sets~--classification--standard practice--behavior of
rotary when hunting; comparison with motor generator
sets--racing--~frequency changing sets~--~parallel operation
of frequency changers~--~cascade converter~--speed--action in
motor armature winding--advantages--how started--comparison of
cascade converter with synchronous converter.
~RECTIFIERS~ 1,495 to 1,530
Classification--~mechanical rectifiers~--essential
features--construction--application--~electrolytic
rectifiers~--principles of operation--Mohawk rectifier--the
term "_valve_"--metals for electrodes--electrolyte--Nodon
valve--Audion valve--Buttner valve--Churcher valve--De Faria
valve--Fleming oscillation valve--Grisson valve--Pawlowski
valve--Giles electric valve--Buttner valve--~mercury vapor
rectifiers~--principles--the terms "arc" and "vapor"--three
phase mercury vapor rectifier--construction--auxiliary
apparatus--series mercury arc rectifier--dissipation
of heat from bulb--replacement of bulb--advantages of
rectifier--precautions in installing--~electromagnetic
rectifiers~--construction and operation.
CHAPTER LI
ALTERNATING CURRENT MOTORS
The almost universal adoption of the alternating current system of
distribution of electrical energy for light and power, and the many
inherent advantages of the alternating current motor, have created the
wide field of application now covered by this type of apparatus.
As many central stations furnish only alternating current, it
has become necessary for motor manufacturers to perfect types of
alternating current motor suitable for all classes of industrial
drive and which are adapted for use on the kinds of alternating
circuit employed. This has naturally resulted in a multiplicity of
types and a classification, to be comprehensive, must, as in the
case of alternators, divide the motors into groups as regarded from
several points of view. Accordingly, alternating current motors may be
classified:
1. With respect to their principle of operation, as
_a._ SYNCHRONOUS MOTORS;
_b._ ASYNCHRONOUS MOTORS:
~1. Induction motors;~
{_series;_
~2. Commutator motors~ {_compensated;_
{_shunt;_
{~repulsion~.
2. With respect to the current as
_a. Single phase;
b. Polyphase;_
[Illustration: FIGS. 1,585 to 1,588.--Synchronous motor principles: I.
_A single phase synchronous motor is not self-starting._ The figures
show an elementary alternator and an elementary synchronous motor,
the construction of each being identical as shown. If the alternator
be started, during the first half of a revolution, beginning at the
initial position ABCD, fig. 1,585, current will flow in the direction
indicated by the arrows, passing through the external circuit and
armature of the motor, fig. 1,586, inducing magnetic poles in the
latter as shown by the vertical arrows. These poles are attracted
by unlike poles of the field magnets, which tend to turn the motor
armature in a counter-clockwise direction. Now, ~before~ _the torque
thus set up has time to_ ~overcome the inertia of the motor armature~
_and cause it to rotate, the alternator armature has completed the
half revolution,_ and beginning the second half of the revolution, as
in fig. 1,587, _the current is reversed_ and consequently _the induced
magnetic poles in the motor armature are reversed also_. This tends to
rotate the armature in the reverse direction, as in fig. 1,588. _These
reversals of current occur with such frequency that the force_ ~does
not act long enough~ _in either direction_ ~to overcome the inertia~
_of the armature; consequently it remains at rest_, or to be exact,
~it vibrates~. Hence, a single phase synchronous motor must be started
by some external force and brought up to a speed that gives the same
frequency as the alternator before it will operate. A single phase
synchronous motor, then, is not self-starting, which is one of its
disadvantages; the reason it will operate after being speeded up to
synchronism with the alternator and then connected in the circuit is
explained in figs. 1,589 to 1,592.]
3. With respect to speed, as
_a._ Constant speed;
_b._ Variable speed.
4. With respect to structural features, as
_a._ Enclosed;
_b._ Semi-enclosed;
_c._ Open;
_d._ Pipe ventilated;
_e._ Back geared;
_f._ Skeleton frame;
_g._ Riveted frame;
_h._ Ventilated; etc.
Of the above divisions and sub-divisions some are self-defining and
need little or no explanation; the others, however, will be considered
in detail, with explanations of the principles of operation and
construction.
~Synchronous Motors.~--The term "synchronous" means _in unison_, that
is, _in step_. A so called synchronous motor, then, as generally
defined, _is one which rotates in unison or in step with the phase of
the alternating current which operates it_.
_Strictly speaking, however, it should be noted that this
condition of operation is only approximately realized as will
be later shown._
Any single or polyphase alternator will operate as a synchronous
motor when supplied with current at the same pressure and frequency
as it produces as a generator, the essential condition, in the case
of a single phase machine, being that it be speeded up to so called
synchronism before being put in the circuit.
In construction, synchronous motors are almost identical with the
corresponding alternator, and consist essentially of two elements:
1. An armature,
2. A field.
[Illustration: FIGS. 1,589 to 1,592.--Synchronous motor principles: II.
_The condition necessary for synchronous motor operation is that the
motor be speeded up until it rotates in synchronism, that is, in step
with the alternator._ This means that the motor must be run at the same
frequency as the alternator (not necessarily at the same speed). In the
figures it is assumed that the motor has been brought up to synchronism
with the alternator and connected in the circuit as shown. In figs.
1,589 and 1,590 the arrows indicate the direction of the current for
the armature position shown. The current flowing through the motor
armature induces magnetic poles which are attracted by the field poles,
thus producing a torque in the direction in which the armature is
rotating. After the alternator coil passes the vertical position, the
current reverses as in fig. 1,591, and the current flows through the
motor armature in the opposite direction, thus reversing the induced
poles as in fig. 1,592. _This brings_ ~like~ _poles near each other,
and since the motor coil has rotated beyond the vertical position_ ~the
repelling action~ _of the_ ~like poles~, _and also_ ~the attraction of
unlike poles~, _produces a torque acting in the direction in which the
motor is rotating_. Hence, when the two armatures move synchronously,
the torque produced by the action of the induced poles upon the field
poles is always in the direction in which the motor is running, and
accordingly, tends to keep it in operation.] either of which may
revolve. The field is separately excited with direct current.
[Illustration: FIGS. 1,593 and 1,594.--Synchronous motor principles:
III. _The current which flows through the armature of a synchronous
motor is that due to the_ ~effective~ _pressure_. Since the motor
rotates in a magnetic field, a pressure is induced in its armature in a
direction opposite to that induced in the armature of the alternator,
and called the _reverse pressure_, as distinguished from the pressure
generated by the alternator called the _impressed pressure_. _At any
instant, the pressure available to cause current to flow through
the two armatures, called the_ ~effective pressure~, _is equal to
the difference between the pressure generated by the alternator or_
~impressed pressure~ _and the_ ~reverse pressure~ _induced in the
motor_. Now if the motor be perfectly free to turn, that is, without
load or friction, the reverse pressure will equal the impressed
pressure and no current will flow. This is the case of real synchronous
operation, that is, not only is the frequency of motor and alternator
the same, but the coils rotate without phase difference. In figs. 1,593
and 1,594, the impressed and reverse pressures are represented by the
dotted arrows Pᵢ and P_{_r_}, respectively. Since in this case these
opposing pressures are equal, the resultant or effective pressure is
zero; hence, there is no current. _In actual machines this condition is
impossible_, because even if the motors have no external load, there
is always more or less friction present; hence, in operation there
must be more or less current flowing through the motor armature to
induce magnetic poles so as to produce sufficient torque to carry the
load. The action of the motor in automatically adjusting the effective
pressure to suit the load is explained in figs. 1,595 and 1,596.]
The principles upon which such motors operate may be explained by
considering the action of two elementary alternators connected in
circuit, as illustrated in the accompanying illustrations, one
alternator being used as a generator and the other as a synchronous
motor.
Suppose the motor, as in figs. 1,585 and 1,586, be at rest when it is
connected in circuit with the alternator. The alternating current will
flow through the motor armature and produce a reaction upon the field
tending to rotate the motor armature first in one direction, then in
another.
[Illustration: FIGS. 1,595 and 1,596.--Synchronous motor principles:
IV--_A synchronous motor adjusts itself to changes of load by changing
the phase difference between current and pressure._ If there be no
load and no friction, the motor when speeded up and connected in the
circuit, will run in true synchronism with the alternator, that is, at
any instant, the coils A B C D and A°B°C°D° will be in parallel planes.
When this condition obtains, no current will flow and no torque will
be required (as explained in figs. 1,593 and 1,594). If a load be put
on the motor, the effect will be to cause A°B°C°D° to lag behind the
alternator coil to some position A"B"C"D" and current to flow. The
reverse pressure will lag behind the impressed pressure equally with
the coil, and the current which has now started will ordinarily take an
intermediate phase so that it is ~behind~ _the impressed pressure but_
~in advance~ _of the reverse pressure_. These phase relations may be
represented in the figure by the armature positions shown, viz.: 1, the
synchronous position A°B°C°D° representing the impressed pressure, 2,
the intermediate position A'B'C'D', the current, 3, the actual position
A"B"C"D" (corresponding to mechanical lag), the reverse pressure.
From the figure it will be seen that the current phase represented
by A'B'C'D' is in advance of the reverse pressure phase represented
by A"B"C"D". Hence, by ~armature reaction~, _the current leading
the reverse pressure weakens the motor field and reduces the reverse
pressure_, thus establishing equilibrium between current and load.
As the load is increased, the mechanical lag of the alternator coil
becomes greater and likewise the current lead with respect to the
reverse pressure, which intensifies the armature reaction and allows
more current to flow. In this way equilibrium is maintained for
variations in load within the limits of zero and 90° mechanical lag.
The effect of armature reaction on motors is just the reverse to its
effect on alternators, which results in marked automatic adjustment
between the machines especially when a single motor is operated from
an alternator of about the same size. In other words, the current
which weakens or strengthens the motor field, strengthens or weakens
respectively the alternator field as the load is varied.]
_Because of the very rapid reversals in direction of the torque thus
set up, there is not sufficient time_ ~to overcome the inertia of
the armature~ _before the current reverses and produces a torque in
the opposite direction_, _hence, the armature remains stationary or,
strictly speaking_, ~it vibrates~.
[Illustration: FIGS. 1,597 and 1,598.--Synchronous motor principles:
V. _The effectiveness of armature reaction in weakening the field
is proportional to the_ ~sine of the angle~ _by which the current
lags behind the impressed pressure._ If a motor be without load or
friction, its armature will revolve synchronously (in parallel planes)
with the alternator armature. In the figures let ABCD represent an
instantaneous position of the motor armature when this condition
obtains; it will then represent the phase relationship of impressed
and reverse pressures for the same condition of no load, no friction,
operation. _Now_, ~if a light load be placed on the motor~ for the
same instantaneous position of alternator armature, ~the motor coil
will drop behind to some position as~ A", fig. 1,597 (part of the
coil only being shown). The reverse pressure will also lag an equal
amount and its phase with respect to the impressed pressure will be
represented by A". The armature current will ordinarily take an
intermediate phase, represented by coil position A'B'C'D', inducing a
field strength corresponding to the 9 lines of force OF, O'F', etc.
_The current being_ ~in advance~ _of the phase of the reverse pressure_
A", _the armature reaction_ ~weakens~ _the field_, thus reducing the
reverse pressure and allowing the proper current to flow to balance
the load. The amount by which the field is weakened may be determined
by resolving the induced magnetic lines OF, O'F', O"F", etc., into
components OG, GF, O'G', G'F', O"G", G"F", etc., respectively
parallel and at right angles to the lines of force of the main field.
Of these components, the field is weakened only by OG, O'G', O"G",
etc. Since by construction, angle OFG = AOA', and calling OF unity
length, OG = sine of angle by which the current lags behind the
impressed pressure. The construction is shown better in the enlarged
diagram. For a heavier load the armature coil will drop back further to
some position as A"', fig. 1,598, and the lag of the current increase
to some intermediate phase as A"B"C"D". By similar construction it
is seen that the component OG (fig. 1,597) has increased to OJ (fig.
1,598), this component thus further weakening the main field, by an
amount _proportional to_ ~the sine of the angle~ _by which the current
lags behind the impressed pressure_. The increased current which is
now permitted to flow, causes the induced field to be strengthened
(as indicated by the dotted magnetic lines M, M', M", etc.), thus
increasing the torque to balance the additional load.]
Now if the motor armature be first brought up to a speed corresponding
in frequency to that of the alternator before connecting the motor in
the circuit, the armature will continue revolving at the same frequency
as the alternator.
The armature continues revolving, because, ~at synchronous speed~,
_the field flux and armature current are always in the same relative
position_, producing a torque which always pulls the armature around in
the same direction.
A polyphase synchronous motor is self starting, because, before the
current has died out in the coils of one phase, it is increasing in
those of the other phase or phases, so that there is always some
turning effort exerted on the armature.
The speed of a synchronous motor is that at which it would have to run,
if driven as an alternator, to deliver the number of cycles which is
given by the supply alternator.
[Illustration: FIGS. 1,599 and 1,600.--Synchronous motor principles:
VI. _A single phase synchronous motor has_ "~dead centers~," _just the
same as a one cylinder steam engine_. Two diagrams of the motor are
here shown illustrating the effect of the current in both directions.
When the plane of the coil is perpendicular to the field, the poles
induced in the armature are parallel to field for either direction of
the current; that is to say, the field lines of force and the induced
lines of force acting in parallel or opposite directions, no turning
effect is produced, just as in analogy when an engine is on the dead
center, the piston rod (field line of force) and connecting rod
(induced line of force) being in a straight line, the force exerted by
the steam on the piston produces no torque.]
For instance a 12 pole alternator running at 600 revolutions per
minute will deliver current at a frequency of 60 cycles a second; an
8 pole synchronous motor supplied from that circuit will run at 900
revolutions per minute, which is the speed at which it would have to be
driven as an alternator to give 60 cycles a second--the frequency of
the 12 pole alternator.
[Illustration: FIGS. 1,601 to 1,604.--Synchronous motor principles VII.
_An essential condition for synchronous motor operation is that the
mechanical lag be less than 90°._ Figs. 1,601 and 1,602 represent the
conditions which prevail when the lag of the motor armature A'B'C'D' is
anything less than 90°. As shown, the lag is almost 90°. The direction
of the current and induced poles are indicated by the arrows. _The
inclination of the motor coil is such that_ ~the repulsion of like
poles~ _produces a torque_ ~in the direction of rotation~, thus tending
to keep motor in operation. Now, in figs. 1,603 and 1,604, for the same
position of the alternator coil ABCD, _if the lag be greater than_
90°, _the inclination of the motor coil_ A'B'C'D' _is such that at
this instant_ ~the repulsion of like poles~ _produces a torque in a
direction_ ~opposite~ _to that of_ ~the rotation~, _thus_ ~tending to
stop~ _the motor_. In actual operation this quickly brings the motor
to rest, having the same effect as a strong brake in overcoming the
momentum of a revolving wheel.]
[Illustration: FIGS. 1,605 to 1,608.--Synchronous motor principles:
VIII. _If the torque and current through the motor armature be kept
constant, strengthening the field will increase the mechanical lag, and
the lead of the current with respect to the reverse pressure._ In the
figures, let A be an instantaneous position of the alternator coil,
A°, synchronous position of motor coil, A', position corresponding to
current phase, A", actual position or mechanical lag of motor coil
behind alternator coil necessary to maintain equilibrium. In fig.
1,606, let A' and A" represent respectively the relation of current
phase and mechanical lag _corresponding to a_ ~certain~ _load and field
strength_. For these conditions OG, O'G', O"G", etc., will represent
the components of the induced lines of force in opposition to the
motor field, that is, _they indicate the intensity of the armature
reaction at the instant depicted_. Now, assume the field strength to be
~doubled~, as in fig. 1,608, _the motor_ ~load~ _and_ ~current~ _being
maintained_ ~constant~. Under these conditions, the armature reaction
must be doubled to maintain equilibrium; that is, the components OG,
O'G', etc., fig. 1,608, must be twice the length of OG, O'G', etc.,
fig. 1,605. Also since the current is maintained constant, the induced
magnetic lines OF, O'F' are of same length in both figures. Hence, in
fig. 1,608 the plane of these components is such that their extremities
touch perpendiculars from G, G', etc., giving the other components FG,
F'G', etc. The plane A', normal to OF, O'F', etc., gives the current
phase. By construction, the phase difference between A° and A' is such
that sin A°OA' (fig. 1,608) = 2 × sin A°OA' (fig. 1,606). That is,
doubling the field strength causes an increase of current lag such
that the sine of the angle of this lag is doubled. Since the intensity
of the armature reaction depends on the lead of the current with
respect to the reverse pressure, the mechanical lag of the coil must be
increased to some position as A" (fig. 1,608), such as will give an
armature reaction of an intensity indicated by the components OG, O'G',
etc.]
The following simple formula gives the speed relations between
generators and motors connected to the same circuit and having
different numbers of poles.
P × S
_s_ = -----
_p_
in which
_s_. Revolutions per minute of the motor;
_p_. Number of poles of the motor;
S. Revolutions per minute of the alternator;
P. Number of poles of the alternator.
~Question. If the field strength of a synchronous motor be altered,
what effect does this have on the speed, and why?~
Ans. The speed does not change (save for a momentary variation to
establish the phase relation corresponding to equilibrium), because the
motor has to run at the same frequency as the alternator.
~Ques. How does a synchronous motor adjust itself to changes of load
and field strength?~
Ans. By changing the phase difference between the current and pressure.
If, on connecting a synchronous motor to the mains, the
excitation be too weak, so that the voltage is lower than that
of the supply, this phase difference will appear resulting
in wattless current, since the missing magnetization has,
as it were, to be supplied from an external source. A phase
difference also appears when the magnetization is too strong.
~Ques. State the disadvantages of synchronous motors.~
Ans. A synchronous motor requires an auxiliary power for starting, and
will stop if, for any reason, the synchronism be destroyed; collector
rings and brushes are required. For some purposes synchronous motors
are not desirable, as for driving shafts in small workshops having
no other power available for starting, and in cases where frequent
starting, or a strong torque at starting is necessary. A synchronous
motor has a tendency to _hunt_[1] and requires intelligent attention;
also an exciting current which must be supplied from an external source.
[1] NOTE.--See Hunting of synchronous motors, page 1,280.
~Ques. State the advantage of synchronous motors.~
Ans. The synchronous motor is desirable for large powers where starting
under load is not necessary. Its power factor may be controlled by
varying the field strength. The power factor can be made unity and,
further, the current can be made to lead the pressure.
[Illustration: FIG. 1,609.--Diagram illustrating method of representing
the performance of synchronous motors. The _V shaped curve_ is obtained
by plotting the current taken by motor under different degrees of
excitation, the power developed by the motor remaining constant. The
current may be made to lag or lead while the load remains constant,
by varying the excitation. By varying the excitation, a certain value
may be reached which will give a minimum current in the armature;
this is the condition of unity power factor. If now the excitation be
diminished the current will lag and increase in value to obtain the
same power; if the excitation be increased the current will _lead_
and increase in value to obtain the same power. The results plotted
for several values of the excitation current will give the V curve as
shown. This is an actual curve obtained by Mordey on a 50 kw. machine
running unloaded as a motor. Other curves situated above this one may
be obtained for various loadings of the motor.]
A synchronous motor is frequently connected in a circuit solely
to improve the power factor. In such cases it is often called a
"condenser motor" for the reason that its action is similar to
that of a condenser.
The design of synchronous motors proceeds on the same lines as
that of alternators, and the question of voltage regulation in
the latter becomes a question of power factor regulation in the
former.
~Ques. For what service are they especially suited?~
Ans. For high pressure service.
High voltage current supplied to the armature does not pass
through a commutator or slip rings; the field current which
passes through slip rings being of low pressure does not give
any trouble.
[Illustration: FIG. 1,610.--Westinghouse self-starting synchronous
motor. Motors of this type are suitable for constant speed service
where starting conditions are moderate, such as driving compressors,
pumps, and large blowers. Synchronous motors can be made to operate
not only as motors but as synchronous condensers to improve the power
factor of the circuit. The field is provided with a combined starting
and damper or _amorlisseur_ winding so proportioned that the necessary
starting torque is developed by the minimum current consistent with
satisfactory synchronous running without hunting. The armature slots
are open and the coils form wound, impregnated, and interchangeable.
Malleable iron finger plates at each end of the core support the
teeth. Ventilating finger plates assembled with the laminations form
air ducts. The frames are of cast iron, box section with openings for
ventilation; shoes and slide rails permit adjustment of position. The
brush holders are of the standard sliding shunt type. Two or more
brushes are provided for each ring.]
~Ques. How do synchronous and induction motors compare as to
efficiency?~
Ans. Synchronous motors are usually the more efficient.
[Illustration: FIG. 1,611.--Mechanical analogy illustrating "hunting."
The figure represents two flywheels connected by a spring susceptible
to torsion in either direction of rotation. If the wheels A and B be
rotating at the same speed and a brake be applied, say to B, its speed
will diminish and the spring will coil up, and if fairly flexible, more
than the necessary amount to balance the load imposed by the brake;
because when the position of proper torque is reached, B is still
rotating slightly slower than A, and an additional torque is required
to overcome the inertia of B and bring its speed up to synchronism with
A. Now before the spring stops coiling up the wheels must be rotating
at the same speed. When this occurs the spring has reached a position
of too great torque, and therefore exerting more turning force on B
than is necessary to drive it against the brake. Accordingly B is
accelerated and the spring uncoils. The velocity of B thus oscillates
above and below that of A when a load is put on and taken off. Owing to
friction, the oscillations gradually die out and the second wheel takes
up a steady speed. A similar action takes place in a synchronous motor
when the load is varied.]
~Hunting of Synchronous Motors.~--Since a synchronous motor runs
practically in step with the alternator supplying it with current when
they both have the same number of poles, or some multiple of the ratio
of the number of poles on each machine, it will take an increasing
current from the line as its speed drops behind the alternator, but
will supply current to the line as a generator if for any reason the
speed of the alternator should drop behind that of the motor, or the
current wave lag behind, which produces the same effect, and due to
additional self-induction or inductance produced by starting up or
overloading some other motor or rotary converter in the circuit.
When the motor is first taking current, then giving current back to the
line, and this action is continued periodically, the motor is said to
be _hunting_.
[Illustration: FIG. 1,612.--Diagram illustrating the use of a
synchronous motor as a condenser. If a synchronous motor be
sufficiently excited the current will lead. Hence, if it be connected
across an inductive circuit as in the figure and the field be over
excited it will compensate for the lagging current in the main, thus
increasing the power factor. If the motor be sufficiently over excited
the power factor may be made unity, the minimum current being thus
obtained that will suffice to transmit the power in the main circuit.
A synchronous motor used in this way is called a _rotary condenser_
or _synchronous compensator_. This is especially useful on long lines
containing transformers and induction motors.]
~Ques. What term is applied to describe the behavior of the current
when hunting occurs?~
Ans. The term _surging_ is given to describe the current fluctuations
produced by hunting.
The mechanical analogy of hunting illustrated in fig.
1,611 will help to an understanding of this phenomenon. In
alternating current circuits a precisely similar action takes
place between the alternators and synchronous motors, or even
between the alternators themselves.
~CHARACTERISTICS OF SYNCHRONOUS MOTORS~
~Starting.~--The motor must be brought up to synchronous speed
without load, a _starting compensator_ being used. If provided with
a self-starting device, the latter must be cut out of circuit at the
proper time. The starting torque of motor with self-starting device is
very small.
~Running.~--The motor runs at synchronous speed. The maximum torque is
several times full load torque and occurs at synchronous speed.
~Stopping.~--If the motor receive a sudden overload sufficient to
momentarily reduce its speed, it will stop; this may be brought about
by momentary interruption of the current, sufficient to cause a loss of
synchronism.
~Effect upon Circuit.~--In case of short circuit in the line the motor
acts as a generator and thus increases the intensity of the short
circuit. The motor impresses its own wave form upon the circuit. Over
excitation will give to the circuit the effect of _capacity_, and under
excitation, that of _inductance_.
~Power Factor.~--This depends upon the field current, wave form and
hunting. The power factor may be controlled by varying the field
excitation.
~Necessary Auxiliary Apparatus.~--Power for starting, or if
self-starting, means of reducing the voltage while starting; also,
field exciter, rheostat, friction clutch, main switch and exciter
switch, instruments for indicating when the field current is properly
adjusted.
~Adaptation.~--If induction motors be connected to the same line with
a synchronous motor that has a steady load, then the field of the
synchronous motor can be over excited to produce a leading current,
which will counteract the effect of the lagging currents induced by the
induction motors. Owing to the weak starting torque, skilled attendance
required, and the liability of the motor to stop under abnormal working
conditions, the synchronous motor is not adapted to general power
distribution, but rather to large units which operate under a steady
load and do not require frequent starting and stopping.
[Illustration: FIGS. 1,613 to 1,625.--Disassembled view of Western
Electric three phase squirrel cage skeleton frame induction motor.]
~Induction (Asynchronous) Motors.~--
_An induction motor consists essentially of an armature and a field
magnet, there being, in the simplest and most usual types, no
electrical connection between these two parts_.[2]
[2] NOTE.--The author prefers the terms armature and field magnet,
instead of "primary," "secondary," "stator," "rotor," etc., as used
by other writers, the armature being the part in which currents are
induced and the field magnet (or magnets) that part furnishing the
field in which the induction takes place.
According to the kind of current that an induction motor is designed to
operate on, it may be classified as:
1. Single phase;
2. Polyphase.
The operation of an induction motor depends on the production of a
magnetic field by passing an alternating current through field magnets.
The character of this field is either
1. Oscillating[3], or
2. Rotating,
according as single phase or polyphase current is used.
[3] NOTE.--"The word _oscillating_ is becoming specialized in its
application to those currents and fields whose oscillations are being
damped out, as in electric 'oscillations.' But for this, we should have
spoken of an oscillating field."--_S. P. Thompson_. The author believes
the word _oscillating_, notwithstanding its other usage, best describes
the single phase field, and should be here used.
[Illustration: FIGS. 1,626 to 1,628.--General Electric base
construction for polyphase induction motors. The base is made of cast
iron. Adjusting gear is provided to slide the motor along the base as
shown in the illustrations, the movement being from 6 to 12 inches
according to size. With this design of base, motors are securely held
in position under all conditions and may be run with an upward pull on
the belt. Close fitting guides moving in an accurately machined slot on
the base preserve a correct alignment of the motor when adjustment of
the latter is required. The same base can be used whether the motor be
supported from the wall or ceiling or located on the floor. A single
adjusting screw is placed under the center line of the motor frame,
which produces an even and balanced draw in either direction on all
parts of the motor when the belt tension is altered. This screw can be
located at either end of the base. The base can be omitted when the
motor is direct connected or when provision for belt adjustment is not
required.]
~Ques. Describe briefly the operation of a single phase motor.~
Ans. A single phase current being supplied to the field magnets,
an _oscillating field_ is set up. A single phase motor is not
self-starting; but when the armature has been set in motion _by
external means_, the reaction between the magnetic field and the
induced currents in the armature being no longer zero, a torque is
produced tending to turn the armature.
The current flowing through the armature produces an
alternating polarity such that the attraction between the
unlike armature and field poles is always in one direction,
thus producing the torque.
[Illustration: FIG. 1,629.--Richmond three phase induction motor on
base fitted with screw adjusting gear for shifting the position of the
motor on the base to take up slack of belt.]
~Ques. Why is a single phase induction motor not self-starting?~
Ans. When the armature is at the rest, the currents induced therein are
at a maximum in a plane at right angles to the magnetic field, hence
there is no initial torque to start the motor.
~Ques. What provision is made for starting single phase induction
motors?~
Ans. Apparatus is supplied for "splitting the phase" (later described
in detail) of the single phase current furnished, converting it
temporarily into a two phase current, so as to obtain a _rotating
field_ which is maintained till the motor is brought up to speed. The
phase splitting device is then cut out and the motor operated with the
_oscillating field_ produced by the single phase current.
[Illustration: FIGS. 1,630 to 1,641.--Terminals for General Electric
polyphase induction motors. In order to prevent any mechanical strain
on the leads being transmitted to the motor windings, the terminal
cables are clamped in insulated bushings with a connector for each
cable.]
~Ques. Describe briefly the operation of a polyphase induction motor.~
Ans. Its operation is due to the production of a _rotating magnetic
field_ by the polyphase current furnished. This field "rotating" in
space about the axis of the armature induces currents in the latter.
The reaction between these currents and the rotating field creates a
torque which tends to turn the armature, whether the latter be at rest
or in motion.
[Illustration: FIGS. 1642 and 1643.--Western Electric end flange
rivets and punchings of riveted frame induction motor. The riveted
frame is constructed of two cast iron flanges between which the stator
laminations of sheet steel are securely clamped and riveted under
hydraulic pressure. This construction exposes the laminations directly
to the air and improves the radiation, thus insuring high overload
capacity and low operating temperatures. The field slots are overhung
or partially closed, affording mechanical protection to the coils.]
~Ques. Why are induction motors called "asynchronous"?~
Ans. Because the armature does not turn in synchronism with the
rotating field, or, in the case of a single phase induction motor,
with the oscillating field (considering the latter in the light of a
rotating field).
~Ques. How does the speed vary?~
Ans. It is slower (more or less according to load) than the "field
speed," that is, than "synchronism" or the "synchronous speed."
[Illustration: FIGS. 1,644 to 1,649.--Construction of General Electric
drawn shell fractional horse power motors. The distinguishing feature
of drawn shell motors is the field construction which consists of a
steel shell or cylinder supporting and clamping together the stator
or field punchings. This method avoids the cast frame work outside
the active magnetic material. A disc is first punched or "blanked"
out of soft steel, fig. 1,644, this disc being faced into the shape,
fig. 1,645, with one end closed. The other end of the shell is then
cut out, leaving the small flange as in fig. 1,646. It is now ready to
receive the core punchings. In the next operation a suitable number
of spacing rings, fig. 1,647, are forced into the shell and seated
against the retaining lip, which may be seen in fig. 1,646. The field
punchings or laminæ, fig. 1,648, are now assembled, after which a
second and equal set of spacing rings are put into place to center the
active field iron. The open edge of the shell is then rolled over the
punchings under heavy pressure, thus preparing the field structure for
the machining and fitting of the end heads and base. Fig. 1,649 shows
a section of the completely assembled field structure, the parts being
cut away to indicate the relation between the field punchings, spacing
rings and shell. After the spacing rings at both frame ends have been
turned true and grooved, the bearing heads, fig. 1,649, are ready for
fastening in place by four fillister headed screws. A complete wound
field is shown in fig. 1,858, with flat base casting attached.]
~Ques. What is the difference of speed called?~
Ans. The _slip_.
This is a vital factor in the operation of an induction
motor, since _there must be slip in order that the armature
inductors shall cut magnetic lines_ to ~induce~ (hence the name
"~induction~" motor) currents therein so as to create a driving
torque.
[Illustration: FIG. 1,650.--Ideal fifteen horse power two phase
induction motor. The armature core is supported by a cast iron
frame carried on a base, with sliding ways and screw adjustment for
tightening the belt. The armature core is provided with ventilating
apertures, with metal spacers between each tooth. The revolving field
is a steel casting with radially projecting poles, to which the pole
shoes are bolted. The overhanging pole tips retain the field coils.
All coils of the smaller sizes are wound with insulated copper wire of
square section, and of the larger sizes, with flat copper, wound on
edge, each turn being insulated by sheet insulation. Motors of this
type are adapted for use in small power plants and isolated plants. The
relatively high speed for which they are designed, reduces considerably
the weight and overall dimensions, and likewise the cost. The exciter
is belt driven. The normal kw. capacity of the exciter usually exceeds
the kw. required for the excitation under normal load conditions to
permit of station lighting. All exciters are built as compound wound
dynamos, capable of delivering the exciter current up to 125 volts,
which is sufficient margin in the field to control the alternating
current line voltage on circuits of unusually low power factor.]
~Ques. What is the extent of the slip?~
Ans. It varies from about 2 to 5 per cent. of synchronous speed
depending upon the size.
~Ques. Why are induction motors sometimes called constant speed motors?~
Ans. They are erroneously and ill advisedly, yet conveniently so called
by builders to distinguish them from induction motors fitted with
special devices to obtain widely varying speeds, and which are known as
_variable speed_ induction motors.
The term _adjustable_ would be better.
~Motor, Constant Speed.~--A motor in which the speed is either
constant or does not materially vary; such as synchronous
motors, induction motors with small slip, and ordinary direct
current shunt motors.--Paragraph 46 of 1907 Standardization
Rules of the A.I.E.E.
~Motor, Variable Speed.~--A motor in which provision is
made for varying the speed as desired. The A.I.E.E. has
unfortunately introduced the term _varying speed motor_, to
designate "motors in which the speed varies with the load,
decreasing when the load increases, such as series motors." The
term is objectionable, since by the expression _variable speed
motor_ a much more general meaning is intended.
[Illustration: FIG. 1,651.--Western Electric core construction and
method of winding field of skeleton frame induction motor. The coils
are wound on forms to give them exact shape and dimensions required.
They are pressed into hot moulds to remove any irregularities and then
the coils are impregnated with hot cement, to bind the layers together
in their permanent shape. The portion of the coil which fits into the
slot is wrapped with varnished cloth and a layer of dry tape is wound
over the entire coil. The coils are then impregnated with an insulating
compound and baked, the process being repeated six times. Coils for
1,100 and 2,200 volt motors have an extra covering of insulation
and double the amount of impregnating and baking. The coils may be
furnished with special insulation and treatment for exceptionally
severe service conditions, such as exposure to excessive moisture,
extreme heat, acid or alkaline fumes, etc. The coils are accessible and
for the final finish are sprayed with black varnish.]
~Ques. Why do some writers call the field magnets and armature the
_primary_ and _secondary_, respectively?~
Ans. Because, in one sense, the induction motor is a species of
transformer, that is, it acts in many respects like a transformer, the
primary winding of which is on the field and the secondary winding on
the armature.
In the motor the function of the secondary circuit is to
furnish energy to produce a torque, instead of producing light
and heat as in the case of the transformer. Such comparisons
are ill advised when made for the purpose of supplying names
for motor parts. There can be no confusion by employing the
simple terms armature and field magnets, remembering that the
latter is _that part that produces the oscillating or rotating
field_ (according as the motor is single or polyphase), and the
former, _that part in which currents are induced_.
[Illustration: FIG. 1,652.--Armature of Allis-Chalmers squirrel cage
induction motor. The frame casting is of the box type and has large
cored openings for ventilation. Lugs are cast on the interior surface
of the frame to support the core, leaving a large air space between.]
~Ques. Why are polyphase induction motors usually presented in text
books before single phase motors?~
Ans. Because the latter must start with a rotating field and come up to
speed before the oscillating field can be employed.
A knowledge then of the production of a rotating field is
necessary to understand the action of the single phase motor at
starting.
~Polyphase Induction Motors.~--As many central stations put out only
alternating current circuits, it has become necessary for motor
builders to perfect types of alternating current motor suitable for
all classes of industrial drive and which are adapted for use on these
commercial circuits. Three phase induction motors are slightly more
efficient at all loads than two phase motors of corresponding size, due
to the superior distribution of the field windings. The power factor is
higher, especially at light loads, and the starting torque with full
load current is also greater. Furthermore, for given requirements of
load and voltage, the amount of copper required in the distributing
system is less; consequently, wherever service conditions will permit,
three phase motors are preferable to two phase.
[Illustration: FIG. 1,653.--Sectional view showing parts of Reliance
polyphase induction motor. A special feature of the squirrel cage
armature construction is the multiplicity of short circuiting rings.
The holes in the rings are bored slightly smaller than the diameter
of the copper rods, and the force fit gives good contact. The rings
having been forced in place are dip soldered in an alloy of tin of
high melting point. The motor parts are: 1, end yoke; 2, shaft;
3, armature short circuiting rings; 4, oil ring; 5, self-aligning
bearing bushing; 6, spider; 7, armature bars; 8, field coils; 9, field
lamination end plate; 10, field laminations; 11, eye bolt; 12, stator
locking key; 13, armature laminations; 14, armature lamination end
plate; 15, armature locking key; 16, dust cap; 17, oil well cover; 18,
oil throws; 19, field frame; 20, squirrel cage armature.]
[Illustration: FIG. 1,654.--Tesla's rotating magnetic field. The figure
is from one of Tesla's papers as given in The Electrician, illustrating
how a rotating magnetic field may be produced with stationary magnets
and polyphase currents. The illustration shows a laminated iron
ring overwound with four separate coils, AA, and BB, each occupying
about 90° of the periphery. The opposite pairs of coils AA and BB
respectively are connected in series and joined to the leads from a two
phase alternator, the pair of coils AA being on one circuit and the
coils BB on the other. The resultant flux may be obtained by combining
the two fluxes due to coils AA and BB, taking account of the phase
difference of the two phase current, as in fig. 1,655.]
The construction of an induction motor is very simple, and since there
are no sliding contacts as with commutator motors, there can be no
sparks during operation--a feature which adapts the motor for use in
places where fire hazards are prominent.
The motor consists, as already mentioned, simply of two parts: _an
armature and field magnets, without any electrical connection between
these parts_. Its operation depends upon:
1. _The production of a rotating field;_
2. _Induction of current in the armature;_
3. _Reaction between the revolving field and the induced currents._
[Illustration: FIG. 1,655.--Method of obtaining resultant flux of
Tesla's rotating magnetic field. The eight small diagrams here seen
show the two components and resultant for eight equivalent successive
instants of time during one cycle. At 1, the vertical flux is at +
maximum and the horizontal is zero. At 2, the vertical flux is still +
but decreasing, and the horizontal is + and increasing, the resultant
is the thick line sloping at 45° upwards to the right. At 3, the
vertical flux is zero, and the horizontal is at its + maximum, and
similarly for the other diagrams. Thus at 8, the vertical flux is +
and increasing, while the horizontal is-and decreasing, the resultant
is the thick line sloping at 45° upwards to the left. At points 2, 4,
6, and 8 the increasing fluxes are denoted by full and the decreasing
by dotted lines. The laminated iron of the ring is indicated by the
circles, and the result is that at the instants chosen the flux across
the plane of the ring is directed inwards from the points 1, 2, 3, 4,
etc., on the inner periphery of the iron. There will, therefore, appear
successively at these points effective north poles, the corresponding
south poles being simultaneously developed at the points diametrically
opposite. These poles travel continuously from one position to the
next, and thus the magnetic flux across the plane of the ring swings
round and round, completing a revolution without change of intensity
during the cycle time of the current.]
~Production of a Rotating Field.~--It should at once be understood that
the term "rotating field" does not signify that part of the apparatus
revolves, the expression merely refers to the magnetic lines of force
set up by the field magnets without regard to whether the latter be the
stationary or rotating member.
A rotating field then may be defined as _the resultant magnetic field
produced by a system of coils symmetrically placed and supplied with
polyphase currents_.
A rotating magnetic field can, of course, be produced by
spinning a horse shoe magnet around its longitudinal axis, but
with polyphase currents, as will be later shown, the rotation
of the field can be produced Without any movement of the
mechanical parts of the electro magnets.
[Illustration: FIG. 1,656.--Arago's rotations. The apparatus necessary
to make the experiment consists of a copper disc M, arranged to rotate
around a vertical axis and operated by belt drive, as shown. By
turning the large pulley by hand, the disc M may be rotated with great
rapidity. Above the disc is a glass plate on which is a small pivot
supporting a magnetic needle N. If the disc now be rotated with a slow
and uniform velocity, the needle is deflected in the direction of the
motion, and stops at an angle of from 20° to 30° with the direction of
the magnetic meridian, according to the velocity of the rotation of the
disc. If the velocity increase, the needle is ultimately deflected more
than 90° and then continues to follow the motion of the disc.]
The original rotating magnetic field dates back to 1823, when Francois
Jean Arago, an assistant in Davy's laboratory, discovered that if a
magnet be rotated before a metal disc, the latter had a tendency to
follow the motion of the magnet, as shown in fig. 290, page 270 and
also in fig. 1,656. This experiment led up to the discovery which
was made by Arago in 1824, when he observed that the _number of
oscillations which a magnetized needle makes in a given time, under
the influence of the earth's magnetism, is very much lessened by the
proximity of certain metallic masses_, and especially of copper, which,
may reduce the number in a given time from 300 to 4.
[Illustration: FIG. 1,657.--Explanation of Arago's rotations. Part of
fig. 1,656 is here reproduced in plan. Faraday was the first to give an
explanation of the phenomena of magnetism by rotation in attributing
it to the induction of currents which by their electro-dynamic action,
oppose the motion producing them; the action is mechanically analogous
to friction. In the figure, let AB be a needle oscillating over a
copper disc, and suppose that in one of its oscillations it goes in the
direction of the arrow from M to S. In approaching the point S, for
instance, it develops there a current in the opposite direction, and
which therefore repels it; in moving away from M it produces currents
which are of the same kind, and which therefore attract, and both these
actions concur in bringing it to rest. Again, suppose the metallic
mass turn from M towards S, and that the magnet be fixed; the magnet
will repel by induction points such as M which are approaching A, and
will attract S which is moving away; hence the motion of the metal
stops, as in Faraday's experiment. If in Arago's experiment the disc be
moving from M to S, M approaches A and repels it, while S, moving away,
attracts it; hence the needle moves in the same direction as the disc.
If this explanation be true, all circumstances which favor induction
will increase the dynamic action; and those which diminish the former
will also lessen the latter.]
The explanation of Arago's rotations is that _the magnetic field
cutting the disc produces eddy currents therein and the reaction
between the latter and the field causes the disc to follow the
rotations of the field_.
The induction motor is a logical development of the experiment of
Arago, which so interested Faraday while an assistant in Davy's
laboratory and which led him to the discovery of the laws of
electromagnetic induction, which are given in Chapter X.
[4]In 1885, Professor ~Ferraris~, of Turin ~discovered~ that _a
rotating field could be produced from stationary coils by means of
polyphase currents_.
[4] NOTE.--Walmsley attributes the first production of rotating fields
to Walter Bailey in 1879, who exhibited a model at a meeting of the
Physical Society of London, but very little was done, it is stated,
until Ferraris took up the subject.
[Illustration: FIG. 1,658.--Experiment made by Faraday being the
reverse of Arago's first observation. Faraday assumed that since the
presence of a metal at rest stops the oscillations of a magnetic
needle, the neighborhood of a magnet at rest ought to stop the motion
of a rotating mass of metal. He suspended a cube of copper by a twisted
thread, which was placed between the poles of a powerful electromagnet.
When the thread was left to itself, it began to spin round with great
velocity, but stopped the moment a powerful current was passed through
the electromagnet.]
[5]This discovery was commercially applied a few years later by Tesla,
Brown, and Dobrowolsky.
[5] NOTE.--The Tesla patents were acquired in the U.S. by the
Westinghouse Co. in 1888, and polyphase induction motors, as they were
called, were soon on the market. Brown of the Oerlikon Machine Works
developed the single phase system and operated a transmission plant
over five miles in length at Kassel, Germany, which operated at 2,000
volts.
_The principles of polyphase motors_ can be best understood by means
of elementary diagrams illustrating the action of polyphase currents
in producing a rotating magnetic field, as explained in the paragraphs
following.
~Production of a Rotating Magnetic Field by Two Phase Currents.~--Fig.
1,659 represents an iron ring wound with coils of insulated wire, which
are supplied with a two phase current at the four points A, B, C, D,
the points A and B, and C and D, being electrically connected.
[Illustration: FIG. 1,659.--Production of a rotating magnetic field
by two phase currents. The figure represents an iron ring, wound with
coils of insulated wire, and supplied with two phase currents at the
four points A, B, C, and D. The action of the two phase current on
the ring in producing a rotating magnetic field is explained in the
accompanying text.]
According to the principles of electromagnetic induction, if only one
current _a_ entered the ring at A, and the direction of the winding be
suitable, a negative pole (-) will be produced at A and a positive pole
(+) at B, so that a magnetic needle pivoted in the center of the ring
would tend to point vertically upward towards A. Now suppose that at
this instant, corresponding to the beginning of an alternating current
cycle, a second current _b_, differing in phase from the first by 90
degrees, is allowed to enter the ring at C. As shown in fig. 1,659,
when the pressure of the current _a_ is at its maximum, that of the
current _b_ is at its minimum; therefore, even a two phase current, at
the beginning of the cycle, the needle will point toward A.
[Illustration: FIG. 1,660.--Production of rotating magnetic field in
a two pole two phase motor. The poles are numbered from 1 to 4 in a
clockwise direction. Phase A winding is around poles 1 and 3, and phase
B winding, around poles 2 and 4. In each case the poles are wound
alternately, that is, if 1 be wound clockwise, 3 will be wound counter
clockwise, thus producing unlike polarity in opposite poles. Now during
one cycle of the two phase current, the following changes take place,
starting with pole 1 of N polarity and 3, of S polarity:
+--------------------------------------------------------------+
| | One Cycle |
|----------+-----------+-------------+------------+------------|
| Degrees | 0° to 90° | 90° to 180° |180° to 270°|270° to 360°|
+----------+-----------+-------------+------------+------------+
| Polarity | 1N - 3S | 2N - 4S | 3N - 1S | 4N - 2S |
+--------------------------------------------------------------+]
[Illustration: FIG. 1,661.--Diagram showing resultant poles due to two
phase current.]
[Illustration: FIG. 1,662.--Diagram of two phase, six pole field
winding. There are six coils in each phase, as shown. The coils of each
phase are connected in series, adjacent coils being joined in opposite
senses, thus, for each phase, first one coil is wound clockwise, and
the next counter clockwise.]
As the cycle continues, however, the strength of _a_ will diminish and
that of _b_ increase, thus shifting the induced pole toward C, until
_b_ attains its maximum and _a_ falls to its minimum at 90° or the end
of the first quarter of the cycle, when the needle will point toward
C. At 90°, the phase _a_ current reverses in direction and produces a
negative pole at B, and as its strength increases from 90° to the 180°
point of the cycle, and that of phase _b_ diminishes, the resultant
negative pole is shifted past C toward B, until _a_ attains its maximum
and _b_ falls to its minimum at 180°, and the needle points in the
direction of B.
[Illustration: FIG. 1,663.--Diagram of two phase, eight pole field
winding. The winding is divided into 16 groups (equal to the product
of the number of poles multiplied by the number of phases). Each group
such as at A comprises a number of coils in series, each coil being
located in a separate pair of slots, the end of one being connected to
the beginning of the next. When the currents are in the same direction,
the currents circulate in the same direction in two adjacent groups, a
pole then with this arrangement being formed by two groups, both phases
contributing to the formation of the pole. After ½ cycle when the
current in each phase reverses, the pole advances the angular distance,
covered by two groups; hence the field completes one revolution in
eight alternations of current.]
[Illustration: FIGS. 1,664 to 1,683.--Sine curves of two phase current
and diagrams showing the physical conception of a ~two phase rotating
magnetic field~. The alternating magnetizing current is assumed to be
of such strength that, at its maximum strength, the field produced may
be represented by 10 lines of force as indicated by the parallel lines.
At the beginning of the rotation, fig. 1,664, phase A magnetization,
according to sine curve is zero, indicated by the solid black poles,
while phase B is of strength 10 with]
[Illustration: current in the direction to produce a south pole at
B. Similarly, in fig. 1,665, the strength of A is 4 lines, and of
B, 9 lines, the resultant magnetization having rotated 22½°. The
direction of the resultant magnetization is indicated by the arrow in
each figure. It should be noted in fig. 1,669, that the polarity of
B is reversed, the current curve now being above the zero line. By
following the arrow through the successive positions the rotation of
the resultant magnetization is clearly seen.]
At the 180° point of the cycle, _b_ reverses in direction and produces
a negative pole at D, and as the fluctuation of the pressure of the
two currents during the second half of the cycle, from 180° to 360°,
bear the same relation to each other as during the first half, the
resultant poles of the rotating magnetic field thus produced carry the
needle around in continuous rotation so long as the two phase current
traverses the windings of the ring.
[Illustration: FIG. 1,684.--Moving picture method of showing motion
of a rotary magnetic field. A number of sheets of paper are prepared,
each containing a drawing of the motor frame and a magnetic needle
in successively advancing angular positions, indicating resultant
directions of the magnetism. The sheets are bound together so that the
axis of the needle on each sheet coincides. When passing the sheets in
one way the revolving field will be seen to rotate in one direction,
while, when moving the sheets backward, the rotation of the magnetic
field is in the opposite direction, showing that the reversal of the
order of the coils has the effect of reversing the rotation of the
magnetic field.]
~Production of Rotating Magnetic Field by Three Phase Current.~--A
rotating magnetic field is produced by the action of a three phase
current in a manner quite similar to the action of a two phase current.
Fig. 1,685 shows a ring suitably wound and supplied with a three phase
current at three points A, B, C, 120° of a cycle apart.
[Illustration: FIG. 1,685.--Production of a rotating magnetic field
by three phase current. A ring wound as shown is tapped at points
A, B, and C, 120° apart, and connected with leads to a three phase
alternator. As described on page 1,304, a rotating magnetic field is
produced in a manner similar to the two phase method.]
[Illustration: FIG. 1,686.--Diagram of three phase, four pole ~Y~
connected field winding.]
At the instant when the current _a_, flowing in at A, is at its
maximum, two currents _b_ and _c_, each one-half the value of _a_, will
flow out B and C, thus producing a negative pole at A and a positive
pole at B and at C. The resultant of the latter will be a positive pole
at E, and consequently, the magnetic needle will point towards A.
[Illustration: FIG. 1,687.--Production of a rotating magnetic field in
a two pole three phase motor. In order to obtain a uniformly rotating
magnetic field, it is necessary to arrange the phase windings in the
direction of rotation, in the sequence ACB, not ABC as indicated on the
magnets. Thus poles 1 and 4 are connected in series to phase A, 2 and 5
in series to phase C, and 3 and 6 in series to phase B. The different
phase windings are differently lined, and it should be noted that they
have a common return wire, though this is not absolutely necessary.
Since the phases of the three currents differ from each other by
one-third of a period or cycle, each of the phase windings will
therefore set up a field between its poles, which at any instant will
differ, both in direction and magnitude, from the fields set up by the
other phase windings. Hence, the three phase windings acting together
will produce a resultant field, and if plotted out, the directions of
this field for various fractions of the period is such that in one
complete period the resultant field will make one complete round of
the poles in a clockwise direction, as indicated by the curved arrow.
The positions of the resultant field during one complete period may be
tabulated as follows:
+--------------------------------------------------------------------+
| | One Cycle |
| |---------+---------+---------+---------+---------+---------|
| |0° to 60°| 60° to | 120° to | 180° to | 240° to | 300° to |
| | | 120° | 180° | 240° | 300° | 360° |
| |---------+---------+---------+---------+---------+---------|
|Polarity| 1N - 4S | 2N - 5S | 3N - 6S | 4N - 1S | 5N - 2S | 6N - 3S |
|--------+---------+---------+---------+---------+---------+---------|]
As the cycle advances, however, the mutual relations of the
fluctuations of the pressures of the three currents, and the time of
their reversals of direction will be such, that when a maximum current
is flowing at any one of the points A, B, and C, two currents each of
one-half the value of the entering current will flow out of the other
two points, and when two currents are entering at any two points, a
current of maximum value will flow out of the other point. This action
will produce one complete rotation of the magnetic field during each
cycle of the current.
[Illustration: FIG. 1,688.--Production of three phase rotating magnetic
field with winding on laminated iron ring. The winding is divided into
twelve sections, which are connected in three groups, A, B, and C, of
four sections each, the sections in each group being evenly placed
round the ring with the sections of the two other groups between
them. One end of each group is to be connected to the line wire and
the other end to the common junction J, from which it follows that
the winding given is an example of "star" winding. With three phase
currents the winding will give at every instant four N poles and four S
poles round the ring, and in actual working these poles will be on the
inner periphery because of the presence of an inner ring or cylinder
of good magnetic iron placed, with the requisite clearance to allow
of rotation, as close as is mechanically possible to the outer ring.
Each one of these eight poles will make a complete revolution round the
ring in four times the periodic time of the currents supplied. Thus, if
the supply current has a frequency of 50, a complete revolution of the
field will take place in .08 (=⁴/₅₀) of a second, which corresponds
to an angular velocity of 750 revolutions per minute in place of 3,000
revolutions per minute, which would be the angular velocity with a
bipolar field at this periodicity. Similarly a continuously wound
Gramme ring tapped at twelve points, joined in three groups of four
each to the supply mains, would give an eight pole rotary field. In
this case the grouping would be a "mesh" grouping, with each side of
the mesh formed of four coils in parallel.]
[Illustration: FIGS. 1,689 to 1,708.--Sine curves of three phase
current and diagrams showing the physical conception of a ~three phase
rotating magnetic field~. The diagrams are constructed in the same
manner as explained in figs. 1,664 to 1,683. It should be noted that
the phase windings are arranged in the direction of rotation in the
sequence ACB, phase C being wound in opposite]
[Illustration: sense to A and B, as indicated by the curves, in that
north poles are produced at A and B when the respective curves are
above the zero line, a south pole being produced at C when its curve
is above the zero line. The rotation of the resultant magnetization is
clearly seen by following the arrow through its successive positions.]
~Slip.~--Instead of the magnetic needle as was used in the preceding
figures, a copper cylinder may be placed in a rotating magnetic field
and it will be urged also to turn in the same direction as the rotation
of the field.
[Illustration: FIG. 1,709.--Diagram of three phase, six pole field
winding. There are 18 groups, and the sequence of phases is ABC in a
counter clockwise direction. For a ~Y~ connection, the middle phase is
reversed, so that a pole will be formed by the three consecutive phases
when the current is in the same direction in A and C, and opposite in
B. The beginning of the middle coil C, and not the end, as with the
other two, is connected to the common point O. In this case the pole
shifts a distance equal to three groups for each alternation, so that
one revolution of the field requires three cycles.]
_The torque tending to turn the cylinder is due to the induction of
currents of opposite polarity in the cylinder._
For simplicity, the rotating magnetic field may be supposed to be
produced by a pair of magnetic poles placed at opposite sides of the
cylinder and _revolved around it_ as in fig. 1,710.
Now, for instance in starting, the cylinder being at rest any element
or section of the surface as the shaded area AB, will, as it comes into
the magnetic field of the rotating magnet, cut
[Illustration: FIG. 1,710.--Copper cylinder and rotating magnet
_illustrating the principle of operation of an induction motor_. The
"rotating magnetic field" which is necessary for induction motor
operation is for simplicity here produced by rotating a magnet as
shown. In starting, the cylinder being at rest, any element as AB, as
it is swept by the field will cut magnetic lines, which will induce a
current upward in direction as determined by applying Fleming's rule
(fig. 132, page 133). The inductive action is strongest at the center
of the field hence as AB passes the center the induced pressure along
AB is greater than along elements more or less remote on either side.
Accordingly a pair of eddy currents will result as shown (see fig.
291, page 271). Applying the right hand rule for polarity of these
eddy currents (see fig. 119, page 117) it will be seen that a S pole
is induced by the eddy on the side of the cylinder receding from the
magnet, and a N pole by the eddy on the side toward which the magnet
is approaching. The cylinder, then, is _attracted_ in the direction of
rotation of the magnet by the induced pole on the receding side, and
_repelled_ in the same direction by the induced pole on the approaching
side. Accordingly, the cylinder begins to rotate. The velocity with
which it turns depends upon the load; it must always turn _slower_
than the magnet, in order that its elements may cut magnetic lines and
induce poles to produce the necessary torque to balance the load. The
difference in speed of the magnet and cylinder is called the _slip_.
Evidently the greater the load, the greater is the slip required to
induce poles of sufficient strength to maintain equilibrium. The
figure is drawn somewhat distorted, so that both eddies are visible.]
magnetic lines of force inducing a current therein, whose direction is
easily determined by applying Fleming's rule.[6]
[6] NOTE.--In order to avoid confusion in applying Fleming's rule, it
may be well to regard the pole as being stationary and the cylinder as
in motion; for, since motion is "purely a relative matter" (see fig.
1,393), the inductive action will be the same as if the pole stood
still while the cylinder revolved from left to right, that is, counter
clockwise, looking down on it. Regarding it thus (pole stationary and
cylinder revolving counter clockwise) Fleming's rule (see fig. 132,
page 133) is easily applied to ascertain the direction of the induced
current, which is found to flow upward in the shaded area as shown.
Since the field is not uniform, but gradually weakens, as shown, on
either side of the shaded area (which is just passing the center), the
pressure induced on either side will be less than that induced in the
shaded area, giving rise to eddy currents (as illustrated in fig. 291,
page 271). These eddy currents induce poles as indicated at the centers
of the whorls, the polarity being determined by applying the right hand
rule (fig. 119, page 117).
[Illustration: FIGS. 1,711 to 1,718.--Parts of Allis-Chalmers polyphase
induction motor with squirrel cage armature.]
By inspection of fig. 1710, it is seen that _the induced pole toward
which the magnet is moving is of the same polarity as the magnet;
therefore it is_ ~repelled~, _while the induced pole from which the
magnet is receding, being of opposite polarity, is_ ~attracted~. _A
torque is thus produced tending to rotate the cylinder._
It must be evident that this torque is greatest when the cylinder is
at rest, because the magnetic lines are cut by any element on the
cylindrical surface at the maximum rate.
Moreover, as cylinder is set in motion and brought up to speed, the
torque is gradually reduced, because the rate with which the magnetic
lines are cut is gradually reduced.
~Ques. What is the essential condition for the operation of an
induction motor?~
Ans. The armature, or part in which currents are induced, must rotate
at a speed slower than that of the rotating magnetic field.
In the elementary induction motor, fig. 1,710, the cylinder is
the armature, and the rotating magnets are the equivalent of a
rotating magnetic field.
~Ques. What is the difference of speed called?~
Ans. _The slip._
~Ques. Why is slip necessary in the operation of an induction motor?~
Ans. If the armature had no weight and there was no friction offered by
the bearings and air, it would revolve in synchronism with the rotating
magnetic field, that is, the slip would be zero; but since weight and
friction are always present and constitute a small load, its speed
of rotation will be a little less than that of the rotating magnetic
field, so that induction will take place, in amount sufficient to
produce a torque that will balance the load.
[Illustration: FIG. 1,719.--General Electric vertical type induction
motor; sectional view showing oiling system. It is provided with ball
thrust bearings and top and bottom guide bearings, and a continuous
flow of oil is maintained through all the bearings by means of a pump
which is made integral with the motor. The ball thrust bearings are
designed to support the weight of the armature only. In cases where
the armature is direct connected a flexible coupling should be used to
prevent additional weight coming on the thrust bearings. In operation,
when the motor starts, the oil, revolving with the pan, flows against
the stationary nozzle and is forced by its velocity at a high pressure
through the oil pipe into the reservoir on top. It then flows down
through the ball bearing and upper guide bearing, through a slot in the
armature spider into the lower guide bearing and thence into the oil
pan. Thus a continuous stream of oil is delivered through all bearings.]
~Ques. How is slip expressed?~
Ans. In terms of synchronism, that is, as a percentage of the speed of
the rotating magnetic field.
The slip is obtained from the following formula:
Slip (rev. per sec.) = S_f_ - Sₐ
or, expressed as a percentage of synchronism, that is, of the
synchronous speed,
(S_f_ - Sₐ) × 100
Slip (%) = -----------------
S_f_
where
S_f_ = Synchronous speed, or R.P.M. of the rotatory magnetic field;
Sₐ = Speed of the armature.
The synchronous speed is determined the same as for synchronous motor
by use of the following formula:
2_f_
S_f_ = ----
P
where
S_f_ = Synchronous speed or R.P.M. of the rotating magnetic field;
P = Number of poles;
_f_ = frequency.
[Illustration: FIG. 1720.--Triumph back geared polyphase induction
motor. A great many applications, especially for direct attachment,
require the use of either a very slow or special speed motor. As these
are quite costly, the preferable arrangement, and one equally as
satisfactory, is the use of a standard speed motor combined with a back
geared attachment. Rawhide pinions are furnished whenever possible,
insuring smooth running with a minimum of noise.]
[Illustration: FIGS. 1,721 to 1,735.--Parts of General Electric small
polyphase induction motors. A, armature; B, key for armature shaft; C,
oil ring; D, bearing lining; E, bearing head, pulley end; F, cap screw
for bearing heads; G, field, complete with winding, terminal plate
and leads; H, motor leads; I, terminal connector for motor leads; J,
soft rubber bushing for motor leads; K, terminal plate; L, screw for
terminal board; M, field coils; N, wooden top sticks for field coils;
O, oil filler; P, bearing head opposite pulley end; Q, screw for oil
well cover; R, oil well cover; S, socket pipe plug for bearing head; U,
motor base; V, yoke for motor base; W, motor base adjusting screw; X,
bolt for motor base and frame (short); Y, cap screw for bearing head;
Z, internal directive fan; Aa, pulley.]
The following table gives the synchronous speed for various frequencies
and different numbers of poles:
~Table of Synchronous Speeds~
+------------------------------------------------------+
| | R.P.M. of the rotating magnetic field, |
| | when number of poles is |
|Frequency|--------------------------------------------|
| | 2 | 6 | 10 | 16 | 20 | 24 |
|---------+-------+-------+-------+------+------+------|
| 25 | 1,500 | 500 | 300 | 188 | 150 | 125 |
| 60 | 3,600 | 1,200 | 720 | 450 | 360 | 300 |
| 80 | 4,800 | 1,600 | 960 | 600 | 480 | 400 |
| 100 | 6,000 | 2,000 | 1,200 | 750 | 600 | 500 |
| 120 | 7,200 | 2,400 | 1,440 | 900 | 720 | 600 |
| 125 | 7,500 | 2,500 | 1,500 | 938 | 750 | 625 |
+------------------------------------------------------+
~Ques. How does the slip vary?~
Ans. It varies from about 1 per cent. in a motor designed for very
close regulation to 40 per cent. in one badly designed, or designed for
some special purpose.
~Ques. Why is the slip ordinarily so small?~
Ans. Because of the very low resistance of the armature, very little
pressure is required to produce currents therein, of sufficient
strength to give the required torque. Hence, the necessary rate of
cutting the magnetic lines to induce this pressure in the armature
is reached with very little difference between the field speed and
armature speed, that is, with very little slip.
~Ques. How does the slip vary with the load?~
Ans. The greater the load the greater the slip.
In other words, if the load increase, the motor will run
slower, and the slip will increase. With the increased slip,
the induced currents and the driving force will further
increase. If the motor be well designed so that the field
strength is constant and the lag of the armature currents
is small, the driving force developed or _torque_ will be
proportional to the slip, that is the slip will increase
automatically as the load is increased, so that the _torque_
will be proportional to the load.
According to Weiner, the slip varies according to the following table:
~SLIP OF INDUCTION MOTORS~
+----------+------------------------+----------+------------------------+
| | Slip at full load | | Slip at full load |
| Capacity | per cent. | Capacity | per cent. |
| of motor |------------------------| of motor |------------------------|
| H. P. | Usual limits | Average | H. P. | Usual limits | Average |
+----------+--------------+---------+----------+--------------+---------+
| ⅛ | 20 to 40 | 30 | 15 | 5 to 11 | 8 |
| ¼ | 10 " 30 | 20 | 20 | 4 " 10 | 7 |
| ½ | 10 " 20 | 15 | 30 | 3 " 9 | 6 |
| 1 | 8 " 20 | 14 | 50 | 2 " 8 | 5 |
| 2 | 8 " 18 | 13 | 75 | 1 " 7 | 4 |
| 3 | 8 " 16 | 12 | 100 | 1 " 6 | 3.5 |
| 5 | 7 " 15 | 11 | 150 | 1 " 5 | 3 |
| 7½ | 6 " 14 | 10 | 200 | 1 " 4 | 2.5 |
| 10 | 7 " 12 | 9 | 300 | 1 " 3 | 2 |
+----------+--------------+---------+----------+--------------+---------+
[Illustration: FIG. 1,736.--Sector method of measuring the slip of
induction motors. A black disc having a number of white sectors
(generally the same as the number of poles of the induction motor) is
fastened with wax to shaft of the induction motor, and is observed
through another disc having an equal number of sector shaped slits
(that is a similar disc with the white sectors cut out) and attached to
the shaft of a small self-starting synchronous motor, which is fitted
with a revolution counter that can be thrown in or out of gear at
will; then the slip (in terms of N_{_r_}) = N ÷ (N_{_s_} ÷ N_{_r_}),
in which: N = number of passages of the sectors; N_{_s_} = number of
sectors; N_{_r_} = number of revolutions recorded by the counter during
the interval of observation. For large values of slip, the observations
may be simplified by using only one sector (N_{_s_} = 1), then N will
equal the slip in revolutions.]
~Ques. Describe one way of measuring the slip.~
Ans. A simple though rough way is to observe simultaneously the speed
of the armature and the frequency, calculating the slip from the data
thus obtained, as on page 1,315.
This method is not accurate, as, even with the most careful
readings, large errors cannot be avoided. A better way is shown
in fig. 1,736.
[Illustration: FIG. 1,737.--Detail of Westinghouse squirrel cage
armature for induction motor. This is an example of _cast on_
construction similar to that of Morse-Fairbanks (see figs. 1,752, 1,753
and 1,915). The inductors are embedded in a special cement.]
~Evolution of the Squirrel Cage Armature.~--In the early experiments
with rotating magnetic fields, copper discs were used; in fact, it was
then discovered that _a mass of copper or any conducting metal, if
placed in a rotating magnetic field, will be urged in the direction of
rotation of the field_.
Ferraris used a copper cylinder as in figs. 1,710 and 1,738, which was
the first step in the evolution of the squirrel cage armature. The
trouble with an armature of this kind is that there is no definite path
provided for the induced currents.
[Illustration: SO CALLED SQUIRREL CAGE]
[Illustration: FIGS. 1,738 to 1,744.--Evolution of the squirrel cage
armature. The early experiments of Arago, Herschel, Babbage and
Baily demonstrated that a mass of copper or any conducting metal, if
placed in a revolving magnetic field, will be urged to revolve in the
direction of the revolving field. They used discs, but Ferraris used
a copper cylinder as shown in figs. 1,710 and 1,738; this was the
first squirrel cage armature. Figs. 1,739 to 1,744 show the gradual
development of the primitive device shown in fig. 1,738; fig. 1,739,
Ferraris' cylinder with slots restricting the path of induced currents;
fig. 1,740, Dobrowolsky's so called squirrel cage which he embedded in
a solid iron core, as in fig. 1,741; fig. 1,742, design with insulated
bars and laminated core to prevent eddy currents in the core; fig.
1,743, laminated core with ventilating ducts; fig. 1,744, modern
squirrel cage armature representing the latest practice as built by
Mechanical Appliance Co. The core is built up of discs punched from No.
29 gauge electrical sheet, insulated from each other and firmly clamped
between end plates locked on the shaft. The slots in the discs are of
the same general form as those in the core. Heavy fibre end pieces,
punched to match the discs are placed at each end of the core, to
prevent the bars coming in contact with the sharp edges of the teeth.
The winding is made up of rectangular copper bars, passing through
slots in the core, and short circuited on each other by means of copper
end rings of special design. The bars are pressed into holes punched
in the end rings, and the contact is then protected from corrosion by
being dipped in a solder bath. The bars are insulated from the iron of
the core by fibre cell projecting beyond the end of the slot. To secure
ventilation the short circuiting rings are set some distance from the
end of the core. In this way the bars between the core and the ring
act as the vanes of a pressure blower, forcing a large volume of air
through the field coils and ventilating openings.]
[Illustration: FIG. 1,745.--Mechanical Appliance Co. solid core discs
as used on small and medium size induction motors.]
[Illustration: FIG. 1,746.--Allis-Chalmers squirrel cage armature
construction. The core laminæ are mounted on a cast iron spider having
arms shaped to act as fan blades for forcing air through the motor. The
spider is pressed on to the shaft. In the smallest sizes the punchings
are mounted directly on the shaft, which is properly machined to hold
them firmly. Copper bars are used as inductors in the larger sizes,
and copper rods in the smaller sizes. The ends of the inductors are
turned down somewhat smaller than the body and fit in holes in the end
rings. The shoulder thus formed fits firmly against the end rings.
Good electrical contact is obtained by expanding the inductors in the
end ring holes. In large armatures both bars and end rings are of
rectangular cross section, the bars and rings being fastened by machine
steel cap screws.]
Obviously, a better result is obtained if, in fig. 1,738, the downward
returning currents of the eddies are led into some path where they will
return across a field of opposite polarity from that across which
they ascended, as in such case, the turning effect will be doubled.
Accordingly the design of fig. 1,738 was modified by cutting a number
of parallel slits which extended nearly to the ends, leaving at each
end an uninterrupted "ring" of metal. This may be called the first
squirrel cage armature, and in the later development Dobrowolsky was
the first to employ a built-up construction, using a number of bars
joined together by a ring at each end, as in fig. 1,740, and embedded
in a solid mass of iron, as in fig. 1,741; he regarding the bars merely
as veins of copper lying buried in the iron.
[Illustration: FIG. 1,747.--Triumph squirrel cage armature. In
construction thin sheet steel laminations, japanned, are built up to
form the core, and are rigidly clamped together by heavy malleable iron
end plates. Semi-enclosed slots are punched in the outer periphery to
receive the windings, so that none of the centrifugal force is carried
by the inductors. These inductors are set edge on, and are riveted and
soldered into resistance rings. These rings are punched to receive the
inductors in such a manner that there is an unbroken strip of metal
completely surrounding them. Moreover, the short circuiting rings are
set some distance from the end of the core, so that the inductors
between the core and ring act as vanes to force air through the coils
for ventilation.]
[Illustration: FIG. 1,748.--General Electric soldered form of end
ring construction on squirrel cage armatures. The armature inductors
or copper bars laid in the core slots are short circuited by these
end rings, which are also made of copper. For the smaller sizes the
rings are thin, but of considerable radial depth and are held apart by
spacing washers. They have rectangular holes punched near their outer
peripheries through which the bars pass. Lips are formed on the rings,
as shown, to which the bars are soldered.]
[Illustration: FIG. 1,749.--General Electric welded form of end ring
construction on squirrel cage armatures. Space limitations make it
difficult to provide multiple soldered rings of sufficient area for
large motors; hence, on such machines welding is resorted to, as shown.
The ring in welded construction is placed beneath the bars at each end
of the armature. Short radial bars are welded to the edges of these
rings and to the inductors or squirrel cage bars, thereby making good
electrical contact.]
A solid cylinder of iron will of course serve as an armature,
as it is magnetically excellent; but the high specific
resistance of iron prevents the flow of induced currents taking
place sufficiently copiously; hence a solid cylinder of iron
is improved by surrounding it with a mantle of copper, or by a
squirrel cage of copper bars (like fig. 1,740), or by embedding
rods of copper (short circuited together at their ends with
rings) in holes just beneath its surface. However, since all
eddy currents that circle round, as those sketched in fig.
1,738, are not so efficient in their mechanical effect as
currents confined to proper paths, and as they consume power
and spend it in heating effects, the core was then constructed
with laminations lightly insulated from each other, and further
the squirrel cage copper bar inductors were fully insulated
from contact with the core. Tunnel slots were later replaced by
designs with open tops.
[Illustration: Figs. 1,750 and 1,751.--Built up core construction
with discs punched in one piece. The spider proper consists of a hub
provided with four radial arms, which fit the inner diameter of the
disc. The hub is bored out so that it fits very tightly on the shaft,
and a key is provided to avoid any chance of turning. The core disks
are clamped firmly in place by two heavy cast iron end plates which are
pressed up and held by the bolts. These bolts pass under the discs, so
that there is no danger of their giving rise to eddy currents. The key
not only prevents the discs turning on the spider but also ensures the
alinement of discs, which is necessary to make the teeth form smooth
slots when the core is assembled.]
Fig. 1,744 shows a modern squirrel cage armature conforming to the
latest practice, other designs being illustrated in the numerous
accompanying cuts.
In the smaller sizes, the core laminæ are of the solid type as shown
in fig. 1,745, but for larger motors the core consists of a spider and
segmental discs as shown in figs. 1,750 and 1,751.
Fig. 1,748 shows a soldered form of end ring construction, and figs.
1,752 and 1,753 the method of welding the end ring to the inductors.
~The Field Magnets.~--The construction of the field magnets, which,
when energized with alternating current produce the rotating magnetic
field, is in many respects identical with the armature construction of
revolving field alternators.
[Illustration: FIG. 1,752.--Fairbanks-Morse squirrel cage armature with
cast-on rings showing inspection grooves. The method consists in fusing
the ends of the inductors into an end ring of a special composition,
thereby producing a perfect electrical and mechanically strong joint.
In this process the armature with its bars in place is put into a mould
and the molten metal poured around the inductors, melting their ends
and effectually fusing them into the body of the ring. The ring is then
turned down to finished size and polished. An inspection groove is cut
as shown to indicate that the fusion is complete and the joint perfect.]
[Illustration: FIG. 1,753.--Section of Fairbanks-Morse "cast-on" joint
showing union of end ring and inductor. The view shows the V-shape
inspection groove as described in fig. 1,752.] Broadly, the field
magnets of induction motors consists of:
1. Yoke or frame;
2. Laminæ, or core stampings;
3. Winding.
[Illustration: FIG. 1,754.--Richmond field construction for polyphase
induction motors, showing style of winding for use with squirrel cage
and wound armature types.]
~Ques. What is the construction of the yoke and laminæ?~
Ans. They are in every way similar to the armature frame and core
construction of revolving field alternators.
[Illustration: FIG. 1,755.--Western Electric squirrel cage armature
of high speed induction motor for centrifugal pump service. This
armature is an example of heavy duty construction. The inductors are
welded to the short circuiting end rings, the latter being located
beneath the inductors, as shown. Fan vanes are provided at one end
for ventilation. In the field construction, the core laminations are
assembled in a closed box frame, and clamped by heavy rings while under
hydraulic pressure. The stator coils are form wound and subjected to a
special insulating process, which renders them especially impervious
to moisture, and capable of operating without breakdown in locations
which are too damp for ordinary motors. The bearing brackets are of
rigid mechanical construction, and the pulley end bracket and bearings
of all sizes are split to facilitate removal of the rotor and complete
inspection. These machines range in size from 50 to 200 horse power,
the rugged construction adapting them to heavy and severe service, such
as is met with in mining, the construction of dams, canals, aqueducts,
tunnels, etc.]
[Illustration: FIG. 1,756.--Wagner squirrel cage armature for polyphase
induction motor, as employed on motors of from 5 to 25 horse power.
The features of construction as seen in the illustration are bar
inductors, ventilating passages through the core laminæ, riveted
connection between inductors and end rings ventilating vanes on end
plate, extra large end rings. The object of making the rings unusually
large is to make the resistance of the rings lower than is desirable
for some classes of service, in order to obtain motors having minimum
slip, increased efficiency, and maximum overload capacity under normal
operation. When the torque required by some very unusual and entirely
abnormal installation exceeds that of the average conditions, it is
an easy matter to reduce the section of the end rings, by turning
them down in a lathe, thereby increasing the resistance and starting
torque.]
~Field Windings for Induction Motors.~--The field windings of induction
motors are almost always made to produce more than two poles in order
that the speed may not be unreasonably high. This will be seen from the
following:
If P be the number of _pairs_ of poles per phase, _f_, the
frequency, and N, the number of revolutions of the rotating
field per minute, then
60 × _f_
N = --------
P
Thus for a frequency of 100 and one pair of poles, N = 60 × 100
÷ 1 = 6,000. By increasing the number of pairs of poles to 10,
the frequency remaining the same, N = 60 × 100 ÷ 10 = 600.
Hence, in design, by increasing the number of pairs of poles
the speed of the motor is reduced.
[Illustration: FIG. 1,757.--Richmond squirrel cage armature. The copper
bars are double riveted at either end to the resistance rings, then
dipped into a solder bath.]
~Ques. State an objection to very high speed of the rotating field.~
Ans. The more rapid the rotation of the field, the greater is the
starting difficulty.
~Ques. Besides employing a multiplicity of poles, what other means is
used to reduce the speed?~
Ans. Reducing the frequency.
~Ques. What difficulty is encountered with low frequency currents?~
Ans. If the frequency be very low, the current would not be suitable
for incandescent lamp lighting, because at low frequency the rise and
fall of the current in the lamps is perceptible.
[Illustration: FIG. 1,758.--Field construction of Crocker-Wheeler
induction motor with _magnetic bridge_. Steel bridges are inserted in
the grooves where the coils are placed, to protect them from dirt and
mechanical injury and at the same time provide a path for the magnetic
flux which has a more uniform reluctance, thereby insuring a better
distribution of the flux in the air gap and at the same time retaining
open slot construction from which the coils can be readily removed.]
~Ques. What is the general character of the field winding?~
Ans. The field core slots contain a distributed winding of
substantially the same character as the armature winding of a revolving
field polyphase alternator.
~Ques. Are the poles formed in the usual way?~
Ans. They are produced by properly connecting the groups of coils and
not by windings concentrated at certain points on salient or separately
projecting masses of iron, as in direct current machines.
~Ques. How are the coils grouped?~
Ans. Three phase windings are usually ~Y~ connected.
[Illustration: FIG. 1,759.--Western Electric squirrel cage armature.
The inductors consist of solid copper bars embedded in the slots of a
laminated core, with their projecting ends securely fitted and soldered
to heavy copper rings.]
~Ques. What other arrangement is sometimes used?~
Ans. In some cases ~Y~ grouping is used for starting and Δ grouping for
running.
* * * * *
~Starting of Induction Motors.~--It must be evident that if the field
winding of an induction motor whose armature is at rest, be connected
directly in the circuit without using any starting device, the machine
is placed in the same condition as a transformer with the secondary
short circuited and the primary connected to the supply circuit. Owing
to the very low resistance of the armature, the machine, unless it be
of very small size, would probably be destroyed by the heat generated
before it could come up to speed. Accordingly some form of starting
device is necessary. There are several methods of starting, as with:
1. Resistances in the field;
2. Auto-transformer or compensator;
3. Resistance in armature.
[Illustration: FIG. 1,760.--Holzer Cabot combination polyphase
induction motor set, consisting of wound frame and three rotors: 1,
squirrel cage armature, 2, wound armature, 3, rotating field. The set
is intended for school demonstration of induction motor phenomena.
The motor operating with the squirrel cage armature has an inherent
constant speed characteristic and on brake tests will show its
exceptionally strong starting torque and ability to take excessive
overloads. This motor can be used as a generator also, in the sense
that if connected to the line and driven above synchronous speed by
some external means, it will act as an asynchronous generator and
return power to the line. For variable speed service, an armature
having a winding upon it similar to that on the frame must be used.
External resistances inserted in the armature circuit may be used to
produce, first, a reduction of starting current, second, an increase
of starting torque, or third, a variation of speed. Thus an extensive
list of experiments can be performed with this phase wound armature
directly along the line of present engineering practice. The phase
wound armature can be used as an alternator in the same sense as
mentioned above for the squirrel cage machine. For synchronous motor
and three phase operation the revolving field with projecting poles and
slip rings would be used, the field being excited from a direct current
supply.]
~Ques. Explain the method of inserting resistances in the field.~
Ans. Variable resistances are inserted in the circuits leading to the
field magnets and mechanically arranged so that the resistances are
varied simultaneously for each phase in equal amounts. These starting
resistances are enclosed in a box similar to a direct current motor
rheostat.
~Ques. Is this a good method?~
Ans. It is more economical to insert a variable inductance in the
circuit, by using an auto-transformer.
[Illustration: FIG. 1,761.--Westinghouse auto-starter. Polyphase
induction motors may be started by connecting them directly to the
circuit with an ordinary switch, and the smaller motors are started
in this way in practice. In the larger motors, however, the starting
torque at normal voltage is several times its full load torque;
therefore, they are started on a reduced voltage, and the full
pressure of the circuit is not applied until they have practically
reached their operating speed. The figure shows connections with a
two phase alternating current circuit. The auto-starter consists of
two auto-transformers T and T', each having only a single winding for
both primary and secondary, which are tapped at certain points by
switches, thus dividing the winding into a number of loops, so that
one of several voltages may be applied for starting, and the starting
torque thus adjusted to the work that has to be performed. At the
highest points tapped by the switches S, and S', the full pressure,
and at the lowest points, the lowest pressure, is applied to the motor
by the operation of the main switch M. This switch has four blades and
three positions. When thrown to the left as indicated, it connects the
auto-transformers T and T', across the circuits A and B respectively,
so that the pressure across the transformer coils, as determined by the
position of the switches S and S', is applied to the motor circuits
A and B. The intermediate position of the switch M interrupts both
circuits. To start the motor, the switch M is thrown to the left and
a reduced pressure applied; after the motor has started and come up
to speed the switch M is thrown to the right, thus cutting out the
transformer and connecting the motor directly to the circuit. The
starting device can be located at a point remote from the motor, thus
eliminating danger from fire due to possible sparks, in case where it
is necessary to install the motors in grain elevators, woolen mills,
or in any place exposed to inflammable gases, or floating particles of
combustible matter. This feature is also valuable in cases where motors
are suspended from the ceiling, or installed in places not easily
accessible.]
~Ques. What is the auto-transformer or compensator method of starting?~
Ans. It consists of reducing the pressure at the field terminals by
interposing an impedance coil across the supply circuit and feeding the
motor from variable points on its windings.
[Illustration: FIG. 1,762.--Auto-transformer or compensator connections
for three phase induction motor. In operation when the double throw
switch is thrown over to starting position, the current for each phase
of the motor flows through an auto-transformer, which consists of a
choking coil for each phase, arranged so that the current may be made
to pass through any portion of it (as 1, 2, 3) to reduce the voltage
to the proper amount for starting. After the motor has come up to
speed on the reduced voltage, the switch is thrown over to running
position, thus supplying the full line voltage to the motor. [7]In
actual construction fuses are usually connected, so that they will be
in circuit in the running position, but not in the starting position,
where they might be blown by the large starting current.]
[7] NOTE.--The construction of starting devices for induction motors
is fully explained later, the accompanying cuts serving merely to
illustrate the principles involved.
~Internal Resistance Induction Motors.~--The armature of this type of
induction motor differs from the squirrel cage variety in that the
winding is not short circuited through copper rings, but, in starting,
is short circuited through a resistance mounted directly on the shaft
in the interior of the armature.
When the motor is thrown in circuit, a very low starting
current is drawn from the line due to the added resistance in
the armature. As the motor comes up to speed, this resistance
is gradually cut out, and at full speed the motor operates as a
squirrel cage motor, with short circuited winding. #/
~Ques. How is the resistance gradually cut out in internal resistance
motors?~
Ans. By operating a lever which engages a collar free to slide
horizontally on the shaft. The collar moves over the internal
resistance grids (located within the armature spider), thus gradually
reducing their value until they are cut out.
[Illustration: FIG. 1,763.--View of armature interior of Wagner
polyphase induction motor with wound armature, showing the centrifugal
device which at the proper speed short circuits all the coils,
transforming the motor to the squirrel cage type. The winding is
connected with a vertical "commutator" so called. Inside the armature
are two governor weights, which are thrown outwards by the centrifugal
force when the machine reaches the proper speed, thus pushing a solid
copper ring (which encircles the shaft) into contact with the inner
ends of the "commutator" bars, thus completely short circuiting the
armature winding.]
~Ques. For what size motors is the internal resistance method suited?~
Ans. Small motors.
~Ques. Why is it not desirable for large motors?~
Ans. The excessive I²R loss in the resistances, if confined within
the armature spider, would produce considerable heating, and on this
account it is best placed external to the motor.
~Ques. On what class of circuit are internal resistance motors
desirable?~
Ans. On circuits devoted to lighting service as well as power service,
where a high degree of voltage regulation is essential.
The initial rush of current when a squirrel cage motor is
thrown on the line is more or less objectionable and there are
central stations which allow only resistance type of induction
motor to be used on their lines.
[Illustration: FIG. 1,764.--Western Electric wound armature for
internal resistance induction motor. In starting the inductors are
short circuited through a resistance which is gradually cut out as the
motor comes up to speed.]
[Illustration: FIGS. 1,765 TO 1769.--Western Electric wound armature
for external resistance, or slip ring induction motor, showing brush
rigging, slip rings and bar winding.]
~External Resistance or Slip Ring Motors.~--In large machines, and
those which must run at variable speed, such as is required in the
operations of cranes, hoists, dredges, etc., it is advisable that the
regulating resistances be placed externally to the motor. Motors having
this feature are commercially known as ~slip ring motors~, because
_connections are made between the external resistances and the armature
inductors by means of slip rings_.
[Illustration: FIG. 1,770.--Richmond slip ring motor.]
[Illustration: FIG. 1,771.--Richmond slip ring armature as used on
motor in fig. 1,770.]
[Illustration: FIG. 1,772.--Western Electric riveted frame slip
ring induction motor for variable speed service; adapted either to
continuous or intermittent operation.]
As with the internal resistance motor the armature winding of a slip
ring motor is not short circuited through copper rings in starting, but
through a resistance, which in this case is located externally.
~Ques. How is the armature winding connected?~
Ans. It is connected in ~Y~ grouping and the free ends connected to
the slip rings, leads going from the brushes to the variable external
resistances, these being illustrated in fig. 1,779.
[Illustration: FIGS. 1,773 to 1,778.--Sprague skeleton type motor
frame with various types of armature. Fig. 1,777, plain squirrel
cage armature; fig. 1,778, internal resistance armature; fig. 1,773
slip ring armature. In the construction of the plain squirrel cage
armature, fig. 1,777, copper bars are inserted in the slots of the
core, and are insulated from the core by enclosing tubes which project
about one-half inch beyond the iron at each side. The bars are short
circuited at their ends by copper rings. These rings are thin, but of
considerable radial depth and are held apart by spacing washers. They
have rectangular holes punched near their outward periphery, through
which the armature bars pass, and to which they are soldered. The
internal resistance armature, fig. 1,778, is provided with a phase
winding, starting (internal) resistance, and switch located on the
shaft. The starting resistance is designed to give approximately full
load torque with full load current at starting. A greater torque than
full load torque can be obtained for starting, if required, by cutting
out resistance. The resistance consists of cast iron grids enclosed
in a triangular cover which is bolted to the end plates holding the
armature laminæ together, and is short circuited by sliding laminated
spring metal brushes along the inside surface of the grids. The
brushes are supported by a metal sleeve sliding on the shaft which
is operated by a lever secured to the bearing bracket and located
just above the bearing. A rod passing through the end of the shaft
operates the short circuiting arrangement in sizes up to about 25 horse
power. The external resistance or slip ring armature, fig. 1,773, is
similar in construction to fig. 1,778, with the exception that slip
rings are provided because of the external location of the resistance.
These rings connect the inductor through brushes to a controlling and
external resistance, two or more carbon brushes being provided for each
ring, as in fig. 1,776.]
~Single Phase Induction Motors.~--The general utility of single phase
motors, particularly the smaller sizes, is constantly being enlarged
by the growing practice of central stations generating polyphase
current, of supplying their lighting service through single phase
distribution, and permitting the use of single phase motors of moderate
capacity on the lighting circuit.
[Illustration: FIG. 1,779.--External resistance or slip ring induction
motor connections. The squirrel cage armature winding is not short
circuited by copper end rings, but connected in ~Y~ grouping and the
three free ends connected to three slip rings, leads going from the
brushes to three external resistances, arranged as triplex rheostat
having three arms rigidly connected as shown, so that the three
resistances may be varied simultaneously and in equal amounts.]
[Illustration: FIG. 1,800.--Allis-Chalmers phase wound external
resistance type or slip ring armature construction. The winding is for
three phases and the terminals are brought out to three slip rings. The
front bracket is slightly modified to make room for these rings on the
inside. For starting duty sufficient resistance is supplied to reduce
the starting current taken by the motor to 1¼ times the normal full
load current. In the running position the resistance is all cut out of
the circuit. For speed regulation sufficient resistance is supplied to
reduce the speed 50% on normal full load torque.]
[Illustration: FIGS. 1,801 to 1,828.--Disassembled view of Western
Electric three phase external resistance or slip ring mill type
induction motor. It is adapted to severe working conditions, such as
are met with in steel mills, crane and hoist service, etc. Designed for
220 or 440 volt, 25 cycle circuits. The frame is divided horizontally
into an upper and a lower steel casting, both of which are bolted
together at the corners by four heavy bolts. The lower casting is
provided with four feet for bolting the motor to its foundation.
The end of the upper frame which covers the slip rings is equipped
with malleable iron covers held in place by lock bolts. The field
and armature are of the usual construction. One end of the armature
winding is protected against mechanical injury by the slip rings which
are of heavy construction and of practically the same diameter as
the armature, and the other end by a detachable flange of the same
diameter as the outside of the winding. The slip rings are mounted
on the same spider as the armature, so that the shaft can be removed
without disturbing any of the connections. The brushes are equipped
with riveted pigtails, and held in brass brush boxes machined to gauge.
Heavy coiled clock springs are used to maintain an even pressure of the
brushes on the slip rings. The armature leads are brought out through
holes in the upper half of the frame, and the field leads are brought
through a block, which fits in an opening in the upper edge of the
lower half.]
The simplicity of single phase systems in comparison with polyphase
systems, makes them more desirable for small alternating current plants.
The disadvantage of single phase motors is that they are not
self-starting.
A single phase motor consists essentially of an _armature and
field magnet having a single phase winding and also some phase
splitting arrangement for starting_.
[Illustration: FIG. 1829.--General Electric single phase induction
motor. It is suitable for constant speed service where full load torque
at starting does not exceed 140 per cent., and in general is adapted to
drive all geared and belted machinery requiring _constant speed with
light or moderate_ starting torque.]
[Illustration: FIG. 1,830.--Simplified diagram showing the principle of
phase splitting for starting single phase induction motors. By the use
of an auxiliary set of coils connected in parallel with the main coils
and having in series a resistance or condenser as shown, the single
phase current delivered by the alternator is "split" into two phases,
which are employed to produce a rotating field on which the motor is
started.]
[Illustration: FIGS. 1,831 to 1,850.--Parts of Sprague single phase
clutch type induction motor. The armature is of the high resistance
smooth core squirrel cage type, the core laminæ being assembled upon
a steel sleeve. On starting the armature revolves _freely_ around the
shaft on roller bearings until it accelerates to about 75% of its
rated speed, when a centrifugal clutch engages with an outer shell
keyed directly on the shaft, thus throwing on the load. This type of
motor is adapted to drive all belted, geared, or direct connected
machinery requiring constant speed with moderate starting torque, such
as generators, blowers, line shafting in machine shops and factories,
drill presses, laundry machinery, baking machinery, and the like. When
greater torque is required at the moment of starting type RI motors
should be used, or clutch couplings may be installed between the motor
and the machine it is to drive. The parts are as follows; A, field
frame; B, field coils; C, terminal block; D, terminal block screws; E,
connectors; F, bearing head pulley end; G, bearing head opposite pulley
end; H, motor clamping bolts; I, oil well cover; J, oil well plug; K,
drain plug; L, oil filter; M, cap bolts; N, bearing lining; O, oil
ring; P, belt tightener screw; Q, armature core; R, latch; S, driving
shell; T, driving shell set screw; U, clutch ring; V, clutch ring
spring; W, spring adjusting screw; X, nut for belt tightener screw;
Y, shaft; Z, driving shell key; Aa, armature bearing; Ba, pulley; Ca,
pulley set screw; Da, pulley key; Ea, sliding base; Fa, yoke.]
~Ques. Why is a single phase motor not self-starting?~
Ans. Because the nature of the field produced by a single phase current
is oscillating and not rotating.
~Ques. How is a single phase motor started?~
Ans. By splitting the phase, a field is set up normal to the axis of
the armature, and nearly 90° displaced in phase from the field in that
axis. This cross field produces the useful torque.
[Illustration: FIG. 1,851.--General Electric high resistance clutch
type smooth core squirrel cage armature of single phase induction
motor. The core laminæ are slotted near the circumference to retain the
bar inductors, which extend beyond the core at either end where they
are permanently connected to heavy short circuiting rings.]
[Illustration: FIGS. 1,852 to 1,855.--Parts of General Electric
centrifugal clutch pulley as used on clutch type, single phase
induction motor. A, clutch; B, friction band; C, adjusting spring; D,
outer clutch shell with pulley sleeve; E, solid removable pulley; F,
internal mechanism comprising parts A, B, and C; G, outer shell and
pulley comprising parts D and E.]
[Illustration: FIGS. 1,856 and 1,857.--Partly assembled clutch pulley.
F, internal mechanism comprising parts A, B, C, of fig. 1,852. G, outer
shell and pulley, comprising parts D and E of fig. 1,852.]
~Phase Splitting; Production of Rotating Field from Oscillating
Field.~--As previously stated, an oscillating field, that is, one
due to a single phase current, does not furnish any starting torque.
It is therefore necessary to provide a rotating field for a single
phase induction motor to start on, which, after the motor has come
up to speed, may be cut out and the motor will then operate with the
oscillating field.
A rotating field may be obtained from single phase current by what is
known as _splitting the phase_.
[Illustration: FIG. 1,858.--Switch end view of General Electric drawn
shell type fractional horse power single phase motor.]
~Ques. Describe one method of splitting the phase.~
Ans. The field of the motor is provided, in addition to the main single
phase winding, with an auxiliary single phase winding, and the two
windings are connected in parallel to the single phase supply mains
with a resistance or a condenser placed in series with the single
phase winding, as shown in diagram fig. 1,830, the two windings being
displaced from each other on the armature about 90 magnetic degrees,
just as in the ordinary two phase motor.
~Ques. What is the construction of the two windings?~
Ans. The main coils are of more turns than the auxiliary, being spread
over more surface, and are heavier because they are for constant use;
whereas the auxiliary coils are used only while starting.
[Illustration: FIGS. 1,859 to 1,862.--Detail construction of clutch
parts of General Electric drawn shell type fractional horse power
single phase motor. The starting switch, which is assembled within the
motor frame, consists essentially of three parts: a rotating member
mounted on the armature and provided with two spring controlled pivoted
levers in contact with an insulated collector ring.]
~Ques. What are the auxiliary coils sometimes called?~
Ans. _Starting coils._
~Ques. What are "shading" coils?~
Ans. Auxiliary coils as placed on fan motors in the manner shown in
fig. 1,863.
~Ques. How can single phase motors be started without the use of
external phase splitting devices?~
Ans. Such apparatus may be avoided by having the auxiliary winding of
larger self-inductance than the main winding.
~Ques. What is the character of the starting torque produced by
splitting the phase?~
Ans. It does not give strong starting torque.
[Illustration: FIG. 1,863.--Single phase fan motor with _shading
coils_ for starting. In addition to the main field coils, one tip of
each pole piece is surrounded by a short circuited coil of wire or
frame of copper, as indicated in the figure. This coil, or copper
frame, is called a _shading coil_ and it causes a phase difference
between the pulsating flux that emanates from the main portion of each
polar projection and the pulsating flux which emanates from the pole
tip, thus introducing a two phase action on the armature which is
sufficiently pronounced to start the motor.]
~Ques. How is the plain squirrel cage armature modified to enable the
motor to start with a heavier load?~
Ans. An automatic clutch is provided which allows the armature to turn
free on the shaft until it accelerates almost to running speed.
This type motor is known as the _clutch type_ of single phase
induction motor. In operation when the circuit is closed, the
armature starts to revolve upon the shaft; when it reaches a
premeditated speed, a centrifugal clutch expands and engages
the clutch disc, which is fastened to the shaft.
[Illustration: FIG. 1,864.--Diagram showing action of shading coil in
alternating current motor. The extremities of these pole pieces are
divided into two branches, one of which a copper ring called a _shading
coil_ is placed as shown, while the other is left _unshaded_. The
action of the shading coils is as follows: Consider the field poles
to be energized by single phase current, and assume the current to be
flowing in a direction to make a north pole at the top. Consider the
poles to be just at the point of forming. Lines of force will tend to
pass downward through the shading coil and the remainder of the pole.
Any change of lines within the shading coil generates an e.m.f., which
causes to flow through the coil a current of a value depending on the
e.m.f. and always in a direction to oppose the change of lines. The
field flux is, therefore, partly shifted to the free portion of the
pole, while the accumulation of lines through the shading coil is
retarded.]
[Illustration: FIGS. 1,865 and 1,866.--Fort Wayne split phase factional
horse power induction motor with stationary armature. The object of
placing the squirrel cage armature winding on the stationary part or
frame is to decrease the radial depth of the latter more than would
be possible with the usual arrangement where the armature forms the
rotating part. The small radial depth of the stationary armature
makes possible a revolving field of maximum diameter giving in turn
an exceptionally large air gap area, which reduces the magnetizing
current, hence improves the power factor of the motor.]
~Ques. Explain in detail the action of the clutch type of motor in
starting.~
Ans. It can start a load which requires much more than full load torque
at starting, because the motor being nearly up to full speed, has
available not only its maximum overload capacity, but also the momentum
of the armature to overcome the inertia of the driven apparatus. In
this it is assisted by a certain amount of slippage in the clutch,
which is the case when the armature speed is pulled down to such a
point as to reduce the grip of the centrifugal clutch.
[Illustration: FIGS. 1,867 and 1,868.--General Electric disassembled
clutch as used on clutch type, single phase (KS) induction motor. In
starting, the armature revolves freely on the shaft until approximately
75 per cent. of normal rated speed is reached. The load is then picked
up by the automatic action of a centrifugal clutch, which rigidly
engages an outer shell, keyed directly to the shaft. The brass friction
band of the clutch is permanently keyed to the pulley end of the
armature.]
~Commutator Motors.~--Machines of this class are similar in general
construction to direct current motors. They have a closed coil winding,
which is connected to a commutator.
There are several types of commutator motor, namely:
1. Series;
2. Shunt;
3. Compensated;
4. Repulsion.
Since, as stated, commutator motors are similar to direct current
motors, the question may be asked: Is it possible to run a direct
current motor with alternating current? If the mains leading to a
direct current motor be reversed, the direction or rotation remains the
same, because the currents through both the field magnets and armature
are reversed. It must follow then that an alternating current applied
to a direct current motor would cause rotation of the armature.
[Illustration: FIG. 1,869.--Wagner single phase variable speed
commutator motor. The commutator is of the regular horizontal type
and the brushes remain in contact all the time. As the torque of
alternating motors varies directly as the square of the applied
pressure, wide speed variation may be obtained by varying the voltage
applied at the motor terminals.]
~Action of Closed Coil Rotating in Alternating Field.~--When a closed
coil rotates in an alternating field, there are several different
pressures set up and in order to carefully distinguish between them,
they may be called:
[Illustration: FIGS. 1,870 and 1,871.--Diagrams illustrating
construction and operation of Wagner "unity power factor" single
phase motor. In the field construction, fig. 1,870, two windings are
used. The main winding 1 produces the initial field magnetization
as heretofore; the auxiliary winding 2 controls the power factor or
"compensates" the motor. The main structural departure is in the
armature, the construction of which is more clearly indicated in fig.
1,871. Here again two windings are employed. The main or principal
winding 4 is of the usual well known squirrel cage type and occupies
the bottom of the armature slots. The second or auxiliary winding 3
is of the usual commuted type, is connected to a standard form of
horizontal commutator and occupies the upper portion of the armature
slots. Between the two is placed a magnetic separator in the form of
a rolled steel bar. Two sets of brushes are provided, as indicated
in the diagram of connections shown in fig. 1,870. The main pair of
brushes 5-6 is placed in the axis of the main field winding 1 and is
short circuited. The auxiliary pair of brushes 7-8 is placed at right
angles to the axis of the main field winding and is connected in series
with it at starting. The auxiliary field winding 2 is permanently
connected to one auxiliary brush 7, and is adapted to be connected to
the other auxiliary brush 8 by means of the switch 9. The purpose of
the peculiar armature construction illustrated in fig. 1,871 and of the
brush arrangement and connections shown in fig. 1,870 is to accentuate,
at starting, the effect of the squirrel cage along the axis 5-6 of the
main field winding 1, while suppressing it as far as possible along
the axis 7-8 at right angles to main winding. The magnetic separator
placed above the squirrel cage winding 4 tends to suppress the effect
of that winding along all axes, by making it less responsive to outside
inductive effects. But the influence of the separator is nullified
along the axis of the main field winding by the presence of the short
circuited brushes 5-6, while no means are provided for nullifying
its effects along the axis at right angles to that of the main field
winding. Thus the main field winding 1 will be able to induce heavy
currents in both armature windings because of the short circuited
brushes in the axis 5-6, and in spite of the magnetic separator; while
the armature winding 3, connected in series with 1, will not be able to
produce heavy currents in the squirrel cage winding 4 along the axis
7-8 because of the magnetic separator between 3 and 4, which shunts
or side tracks the inducing magnetic flux. In operation, at starting,
switch 9 of fig. 1,870 is open, the commuted winding 3 along the axis
7-8 being connected in series with the main field winding 1 and across
the mains. The winding 1 induces a large current in the armature
windings 3 and 4 along the axis 5-6, and the winding 3 produces a
large flux along the axis 7-8. The armature currents in the main axis
co-acting with the flux threading the armature along the auxiliary axis
yield the greater part of the starting torque. As the motor speeds
up, the squirrel cage gradually assumes those functions which it
performs in the ordinary single phase, squirrel cage motor, developing
a magnetic field of its own along the axis 7-8 and a correspondingly
powerful torque, which increases very rapidly as synchronism is
approached, but falls suddenly to zero at or near actual synchronism.
It is known that the magnetizing currents circulating in the bars of
the squirrel cage of a single phase motor have, at synchronism, double
the frequency of the stator currents; the fluxes they produce must
therefore also be of double frequency. Now, the magnetic separator is
made of solid steel, and, while this separator forms a sufficiently
effective shunt for the fluxes of line frequency induced from the
field, it is quite ineffective as a shunt for the double frequency
fluxes produced by the armature. With respect to the squirrel cage, the
effect of this magnetic separator diminishes with increasing speed, and
at synchronism the machine operates practically in the same manner as
if the magnetic separator did not exist.]
1. The transformer pressure;
2. The generated pressure;
3. The self-induction pressure.
These pressures may be defined as follows:
~The transformer pressure~ _is that pressure induced in the armature by
the alternating flux from the field magnets._
[Illustration: FIG. 1,872.--Diagram of ring armature in alternating
field illustrating the principles of commutator motors.]
For instance, assuming in fig. 1,872 the armature to be at
rest, as the alternating current which energizes the magnets
rises and falls in value, the variations of flux which threads
through the coils of the ring winding, induce pressure in them
in just the same way that pressure is induced in the secondary
of a transformer.
A ring winding is used for simplicity; the same conditions
obtain in a drum winding.
~The generated pressure~ is that pressure _induced in the armature by
the cutting of the flux when the armature rotates_.
~The self-induction pressure~ _is that pressure induced in both the
field and armature by self-induction._
~Nature of the Generated Pressure.~--In fig. 1,872, the
generated pressure induced by the rotation of the armature is
minimum at the neutral plane C D and maximum at A B. It tends
to cause current to flow up each half of the armature from D to
C, producing poles at these points.
[Illustration: FIG. 1,873.--Wagner single phase repulsion induction
commutator motor. Its working principle is _repulsion start_ and
_induction operation_. Starting with the machine at rest, brushes in
pairs cross connected through a low resistance conductor, bear upon
the commutator, temporarily short circuiting the armature winding then
developing a strong starting torque on the repulsion principle. On
attaining full load speed the individual segments of the commutator
are all positively connected together by the operation of an automatic
centrifugal governor, thereby transforming the armature winding to the
squirrel cage form, the motor then continuing as an induction motor.
The governor at the same time removes the brushes from contact with
the commutator to save wear. If the power service should fail for any
reason, the motor returns to the starting condition, and picks up its
load when the power comes on again without attention of the operator.]
~Nature of the Transformer Pressure.~--This is caused by
variations of the flux passing through each coil of the
armature winding. Evidently this variation is least at the
plane A B because at this point the coils are inclined very
acutely to the flux, and greatest at the plane C D where
the coils are perpendicular to the flux. Accordingly, the
transformer pressure induced in the armature winding is least
at A B and greatest at C D.
The transformer pressure acts in the same direction as the
generated pressure as indicated by the long arrows and gives
rise to what may be called _local armature currents_.
[Illustration: FIGS. 1,874 and 1,875.--Armature of Wagner single phase
repulsion-induction commutator motor as seen from the commutator
and rear ends, showing the vertical commutator and type of governor
employed on the smaller sizes. The operation is explained in fig.
1,873.]
~Nature of the Self-induction Pressure.~--The self-induction
pressure, being opposite in direction to the impressed
pressure, it must be evident that in the operation of an
alternating current commutator motor, the impressed pressure
must overcome not only the generated
[Illustration: FIG. 1,876.--General Electric single phase compensated
repulsion motor. The frame is of the riveted form and the field winding
consists of distributed concentric coils, each being separately
insulated and taped up to each core slot. The compensating winding
(depending usually on the size of frame), forms either the center
portion of the main winding or a separate winding concentric therewith.
The polar groupings are arranged for a frequency of 25 and 60. There
are four terminal leads permitting interchangeability of operation
on 110 or 220 volt circuits. By connecting adjacent pairs of these
terminals in multiple, motors of this type are made adaptable for 110
volt service; for double this pressure the four leading in wires are
connected in series. The motor will operate satisfactorily where the
arithmetical sum of voltage and frequency variation does not exceed
10 per cent.; that is, the voltage may be 10 per cent. high if the
frequency remain at normal, or the frequency may be 10 per cent.
high assuming no variation in voltage. A decrease of 5 per cent. in
frequency accompanied by a similar increase in voltage is permissible
or, as above stated, any similar combination whose arithmetical sum is
within 10 per cent. of normal. The armature winding is of the series
drum type connected to a commutator carrying two sets of brushes,
each set being displaced electrically from the other by 90 degrees.
The first set, known as the ~energy brushes~, is permanently short
circuited and disposed at an angle to the lines of field or primary
magnetization, as in an ordinary repulsion motor. The second set, or
~compensating brushes~, is connected to a small portion of the primary
winding included in the field circuit, so as to impress upon the
armature an electromotive force, which serves both to raise the power
factor and at the same time maintain approximately synchronous speed at
all loads. The armature laminations are built up on a cast iron sleeve
having the same inside bore as the commutator. In case the shaft become
damaged or worn, it can be readily pressed out and replaced without
disturbing the commutator or windings. The motor is connected to run
counter clockwise. Clockwise rotation is obtained by interchanging
the leads to the compensating brushes and slightly shifting the brush
holder yoke. This type motor may be thrown on the line without the use
of a rheostat, and is suitable for operating refrigerating machines,
air compressors, house pumps or similar apparatus where a float switch
or pressure regulator is used to close or open the supply circuit.] /#
pressure but also the self-induction pressure. Hence, as compared to an
equivalent direct current motor, the applied voltage must be greater
than in the direct current machine, to produce an equal current.
[Illustration: FIG. 1,877.--Armature of General Electric single phase
compensated repulsion motor, assembled ready for dip and banding.]
[Illustration: FIG. 1,878.--Cast brush rigging of General Electric
single phase compensated repulsion motor as used for the 3 and 5 horse
power motors.]
~The Local Armature Currents.~--These currents produced by the
transformer pressure occur in those coils undergoing commutation. They
are large, because the maximum transformer action occurs in them, that
is, in the coils short circuited by the brushes.
~Ques. Why do the local armature currents cause sparking?~
Ans. Because of the sudden interruption of the large volume of current,
and also because the flux set up by the local currents being in
opposition to the field flux, tends to weaken the field just when and
where its greatest strength is required for commutation.
[Illustration: FIG. 1,879.--Field of Sprague single phase compensated
repulsion motor. The frame is of the skeleton form which exposes the
core, giving effective heat radiation. The single phase field winding
is of the distributed concentric type. To facilitate connection to
circuits of either 110 or 220 volts, four plainly tagged leads are
brought out to the back of the removable terminal board.]
~Ques. What is the strength of the local current?~
Ans. They may be from 5 to 15 times the strength of the normal armature
current.
~Ques. Upon what does the local armature current depend?~
Ans. Upon the number of turns of the short circuited coils, their
resistance, and the frequency.
~Ques. How can the local currents be reduced to avoid heavy sparking?~
Ans. 1. By reducing the number of turns of the short circuited coils,
that is, providing a greater number of commutator bars; 2, reducing the
frequency; and 3, increasing the resistance of the short circuited coil
circuit: _a_, by means of high resistance connectors; or _b_, by using
brushes of higher resistance.
[Illustration: _Figs._ 1,880 to 1,884.--Assembly and disassembled view
of short circuiting device as used on Bell single phase repulsion
induction motor. The armature, which is wound in a similar manner to
those used in direct current motors, has a commutator, and brushes,
which being short circuited on themselves, allow great starting
torque, with small starting current. The motor starts by the repulsion
principle, and on reaching nearly full speed, a centrifugal governor
pushes the copper ring against the commutator segments, thereby
short circuiting them, and the motor then operates on the induction
principle.]
~Ques. What are high resistance connectors?~
Ans. The connectors between the armature winding and the commutator
bars, as shown in fig. 1,885.
~Ques. Does the added resistance of preventive leads, or high
resistance brushes, materially reduce the efficiency of the machine?~
Ans. Not to any great extent, because it is very small in comparison
with the resistance of the whole armature winding.
[Illustration: FIG. 1,885.--Section of ring armature of commutator
motor showing local current set up by transformer action of the
alternating flux.]
~Ques. What is the objection to reducing the number of turns of the
short circuited coils to diminish the tendency to sparking?~
Ans. The cost of the additional number of commutator bars and
connectors as well as the added mechanism.
~Ques. What effect has the inductance of the field and armature on the
power factor?~
Ans. It produces phase difference between the current and impressed
pressure resulting in a low power factor.
~Ques. What is the effect of this low power factor?~
Ans. The regulation and efficiency of the system is impaired.
The frequency, the field flux and the number of turns in the
winding have influence on the power factor.
~Ques. How does the frequency affect the power factor?~
Ans. Lowering the frequency tends to improve the power factor.
The use of very low frequencies has the disadvantage of
departing from standard frequencies, and the probability that
the greater cost of transformers and alternators would offset
the gain.
[Illustration: FIG. 1,886.--General Electric 5 H.P., 6 pole adjustable
speed single phase compensated repulsion motor. This type is suitable
for service requirements demanding the use of a motor whose speed can
be adjusted over a considerable range, this speed at a fixed controller
setting remaining practically unaffected by any load within the
motor's rated capacity. With the controller on the high speed points,
the motor possesses an inherent speed regulation between no load and
full load of approximately 6 per cent. At the low speed points, under
similar load conditions, the speed variation will be approximately 20
per cent. To secure adjustable speed control, the armature circuits
employ transformers, whose primaries are excited by the line circuit.
The secondaries of these transformers are divided into two sections;
the first or "regulating" circuit is placed across the _energy_
brushes; the other section, since it is connected in series with
the compensating winding, maintains the high power factor and speed
regulation obtained in the constant speed type. The speed range is 2:1,
approximately one-half of this range being below and one-half above
synchronous speed.]
~Series Motors.~--This class of commutator motor is about the simplest
of the several types belonging to this division. In general design the
series motor is identical with the series direct current motor, but all
the iron of the magnetic circuit must be laminated and a _neutralizing
winding_ is often employed.
It will be readily understood that the torque is produced in the same
way as in the direct current machine, when it is remembered that the
direction of rotation of the direct current series motor is independent
of the direction of the voltage applied.
At any moment the torque will be proportional to the product of the
current and the flux which it is at that moment producing in the
magnetic system, and the average torque will be the product of the
average current and the average flux it produces, so that if the iron
parts be unsaturated, as they must be if the iron losses are not to be
too high, _the torque will be proportional simply to the square of the
current_, there being no question of power factor entering into the
consideration.
[Illustration: FIG. 1,887.--Diagram of single phase series commutator
motor. It is practically the same as the series direct current motor,
with the exception that all the metal of the magnetic circuit must be
laminated.]
~Ques. What are the characteristics of the series motor?~
Ans. They are similar to the direct current series motor, the torque
being a maximum at starting and decreasing as the speed increases.
~Ques. For what service is the series motor especially suited?~
Ans. On account of its powerful starting torque it is particularly
desirable for traction service.
~Neutralized Series Motor.~--A chief defect of the series motor is the
excessive self-induction of the armature, hence in almost every modern
single phase series motor a neutralizing coil is employed _to diminish
the armature self-induction_.
The neutralizing coil is wound upon the frame 90 magnetic degrees
or half a pole pitch from the field winding and arranged to carry a
current equal in magnetic pressure and opposite in phase to the current
in the armature.
[Illustration: FIG. 1,888.--Diagram of neutralized series motor;
~conductive method~. In the simple series motor, there will be
a distortion of the flux as in the direct current motor. As the
distorting magnetic pressure is in phase with that of the magnets, the
distortion of the flux will be a fixed effect. If the poles be definite
as in direct current machines, this distortion may not seriously affect
the running of the motor, but with a magnetizing system like that
universally adopted in induction motors the flux will be shifted as a
whole in the direction of the distortion, which will produce the same
effect as if in the former case the brushes had been shifted forward,
whereas for good commutation they should have been shifted backward.
As in direct current machines, this distortion is undesirable since it
is not conducive to sparkless working, and also reduces to a more or
less extent the torque exerted by the motor. The simplest remedy is
to provide _neutralizing coils_ displaced 90 magnetic degrees to the
main field coils as shown. The neutralizing current is obtained by the
method of connecting the _neutralizing coils_ in series in the main
circuit.]
The current through the neutralizing winding may be obtained, either
1. Conductively; or
2. Inductively.
In the conductive method, fig. 1,888, the winding is connected
in series as shown.
In the inductive method, fig. 1,889, the winding is short
circuited upon itself and the current obtained inductively,
the neutralizing winding being virtually the secondary of a
transformer, of which the armature is the primary.
~Ques. When is the conductive method to be preferred?~
Ans. When the motor is to be used on mixed circuits.
[Illustration: FIG. 1,889.--Diagram of neutralized series motor;
~inductive method~. Although the conductive method of neutralization
is employed in nearly all machines, it is possible merely to short
circuit the neutralizing winding upon itself, instead of connecting
it in series with the armature circuit. In this case the flux due to
the armature circuit cannot be eliminated altogether, as sufficient
flux must always remain to produce enough pressure to balance that
due to the residual impedance of the neutralizing coil. It would be
a mistake to infer, however, that on this account this method of
neutralization is less effective than the conductive one, since the
residual flux simply serves to transfer to the armature circuit a drop
in pressure precisely equivalent to that due to the resistance and
local self-induction of the neutralizing coil in the conductive method.]
~Shunt Motors.~--The simple shunt motor has inherently many properties
which render it unsuitable for practical use, and accordingly is of
little importance. Owing to the many turns of the field winding there
is large inductance in the shunt field circuit.
[Illustration: FIG. 1,890.--Diagram of simple shunt commutator motor.
Owing to its many inherent defects it is of little importance.]
[Illustration: FIG. 1,891.--Compensated shunt induction single phase
motor. The transformer shown in the arrangement is capable of being
replaced by a coil placed on the frame having the same axis as the
field winding, so that the flux produced by the field winding induces
in the coil a pressure in phase with the supply pressure. Such a coil
will now be at right angles to the circuit to which it is connected. In
a similar manner a coil at right angles to the armature circuit, that
is, the circuit parallel to the stator axis, if connected in series
with that circuit, will also serve to compensate the motor.]
The inductance of the armature is small as compared with that of the
field; accordingly, the two currents differ considerably in phase.
The phase difference between the field and armature currents and the
corresponding relation between the respective fluxes results in a weak
torque.
[Illustration: FIG. 1,892.--Fynn's shunt conductive single phase motor.
In order to supply along the stator axis a constant field, suitable for
producing the cross flux to which the torque is due by its action on
the circuit perpendicular to the stator axis, the "armature circuit,"
as it may be called, has a neutralizing coil in series with it, so that
the armature circuit and neutralizing coil together produce no flux.
In addition to this, there is a magnetizing coil along the same axis,
which is connected across the mains and so produces the same flux as
the primary coil in a shunt induction machine. Fynn has proposed a
number of methods of varying the speed and compensating this machine.
It is, however, complicated in itself, and is only suited for very
low voltages, so that on ordinary circuits it would need a separate
transformer.]
_It is necessary to use laminated construction in the field circuit to
avoid eddy currents_, which otherwise would be excessive. Fig. 1,890 is
a diagram of a simple shunt commutator motor.
~Repulsion Motors.~--In the course of his observations on the effects
of alternating currents, in 1886-7, Elihu Thomson observed that a
copper ring placed in an alternating magnetic field tends either to
move out of the field, that is, it is _repelled_ by the field (hence
the name ~repulsion motor~), or to return so as to set itself edgeways
to the magnetic lines.
The explanation of the repulsion phenomenon is as follows:
When a closed coil is suspended in an alternating field so that lines
of force pass through it, as in fig. 1,893, an alternating pressure
will be induced in the coil which will be 90° later in phase than
the inducing flux, and since every coil contains some inductance the
resulting current will lag more or less with respect to the pressure
induced in the coil.
[Illustration: FIG. 1,893.--Effect of alternating field on copper ring.
_If a copper ring be suspended in an alternating field so that the
plane of the ring is oblique to the lines of force, it will turn until
its plane is parallel to the lines of force_, that is, to the position
in which it does not encircle any lines of force. The turning moment
acting upon the ring is proportional to the current in it, to the
strength of the field, and to the cosine of the angle ß.
Hence it is proportional to the product sin ß cos ß. The tendency to
turn is zero both at 0° and at 90°; in the former case because there
is no current, in the latter because the current has no leverage. It
is a maximum when ß = 45°. Even in this position there would be no
torque if there were no lag of the currents in the ring, for the phase
of the induced pressure is in quadrature with the phase state of the
field. When the field is of maximum strength there is no pressure, and
when the pressure reaches its maximum there is no field. If there be
self-induction in the ring causing the current to lag, there will be a
net turning moment tending to diminish ß. The largest torque will be
obtained when the lag of the current in the ring is 45°.]
The cosine of this phase relation becomes a negative quantity which
means that the coil is ~repelled~ by the field.
_It is only when the ring is in an oblique position that it tends to
turn._ If it be placed with its plane directly at right angles to
the direction of the magnetic lines, it will not turn; if ever so
little displaced to the right or left, it will turn until its plane is
parallel to the lines.
[Illustration: FIGS. 1,894 to 1,908.--Parts of General Electric
single phase compensated repulsion motor. The field frame employs the
riveted form of construction, so that the ends of the laminations are
exposed directly to the air, insuring low operating temperatures and
high overload capacity. The field winding consists of a main winding
of the distributed concentric type and a compensating winding. The
series type of winding is employed, and the completed rotor is treated
with a special insulating compound, which renders the coils moisture
proof under ordinary conditions. On motors of more than 2 horse power
capacity a ventilating fan is attached to the rotor which provides a
continuous supply of cool air while the motor is in operation. Two
types of brush holder yoke are used. The smaller motors use a moulded
yoke of insulting compound, reinforced by a cast iron L section ring
embedded in the moulded structure. Cast iron yokes are used on larger
motors. The brushes are of carbon with copper pigtails, which carry all
the current. The brushes in this machine remain permanently in contact
with the commutator. The parts are: A, field; B, field winding; C,
line terminal; D, tube terminal; E, compensating terminal; F, terminal
board; G, brush yoke; H, brush holder; I, carbon brush; J, brush stud;
K, short circuit connection; L, armature; M, commutator, N, shaft; O,
fan; P, commutator end shield; Q, pulley end shield; R, oil well cover;
S, oil plugs; T, oil gauge; U, bearing lining; V, oil ring; W, pulley;
X, pulley set screw; Y, commutator end shield holding bolts; Z, pulley
end shield holding bolts; AA, base; BB, float bolts; CC, belt tightener
screw.]
The production of torque may be explained by saying that the current
induced in the ring produces a cross field which being out of phase
with, and inclined to the field impressed by the primary alternating
current, causes a rotary field, and this in turn, reacting on the
conductor, a turning moment results.
[Illustration: FIG. 1,909.--Fynn's compensated shunt induction motor.
This is a combination of the compensated shunt induction motor with the
ordinary squirrel cage form. In one form, in addition to the ordinary
drum winding on the armature, there is another three phase winding
into the "star," of which the drum winding is connected. This second
winding is connected to three slip rings which are short circuited when
the machine is up to speed. Upon the commutator are placed a pair of
brushes connected to an auxiliary winding placed on the frame in such a
position that the flux from the primary coil induces in it a pressure
of suitable phase to produce compensation. The same pair of brushes is
also used for starting.]
Elihu Thompson took an ordinary direct current armature, placed
it in an alternating field, and having short circuited the
brushes, placed them in an oblique position with respect to the
direction of the field. The effect was to cause the armature to
rotate with a considerable torque.
The inductors of the armature acted just as an obliquely
placed ring, but with this difference, that the obliquity
was continuously preserved by the brushes and commutator,
notwithstanding that the armature turned, and thus the rotation
was continuous. This tendency of a conductor to turn from
an oblique position was thus utilized by him to get over
the difficulty of starting a single phase motor. With this
object in view he then constructed motors in which the use of
commutator and brushes was restricted to the work of merely
starting the armature, which when so started was then entirely
short circuited on itself, though disconnected from the rest of
the circuit, the operation then being solely on the induction
principle.
[Illustration: Fig. 1,910.--Diagram of connection of Sprague single
phase compensated repulsion motor. To reverse direction of rotation
interchange leads C₁ and C₂ and slightly shift the brush holder yoke.
Brushes E₁ and E₂ are permanently short circuited. This diagram of
connections applies also to fig. 1,911.]
~Ques. What difficulty was experienced with Thomson's motor?~
Ans. Since an open coil armature was used, the torque developed was due
to only one coil at a time, which involved a necessarily high current
in the short circuited coil resulting in heavy sparking.
~Ques. How was this remedied?~
Ans. By the use of closed coil armatures in later construction.
~Ques. Did this effectually stop sparking?~
Ans. No.
~Ques. What other means is employed in modern designs to reduce
sparking?~
Ans. Compensation and the use of a distributed field winding, high
resistance connectors, high resistance brushes, etc.
~Ques. What are the names of the two classes of repulsion motor?~
Ans. The simple and the compensated types.
~Ques. Describe a simple repulsion motor.~
Ans. It consists essentially of an armature, commutator and field
magnets. The armature is wound exactly like a direct current armature,
and the windings are connected to a commutator. The carbon brushes
which rest on this commutator are not connected to the outside line,
however, but are all connected together through heavy short circuiting
connectors. The brushes are placed about 60° or 70° from the neutral
axis. The field is wound exactly like that of the usual induction motor.
~Ques. What is the action of this type of motor?~
Ans. If nothing be done to prevent, the motor will increase in speed at
no load until the armature bursts, just as it will in a series direct
current motor.
~Ques. What provision is made to avoid this danger?~
Ans. A governor is usually mounted on the armature which short circuits
the windings, after the motor has been started. The motor then runs as
a squirrel cage induction motor. As a rule the brushes are lifted off
the commutator when the armature is short circuited, so as to prolong
their life.
This is a very successful motor, but it is of course more
costly than the simple squirrel cage motor used on two and
three-phase circuits.
[Illustration: FIG. 1,911.--Diagram of connections of Sprague variable
speed single phase compensated repulsion motor and controller. The
controller is designed to give speed reduction and speed increase as
resistance or reactance is inserted in the energy and compensating
circuits. With the exception of the leads brought out from these
circuits, the constant speed and variable speed motors are identical.
The standard controller gives approximately 2:1 speed variation.]
~Ques. What name may appropriately be applied to the motor?~
Ans. It may be called the _repulsion induction motor,_ because it is
constructed for repulsion start and induction running.
~Ques. Describe a compensated repulsion motor.~
Ans. In its simplest form it consists of a simple repulsion motor in
which there are two independent sets of brushes, one set being short
circuited, while the other set is in series with the field magnet
winding, as in the series alternating current motor.
~Ques. What names are given to the two sets of brushes on a compensated
repulsion motor?~
Ans. The _energy_ or main short circuiting brushes, and the
_compensating_ brushes.
[Illustration: Fig. 1,912.--Diagram of connections of Sprague reversing
type of single phase compensated repulsion motor. As shown, there is a
special reverse field winding having terminals for connection to a four
pole double throw switch.]
~Ques. What is the behavior of the armature of a compensated repulsion
motor at starting?~
Ans. It possesses at starting most of the apparent reactance of the
motor, and the effect of speed is to decrease such apparent reactance,
the latter becoming zero at either positive or negative synchronism,
and negative at higher speeds in either direction.
~Ques. What is the nature of the field circuit of the compensated
repulsion motor at starting?~
Ans. At starting it is practically non-inductive, the effect of speed
being to introduce a spurious resistance which increases directly with
the speed, and becomes negative when the speed is reversed.
~Ques. For what use is the compensated repulsion motor especially
adapted?~
Ans. For light railroad service.
~Ques. When employed thus what is the method of control?~
Ans. A series transformer is used in the field circuit.
~Ques. What frequencies are employed with this motor?~
Ans. 25 to 60, the preferred frequency being 40.
~Ques. To what important use is the repulsion principle put?~
Ans. It is sometimes employed for starting on single phase induction
motors.
In this method, after bringing the motor up to speed, the
winding is then short circuited upon itself, and the motor then
operates on the induction principle.
~Ques. What name is given to this type of motor?~
Ans. It is called the repulsion induction motor.
~Power Factor of Induction Motors.~--In the case of a direct current
motor, the energy supplied is found by multiplying the current
strength by the voltage, but in all induction motors the effect of
self-induction causes the current to lag behind the pressure, thereby
increasing the amount of current taken by the motor. Accordingly, as
the increased current is not utilized by the motor in developing power,
the value obtained by multiplying the current by the voltage represents
an _apparent energy_ which is greater than the real energy supplied to
the motor.
[Illustration: Fig. 1,913.--Fairbanks-Morse squirrel cage armature,
showing ball bearings.]
It is evident, that if it were possible to eliminate the lag entirely,
the real and apparent watts would be equal, and the power factor would
be unity.
The importance of power factor and its effect upon both alternator
capacity and voltage regulation is deserving of the most careful
consideration with all electrical apparatus, in which an inherent phase
difference exists between the pressure and the current, as for instance
in static transformers and induction motors.
While the belief is current that any decrease in power factor from
unity value does not demand any increase of mechanical output, this
is not true, since all internal alternator and line losses manifest
themselves as heat, the wasted energy to produce this heat being
supplied by the prime mover.
Apart from the poor voltage regulation of alternating current
generators requiring abnormal field excitation to compensate for
low power factor, some of the station's rated output is rendered
unavailable and consequently produces no revenue. The poor steam
economy of underloaded engines is also a serious source of fuel wastage.
[Illustration: Fig. 1,914.--Fairbanks-Morse 20 horse power squirrel
cage induction motor connected to a 20 inch self-feed rip and
chamfering saw. The absence of commutator and brushes on the squirrel
cage armature eliminates sparking and therefore renders this type of
motor particularly adapted for use in places where sparking would be
dangerous, such as in wood working plants, textile mills, etc.]
Careful investigations have shown that the power factor of
industrial plants using induction motor drive with units
of various sizes will average between 60 and 80 per cent.
With plants supplying current to underloaded motors having
inherently high lagging current values, a combined factor as
low as 50 per cent. may be expected. Since standard alternators
are seldom designed to carry their rated kilowatt load at less
than 80 per cent. power factor, the net available output is,
therefore, considerably increased.
[Illustration: Fig. 1,915.--Method of casting end rings on squirrel
cage armatures of Fairbanks-Morse induction motors. The metal being
fused to the bars at a temperature in excess of 1,832 degrees Fahr., it
is readily seen that the destructive effect of any subsequent heating
is eliminated. While giving the most intimate contact at the joints, a
multiplicity of joints is avoided as well as solder.]
~Speed and Torque of Motors.~--The speed of an induction motor depends
chiefly on the frequency of the circuit and runs within 5 per cent. of
its rated speed; it will produce full torque if the line voltage do not
vary more than 5 to 10 per cent.
At low voltage the speed will not be greatly reduced as in a direct
current motor, but as the torque is low the motor is easily stopped
when a light load is thrown on.
The current taken by an induction motor from a constant pressure
line varies with the speed as in a direct current motor. When a
load is thrown on, the speed is reduced correspondingly and as the
self-induction or reactance is diminished, more current circulates in
the squirrel cage winding, which in turn reacts on the field coils in
a similar manner and more current flows in them from the line. In this
manner the motor automatically takes current from the line proportional
to the load and maintains a nearly constant speed.
The so-called constant speed motors require slight variations in speed
to automatically take current from the line when the load varies.
Induction motors vary in speed from 5 to 10 per cent., while
synchronous motors vary but a fraction of one per cent.
Single phase motors to render efficient service must be able,
where requisite, to develop sufficient turning moment or torque
to accelerate, from standstill, loads possessing large inertia or
excessive static friction; for example, meat choppers and grinders,
sugar or laundry centrifugals; heavy punch presses; group driven
machines running from countershafts with possibly over taut belting,
poor alignment, lubrication, etc.
CHAPTER LII
TRANSFORMERS
The developments in the field of electrical engineering which have
rendered feasible the transmission of high pressure currents over long
distances, together with the reliability and efficiency of modern
generating units, have resulted in notable economies in the generation
and distribution of electric current.
This has been accomplished largely by the use of distant water power or
the centralization of the generating plants of a large territory in a
single power station.
The transformer is one of the essential factors in effecting the
economical distribution of electric energy, and may be defined as _an
apparatus used for changing the voltage and current of an alternating
circuit_. A transformer consists essentially of:
1. A primary winding;
2. A secondary winding;
3. An iron core.
~Basic Principles.~--If a current be passed through a coil of wire
encircling a bar of soft iron the iron will become a magnet; when the
current is discontinued the bar loses its magnetization.
_Conversely:_ If a bar of iron carrying a coil of wire be magnetized
in a direction at right angles to the plane of the coil a momentary
electric pressure will be induced in the wire; if the current be
reversed, another momentary pressure will be induced in the opposite
direction in the coil.
These actions are fully explained in chaps. X and XI, and as they are
perfectly familiar phenomena, a detailed explanation of the principles
upon which they depend is not necessary here.
From the first two statements given above it is evident that
if a bar of iron be provided with two coils of wire, one of
which is supplied from a source of alternating current, as
shown diagrammatically by fig. 1,916, at each impulse of the
exciting current a pressure will be induced in the secondary
coil, the direction of these impulses alternating like that of
the exciting current.
~Ques. What name is given to the coil through which current from the
source flows?~
Ans. _The primary winding._
[Illustration: Fig. 1,916.--Diagram of elementary transformer with
non-continuous core and connection with single phase alternator. The
three essential parts are: primary winding, secondary winding, and an
iron core.]
~Ques. What name is given to the coil in which a current is induced?~
Ans. _The secondary winding._
Similarly, the current from the source (alternator) is called
the _primary current_ and the induced current, the _secondary
current_.
~Ques. What is the objection to the elementary transformer shown in
fig. 1,916?~
Ans. The non-continuous core. With this type core, the flux emanating
from the north pole of the bar has to return to the south pole through
the surrounding air; and as the reluctance of air is much greater than
that of iron, the magnetism will be weak.
~Ques. How is this overcome?~
Ans. By the use of a continuous core as shown in fig. 1,917.
~Ques. Is this the best arrangement, and why?~
Ans. No. If the windings were put on as in fig. 1,917, the leakage of
magnetic lines of force would be excessive, as indicated by the dotted
lines. In such a case the lines which leak through air have no effect
upon the secondary winding, and are therefore wasted.
[Illustration: Fig. 1,917.--Diagram of elementary transformer with
continuous core and connections with alternator. The dotted lines show
the leakage of magnetic lines. To remedy this the arrangement shown in
fig. 1,918 is used.]
~Ques. How is the magnetic leakage reduced to a minimum in commercial
transformers?~
Ans. In these, and even in ordinary induction coils (the operating
principle of which is the same as that of transformers) the magnetic
leakage is reduced to the lowest possible amount by arranging the coils
one within the other, as shown in cross section in fig. 1,918.
~The Induced Voltage.~--The pressure induced in the secondary winding
will depend on the _ratio_ between the number of turns in the two
windings. For example, a transformer with 500 turns of wire in its
primary winding and 50 turns in its secondary winding would have a
transformation ratio of 10 to 1, and if it were supplied with primary
current at 1,000 volts, the secondary pressure at no load would be 100
volts.
[Illustration: Fig. 1,918.--Cross section showing commercial
arrangement of primary and secondary windings on core. One is
superposed on the other. This arrangement compels practically all of
the magnetic lines created by the primary winding to pass through the
secondary winding.]
EXAMPLE.--If ten amperes flow in the primary winding and the
transformation ratio be 10, then 10 × 10 = 100 amperes will
flow through the secondary winding.
Thus, a direct proportion exists between the pressures and
turns in the two windings and an inverse proportion between the
amperes and turns, that is:
_primary voltage: secondary voltage = primary turns: secondary turns_
_primary current: secondary current = secondary turns: primary turns_
From the above equations it is seen that the watts of the primary
circuit equal the watts of the secondary circuit.
~Ques. Are the above relations strictly true, and why?~
Ans. No, they are only approximate, because of transformer losses.
In the above example, the total wattage in the primary circuit
is 1,000 × 10 = 10 kw., and that in the secondary circuit
is 100 × 100 = 10 kw. Hence, while both volts and amperes
are widely different in the two circuits, the watts for each
are the same in the ideal case, that is, assuming perfect
transformer action or 100% efficiency. Now, the usual loss in
commercial transformers is about 10%, so that the actual watts
delivered in the secondary circuit is (100 × 100) × 90% = 9 kw.
[Illustration: FIG. 1,919.--Wagner transformer coil formed, ready for
taping. These are known as "pan cake" coils. They are wound with flat
cotton covered copper strip. In heavy coils, several strips in parallel
are used per turn in order to facilitate the winding and produce a more
compact coil.]
~The No Load Current.~--When the secondary winding of a transformer is
open or disconnected from the secondary circuit no current will flow in
the winding, but a very small current called the _no load current_ will
flow in the primary circuit.
The reason for this is as follows: The current flowing in
the primary winding causes repeated reversals of magnetic
flux through the iron core. These variations of flux induce
pressures in both coils; that induced in the primary called the
_reverse pressure_ is opposite in direction and very nearly
equal to the impressed pressure, that is, to the pressure
applied to the primary winding. Accordingly the only force
available to cause current to flow through the primary winding
is the difference between the impressed pressure and reverse
pressure, the _effective pressure_.
[Illustration: FIG. 1,920.--Wagner coils with insulation ready for core
assembly. The flat coils, sometimes called pancake coils are wound
of flat, cotton covered, copper strip with ample insulation between
layers. In heavy coils several flat strips in multiple are used per
turn in order to facilitate the winding and produce a more compact
coil. In many cases normal current flow per high tension coil is very
low and could be carried with a very small cross sectional area of
copper; however, flat strip is almost always used on account of the
increased mechanical stability thus obtained.]
~The Magnetizing Current.~--The magnetizing current of a transformer
is sometimes spoken of as that current which the primary winding takes
from the mains when working at normal pressure. The _true magnetizing
current_ is only that component of this total no load current which is
in quadrature with the supply pressure. The remaining component has
to overcome the various iron losses, and is therefore an "in phase"
component. The relation between these two components determines the
power factor of the so called "magnetizing current."
[Illustration: FIGS. 1,921 and 1,922.--Assembled coils of Westinghouse
10 and 15 kva. transformers; views showing ventilating ducts.]
The true magnetizing component is small if the transformer be well
designed, and be worked at low flux density.
~Action of Transformer with Load.~--If the secondary winding of a
transformer be connected to the secondary circuit by closing a switch
so that current flows through the secondary winding, the transformer is
said to be _loaded_.
The action of this secondary current is to oppose the
magnetizing action of the slight current already flowing in the
primary winding, thus decreasing the maximum value reached by
the alternating magnetic flux in the core, thereby decreasing
the induced pressure in each winding.
The amount of this decrease, however, is _very small_,
inasmuch as a very small decrease of the induced pressure in
the primary coil greatly increases the difference between the
pressure applied to the primary coil and the opposing pressure
induced in the primary coil, so that the primary current is
greatly increased. In fact, _the increase of primary current
due to the loading of the transformer is just great enough
(or very nearly) to exactly balance the magnetizing action of
the current in the secondary coil;_ that is, the flux in the
core must be maintained approximately constant by the primary
current whatever value the secondary current may have.
When the load on a transformer is increased, the primary of the
transformer automatically takes additional current and power
from the supply mains in direct proportion to the load on the
secondary.
When the load on the secondary is reduced, for example by
turning off lamps, the power taken from the supply mains by
the primary coil is automatically reduced in proportion to the
decrease in the load. This automatic action of the transformer
is due to the balanced magnetizing action of the primary and
secondary currents.
[Illustration: FIG. 1,923.--Rear view of Fort Wayne distributing
transformer, showing hanger irons for attaching to pole cross arm.]
~Classification of Transformers.~--As in the case of motors, the great
variety of transformer makes it necessary that a classification, to be
comprehensive, must be made from several points of view, as:
1. With respect to the transformation, as
_a._ Step up transformers;
_b._ Step down transformers.
2. With respect to the arrangement of the coils and magnetic circuit, as
_a._ Core transformers;
_b._ Shell transformers;
_c._ Combined core and shell transformers.
3. With respect to the kind of circuit they are to be used on, as
_a._ Single phase transformers;
_b._ Polyphase transformers.
4. With respect to the method employed in cooling, as
_a._ Dry transformers;
_b._ Air cooled transformers {natural draught;
{forced draught, or air blast;
_c._ Oil cooled transformers;
_d._ Water cooled transformers.
5. With respect to the nature of their output, as
_a._ Constant pressure transformers;
_b._ Constant current transformers;
_c._ Current transformers;
_d._ Auto-transformers.
6. With respect to the kind of service, as
_a._ Distributing;
_b._ Power.
7. With respect to the circuit connection that the transformer
is constructed for, as
_a._ Series transformers;
_b._ Shunt transformers.
~Step Up Transformers.~--This form of transformer is used to transform
a low voltage current into a high voltage current. Such transformers
are employed at the generating end of a transmission line to raise the
voltage of the alternators to such value as will enable the electric
power to be economically transmitted to a distant point.
[Illustration: FIG. 1,924.--Diagram of elementary _step up_
transformer. As shown the primary winding has two turns and secondary
10 turns, giving a ratio of voltage transformation of 10 ÷ 2 = 5.
Since only ⅕ as much current flows in the secondary winding as in the
primary, the latter requires heavier wire than the former.]
~Copper Economy with Step Up Transformers.~--To comprehend
fully the bearing of the matter, it must be remembered that the
energy supplied per second is the product of two factors, the
current and the pressure at which that current is supplied;
the magnitudes of the two factors may vary, but the value of
the power supplied depends only on the product of the two; for
example, the energy furnished per second by a current of 10
amperes supplied at a pressure of 2,000 volts is exactly the
same in amount as that furnished per second by a current of 400
amperes supplied at a pressure of 50 volts; in each case, the
product is 20,000 watts.
Now the loss of energy that occurs in transmission through a
well insulated wire depends also on two factors, the current
and the resistance of the wire, and in a given wire is
proportional to the square of the current. In the above example
the current of 400 amperes, if transmitted through the same
wire as the 10 amperes current, would, because it is forty
times as great, waste sixteen hundred times as much energy in
heating the wire. It follows that, for the same loss of energy,
the 10 ampere current at 2,000 volts may be carried by a wire
having only ¹/₁₆₀₀th of the sectional area of the wire used
for the 400 ampere current at 50 volts.
The cost of copper conductors for the distributing lines is
therefore very greatly economized by employing high pressures
for distribution of small currents.
[Illustration: FIG. 1,925.--Diagram of elementary step down
transformer. As shown the primary winding has 10 turns and the
secondary 2, giving a ratio of voltage transformation of 2 ÷ 10 = .2.
The current in the secondary being 5 times greater than in the primary
will require a proportionately heavier wire.]
~Step Down Transformers.~--When current is supplied to consumers
for lighting purposes, and for the operation of motors, etc.,
considerations of safety as well as those of suitability, require the
delivery of the current at comparatively low pressures ranging from 100
to 250 volts for lamps, and from 100 to 600 volts for motors.
This involves that the high pressure current in the transmission
lines must be transformed to low pressure current at the receiving
or distributing points by _step down transformers_, an elementary
transformer being shown in fig. 1,925.
[Illustration: FIGS. 1,926 and 1,927.--Core type transformer. It
consists of a central core of laminated iron, around which the coils
are wound. A usual form of core type transformer consists of a
rectangular core, around the two long limbs of which the primary and
secondary coils are wound, the low tension coil being placed next the
core.]
Transformers of this type have a large number of turns in the primary
winding and a small number in the secondary, in ratio depending on the
amount of pressure reduction required.
~Core Transformers.~--This type of transformer may be defined as one
having an iron core, upon which the wire is wound in such a manner that
the iron is enveloped within the coils, the outer surface of the coils
being exposed to the air as shown in figs. 1,926 and 1,927.
~Shell Transformers.~--In the shell type of transformer, as shown in
fig. 1,928, the core is in the form of a shell, being built around and
through the coils. A shell transformer has, as a rule, fewer turns and
a higher voltage per turn than the core type.
~Ques. What is the comparison between core and shell transformers?~
Ans. The relative advantages of the two types has been the subject of
considerable discussion among manufacturers; the companies who formerly
built only shell type transformers, now build core types, while with
other builders the opposite practice obtains.
[Illustration: FIG. 1,928.--Shell type transformer. In construction
the laminated core is built around and through the coils as shown. For
large ratings this type has some advantages with respect to insulation,
while for small ratings the core type is to be preferred in this
respect. The shell arrangement of the core gives better cooling; with
this arrangement minimum magnetic leakage is easily obtained.]
~Ques. Upon what does the choice between the two types chiefly depend?~
Ans. Upon manufacturing convenience rather than operating
characteristics.
The major insulation in a core type transformer consists of
several large pieces of great mechanical strength, while in
the shell type, there are required an extremely large number
of relatively small pieces of insulating material, which
necessitates careful workmanship to prevent defects in the
finished transformer, when thin or fragile material is used.
Both core and shell transformers are built for all ratings; for
small ratings the core type possesses certain advantages with
reference to insulation, while for large ratings, the shell
type possesses better cooling properties, and has less magnetic
leakage than the core type.
[Illustration: FIG. 1,929.--View illustrating the construction of cores
and coils of Maloney transformers.]
[Illustration: FIG. 1,930.--Maloney mica shield between primary and
secondary coils, showing lapping feature which prevents the wrinkling
and cracking of the mica.]
~Combined Core and Shell Transformers.~--An improved type of
transformer has been introduced which can be considered either as two
superposed shell transformers with coils in common, or as a single core
type transformer with divided magnetic circuit and having coils on only
one leg. It is best considered however, as a combined core and shell
transformer, and for small sizes it possesses most of the advantages
of both types. It can be constructed at less cost than can either a
core or a shell transformer having the same operating characteristics
and temperature limits.
[Illustration: FIGS. 1,931 and 1,932.--The Berry combined core and
shell transformer. It consists of a number of inner and outer vertical
and radial laminated iron blocks built up of the usual thin sheet iron,
with the coils between. The magnetic circuit is completed at the top
and bottom by other laminated blocks placed horizontally, and the whole
is held together between top and bottom cast iron frame plates by a
bolt passing right down the center. Fig. 1,931 gives a general view,
W being the winding, and B, B, B, etc., the outer laminated blocks.
The construction will be better understood from fig. 1,932, where it
may be supposed that the top cap and laminated cross pieces have been
removed. Here I, I, I and O, O, O are respectively the inner and outer
radial vertical blocks, P the primary, and S, S the secondary; the
latter being in two sections with the primary sandwiched between, as an
extra precaution against shock. It will be evident that this form of
transformer possesses excellent ventilation; and this is still further
enhanced by opening out the winding at intervals to leave ventilating
apertures, as at A, A, A. Fig. 1,932 shows only 6 sets of radial
blocks, but the usual plan is to provide 24 or 36, according to the
size of the transformer.]
Fig. 1,932 shows a cross section of the first transformer of this type
to be developed commercially, and known as an "iron clad" transformer;
this construction has been used in England for some time. Fig. 1,933
shows the American practice.
[Illustration: FIG. 1,933.--Plan of core of General Electric combined
core and shell transformer. The core used contains four magnetic
circuits of equal reluctance, in multiple; each circuit consisting of a
separate core. In this construction one leg of each circuit is built up
of two different widths of punchings forming such a cross section that
when the four circuits are assembled together they interlock to form
a central leg, upon which the winding is placed. The four remaining
legs consist of punchings of equal width. These occupy a position
surrounding the coil at equal distances from the center, on the four
sides; forming a channel between each leg and coil, thereby presenting
large surfaces to the oil and allowing its free access to all parts
of the winding. The punchings of each size transformer are all of the
same length, assembled alternately, and forming two lap joints equally
distributed in the four corners of the core, thereby giving a magnetic
circuit of low reluctance.]
~Ques. How is economy of construction obtained in designing combined
core and shell transformers?~
Ans. The cross section of iron in the central leg of the core is made
somewhat less than that external to the coils, in order to reduce the
amount of copper used in the coils.
~Single and Polyphase Transformers.~--A single phase transformer may be
defined as _one having only one set of primary and secondary terminals,
and in which the fluxes in the one or more magnetic circuits are all in
phase_, as distinguished from a polyphase transformer, or combination
in one unit of several one phase transformers with separate electric
circuits but having certain magnetic circuits in common. In polyphase
transformers there are two or more magnetic circuits through the core,
and the fluxes in the various circuits are displaced in phase.
~Ques. Is it necessary to use a polyphase transformer to transform a
polyphase current?~
Ans. No, a separate single phase transformer may be used for each phase.
[Illustration: FIGS. 1,934 and 1,935.--Top view showing core and
coils in place, and view of coils of Westinghouse distributing
transformer. The coils are wound from round wire in the smaller sizes
of transformers and from strap copper in the larger sizes. Strap wound
coils allow a greater current carrying conductor section than coils
wound from large round wire, as there is little waste space between the
different turns of the conductor. The coils are arranged concentrically
with the high tension winding between the two low tension coils, this
arrangement giving the fine regulation found in these transformers. The
low tension coils are wound in layers which extend across the whole
length of the coil opening in the iron, while the high tension coils
are wound in two parts and placed end to end. This construction reduces
the normal voltage strains to a value which will not give trouble
under any condition of service. The magnetic circuit is built up of
laminated, alloy steel punchings, each layer of laminæ being reversed
with reference to the preceding layer and all joints butted. This gives
a continuous magnetic circuit of low reluctance, low iron loss and low
exciting current. When assembled, the magnetic circuit consists of four
separate parallel circuits encircling the coils and protecting the
windings from mechanical injury. Separate high and low tension terminal
blocks of glazed porcelain are mounted upon extensions of the upper
end frames. All danger of confusing the leads or inadvertently making
an electrical connection between the high and low tension sides of the
transformer is thus averted. The high tension winding has four leads
brought to the studs in the terminal block. Adjustable brass connectors
or links between the studs provide for series or multiple connections
between two points of the high tension winding. The position of the
studs and the length of the links are so proportioned that wrong
connections on the block are impossible. Barriers on the porcelain
block separate the studs and prevent danger of arcing. Leads with means
of preventing creeping of oil by capillary action are attached to these
studs and brought out of the core through porcelain bushings.]
~Ques. Is there any choice between a polyphase transformer and separate
single phase transformers for transforming a polyphase current?~
Ans. Yes, the polyphase transformer is preferable, because less iron
is required than would be with the several single phase transformers.
The polyphase transformer therefore is somewhat lighter and also more
efficient.
[Illustration: FIGS. 1,936 and 1,937.--Core and shell types of three
phase transformer. In the core type, fig. 1,936, there are three cores
A, B, and C, joined by the yokes D and D'. This forms a three phase
magnetic circuit, since the instantaneous sum of the fluxes is zero.
Each core is wound with a primary coil P, and a secondary coil S. As
shown, the primary winding of each phase is divided into three coils
to ensure better insulation. The primaries and secondaries may be
connected _star_ or _mesh_. The core B has a shorter return path than
A and C, which causes the magnetizing current in that phase to be less
than in the A and C phases. This has sometimes been obviated by placing
the three cores at the corners of an equilateral triangle (as in figs.
1,939 and 1,940), but the extra trouble involved is not justified, as
the unbalancing is a no load condition, and practically disappears
when the transformer is loaded. The shell type, fig. 1,937, consists
practically of three separate transformers in one unit. The flux paths
are here separate, each pair of coils being threaded by its own flux,
which does not, as in the core type, return through the other coils.
This gives the shell type an advantage over the core type, for should
one phase burn out, the other two may still be used, especially if the
faulty coils be short circuited. The effect of such short circuiting is
to prevent all but a very small flux from threading the faulty coil.]
~Ques. Name two varieties of polyphase transformer?~
Ans. The core, and the shell types as shown in figs. 1,936 and 1,937.
~Ques. How should a three phase transformer be operated with one phase
damaged?~
Ans. The damaged windings should be separated electrically from the
other coils.
The pressure winding of the damaged phase should be short
circuited upon itself and the corresponding low pressure
winding should also be short circuited upon itself. The winding
thus short circuited will choke down the flux passing through
the portion of the core surrounded by them without producing
in any portion of the winding a current greater than a small
fraction of the current which would normally exist in such
portion at full load.
~Transformer Losses.~--As previously mentioned, the ratio between
the applied primary voltage and the secondary terminal voltage of a
transformer is not always equal to the ratio of primary to secondary
turns of wire around the core.
The commercial transformer is not a perfect converter of energy, that
is, the ~input~, or watts applied to the primary circuit is always more
than the ~output~ or watts delivered from the secondary winding.
[Illustration: FIG. 1,938.--Interior of General Electric oil cooled 500
kva. 33,000 volt outdoor transformer showing lifting arrangement.]
This is due to the various losses which take place, and the difference
between the input and the output is equal to the sum of these losses.
They are divided into two classes:
1. The ~iron~ or ~core~ losses;
2. The ~copper~ losses.
The iron or core losses are due to
1. Hysteresis;
2. Eddy currents;
3. Magnetic leakage (negligibly small).
[Illustration: FIGS. 1,939 and 1,940.--Triangular arrangements of cores
of three phase transformer. Fig. 1,939, form with three cornered yokes
at bottom and top of cores; fig. 1,940, form with circular yokes. While
these designs give perfect symmetry for the three phases, there is some
trouble in the mechanical arrangement of the yokes. If these be stamped
out triangularly and inserted horizontally between the three cores, it
is necessary to interpose a layer of insulation, otherwise there would
be objectionable eddy currents formed in the stampings.]
Those which are classed as copper losses are due to
1. Heating the conductors (the I²R loss);
2. Eddy currents in conductors.
~Hysteresis.~--In the operation of a transformer the alternating
current causes the core to undergo rapid reversals of magnetism. This
requires an expenditure of energy which is converted into heat.
[Illustration: FIG. 1,941.--View showing mechanical construction of
coil and core of Moloney pole type ½ to 50 kw. transformer. Moloney
standard transformers of these sizes are regularly wound for 1,100 to
2,200 primary volts. For 1,100 volts the primary coils are connected
in parallel by means of connecting links; for 2,200 volts, they are
connected in series. The porcelain primary terminal board is provided
with two connecting links so that connections can be made for either
1,100 or 2,200 volts.]
This loss of energy as before explained is due to the work required
to change the position of the molecules of the iron, in reversing the
magnetization. Extra power then must be taken from the line to make up
for this loss, thus reducing the efficiency of the transformer.
~Ques. Upon what does the hysteresis loss depend?~
Ans. Upon the quality of the iron in the core, the magnetic density at
which it is worked and the frequency.
~Ques. With a given quality of iron how does the hysteresis loss vary?~
Ans. It varies as the 1.6 power of the voltage with constant frequency.
~Ques. In construction, what is done to obtain minimum hysteresis loss?~
Ans. The softest iron obtainable is used for the core, and a low degree
of magnetization is employed.
[Illustration: FIG. 1,942.--Fort Wayne transformer coils and core
complete.]
[Illustration: FIG. 1,943.--Top view of Fort Wayne (type A) transformer
cover removed, showing assembly of coils and core and disposition of
leads.]
~Eddy Currents.~--The iron core of a transformer acts as a closed
conductor in which small pressures of different values are induced in
different parts by the alternating field, giving rise to eddy currents.
Energy is thus consumed by these currents which is wasted in heating
the iron, thus reducing the efficiency of the transformer.
~Ques. How is the loss reduced to a minimum?~
Ans. By the usual method of laminating the core.
The iron core is built up of very thin sheet iron or steel
stampings, and these are insulated from each other by varnish
and are laid face to face at right angles to the path that the
eddy currents tend to follow, so that the currents would have
to pass from sheet to sheet, through the insulation.
~Ques. In practice, upon what does the thickness of the laminæ or
stampings depend?~
Ans. Upon the frequency.
The laminæ vary in thickness from about .014 to .025 inch,
according as the frequency is respectively high or low.
[Illustration: FIG. 1,944.--General Electric 10 kva., (type H)
transformer removed from tank. That part of the steel core composing
the magnetic circuit outside of the winding is divided into four
equal sections. Each section is located a sufficient distance from
the winding so that all portions of the winding and core are equally
exposed to the cooling action of the oil. On all except the very
smallest sizes the winding is divided by channels and ducts through
which a continual circulation of oil is maintained. The result is
uniform temperature throughout the transformer, thus eliminating the
detrimental effects of unequal expansion in the coils with consequent
rubbing and abrasion of the insulation.]
~Ques. Does a transformer take any current when the secondary circuit
is open?~
Ans. Yes, a "no load" current passes through the primary.
~Ques. Why?~
Ans. The energy thus supplied balances the core losses.
[Illustration: FIG. 1,945.--Cover construction of Wagner 350 kva., oil
filled 1,100-2,200 volt transformer. In transformers with corrugated
cases, the base and top ring are cast to the corrugated iron sheets.]
~Ques. Are the iron or copper losses the more important, and why?~
Ans. The iron losses, because these are going on as long as the primary
pressure is maintained, and the copper losses take place only while
energy is being delivered from the secondary.
Strictly speaking, on _no load_ (that is when the secondary
circuit is open) a slight copper loss takes place in the
primary coil but because of its smallness is not mentioned.
It is, to be exact, included in the expression "iron losses,"
as the precise meaning of this term signifies _not only the
hysteresis and eddy current losses but the copper loss in the
primary coil when the secondary is open_.
The importance of the iron losses is apparent in noting that in
electric lighting the lights are in use only a small fraction
of the 24 hours, but the iron losses continue all the time,
thus the greater part of each day energy must be supplied to
each transformer by the power company to meet the losses,
during which time no money is received from the customers.
Some companies make a minimum charge per month whether any
current is used or not to offset the no load transformer losses
and rent of meter.
[Illustration: FIGS. 1,946 to 1,948.--Methods of connecting the low
tension sides of Westinghouse transformers using the connectors
illustrated in figs. 1,949 to 1,953.]
~Ques. How may the iron losses be reduced to a minimum?~
Ans. By having short magnetic paths of large area and using iron or
steel of high permeability. The design and construction must keep the
eddy currents as low as possible.
As before stated the iron losses take place continually, and
since most transformers are loaded only a small fraction of a
day it is very important that the iron losses should be reduced
to a minimum.
With a large number of transformers on a line, the magnetizing
current that is wasted, is considerable.
During May, 1910, the U. S. Bureau of Standards issued a
circular showing that each watt saving in core losses was a
saving of 88 cents, which is evident economy in the use of high
grade transformers.
~Copper Losses.~--Since the primary and secondary windings of a
transformer have resistance, some of the energy supplied will be lost
by heating the copper. The amount of this loss is proportional to
square of the current, and is usually spoken of as the I²R loss.
[Illustration: FIGS. 1,949 to 1,953.--Westinghouse low tension
transformer connectors for connecting the low tension leads to the
feeder wires. The transformers of the smaller capacities have knuckle
joint connectors and those of the larger sizes have interleaved
connectors. These connectors form a mechanically strong joint of high
current carrying capacity. Since the high tension leads are connected
directly to the cut out or fuse blocks, connectors are not required on
these leads. The use of these connectors allows a transformer to be
removed and another of the same or a different capacity substituted
usually without soldering or unsoldering a joint. The connectors also
facilitate changes in the low tension connections.]
~Ques. Define the copper losses.~
Ans. The copper losses are the sum of the I²R losses of both the
primary and secondary windings, and the eddy current loss in the
conductors.
~Ques. Is the eddy current loss in the conductors large?~
Ans. No, it is very small and may be disregarded, so that the sum of
the I²R losses of primary and secondary can be taken as the total
copper loss for practical purposes.
~Ques. What effect has the power factor on the copper losses?~
Ans. Since the copper loss depends upon the current in the primary and
secondary windings, it requires a larger current when the power factor
is low than when high, hence the copper losses increase with a lowering
of the power factor.
[Illustration: FIG. 1,954.--Method of bringing out the secondary leads
in Wagner central station transformers. Each primary lead is brought
into the case through a similar bushing. Observe the elimination of all
possibility of grounding the cable on the case or core.]
~Ques. What effect other than heating has resistance in the windings?~
Ans. It causes poor regulation.
This is objectionable, especially when incandescent lights are
in use, because the voltage fluctuates inversely with load
changes, that is, it drops as lamps are turned on and rises
as they are turned off, producing disagreeable changes in the
brilliancy of the lamps.
~Cooling of Transformers.~--Owing to the fact that a transformer is a
stationary piece of apparatus, not receiving ventilation from moving
parts, its efficient cooling becomes a very strong feature of the
design, especially in the case of large high pressure transformers. The
effective cooling is rendered more difficult because transformers are
invariably enclosed in more or less air tight cases, except in very dry
situations, where a perforated metal covering may be permitted.
[Illustration: FIGS. 1,955 and 1,956.--Westinghouse transformer
terminal blocks for high and low tension conductors.]
The final degree to which the temperature rises after continuous
working for some hours, depends on the total losses in iron and copper,
on the total radiating surface, and on the facilities afforded for
cooling.
There are various methods of cooling transformers, the cooling mediums
employed being
1. Air;
2. Oil;
3. Water.
The means adopted for getting rid of the heat which is inevitably
developed in a transformer by the waste energy is one of the important
considerations with respect to its design.
~Ques. What is the behaviour of a transformer with respect to heating
when operated continuously at full load?~
Ans. The temperature gradually rises until at the end of some hours it
becomes constant.
The difference between the constant temperature and that of the
secondary atmosphere is called the temperature rise at full
load. _Its amount constitutes a most important feature in the
commercial value of the transformer._
[Illustration: FIGS. 1,957 to 1,960.--Porcelain bushing for
Westinghouse transformers.]
~Ques. Why is a high rise of temperature objectionable?~
Ans. It causes rapid deterioration of the insulation, increased
hysteresis losses, and greater fire risk.
~Dry Transformers.~--This classification is used to distinguish
transformers using air as a cooling medium from those which employ a
liquid such as water or oil to effect the cooling.
~Air Cooled Transformers.~--This name is given to all transformers
which are cooled by currents of air without regard to the manner in
which the air is circulated. There are two methods of circulating the
air, as by
1. Natural draught;
2. Forced draught, or blast.
~Ques. Describe a natural draught air cooled transformer.~
Ans. In this type, the case containing the windings is open at the top
and bottom. The column of air in the case expands as its temperature
rises, becoming lighter than the cold air on the outside and is
consequently displaced by the latter, resulting in a circulation of air
through the case. The process is identical with furnace draught.
[Illustration: FIGS. 1,961 to 1,963.--Fuse blocks for Westinghouse
transformers. The fuses furnished with the transformers are mounted in
a weather proof porcelain fuse box of special design. The stationary
contacts are deeply recessed in the porcelain and are well separated
from each other. The contacts are so constructed that the plug is held
securely in place by giving it a partial turn after inserting it. When
the plug is in position, the fuse is in sight and its condition can be
noted which eliminates all danger of pulling the fuse while same is
still intact and the transformer is under load.]
~Ques. Describe a forced draught or air blast transformer.~
Ans. The case is closed at the bottom and open at the top. A current of
air is forced through from bottom to top as shown in fig. 1,964 by a
fan.
~Ques. How are the coils best adapted to air cooling?~
Ans. They are built up high and thin, and assembled with spaces between
them, for the circulation of the air.
~Ques. What are the requirements with respect to the air supply in
forced draught transformers?~
Ans. Air blast transformers require a large volume of air at a
comparatively low pressure. This varies from one-half to one ounce
per square inch. The larger transformers require greater pressure to
overcome the resistance of longer air ducts.
[Illustration: FIG. 1,964.--Forced draught or "air blast" transformer.
As is indicated by the classification, this type of transformer is
cooled by forcing a current of air through ducts, provided between the
coils and between sectionalized portions of the core. The cold air
is forced through the interior of the core containing the coils by a
blower, the air passing vertically through the coils and out through
the top. Part of the air is sometimes diverted horizontally through
the ventilating ducts provided in the core, passing off at one side of
the transformer. The amount of air going through the coils, or through
the core, may be controlled independently by providing dampers in the
passages.]
~Ques. How much air is used ordinarily for cooling per kw. of load?~
Ans. About 150 cu. ft. of air per minute.
In forced draught transformers, the air pressure maintained
by the blower varies from ½ to 1½ oz. per square inch. Forced
draught or air blast transformers are seldom built in small
sizes or for voltages higher than about 35,000 volts.
~Oil Cooled Transformers.~--In this type of transformer the coils and
core are immersed in oil and provided with ducts to allow the oil to
circulate by convection and thus serve as a medium to transmit the heat
to the case, from which it passes by radiation.
[Illustration: FIG. 1965.--Looking down into a Wagner central station
transformer, showing the connection board, which provides facility for
varying the ratio of transformation and also for interchanging the
primaries.]
~Ques. Explain in detail the circulation of the oil.~
Ans. The oil, heated by contact with the exposed surfaces of the core
and coils, rises to the surface, flows outward and descends along the
sides of the transformer case, from the outer surface of which the heat
is radiated into the air.
~Ques. How may the efficiency of this method of cooling be increased?~
Ans. By providing the case with external ribs or fins, or by "fluting"
so as to increase the external cooling surface.
[Illustration: FIG. 1966.--Section through Westinghouse ½ kilovolt
ampere type S transformer. FIG. 1967.--Section through Westinghouse 50
kilovolt ampere type S transformer showing large oil ducts.]
~Ques. In what types of transformer is this mode of oil cooling used?~
Ans. Lighting transformers.
In such transformers, the large volume of oil absorbs
considerable heat, so that the rise of temperature is retarded.
Hence, for moderate periods of operation, say 3 or 4 hours,
the average lighting period, the maximum temperature would not
be reached.
~Ques. In what other capacities except that of cooling agent, does the
oil act?~
Ans. It is a good insulator, preserves the insulation from oxidation,
increasing the breakdown resistance of the insulation, and generally
restores the insulation in case of puncture.
[Illustration: FIG. 1,968.--Wagner 300 kva, 4,400 volt three phase oil
cooled transformer. In this type of transformer the case is filled with
oil and fluted so as to increase the cooling surface, an oil drain
valve is screwed to a wrought iron nipple cast into the base, the duct
to which is in such a position as to make it possible not only to drain
all of the oil from the transformer, but when desirable, to draw off a
small quantity from the bottom. Should any moisture be in the oil it is
therefore drawn off first.]
~Ques. What is the special objection to oil?~
Ans. Danger of fire.
~Ques. What kind of oil is used in transformers?~
Ans. Mineral oil.
~Ques. What are the requirements of a good grade of transformer oil?~
Ans. It should show very little evaporation at 212° Fahr., and should
not give off gases at such a rate as to produce an explosive mixture
with the air at a temperate below 356°. It should not contain moisture,
acid, alkali or sulphur compounds.
[Illustration: FIG. 1,969.--Section through Fort Wayne (type A)
transformer showing interior of case, core conductors, and insulation,
also division of laminæ.]
The presence of moisture can be detected by thrusting a red
hot nail in the oil; if the oil "crackle," water is present.
Moisture may be removed by raising the temperature slightly
above the boiling point, 212° Fahr., but the time consumed
(several days) is excessive.
~Water Cooled Transformers.~--A water cooled transformer is one in
which water is the cooling agent, and, in most cases, oil is the
medium by which heat is transferred from the coils to the water. In
construction, pipes or a jacketed casing is provided through which the
cooling water is passed by forced circulation, as shown in figs. 1,970
and 1,971.
[Illustration: FIG. 1,970.--Water cooled transformer with internal
cooling coil, that is, with cooling coil within the transformer case.
In this type, the cooling coil, through which the circulating water
passes, is placed in the top of the case or tank, the latter is
filled with oil so that the coil is submerged. The oil acts simply
as a medium to transfer the heat generated by the transformer to the
water circulating through the cooling coil. In operation a continual
circulation of the oil takes place, as indicated by the arrows, due
to the alternate heating and cooling it receives as it flows past the
transformer coils and cooling coil respectively.]
In some cases tubular conductors are provided for the circulation of
the water.
Water cooled transformers may be divided into two classes, as those
having:
1. Internal cooling coils;
2. External cooling coils.
~Ques. Describe the first named type.~
Ans. Inside the transformer case near the top is placed a coil of
wrought iron pipe, through which the cooling water is pumped. The case
is filled with oil, which by _thermo-circulation_ flows upward through
the coils, transferring the heat absorbed from the coils to the water;
on cooling it becomes more dense (heavier) and descends along the
inside surface of the casing.
[Illustration: FIG. 1,971.--Water cooled transformer with external
cooling coil. In this arrangement the cooling coil is placed in a
separate tank as shown. Here forced circulation is employed for both
the heat transfer medium (oil) and the cooling agent (water), two
pumps being necessary. The cool oil enters the transformer case at
the lowest point and absorbing heat from the transformer coils it
passes off through the top connection leading to the cooling coil and
expansion tank. Since the transformer tank is closed, an expansion tank
is provided to allow for expansion of the oil due to heating. The water
circulation is arranged as illustrated.]
~Ques. How much circulating water is required?~
Ans. It depends upon the difference between the initial and discharge
temperatures of the circulating water.
[Illustration: FIG. 1,972.--Interior of General Electric water cooled
140,000 volt transformer showing cooling coil.]
~Ques. In water cooled transformers how much cooling surface is
required for an internal cooling coil?~
Ans. The surface of the cooling coil should be from .5 to 1.3 sq. in.
per watt of total transformer loss, depending upon the amount of heat
which the external surface of the transformer case will dissipate.
For a water temperature rise of 43° Fahr., 1.32 lbs. of water
per minute is required per kw. of load.
~Transformer Insulation.~--This subject has not, until the last few
years, been given the same special attention that many other electrical
problems have received, although the development of the transformer
from its original form, consisting of an iron core enclosed by coils of
wire, to its present degree of refinement and economy of material, has
been comparatively rapid.
In transformer construction it is obviously very important that
the insulation be of the best quality to prevent burn outs and
interruptions of service.
~Ques. What is the "major" insulation?~
Ans. The insulation placed between the core and secondary (low
pressure) coils, and between the primary and secondary coils.
[Illustration: FIG. 1,973.--Assembled coils of General Electric water
cooled 500 kva., 66,000 volt transformer.]
It consists usually of mica tubes, sometimes applied as sheets
held in place by the windings, when no ventilating ducts are
provided, or moulded to correct form and held between sheets of
tough insulating material where ducts are provided for air or
oil circulation.
~Ques. Describe the "minor" insulation.~
Ans. It is the insulation placed between adjacent turns of the coils.
Since the difference of pressure is small between the adjacent
turns the insulation need not be very thick. It usually
consists of a double thickness of cotton wrapped around each
conductor. For round conductors, the ordinary double covered
magnet wire is satisfactory.
~Ques. What is the most efficient insulating material for transformers?~
Ans. Mica.
It has a high dielective strength, is fireproof, and is the
most desirable insulator where there are no sharp corners.
[Illustration: FIG. 1,974.--Three Westinghouse 20 kva, outdoor
transformers, for irrigation service. These are mounted on a drag
so that they may be readily transported from place to place. 33,000
volts high tension; 2,200 and 440 volts low tension, 50 cycles. These
outdoor transformers are of the oil immersed, self-cooling type and
have been developed to meet the requirements for transformers of
capacities greater or of voltages higher than are usually found in
distribution work. They are in reality distributing transformers for
high voltage, outdoor installations, single or three phase service,
for voltages up to 110,000. Where the magnitude of the load does not
warrant an expensive installation, transformers of the outdoor type are
particularly applicable. The cost of a building and outlet bushings
which is often the item of greatest expense is eliminated where outdoor
type transformers are installed.]
~Oil Insulated Transformers.~--High voltage transformers are insulated
with oil, as it is very important to maintain careful insulation not
only between the coils, but also between the coils and the core. In the
case of high voltage transformers, any accidental static discharge,
such as that due to lightning, which might destroy one of the air
insulated type, might be successfully withstood by one insulated with
oil, for if the oil insulation be damaged it will mend itself at once.
By providing good circulation for the oil, the transformer can get rid
of the heat produced in it readily and operate at a low temperature,
which not only increases its life but cuts down the electric resistance
of the copper conductors and therefore the I²R loss.
~Efficiency of Transformers.~--The efficiency of transformers is _the
ratio of the electric power delivered at the secondary terminals to the
electric power absorbed at the primary terminals_.
Accordingly, the output must equal the input minus the losses. If the
iron and copper losses at a given load be known, their values and
consequently the efficiency at other loads may be readily calculated.
EXAMPLE.--If a 10 kilowatt constant pressure transformer at full load
and temperature have a copper loss of .16 kilowatt, or 1.6 per cent.,
and the iron loss be the same, then its
output 10
efficiency = ------ = -------------- × 100 = 96.9 per cent.
input 10 + .16 + .16
At three-quarters load the output will be 7.5 kilowatts; and as the
iron loss is practically constant at all loads and the copper loss is
proportional to the square of the load, the
output 7.5
efficiency = ------ = --------------- × 100 = 96.8 per cent.
input 7.5 + .16 + .09
The matter of efficiency is important, especially in the case of large
transformers, as a low efficiency not only means a large waste of power
in the form of heat, but also a great increase in the difficulties
encountered in keeping the apparatus cool. The efficiency curve shown
in fig. 1,975, serves to indicate, however, how slight a margin
actually remains for improvement in this particular in the design and
construction of large transformers.
[Illustration: FIG. 1,975.--Efficiency curve of Westinghouse 375 kw.,
transformer. Pressure 500 to 15,000 volts; frequency 60. Efficiencies
at different loads: full load efficiency, 98%; ¾ full load efficiency,
98%; ½ full load efficiency, 97.6%; ¼ full load efficiency, 96.1%;
regulation non-inductive load, 1.4%; load having .9 power factor, 3.3%.]
The efficiency of transformers is, in general, higher than that
of other electrical machines; even in quite small sizes it
reaches over 90 per cent., and in the largest, is frequently as
high as 98.5 per cent.
To measure the efficiency of a transformer directly, by
measuring input and output, does not constitute a satisfactory
method when the efficiency is so high. A very accurate
result can be obtained, however, by measuring separately, by
wattmeter, the core and copper losses.
The core loss is measured by placing a wattmeter in circuit
when the transformer is on circuit at no load and normal
frequency.
The copper loss is measured by placing a wattmeter in circuit
with the primary when the secondary is short circuited, and
when enough pressure is applied to cause full load current to
flow.
If it be desired to separate the load losses from the true
I²R loss, the resistances can be measured, and the I²R loss
calculated and subtracted from the wattmeter reading. The
losses being known, the efficiency at any load is readily found
by taking the core loss as constant and the copper loss as
varying proportionally to the square of the load. Thus,
output
efficiency = --------------- × 100
output + losses
~All Day Efficiency of Transformers.~--This denotes the ratio of the
total watt hour output of a transformer to the total watt hour input
taken over a working day. To compute this efficiency it is necessary to
know the load curve of the transformer over a day. Suppose that this is
equivalent to 5 hours at full load, and 19 hours at no load. Then, if
W₁ be the core loss in watts, W₂ the copper loss at rated load, and W
the rated output,
output = 5 × W,
losses = 5 (W₁ + W₂) + 19 W₁,
input = 5 (W + W₁ + W₂) + 19 W₁,
and the all day efficiency is equal to
5W × 100
--------------------- per cent.
5(W + W₁ + W₂) + 19W₁
Commercial or all day efficiency is a most important point in a good
transformer. The principal factor in securing a high all day efficiency
is to keep the core loss as low as possible. The core loss is
constant--it continues while current is supplied to the primary, while
copper loss takes place only when the secondary is delivering energy.
In general, if a transformer is to be operated at light loads the
greater part of the day, it is much more economical to use one designed
for a small iron loss than for a small full load copper loss.
[Illustration: FIGS. 1,976 and 1,977.--Westinghouse double pole fuse
box; views showing box open with tubes in place, and with tubes
removed.]
~Transformer Fuse Blocks.~--These may be of either the single pole or
double pole type. Fig. 1,976 shows a double pole fuse box opened, and
fig. 1,977, the fuse box opened and the tubes removed. Of the four
wires, W, W, W, W, entering the box from beneath, two are from the
primary mains, and two lead to the primary coil of the transformer.
These wires terminate in metallic receptacles R, R, R, R, in the
porcelain plate P, fig. 1,977, which are bridged over in pairs by fuse
wires placed inside porcelain tubes T, T, as shown in fig. 1,976. These
tubes are air tight except for a small outlet O in each, which fit into
the receptacles B, B, in the porcelain plate and open out at the back
of the block, as shown in fig. 1,977.
The fuse wires are connected between metallic spring tubes S, S, S, S,
which fit into the receptacles R, R, R, R.
If a sudden load or a short circuit occur in the transformer, the
intense heat, accompanying the melting or blowing of the fuse,
causes a rapid expansion of the air inside the tube, so that a strong
blast of air rushes through the outlet O of the tube and immediately
extinguishes the arc.
By this arrangement, sustained arcing is prevented, as the
action of the tube causes the arc to extinguish itself
automatically when the current is interrupted.
The porcelain tubes are held in position by the spring K, and
the primary of the transformer becomes entirely disconnected
from the circuit when the tubes are lifted out.
This form of construction enables the lineman to detach
the tubes from the fuse box, and insert the fuse at his
convenience. Furthermore, when inserting a fuse in a short
circuited line, he does not run the risk of being hurt, as
the heated vapor of the exploding fuse can escape through
the outlet provided for that purpose, and in a predetermined
direction.
The method of attaching the lid not only permits of quick
access to the interior of the box, but enables the lineman to
tighten the joints by means of the thumb screws L, L, so as to
keep the box waterproof.
[Illustration: FIG. 1,978.--Diagram illustrating connections and
principles of auto-transformers as explained in the accompanying text.]
~Auto-transformers.~--In this class of transformer, there is only one
winding which serves for both primary and secondary. On account of its
simplicity it is made cheaply.
Auto-transformers are used where the ratio of transformation
is small, as a considerable saving in copper and iron can be
effected, and the whole transformer reduced in size as compared
with one having separate windings.
Fig. 1,978 illustrates the electrical connections and the
relations between the volts and number of turns.
By using the end wire and tapping in on turn No. 20 a current
at 20 volts pressure is readily obtained which may be used for
starting up motors requiring a large starting current and yet
not draw heavily on the line.
Since the primary is connected directly to the secondary it
would be dangerous to use an auto-transformer on high pressure
circuits. This type of transformer has only a limited use,
usually as compensator for motor starting boxes.
[Illustration: FIGS. 1,979 and 1,980.--Two winding transformer and
single winding or auto-transformer. Fig. 1,979 shows a 200:100 volt
transformer having a 10 amp. primary and a 20 amp. secondary, the
currents being in opposite directions. If these currents be superposed
by using one winding only, the auto-transformer shown in fig. 1,980
is obtained where the winding carries 10 amp. only and requires only
one-half the copper (assuming the same mean length of turn). If R be
the ratio of an auto-transformer, the relative size of it compared
with a transformer of the same ratio and output is ((R - 1) / R):1.
For example, a 10 kw. transformer of 400 volts primary and 300 volts
secondary could be replaced by an auto-transformer of
10 × (1.33 - 1) / 1.33 = 2.5 kw.; or, in other words, the amount
of material used in a 2½ kw. transformer could be used to wind an
auto-transformer of 400:300 ratio and 10 kw. output.]
~Constant Current Transformers for Series Arc Lighting.~--The
principle of the constant current transformer as used for series arc
lighting is readily understood by reference to the elementary diagram
shown in fig. 1,981. A constant alternating current is supplied to
the stationary primary coil which induces a current in the movable
secondary coil. The pressure induced in the coil will depend on the
number of lines of flux which pass through it and by changing its
position in the magnetic field over the primary a variable e.m.f. can
be produced and a constant current maintained in the lighting circuit
when the lamps are turned on or off, or if the resistance of the
circuit be lowered by the consumption of the carbons.
[Illustration: FIG. 1,981.--Elementary diagram illustrating the
principles of constant current transformer as used for series arc
lighting.]
Since the induced currents in the secondary are repelled by the primary
there is a tendency for the secondary coil to jump out of the primary
field, and in case of a very large current due to a short circuit in
the lamp circuit, the secondary current is quickly reduced to normal by
the rapid movement of the coil upward.
By adjusting the counterweight for a given number of amperes required
by the arc, the current will be maintained constant by the movement of
the secondary coil.
The magnetic field produced by the primary must be kept the same by a
constant current from the alternator, therefore, when the lamp load is
increased the primary voltage increases similar to that of an ordinary
series wound direct current dynamo. In other words the alternator and
regulating transformer supply a constant current and variable voltage.
[Illustration: FIG. 1,982.--Mechanism of General Electric air cooled
constant current transformer. It operates on the principle explained in
the accompanying text and is built to supply 25 to 100 arc lamps at 6.6
to 7.5 amperes. The transformers are interchangeable and will operate
on 60 or 125 cycles. The relative positions of the two coils may be
changed in order to regulate the strength of the current more closely,
by shifting the position of the arc carrying the counterbalance by
means of the adjusting screw on it. A dash pot filled with special oil
prevents sudden movements of the secondary coil and keeps the current
through the lamps nearly constant, when they are being cut in or out
of the circuit. In starting up a constant current transformer, it is
necessary to separate the two coils as far as possible and then close
the primary circuit switch and allow the two coils to come together.
If the primary circuit be thrown directly on the generator the heavy
rush of current which will follow due to the two coils being too close
together might injure the lamps.]
Constant current incandescent lighting systems for use in small towns
also use this method for automatically regulating the current.
~Regulation.~--This term applies to the means adopted either to obtain
constancy of pressure or current. In the transformer, regulation is
_inherent_, that is, the apparatus automatically effects its own
regulation. The regulation of a transformer means, _the change of
voltage due to change of load on the secondary_; it may be defined more
precisely as: _the percentage increase in the secondary voltage as the
load is decreased from its normal value to zero_. Thus, observation
should be made of the secondary voltage, at full load and at no load,
the primary pressure being held constant at the normal value.
[Illustration: FIG. 1,983.--General Electric air cooled constant
current transformer. View showing external appearance with case on.]
The regulation is said to be "good" or "close," when this
change is small. In the design of a transformer, good
regulation and low iron losses are in opposition to one another
when the best results are desired in both. A well designed
transformer, however, should give good results, both as to
regulation and iron losses, the relative value depending upon
the class of work it has to do, and size.
~Transformer Connections.~--The alternating current has the advantage
over direct current, in the ease with which the pressure and current
can be changed by different connections of transformers.
On single phase circuits the transformer connections can be varied to
change current and pressure, and in addition on polyphase circuits the
phases can also be changed to almost any form.
~Single Phase Connections.~--The method of connecting ordinary
distributing transformers to constant pressure mains is shown by the
elementary diagram, fig. 1,984, where a transformer of 10 to 1 ratio is
indicated with its primary winding connected to a 1,000 volt main, and
a secondary winding to deliver 100 volts.
[Illustration: FIG. 1,984.--Single phase transformer connection with
constant pressure main.]
[Illustration: FIG. 1,985.--Usual method of single phase transformer
connections for residence lighting with three wire secondaries. A
balancing transformer is connected to the three wire circuit near the
center of distribution as shown.]
Fig. 1,986 shows a transformer with each winding divided into two
sections. Each primary section is wound for 1,000 volts, and each
secondary section for 50 volts. By connecting the entire primary
winding in series, the transformer may be supplied from a 2,000 volt
main, as indicated, and if the secondary winding be also connected
all in series, as shown, the no load voltage will be 100 between the
secondary terminals.
[Illustration: FIG. 1,986.--Diagram of single phase transformer having
primary and secondary windings in two sections, showing voltages per
section with series connections.]
The sections of the primary winding may be connected in parallel to
a 1,000 volt main, and 100 volts obtained from the secondary, or the
primary and secondary windings may be connected each with its two
sections in parallel, and transformations made from 1,000 to 50 volts
as represented in fig. 1,987.
This is a very common method of construction for small
transformers, which are provided with convenient terminal
blocks for combining the sections of each winding to suit the
requirements of the case. When the two sections of either
winding are connected in parallel as shown in fig. 1,987, _care
must be taken to connect_ ~corresponding ends~ _of the two
sections together_.
~Combining Transformers.~--Two or more transformers built to operate
at the same pressure and frequency may be connected together in a
variety of ways; in fact, the primary and secondary terminals may each
be considered exactly as the terminals of direct current dynamos, with
certain restrictions.
[Illustration: FIG. 1,987.--Diagram of single phase transformer with
primary and secondary windings of two sections each, showing voltages
per section with parallel connection.]
~Ques. What are the two principal precautions which must be observed in
combining transformer terminals?~
Ans. The terminals must have the same polarity at a given instant, and
the transformers should have practically identical characteristics.
The latter condition is not absolutely essential, but it
is emphatically preferable. For example, if a transformer,
which has 2 per cent. regulation, be connected in parallel,
as indicated in fig. 1,988, with one which has 3 per cent.
regulation, at no load the transformers will give exactly the
same voltage at the secondary terminals, but at full load one
will have a secondary pressure of, say, 98 volts, while the
other has 97 volts. The result is that the transformer giving
only 97 volts will be subject to a reverse pressure of one
volt from its mate. This will not cause excessive current to
flow backward through the secondary winding of the low voltage
transformer, but it will disturb the phase relations and lower
the power factor and efficiency of the combination. In such a
case it is much better to work the secondary circuits of the
two transformers separately.
In case the transformers have practically the same
characteristics it is necessary, as stated above, to make sure
that the secondary terminals connected together have the same
polarity at a given instant; it is not necessary to find out
definitely what the polarity is, merely that it is the same for
both terminals. This can be easily done as shown in fig. 1,989.
[Illustration: FIG. 1,988.--Diagram showing unlike single phase
transformers in parallel.]
~Ques. What may be said with respect to operating transformer
secondaries in parallel?~
Ans. It is seldom advantageous. Occasionally it may be necessary as a
temporary expedient, but where the load is such as to require a greater
capacity than that of a transformer already installed, it is much
better to replace it by a large transformer than to supplement it by an
additional transformer of its own size.
~Ques. How are the secondaries arranged in modern transformers and why?~
Ans. The secondary windings are divided into at least two sections so
that they may be connected either in series or parallel.
~Ques. Explain how secondary connections are made for different
voltages.~
Ans. If, for instance, the secondary pressure of a transformer having
two sections be 100 volts with the terminals in parallel, as in fig.
1,990, then connecting them in series will give 200 volts at the free
secondary terminals, as indicated in fig. 1,991.
~Ques. What precaution should be taken in connecting secondary sections
in parallel in core type if the two sections be wound on different
limbs of the cores?~
[Illustration: FIG. 1,989.--Method of comparing instantaneous polarity.
Two of the terminals are connected as shown by a small strip of fuse
wire, and then touching the other two terminals together. If the fuse
blows, then the connections must be reversed; if it does not, then they
may be made permanent.]
Ans. It will be advisable to make the connections ample and permanent,
so that there will not be any liability to a difference between the
current flowing in one secondary winding and that flowing through the
other.
~Two Phase Connections.~--In the case of two phase distribution each
circuit may be treated as entirely independent of the other so far as
the transformers are concerned. Two transformers are used, one being
connected to one primary phase and supplying one secondary phase, the
other being connected to the other primary phase and supplying the
other secondary phase as indicated in fig. 1,996, exactly as though
each primary and secondary phase were an ordinary single phase system,
independent of the other phase.
[Illustration: FIGS. 1,990 and 1,991.--Methods of altering the
secondary connections of a transformer having two sections in the
secondary to obtain a different voltage. Fig. 1,990 shows the two
sections in parallel giving say 100 volts; fig. 1,991 shows the two
sections in series giving 200 volts.]
~Ques. Is the above method usually employed?~
Ans. No, the method shown in fig. 1,997 is generally used.
~Three Phase Connections.~--There is not so much freedom in making
three phase transformer connections, as with single or two phase,
because the three phases are inseparably interlinked. However, the
system gives rise to several methods of transformer connection, which
are known as:
1. Star;
2. Delta;
3. Star-delta;
4. Delta-star.
[Illustration: FIGS. 1,992 to 1,995.--Three phase transformer
connections. Fig. 1,992 ~delta~ connection; fig. 1,993 ~star~
connection; fig. 1,994 ~delta star~ connection; fig. 1,995 ~star-delta~
connection.] /#
~Delta Connection.~--In the delta connection both primaries and
secondaries are connected in delta grouping, as in fig. 1,992.
~Star Connection.~--This method consists in connecting both the
primaries and secondaries in star grouping, as in fig. 1,993.
~Delta-star Connection.~--In this method the primaries are connected in
delta grouping and the secondaries in star grouping, as in fig. 1,994.
~Star-delta Connection.~--This consists in connecting the primaries in
star grouping, and the secondaries in delta grouping, as in fig. 1,995.
[Illustration: FIG. 1,996.--Two phase transformer connections. Two
single phase transformers are used and connections made just as though
each phase were an ordinary single phase system.]
~Ques. What advantage has the star connection over the delta
connection?~
Ans. Each star transformer is wound for only 58% of the line voltage.
In high voltage transmission, this admits of much smaller transformers
being built for high pressure than possible with the delta connection.
~Ques. What advantages are obtained with the delta connection?~
Ans. When three transformers are delta connected, one may be removed
without interrupting the performance of the circuit, the two remaining
transformers in a manner acting in series to carry the load of the
missing transformer.
[Illustration: FIG. 1,997.--Two phase transformer connections, with
secondaries arranged for three wire distribution, the primaries
being independently connected to the two phases. In the three wire
circuit, the middle or neutral wire is made about one-half larger than
each of the two outer wires. In fig. 1,996 it makes no difference
which secondary terminal of a transformer is connected to a given
secondary wire, so long as no transformers are used in parallel. For
example, referring to the diagram, the left hand secondary terminal
of transformer, A, could just as well be connected to the lower wire
of the secondary phase, A, and its right hand terminal connected to
the upper wire, the only requirement being that the two pairs of mains
shall not be "mixed;" that is, transformer, A, must not be connected
with one secondary terminal to phase, A and the other to phase, B. In
the case shown by fig. 1,997, there is not quite so much freedom in
making connections. One secondary terminal of each transformer must be
connected to one of the outer wires and the other two terminals must be
both connected to the larger middle wire of the secondary system. It
makes no difference, however, which two secondary terminals are joined
and connected to the middle wire so long as the other terminal of each
transformer is connected to an outer wire of the secondary system.]
The desire to guard against a shut down due to the disabling
of one transformer has led to the extensive use of the delta
connection, especially for the secondaries or low pressure
side.
It should be noted that if one transformer be disabled, the
efficiency of the other two will be greatly reduced. To operate
a damaged three phase transformer, the damaged windings must
be separated electrically from the other coils, the damaged
primary and secondary being respectively short circuited upon
themselves.
~Ques. What kinds of transformers are used for three phase current?~
Ans. Either a three phase transformer, or a separate single phase
transformer for each phase.
[Illustration: FIG. 1,998.--Three wire connections for transformer
having two secondary sections on different legs of the core. If the
secondary terminals be connected up to a three wire distribution, as
here shown diagrammatically, it is advisable to make the fuse, 2,
in the middle wire, considerably smaller than necessary to pass the
normal load in either side of the circuit, because, should the fuse,
1, be blown, the secondary circuit through the section, Sa, will be
open, and the corresponding half of the primary winding, Pa, will
have a much higher impedance than the half of the primary winding,
Pb, the inductance of which is so nearly neutralized by the load on
the secondary winding, Sb. The result will be that the voltage of
the primary section, Pa, will be very much greater than that of the
section, Pb, and as the sections are in series the current must be the
same through both halves of the winding; the drop or difference of
pressure, therefore, between the terminals of Pa will be much higher
than that between the terminals of Pb, consequently, the secondary
voltage of Sb will be greatly lowered and the service impaired. As the
primary winding, Pa, is designed to take only one-half of the total
voltage, the unbalancing referred to will subject it to a considerably
higher pressure than the normal value; consequently, the magnetic
density in that leg of the transformer core will be much higher than
normal, and the transformer will heat disastrously. If the fuse, 2,
in the middle wire be made, say, one-half the capacity of each of the
other fuses, this condition will be relieved by the blowing of this
fuse, and as the lamps in the live circuit would not be anywhere near
candle power if the circuit remained intact, the blowing of the middle
fuse will not be any disadvantage to the user of the lamps. Some makers
avoid the contingency just described by dividing each secondary coil
into two sections and connecting a section on one leg in series with a
section on the other leg of the core, so that current applied to either
pair of the secondary terminals will circulate about both legs of the
core.]
[Illustration: FIGS. 1,999 to 2,002.--Three phase delta, and star
connections using three transformers. There are two ways of connecting
up the primaries and secondaries, one known as the "delta" connection,
and illustrated diagrammatically by fig. 1,999, and the other known
as the "star" connection, and illustrated by fig. 2,001. In both
diagrams the line wires are lettered, A, B and C. Fig. 2,000 shows the
primaries and secondaries connected up delta fashion, corresponding
to fig. 1,999, and fig. 2,002 shows them connected up star fashion,
corresponding to fig. 2,001. In both of the latter sketches the
secondary wires are lettered to correspond with the respective primary
wires. When the primaries are connected up delta fashion, the voltage
between the terminals of each primary winding is the same as the
voltage between the corresponding two wires of the primary circuit,
and the same is true of the secondary transformer terminals and
circuit wires. The current, however, flowing through the transformer
winding is less than the current in the line wire, for the reason
that the current from any one line wire divides between the windings
of two transformers. For example, in figs. 1,999 and 2,000, part of
the current from the line wire, A, will flow from A to B through the
left hand transformer, and part from A to C through the right hand
transformer; if the current in the line wire, A, be 100 amperes, the
current in each transformer winding will be 57.735 amperes. When
transformers are connected up star fashion, as in figs. 2,001 and
2,002, the current in each transformer winding is the same as that
in the line wire to which it is connected, but the voltage between
the terminals of each transformer winding is 57.735 per cent. of the
voltage from wire to wire on the circuit. For example, if the primary
voltage from A to B is 1,000 volts, the voltage at the terminals of
the left hand transformer (from A to J) will be only 577.35 volts,
and the same is true of each of the other transformers if the system
is balanced. These statements apply, of course, to both primary and
secondary windings, from which it will become evident that if the
three transformers of a three phase circuit be connected up star
fashion at the primaries, and delta fashion at the secondaries, the
secondary voltage will be lower than if both sides are connected up
star fashion. For example, if the transformers be wound for a ratio
of 10 to 1, and are connected up with both primaries and secondaries
alike, no matter whether it be delta fashion or star fashion, the
secondary voltage will be one-tenth of the primary voltage; but if the
primaries be connected up star fashion on a 1,000 volt circuit, and
the secondaries be connected up delta fashion, the secondary voltage
will be only 57.735 volts, instead of 100 volts. The explanation of the
difference between the voltage per coil in a delta system and that in
a star system is that in the former each winding is connected directly
across from wire to wire; whereas in the star system, two windings are
in series between each pair of line wires. The voltage of each winding
is not reduced to one-half, however, because the pressures are out of
phase with each other, being 120°, or one-third of a cycle, apart;
consequently, instead of having 500 volts at the terminals of each coil
in fig. 2,001 the voltage is 577.35. The same explanation applies to
the current values in a delta system. The current phase between A and
B, in fig. 1,999, is 120° removed from that in the winding between A
and C; consequently the sum of the two currents, in the wire, A, is
1.732 times the current in each wire; or, to state it the opposite way,
the current in each winding is 57.735% of the current in the wire, A.
It will be well for the reader to remember that in all cases pressures
differing in phase when connected in series, combine according to the
well-known law of the parallelogram of forces; currents differing in
phase, and connected in parallel, combine according to the same law.]
~Ques. What points are to be considered in choosing between three
phase and single phase transformers for the three phase current
transformation?~
Ans. No specific rule can be given regarding the selection of single
phase or three phase transformers since both designs are equally
reliable; local conditions will generally determine which type is
preferable.
The following general remarks may, however, be helpful:
Single phase transformers are preferable where only one
transformer group is installed and where the expense of a spare
transformer would not be warranted. In such installations
the burn out of one phase of a three phase unit would cause
considerable inconvenience for the reason that the whole
transformer would have to be disconnected from the circuit
before repairs could be made.
If single phase transformers be used and connected in delta
on both primary and secondary, the damaged transformer can
be cut out with a minimum amount of trouble and the other two
transformers can be operated at normal temperature open delta
at 58 per cent. of the normal capacity of the group of three
transformers, until the third unit can be replaced.
With a three phase shell type transformer, if both the primary
and secondary be delta connected, trouble in one phase will not
prevent the use of the other two phases in open delta. By short
circuiting both primary and secondary of the defective phase,
and cutting it out of circuit the magnetic flux in that section
is entirely neutralized. This cannot be done, however, with any
but delta connected shell type transformers.
[Illustration: FIG. 2,003.--Diagram showing three wire secondary
connections General Electric (type H) transformer. As will be seen,
the method adopted consists of distributing equally, on each side of
the primary coil, both halves of the secondary winding, so that each
secondary throughout its length is closely adjacent to the entire
primary winding. In order to insure the exact equality of resistance
and reactance in the two secondary windings necessary to obtain perfect
regulation of the two halves, the inside portion of the secondary
winding on one side of the primary coil is connected in series with the
outside portion of that on the other side. As a result, the drop of
voltage in either side of the secondary under any ordinary conditions
of unbalanced load, does not exceed the listed regulation drop. This
particular arrangement is used because it is the simplest and best
method for this construction.]
Where a large number of three phase transformers can be used,
it is generally advisable to install three phase units, the
following advantages being in their favor as compared with
single phase units:
1. Require less floor space than three single phase units;
2. Weigh less than the single phase units;
3. Simpler connections, as only three primary and three
secondary leads are generally brought out;
4. Transformer presents a symmetrical and compact appearance.
~Ques. What is the character of the construction of three phase
transformers?~
Ans. The three phase transformer is practically similar to that of
the single phase, except that somewhat heavier and larger parts are
required for the core structure.
~COMPARISON OF AIR BLAST, WATER COOLED, AND OIL COOLED
TRANSFORMERS~
======================+=======================+=======================
~Air blast type~ | ~Water cooled type~ | ~Oil cooled type~
----------------------+-----------------------+-----------------------
~1. COST~
----------------------+-----------------------+-----------------------
~A. First cost~ | |
Necessarily more |Least expensive of all |Necessarily more
expensive than the |types. |expensive than the air
water cooled type of | |blast and water cooled
similar rating. | |type of similar rating.
| |
~B.~ | |
The installation is |Being heavier than the |Being heavier than the
extremely simple. |air blast type, these |air blast and water
|transformers, as a |cooled type, these
Moisture that may |rule, require heavier |transformers require
have collected on the |apparatus for |heavier apparatus for
surfaces during |installing. Both |installing. Both
transportation |transformer and tank |transformer and tank
or storage should be |should be thoroughly |should be thoroughly
thoroughly dried out. |dried out before being |dried out before being
|filled with oil. |filled with oil.
| |
|The oil is usually |
|supplied in 50 gal. |
|hermetically sealed |
|steel barrels to |
|minimize possibility of|
|moisture during |
|transportation. |
----------------------+-----------------------+-----------------------
~C. Auxiliary | |
apparatus~| |
A duct, or chamber, |In most cases, cooling |Do not require cooling
of considerable size |water may be obtained |water or blower.
is required under the |with sufficient natural|
transformers in order |head. However, there |
to conduct the cooling|are frequent cases in |
air to them. |which it can be |
|obtained only by the |
A blower outfit for |use of pumps. |
supplying air is | |
required. |A system of piping for |
|the cooling water and |
|oil drainage is |
|required, the cost of |
|which depends, of |
|course, on the station |
|layout. |
~D. Maintenance~ | |
An occasional |A water pumping outfit |No air or water
cleaning, for which a |would possibly require |circulation to demand
supply of compressed |a trifle more attention|attention.
air at about 20 lb. |than a blower outfit in|
pressure is |which there are no |
recommended. |valves or piping. |
| |
The blower outfit | |
requires no more care | |
than any other similar| |
apparatus. | |
======================+=======================+=======================
~2. FLOOR SPACE~
----------------------+-----------------------+-----------------------
Always requires space |Extra space only |Only require space for
for cooling apparatus.|required when auxiliary|the transformer as no
|pumping apparatus is |extra apparatus is
|necessary. |necessary
======================+=======================+=======================
~3. LOCATION~
----------------------+-----------------------+-----------------------
As the transformers |Transformers are |Transformers are
are open at the top |completely enclosed but|completely enclosed but
they should not be |location should be such|location should be such
located where there is|that no water will fall|that no water will fall
much dust or dirt nor |on leads or bushings. |on leads or bushings.
where water from any | |
source is liable to | |The building should be
fall on them. | |well ventilated.
| |
| |There is no auxiliary
| |apparatus.
The blower should be |Location of auxiliary |
so situated as to |apparatus will depend |
obtain clean dry air |on the station layout. |
of a temperature not | |
greater than 77° Fahr.| |
======================+=======================+=======================
~4. GENERAL APPEARANCE~
----------------------+-----------------------+-----------------------
Terminal leads may |Leads are brought out |Leads are brought out
be located in the |of the top of the |of the top of the
base and the air |transformers. |transformers.
chamber may be used | |
for conducting and |Water cooling pipes |
distributing the |are connected at the |
connecting wiring. |top in most cases. |
| |
The absence of | |
overhead wiring aids | |
in simplifying the | |
appearance of the | |
station. | |
======================+=======================+=======================
~5. OPERATION~
----------------------------------------------------------------------
Equal reliability in all three types.
While full load efficiencies are practically equal in the three
designs, it is necessary to change the proportion of iron and copper
losses somewhat as the copper loss of the air blast transformer is a
smaller part of its total loss than of the water cooled and oil cooled
types. As a result, the regulation of the air blast transformer is a
trifle better.
======================================================================
~6. GENERAL~
----------------------------------------------------------------------
The above information regarding selection of type is not applicable
to air blast transformers for circuits materially in excess of 33,000
volts.
On account of the great thickness of the solid insulation needed and
the consequent difficulty in radiating heat from the copper, it is
impracticable to design the air blast type for more than this voltage.
The oil immersed designs are therefore recommended for transformers
above 33,000 volts.
Both oil cooled and water cooled types are available for all
voltages, being restricted in this respect only by the limitations of
transmission facilities.[8]
======================================================================
[8] NOTE.--No special foundations are necessary for any type of
transformer other than a good, even floor, having sufficient strength
to support the weight.
[Illustration: FIGS. 2,004 to 2,011.--Connections of standard
transformers. All stock transformers are wound for some standard
transformation ratio, such as 10 to 1, but various leads are brought
out by means of which ratios of 5, 10 and 20 to 1 may be obtained for
one transformer. The figures show the voltage combinations possible
with a standard transformer.]
~Ques. How are transformers connected for four wire three phase
distribution?~
Ans. When the secondaries of three transformers are star connected, a
fourth wire may be run from the neutral point, thus obtaining the four
wire system.
[Illustration: FIG. 2,012.--Method of determining core loss. Connect
voltmeter and wattmeter as shown in the illustration to the low tension
side of the transformer. By means of a variable voltage transformer
bring the applied voltage to the point for which the transformer
is designed. The wattmeter indicates directly the core loss, which
includes a very small loss due to the current in the copper.
~Cautions.~--1. Make sure of the voltage and frequency. The
manufacturers' tabulated statements refer to a definite voltage and
frequency and these have a decided influence upon the core loss. 2. The
high tension circuit must remain open during the test.]
The voltage between any main wire and the neutral will be
57 per cent. of the voltage between any two main wires. For
general distribution this system is desirable, requiring less
copper and greater flexibility than other systems.
Three phase 200 volt motors may be supplied from the main wires
and 115 volt lamps connected between each of the three main
wires and the neutral; if the lamp load be very nearly balanced
the current flowing in the neutral wire will be very small, as
in the case of the ordinary three wire direct current system.
~How to Test Transformers.--~The troubles incident to gas or water
service have their parallels in electric power distribution.
[Illustration: FIG. 2,013.--Method of determining copper loss. Connect
ammeter and wattmeter to high tension side of transformer short circuit
secondary leads, as shown in the illustration, and by means of a
variable voltage, adjust current to the full load value for which the
transformer is intended. The wattmeter reading shows the copper loss at
full load. The full load primary current of any transformer is found
from the following equation.
_full load current = full load watts ÷ primary volts_
EXAMPLE: To find proper full load current on a five kw. 2,200 volt
transformer, divide 5,000 watts by 2,200 volts, the full load current
will then be 2.27 amperes. A slight variation in primary current
greatly increases or decreases the copper loss.
~Remarks.~--Copper loss increases with temperature because the
resistance of the metal rises. Do not overload the current coil of the
wattmeter. For greater accuracy the I²R drop of potential method should
be used.]
Companies engaged in the former, credit a large percentage of their
losses to leaky valves and defective mains. The remedy may involve
heavy expense and the loss is often tolerated as the lesser of two
evils.
In electric power distribution the transformer takes in part the place
of the valve and pipe system. An inferior or defective transformer
usually treats both the central station and its customers badly, being
in this respect more impartial than the gas or water pipe which may
annoy but one of the interested parties at a time.
Like a neglected or defective gas fixture a transformer can
menace life, failing, however, to give the warning the former
gives, and with a more hidden threat on account of its location.
[Illustration: FIG. 2,014.--Diagram of connections for regulation
test. Connect transformer under test to high tension supply circuit.
A second transformer with same or other known change ratio is also
to be connected up, as illustrated. By means of a double pole double
throw switch, the voltmeter can be made to read the pressure on the
secondary of either transformer. Supposing the same change ratio it
is evident that if both remain unloaded the voltmeter will indicate
the same pressure. A gradually increasing lamp load up to the limit
of the transformer capacity, will be attended by a drop in pressure
at the terminals. This drop can be read as the difference of the
voltmeter indications, and when expressed in per cent. of secondary
voltage stands for "regulation." ~Remarks:~ The auxiliary transformer
is necessary in order to make sure of the high tension line voltage. A
large transformer under test may cause primary drop in taking power.
This must be set down against it in testing regulation. The second
transformer gives notice of such drop, whatever be the cause. Figs.
2,012 to 2,014 used by courtesy of the Moloney Electric Co.]
Apart from this, corresponding to an exasperated customer who
complains at home and to his friends of dim lamps, blackened
lamps, you will find in the power station the manager,
who, also worried and in no better humor, contemplates the
difference in meter readings at the end of the line.
His business does not increase and would not increase even if
he could lower the rates, which he cannot do because of these
meter readings.
He may be confident of his engines and generators, and that
his line is up and all right, but he very seldom knows what
the transformers are doing on top of the poles. Perhaps he
feels that this waste is so slight that it makes no material
difference. This can be readily ascertained by means of a set
of testing instruments.
Perhaps the transformers were purchased because of their attractive
prices and never tested.
Water, plumbing, gas and steam fittings are subjected to test. Why not
transformers? Even more so because transformers take constant toll from
the company installing them, while gas and water fittings, once passed,
are off the contractor's hands.
The busy manager has little time for complicated treatises and
monographs on electrical measurements and even handbooks confront
him with forbidding formulæ. Accordingly the methods of transformer
testing, which are very simple, are illustrated in the accompanying
cuts. Managers of electric power and lighting companies should study
them carefully.
[Illustration: FIG. 2,015.--Wagner central station core type
transformer repair unit consisting of one half set of primary and
secondary windings together with the section of the iron core upon
which the coils are wound.]
An ammeter, voltmeter and wattmeter are required to make the tests.
Losses are small in ~good~ transformers and hence the instruments
~should be accurate~. For the same reason instruments should be chosen
of the proper capacity to give their best readings. If there be any
doubt about the testing instruments being correct, they should be
calibrated before being used. The testing circuits should be properly
fused for the protection of the instruments. It is hardly logical, but
a very common practice is to mistrust meters and to watch them closely,
while the transformers are guilty of theft unchallenged, and keep
busily at it on a large scale.
[Illustration: FIG. 2,016.--Moloney tubular air draft oil filled
transformer. The case is made of cast iron, with large steel tubes
passing from the bottom through the top. In operation the air in the
tubes becomes hot and expands; a draft is thus produced which carries
away considerable heat.]
~Transformer Operation with Grounded Secondary.~--The operation of a
transformer with a grounded secondary has been approved by the American
Institute of Electrical Engineers, and by the National Board of Fire
Underwriters.
This method of operation effectually prevents a high voltage occurring
upon the low tension wires in case of a breakdown or other electrical
connections occurring between the primary and secondary windings.
[Illustration: FIG. 2,017.--Moloney pressure transformer adapted for
switchboard work in connection with voltmeters, wattmeters the, etc.,
in sizes from 25 to 500 watts.]
In case of a breakdown without the secondary grounded, any one touching
a part of the low tension system, such as a lamp socket, might receive
the full high pressure voltage. With the low tension grounded, the fuse
in the high tension circuit will blow and the fault be discovered upon
replacing it.
~Transformer Capacity for Motors.~--The voltage regulation of a well
designed transformer is within 3 per cent. of its rated voltage on a
non-inductive load such as incandescent lamps, but when motors are
connected to the circuit their self-induction causes a loss of 5 per
cent. or more, and if the load be fluctuating, it is better to use
independent transformers for the motor, which will prevent considerable
fluctuations in the incandescent lamps. Arc lamps do not show slight
voltage changes as much as incandescent lamps. The proper rating of
transformers for two phase and three phase induction motors is given in
table on the next page.
[Illustration: FIG. 2,018.--Moloney current transformer switchboard or
indoor type. It is used ordinarily for insulating an ammeter, a current
relay, the current coil of a watt meter or watt hour meter from a high
tension circuit, for reducing the line current to a value suitable for
these instruments.]
A three phase induction motor may be operated from three single phase
transformers or one three phase transformer. While the one three phase
transformer greatly reduces the space and simplifies the wiring, the
use of three single phase transformers is more flexible and, in case
one transformer burns out, the connection can be readily changed so
that two transformers will operate the motor at reduced load until the
burned out transformer is replaced or repaired.
[Illustration: FIG. 2,019.--Diagram showing a method of operating a
three phase motor on a two phase circuit, using a transformer having
a tap made in the middle of the secondary winding, so as to get the
necessary additional phase. While this does not give a true balanced
three phase secondary, it is close enough for motor work. In the above
arrangement, the main transformer supplies 54 per cent. of the current
and the other with the split winding 46 per cent.]
It is well to allow one kilowatt per horse power of the motor in
selecting the size for the transformers, excepting in the small sizes
when a little larger kilowatt rating is found to be the most desirable.
~Transformers for Two and Three Phase Motors~
-----------+-------------------------------------------------
| Single phase transformer voltages
Delivered +------------------------+------------------------
voltage of | 110 volt motor | 220 volt motor
circuit +-----------+------------+----------+-------------
| Primary | Secondary | Primary | Secondary
-----------+-----------+------------+----------+-------------
1,100 | 1,100 | 122 | 1,100 | 244
2,200 | 2,200 | 122 | 2,200 | 244
-----------+-----------+------------+----------+-------------
Very small transformers should not be used, even when the motor is
large compared to the work it has to do, as the heavy starting current
may burn them out.
The following tables give the proper sizes of transformer for three
types of induction motor and the approximate current taken by three
phase induction motors at 220 volts.
-----------------------------------------------------------
~Capacities of Transformers for Induction Motors~
---------------+-------------------------------------------
| Kilowatts per transformer
Size of motor +--------------+--------------+-------------
horse power | Two single | Three single | One three
| phase | phase | phase
| transformers | transformers | transformer
---------------+--------------+--------------+-------------
1 | 0.6 | 0.6 |
2 | 1.5 | 1.0 | 2.0
3 | 2.0 | 1.5 | 3.0
5 | 3.0 | 2.0 | 5.0
7 | 4.0 | 3.0 | 7.5
10 | 5.0 | 4.0 | 10.0
15 | 7.5 | 5.0 | 15.0
20 | 10.0 | 7.5 | 20.0
30 | 15.0 | 10.0 | 30.0
50 | 25.0 | 15.0 | 50.0
75 | 40.0 | 25.0 | 75.0
100 | 50.0 | 30.0 | 100.0
---------------+--------------+--------------+-------------
~Current taken by Three Phase Induction Motors at 220 Volts~
-------------+-----------------+------------+----------------
Horse power |Approximate full |Horse power |Approximate full
of motor | load current | of motor |load current
-------------+-----------------+------------+----------------
1 | 3.2 | 20 | 50.
2 | 6.0 | 30 | 75.
3 | 9.0 | 50 | 125.
5 | 14.0 | 75 | 185.
10 | 27.0 | 100 | 250.
15 | 40.0 | 150 | 370.
-------------+-----------------+------------+----------------
~Transformer Connections for Motors.~--Fig. 2,020 shows the connection
of a three phase so called delta connected transformer with the three
primaries connected to the lines leading from the alternator and the
three secondaries leading to the motor.
The connections for a three phase motor using two transformers is shown
in fig. 2,021 and is identical with the previous arrangement, except
that one transformer is left out and the other two made correspondingly
larger.
The copper required in any three wire three phase circuit for a given
power and loss is 75 per cent. that necessary with the two wire single
phase or four wire two phase system having the same voltage between
lines.
[Illustration: FIG. 2,020.--Three phase motor transformer connections;
the so-called Delta connected transformers.]
The connections of three transformers for a low tension system of
distribution by the four wire three phase system are shown in fig.
2,022. The three transformers have their primaries joined in delta
connection and the secondaries in "~Y~" connection. The three upper
lines of the secondary are the three main three phase lines, and the
lowest line is the common neutral.
[Illustration: FIG. 2,021.--Three phase motor connections using two
transformers.]
The voltage across the main conductors is 200 volts, while
that between either of them and the neutral is 115 volts; 200
volt motors should be joined to the mains while 115 volt lamps
are connected between the mains and neutral. The arrangement
is similar to the Edison three wire system and the neutral
carries current only when the lamp load is unbalanced.
The voltage between the mains should be used in calculating the
size of wires, and the size of the neutral wire should be made
in proportion to each of the main conductors that the lighting
load is to the total load.
[Illustration: FIG. 2,022.--Delta-star connection of three transformers
for low pressure, three phase, four wire system.]
When lights only are used the neutral should be the same as the
main conductors. The copper required in such a system for a
given power and loss is about 33.3 per cent. as compared with
a two wire single phase system or a four wire two phase system
using the same voltage.
[Illustration: FIG. 2,023.--Diagram of transformer connections for
motors on the monocyclic system.]
~Monocyclic Motor System.~--Motors on the monocyclic system are
operated from two transformers connected as shown in fig. 2,023. In
the monocyclic system the single phase current is used to supply the
lighting load and two wires only are necessary, but if a self-starting
induction motor be required, a third or _teaser wire_ is brought to the
motor and two transformers used.
The teaser wire supplies the quarter phase current required to start
the motor, which afterwards runs as a single phase synchronous motor
and little or no current flows through the teaser circuit as long as
the motor keeps in synchronism; in case it fall behind, the teaser
current tends to bring it up to speed instead of the motor stopping, as
would be the case of a single phase motor.
[Illustration: FIG. 2,024.--Moloney flaming auto type arc lamp
transformer for 110 volts primary to 55 volts secondary. A hook in
bottom of case provides means for suspension of lamp. The transformer
may be operated on circuits from 100 to 120 volts primary, 50 to 60
volts secondary. The secondary capacity is 8 to 12 amperes.]
The voltage of the transformers should be tested by means of a
voltmeter or two incandescent lamps joined in series, before starting
up the motor, to see if the proper transformer connections have been
made and prevent an excessive flow of current.
If one of the transformers be reversed the voltage will be
almost doubled; in fact, it is a good plan to check up all the
transformer connections with the voltmeter or lamps which will
often save a burn out.
+------------------+-----------+-----------+-----------------------+
| | | |Ratio of transformation|
| | | | at no load |
| | | +-----------+-----------+
| Arrangement of | Primary | For | with | with |
| links on the | coils | circuit | secondary | secondary |
| connecting board | will be | voltage | coils in | coils in |
| | connected | normal at | multiple | series. |
| | in | | | |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Multiple | 1,100 | 10:1 | 5:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Multiple | 1,100 | 9.05:1 | 4.52:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Series | 2,200 | 20:1 | 10:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Series | 2,200 | 19.05:1 | 9.5:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Series | 2,200 | 18.1:1 | 9.05:1 |
|------------------+-----------+-----------+-----------+-----------+
FIGS. 2,025 to 2,029.--Diagrams of Wagner transformer connection
board, and table showing various arrangements of the terminal links,
corresponding transformation ratios, and suitable primary voltages. */
[Illustration: FIG. 2,030.--Installation of a transformer on pole;
view showing method of attachment and disposition of the primary and
secondary leads, cutouts, etc.]
[Illustration: FIG. 2,031.--Diagram of static booster or regulating
transformer. It is used for regulating the pressure on feeders. In
the figure, B are the station bus bars, R the regulable transformer,
F the two wire feeders, and T a distant transformer feeding into the
low pressure three wire distributing network N. The two ends of the
primary, and one end of the secondary of R, are connected to the bus
bars as shown. The other end of the secondary, as well as a number of
intermediate points, are joined up to a multiple way switch S, to which
one of the feeder conductors is attached, the other feeder main being
connected to the opposite bus bar. As will be evident from the figure,
by manipulating S extra volts may be added to the bus bar pressure at
will, and the drop along F compensated for. R is a step transformer,
the total secondary difference of pressure being comparatively small.
The above device possesses rather serious drawbacks, in that the switch
S has to carry the main current, and that the supply would be stopped
if the switch got out of order. Kapp improved on the arrangement by
putting the switch in the primary circuit.]
CHAPTER LIII
CONVERTERS
The alternating current must change to a direct current in many cases
as in railroad work because the induction motor is not so satisfactory
as the direct current series motor and the alternating current series
motor is slow in coming into general use.
In all kinds of electrolytic work, transformation must be made, and
in many cities where the direct current system was started, it is
still continued for local distribution, but the large main stations
generating alternating currents and frequently located some distance
away from the center of distribution have replaced a number of small
central stations.
Transformation may be made by any of the following methods:
1. Rotary converters;
2. Motor generator sets;
3. [A]Mercury vapor rectifiers;
4. [9]Electrolytic rectifiers.
[9] NOTE.--Rectifiers are explained in detail in Chapter LIV.
Strictly speaking, _a converter is a revolving apparatus for converting
alternating current into direct current or vice versa_; it is usually
called a rotary converter and is to be distinguished from the other
methods mentioned above.
Broadly, however, a converter may be considered as _any species of
apparatus for changing electrical energy from one form into another_.
According to the standardization rules of the A. I. E. E. converters
may be classified as:
1. Direct current converters;
2. Synchronous converters;
3. Motor converters;
4. Frequency converters;
5. Rotary phase converters.
[Illustration: FIGS. 2,032 and 2,033.--Gramme ring dynamo and
alternator armatures illustrating converter operation. The current
generated by the dynamo is assumed to be 100 amperes. Now, suppose, an
armature similar to fig. 2,032 to be revolving in a similar field, but
let its windings be connected at two diametrically opposite points to
two slip rings on the axis, as in fig. 2,032. If driven by power, it
will generate an alternating current. As the maximum voltage between
the points that are connected to the slip rings will be 100 volts, and
the virtual volts (as measured by a voltmeter) between the rings will
be 70.7 (= 100 ÷ √2̅), if the power applied in turning this armature
is to be 10 kilowatts, and if the circuit be non-inductive, the output
in virtual amperes will be 10,000 ÷ 70.7 = 141.4. If the resistances
of each of the armatures be negligibly small, and if there be no
frictional or other losses, the power given out by the armature which
serves as motor will just suffice to drive the armature which serves as
generator. If both armatures be mounted on the same shaft and placed in
equal fields, the combination is a ~motor dynamo~. In actual machines
the various losses are met by an increase of current to the motor.
Since the armatures are identical, and as the similarly placed windings
are passed through identical magnetic fields, one winding with proper
connections to the slip rings and commutator will do for both. In this
case only one field is needed; such a machine is called a ~converter~.]
~A direct current converter~ converts from a direct current to
a direct current.
~A synchronous converter~ (commonly called a _rotary
converter_) converts from an alternating current to a direct
current.
~A motor converter~ is a combination of an induction motor with
a synchronous converter, the secondary of the former feeding
the armature of the latter with current at some frequency other
than the impressed frequency; that is, it is a synchronous
converter in combination with an induction motor.
~A Frequency Converter~ (preferably called a _frequency
changer_) converts alternating current at one frequency into
alternating current of another frequency with or without a
change in the number of phases or voltages.
~A Rotary Phase Converter~ changes alternating current of one
or more phases into alternating current of a different number
of phases, but of the same frequency.
[Illustration: FIG. 2,034.--Diagram of ring wound single phase rotary
converter. It is a combination of a synchronous motor and a dynamo. The
winding is connected to the commutators in the usual way, and divided
into two halves by leads connecting segments 180° apart to collector
rings. A bipolar field is shown for simplicity; in practice the field
is multipolar and energized by direct current.]
~Rotary Converters.~--The synchronous or rotary converter consists of
a synchronous motor and a direct current generator combined in one
machine. It resembles a direct current generator with an unusually
large commutator and an auxiliary set of collector rings.
~Ques. In general, how does a rotary converter operate?~
Ans. On the collector ring side it operates as a synchronous motor,
while on the commutator side, as a dynamo.
Its design in certain respects is a compromise between
alternating current and direct current practice most noticeably
with respect to the number of poles and speed.
~Ques. Upon what does the speed depend?~
Ans. Since the input side consists of a synchronous motor, the speed is
governed by the frequency of the alternating current supplied, and the
number of poles.
[Illustration: FIG. 2,035.--Diagram of two phase rotary converter. This
is identical with the single phase machine with the exception that
another pair of collector rings are added, and connected to points
on the winding at right angles to the first, giving four brushes on
the alternating side for the two phase current. The pressure will be
the same for each phase as in the single phase rotary. Neglecting
losses the current for each phase will be equal to the direct
current × 1 ÷ / √2̅ = direct current × .707.]
Fig. 2,034 is a diagram of a ring wound rotary converter. This
style winding is shown to simplify the explanation. In practice
drum wound armatures are used, the operation, however, is the
same.
With this simple machine the following principles can be
demonstrated:
1. If the coil be rotated, alternating currents can be taken
from the collector rings and it is called an alternator.
2. By connecting up the wires from the commutator segments,
a direct current will flow in the external circuit making a
dynamo.
3. Two separate currents can be taken from the armature, one
supplying alternating current and the other direct current;
such a machine is called a _double current generator_.
4. If a direct current be sent in the armature coil through the
commutator, the coil will begin to rotate as in a motor and an
alternating current can be taken out of the collector rings.
Such an arrangement is called an _inverted rotary converter_.
5. If the machine be brought up to synchronous speed by
external means and then supplied with alternating current at
the collector rings, then if the direction of the current
through the armature coil and the pole piece have the proper
magnetic relation, the coil will continue to rotate in
synchronism with the current. A direct current can be taken
from the commutator, and when used thus, the machine is called
simply a _rotary converter_.
[Illustration: FIG. 2,036.--Diagram of three phase rotary converter. In
this type, the winding is tapped at three points 120° distant from each
other, and leads connected with the corresponding commutator segments.]
[Illustration: FIGS. 2,037 to 2,041.--Various rotary converter and
transformer connections. Fig. 2,037 two phase connections; fig. 2,038
three phase delta connections; fig. 2,039 three phase ~Y~ or star
connections; fig. 2,040 six phase delta connections; fig. 2,041 six
phase ~Y~ connections.]
~Ques. What is the relation between the impressed alternating pressure
and the direct pressure at the commutator?~
Ans. The ratio between the impressed alternating pressure and the
direct current pressure given out is theoretically constant, therefore,
the direct pressure will always be as 1 to .707 for single phase
converters or if the pressure of the machine used above indicate 100
volts at the direct current end, it will indicate 70.7 volts at the
alternating current side of the circuit.
~Ques. Name two different classes of converter.~
Ans. Single phase and polyphase.
~Ques. What is the advantage of polyphase converters?~
Ans. In the majority of cases two or three phase converters are used on
account of economy of copper in the transmission line.
~Ques. How is the armature of a polyphase converter connected?~
Ans. Similar to that of an alternator with either delta or ~Y~
connections.
Figs. 2,037 to 2,041 show various converter connections between
the collector rings and commutator.
Fig. 2,037 indicates how the armature is tapped for two phase
connections.
Fig. 2,038 shows three phase delta connections, and fig. 2,039
the three phase ~Y~ or star connections.
Six phase delta and ~Y~ connections are frequently used as
shown in fig. 2,040 and fig. 2,041, both of which require
two secondary coils in the transformer, one set of which is
reversed, so as to supply the current in the proper direction.
~Ques. With respect to the wave, what is the relation between the
direct and alternating pressures?~
Ans. The direct current voltage will be equal to the crest of the
pressure wave while the alternating voltage will depend on the virtual
value of the maximum voltage of the wave according to the connections
employed.
Table of
Alternating Current and Voltage in Terms of Direct Current
(According to Steinmetz)
------------------------------------------------------------------------
| | DIRECT| SINGLE | TWO | THREE |
| |CURRENT| PHASE | PHASE | PHASE |
|-----------------|-------|-------------|-------------|----------------|
|VOLTS BETWEEN | | | | |
|COLLECTOR RING | 1 | | | |
|AND NEUTRAL POINT| |1/(2√2̅)=.354|1/(2√2̅)=.354| 1/(2√2̅)=.354 |
|-----------------|-------|-------------|-------------|----------------|
|VOLTS BETWEEN | | | | |
| ADJACENT | 1 | 1/√2̅=.707 |½=.5 | √3̅/(2√2̅)=.612|
|COLLECTOR RINGS | | | | |
|-----------------|-------|-------------|-------------|----------------|
| | | | | |
| AMPERES | 1 | √2̅=1.414 | 1/√2̅=.707 | (2√2̅)/3=.943 |
| PER LINE | | | | |
|-----------------|-------|-------------|-------------|----------------|
| | | | | |
|AMPERES BETWEEN | 1 | √2̅=1.414 |½=.5 |(2√2̅)/(3√3̅)=.545|
|ADJACENT LINES | | | | |
------------------------------------------------------------------------
-------------------------------------------------------------
| | SIX | TWELVE | n |
| | PHASE | PHASE | PHASE |
|-----------------|-------------|-----------|---------------|
|VOLTS BETWEEN | | | |
|COLLECTOR RING | | | |
|AND NEUTRAL POINT|1/(2√2̅)=.354|1/(2√2̅)=.354|1/(2√2̅)=.354|
|-----------------|-------------|-----------|---------------|
|VOLTS BETWEEN | | | |
| ADJACENT |1/(2√2̅)=.354| .183 | (SIN(π/n))/√2̅|
|COLLECTOR RINGS | | | |
|-----------------|-------------|-----------|---------------|
| | | | |
| AMPERES | √2̅/3=.472 | .236 |(2√2)̅/n |
| PER LINE | | | |
|-----------------|-------------|-----------|---------------|
| | | | |
|AMPERES BETWEEN | √2̅/3=.472 | .455 |(√2̅SIN(π/n))/n|
|ADJACENT LINES | | | |
-------------------------------------------------------------
In a single phase rotary, the value of the direct pressure is 1
to .707, therefore a rotary which must supply 600 volts direct
current must be supplied by 600 × .707 = 424 volts alternating
current. For three phase rotaries the ratio is 1 to .612, or
in order to produce 600 volts direct current, 600 × .612 = 367
volts on the alternating current side of the rotary is required.
[Illustration: FIG. 2,042.--Westinghouse rotary converter armature
coils. These are wound from bar copper and are interchangeable. The
armature coils are heavily insulated to withstand the tests specified
in the standardization rules of the American Institute of Electrical
Engineers.]
Fig. 2,034 shows a complete diagram of the electrical
connections. A single phase rotary is illustrated so as to
simplify the wiring.
The table of Steinmetz on page 1,464 gives the values of the
alternating volts and amperes in units of direct current.
~Ques. How is the voltage of a rotary varied on the direct current
side?~
Ans. Pressure or potential regulators are put in the high tension
alternating current circuit and may be regulated by small motors
operated from the main switchboard or operated by hand.
~Ques. What is the advantage of unity power factor for rotary
converters?~
Ans. It prevents overheating when the rotary is delivering its full
load in watts.
~Ques. What greatly influences the power factor of the high tension
line?~
Ans. The strength of the magnetic field.
[Illustration: FIG. 2,043.--Westinghouse rotary converter armature
spider. It is made of cast iron or cast steel. The dovetail grooves
are machined in the feet or ends of the arms and in these slots the
laminations forming the armature coil engage.]
~Ques. Does variation of the field strength materially affect the
voltage?~
Ans. No.
Since variation of the field strength does not materially
affect the voltage, by adjusting the resistance in series with
the magnetic circuit, the strength of the field can be changed
and the power factor kept 1 or nearly 1 as different loads are
thrown on and off the rotary.
~Ques. What is the effect of a field too strong or too weak?~
Ans. If too strong, a leading current is produced, and if too weak,
the current lags, both of which reduce the power factor and are
objectionable.
Usually there is a power factor meter connected up in the main
generating station and one also in the rotary substation, and
it is the duty of the attendant at the substation to maintain
the proper power factor.
~Ques. What is the ordinary range of sizes of rotaries~?
Ans. From 3 kw. to 3,000 kw.
[Illustration: FIG. 2,044.--Equalizer connections of Westinghouse
rotary converter. The armature coils are cross connected at points
of equal voltage and taps are led out from the winding at suitable
points to the slip rings. This construction insures a uniform armature
saturation below each pole piece and eliminates one cause of sparking
at the commutator.]
~Ques. What is the general construction of a rotary converter?~
Ans. It is built similar to a dynamo with the addition of suitable
collector rings connected to the armature windings at points having the
proper phase relations.
Standard rotary converters have been developed for 25 and 60
cycles. The standard railway machines are compound wound, the
series field being designed for a compounding of 600 volts at
no load and full load when supplied from a source of constant
pressure with not more than 10 per cent. resistance drop and
with 20 to 30 per cent. reactance in the circuit. The large
size machines are usually wound for six phase operation.
[Illustration: FIGS. 2,045 and 2,046.--Westinghouse pole construction
for converters. Fig. 2,045, pole without windings; fig 2,046, pole with
windings. Poles are built up of steel laminations held together with
rivets. Projections on the inner ends of the poles form seats for the
field coils and hold them in position. Copper dampers set in slots in
the pole faces insure stable operation. Rotary converters for railway
service are almost invariably compound wound. The series windings are
formed of bare copper strap. The shunt windings are of insulated copper
strap or wire. Spaces between coil turns and sections are provided for
ventilation.]
~Compounding of Rotary Converters.~--Compounding is desirable where the
load is variable, such as is the case with interurban railway systems.
The purpose of the compounding is to compensate automatically for the
drop due to line, transformer, and converter impedance.
On account of the low power factor caused by over compounding, and
the fact that substations are customarily connected to the trolley at
its nearest point without feeder resistance, over compounding is not
recommended. An adjustable shunt to the series field is provided with
each machine.
Shunt wound converters are satisfactory for substations in large
cities and similar installations where due to the larger number of car
units demanding power, the load is more nearly constant.
~Ratio of Conversion.~--The relation between the alternating and
direct current voltages varies slightly in different machines, due to
differences in design. The best operating conditions exist when the
desired direct current voltage is obtained with unity power factor at
the converter terminals when loaded.
[Illustration: FIG. 2,047.--Westinghouse rotary converter brush rigging
showing method of bracing the brushes. The brushes are supported by
a rigid cast iron rocker ring which fits accurately in the frame.
A handwheel worm and screw arrangement for shifting the brushes is
provided. Cast iron arms bolted to, but insulated from the rings, carry
the rods on which the brush holders are mounted. Brush holders are of
brass cast in one piece, of the sliding type and have braided copper
shunts. Brush tension is adjustable.]
[Illustration: FIG. 2,048.--Westinghouse commutating pole rotary
converter. The construction details are substantially the same as for
the railway converter, with exception of the commutating poles. The
application of commutating pole converters is particularly desirable
where special requirements such as great overload capacity or large
capacity and low voltage enable them to show to the greatest advantage.
Commutating poles as applied to rotary converters fulfill the same
functions as in the more familiar applications to dynamos and motors.
That is, the commutating pole insures sparkless commutation from no
load to heavy overloads with a fixed brush position. Brush shifting
devices are not furnished on commutating pole converters. Commutating
pole rotary converters for railway service are normally arranged for
automatic compounding which is effected by the proper combination of
series excitation and inductance between the generator and the rotary
converter. This inductance is normally included in the transformer
but in special cases may be partly in a transformer and partly in a
separate reactance. It is possible to produce by this means a slight
increase in the direct current voltage provided the voltage drop
in the alternating current line be not excessive. Usually it is so
arranged that the compounding that can be obtained is just sufficient
to overcome the alternating current line voltage drop. The standard
Westinghouse method of starting is alternating current self-starting.
With this method of self-starting, the brushes of a commutating pole
rotary converter must be lifted from the commutator during the starting
operation to prevent sparking. A mechanical device, as shown in fig.
2,050, is provided which accomplishes this. With direct current or
motor starting a brush lifting device is not necessary.]
~Ques. Upon what does the ratio of conversion depend?~
Ans. Upon the number of phases and method of connecting the windings.
For single phase or two phase machines it is 1 to .7; for three
phase, 1 to .612, or six phase, 1 to .7 or 1 to .613 depending
upon the kind of connection used for the transformer.
For example, a two phase rotary receiving alternating current
at 426 volts will deliver direct current at 600 volts, while a
three phase rotary receiving alternating current at 367 volts
will deliver direct current at 600 volts.
[Illustration: FIG. 2,049.--Commutating pole of Westinghouse
commutating pole rotary converter. The commutating poles are similar in
general construction to the main poles. The coils are of bare copper
strap wound on edge. Ventilating spaces are provided between the pole
and coil and between turns. The copper winding is bare except for a few
turns at each end. Insulating bolts retain the turns in their proper
position.]
~Ques. What difficulty would be encountered if other ratios of
conversion than those given above were required?~
Ans. An armature with a single winding could not be used.
It would be necessary to use a machine with two distinct
armature windings or else a motor generator set.
~Ques. What change in voltage is necessary between a converter and the
alternator which furnishes the current?~
Ans. The voltage must be reduced to the proper value by a step down
transformer.
~Voltage Regulation.~--As the ratio of the alternating to the direct
current voltage of a converter is practically constant, means must be
provided to compensate for voltage variation due to changes of load in
order to maintain the direct current pressure constant.
[Illustration: FIG. 2,050.--Westinghouse brush lifting device for
commutating pole rotary converter. A rack is attached to each brush as
shown. Into this rack the spring hinged lifting hook of the raising
device engages only when the lifting lever is shifted toward the
raised position. The lifting arrangement is independent of the brushes
during normal running, so it can in no way affect the operation of the
machine. Each brush is merely raised and lowered within its own holder
so the brush position or commutation is not altered.]
There are several methods of doing this, as by:
1. Shifting the brushes (objectionable);
2. Split pole method;
3. Regulating pole method;
4. Reactance method;
5. "Multi-tap" transformer method;
6. Synchronous regulator.
~Shifting the Brushes.~--Were it not for the difficulties encountered,
this would be a most convenient method of voltage regulation, since by
this procedure the direct current voltage may be varied from maximum to
zero. It is, however, not practical because of the excessive sparking
produced when the brushes are shifted out of the neutral plane.
[Illustration: FIGS. 2,051 to 2,053.--Woodbridge split pole rotary
converter. Each pole is split into three sections and provided with
windings as indicated in fig. 2,051. When excited as in fig. 2,052, the
commutator voltage is at its highest value; when excited as in fig.
2,053, the commutator voltage is low. The change in commutator voltage
for constant collector ring voltage is in virtue of the property of
rotary converters that the ratio of these two voltages is a function of
the width of the pole arc.]
~Split Pole Method.~--In order to overcome the difficulty encountered
in shifting the brushes the split pole method was devised by Woodbridge
in which each field pole is split into two or three parts.
The effect of this is the same as shifting the brushes except that no
sparking results.
The other part is arranged so that its excitation may be varied, thus
shifting the resultant plane of the field with respect to the direct
current brushes.
One of these parts is permanently excited and it produces near its edge
the fringe of field necessary for sparkless commutation.
~Regulating Pole Method.~--As applied to the rotary converter
regulating poles fulfill the same functions as commutating or
interpoles (see page 385) on motors and dynamos, that is, they insure
sparkless commutation from no load to heavy overloads with a fixed
brush position.
[Illustration: FIG. 2,054.--General Electric regulating pole rotary
converter. The field structure is divided into two parts, a main pole
and a regulating pole. The ratio between the voltages on the direct
current and alternating current sides may be readily varied by varying
the excitation of the regulating poles, the only auxiliary apparatus
required being a field rheostat for controlling the exciting current.
Where automatic regulation is required, machines may be provided with
compound windings, or automatic field regulators may be used responsive
to either voltage or current. These converters are adapted for a
variety of purposes where a variable conversion ratio is required,
either to maintain constant D. C. voltage with varying A. C. voltage
or to vary the D. C. voltage as required. Converters may be operated
inverted where it is required to furnish constant or variable A. C.
voltage from a D. C. source. Where converter and inverted converter
operation are desired, an opposite direction of rotation is required
for the inverted operation. Converters of this type are built in
capacities from 300 kw. up to 3,000 kw., and constructed to give a
voltage range between 240 and 300 volts, to cover the usual lighting
circuit requirements. In design, they are similar to standard rotary
converters, with the exception that the regulating poles are located
next to the main pole pieces and a slightly different form of pole
piece bridge is used for the main poles, in order to allow the
auxiliary poles to be readily removed or assembled.]
The regulating poles are used in order to vary the ratio between the
alternating current collector rings and the direct current side without
the use of auxiliary apparatus such as induction regulators or dial
switches which involve complicated connections and many additional
wires. The regulating poles are arranged with suitable connection so
that the current through them can be raised, lowered or reversed.
[Illustration: FIG. 2,055.--Detail of Westinghouse commutating pole
rotary converter brush, showing rack. The brush lifting mechanism and
its operation is explained in fig. 2,050.]
The characteristics of the regulating pole converter being novel, a
detailed explanation of the principles involved is given to facilitate
a clear understanding of its operation.
Consider a machine with a field structure as shown in fig.
2,056 resembling in appearance a machine with commutating
poles, but with the brushes so set that one of the regulating
poles adds its flux to that of one main pole, cutting the
inductors between two direct current brushes. The regulating
pole is shown with a width equal to 20 per cent. of that of the
main pole.
To obtain definite figures, it will be assumed that the machine
at normal speed, with the main poles excited to normal density,
but with no excitation on the regulating poles, gives 250 volts
direct current pressure. Then with each regulating pole excited
to the same density as the main poles, and with a polarity
corresponding to that of the main pole in the same section
between brushes, the direct current pressure will rise to 300
volts at the same speed, since the total flux cutting the
inductors in one direction between brushes has been increased
20 per cent.
If, on the other hand, the excitation of the regulating poles
be reversed and increased to the same density as that of the
main poles, the direct current pressure will fall to 200
volts, since in this case the regulating poles give a reverse
pressure, that is, a pressure opposing that generated by the
main poles.
[Illustration: FIG. 2,056.--Diagram of field of regulating pole
converter illustrating principles explained in the accompanying text.]
Now, if the machine be equipped with collector rings, that is,
if it be a converter, this method of varying the direct current
voltage from 200 to 300 volts does not give nearly as great
a variation of the alternating current voltage; in fact, the
latter voltage will be the same when delivering 200 volts as
when delivering 300 volts direct current pressure, if the field
excitation be the same.
This may be seen by reference to fig. 2,057, which is a diagram
of the alternating current voltage developed in the armature
windings by the two sets of poles.[10]
[10] NOTE.--In the Burnham split pole rotary converter, each pole is
divided into only two sections, one larger than the other. A main shunt
winding is arranged on the large sections, and a winding for providing
the voltage regulation is placed on the other section. When the current
is sent through this latter winding in one direction the voltage is
raised, when in the other direction the voltage is lowered.
The horizontal line OA represents the alternating current
voltage generated by the main poles, alone, with the
regulating poles unexcited, that is, when delivering 250 volts
direct current pressure.
For a six phase converter OA measures about 180 volts
diametrically, that is, between electrically opposite collector
rings.
If now the regulating poles be excited to full strength,
to bring the direct current pressure up to 300 volts, the
alternating current voltage generated by the regulating poles
will be 90 degrees out of phase with that generated by the main
poles (since they are placed midway between the main poles),
and will be about 40 volts as shown by the line AB.
The resultant alternating current volts across the collector
rings will be represented by the line OB with a value equal to
184.
[Illustration: FIG. 2,057.--Voltage diagram for regulating pole
converter illustrating principles explained in the accompanying text.]
Again, if the regulating poles be reversed at full strength,
to cut the direct current pressure down to 200 volts, the
alternating current voltage of the main and regulating poles
will be OA and AC respectively, giving the resultant OC equal
to OB with a value of 184 volts. Accordingly, the direct
current pressure may be either 200 or 300 volts with the same
alternating current pressure, and if the main field be kept
constant, the direct current pressure may range between 200 or
300 volts, while the alternating current pressure varies only
between 180 and 184 volts.
The alternating current pressure can be kept constant through
the full range of direct current voltage by changing the main
field so as always to give an equal and opposite flux change to
that of the regulating field. A constant total flux may thus
be obtained equal to the radius of the arc BC, fig. 2,057. In
this case the line OA, representing the main field strength,
will equal OB when the regulating field is not excited, and 250
volts can only be obtained at this adjustment.
This method of operation gives unity power factor with a
constant impressed pressure of 184 volts alternating current
with a range of direct current voltage from 200 to 300 volts.
[Illustration: FIGS. 2,058 and 2,059.--Diagrams illustrating the effect
on the alternating current voltage due to varying the regulating field
strength (of a machine proportioned according to fig. 2,060), from a
density equal to that in the main poles to the same density reversed,
the main field strength remaining constant. The D. C. voltage in this
case varies from 30 per cent. above that produced by the main field
alone to 30 per cent. below, or from 325 to 175 volts, while the A. C.
voltage varies only from 200 to 175 volts. To keep the A. C. voltage
constant with such a machine the main field must be strengthened as the
regulating field is weakened or reversed to reduce the D. C. voltage.
This strengthening increases the core loss particularly on low direct
current voltages, which however, are rarely required, hence a machine
proportioned as in fig. 2,060, would not be operated through so wide a
range as 175 to 325 volts. Assume that the range is 240 to 300 volts,
and that at the highest voltage, both main and regulating fields have
the same density, presenting to the armature practically one continuous
pole face of uniform flux intensity. The diagram of A. C. component
voltages to give constant A. C. resultant voltage across the rings for
the case, is shown in fig. 2,059. At 300 volts D. C., the main field
produces an A. C. voltage OA, and the regulating field, a voltage AB,
with a resultant OB, equal to about 200 volts A. C. At 270 volts D. C.,
the main field produces an A. C. voltage OA, and a regulating field
voltage AB, giving a resultant A. C. voltage OB, equal to 200 volts.
Similarly, at 240 volts D. C., the main field produces an A. C. voltage
OA, and the regulating field (now reversed) produces the reverse
voltage AB, giving the resultant OB again equal to 200 volts. It will
be noted that, theoretically the main field strength must be increased
about 15 per cent. above its value at 300 volts D. C. in order to keep
the D. C. voltage at 250 volts.]
~Ques. Where should the regulating poles be located for best results?~
Ans. A better construction is obtained by placing them closer to the
corresponding main pole, as in fig. 2,060, than when spaced midway
between the main poles as in fig. 2,056.
~Ques. When the regulating poles are spaced as in fig. 2,060, what is
the effect on the direct current voltage?~
Ans. The effect is the same as for the midway position (fig. 2,056)
except for magnetic leakage from the main poles to the regulating poles
when the latter is opposed to the former, that is, when the direct
current voltage is being depressed.
[Illustration: FIG. 2,060.--Diagram illustrating placement of
regulating poles. In practice machines are not built as indicated
diagrammatically in fig. 2,056, that is, with regulating poles spaced
midway between the main poles, because a better construction is
obtained by placing the regulating pole closer to the corresponding
main pole, as shown above.]
~Ques. What is the effect on the alternating current voltage?~
Ans. It is somewhat altered as explained in figs. 2,058 and 2,059.
~Reactance Method.~--This consists in inserting inductance in the
supply circuit and running the load current through a few turns around
the field cores. This method is sometimes called _compounding_, and
as it is automatic it is generally used where there is a rapidly
fluctuating load.
[Illustration: FIG. 2,061.--Westinghouse 300 kw., 1,500 volt, three
phase, 25 cycle, commutating pole rotary converter. The illustration
shows clearly the commutating, and main poles and the relative sizes,
also arrangement of the terminal connections.]
If a lagging current be passed through an inductance, the collector
ring voltage will be lowered, but will be raised in case of a leading
current. The degree of excitation governs the change in the phase of
the current to the converter, the excitation, in turn, being regulated
by the load current. Accordingly with series inductance, the effect of
the series coils on the field of the converter is quite similar to that
of the compounding of the ordinary railway dynamo.
~Multi-tap Transformer Method.~--The employment of a variable ratio
step down transformer for voltage regulation is a non-automatic method
of control and, accordingly, is not desirable except in cases where the
load is fairly constant over considerable periods of time. It requires
no special explanation.
[Illustration: FIG. 2,062.--Mechanical oscillator and speed limit
device of Westinghouse commutating pole rotary converter. It
automatically prevents the armature of the converter remaining in
one position and thus not allowing brushes to wear grooves in both
commutator and collector rings. The oscillator is a self-contained
device carried at one end of the shaft. The operating parts consist
of a hardened steel ball and a steel plate with a circular ball
race, backed by a spring. The machine is so installed with a slight
inclination toward the end carrying the oscillator. As the armature
revolves the ball is carried upward and owing to the convergence of
the steel race and shaft face, the spring is compressed. The reaction
of the spring forces the armature away from its natural position and
allows the ball to drop back to the lowest point of the race.]
~Synchronous Booster Method.~--This consists of combining with the
converter a revolving armature alternator having the same number of
poles.
~Ques. How is the winding of the booster alternator armature connected?~
Ans. It is connected in series with the input circuits on the converter.
[Illustration: FIG. 2,063.--Westinghouse 2,000 kw., 270 volt, direct
current, 6 phase, 167 R.P.M., synchronous booster rotary converter,
having a voltage range from 230 to 310 volts. It consists of a standard
rotary converter in combination with a revolving armature alternator
mounted on the same shaft with the rotary converter and having the same
number of poles. By varying the field excitation of the alternator,
the alternating current voltage impressed on the rotary converter
can be increased or decreased as desired. The direct current voltage
delivered by the converter is thereby varied accordingly. The principle
of operation of the booster converter is therefore very simple and
easily understood. It is simply a combination of two standard pieces
of electrical apparatus, accordingly there are incorporated in it no
details of construction essentially different from those encountered
in standard rotary converters and alternators. The only novelty is in
their combination. The frames may be supported either from the rotary
converter frame, as in the small units, or from the bed plate, as in
the larger ones. A synchronous booster converter can be built, if
necessary, with a vertical shaft to satisfy special floor space and
head room requirements.]
~Ques. How are the field windings connected?~
Ans. They are either fed with current regulated by means of a motor
operated field circuit rheostat, or joined in series with the
commutator leads of the converter.
[Illustration: FIG. 2,064.--Armature of Westinghouse synchronous
booster converter. Heavy cast yokes form the frames. They are
proportioned to rigidly support the laminated steel field poles. The
poles are fastened to the frame with through bolts. A lifting hook is
provided on all frames. The bed plates are in one piece for the smaller
machines but two piece bed plates are used for the larger ones. The
bearings are ring oiling and have babbitt wearing surfaces that are
renewable.]
[Illustration: FIG. 2,065.--Westinghouse field frames and bearings for
synchronous booster rotary converter. The frames consist of heavy cast
yokes. The poles are fastened to the frames with through bolts. The bed
plate is in one piece for the small machines and in two pieces for the
large ones.]
~Ques. For what service is the synchronous booster method desirable?~
Ans. For any application where a relatively wide variation in direct
current voltage is necessary.
It is particularly desirable 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.
[Illustration: FIG. 2,066.--General Electric motor generator set
consisting of 2,300 volt synchronous motor and 550 volt dynamo.]
~Motor Generator Sets.~--The ordinary rotary converter is the most
economical machine for converting alternating currents into direct
currents, and where slight variations in the direct current voltage is
necessary, they are mostly used on account of their high efficiency,
and because they are compact.
[Illustration: FIG. 2,067.--General Electric motor generator set
consisting of 230 volt induction motor and 125 volt dynamo.]
In many central stations where they supply a great variety of
apparatus, the motor generator sets are employed as the generator is
independent of the alternating current line voltage and any degree of
voltage regulation can be performed.
~Motor Generator Combinations.~--The following combinations of motor
generators are made and used to suit local conditions:
Synchronous motor dynamo
Induction motor dynamo
Direct current motor dynamo
Direct current motor alternator
Synchronous motor alternator
Induction motor alternator
[Illustration: FIG. 2,068.--General Electric generator set, as
installed for the Cleveland Electric Illuminating Company, Cleveland,
Ohio. It consists of 11,431 volt motor and 275 volt generator. Speed,
360 revolutions per minute.]
Standard practice has adopted high tension alternating current for
transmission systems, but direct current distribution is very
frequently used. This is particularly true where alternating current
apparatus has been introduced in old direct current lighting systems.
The synchronous motor or the induction motor connected to a generator
stands next in importance to the rotary converter because it is easy
to operate and the pressure may be changed by a rheostat placed in the
field circuit of the generator.
The line wires carrying full voltage can usually be connected direct
to the motor and thus do away with the necessary step-down transformer
required by the rotary.
~Ques. What is the behavior of a rotary converter when hunting?~
Ans. It is liable to flash over at the direct current brushes, which is
common in high frequency converters where there are a great number of
poles and the brushes are necessarily spaced close together around the
commutator.
~Ques. Is this fault so pronounced with motor generator sets?~
Ans. The motor generator operating on a high frequency circuit, the
generator can be designed with a few poles and the brushes set far
apart which will greatly reduce the chance of flashing over.
A synchronous motor will drive a generator at a constant speed
during changes in load on it, and by having a field regulating
resistance it can be used to improve the power factor of the
system.
When an induction motor is used its speed drops off slowly
as the load comes on the generator, and it is necessary to
regulate the voltage of the generator by means of a field
rheostat, or compound wound machines may be used.
While an induction motor requires no separate excitation of the
field magnets like the synchronous motor, its effect on the
power factor of the system is undesirable.
Although it is seldom necessary to convert direct current to
alternating, such an arrangement of a direct current motor
driving an alternator is often justified in place of an
inverted rotary converter, as in this case the alternating
current voltage can be changed independent of the direct
current voltage.
The racing of an inverted rotary under a heavy inductive load
or short circuit does not take place in motor generator set
mentioned above.
[Illustration: FIG. 2,069.--General Electric frequency changer set,
consisting of a 11,000 volt synchronous motor with direct connected
exciter and a 2,300 volt alternator.]
~Frequency Changing Sets.~--A frequency of 25 cycles is generally used
on railway work and in large cities using the Edison three wire system,
and as a 25 cycle current is not desirable for electric lighting it is
necessary to change it to 60 cycles by means of a frequency changer
shown in fig. 2,069 for distribution in the outlying districts.
The two machines in this combination are of the same construction, only
the synchronous motor would have eight poles and have the 25 cycle
current passing through it, while the generator would have 20 poles
and produce 62½ cycles per second at 300 revolutions per minute. By
supplying the motor with 24 cycles, the generator would produce 60
cycles.
[Illustration: FIG. 2,070.--General Electric four unit frequency
changer set consisting of a 11,000 volt synchronous motor, 13,200 volt
alternator, 250 volt exciter, and 440 volt starting induction motor.
Where parallel operation is required between synchronous motor driven
frequency changers, a mechanical adjustment is necessary between
the fields or armatures of the alternator and motor to obtain equal
division of the load. The adjustment can be obtained by shifting the
keyway, or by special cradle construction. In the latter method, one
machine is bolted to a cradle fastened to the base. By taking out the
bolts, the frame can be turned around through a small angle relative to
the cradle and therefore to the armature frame of the other machine,
when the bolts can be replaced.]
It will be seen from the figure that the separate exciter is fastened
on the base plate and has its armature directly connected to the shaft.
~Parallel Operation of Frequency Changers.~--It is very difficult to
construct two or more frequency changers and join them to synchronous
motors so that the current wave of one machine will be in phase with
the other, since the speed of the motor will depend on the frequency of
the line and be independent of the load thrown on it.
When alternators are run in parallel, if one machine lag behind, the
other carries the load with the result that the lightly loaded machine
will speed up and get in step with the other, or in other words a
synchronizing current will flow between the two alternators and tend to
keep them in proper relation with respect to phase and load.
[Illustration: FIG. 2,071.--Diagram of "Cascade" motor generator
set or motor converter, as it is called in England where it is used
extensively for electric railway work. In the diagram of motor armature
winding, some of the connections are omitted for simplicity. The
windings are ~Y~ connected, and as they are fed by wires joined to the
slip rings at the right and center, the rest of the power passes to the
converter windings back to rotor winding and out to the slip rings so
that part of the power enters the rotor and part through the converter.]
~Cascade Converter.~--This piece of apparatus was introduced by Arnold
and La Cour. Briefly, it consists of a combination of an induction
motor having a wound armature and a dynamo, the armatures being placed
on the same shaft. The windings are joined in cascade, that is, in
series with those of the armature of the induction motor. The line
supplies three phase currents at high voltage direct to the field of
the induction motor and drives it, generating in it currents at a lower
voltage depending on the ratio of the windings.
[Illustration: FIG. 2,072.--General Electric shunt wound booster set.
Sets of this class are used in railway stations to raise the pressure
of the feeders extending to distant points of the system, for storage
battery charging and regulation, and in connection with the Edison
three wire lighting system. The design of the various sets is closely
dependent upon their application. Booster sets are constructed in
either series or shunt wound types and they may be arranged for either
automatic or hand regulation, depending on the nature of the service
required. Where there are a number of lighting feeders connected and
run at full load for only a short time each day it will generally be
economical to install boosters rather than to invest in additional
feeder copper. It is important, however, to consider each case where
the question of installing a booster arises, as a separate problem, and
to determine if the value of the power lost represents an amount lower
than the interest charge on the extra copper necessary to deliver the
same voltage without the use of a booster.]
Part of the current thus generated in the armature passes into the
armature of the dynamo and is _converted_ by the commutator into
direct current as in a rotary converter, but is also increased by the
current induced in the winding of the dynamo armature.
[Illustration: FIG. 2,073.--General Electric 9.5 kw. 1,000 R.P.M.
balancer set. This is a form of direct current compensator, the units
of which it is composed may be alternately motor or generator, and the
secondary circuit is interconnected with the primary. Balancer sets
are widely used to provide the neutral of Edison three wire lighting
systems. They are also installed for power service in connection with
the use of 250 volt motors on a 500 volt service or 125 volt motors
on a 250 volt service. Although the balancer set may be manufactured
for more than one intermediate voltage, the General Electric Company
recommends the three wire system as it is simpler. The motors may
be more equally divided and the reserve margin of the motor is more
conservative for that class of work requiring increase in torque
with decrease in speed. The voltage of the system being given, the
capacity of a three wire balancer set is fixed by the maximum current
the neutral wire is required to carry. This figure is a more definite
specification of capacity than a statement in per cent. of unbalanced
load. As designed for power work and generally for lighting service,
the brushes of each machine are set at the neutral point in order to
get the best results for operating alternately either as a generator or
motor. Where the changes of balance are so gradual as to permit of hand
adjustment if desired, a considerable increase in output is obtainable.]
~Ques. At what speed does the machine run?~
Ans. Assuming equal numbers of pole, the armatures rotate at a speed
corresponding to one half the circuit frequency.
[Illustration: FIGS. 2,074 and 2,075.--General Electric charging sets.
Fig. 2,074 set consists of dynamo and direct current motor; fig. 2,075
set consists of a dynamo and alternating current motor. Fig. 2,074 set
is equipped with 125 volt, shunt wound dynamo and 230 or 550 volt motor
and range in capacity from .125 kw. to 13 kw., the speed varying from
2,250 R.P.M. in the .125 kw. set to 925 R.P.M. in the 13 kw. set with
230 volt motor. Both motor and dynamo have the same type and size of
frame; these are bolted together and form a compact and symmetrical
outfit, no base being necessary. Sets of the type shown in fig. 2,075
range in capacity from .2 kw. to 10 kw. and are equipped with 125 volt
shunt wound dynamo and 110, 220, 440 or 550 volt two or three phase
motors. If desired they can be furnished with single phase motors
wound for 110 or 220 volts. The speed of this type is 1,800 R.P.M.
When a motor generator set is used to charge only one battery, the
insertion of a resistance between the charging dynamo and the battery
is not necessary, inasmuch as all adjustments of voltage can be made
by varying the field strength of the dynamo, and, therefore, there are
no large losses due to resistance since the loss in the dynamo field
rheostat is very small. When a motor generator set is used to charge
two or more batteries of different capacities, or voltages, or which
are in different conditions of charge, it is necessary to insert a
resistance in series with each battery, in order that the current may
be properly adjusted for each particular battery.]
Thus if the motor have six poles and the frequency be 50, the
rotary field revolves at 50 × 60 ÷ 3 = 1,000 R.P.M. and the
motor will revolve at one-half that speed or 1,000 ÷ 2 = 500
R.P.M.
Since the connections are so arranged that these currents
tend to set up in the armature a revolving field, rotating at
half speed in a sense opposite to that in which the shaft is
rotating at half speed, it follows that by the super-position
of this revolving field upon the revolutions of the machine,
the magnetic effect is equivalent to a rotation of the armature
at whole speed, so that it operates in synchronism, as does the
armature of a rotary converter.
Half the electric input into the motor part is, therefore,
turned into mechanical energy to drive the shaft, the other
half acts inductively on the armature winding, generating
currents therein.
As to the dynamo part it is half generator, receiving
mechanical power by transmission along the shaft to furnish
half its output, and it is half converter, turning the currents
received from the armature into direct current delivered at the
brushes.
~Ques. What action takes place in the motor armature winding?~
Ans. Since it runs at one-half synchronous speed, it generates
alternating current of half the supply current frequency, delivering
these to the armature of the dynamo.
~Ques. What claim is made for this type of apparatus?~
Ans. The cost is said to be less than a motor generator set, and it is
claimed to be self-synchronizing and to require no special starting
gear, also to be 2.5 per cent. more efficient than a motor generator.
~Ques. How is the machine started from the high pressure side?~
Ans. The field winding is connected directly to the high pressure
leads. The three slip ring brushes are connected with external
resistances which are used while starting, the external resistances
being gradually cut out of the circuit as the machine comes up to speed
(the same as with an ordinary slip ring motor).
~Ques. How does a cascade converter compare with a synchronous
converter?~
Ans. It is about equally expensive as the synchronous converter with
its necessary bank of transformers, but is about one per cent. less
efficient. It is claimed to be more desirable for frequencies above 40
on account of the improved commutation at the low frequency used in
the dynamo member. For lower frequencies the synchronous converter is
preferable.
CHAPTER LIV
RECTIFIERS
The purpose of a rectifier is to change alternating current into a
uni-directional or pulsating current. There are several classes of
apparatus to which the term rectifier may be applied, as
1. Mechanical rectifiers;
2. Electrolytic rectifiers;
3. Mercury vapor rectifiers, or, mercury arc rectifiers;
4. Electro-magnetic rectifiers.
~Mechanical Rectifiers.~--By definition, a mechanical rectifier is a
form of commutator operating in synchronism with the generator and
commutating or rectifying the negative waves of the alternating current
as shown graphically in figs. 2,076 and 2,078. The essential features
of construction are shown in fig. 2,079.
~Ques. Mention some application of a mechanical rectifier.~
Ans. It is used on a compositely excited alternator as illustrated on
page 1,192.
~Electrolytic Rectifiers.~--If two metals be placed in an electrolyte
and then subjected to a definite difference of pressure, _they will_
(under certain conditions) _offer greater resistance to the passage of
a current in one direction, than in the other direction._ On account
of this so called valve effect, electrolytic rectifiers are sometimes
called "valves."
~Ques. What metal is generally used for the cathode?~
Ans. Aluminum.
[Illustration: FIGS. 2,076 to 2,078.--Diagrams showing alternating
currents, and partial and complete rectification.]
~Ques. What is generally used for the other electrode?~
Ans. Lead or polished steel.
Metals of low atomic weight exhibit the valve effect at high
differences of pressure, and heavier metals at low differences
of pressure.
~Ques. Describe the "Nodon valve."~
Ans. The cathode is of aluminum or aluminum alloy, and the other
electrode, which has considerably more surface, is the containing
vessel. The electrolyte is a neutral solution of ammonia phosphate.
~Ques. Describe its action.~
Ans. It is due to the formation of a ~film~ of normal hydroxide of
aluminum, over the surface of the aluminum electrode. This film
presents a very high resistance to the current when flowing in one
direction but very little resistance, when flowing in the reverse
direction.
[Illustration: FIG. 2,079.--Mechanical rectifier. The rectifier
consists of two castings M and S with teeth which fit together as
shown, being insulated so they do not come in contact with each
other. Every alternate tooth, being of the same casting, is connected
together, the same as though joined by a conducting wire. There are as
many teeth as there are poles. The part M of the rectifier is connected
to one of the collector rings by F, and the part S to the other ring by
G.]
~Ques. What is the effect when a Nodon cell is supplied with
alternating current?~
Ans. Half of the wave will be suppressed and an intermittently
pulsating current will result as shown in fig. 2,077.
[Illustration: FIGS. 2,080 and 2,081.--Two views of Nodon valve.
This is an electrolytic rectifier in which the cathode is a rod
of aluminum alloy held centrally in a leaden vessel which forms
the anode and contains the electrolyte, a concentrated solution of
ammonium phosphate. Only a short portion at the lower end of the
cathode is utilized, the rest, which is rather smaller in diameter,
being protected from action by an enclosing glass sleeve. The current
density at the cathode ranges from 5 to 10 amp. per sq. dm. In the
larger sizes, the cells are made double, and a current of air is kept
circulating between the walls by means of a motor driven fan. In
order to utilize both halves of the supply wave, the Gratz method of
connection is adopted. The maximum efficiency is obtained at about
140 volts, and the efficiency lies between 65 and 75 per cent., and
is practically independent of the frequency between the limits of 25
~ and 200 ~. Above a pressure of 140 volts, the efficiency falls off
very rapidly, owing to breakdown of the film. The pressure difference
is high, being over 90 per cent. at full load. Temperature largely
influences the action of the valve, and should never exceed 122° Fahr.]
[Illustration: FIG. 2,082.--Oscillograph record from Nodon valve
showing original supply voltage and the corresponding pulsating current
at the terminals of such a valve.]
~Ques. How may both halves of the alternating waves be utilized?~
Ans. By coupling a series of cells in opposed pairs as in fig. 2,080.
[Illustration: FIG. 2,083.--Performance curves of five ampere Nodon
valve. Constant secondary voltage test. Loaded on non-inductive
resistances. Frequency 50. Maximum power factor on valve .7.]
~Ques. Upon what does the efficiency of the film depend?~
Ans. Upon the temperature.
It should not for maximum efficiency exceed 86 degrees Fahr.
There is also a certain critical voltage above which the film
breaks down locally, giving rise to a luminous and somewhat
disruptive discharge accompanied by a rapid rise of temperature
and fall in efficiency.
[Illustration: FIG. 2,084.--Mohawk electrolytic rectifier and
switchboard; diagram showing connections for charging storage battery.
~Operating instructions:~ After assembling battery as in fig. 2,085,
the ~film~ must be formed on the aluminum alloy electrodes so that
the rectifier will pass current only in the right direction. Open
switch B, close switch T to the right; discharge lever can be in any
position; charging regulator lever must be to the extreme left, the
zero position; now close main switch M. Moving regulator lever R from
the zero position to the first button or contact, let it remain there
for a time, ~not less than five minutes;~ _this is important_, as the
proper rectification of the current depends on the film formed on the
aluminum rods. The ammeter after the first rush of current may not
show any current as passing, or it may show a reverse current. In the
latter case, leave the contact finger on the first button until the
needle comes back to zero. This may take some time, but the needle
will eventually come back; it also indicates that the film is properly
formed when the needle returns to zero. Move regulator R to the extreme
right step by step and note that the ammeter continues to return to
zero, which indicates that the film on rectifier electrodes is formed
properly. Move regulator R to zero, close switch T to the left in
normal charging position. Close charging switch B. To regulate the
flow of current through the battery move charging lever R to the right
slowly until ammeter indicates the correct charging current. After the
batteries are charged and ready for use, discharge lever can be moved
to connect either set of storage batteries to the load terminal. The
voltage of the batteries can be read at any time, by pressing the strap
key. The discharge lever connects the batteries to the volt meter and
it is possible by moving it to measure the voltage of either set of
battery, charging or discharging. Trouble in the rectifier demonstrates
itself by the solution becoming heated. The condition of the rectifier
can be tested any time in a few seconds by opening switch B and closing
switch T to the right. If the rectifier be in proper condition the
ammeter will read zero. And if it be not rectifying and permitting
A. C. current to flow through the rectifier, the ammeter will read
negative or to the left of the zero. An old solution that is heating
and not rectifying properly will turn a reddish brown color.]
~Ques. When an electrolytic rectifier is not in use for some time what
happens?~
Ans. The electrodes will loose the film.
[Illustration: FIGS. 2,085 and 2,086.--Mohawk electrolytic rectifier.
To put in commission, clean out the jar. Fill with distilled or rain
water. Add six pounds of electro salts, stir and after all salts are
dissolved place the cover in position. The specific gravity of the
solution should be 1.125. The middle iron electrode must hang straight
down in the solution and not touch either of the other aluminum alloy
electrodes. The aluminum alloy electrodes are mounted on an insulated
bracket that slides up and down on a ¼" rod. This rod screws in the
hole taped in the middle of the cover. The electrodes give the best
results only when perfectly smooth. Should they get rough, covered
with a deposit or a white coating remove from the solution, and clean
with fine sand paper. Finish with fine sand paper. Form the film
again and the electrodes will be as good as new. Clean iron electrode
occasionally.]
~Ques. What must be done in such case?~
Ans. The electrodes must be reformed.
~Ques. How is the loss of film prevented?~
Ans. By Removing the electrodes from the electrolyte and drying them.
[Illustration: FIG. 2,087.--The Fleming oscillation valve. It depends
for its action on the well-known Edison effect in glow lamps. The valve
consists of a carbon filament glow lamp with a simple central horseshoe
filament. Around this filament inside the exhausted bulb is fixed a
small cylinder of nickel, which is connected by means of a platinum
wire sealed through the bulb to a third terminal. ~The valve is used as
follows~: _The carbon loop is made incandescent by a suitable battery.
The circuits in which the oscillations are to be detected is joined
in series with a sensitive mirror galvanometer, the nickel cylinder
terminal and the negative terminal of the filament of the valve being
used._ The galvanometer will then be traversed by a series of rapid
discharges all in the same direction, those in the opposite direction
being entirely suppressed.]
~Ques. What attention must be given to the electrolyte?~
Ans. Water must be added from time to time to make up for evaporation.
This is necessary to keep the solution at the proper density.
~Ques. What is the indication that the rectifier needs recharging?~
Ans. Excessive heating of the solution with normal load.
~Ques. What is the indication that a rectifier is passing alternating
current?~
Ans. It will heat, and if the solution be very weak, it will cause a
buzzing sound.
[Illustration: FIG. 2,088.--The Churcher valve. This is of the modified
Nodon type. It differs from the latter in that it has two cathodes of
aluminum and an anode of lead or platinum, suspended in the one cell.
This permits the complete utilization of both halves of the supply wave
with one cell instead of the four required in the Gratz method. The
connections of such a cell are shown in the figure. The secondary of
the transformer carries a central tapping, and is connected through the
direct current load to the central anode, while each of the cathodes
is connected to the ordinary terminals of the transformer itself. The
practical limits of the cell are 50 volts direct current, or 130 volts
at the transformer terminals AB. F, is the anode; C, cathode I; D,
cathode II.]
~Ques. What harm is caused by operating a rectifier with a weak
electrolyte?~
Ans. The electrodes will eat away.
A few of the so called electrolytic valves are here briefly
described:
~The Audion Valve.~--This valve was invented by De Forest in
1900 and is practically identical with the Fleming oscillation
valve, the latter being illustrated in fig. 2,086.
~Grisson Valve.~--In this valve the cathode is a sheet of
aluminum, and the anode, a sheet of lead, supported, in
the original form, horizontally in a vessel containing the
electrolyte, consisting of a solution of sodium carbonate.
Cooling is effected by circulating water through metal tubes in
the electrolyte itself.
[Illustration: FIGS. 2,089 and 2,090.--The De Faria valve. This is an
aluminum lead rectifier. The cathode is a hollow cylinder of aluminum
placed concentrically in a larger cylinder of lead, and the whole
immersed in electrolyte of sodium phosphate in an ebonite containing
vessel. Cooling is effected by promoting automatic circulation of
the electrolyte by providing the lead cylinder with holes near its
extremities; the heated electrolyte then rises in the lead cylinder,
passes out at the upper holes, is cooled by contact with the walls of
the containing vessel, and descends outside the lead cylinder. It is
claimed that this cooling action is sufficient to allow of a current
density of 8 amp. per sq. dm. of aluminum.]
~Pawlowski Valve.~--This is an electrolytic valve employing
a solid electrolyte. It consists of a copper plate which has
been coated with a crystalline layer of carefully prepared
copper hemisulphide, prepared by melting sulphur and copper
together out of contact with air. The prepared plate is placed
in contact with an aluminum sheet and the combination is then
_formed_ by submitting it to an alternating pressure until
sparking, which at first occurs, ceases.
~Giles Electric Valve.~--This consists of a combination of
spark gaps and capacity used to protect electrical apparatus
against damage due to atmospheric discharges and resonance
surges. The spark gaps are formed between the edges of sharp
rimmed discs of non-arcing metal. These discs are insulated
from each other, and from the central tube, which provides a
support for the apparatus and also an earth. The condenser
effect is obtained by means of the annular discs and the tube;
an adjustable spark gap, a high resistance, and a fuse all
connected in series, complete the valve.
[Illustration: FIG. 2,091.--75 light Westinghouse-Cooper Hewitt mercury
vapor rectifier constant current regulating transformer. View showing
assembly in case.]
~Buttner Valve.~--It is of the Nodon type employing a cathode
of magnesium-aluminum alloy, and probably iron or lead as
anode, with an electrolyte of ammonium borate. Buttner claims
that the borate is superior to the phosphate in that it does
not attack iron, and will keep in good working condition for
longer periods.
[Illustration: FIG. 2,092.--75 light Westinghouse-Cooper Hewitt mercury
vapor rectifier constant current regulating transformer with case
removed. The transformer is of the repulsion coil type, oil cooled
and oil insulated. It is so arranged as to give a constant secondary
current and to insulate the arc lines from the primary circuit. The
regulating transformer contains two stationary secondary coils and
two moving primary coils balanced against each other. Each secondary
coil of the 75 light regulator is wound in two parts, owing to the
use of two rectifier bulbs in series in outfits of this capacity. The
repulsion between the primary and secondary coils changes the distance
between them according to the variation of load, and the induced
current in the secondary is thus kept constant. An increase in current
causes the primary and secondary coils to separate, and a decrease in
current permits them to approach each other, until the normal balance
is restored. The moving coils are hung from sheave wheels having roller
bearings and are balanced so that they are sensitive to the slightest
impulse tending to separate them or draw them closer together. (See
figs. 1,981-2, and 2,111.) The windings are insulated for a voltage
considerably in excess of that existing in normal service. Several
taps are provided to take care of different voltages and wave forms. A
combination of taps will be found which will be suitable for any wave
form coming within the American Institute of Electrical Engineer's
limits for a sine wave. The secondary coils are also provided with taps
for 85 per cent. of normal load, so that less than normal load can be
taken care of at a good power factor. Any part of the full load can be
carried temporarily with the full load connections of the transformer,
but at permanent light loads the power factor and efficiency will be
improved by using the 85 per cent. connections. Standard regulating
transformers are wound for 6.6 and 4 amperes, and for primary circuits
of 220, 440, 1,100 2,200, 6,600 and 13,200 volts. Regulators can
be specially wound for 5.5 amperes. For three phase circuits three
regulators can be used, one on each phase, or they can be furnished
in pairs with an auxiliary auto-transformer to give a balanced load.
The regulators can be connected, in cases where the unbalancing is not
objectionable, to separate phases.]
~Mercury Vapor Rectifiers.~--The Cooper Hewitt mercury vapor rectifier,
as shown in fig. 2,093 consists essentially of a _hermetically
sealed glass bulb filled with mercury vapor and provided with four
electrodes_. The two upper electrodes are of solid material and the two
lower of mercury.
The solid electrodes are the positive electrodes; the mercury
electrodes are the negative electrodes.
The mercury pools of the two lower electrodes are not in contact when
the bulb is vertical, but the bulb is so mounted that it can be tilted
to bring these two pools temporarily in contact for starting.
[Illustration: FIG. 2,093.--Diagram of connections of
Westinghouse-Cooper Hewitt mercury vapor rectifier arc light circuit.]
The bulb contains highly attenuated vapor of mercury, which, like other
metal vapors, is an electrical conductor under some conditions. The
positive electrodes are surrounded by this vapor. Current can readily
pass from either of the solid electrodes to the mercury vapor and from
it to the mercury electrode, but when the direction of flow tends
to reverse, so that current would pass from the vapor to the solid
electrode, there is a resistance at the surface of the electrode, which
entirely prevents the flow of current.
[Illustration: FIG. 2,094.--Cooper Hewitt mercury vapor rectifier.
The mercury vapor rectifier as developed by Peter Cooper Hewitt for
changing alternating current into direct current is the result of a
series of careful experiments and investigations of the action going
on in his mercury vapor lamp for electric lighting used on direct
current circuits only. While many attempts have been made to produce
an alternating current lamp, up to the present time, they have been
unsuccessful. The difficulty of operating a lamp on the alternating
current circuit lies in the fact that while a current will flow freely
through it in one direction, when the current reverses the negative
electrode or cathode acts as an electric valve and stops the current,
thus breaking the circuit and putting out the light. By following
up this new electrical action, Hewitt applied the principle in the
construction of a vacuum tube with suitable electrodes, and by using
two electrodes of iron or graphite for the positive or incoming current
and one of mercury for the negative or where the current leaves the
tube, the circuits could be arranged so that a direct current would
flow from the mercury electrode and be used for charging storage
batteries, electro-chemical work or operating direct current flame arc
lamps. As shown in the figure, ~the rectifier consists essentially~
_of a glass bulb into which are sealed two iron or graphite anodes
and one mercury cathode, and a small starting electrode_. The bulb
is filled with mercury vapor under low pressure. The action of this
device depends on _the property of ionized mercury vapor of conducting
electricity in one direction only_. ~In operation~ no current will flow
until the starting or negative electrode resistance has been overcome
by the ionization of the vapor in its neighborhood. To accomplish this,
the voltage is raised sufficiently to cause the current to jump the gap
between the mercury cathode and the starting cathode, or by bringing
the cathode and starting electrode together in the vapor by tilting
and then separating them, thus drawing out the arc. When this has been
done, current will only flow from the anode to the mercury cathode, and
not in the reverse direction. In order to maintain the action, a lag is
produced in each half wave by the use of a reactive or sustaining coil;
hence the current never reaches its zero value, otherwise the arc would
have to be restarted. ~There are two kinds of losses in the tube:~ 1,
_arcing, or leakage from one anode to the other_, and 2, _the mercury
arc voltage drop_. This drop does not depend on the load, the energy
represented by the drop being converted into heat, which is dissipated
at the surface of the containing vessel. According to Steinmetz, the
limit of voltage must be very high, as 36,000 volts has been rectified.
The current output is limited principally by the leading-in wires to
the electrodes, it being a difficult problem to seal into the glass
container the large masses of metal required for the conduction of
large currents. Frequency has but little influence. The direct current
voltage ranges from 20 to 50 per cent. that of the arc supply. The
life of the valve depends somewhat upon its size, being longer in
the small sizes and never, with fair usage, less than 1,000 hours.]
[Illustration: FIG. 2,095.--Three phase mercury arc rectifier. The
rectifier bulb is provided with three positive electrodes or anodes,
a negative electrode or cathode, and a starting anode, as shown. The
three phase leads are connected to the anodes at the top of the bulb,
a branch from one phase being brought down to the starting anode, a
resistance being placed in the circuit to prevent excessive current on
account of the proximity of the two lower electrodes. Since there is
always a pressure on one of the three anodes in the right direction, a
reactance coil is not necessary. The apparatus is started in the usual
way by tilting.]
The alternating current supply circuit is connected to the two positive
electrodes as shown in the diagram, and as the electrodes will allow
current to flow in only one direction and oppose any current flow in
the opposite direction, the pulsations of the current pass alternately
from one or the other of the positive electrodes into the mercury.
As these currents cannot pass from the vapor into either positive
electrode, they are constrained to pass out all in one direction
through the mercury electrode, from which they emerge as a
uni-directional current. The positive electrodes of the rectifier thus
act as check valves, permitting current to pass into the mercury vapor
but not allowing it to pass from the vapor to the solid electrodes.
[Illustration: FIGS. 2,096 to 2,098.--Westinghouse diagrams showing
comparative efficiencies of different systems of series arc lighting.]
~Ques. What condition prevails before the bulb starts to rectify?~
Ans. There appears to be a high resistance at the surface
[Illustration: FIG. 2,099.--Westinghouse-Cooper Hewitt mercury vapor
rectifier bulb. It consists essentially of a hermetically sealed glass
bulb filled with highly attenuated vapor of mercury, and provided with
electrodes. Its operation is fully explained in the accompanying text.]
[Illustration: FIG. 2,100.--Westinghouse-Cooper Hewitt mercury vapor
rectifier bulb and box. The life of the bulb is materially increased
by operating it at certain temperatures and for this reason the bulbs
in the arc light rectifier outfits are immersed in oil and mounted in
the same tank with the regulating transformer. Two bulbs in series
are used with the 75 light outfit. The bulb is mounted on tilting
trunnions in a box which can be lifted out through a door in the top
of the tank without disconnecting any leads. The containing box has
contacts on the bottom so arranged that when it is lowered into place,
the bulb is automatically connected in circuit. To replace a defective
bulb it is only necessary to lift out the containing box by its handle
through the door in the top of the case and mount a new bulb in it,
after which the box can be lowered into place. It is desirable to have
at each installation, a spare bulb box in which a bulb can be kept,
connected ready for use. If this be done, it is only necessary, in case
of trouble with the bulb, to withdraw the old bulb and box and replace
them with the spare set. This avoids having the lamps out of service.]
of the mercury, which must be broken down so that the current can pass.
~Ques. What is this apparent surface resistance called?~
Ans. _The negative electrode resistance._
~Ques. What must be done before any current can pass?~
Ans. The negative electrode resistance must be overcome.
When once started the current will continue to flow, meeting
with practically no resistance as long as the current is
uninterrupted.
~Ques. What will happen if the current be interrupted even for the
smallest instant of time?~
Ans. The negative electrode resistance will re-establish itself, and
stop the operation of the bulb.
~Ques. How is the negative electrode resistance overcome?~
Ans. The bulb is tilted or shaken so that the space between the mercury
electrodes is bridged by the mercury.
~Ques. What happens when the bulb is tilted?~
Ans. Current then passes between the two mercury electrodes from the
starting transformer and the little stream of mercury which bridges
the space between the electrodes breaks with a spark as the bulb is
returned to its vertical position.
~Ques. What duty is performed by the spark?~
Ans. It breaks down the negative electrode resistance.
~Ques. What conditions are now necessary for continuous operation of
the rectifier?~
Ans. The rectifier will now operate indefinitely as long as the current
supply is uninterrupted and the direct current load does not fall below
the minimum required for the arc.
~Ques. Is the rectifier self-starting?~
Ans. After the bulb has been started a few times, as described above,
it becomes self-starting, so that under all ordinary operating
conditions it will commence to operate when the switches connecting it
with the load and the alternating current supply are closed.
~Ques. What provision is made in the Westinghouse-Cooper Hewitt
rectifier to render it self-starting?~
Ans. It is rendered self-starting by means of a condenser.
[Illustration: FIG. 2,101.--General Electric 50 light double tube
combined unit series mercury arc rectifier outfit; front view. This
unit consists of a constant current transformer, reactance, tube tank
and exciting transformer mounted on a common base; also a static
discharger and pilot lamp mounted on top of the transformer. This
arrangement makes the rectifier outfit, with the exception of the
switchboard panel, complete in itself.]
~Ques. Describe the arrangement and operation of the condenser.~
Ans. The condenser is connected between one of the positive electrodes
and a coating of tinfoil outside [Illustration: FIG. 2,102.--General
Electric 2,200 volts, 60 cycle, primary, 6.6 ampere, secondary, 75
light, double tube mercury arc rectifier outfit with automatic shaking
device, the case being removed to show parts. The constant current
transformer is air cooled. The winding which supplies energy for the
exciter transformer is located at the top of, and around the core of
the constant current transformer. The exciting transformer is mounted
on the base of the constant current transformer inside of the casing.
It supplies low pressure currents to the starting anodes of the
rectifier tube. This current establishes an auxiliary arc when the
tube is shaken, which is necessary in order to start the rectifier.
The exciting transformer is wound for 110 volts and it consumes about
200 volt-amperes. The direct current reactance is mounted on the base
of the transformer and enclosed in the same casing. It is connected
in series with the lamp load and its function is to reduce the
pulsations of the circuit to a value most satisfactory for operation.
The tube tank for holding the oil is mounted on the same base as the
transformer. It is provided with a cooling coil; a tube carrier is
provided for raising or lowering the tube in the tank. A thermometer
is provided to gauge the temperature of the oil in the tank. The
static dischargers consisting of horn gaps in series with resistance,
are connected between the anodes and the cathode in order to protect
the tubes and other apparatus from excessive electrical strains. The
horn gaps open the circuit after discharge, and in case the resistance
becomes damaged the discharge passes across the spark gap provided,
thereby shunting the resistance.] the part of the bulb containing the
mercury, and induces static sparks on the surface of the mercury which
break down the negative electrode resistance.
The action of the rectifier will be better understood by
reference to the diagram of current waves and impressed
pressure as shown in figs. 2,103 to 2,106.
[Illustration: FIGS. 2,103 to 2,106.--Diagram of current waves and
impressed pressure of Westinghouse-Cooper Hewitt mercury vapor
rectifier. The whole of the alternating current wave on both sides of
the zero line is used. The two upper curves in the diagram show the
current waves in each of the two positive electrodes, and the resultant
curve III represents the rectified current flowing from the negative
electrode. Curve IV shows the impressed alternating current pressure.
It is evident that if the part of the wave below the zero line were
reversed, the resulting current would be a pulsating direct current
with each pulsation varying from zero to a positive maximum. Such a
current could not be maintained by the rectifier, because as soon as
the zero value was reached the negative electrode resistance of the
rectifier would be re-established and the circuit would be broken. To
avoid this condition, reactance is introduced into the circuit, which
causes an elongation of current waves so that they overlap before
reaching the zero value. The overlapping of the rectifier current waves
reduces the amplitude of the pulsations and produces a comparatively
smooth direct current as shown in curve III. In this way the whole of
the alternating current is transformed to direct current because each
of the alternations in both directions is alternately rectified.]
~Ques. Describe a mercury vapor rectifier outfit for series arc
lighting.~
Ans. It consists of a constant current regulating transformer, a
rectifier bulb, and a control panel containing the necessary switches,
meters, etc. The transformer and rectifier bulb are mounted in the same
tank.
[Illustration: FIG. 2,107.--General Electrical rectifier tube as used
on series mercury arc rectifier. The tube rectifies the alternating
current into direct current. It consists of an exhausted glass vessel
containing one anode or positive terminal in each of the two upper
arms, two mercury starting anodes and a cathode or negative terminal of
mercury at the bottom of the tube. It is submerged in oil and supported
in a removable carrier. The tube is put into operation by slightly
shaking it. In the combined unit set, this shaking is accomplished by
an electromagnet mounted above the tube tank and operated from a pull
button switch on the panel. An automatic shaker is sometimes installed
which will automatically start the tube when the set is started, or if
its operation should become interrupted while in service. The energy
for the operation of this magnet (110 volts alternating current) is
obtained from the small auxiliary winding on the main transformer which
also supplies energy to the exciting transformer. The oil in which the
tube is placed is cooled by a circulation of water through the cooling
coils on the inside of the tube tank. The amount of water necessary
for cooling the rectifier tubes varies according to local conditions,
depending upon the temperature of the water and that of the air in the
station but under the most favorable conditions no water is required.
As rectifiers are commonly installed in steam driven stations, the
drip from the tube tanks is usually piped to the boiler supply thereby
eliminating any loss for cooling water.]
[Illustration: FIG. 2,108.--Elementary diagram of mercury arc rectifier
connections. A.A., graphite anodes; B, mercury cathode; C, small
starting electrode; D, battery connection; E and F, reactance coils; G
and H, transformer terminals; J, battery.]
~Ques. Describe the construction and operation of the mercury arc[11]
rectifier shown in fig. 2,108.~
[11] NOTE.--The terms _vapor_ and _arc_ as applied to rectifiers, do
not indicate a different principle; the Westinghouse Co. employ the
former, and the General Electric Co., the latter.
Ans. Fig. 2,108 is an elementary diagram of connections. The rectifier
tube is an exhausted glass vessel in which are two graphite anodes
A, A', and one mercury cathode B. The small starting electrode C is
connected to one side of the alternating circuit, through resistance;
and by rocking the tube a slight arc is formed, which starts the
operation of the rectifier tube. At the instant the terminal H of the
supply transformer is positive, the anode A is then positive, and
the arc is free to flow between A and B. Following the direction of
the arrow still further, the current passes through the battery J,
through one-half of the main reactance coil E, and back to the negative
terminal G of the transformer. When the impressed voltage falls below
a value sufficient to maintain the arc against the reverse pressure of
the arc and load, the reactance E, which heretofore has been charging,
now discharges, the discharge current being in the same direction as
formerly. This serves to maintain the arc in the rectifier tube until
the pressure of the supply has passed through zero, reversed, and
built up such a value as to cause the anode A to have a sufficiently
positive value to start the arc between it and the cathode B. The
discharge circuit of the reactance coil E is now through the arc A'B
instead of through its former circuit. Consequently the arc A'B is now
supplied with current, partly from the transformer, and partly from the
reactance coil E. The new circuit from the transformer is indicated by
the arrows enclosed in circles.
[Illustration: FIG. 2,109.--Diagram showing connections of General
Electric combined unit mercury arc ~single tube~ rectifier outfit with
remote controlled non-automatic shaking device.]
[Illustration: FIG. 2,110.--Diagram showing connections of General
Electric combined unit mercury arc ~double tube~ rectifier outfit with
remote controlled non-automatic shaking device.]
[Illustration: FIG. 2,111.--Diagram showing connections of General
Electric series mercury arc rectifier.]
[Illustration: FIGS. 2,112 and 2,113.--Diagram of current waves
showing effect of reactance coil. If the alternating current wave
could be rectified without the use of the reactance coil, the direct
current produced would consist of a series of impulses which would
rise and fall from the zero line as illustrated in fig. 2,112. The
action of the reactance coil not only maintains the current through the
tube while the supply current is passing through zero, but helps to
smooth out the pulsations of the direct current which is passing out
of the cathode terminal of the tube to the batteries, or other direct
current apparatus put in its circuit. The smoothing out effect of the
reactance is shown in fig. 2,113. It will be seen from the diagram
that the current does not drop down to zero and the pulsations of the
direct current are greatly reduced. The waves A, A, etc., are from
the positive waves of the alternating current supply, while B, B, are
from the negative waves, and together they form the rectified current,
flowing in the same direction to the external direct current circuit
shown at B in the diagram, fig. 2,108.]
~Ques. How is a mercury arc rectifier started?~
Ans. A rectifier outfit with its starting devices, etc., is shown in
figs. 2,114 to 2,116. To start the rectifier, close in order named
line switch and circuit breaker; hold the starting switch in opposite
position from normal; rock the tube gently by rectifier shaker. When
the tube starts, as shown by greenish blue light, release starting
switch and see that it goes back to normal position. Adjust the
charging current by means of fine regulation switch on the left; or, if
not sufficient, by one button of coarse regulation switch on the right.
The regulating switch may have to be adjusted occasionally during
charge, if it be desired to maintain charging amperes approximately
constant.
[Illustration: FIGS. 2,114 to 2,116.--General Electric mercury arc
rectifier outfit, or charging set. The cut shows front, rear, and side
views of the rectifier, illustrating the arrangement on a panel, of the
rectifier tube with its connection and operating devices.]
[Illustration: FIGS. 2,117 and 2,118.--General Electric mercury arc
bulbs or tubes for 200 and 10,000 volt circuits.]
~Ques. In the manufacture of rectifiers, could other metals be used for
the cathode in place of mercury?~
Ans. Yes.
~Ques. Why are they not used?~
Ans. Because, on account of the arc produced, they would gradually wear
away and could not be replaced conveniently.
In the case of mercury, the excess vapor is condensed to liquid
form in the large glass bulb or condensing chamber of the tube
and gravitates back to the cathode, where it is used over and
over again.
[Illustration: FIG. 2,119.--General Electric series mercury arc
rectifier outfit; view showing method of replacing a tube. The
illustration also shows tube carrier and drip tray.]
~Ques. In the operation of rectifiers, how is the heat generated in the
bulb dissipated?~
Ans. In small rectifier sets the heat generated is dissipated through
the tube to the air, and in large tubes such as used in supplying 40 to
60 kw. for constant current flaming arc lights operating at 4 or 6.6
amperes, the tubes are immersed in a tank of oil, and cooled similar
to the arrangement used for oil insulated water cooled transformers.
~Ques. What results are obtained with oil cooled tubes?~
Ans. In practice it is found that the life of oil cooled tubes is
greatly increased and temperature changes do not affect the ability to
start up as in the air cooled tubes.
[Illustration: FIG. 2,120.--General Electric 100 volt mercury arc
rectifier tube. A,A, anodes; B, cathode; C, starting anode; D, tube or
bulb.]
~Ques. In the operation of a rectifier, name an inherent feature of the
mercury arc.~
Ans. A reverse pressure of approximately 14 volts is produced, which
remains nearly constant through changes of load, frequency, and
voltage. Its effect is to decrease the commercial efficiency slightly
on light loads.
~Ques. What is the advantage of a rectifier set over a motor generator
set?~
Ans. Higher efficiency and lower first cost.
~Ques. What is the capacity of a rectifier tube?~
Ans. 40 to 50 amperes.
~Ques. How is greater capacity obtained?~
Ans. When a greater ampere capacity is required, two or more rectifier
sets can be joined to one circuit.
The rectifier may be joined in series for producing an
increased voltage or two tubes can be connected in series in a
single set.
~Ques. For what service is a rotary converter better adapted than a
rectifier?~
Ans. For power distribution and other cases where a great amount of
alternating current is to be converted into direct current, the rotary
converter or large motor generator sets are more practical.
~Ques. For what service is a rectifier especially adapted?~
Ans. It is very desirable for charging storage batteries for
automobiles from the local alternating current lighting circuit.
When the consumer installs and operates the apparatus for his
own use and wear, there is considerable saving over motor
generator sets because a small one to two horse power motor
generator outfit has an efficiency of only 40 to 50 per cent.
while mercury vapor rectifiers will have from 75 to 80 per cent.
~Ques. What precautions should be taken in installing a rectifier?~
Ans. It should be installed in a dry place and care should be taken
to avoid dangling wires near the tube to prevent puncturing. If the
apparatus be installed in a room of uniform moderate temperature very
little trouble will be experienced in starting, while extreme cold will
make starting more difficult.
[12]~Electro-magnetic Rectifiers.~--Devices of this class consist
essentially of a double contact rocker which rocks on pivot (midway
between the contacts), in synchronism with the frequency of the
alternating current, so changing the connections at the instants of
reversals of the alternating current that a direct current is obtained.
[12] NOTE.--The Edison electromagnetic rectifier is described in detail
in GUIDE No. 4, pages 942 to 945.
Fig. 2,121 is a combined sketch and diagram of connections of a type
of _electromagnetic rectifier_ that has been introduced for changing
alternating into direct current. The actual apparatus consists of a
box, with perforated metal sides, about ten inches square and six
inches deep. This box contains the step down transformer P,S,S', and
the condensers K and K', the magnets and contact making device about
to be described being fixed on the polished slate top of the box,
exactly as shown in the figure. The transformer primary winding P may
be connected through a switch _s_ with a pair of ways on the nearest
distribution box, or to a plug connection or lamp-holder, and the
apparatus will give a rectified current of 6 or 12 amperes at 20 volts,
according to the size.
S and S' is the secondary winding of the transformer, with a tapping
_t_ midway, joining it to a series circuit containing two alternating
current electromagnets E and E', whose cores are connected by the long
soft iron yoke Y. Pivoted at P' is a steel bar SB, which is polarized
by the two coils C and C' the current being supplied by a cell A.
Fixed rigidly to SB, and moving with it, is a double contact piece CP
with platinum contacts opposite similar ones on the fixed studs CS, CS'.
CP is flexibly connected through F to one of the direct current
terminals T, to which also are joined up one coating of each condenser
K and K'.
[Illustration: FIG. 2,121.--Diagram showing essential features of
Premier Ampero electromagnetic rectifier. Details of construction and
principles of operation are given in the accompanying text.]
The other direct current terminal T' is connected to the center of the
transformer secondary at _t_; and CS and CS' are respectively joined up
to either end of the secondary winding and to the other coatings of the
condensers.
[Illustration: FIG. 2,122.--Diagram of General Electric (Batten)
electromagnetic rectifier. It is desirable for light and occasional
service, where direct current is required but only an alternating
supply is available, being used for charging storage batteries,
exciting spark coils, performing electrolytic work, etc. The rectifier
consists of a step down static transformer T, by means of which the
circuit pressure is reduced to about 50 volts; also, a polarized relay
R, the contact tongue C of which moves to one side or the other in
sympathy with the alternations of the current in the primary winding P,
the secondary current induced in the winding S being thereby rendered
direct in the outer circuit. T', T' are the main terminals which are
connected to the alternating current supply through the wires W. Lamps
inserted at L are used as resistances in the primary circuit, the
reduction of the voltage already alluded to being effected by this
means. In charging storage batteries where a low pressure is required,
a lamp (or lamps) should be connected in the secondary circuit as
shown, S B being the storage battery, and L' L' the lamp resistances in
series therewith, the battery has one end of the secondary S connected
to its middle. Thus the alternating current leaving the transformer
by the wire 1, passes by flexible connection 2, to the vibrating
contact tongue C of the relay, the latter causing the currents in
either direction to flow through the two halves H, H' of the battery,
whence the current re-enters the secondary of the transformer by the
wire 3. The soft iron core of the relay is in two halves S' S' and
the armature A, carrying C, vibrates between their polar extremities.
M, M' are two permanent magnets with their like poles together at
the center C' where A is pivoted. Supposing these poles are north as
indicated, the extremities of A will be south. The south ends of M, M
being in juxtaposition with the centers of the soft iron cores S', S'
will render their extremities facing the ends of A of north polarity.
The windings on S', S' are connected in series with each other, and
in shunt with P across the main terminals T', T'. Then because of
the polarization of A and S', S', the former will vibrate rapidly in
sympathy with the alternations of the current. K is a condenser shunted
by a lamp resistance L", this being found to improve the working of
R.]
When the alternating current circuit is broken, the springs SP, SP,
carried by SB and bearing against the adjustable studs, keep SB, CS and
CS'. The apparatus thus acts also as a _no voltage circuit breaker_,
for should the supply fail, the storage battery A' under charge will be
left on open circuit.
The action of the device is briefly as follows:
Owing to the direct current in the magnetizing coils C and C' one end
of SB will be permanently of north and the other of south polarity;
and since the polarities of the poles E and E' will alternate with the
alternations of the transformer secondary current, SB will rock rapidly
on its pivot, and contact will be made by turns with CS and CS'.
The purpose of the condensers K and K' is to reduce the sparking at
these points. When contact is made at CS, the direct current terminals
T and T' are connected to the S half of the secondary winding; and
when contact is made at CS', they are connected to the S' half. Thus a
rectified uni-directional current will flow from T and T', and it may
be used to charge the battery A', work a small motor or for various
other purposes requiring direct current.
When the rectifier is used for charging storage batteries, the separate
cell A may sometimes be dispensed with, the winding C,C' being
connected to one of the cells under charge.
The rectifier is adjusted to suit the frequency of the supply circuit
by altering the distance of the poles of E and E' from the ends of the
polarized armature SB; and also by changing the tension of SP, SP by
means of the screw studs against which they bear.
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~.
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