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Title: Hawkins Electrical Guide Number 2 - 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
As this book started as an ASCII text book there are no pictures available.
*** Start of this LibraryBlog Digital Book "Hawkins Electrical Guide Number 2 - 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
HAWKINS
ELECTRICAL GUIDE
NUMBER
TWO
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
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. 2.
=THE ARMATURE= 221 to 228
Definition--=how continuous current is obtained=--type
of armature--comparison ring and drum armatures--=why drum
armature is the prevailing type=--disc armatures--why disc
armatures were abandoned.
=ARMATURE WINDINGS= 229 to 256
Preliminary considerations--=winding diagrams and winding
tables=--lap and wave winding--angular pitch or spread of
drum coils--parallel or =lap winding=--series or =wave
winding=--double-windings--Siemens winding--objection
to Siemens winding--=chord winding=--multiplex
windings--number of brushes required--number of armature
circuits--=equalizer rings=--=drum winding
requirements=.
=THEORY OF THE ARMATURE= 257 to 282
Current distribution in ring and drum armatures--=connection
of brushes=--variation of voltage around the
commutator--=cross magnetization=; field
distortion--remedies for field distortion--angle of
lead--demagnetizing effect of armature reaction--=effect of
lead=--=eddy currents=; lamination--remedy for eddy
currents--magnetic drag on the armature--smooth and slotted
armatures--comparison of smooth and slotted armatures--magnetic
=hysteresis= in armature cores--=core loss= or iron
loss--dead turns--friction.
=COMMUTATION AND THE COMMUTATOR= 283 to 302
=Period of commutation=--commutating
plane--normal neutral plane--neutral plane--plane
of maximum induction--commutation--position
of the brushes--=sparking=--effect of
self-induction--construction of commutators--=points
relating= to commutators--types of commutator.
=BRUSHES AND THE BRUSH GEAR= 303 to 320
Classification--gauze brushes--wire brushes--strip
brushes--=carbon brushes=--adjustment--comparison of
copper and carbon brushes--size of brushes--number--=contact
angle of brush=---brush contact--drop in voltage at
brushes--=brush holders=--brush rigging--multipolar brush
gear.
=ARMATURE CONSTRUCTION= 321 to to 348
Parts--shaft--core--slotted core--=core
laminations=--core bolts--attachment to
shaft--insulation of core discs--teeth--advantages and
defects of slotted armatures--slotted cores; built
up construction--=ventilation=--=insulation
of core=--armature windings--construction of
inductors--objection to copper bars--=various windings=:
hand winding--evolute or butterfly winding--connectors--barrel
winding--bastard winding--former winding--former
coils--peculiarity of evolute coil--"straight out"
coil--=coil retaining devices=--driving horns.
=MOTORS= 349 to 388
Definition--=principles=--propelling drag--essential
requirements of construction--=the reverse electromotive
force=--hydraulic analogy--action of current supplied
to motor--=armature reaction= in motors--method
of starting a motor---classes of motor; =series,
shunt, and compound=--power of a motor--brake horse
power--mutual relations of motor torque and speed--=speed
regulation=--series parallel controller--=interpole
motors=.
=SELECTION AND INSTALLATION= 389 to 406
=General conditions governing
selection=--=construction=--efficiency--adaptation of
series and shunt motors--location--foundations--=erection
of dynamos and motors=--connecting up dynamos--marine
generating set--belt clamp--belt lacing--belt
speed--=points relating to belts=--gear drive--friction
drive--=electrical connections=.
=AUXILIARY APPARATUS= 407 to 430
=Switches=--switch classification and
construction--difficulty encountered in opening the
circuit--=various switches=: knife, snap, and
quick break types--=fuses=--circuit breakers:
maximum, minimum, reverse current, maximum and reverse
current, no voltage breaker--=discriminating cut
out=--time limit attachments--=rheostats=--starting
boxes--=switchboards=.
CHAPTER XVII
THE ARMATURE
The armature of a dynamo consists of coils of insulated wire wound
around an iron core, and so arranged that electric currents are induced
in the wire when the armature is rotated in a magnetic field or the
field magnets rotated and armature held stationary.
The commutator is in fact a part of the armature, but is of
sufficient importance to be considered in a separate chapter.
=Ques. What are the practical objections to the elementary armature,
described in fig. 165?=
Ans. It induces a very feeble current, which is not of constant
pressure, but pulsating; that is, it consists of two pronounced
impulses in each revolution as shown in fig. 168.
=Ques. Why does the elementary armature produce a pulsating
current?=
Ans. The pulsations are due to the coil moving alternately into, and
out of, the positions of best and least action in the magnetic field.
=Ques. How is a continuous current, or one of uniform pressure
obtained?=
Ans. If an additional coil be added to the elementary armature, at
right angles to the existing coil, and its ends suitably connected
to a four part commutator, as in fig. 185, so that one coil is in
the position of best action, while the other is in the position of
least action, the pulsations of the resulting current will be of
less magnitude. By increasing the coils and suitably altering the
construction of the commutator to accommodate the ends of these coils,
the resultant current may be represented by practically a straight
line, indicating the so called _continuous current_, instead of the
wavy resultant curve No. 6, as illustrated in fig. 187.
[Illustration: FIG 247.--Ring armature of four pole dynamo:
diagram of winding and connections, showing direction of the induced
currents. The currents in the windings under the upper N and S poles
are opposed to each other and flow to the external circuit by the
positive brush 1, and back to this half of the armature by the negative
brushes 3 and 4. At the same instant the opposed currents in the lower
windings flow to the external circuit by positive brush 2 and return
to the armature through negative brushes 3 and 4. The armature is
thus divided into four circuits and four brushes are required which
must be placed between the poles so as to short circuit the coils as
they pass through the neutral space. In this form of winding there is
no difference of potential between the + brushes, so that they are
connected in parallel, as are also the negative brushes, and then
to the external circuit. In multipolar machines there are as many
brushes as pole pieces. Since opposite commutator bars are of the
same potential on this four pole dynamo they may be joined by a cross
connecting wire and two brushes, as 2 and 4, dispensed with. This can
only be done when there is an even number of coils. The armature is
said to be "cross connected."]
An armature for practical use has a large number of coils,
suitably arranged upon an iron core, so that a large proportion
of them are always actively cutting the lines of force, or
moving into the positions of best action in the magnetic field.
=Types of Armature.=--Although there are many forms of armature,
all may be divided into three classes, according to the arrangement of
the coils or winding on the core, as:
1. Ring armatures;
2. Drum armatures;
3. Disc armatures.
Each of these forms of armature has its own special advantages
for particular purposes, the disc type being least in favor and
not having had any extensive application in this country.
[Illustration: FIG. 248.--Early form of Gramme ring armature,
the core being shown cut through, and some of the coils displaced to
make it clearer. The core, F, consists of a quantity of iron wire
wound continuously to form a ring of the shape shown by the section.
Over this is wound about thirty coils of insulated copper wire, B C D,
etc., the direction of the winding of each being the same, and their
adjacent ends connected together. The commutator segments consist of
a corresponding number of brass angle pieces, _m_, _n_, which are
fixed against the wooden boss, _o_, carried on the driving shaft.
The junction of every two adjacent coils is connected to one of the
commutator segments, as shown at _n_.]
=Ques. What is the comparison between ring and drum armatures?=
Ans. The drum armature is electrically and mechanically the more
efficient, possessing, as it does, possibilities in the way of better
mechanical construction of the core, and in the arrangement and fixing
of the inductors thereon not to be found in the ring form. Less wire
and magnetizing current are required for the field magnets for a given
output than with the ring armature. Drum winding is not so simple as
ring winding, and it is more difficult to ventilate a drum than a ring
armature, it being necessary to provide special ventilating ducts.
[Illustration: FIG. 249.--Modern form of Gramme ring armature.
The core consists of a number of thin flat rings of well annealed
charcoal iron, the outer diameter of each ring or disc being 11½
inches, and its inner diameter 9¼ inches. Sheets of thin paper
insulate each disc from its neighbors to prevent the flow of eddy
currents. The armature is mounted on a steel shaft to which is keyed
a four armed metal "spider," the extremities of whose arms fit into
notches cut in the inner edges of the soft iron core rings, so that a
good mechanical connection is obtained between the core and the shaft.
The spider is made of a non-magnetic metal, to reduce the tendency to
leakage of lines of force across the interior of the armature. The
armature inductors consist of cotton covered copper wire of No. 9
standard wire gauge, wound around the core in one layer, and offering
a resistance, from brush to brush, of 0.048 ohm. There are two
convolutions in each section, the adjacent ends of neighboring sections
being soldered to radial lugs projecting from the commutator bars.]
=Ques. Describe a ring armature.=
Ans. It consists essentially of an iron ring, around which is wound
a number of coils. These various coils are wound on separately, the
wire being carried over the outside of the ring, then through the
center opening and again around the outside, this operation being
repeated until the winding for that individual section is completed.
The adjacent coil is then wound in the same way, the ends of each being
brought out to the commutator side of the armature, the arrangement of
the coils on the ring and connections with the commutator being shown
in fig. 247, examples of actual construction being shown in figs. 248
and 249.
=Ques. For what conditions of operation is the ring armature
specially adapted, and why?=
Ans. It is well suited to the generation of small currents at high
voltage, as for series arc lighting, because the numerous coils can be
very well insulated.
[Illustration: FIG. 250.--Distribution of magnetic lines of
force through a Gramme ring. Since the metal of the ring furnishes a
path of least reluctance, most of the magnetic lines will follow the
metal of the ring and very few will penetrate into the aperture of
the interior. This condition causes a serious defect in the action of
ring armatures rendering the winding around the interior useless for
the production of electromotive force. Hence, in ring armatures only
about half of the winding is effective, the rest or "dead wire," adding
its resistance to the circuit, thus decreasing the efficiency of the
machine.]
=Ques. Why does a ring armature require more copper in the winding
than a drum armature?=
Ans. For the reason that those inductors which lie on the inner side of
the iron ring, being screened from practically all the lines of force,
as shown in fig. 250, do not generate any current.
Numerous attempts have been made to utilize this part of the
winding by making the pole pieces extend around the ring in
such a manner that lines of force will pass to the inside of
the ring, also by arranging an additional pole piece on the
inside of the armature, but mechanical considerations have
shown these methods to be impractical.
=Ques. Is any portion of the winding of a drum armature inactive?=
Ans. Yes; the end connectors do not generate any current.
[Illustration: FIG. 251.--Illustrating the principle of
Siemens' drum winding. In order to make the winding and connections
clear, one coil and the commutator is shown assembled, although the
latter is not put in place until after all the sections have been
wound, the ends of the wires being temporarily twisted together until
all can be soldered to the risers. The cores of these early machines
were of wood overspun circumferentially with iron wire before receiving
the longitudinal copper windings.]
=Ques. What is the chief advantage of the drum armature?=
Ans. It reduces considerably the large amount of dead wire necessary
with the ring type.
=Ques. How is this accomplished?=
Ans. By winding the wire entirely on the outer surface of a cylinder or
_drum_, as it is called, as shown in fig. 251, thus none of the wire is
screened by the metal of the core.
[Illustration: FIG. 252.--Elementary four coil drum winding,
showing the connections with the commutator segments, and directions of
currents in the several coils. The action of this type of armature is
fully explained in the text.]
Fig. 252 shows an elementary four coil drum armature. Starting
from the point _a_ and following the winding around without
reference at first to the commutator, it will be found that the
rectangular turns of the wire form a closed circuit, and are
electrically in series with one another in the order of the
numbers marked on them.
With respect to the connections to the four segments _w_, _x_,
_y_, _z_, of the commutator it will be found that at two of
these, _x_ and _y_, the pressures in the windings are both
directed _from_, or both directed _toward_ the junction with
the connecting wire. At the other two segments, _z_ and _w_,
one pressure is toward the junction and the other directed from
it. If, therefore, the brushes be placed on _x_ and _y_ they
will supply current to an external circuit, _z_ and _w_, for
the moment being idle segments.
=Disc Armatures.=--The inductors of a disc armature move in a
plane, perpendicular to the direction of the lines of force, about an
axis parallel to them as shown in fig. 253. The main difficulty with
this type has been in constructing it so that it will be strong and
capable of resisting wear and tear. It was introduced in an effort to
avoid the losses due to eddy currents and hysteresis present in the
other types of armature.
[Illustration: FIG. 253.--Disc armature of Niaudet. It is
equivalent to a ring armature, having the coils turned through an angle
of 90°, so that all the coils lie in a plane perpendicular to the axis
of rotation. The connections of the coils with each other and with
the commutator remain the same, the beginning and the end of adjacent
coils leading to a common commutator bar as shown. The magnetic field
is arranged by the use of two magnets, so arranged as to present the
north pole of one to the south pole of the other, and _vice versa._ In
the figure one of these magnets is considered as above the paper, and
the other below. If this armature be rotated through the magnetic field
as shown, a reversal of current takes place in each coil, when it is in
such a position that one of its diameters coincides with the pole line,
_NS_. If the brushes be set so as to short circuit the coils that are
in this position, the armature will be divided into two branchings, the
current flowing in an opposite direction in each, and a direct current
will flow in the exterior circuit.]
On account of the nature of the construction of a disc
armature, it is necessary that the coils subject to induction
occupy as small a space as possible in the direction of their
axes. This requirement, as well as the connection of the
inductors with each other and with the commutator, prevented
the general adoption of this form of armature, and subsequent
experience failed to justify the existence of the type.
CHAPTER XVIII
ARMATURE WINDINGS
To connect up rightly the inductors on an armature so as to produce
a desired result is a simple matter in the case of ring winding, for
bipolar or multipolar machines. It is a less easy matter in the case
of drum winding, especially for multipolar machines. Often there are
several different ways of arriving at the same result, and the fact
that methods which are electrically equivalent may be geometrically and
mechanically different makes it desirable to have a systematic method
of treating the subject.
The elementary arrangement of drum and disc armatures has already been
considered, which is sufficient explanation for small armature coils
of only a few turns of wire, but in the case of larger machines which
require many coils, further treatment of the subject is necessary.
For example, in order to direct the winder how to make the
connections for, say a four pole machine having 100 bars spaced
around its armature, some plain method of representing all the
connections so that they may be easily understood is necessary.
From this the workman finds out whether he is to connect the
_front_[A] end of bar No. 1 across to 50 or across a quarter
of the circumference to 24, or across three quarters of it to
bar 75. Again, he ascertains to which bar he is to connect the
_back_[1] end of the bar, and how the bars are to be connected
to the commutator.
[1] NOTE:--The "front" end means the end at which the commutator is
located. Armatures are most conveniently regarded from this end, the
opposite end being known as the "back" end.
=Winding Diagrams and Winding Tables.=--In the construction of
armatures, instructions to winders are given in the form of diagrams
and tables. In the tables the letters F and B stand for _front_ and
_back_, meaning _toward_ the front end, and _from_ the front end
respectively. The letters U and D stand for _up_ and _down_.
[Illustration: FIG. 254.--End of ring winding for a four
pole machine. An end view is simply a view showing the arrangement
of the armature inductors and connections _looking from the front or
commutator end_. A developed view of the above winding is shown in fig.
257.]
There are three kinds of winding diagram:
1. End view diagram;
2. Radial diagram;
3. Developed diagram.
The end view is simply a view showing the arrangement of the armature
inductors and connections looking from the front or commutator end,
such as shown in fig. 254.
In the radial diagram the inductors of the armature are represented by
short radial lines, while the end connectors are represented by curves
or zigzags, those at one end of the armature being drawn within, those
at the other end, without the circumference of the armature. With the
radial diagram it is easier to follow the circuits and to distinguish
the back and front pitch of the winding.
[Illustration: FIG. 255.--Partial sketch of a four pole
machine laid on its side. If the observer imagine himself placed at the
center, and the panorama of the four poles to be then laid out flat,
the developed view thus obtained would appear as in fig. 256.]
The developed diagram is a mode of representation, originally suggested
by Fritsche of Berlin, in which the armature winding is considered as
though the entire structure had been developed out of a flat surface.
This is best explained by aid of figs. 255 and 256.
If in fig. 255, which represents an armature core and a four
pole field, wires _a_ and _c_ be placed parallel to the axis of
the armature to represent two of the armature inductors, and
moved along the air gap space clockwise past the S poles,
they will cut magnetic lines inducing electromotive forces in
the directions indicated. To attempt to show a large number of
inductors in a drawing of this kind would be unintelligible.
Accordingly, the observer is considered as being placed at the
center of the armature, and the panorama of the four poles
surrounding him to be then laid out flat or "developed" as in
fig. 256.
The faces of the N and S poles are shaded obliquely for
distinction. By choosing the proper directions for these
oblique lines, a piece of paper having a narrow slit to
represent the wire may be laid over the drawing of the pole and
when moved, as indicated by the dotted arrows to the right, the
slit in passing over the oblique lines will cause an apparent
motion in the direction in which the current in reality tends
to flow. It is easily remembered which way the oblique lines
must slope, for those on the N pole slope parallel to the
oblique part of the letter N.
=Lap Winding and Wave Winding=.--In winding armatures there are
two distinct methods employed, known respectively as _lap_ and _wave
winding_. The distinction arises in the following manner: Since the
inductors, in passing a north pole generate electromotive forces in one
direction, and in passing a south pole generate electromotive forces in
the opposite direction, it is evident that an inductor in one of these
groups ought to be connected to one in nearly a corresponding position
in the other group, so that the current may flow down one and up the
other in agreement with the directions of the electromotive forces. The
order followed in making these connections gives rise to lap and wave
windings.
[Illustration: FIG. 256.--Developed view of the four pole
field shown in perspective in fig. 255.]
=Ques. What is lap winding?=
Ans. _One in which the ends of the coils come back to adjacent segments
of the commutator; the coils of such a winding lap over each other._
=Ques. What is a wave winding?=
Ans. _One in which the coil ends diverge and go to segments widely
separated, the winding to a certain extent resembling a wave._
[Illustration: FIG. 257.--Development of ring winding of four
pole machine shown in fig. 254. The dead wire or inactive inductors
on the inside of the ring are shown in dotted lines, the full lines
representing the active portion of the winding.]
=Angular Pitch or Spread of Drum Coils.=--Before taking up
the winding as a whole, the form of the individual coil should be
considered. Fig. 260 shows an end view of one coil in position on a
drum armature of a multipolar machine. The two slots X and Y contain
the sides of the coil and the distance between them on the surface
of the drum is called the _angular pitch_ or _spread of the coil_.
Theoretically this is equal to the pitch of the poles, represented by
the angle M, which is the angle between the pole centres.
[Illustration: FIGS. 258 and 259.--Wooden armature core and
winding table for practice in armature winding. By using strings of
different colors to represent the various coils, the path of each coil
is easily traced when the winding is completed, as in fig. 263.]
For instance, on a four pole machine the pitch would be 90°,
on a six pole machine, 60°, etc. Usually the angular pitch of
the coil is made just a little less than the pole pitch of the
machine, in order to shorten the end connections of the coils
from slot to slot. However, if the angular pitch be made too
small trouble will be encountered in commutation.
In addition to the angular pitch there is the _commutator
pitch_ which relates to the distance around the commutator
bridged by the ends of the coil. Thus, if the commutator
segments were numbered consecutively 1, 2, 3, etc., and the
_commutator pitch_ say is 10, it would signify that one end
of the coil was connected to segment 1 and the other end to
segment 11; the ends of the next coil in order then would be
connected to segments 2 and 12, in each case there would be ten
segments between the two segments connecting with the coil ends.
[Illustration: FIG. 260.--End view of drum armature of a
multipolar machine showing one coil in position to illustrate the
_angular pitch_ or spread of drum coils.]
=Parallel or Lap Drum Winding.=--In order to avoid much of the
difficulty usually experienced by students of drum winding, the
beginner should construct for himself a wooden armature core upon
which he can wind strings of various colors, or wires with distinctive
insulation, to represent the numerous coils that are used on real
armatures. A few windings attempted in this way will make clear many
points that cannot be so easily grasped from a written description.
The type of drum core best adapted for this work is the slotted variety
as shown in fig. 258, as it will facilitate the winding. The core as
shown in the illustration has twelve slots and six commutator segments,
the number of each required for the example of lap winding indicated in
the winding table fig. 259.
In making the wooden core, the slots may be formed by nailing a
series of thin strips around a cylindrical piece of wood, thus
avoiding the trouble of cutting grooves. In the illustrations
the commutator segments are shortened (leaving no room for
brushes) in order to show the connections as clearly as
possible.
[Illustration: FIG. 261.--Developed view of a typical lap
winding. From the figure it is seen that at the back of the armature
each inductor is united to one five places further on, that is, 1 to 6,
3 to 8, etc., and at the front end of the winding, after having made
one "element," as for example _d_-7-12-_e_, then forms a second element
_e_-9-14-_f_ which "laps" over the first, and so on all around until
the winding returns on itself.]
=Ques. Describe the simple lap winding fig. 259.=
Ans. As given in the table, it consists of six loops of wire presenting
twelve inductors on the cylindrical surface of the core or drum. In the
table, six wires are shown, having distinctive and varied insulation
so as to readily distinguish the different coils. Opposite these are
letters and figures designating the path and connections of each coil.
=Ques. What is the path of the first coil?=
Ans. According to the table it is:
A — 1 — 6 — B
that is, one end of the wire is connected to commutator segment A (fig.
262) and then wound to the back of the drum through slot 1, across
the back of the drum to slot 6, returning through this slot, and then
connected with commutator segment B.
[Illustration: FIG. 262.--Skeleton view of wooden armature
core showing in position the first two coils of the winding indicated
in the table fig. 259.]
=Ques. Describe the path of the second coil.=
Ans. The second coil, having the block insulation, is wound according
to the table, in the order:
B--3--8--C
that is, beginning at segment B, thence to back of drum through slot 3,
across the back to slot 8, returning through this slot and ending at
segment C.
The completed winding of the first two coils are shown in fig.
262, the drum being shown in dotted lines so that all of each
coil may be visible.
[Illustration: FIG. 263.--View of completed winding as
indicated in the table fig. 259. Thus the path of the first coil,
according to the table is A-1-6-B which means that the coil begins at
segment A of the commutator, rises to slot 1, and proceeds through the
slot to the back of the drum; thence across the back to slot 6, through
the slot and ending at segment B. The other coils are wound in similar
order as indicated in the table.]
=Ques. How are the remaining coils wound on the drum?=
Ans. Each of the succeeding coils are wound as indicated in the table,
the last connection being made to segment A, the one from which the
winding started.
=Ques. What is the general form of the completed winding?=
Ans. It may be considered simply as a wire wound spirally around the
drum, with loops brought down to the commutator segments, and ending at
the segment from which the start was made.
The completed winding as indicated by the table is shown in
fig. 263. Here the path of each coil is easily distinguished by
means of the varied insulations although in part hidden by the
drum. Fig. 264 shows a developed view of the winding.
[Illustration: FIG. 264.--Developed view of the winding shown
in perspective in fig. 263.]
=Ques. What condition must obtain in winding an even number of
coils?=
Ans. The wire must not be wound around the drum to diametrically
opposite positions, as for instance 1 to 7 in fig. 265.
=Ques. Why is this?=
Ans. The reason will be clearly seen by attempting the winding on the
wooden core. A winding of this kind on the drum fig. 258, would proceed
as follows:
A--1-- 7--B
B--3-- 9--C
C--5--11--D
In order now to continue winding in a regular way, the wire from
segment _d_ should pass to the rear of the armature along space 7,
but this space is already occupied by the return of the first coil.
Continuing the winding from this point, it would be necessary to carry
the wire from segment _d_ to 6 or 8, _resulting in an unbalanced
winding_.
[Illustration: FIG. 265.--Lap winding for bipolar machine,
with uneven number of coils; in this case the rear connectors may be
made directly across a diameter as shown.]
=Ques. How is a symmetrical winding obtained having an even number of
coils?=
Ans. The inductors, in passing from the front to the rear of the
armature, fig. 263, must occupy positions 1, 3, 5, 7, 9, 11, and the
even numbered positions will then serve as the returns for these wires.
In the example here shown there are six coils, comprising
twelve inductors and six commutator segments; it should be
noted, however, that if there were an uneven number of coils,
the rear connections could be made directly across a diameter
as shown in fig. 265, which would give a symmetrical winding.
With ten slots as shown in the figure, the drum would be wound,
for a bipolar machine, according to the following table:
A--1-- 6--B
B--3-- 8--C
C--5--10--D
D--7-- 2--E
E--9-- 4--F
[Illustration: FIG. 266.--Developed view of a typical wave
winding. This winding, instead of lapping back toward the commutator
segment from whence it came, as in lap winding, turns the other way.
For instance, _d_-7-12 does not return directly to _e_, but goes on to
_i_, whence another element _i_-17-4-_e_ continues in a sort of zigzag
_wave_.]
=Ques. Are coils such as shown in figs. 263 and 265 used in
practice?=
Ans. No, for practical use each coil would consist of several turns,
the diagram then merely indicates the end connections and slots for the
several turns of each coil.
=Series or Wave Drum Winding.=--In this mode of winding, the
inductors are arranged around the armature so that they do not turn
back, thus describing a zigzag or _wave-like_ path; that is, the coil
ends instead of connecting with adjacent segments of the commutator,
are attached to segments more or less remote.
=Ques. Describe the circuits of a simple or simplex wave winding.=
Ans. Only two sets of brushes are required for such a winding, but as
many brushes as there are poles can be used.
[Illustration: FIG. 267.--Five coil wave winding for a four
pole machine. In this winding only two brushes are used, there being
only two paths through the armature.]
=Ques. For what service are wave windings adapted?=
Ans. They are generally used on armatures designed to furnish a current
of high voltage and low amperage.
An example of wave drum winding for a four pole machine is
shown in fig. 267. For simplicity, very few coils are taken,
there being only five as shown in the illustration. To make the
winding, one strip should be removed from the wooden core and
the others spaced equally around the cylindrical surface. This
will give ten slots, the number required for the five coils.
The winding is indicated in the following table:
A--1-- 4--C
B--3-- 6--D
C--5-- 8--E
D--7--10--A
E--9-- 2--B
Accordingly the first coil starting at segment A, is carried
to the back of the drum through slot 1, thence across the back
and returning through slot 4, ending at segment C the starting
point of the second coil. Each coil is wound on in similar
manner, the last coil ending at segment A, the starting point
of the first coil. A developed view of the winding is shown in
fig. 268.
=Double Windings.=--In the various drum windings thus far
considered, each coil had its individual slots, that is, no two
occupied the same two slots. This arrangement gave twice the number of
slots as commutator segments.
[Illustration: FIG. 268.--Developed view of the five coil wave
winding shown in fig. 267.]
In a double winding there are as many segments as slots, each of the
latter containing two inductors, comprising part of two coils.
=The Siemens Winding.=--In winding drum armatures for bipolar
dynamos of two horse power or less, and especially for very small
machines as used in fan or sewing machine motors, a form of winding,
known as the Siemens winding, which is shown in fig. 271, is largely
used. It consists in dividing the surface of the armature core in one
equal number of slots, say 16, and using a 16 part commutator.
In the Siemens winding, the end of the wire used at the start
is to be connected to the first commutator bar, but must be
fastened to the armature core out of the way so as not to
interfere with the winding of the coils.
If eight turns of wire be required to fill a slot with one
layer, then the wire is carried from front to back and bent
aside so as to clear the shaft; after passing across the back
or pulley end of the armature, it is wound in the diametrically
opposite section and brought to the front, then across the
commutator end and up close to the beginning of the coil.
[Illustration: FIG. 269.--Series connected wave wound ring
armature for a four pole machine. The coils are so connected that only
two brushes are necessary.]
Since eight turns are to be used, the process of winding
is continued until the section is full and the end of the
coil will lie in a position ready to begin the next section.
Sometimes the wire is cut at this part of the coil leaving 3 or
4 inches projecting for connecting to the commutator bar 2, or
next to the first bar where the winding was started.
The usual practice is, however, to make a loop of the wire of
sufficient length to make the connection to the commutator
and it has the advantage that since all of the coils on the
armature are joined in series, the ending of one coil is joined
to the beginning of the next which avoids making mistakes in
making the commutator connections.
If the ends be cut they should be marked "beginning" and
"end" to avoid trouble, because if they get mixed, it will be
necessary to test each coil with a battery and compass needle
in order to determine the polarity produced and find which is
the beginning of the coil and which the end. With 32 ends
of the wire projecting from the end of the armature, it is
confusing and mistakes are often made in the connections, so
that one or more coils may oppose each other which would reduce
the voltage.
After the surface of the armature is covered with one layer it
will be noticed that the number of leads from the coils to the
commutator bars is only one-half the number of bars and that
they lie on one-half of the armature.
In order to complete the winding the first layer should be
insulated and the second layer wound on. The beginning of the
new coil will be directly over the first coil put on, but the
beginning of the new coil will be diametrically opposite the
beginning of the first coil wound.
The winding is now continued section by section and as each
coil is finished a loop or pair of leads is left to connect to
each bar. When the last coil is wound, its end will be found
lying next to the wire used in starting and should be joined to
it and finally connected to bar number one where the start was
made.
[Illustration: FIG. 270.--Developed view of the series
connected wave wound ring armature shown in fig. 269.]
With the winding and commutator connected, all of the coils are
in series and the beginning of the first coil joins the end of
the last coil.
If a pair of brushes be now placed on the commutator at
opposite points the current will flow into the bar and then
divide between the two leads connected to it, half of the
current flowing around one side and the other half flowing
around the other half of the armature or in other words, the
two halves of the armature are joined in parallel.
=Ques. What is the objection to the Siemens winding just
described?=
Ans. It produces an unsightly head where the wires pass around the
shaft and requires considerable skill to make it appear workmanlike.
=Ques. How may this be avoided?=
Ans. By using the chord windings of Froehlich or Breguet, which are
improvements over the Siemens in appearance and are more easily carried
out.
[Illustration: FIG. 271.--End view of an armature, showing the
distinction between Siemens' winding and chord winding.]
=Chord Winding.=--In cases where the front and back pitches[2] are
so taken that the average pitch differs considerably from the value
obtained by dividing the number of inductors by the number of poles,
the arrangement is called a chord winding.
[2] NOTE--The term _back pitch_ means the number of _spaces_ between
the two inductors of a coil. For instance, in fig. 267, the pitch
is 3; that is, there are three spaces between say inductors 1 and 4
which form part of the coil A--1--4--C. It is called the back pitch
in distinction from the _front_ or commutator pitch, which in this
instance is 2.
In this method each coil is laid on the drum so as to cover an arc of
the armature surface nearly equal to the angular pitch of the poles; it
is sometimes called _short pitch winding_.
=Ques. What is the difference between the Siemens winding and the
chord winding?=
Ans. This is illustrated in fig. 271, which shows one end of an
armature. In the Siemens winding, a wire starting, say at A, crosses
the head and enters the slot marked B. If it enters slot C it is a
chord winding.
=Ques. Describe a chord winding.=
Ans. The winding is started in the same manner as described in the
Siemens method, only instead of crossing the head and returning in
the section diametrically opposite, the section _A C_, fig. 271, next
to it is used for the return of the wire to the front end. Leads for
connecting to the commutator are left at the beginning and end of
each section as before stated and the only difference between the two
methods will be noticed when the first layer is nearly complete in that
two sections lying next to each other have no wire in them. This will
cause the winder to think he has made a mistake, but by continuing the
winding and filling in these blank spaces in regular order when the two
layers are completed, all the sections will be filled with an equal
number of turns and there will be the required number of leads from the
coils to connect up to the commutator bars.
=Ques. How many paths in the chord winding just described?=
Ans. Two.
=Multiplex Windings.=--An armature may be wound with two or more
independent sets of coils. Instead of independent commutators for the
several windings, they are combined into one having two or more sets
of segments interplaced around the circumference. Thus, in the case of
two windings, the brush comes in contact alternately with segments of
each set. The brush then must be large enough to overlap at least two
segments, so as to collect current from both windings simultaneously.
Both windings then are always in the circuit in parallel.
=Ques. What is the effect of a multiplex winding?=
Ans. It reduces the tendency to sparking, because only half of the
current is commutated at a time, and also because adjacent commutator
bars belong to different windings.
[Illustration: FIG. 272.--A progressive wave winding. _If
the front and back pitches of a wave winding be such that in tracing
the course of the winding through as many coils as there are pairs of
poles, a segment is reached in advance of the one from which the start
was made, the winding is said to be progressive._ The figure shows
three coils of a winding having 18 inductors. From the definition, the
number of coils to consider to determine if the winding be progressive
is equal to the number of poles divided by 2, which in this case is
equal to 2. These coils are shown in the figure as follows: A--1--4--F
and F--11--14--B. The second coil ends at segment B which is in
_advance_ of segment A from which the winding began, indicating that
the winding is progressive. Fig. 272 is given simply to illustrate the
definition of a progressive winding, and not to represent a practical
winding.]
=Ques. Does an accident to one winding disable the machine?=
Ans. No, it simply reduces its current capacity.
=Ques. Can multiplex windings have more than two windings?=
Ans. Yes, there may be three or four windings.
=Ques. What is the objection to increasing the number of windings?=
Ans. It involves an increased number of inductors and commutator
segments, which is undesirable in small machines, but for large ones
might be allowable.
[Illustration: FIG. 273.--A retrogressive wave winding. _If
the pitches be such that in tracing the winding through as many coils
as there are pairs of poles, the first segment of the commutator is not
encountered or passed over, the winding is said to be retrogressive._
The number of coils to consider is two, as follows: A-1-4-D and
D-7-10-G. The second coil ends at G, hence, since the segment A where
the start was made has not been reached or passed over the winding is
retrogressive. Fig. 273 is given simply to illustrate the definition of
retrogressive winding, and not to represent a practical winding.]
When there are two independent windings the arrangement is
called _duplex_, with three windings, _triplex_, and with four,
_quadruplex_.
=Ques. What loss is reduced with multiplex windings?=
Ans. In these windings, the division of what otherwise would be very
stout inductors into several smaller ones, has the effect of reducing
eddy current loss.
=Ques. For what service are machines with multiplex windings
specially adapted?=
Ans. Multiplex windings are used in machines intended to supply large
currents at low voltages, such as is required in electrolytic work.
=Number of Brushes Required.=--The number of places on the
commutator at which it is necessary or advisable to place a set of
collecting brushes can be ascertained from the winding diagrams. All
that is necessary is to draw arrows marking the directions of the
induced electromotive forces. Wherever two arrow heads meet at any
segment of the commutator, a positive brush is to be placed, and at
every point from which two arrows start in opposed directions along the
winding, a negative brush should be placed.
=Ques. How many brushes are required for lap windings and ordinary
parallel ring windings?=
Ans. There will be as many brushes as poles, and they will be situated
symmetrically around the commutator in regular order and at angular
distances apart equal to the pole pitch.
It should be noted that the number of brush sets does not
necessarily show the number of circuits through the armature.
=Ques. How many brushes are required for wave windings?=
Ans. If arrows be drawn marking the direction of the induced
electromotive forces to determine the number of brushes, it will be
found that only two brushes are required for any number of poles.
=Ques. What is the angle between these two brushes?=
Ans. It is the same as the angle between any north and south pole.
For instance, in a ten pole machine with wave winding the pitch
between the brushes may be any of the following angles:
360 ÷ 10 = 36°
3 × 36° = 108°
5 × 36° = 180°
[Illustration: FIGS. 274 and 275.--Right and left hand
windings. These consist respectively of turns which pass around the
core in a right or left handed fashion. Thus in fig. 274, in passing
around the circle clockwise from _a_ to _b_, the path of the winding
is a right handed spiral. In fig. 275, which shows one coil of a drum
armature, if _a_ be taken as the starting point, in going to _b_,
_a_ must be connected by a spiral connector across the front end of
the drum to one of the descending inductors such as M, from which at
the back end another connector must join it to one of the ascending
inductors, such as S, where it is led to _b_, thus making one right
handed turn.]
Sometimes with lap winding it is desirable to reduce the number of
brushes. In fig. 276, is shown the distribution of currents in a four
pole lap wound machine having four brushes and generating 120 amperes.
In each of the four circuits the flow is 30 amperes, and the current
delivered to each brush is 60 amperes. If now two of the brushes be
removed, the current through each of the remaining two will be 120
amperes, while internally there will be only two circuits as shown
in fig. 277. It should be noted, however, that these two circuits do
not take equal shares of the current since, though the sum of the
electromotive forces in each circuit is the same, the resistance of
one is three times that of the other, giving 90 amperes in one and
30 amperes in the other, as indicated in the figure. If no spark
difficulties occur in collecting all the current with only two brushes,
the arrangement will work satisfactorily, but the heat losses will be
greater than with four brushes.
[Illustration: FIG. 276.--Distribution of armature currents
in a four pole lap wound dynamo having four brushes and generating 120
amperes.]
=Ques. Are more than two brushes ever used with wave winding?=
Ans. It is sometimes advisable to use more than two brushes with wave
windings, especially when the current is very large.
For instance, in the case of a singly re-entrant[3] simplex
wave winding for an eight pole machine, whenever any brush
bridges adjacent bars of the commutator, it short circuits one
round of the wave winding and this round is connected at three
intermediate points to other bars of the commutator. Hence, if
the short circuiting brush be a positive brush, no harm will
be done by three other positive brushes touching at the other
points. If these other brushes be broad enough to bridge across
two commutator bars, they may effect commutation, that is,
three rounds instead of one undergoing commutation together.
[3] NOTE.--A re-entrant winding is one in which both ends re-enter or
lead back to the starting point; a closed winding.
=Number of Armature Circuits.=--It is possible to have windings
that give any desired even number of circuits in machines having any
number of poles.
[Illustration: FIG. 277.--Showing effect of removing two of
the brushes in fig. 275. If no spark difficulties occur in collecting
the current with only two brushes, the arrangement will work
satisfactorily, but the heat losses will be greater than with four
brushes.]
=Ques. How many paths are possible in parallel?=
Ans. For a simplex spirally wound ring, _the number of paths in
parallel is equal to the number of poles_, and for a simplex series
wound ring, _there will be two paths_. In the case of multiplex
windings _the number of paths is equal to that of the simplex winding
multiplied by the number of independent windings_.
In large multipolar dynamos it is, as a rule, inadvisable to
have more than 100 or 150 amperes in any one circuit, except in
the case of special machines for electro-chemical work. Such
considerations are factors which govern the choice of number of
circuits.
=Equalizer Rings.=--These are rings resembling a series of
hoops provided in a parallel wound armature to eliminate the effects
of "unbalancing," by which the current divides unequally among the
several paths through the armature. By means of leads, equalizer rings
connect points of equal potential in the winding and so preserve an
equalization of current.
[Illustration: FIG. 278.--Rear view of armature of a large
dynamo built by the General Electric Co., showing equalizer rings.]
=Ques. In multipolar machines what points are connected by equalizer
rings?=
Ans. Any two or more points in the winding, that during the rotation,
are at nearly equal potentials.
If there were perfect symmetry in the field system, no currents
would flow along such connectors; however, owing to imperfect
symmetry, the induction in the various sections of the winding
may be unequal and the currents not equally distributed.
=Drum Winding Requirements.=--There are several conditions that
must be satisfied by a closed coil drum winding:
1. There cannot be an odd number of inductors;
An odd number of inductors would be equivalent to not having
a whole number of coils. The even numbered inductors may be
regarded as the returns for the odd numbered inductors.
2. Both the front and back pitches must be odd in simplex windings.
3. The average pitch should be approximately equal to the number of
inductors divided by the number of poles.
This condition =must= obtain in order that the electric
pressures induced in inductors moving simultaneously under
poles of opposite sign, will be added. The smallest pitch
meeting this condition would stretch completely across a pole
face, while the largest would stretch from the given pole tip
to the next pole tip of like polarity.
The choice of front and back pitch for a given number of inductors
should, with lap and wave windings in general, comply with the
following conditions:
1. All the coils composing the winding must be similar, both
mechanically and electrically, and must be arranged symmetrically upon
the armature.
2. Each inductor of a simplex winding must be encountered once only,
and the winding must be re-entrant.
3. Each simplex winding composing a multiplex winding must fulfill the
requirement for a simplex winding.
4. A singly re-entrant multiplex winding must as a whole satisfy the
requirement for a simplex winding.
In addition to the above requirements for lap and wave windings in
general, lap windings must comply with the following conditions:
1. The front and back pitches must be opposite in sign;
2. The front and back pitches must be unequal;
If they be equal, the coil would be short circuited upon itself.
3. The front and back pitches must differ by two;
4. In multiplex windings, the front and back pitches must differ by two
multiplied by the number of independent simplex windings composing the
multiplex winding;
5. The number of slots on a slotted armature may be even or odd;
6. The number of inductors must be an even number; it may be a multiple
of the number of slots;
In the case of wave windings the several conditions to be fulfilled may
be stated as follows:
1. The front and back pitches must be alike in sign;
2. The front and back pitches may be equal or they may differ by any
multiple of two.
They are usually made nearly equal to the number of inductors
divided by the number of poles.
CHAPTER XIX
THEORY OF THE ARMATURE
=Current Distribution in Ring and Drum Armatures.=--In studying
the actions and reactions which take place in the armature, the student
should be able to determine the directions of the induced currents. The
basic principles of electromagnetic induction were given in chapter
X, from which, for instance, the distribution of current in the
gramme ring armature, shown in fig. 279, is easily determined by the
application of Fleming's rule.
Tracing the current from the negative to the positive brush, it
will be seen that it divides, half going through coils 1, 2, 3,
and half through coils I, II, III, these two currents ascend to
the top of the ring, uniting at the positive brush.
=Ques. In the Gramme ring armature (fig. 279) what is the
distribution of armature currents?=
Ans. There are two paths in parallel as indicated in fig. 279.
=Ques. How does the voltage vary in the coils?=
Ans. It varies according to the position of the coils, being least when
vertical and greatest when horizontal in a two pole machine arranged as
in fig. 279.
The upper and lower coils in the right hand half of the ring
armature, fig. 279, will have about the same electromotive
force induced in them, say 2 volts each, while the two coils
between them will have a higher electromotive force, at the
same instant, say 4 volts each, since they occupy nearly the
positions of the maximum rate of change of the magnetic lines
threading through them. These eight coils may be represented by
two batteries connected in parallel, each battery consisting of
two 2 volt cells and two 4 volt cells as shown in fig. 280. The
voltage of each battery then will be
2 + 4 + 4 + 2 = 12 volts
[Illustration: FIG. 279.--Current distribution in a gramme
ring armature. There are two paths for the current between the brushes,
half going up each side of the ring as indicated by the arrows, thus
giving two paths in parallel as indicated in fig. 281.]
[Illustration: FIG. 280.--Battery analogy illustrating current
distribution in a ring armature. The eight coils of the armature, fig.
279, are represented by two batteries of four cells each. The action
of the two units thus connected is indicated by the arrows. In the
external circuit the voltage is equal to that of one battery and the
current is equal to the sum of the currents in each battery.]
The two batteries being connected in parallel, the voltage at
the terminals will be the same, but the current will be the sum
of the currents in each battery.
=Ques. How may the number of paths in parallel be increased?=
Ans. By increasing the number of poles.
For instance, in a four pole machine, as in fig. 283, there are
four paths in parallel. In this case the armature may be used
to furnish two separate currents, though this is not desirable.
[Illustration: FIG. 281.--Diagram showing distribution of
current in the gramme ring armature of fig. 279. The current flows in
two parallel paths as indicated.]
[Illustration: FIG. 282.--Diagram showing current distribution
through armature of a four pole machine with like brushes connected.
There are four paths in parallel, hence the induced voltage will equal
that of one set of coils, and the current will be four times that
flowing in one set of coils.]
=Ques. How are the brushes connected?=
Ans. Usually all the positive brushes are connected together, and all
the negative brushes as in fig. 283, giving four paths in parallel
through the armature as indicated in fig. 282.
[Illustration: FIG. 283.--Brush connections for four pole
dynamo. It is usual to connect all the positive brushes to one terminal
and all the negative brushes to the other which gives four parallel
paths as shown in the diagram, fig. 282. In a four pole machine, two
separate currents can be obtained by omitting the parallel brush
connections.]
=Ques. How does this method of brush connection affect the
voltage?=
Ans. The voltage at the terminals is equal to that of any of the sets
of coils between one positive brush and the adjacent negative brush.
Thus in the four pole machine, fig. 283, the coils of the
four quadrants are in four parallels, which gives an internal
resistance equal to one-sixteenth that of the total resistance
of the entire ring.
When the coils are connected in two circuits or series
parallel, it requires only two brushes at two neutral points on
the commutator, for any number of poles; this arrangement is
shown in fig. 269.
=Ques. In general what may be said about the current paths through an
armature?=
Ans. The paths may be in parallel or series parallel according as the
winding is of the lap or wave type.
[Illustration: FIG. 284.--Morday's method of measuring the
variation of voltage around the commutator by use of a single exploring
brush and volt meter. It consists in connecting one terminal of the
volt meter (preferably an electrostatic one) to one brush of the
machine, and the other terminal to the exploring brush, which can be
moved from point to point, readings being taken at each point.]
=Variation of Voltage Around the Commutator.=--There are numerous
ways of determining the value of the induced voltage in an armature
at various points around the commutator. In the method suggested by
Morday, it can be measured by the use of a single exploring brush and a
volt meter as shown in fig. 284.
In this method, one terminal of the volt meter is connected
to one of the brushes of the dynamo, and the other terminal
is joined by a wire to a small pilot brush which can be
pressed against the commutator at any desired part of its
circumference. With the machine running at its rated speed, the
exploring brush is placed in successive positions between the
two brushes of the machine. In each position a reading of the
volt meter is taken and the angular position of the exploring
brush noted.
[Illustration: FIG. 285.--Cross magnetization. This is defined
as lines of magnetic force set up in the windings of a dynamo armature
which oppose at right angles the lines of force created between the
poles of the field magnet. The figure shows this cross flux which is
due to the armature current alone.]
=Ques. How does the voltage vary between successive pairs of
commutator segments?=
Ans. The variation is not constant.
=Cross Magnetization; Field Distortion.=--In the operation of
a dynamo with load, the induced current flowing in the armature
winding, converts the armature into an electromagnet setting up a field
across or at right angles to the field of the machine. This cross
magnetization of the armature tends to distort the field produced by
the field magnets, the effect being known as _armature reaction_. To
understand the nature of this reaction it is best to first consider the
effect of the field current and the armature current separately.
Fig. 285 represents the magnetic flux through an armature at rest,
where the field magnets are separately excited. If the armature be
rotated clockwise, induced currents will flow upward through the two
halves of the winding between the brushes, making the lower brush
negative and the upper brush positive.
=Ques. If, in fig. 285, the current in the field magnet be shut off,
and a current be passed through the armature entering at the lower
brush, what is the effect?=
Ans. The current will divide at the lower brush, flowing up each side
to the top brush. These currents tend to produce north and south poles
on each half of the core at the points where the current enters and
leaves the armature. Hence, there will be two north poles at the top of
the ring and two south poles at the bottom.
=Ques. What effect is produced by the like poles at the top and
bottom of the ring?=
Ans. The external effect will be the same as though there were a single
north and south pole situated respectively at the top and bottom of the
ring.
=Ques. In the operation of a dynamo, how do the poles induced in the
armature affect the magnetic field of the machine?=
Ans. They distort the lines of force into an oblique direction as shown
exaggerated in the diagram fig. 286.
[Illustration: FIG. 286.--Distortion of magnetic field due
to cross magnetization. For clearness, the effect is shown somewhat
exaggerated. A drag or resistance to the movement of the armature is
caused by the attraction of the north and south poles on the armature
and pole pieces respectively.]
=Ques. What effect has the presence of poles in the armature on the
operation of the machine?=
Ans. In fig. 286, the resultant north pole _n_, _n_, _n_, where the
lines emerge from the ring, attracts the south pole, _s_, _s_, _s_,
where the lines enter the field magnet, hence a load is brought upon
the engine, which drives the dynamo, in dragging the armature around
against these attractions. The stronger the current induced in the
armature, the greater will be the power necessary to turn it.
=Ques. Why does this reaction in the armature require more power to
drive the machine?=
Ans. The effect produced by the armature reaction is in accordance
with Lenz's law which states that: _In electromagnetic induction,
the direction of the induced current is such as to oppose the motion
producing it._
[Illustration: FIG. 287.--Actual distortion of field resulting
from cross magnetization, as shown by iron filings.]
=Remedies for Field Distortion.=--Since the distortion of the
magnetic field of a dynamo causes unsatisfactory operation, numerous
attempts have been made to overcome this defect, as for instance, by:
1. Experimenting with different forms of pole piece;
The reluctance of the pole piece should be increased in the
region where the magnetic flux tends to become most dense.
The trailing horn of the pole piece may be made longer than
the advancing horn and cut farther from the surface of the
armature, so as to equalize the distribution of the magnetic
flux.
2. Lengthening the air gap;
This increases the reluctance, and also necessitates more
ampere turns in the field winding. The field distortion,
however, will not be so great, as it would be if the magnetic
field of the machine were weaker.
3. Slotting the pole pieces;
Both longitudinal gaps and oblique slots have been tried. The
reduction of cross section of the pole piece causes it to
become highly saturated and to offer large reluctance to the
cross field.
4. The use of auxiliary poles.
These are small poles placed between the main poles and so
wound and connected that their action opposes that of the cross
field.
=Normal Neutral Plane.=--This may be defined as _a plane passing
through the axis of the armature perpendicular to the magnetic field of
the machine when there is no flow of current in the armature_, as shown
in fig. 288. It is the plane in which the brushes would be placed to
prevent sparking when the machine is in operation were the field not
distorted by armature reaction, and there were no self-induction in the
coils.
=Commutating Plane; Lead of the Brushes.=--It has been found that
in order to reduce sparking to a minimum, the brushes must be placed in
certain positions found by trial and designated as being located in the
_neutral plane_.
When the brushes are in the neutral plane, they are in contact with
commutator segments connecting with coils that are cutting the lines of
force at the minimum rate.
=Ques. Define the term "commutating plane."=
Ans. This is a plane passing through the axis of the armature and
through the center of contact of the brushes as shown in figs. 289 and
300.
=Ques. What is the angle of lead?=
Ans. The angle between the normal neutral plane and the commutating
plane.
In the operation of a dynamo since the field, on account of
armature reaction, is twisted around in the direction of
rotation, the proper position for the brushes is no longer in
the normal neutral plane, but lies obliquely across, a few
degrees in advance. Hence, for sparkless commutation, the
commutating plane is a little in advance of the normal neutral
plane, the lead being measured by the angle between these
planes, as stated in the definition.
[Illustration: FIG. 288.--Normal neutral plane. This is
a reference plane from which the lead is measured. As shown, the
normal neutral plane lies at right angles to the lines of force of an
undistorted field.]
=Ques. What may be said with respect to the angle of lead?=
Ans. _For sparkless commutation, the angle of lead varies with the
load._
If the field be much altered at full load, it is evident that
at half or quarter load it will not be nearly so much twisted,
hence the necessity for mounting the brushes on some kind of
rocking device which will allow them to be shifted in different
positions for different loads. A desirable point, then, in
dynamo design is to make the angle of lead at full load so
small that it will not be necessary to shift the brushes much
for variation of load. This can be accomplished by making the
field magnet field considerably more powerful than the armature
field.
[Illustration: FIG. 289.--Diagram illustrating the
demagnetizing effect of armature reaction. This results from
the forward lead given the brushes in order to secure sparkless
commutation.]
=Demagnetizing Effect of Armature Reaction.=--In the operation
of a dynamo, as previously explained, the position of the brushes for
sparkless commutation must be varied with the load; that is, for light
load they should occupy a position practically midway between the poles
and for a heavy load they must be moved a few degrees in the direction
of rotation. In other words, the commutating plane must be more or less
in advance of the normal neutral plane as shown in fig. 289.
=Ques. What is the effect of lead?=
Ans. It produces a demagnetizing effect which tends to weaken the field
magnets.
=Ques. Describe this demagnetizing effect in detail.=
Ans. Tracing the armature currents, in fig. 289 according to Fleming's
rule, it will be seen that current in inductors 1 to 18 flow _from_
the observer indicated by crosses representing the tails of retreating
arrows and in inductors 19 to 36, _toward_ the observer from the back
of armature, indicated by dots representing the points of approaching
arrows. In determining these current directions the inductors to the
right of the neutral line are considered as moving downward, and those
to the left as moving upward. The current in inductors 1 to 15 and 19
to 33, tends to cross magnetize the magnetic field of the machine, but
the current in inductors 34 to 36 and 16 to 18 tends to produce north
and south poles as indicated. These poles are in opposition to the
field poles and tend to demagnetize them. Hence, the inductors lying
outside the two upright lines are known as _cross magnetizing turns_,
and those lying inside, as _demagnetizing turns_.
The breadth of the belt of demagnetizing turns included between
the two upright lines is clearly proportional to the angle of
lead; therefore, the demagnetizing effect increases with the
lead.
=Eddy Currents; Lamination.=--Induced electric currents, known
as eddy currents, occur when a solid metallic mass is rotated in a
magnetic field. They consume considerable energy and often occasion
harmful rise in temperature. Armature cores, pole pieces, and field
magnet cores are specially subject to these currents.
[Illustration: FIG. 290.--Arago's experiment illustrating
eddy currents. Arago found that if a copper disc be rotated in its
own plane underneath a compass needle, the needle was dragged around
as by some invisible friction. The explanation of this phenomenon,
known as _Arago's rotations,_ is due to Faraday, who discovered that
it was caused by induction. That is, a magnet moved near a solid mass
of metal, induces in it currents, which, in flowing from one point to
another, have their energy converted into heat, and which, while they
last, produce (in accordance with Lenz's law) electromotive forces
tending to stop the motion. Thus, in the figure, there are a pair of
eddies in the part passing between the poles, and these currents oppose
the motion of the disc. Foucault showed by experiment the heating
effect of eddy currents, but such currents were known years before
Foucault's experiments, hence they are incorrectly called Foucault
currents.]
=Ques. Describe the formation of eddy currents.=
Ans. In fig. 291, a bar inductor is seen just passing from under the
tip of the pole piece N of the field magnet. Noting the distribution of
the lines of force, it will be seen that the edge _c d_ is in a weaker
field than the edge _a b_, hence, since the two edges move with the
same velocity, the electromotive force induced along _c d_ will be less
than that induced along _a b_. This gives rise to whirls or current
eddies in the copper bar as shown.
[Illustration: FIG. 291.--Formation of eddy currents in a
solid bar inductor. On account of its appreciable size, the field
is sometimes weaker at one point than another, hence the unequal
electromotive forces thus produced will induce eddy currents.]
=Ques. What should be noted in seeking a remedy for eddy currents?=
Ans. It should be noted that eddy currents are due to very small
differences of pressure and that the currents are large only because of
the very low resistance of their circuits.
=Ques. What is the best means of reducing eddy currents?=
Ans. Lamination.
=Ques. Explain this mode of construction with respect to the bar
inductor fig. 291.=
Ans. In the case of a large bar inductor such as shown in fig. 291, it
could be replaced by a number of small wires soldered together only at
the ends. The layer of dirt or oxide on the outside of the wires will
furnish sufficient resistance to practically prevent the eddy currents
passing from wire to wire.
[Illustration: FIG. 292.--Eddy currents induced in a solid
armature core. Eddy currents always occur when a solid metallic mass
is rotated in a magnetic field, because the outer portion of the metal
cuts more lines of force than the inner portion, hence the induced
electromotive force not being uniform, tends to set up currents between
the points of greatest and least potential. Eddy currents consume
a considerable amount of energy and often occasion harmful rise in
temperature.]
=Ques. How should an armature core be laminated to avoid eddy
currents?=
Ans. It should be laminated at right angles to its axis.
Fig. 292 shows the induced eddy currents in a solid armature
core, and fig. 293 shows the manner in which the paths of these
currents are interrupted and the losses due to their effect
diminished by the use of laminated cores.
[Illustration: FIG. 293.--Armature core with a few laminations
showing effect on eddy currents. In practice the core is made up
of a great number of thin sheet metal discs, about 18 gauge, which
introduces so much resistance between the discs that the formation of
eddy currents is almost entirely prevented.]
In fig. 293, only five laminations or plates are indicated,
so as to show the sub-division of the eddy currents, but in
practical armatures, the number of laminations or punchings
ranges from 40 to 66 to an inch, and brings the eddy current
loss down to about one per cent. A greater increase in the
number of laminations per inch is not economical, however,
owing to the difficulties encountered in the punching and
handling of extremely thin sheets of iron, and the loss of
space between the plates.
Armature cores constructed of the number of plates stated,
and forced together by means of screws and heavy hydraulic
pressure, contain from 80 to 90 per cent. of iron, and have a
magnetic flux carrying capacity only from 5 to 15 per cent.
less than when they are made of an equal volume of solid iron.
=Magnetic Drag on the Armature.=--Whenever a current is induced
in an armature coil by moving it in the magnetic field so as to cut
lines of force, the direction of the induced current is such as to
oppose the motion producing it. Hence, in the operation of a dynamo,
considerable driving power is required to overcome this magnetic drag
on the armature.
[Illustration: FIG. 294.--Circular concentric magnetic field
surrounding a conductor carrying a current. If this conductor be moved
across a magnetic field, as between the poles of a magnet, the lines of
force will be distorted as in fig. 295, which will oppose the motion of
the conductor.]
A conductor carrying a current is surrounded by a circular concentric
magnetic field. If now such a conductor, with current flowing toward
the observer as in fig. 294, be placed in a uniform magnetic field, a
distortion of the magnetic lines will occur as shown in fig. 295. The
resulting mechanical actions are easily determined by remembering
that _the magnetic lines act like elastic cords tending to shorten
themselves_. There is in fact a tension along the magnetic lines and a
pressure at right angles to both, proportional at every point to the
square of their density.
[Illustration: FIG. 295.--Illustrating drag on armature
inductors. In moving a wire carrying a current through a magnetic
field, the lines of force are distorted, and the effect on the wire
is the same as though the magnetic lines were elastic cords tending
to shorten themselves. They, therefore, oppose the motion of the
wire; hence, in dynamo operation, more or less power is absorbed in
overcoming this drag on the numerous inductors. In the figure the
inductor is being moved upward against the "drag" due to the magnetic
field.]
It is evident by inspection of the lines in fig. 295, that there is a
drag upon the conductor in the direction shown by the arrow.
=Smooth and Slotted Armatures.=--The inductors of an armature may
be placed on a smooth drum or in slots cut in the surface parallel to
the axis.
In the first instance, the magnetic drag comes on the inductors and in
the case of slots, upon the teeth.
The effect of embedding the armature inductors in slots is
to distort the magnetic field as shown in fig. 296. Most of
the lines of force pass through the teeth, thus, not only are
the inductors better placed for driving purposes, but, being
screened magnetically by the teeth, the forces acting on them
are reduced, the greater part of the magnetic drag being taken
up by the core.
It should be noted that, although screened from the field, the
inductors in a slotted armature cut magnetic lines precisely
as if they were not protected. The effect is as though the
magnetic lines flashed across the slots from tooth to tooth,
instead of passing across the intermediate slot at the ordinary
angular velocity.
=Comparison of Smooth and Slotted Armatures.=--The slotted
armature has the following advantages over the smooth type:
1. Reduced reluctance of the air gap;
2. Better protection for the winding;
3. Inductors held firmly in place preventing slippage;
4. No magnetic drag on inductors;
5. No eddy currents in inductors;
6. Better ventilation;
7. Opposition to armature reaction.
Due to increased density of flux through the teeth.
The disadvantages of slotted armatures may be stated as follows:
1. Tendency of the teeth to induce eddy currents in the pole pieces;
2. Increased self-induction of the armature coils;
3. Greater hysteresis loss on account of denser flux in the teeth;
4. Leakage of lines of force through the core, especially in the case
of partially enclosed slots.
[Illustration: FIG. 296.--Effect of slotted armature. The
teeth, as they sweep past the pole face, cause oscillations of the
magnetic flux in the iron near the surface because the lines in the
pole piece PP tend to crowd toward the nearest teeth, and will be
less dense opposite the slots. This fluctuation of the magnetic lines
produce eddy currents in the pole faces unless laminated. The armature
inductors, being screened from the field, are relieved of the drag
which is taken by the teeth.]
=Magnetic Hysteresis in Armature Cores.=--When the direction
or density of magnetic flux in a mass of iron is rapidly changed a
considerable expenditure of energy is required which does not appear as
useful work. For instance, when an armature rotates in a bipolar field,
the armature core is subjected to two opposite magnetic inductions in
each revolution; that is, at any one instant a north pole is induced
in the core opposite the south pole of the magnet and a south pole in
the core opposite the north pole of the magnet as indicated in fig.
297 by _n_ and _s_. Accordingly, if the armature rotate at a speed of
1,000 revolutions per minute, the polarity of the armature will be
changed 2,000 times per minute, and result in the generation of heat at
the expense of a portion of the energy required to drive the armature.
This loss of energy is due to the work required to change the position
of the molecules of the iron, and takes place both in the process of
magnetizing and demagnetizing; the magnetism in each case lagging
behind the force.
=Core Loss or Iron Loss.=--These terms are often employed to
designate the total internal loss of a dynamo due to the combined
effect of eddy currents and hysteresis, but as the losses due to the
former are governed by laws totally different from those applicable to
the latter, special analysis is required to separate them.
The eddy current loss per pound of iron in the armature core diminishes
with the thinness of the laminated sheets, and may be made indefinitely
small by the use of indefinitely thin iron plates, were it not for
certain mechanical and economical reasons.
The loss due to hysteresis per pound of iron in the core, does not vary
with the thinness of the core plates; it can be reduced only by the use
of a material having a low hysteretic coefficient.
=Dead Turns.=--The voltage generated in a dynamo with a given
degree of field excitation is not strictly proportional to the speed,
but somewhat below on account of the various reactions. That is, the
machine acts as though some of its revolutions were not effective in
inducting electromotive force.
The name _dead turns_ is given to the number of revolutions by which
the actual speed exceeds the theoretical speed for any output.
Again, this term is sometimes used to denote that portion of the wire
on an armature which comes outside the magnetic field and is therefore
rendered ineffective in inducing electromotive force. The number of
dead turns is about 20% of the total number of turns.
[Illustration: FIG. 297.--Magnetic hysteresis in armature
core. Unlike poles are induced in the core opposite the poles of the
field magnet. Since on account of the rotation of the core the induced
poles are reversed a thousand or more times a minute, considerable
energy is required to change the positions of the molecules of the iron
for each reversal, resulting in the generation of heat at the expense
of a portion of the energy required to drive the armature.]
=Self-induction in the Coils; Spurious
Resistance.=--Self-induction opposes a rapid rise or fall of an
electric current in just the same way that the inertia of matter
prevents any instantaneous change in its motion. This effect is
produced by the action of the current upon itself during variations in
its strength.
In the case of a simple straight wire, the phenomenon is almost
imperceptible, but if the wire be in the form of a coil, the adjacent
turns act inductively upon each other upon the principle of the mutual
induction arising between two separate adjacent circuits.
=Ques. What effect has self-induction on the operation of a
dynamo?=
Ans. It prevents the instantaneous reversal of the current in the
armature coils. That is, the current tends to go on and in fact does
actually continue for a brief time after the brush has been reached.
[Illustration: FIG. 298.--Distribution of magnetic lines
through a ring armature. Since the lines follow the metal of the ring
instead of penetrating the interior, no electromotive force is induced
in that portion of the winding lying on the interior surface of the
ring. There is, therefore, a large amount of dead wire or wire that
is ineffective in inducing electromotive force; this is the chief
objection to the ring type of armature.]
=Ques. What becomes of the energy of the current at reversal?=
Ans. The energy of the current in the section of the winding undergoing
commutation is wasted in heating the wire during the interval when it
is short circuited, and as it passes on, energy must again be spent in
starting a current in it in the reverse direction. There is, then, a
lagging of the current in the armature coils due to self-induction.
=Ques. What is spurious resistance?=
Ans. This is an apparent increase of resistance in the armature
winding, which is proportional to the speed of the armature, and due to
the lagging of the current.
[Illustration: FIG. 299.--Distribution of magnetic lines
through solid drum armature of a four pole machine.]
=Armature Losses.=--The mechanical power delivered to the pulley
of a dynamo is always in excess of its electrical output on account of
numerous mechanical and electrical losses. Mechanical losses result
from:
1. Friction of bearings;
2. Friction of commutator brushes;
3. Air friction.
The electrical losses may be classified as those due to:
1. Armature resistance;
2. Hysteresis;
3. Eddy currents.
=Ques. How do the mechanical and electrical losses compare?=
Ans. The mechanical losses are small in comparison with the electrical
losses.
=Ques. What may be said with respect to friction?=
Ans. The bearing friction varies with the load. In calculating this
loss not only must the weight of the armature be considered but also
the belt tension and magnetic attraction in order to get the resultant
thrust on the bearing. Friction of the brushes is very small and may be
neglected. A small loss of power is caused by the friction of the air
on the armature. The latter, since it revolves rapidly, acts to some
extent as a fan, and in some machines this fan action is made use of
for ventilation and cooling.
=Ques. How are the other losses determined?=
Ans. The loss of power due to armature resistance is easily found
by Ohm's law, but the hysteresis and eddy current losses, known
collectively as _iron losses_, are not so easily determined. If the
magnetization curve of the particular quality of iron used for armature
plates be known, the hysteresis loss may be calculated approximately.
Eddy current losses are the most important, especially in large
machines. As previously explained, in all the moving metal masses
unless laminated, there will be eddy currents set up if they cut
magnetic lines. Power may be lost from this cause even in the metal of
the shaft if there be leakage of magnetic lines into it.
CHAPTER XX
COMMUTATION AND THE COMMUTATOR
The act of commutation needs special study. If it be incorrectly
performed, the imperfection at once manifests itself by sparks which
appear at the brushes. In the study of this chapter on commutation it
would be advisable for the student to first review the basic principles
of commutation as given in chapter XIV, which contains a brief and
simple explanation of how the alternating current in the armature is
converted into direct current by the action of the commutator.
=Ques. What is the period of commutation?=
Ans. The time required for commutation, or the angle through which the
armature must turn to commute the current in one coil.
=Ques. Upon what does the period of commutation depend?=
Ans. Upon the width of the brushes as shown in fig. 300.
This fixes the angle through which the armature must revolve
to commute the current in one coil. This angle is formed, as
shown in the figure, by two intersecting planes, M and S, which
pass through the axis of the armature and the two edges of the
brush. Commutation then, begins at M and ends at S.
=Ques. What is the position of the commutating plane with respect to
M and S, in fig. 300?=
Ans. It bisects the angle formed by the planes M and S.
[Illustration: FIG. 300.--Armature with one brush in position
to illustrate the _period of commutation_ and _commutating plane_. The
latter is called "commutating line" by some writers. The period of
commutation depends on the thickness of the brush end in contact with
the commutator. Careful distinction should be made between _commutating
plane_, _neutral plane_, and _normal neutral plane_ as defined
elsewhere.]
=Ques. What is the the commutating plane?=
Ans. An imaginary plane passing through the axis of the armature and
the center of contact of the brush.
=Ques. What two planes are referred to in stating the position of the
brushes?=
Ans. The normal neutral plane and the commutating plane.
The angle intercepted by these two planes represents the
_lead_, thus in stating that the brushes have a lead of 6°,
means that the angle intercepted by the normal neutral plane
and the commutating plane is 6°.
[Illustration: FIG. 301.--The proper position of the brushes,
if there were no field distortion and self-induction in the armature
coils, would be in the normal neutral plane. In the actual dynamo
these two disturbing effects are present which makes it necessary to
advance the brushes as shown in figs. 302 and 303 to secure sparkless
commutation.]
=Ques. What is the difference between the normal neutral plane and
the neutral plane?=
Ans. This is illustrated in figs. 301 and 302. _The normal neutral
plane is the position of zero induction assuming no distortion of
the field_ as in fig. 301. _The neutral plane is the position of zero
induction with distorted field_ as in fig. 302 and as is found in
the actual machine; the distortion is exaggerated in the figure for
clearness.
[Illustration: FIG. 302.--Brush adjustment for field
distortion. The effect of the latter is to twist the lines of force
around in the direction of rotation, thus maximum induction takes place
in an inclined plane. The brushes then must be advanced to the _neutral
plane_ which is at right angles to the plane of maximum induction. This
gives the proper position of the brushes _neglecting self-induction_.]
=Ques. What is the normal plane of maximum induction?=
Ans. _A plane, 90° in advance of the normal neutral plane_, being the
position of maximum induction with no distortion of field, as in fig.
301.
=Ques. What is the plane of maximum induction?=
Ans. _A plane 90° in advance of the neutral plane_, being the position
of maximum induction in a distorted field as in fig. 302.
[Illustration: FIG. 303.--Brush adjustment for self-induction.
For convenience an electric current is regarded as having weight and
hence possessing the property of inertia. The current then during
commutation cannot be instantly brought to rest and started in the
reverse direction but these changes must be brought about gradually
by an opposing force. Hence by advancing the brushes beyond the
neutral plane as illustrated, commutation takes place with the short
circuited coil cutting the lines of force so as to induce a current in
the opposite direction; this opposes the motion of the current in the
short circuited coil, brings it to rest and starts it in the opposite
direction, thus preventing sparks. Figs. 301 to 303 should be carefully
compared and thoroughly understood.]
=Ques. What should be noted with respect to the different planes?=
Ans. The commutating plane should be carefully distinguished from the
normal neutral plane and from the neutral plane, as shown in fig. 303.
=Commutation.=--In order to understand just what happens during
commutation, a section of a ring armature may be used for illustration,
such as shown in fig. 304. Here the coils A, B, C, D, E, are connected
to commutator segments 1, 2, 3, 4, and the positive brush is shown
in contact with two segments 2 and 3, the brush being in the neutral
position. Currents in the coils on each side of the neutral line flow
to the brush through segments 2 and 3; the brush then is positive.
[Illustration: FIG. 304.--Commutation. This takes place during
the brief interval in which any two segments of the commutator are
bridged by the brush. The coil connecting with the two segments under
the brush is thus short circuited. During commutation the current in
the short circuited coil is brought to rest and started again in the
reverse direction against the opposition offered by its so called
inertia, or effect produced by self-induction.]
Now, as the armature turns, the commutator segments come successively
into contact with the brush. In the figure, segment 3 is just leaving
the brush and 2 is beginning to pass under it, hence, for an instant
the coil C is short circuited.
=Ques. In fig. 304, what are the current conditions?=
Ans. Previous to contact with segment 2, current flowed in coil C in
the same direction as in coil B.
=Ques. What occurs while the brush is in contact with segments 2 and
3?=
Ans. During this brief interval, the current in C is stopped and
started again in the opposite direction.
Similarly each coil of the armature as it passes the brush will
be short circuited and have its current reversed. This is known
as _commutation_.
=Ques. What is the effect of field distortion with respect to
commutation?=
Ans. The neutral plane no longer coincides with the normal neutral
plane but is advanced in the direction of rotation of the armature as
shown in fig. 302.
The reaction of the poles N' and S' of the armature field on
the poles S and N of the main magnetic field tends to crowd
the lines of force into the upper pole face of the south pole
of the magnet, and into the lower pole face of the north pole.
This effect is due to the strong magnetic attraction between
the opposite poles S and N' and N and S', and the equally
strong repulsion between like poles N and N' and S and S'.
Hence, the plane of maximum induction no longer coincides
with the normal plane of maximum induction, but is advanced
in the direction of rotation, depending upon the strength of
the armature current, being shifted forward for an increase
of current, and backward for a decrease of current. This
distortion of the field and the consequent shifting of the
plane of maximum induction naturally results in the shifting of
the neutral plane from the vertical position to the inclined
position as shown.
=Position of the Brushes; Sparking.=--In accordance with the
laws of electromagnetic induction, if the bipolar ring armature shown
in fig. 301 be rotated in the direction indicated by the arrow the
armature current entering at the brush E will divide, one part passing
through the coils on the right half of the ring, and the other part
through the coils on the left half of the ring, to the brush F, from
which the total current will pass out, urged by the full value of the
electromotive force induced in all the coils on both halves of the ring.
[Illustration: FIGS. 305 to 308--Improper brush adjustment
resulting in excessive sparking. When the brushes are not advanced far
enough, commutation takes place before the short circuited coil reaches
the neutral plane, hence, its motion is not changed with respect
to the magnetic field so as to induce a reverse current till after
commutation. There is then no opposing force, during commutation, to
stop and reverse the current in the short circuited coil, and when the
brush breaks contact with segment 1, as in fig. 308, the "_momentum_"
of the current in coil F causes it to jump the air gap from segment 1
to segment 2 and the brush, against the enormous resistance of the air,
thus producing a spark whose intensity depends on the momentum of the
current in coil F. Sparking, if allowed to continue, will injure the
brushes and commutator segments.]
Again, if the brushes be placed at the points G and H, each half of a
current entering at G, will pass through one-half of the coils on the
left side and one-half of the coils on the right side of the ring, so
that each half of the current will be urged forward by an electromotive
force equal to the electromotive force tending to force it back, and
therefore, no current will pass in or out through the brushes. From
these considerations it is obvious that the proper position for the
brushes would be in the normal neutral plane, _were it not for the
disturbing effects of armature reaction and self induction of the
current_.
=Ques. Should the brushes of a dynamo be placed in the neutral
plane?=
Ans. No.
=Ques. Why not?=
Ans. The brushes must be advanced beyond the neutral plane to prevent
sparking.
=Ques. What is the cause of sparking at the brushes?=
Ans. It is due to _self-induction_ in the coil undergoing commutation.
=Ques. Explain the effect of self-induction in detail.=
Ans. When commutation takes place with the brushes in the neutral
plane as in fig. 304, there will be no voltage induced in the short
circuited coil C. The current, therefore, which flowed in coil C before
it was short circuited will cease, and as segment 3 breaks contact with
the brush, it will be thrown as a perfectly idle coil upon the right
hand half of the ring in which a current is flowing toward the brush.
Moreover, the current which was flowing through D and 3 directly to the
brush, must suddenly traverse the longer path through the idle coil C.
Now, on account of self-induction, _the current acts in precisely the
same manner as though it had weight_; that is:
_It cannot be instantly stopped or started._
[Illustration: FIGS. 309 to 313.--How sparkless commutation is
obtained by advancing the brushes beyond the neutral plane; commutation
progressively shown.]
[Illustration: FIG. 309.--Commutation begins; current flows
up both sides of the armature, uniting at S and flowing to the brush
through commutator segment 1 as indicated by the arrow.]
[Illustration: FIG. 310.--Segment 2 has come into contact
with the brush and coil F, in which commutation is taking place, is
now short circuited. The current now divides at M, part passing to
the brush through segment 2, and part through coil F and segment 1.
Although coil F is short circuited and having passed the neutral plane,
is cutting the lines of force so as to induce a current in the opposite
direction, it still continues to flow with unchanged direction against
these opposing conditions. This is due to _self-induction_ in the coil
which resists any change just as the momentum of a heavy moving body,
such as a train of cars, offers resistance to the action of the brakes
in retarding and stopping its motion.]
Therefore, when segment 3 leaves the brush, the current will not
instantly change its path and flow through C, but will be urged by its
"_momentum_," and jump the air gap between the brush and segment 3,
thus producing a spark.
[Illustration: FIG. 311.--Segment 2 has moved further under
the brush, and the opposition offered to the forward flow of the
current in the short circuited coil F by the reverse induction in the
magnetic field to the right of the neutral plane has finally brought
the current in F to rest. The currents from each side of the armature
now flow direct to the brush through their respective end segments 1
and 2.]
[Illustration: FIG. 312.--Segment 1 is now almost out of
contact with the brush. A current has now been started in the coil F
in the reverse direction due to induction in the magnetic field to the
right of the neutral plane; it flows to the brush through segment 2.
The current has not yet reached its full strength in F, accordingly,
part of the current coming up from the right divides at S and flows to
the brush through segment 1.]
[Illustration: FIG. 313.--Completion of commutation in
segments 1 and 2; the brush is now in full contact with segment 2, the
current in coil F has now reached its full value, hence the current
flowing up from the right no longer divides at S but flows through F
and segment 2 to the brush. If the current in F had not reached its
full value, at the instant segment 1 left contact with the brush, it
could not immediately be made to flow at full speed any more than could
a locomotive have its speed instantly changed. This, as previously
explained, is due to self-induction in the coil or the so called
"inertia" of the current which opposes any sudden change in its rate of
flow or direction. Accordingly that portion of the current which was
flowing up from the right and passing off at S to the brush through
segment 1 as in fig. 312, would, when this path is suddenly cut off as
in fig. 313, encounter enormous opposition in coil F. Hence, it would
momentarily continue to flow through segment 1 and jump the air gap
between this segment and the brush, resulting in a more or less intense
spark depending on the current conditions in coil F.]
=Ques. How may this sparking be prevented?=
Ans. If the brushes be given additional lead, that is shifted further
to the right to some position as N N, fig. 304, coil C will not
remain idle during the interval it is short circuited, _but will
cut the magnetic lines in such a way as to induce a current in the
reverse direction through it_. Under these conditions, when segment 3
breaks contact with the brush, the current flowing through D does not
encounter an idle coil, but one in which a current is flowing in the
same direction, hence, the tendency to jump the air gap and produce a
spark is reduced; with proper adjustment of the brushes, there will be
no sparking.
=Ques. What is the objection to very thin brushes?=
Ans. Time must be allowed for reversal of the current, hence the
brushes must not be so thin as merely to bridge the insulation between
segments.
=Ques. What is the effect of lead?=
Ans. There is usually much sparking when the lead is too small; a
little sparking when too great, and no sparking when just right. If the
lead be excessive, there is a waste of energy due to the generation of
a larger reverse current in the short circuited coil than is necessary.
=Fixed Position of Brushes.=--The condition for sparkless
commutation is that the current in the short circuited coil be reduced
to zero, and increased in the opposite direction up to the same value
as that in the next coil leading. If the brushes are to remain in a
fixed position, this condition will only be realized at the particular
load for which the brushes are set. Thus, if the brushes be set for
the average load, the reversing field will not be correct for either
a weaker or stronger load. Hence, sparkless commutation with fixed
brushes must be due to some other factor.
=Ques. What may be said with respect to carbon brushes?=
Ans. Since carbon possesses a high resistance, the drop will vary
greatly with the contact area, thus affecting a difference of potential
in the two segments passing under the brush and it is largely to this
that sparkless commutation is due.
=Ques. What is the effect of resistance on commutation?=
Ans. In fig. 304 during commutation, that is, while the brush contacts
with any two segments, as 2 and 3, the currents coming up through the
winding on either side of the neutral plane are offered two paths to
the brush: 1, direct to brush through the connecting segment, or 2,
across the short circuited coil and adjacent segment. Thus, on the
right side: to brush through segment 3, or across coil C and adjacent
segment 2.
_The current will take the path of least resistance._
At the beginning of commutation, almost the entire brush area being in
contact with segment 3, the contact resistance of this segment will
be much less than for segment 2; hence, not only will the current
at the right flow through 3, but also the current at the left after
first traversing the short circuited coil. As commutation progresses,
the area of contact of 3 decreases while that of 2 increases, and
the respective resistances vary in inverse proportion. Likewise the
tendency of the current in the left half of the winding to take the
longer path through coil C and segment 3 to the brush gradually
decreases, becoming zero when the two contact areas become equal.
During the second half of the period of commutation, the contact area
of segment 2 becomes greater and of 3, less; thus the resistance of 2
is lowered, and that of 3 increased. Accordingly, all of the current
at the left will flow through segment 2, and the current at the right
will flow through C and 2 rather than through 3. In this way the
current is reversed in C, and, if the brush be broad enough to allow
a sufficient time interval, the current in C is built up to its full
value before segment 3 leaves the brush, thus securing sparkless
commutation.
This contact resistance factor in sparkless commutation
is illustrated in figs. 314 to 318, it being assumed that
during commutation, the brush contact resistance is inversely
proportional to the area of contact, and that the winding is
free of resistance and inductance. The current is taken as 40
amperes, in which case 20 amperes will flow from each side of
the winding to the brush.
In fig. 314 the instant before commutation begins all the
current will flow through segment A. At the end of the first
quarter of the period of commutation, fig. 315, 30 amperes will
flow from the right to brush through A, and from the left, 10
amperes through the short circuited coil via A and 10 amperes
through B.
At the end of the second quarter or half period, fig, 316, the
current through each half of the winding will flow to the brush
through these respective segments.
At the end of the third quarter, fig. 317, the current from the
right will divide, 10 amperes going through A, and 30 amperes
traversing the short circuited coil and out through B. The
entire current from the left will flow through segment B.
At the end of the fourth quarter, fig. 318, or completion of
the period the current from each half of the winding will flow
to the brush through B.
[Illustration: FIGS. 314 to 318.--Brush contact resistance
theory of commutation, neglecting self-induction and resistance in the
coils. The total current is assumed to be 40 amperes made up of 20
amperes flowing toward the brush from the coils on the right and 20
amperes from the coils on the left. During commutation, that is, the
interval during which the brush contacts with any two adjacent segments
of the commutator, the current is assumed to vary directly as the
contact area.]
[Illustration: FIG. 314.--Beginning of commutation; segment A
is entirely under the brush, and B is at the initial point of contact.
For this position the currents from both sides flow to the brush
through segment A.]
[Illustration: FIG. 315.--One-quarter period of commutation.
One quarter of the brush area is in contact with B and three quarters
in contact with A; hence, 10 amperes will flow through B and 30 amperes
through A.]
[Illustration: FIG. 316.--Second quarter of commutation
period. The brush now contacts equally with both segments, hence 20
amperes will flow through each segment.]
[Illustration: FIG. 317.--Third quarter of commutation period.
Three quarters of the brush area is in contact with segment B and one
quarter with segment A; accordingly, 30 amperes will flow through B and
10 amperes through A.]
[Illustration: FIG. 318.--Completion of commutation. The brush
is in full contact with segment B and at the point of breaking contact
with A, hence the entire current from both sides or 40 amperes will
flow through B.]
[Illustration: FIGS. 319 to 323.--Brush contact theory of
commutation for case in which the brush covers two segments of the
commutator. Fig. 319 beginning of commutation; fig. 320 one-quarter
period; fig. 321, one-half period; fig. 322, three-quarter period; fig.
323 completion of commutation.]
=Ques. What is the effect of increasing the degree of contact of the
brushes?=
Ans. It lengthens the period of commutation, and permits it to start
in one coil before the preceding coil has entirely passed through this
stage.
The effect of changing the degree of contact is shown in figs.
319 to 323, in which the width of the brush is made equal to
that of two segments.
=Construction of Commutators.=--The commutator for a closed coil
armature consists of a number of segments or L-shaped bars C of drop
forged hard drawn copper assembled around a tubular iron hub as shown
in figs. 324 and 325. The bars are held in position by the nuts E, and
washers F, screwed on the ends of the tube D. The bars are insulated
from each other and from the washers by mica as shown by the heavy
lines G, and they are also insulated from the tube either by a tube
of mica H, or by a sufficient air space. The ends of the sections
of winding are connected to the vertical portions of the bars K, by
insertion in the slots L, where they are securely held in place by
means of the binding screws, which for greater security are soldered
together, and may be released from the slots, whenever necessary, by
the application of a hot soldering iron.
[Illustration: FIGS. 324 and 325.--Side and end sectional
views of commutator showing construction. The parts are: C, segments;
D, tubular iron hub; E, end nuts; F, clamps; G, insulation; L, riser
connection.]
It is very important that all the parts of the commutator should be
fitted together perfectly and screwed up tightly, in order to prevent
looseness. Commutator segments are often made with the washers E,
projecting beyond the ends, but such construction reduces the effective
length of the commutator, therefore the under cut form of bar is
preferable.
In the construction of commutators, the conditions of operation require
that there be:
1. Adequate insulation;
It is necessary to have good insulation between each segment,
and a specially good insulation between the segments and the
hub or sleeve on which they are mounted; also between the
segments and end clamps. The insulating material must not
absorb moisture, hence asbestos, plaster, or vulcanized fibre
are not used. The end insulating rings are usually built up
of mica and shellac, moulded while hot under pressure to the
correct shape.
[Illustration: FIG. 326.--Front view of Western Electric
Commutator for bar wound armature. This commutator is made of hard
drawn copper and insulated throughout, ventilating spaces being
provided near the shaft.]
2. Rigidity against centrifugal force;
Since the segments are subject to centrifugal force, they
must be securely clamped in place. Screws cannot be used, for
that would destroy the insulation. They are therefore held in
place by insulated clamps as shown in fig. 324. These clamps
should be strong and capable of holding the segments firmly in
position, for if a segment should rise out of its place through
centrifugal force, it would disturb the action of the brush and
cause sparking.
3. Provision for wear.
The segments should be of considerable radial depth, so that
the commutator may be turned down from time to time to preserve
its circular form.
[Illustration: FIG. 327.--Sectional view of a General
Electric commutator. The segments are of rolled or forged copper and
are separated by soft mica insulating sheets. This mica must wear
down evenly with the copper, hence its consistency is important. The
segments are wedge shaped so that when drawn radially inward they
support each other like the stones of an arch. They are drawn together
by hollow cone collars which bear upon lugs projecting from the ends
of the segments. These lugs are turned to form a smooth cone after the
segments are assembled. The collars are insulated with mica from the
segments and they are held in place by nuts upon the commutator shell
or by bolts passing from end to end under the segments. The segments
are also provided with lugs for connection to the windings.]
=Points Relating to Commutators.=--1. The number of commutator
segments depends on the scheme of winding and on the number of sections
in the armature winding.
2. Increasing the number of bars diminishes the tendency to spark, and
lessens the fluctuations of the current.
There are two practical reasons for not using a very great
number of segments: it increases the cost, and in small
machines the segments would be so thin that a brush of the
proper thickness to collect the current would lap over, or
bridge several segments.
=Types of Commutator.=--Commutators are made in various forms, but
they may be grouped into two general types:
1. Commutators for closed coil armatures;
These consist of a large number of segments or bars, insulated
from each other and varying in number according to the scheme
of armature winding, and on the number of sections into which
that winding is grouped.
[Illustration: FIG. 328.--A large current low voltage bipolar
dynamo built for electrolytic work and here shown to illustrate the
large size commutator and brushes necessary to collect the large
current. Carbon brushes would not be suitable for this class of machine
because even with copper brushes, whose conductivity is much higher
than carbon, the commutator must be of considerable size to give the
required brush contact area. The contrast between the axial lengths of
the armature and the commutator is very marked. The rocker construction
is of the ordinary type, and heavy flexible cables conduct the current
from the brush holders to the fixed terminals. The machine here
illustrated gives 310 amperes at 7 volts when running at a speed of
1400 R. P. M., corresponding to an output of 2.17 kilowatts.]
2. Commutators for open coil armatures;
This form of commutator is used on some machines designed
especially for arc lighting, such as the Brush and
Thompson-Houston machines. They consist of a comparatively
small number of segments each of which covers a wide angle, and
are separated from each other by air gaps.
3. The segments should be of considerable depth to permit returning
occasionally so that their circular form may be preserved;
4. The insulating material must be such that it will not absorb oil or
moisture;
Mica is best adapted for insulation, but as there are a great
many varieties, differing greatly in hardness and other
equalities, it is important to select the kind that wears at
the same rate as the segments. If the mica be too hard, the
wearing of the segments will leave it projecting and prevent
proper contact with the brushes; again, if the mica be too
soft, it will result in furrows or depressions between the
segments into which copper dust will collect, causing short
circuits.
CHAPTER XXI
=BRUSHES AND THE BRUSH GEAR=
With respect to construction, brushes may be broadly classified as: 1,
those made of metal, and; 2, those made of carbon. There are several
varieties of metal brush, such as:
1. Gauze brushes;
2. Wire brushes;
3. Laminated or strip brushes.
=Gauze Brushes.=--These are very flexible and yielding, their use
being attended with little wear of the commutator.
=Ques. What is the construction of a gauze brush?=
Ans. A gauze brush is made up of a sheet of copper gauze, folded
several times, with the wires running in an oblique direction, so
as to form a solid flat strip of from ¼ to ½ inch in thickness,
increasing with the volume of the current to be collected.
=Ques. What is the object of folding the gauze with the wires running
in oblique directions?=
Ans. It is to prevent the ends of the brush fraying or threading out,
which would be the case if the gauze were folded up in any other
manner.
=Ques. What are the features of gauze brushes?=
Ans. They make good contact, but are quite expensive. They may be set
either tangentially or radially, the latter preferably, since the point
of contact remains the same as the brushes wear away.
[Illustration: FIGS. 329 to 332.--Various forms of brush. Fig.
329 gauze brush; fig. 330 laminated or strip brush; fig. 331 strip and
wire brush as used on the early Edison machines; fig. 332 carbon brush.
Carbon is preferred to copper for brushes on account of the reduction
of sparking secured by its use.]
=Wire Brushes.=--This class of brush, which was extensively used
before the invention of the gauze brush, is made up of a bundle of
brass or copper wires, laid side by side and soldered together at one
end. Since wire brushes are harder than the gauze brush, they are more
liable to cut or score the commutator, and are also more troublesome to
trim.
=Laminated or Strip Brushes.=--These probably represent the
simplest form of brush, but are not extensively used owing to the lack
of flexibility. They consist of a number of strips of copper or brass,
laid one upon the other and soldered at one end, as in fig. 330.
[Illustration: FIG. 333.--General Electric brush holder.
The brush holder yoke consists of a cast iron ring of elliptical
section, supported from the bracket of the end shield in such a manner
as to facilitate the shifting of the brushes. It is provided with a
suitable handle, and may be fastened in any position by means of a
thumb nut on the outside of the bracket. It is so constructed that the
tension on the individual brush can be adjusted without lifting the
brush from the commutator and without the use of tools. The brush can
be removed while the machine is running, without moving the holder
on the stud and without disturbing any other brush. Removal of the
brushes for inspection can be accomplished without permanent change
in the adjustment of the tension of the brush holder spring. The
connection between the brush and stud is made through a flexible copper
connection.]
=Ques. What name is generally given to strip brushes?=
Ans. They are commonly and erroneously called _tangential brushes_,
but they are really beveled at the end and set inclined to the line of
tangency so that the ends of all the sheets will make contact.
In the Brush and Thomson-Houston arc dynamos, in which the
current is limited to ten amperes, the brushes consist of a
simple strip of flexible sheet copper, the ends of which are
slit in a number of places so as to insure contact at several
points.
[Illustration: FIG. 334.--Crocker-Wheeler brush holder. The
carbon brush B is firmly clamped in the "box" C by two screws which
bear on a sheet of brass to protect the carbon from being broken by the
ends of the screws. The box C is carried by four flexible springs S S,
one at each corner and formed of hard copper leaves. These are fixed at
one end to the box and at the other to the solid base which is in one
piece with the spoke attached to the rocker ring. An adjusting screw
passes through appropriate lugs on the box C and loosely through the
head A of a fixed arm _a_. Between the lower surface of _a_ and the
upper lug on the box C is placed the pressure spring.]
=Carbon Brushes.=--When metallic brushes are used upon the
commutators of high tension machines, they frequently give rise to
excessive sparking and also heating of the armature, the metallic dust
given off appearing to lodge between the segments of the commutator,
thus partially short circuiting the armature. To obviate this, carbon
brushes are extensively used in such dynamos, this material being found
very effectual in the prevention of sparking.
=Ques. What is the usual form of carbon brushes?=
Ans. They are usually in the form of oblong blocks.
[Illustration: FIG. 335.--Perspective views of Crocker-Wheeler
brush holder. This holder is of the parallel type in which the brushes
may be adjusted without affecting the lead. Each brush is held rigidly
in its box and there are no sliding contacts in the path of the
current. The holder is further described under fig. 334.]
=Ques. How are they adjusted on the commutator?=
Ans. They are set "butt" end on the commutator, and fed forward as they
wear away by means of a spring holder.
=Ques. Why are carbon brushes so extensively used?=
Ans. Because they are the only form of brush that will give good
commutation with fixed lead.
=Ques. What may be said of the different grades of carbon in use for
brushes?=
Ans. The very soft carbon leaves a layer of graphitic matter on the
commutator, and at high voltages, this may cause sparking; such grade
of carbon should only be used on low voltage machines.
[Illustration: FIG. 336.--Western Electric box type brush
holder. The box which holds the brush is broached to allow the brush
to slide freely, but not loosely, to and from the commutator against
which it is normally held by a lever acting directly upon the brush
head. This avoids the possibility of uneven bearing on the commutator,
as the brushes are allowed very slight lateral or angular motion. The
adjustment of a brush is also simplified after it has been removed
and then replaced. Tension on the brush head is obtained by a special
spring which maintains any given tension for which it may be set. An
auxiliary flat steel spring on the lower side of the lever acts as a
shock absorber between the lever and the brush head, absorbing all
minor vibrations caused by a worn commutator. Side contact between
brush and brush holder is not relied upon to carry the current,
flexible copper pigtails performing this function to the exclusion of
sliding contacts or tension springs, in order to reduce the brush loss.
It is not necessary to take the brush rigging apart or loosen cable
connections when it is desired either to remove or reverse the brushes
to change the direction of armature rotation.]
=Ques. How are the ends of carbon brushes treated and why?=
Ans. They are usually covered at their upper part with a coating of
electro-deposited copper to insure good contact with the holder.
=Comparison of Copper and Carbon Brushes.=--Copper brushes tend to
tear and roughen the surface of the commutator, while carbon brushes
tend to keep the surface smooth. Copper causes more wear of the
commutator than carbon. With carbon brushes, the armature may be run in
either direction. The resistance of carbon being greater than copper,
there is less short circuiting caused by carbon particles than by those
of copper.
[Illustration: FIG. 337--Westinghouse brush holder. It is made
of brass, cast in one piece, and of standard sliding type with a shunt
of braided copper wire directly connected to a clamp on each brush and
to the solid portion of the holder, where it is held by a screw. This
shunt relieves the spring of heavy currents. The holder is so arranged
as to be easily accessible for adjustment, cleaning and renewal of
carbons. Proper tension is provided by spiral strap springs so mounted
as to eliminate friction and give uniform pressure over a wide working
range. The spring tension is readily adjusted by a simple ratchet
arrangement.]
=Ques. What is the chief merit of carbon brushes?=
Ans. They give less sparking than other types.
=Ques. How has the construction of carbon brushes been varied?=
Ans. Since, for minimum sparking, it is only necessary that the brush
have high resistance in the region near its edge, attempts have been
made to increase the conductivity of the other portions by combining
with the carbon, copper sheets or wires.
=Ques. What are the objections to carbon brushes?=
Ans. They are easily broken and not being flexible, vibration, or any
roughness of the commutator will cause bad contact.
[Illustration: FIG. 338.--Holzer Cabot multiple brush holder.
Each brush is fastened securely to a machined surface by one or two
machine screws, making a positive contact. Several strips of flexible
copper of ample section to carry the current are interposed between
the part of the holder carrying the brush and the portion clamping the
stud, no sliding contact or spring being therefore required to carry
any current. The brushes are proportioned for a carrying capacity of
not more than 25 amperes per square inch of brush surface. The brush
can be adjusted to any degree of tension during the operation of the
machine if necessary. Each holder is insulated in such a manner that no
short circuit can occur if the holder be accidentally tipped backward
while the operator is changing a brush or cleaning the commutator
during a run.]
=Ques. For what class of machine are carbon brushes specially
adapted?=
Ans. For machines furnishing a small current at high pressure.
When carbon brushes are used, it is desirable that the current
be small, because, on account of the low conductivity of the
carbon, more contact area is necessary than with copper for
equal current transmission. For fixed lead and fluctuating
currents, carbon brushes should be used.
=Ques. For what class of machine are copper brushes especially
adapted?=
Ans. For machines furnishing large current at low pressure, as in fig.
328.
=Size of Brushes.=--The number of brush sets depends upon the
number of poles of the machine, but there may be several brushes in
each set. It is usual, except in the smallest machines, to place at
least two brushes exactly similar side by side instead of one broad
brush, thus allowing one brush to be removed for trimming or renewal
while the machine is running. Moreover, better contact is secured by
this sub-division, because a slight elevation in the commutator surface
at one point may slightly raise one brush of a set at each revolution
without much harm, while with one broad brush, the entire brush would
be lifted, causing bad sparking.
=Ques. What determines the number of brushes in each set?=
Ans. It depends upon the current capacity, size of machine, and
judgment of the designer.
=Ques. What may be said with respect to the dimensions of the
brushes?=
Ans. No general rule can be given for breadth and thickness of brush.
The contact face must clearly be wider than the thickness of the
insulation between commutator segments, since the period of commutation
must last an appreciable interval of time on account of self-induction.
=Ques. What should be the minimum width of the brush contact face?=
Ans. It may be taken as one and one-half times the thickness of the
commutator segments.
=Ques. How wide should a carbon brush contact be?=
Ans. The brush should be thick enough to cover two and one-half
commutator segments. The thickness should in no case be excessive on
account of the loss due to heating, which results from the difference
of potential at the forward and rear edge of the brush.
[Illustration: FIG. 339.--Contact angle for the different
types of brush. At A is shown a brush with tangential contact, and at
B, a so called tangent brush; the latter is properly called an inclined
brush. Sheet copper brushes are set tangentially as at A, and gauze
brushes inclined as at B. Carbon brushes are placed radially as at C
when mounted in box holders, and inclined opposite to the direction of
rotation when used with reaction holders.]
=Contact Angle of Brush.=--This may be defined as the angle which
the brushes make with the commutating plane as shown in fig. 339.
The several kinds of brush, together with the varied conditions of
operation require different contact angles ranging from zero to 90°.
Thus in the figure, a copper strip brush may lie at 90° or
tangentially as at A.
Wire or gauze brushes should make a more or less acute angle as
at B, in order to present the end and not the side of the brush
to the commutator.
Carbon brushes may be placed end on or radially as at C, which
is the position almost universally used in the case of traction
or other reversing motors.
Sometimes the carbon brush is inclined as at D, in order that
the revolving commutator may tend to push the brush against its
supports and thus ensure better contact.
=Brush Contact.=--The relation between _contact pressure, contact
resistance_, and _friction_ of brushes varies greatly for different
kinds of brush. Copper brushes will carry from 150 to 200 amperes
per square inch of contact surface; and carbon brushes from 40 to 70
amperes per square inch. The usual contact pressure is 1.25 to 1.5
pounds per square inch for copper brushes, and 1.5 to 2 pounds per
square inch for carbon brushes. The rim velocities of commutators vary
from 1,500 to 2,500 feet per minute, the velocity usually increasing
with the size of the machine.
[Illustration: FIG. 340.--Bissell double brush holder.
Flexible cables carry the current between the brushes and holders.
This holder works equally well for forward or reverse rotation. Two or
more holders are used on each stud except for the two smallest frames.
The construction permits of adjustment or renewal of brush while the
machine is in operation. Sufficient contact area of brush is provided
to permit running on one carbon at ordinary loads in case the other
become worn or inoperative.]
=Ques. What is the drop in voltage at the brushes?=
Ans. For carbon brushes it is about O.8 to 1.0 volt at each contact,
or 1.6 to 2.0 volts for the two, positive and negative, contacts of a
machine.
This value is not materially affected by placing a number of
brushes in parallel or by using several sets, as in the case
of multipolar machines, as such arrangement merely reduces the
current density, and since the contact resistance varies in the
inverse ratio, their product remains nearly constant.
=Ques. What may be said of the friction of the brushes?=
Ans. The coefficient of friction of brushes is about .2 to.25 for
copper and .3 for carbon.
[Illustration: FIG. 341--Western Electric brush gear. The
brush holders carry carbon brushes and are so designed that the brushes
may be firmly clamped in position and also be capable of independent
adjustment. Any brush can be removed while the machine is in operation
without disturbing the others and without moving the holder on the
stud.]
=Ques. How many watts are lost at the brushes?=
Ans. The watt loss is equal to 1.6 to 2. volts for carbon multiplied by
the total current carried.
The watt lost on account of friction may be calculated by the
formula: ((.3 × 746)/33000) × (P × S) = watts lost by carbon
friction, in which P is the total pressure in pounds on the
commutator, and S, the rim velocity of the commutator in feet
per minute.
The losses due to contact resistance and brush friction are
very liable to be greatly increased above the values that may
be obtained by the preceding methods, if the commutator and
brushes are dirty and rough, or not in good condition.
[Illustration: FIGS. 342 to 345.--Various types of brush
holder. Fig. 342, arm or lever type; fig. 343, spring arm type; fig.
344, box type; fig. 345, reaction type.]
=Brush Holders.=--These are devices employed to hold the
brushes against the commutator with the proper pressure. They differ
considerably in various types of machine, hence, no general rules can
be given with respect to their construction or use, but any brush
holder must fulfill the following requirements:
1. It must hold the brush securely and at the same time feed it forward
as it wears away so as to maintain a proper contact;
2. It must hold the brush at the proper contact angle;
3. It must be capable of being raised from the commutator, and held out
of contact by some form of catch;
4. It must be so constructed that the brush can be easily removed for
cleaning or renewal;
5. The spring pressure must be adjustable;
6. The brush holders themselves must be carried on a rocker arm, or
rocker ring.
It is desirable that brush holders be capable of individual adjustment,
so that each may be set at its own point of minimum sparking. A few
forms of brush holder are illustrated in figs. 342 to 345.
The various kinds of brush holder may be divided into four types:
1. Arm or lever type;
2. Spring arm type;
3. Box type;
4. Reaction type.
In the arm or lever type the brush is firmly attached to the extremity
of a rigid arm capable of movement about the brush spindle, except in
so far as it is restrained by a spring as in fig. 342.
Fig. 343 shows a brush holder of the spring arm type. The brush is
firmly attached to the extremity of a spring arm, the other end of
which is secured to the brush spindle, and when once adjusted is not
capable of movement about the brush spindle.
In the box type of brush holder as illustrated in fig. 344, the brush
is free to move up and down in the brush box, so far as it is not
restrained by a spring rigidly secured to the arm which carries the
brush box at its extremity.
[Illustration: FIG. 346.--Fort Wayne type MPL dynamo; view
showing details of armature, commutator and brush rigging of large
machine. The laminations of the armature core are punched from thin
sheet steel, annealed and japanned. Spacing ribs are built into the
core at proper intervals forming air passages for ventilation. In
addition, there are recesses in the inside of the flanges which permit
the passage of air from the interior around the ends of the core to
the openings in the end flanges. The armature coils are constructed of
round or bar copper on standard forms. The coils are laid in slots in
the surface of the core. The commutator is constructed of bars of hard
drawn copper of uniform size and shape, supported and clamped at either
end between beveled rings and securely seated on the commutator drum.
The drum is connected by radial arms to the commutator sleeve which is
mounted and keyed on the armature hub extension.]
Fig. 345 shows the reaction type of brush holder, in which the movement
of the brush is constrained in one direction by the surface of a part
rigidly secured to the brush spindle, and is further constrained by
a spring controlled arm, the pressure of which is capable of ready
adjustment.
Among the special forms of brush holder may be mentioned
1. Scissor type of brush holder, used for slip rings, and
consisting of two arms pivoted together like a pair of
scissors. The lower ends of the arms carry the brushes,
suitably mounted, and the upper ends are drawn together by a
spring, which thus exerts pressure on the brushes.
2. Clock spring type of brush holder in which the necessary
contact pressure is applied to the brush by means of a clock
spring, which, with the aid of a ratchet may be wound up and
adjusted to any desired pressure.
[Illustration: FIG. 347.--Western Electric brush holder. This
holder consists of a rugged iron casting, elliptical in section, and
supported from the commutator end bearing bracket in such a manner as
to provide for the shifting of the brushes. A handle attached to the
yoke aids in this shifting and a thumb nut on the outside holds the
whole brush gear in the desired position. The brush is fed through an
accurately broached slot by a spring which maintains uniform pressure
against the commutator throughout the wearing length of the brush. The
long lever arm of the spring is sufficiently flexible to take up any
minor vibrations of the brush. The tension of the brush may be adjusted
without lifting it from the commutator or disturbing any of the other
holders. The brush, may be removed for inspection by throwing the
spring out of notch. The brush is connected to the holder by flexible
copper pig tails of ample current carrying capacity.]
=Ques. How are brush holders carried?=
Ans. They are carried by a _rocker arm_ for bipolar, and by a _rocker
ring_ for multipolar machines, which is mounted upon one of the main
bearings, or upon a support specially provided for it, being pivoted
to revolve from the same center as the shaft, to permit shifting the
brushes.
=Ques. Mention one trouble sometimes encountered with brush
holders.=
Ans. There is sometimes trouble resulting from the current passing
through the spring which heats it and destroys its elasticity.
[Illustration: FIG. 348.--Western Electric parallel spring
brush holder as used on the larger machines.]
=Ques. How may this be avoided?=
Ans. By insulating one end of the spring, and carrying the entire
current directly from the brush itself to the main conductors by a
flexible copper strip or cable firmly connected to both.
=Ques. What may be said with respect to brush construction on
machines for electrolytic work?=
Ans. The collection of large currents at low voltage, generated by
comparatively small machines, requires careful design of brushes
and brush holders. The commutator is longer than the commutators
on machines of equal capacity at higher voltages, and as a rule the
commutator segments are thicker and fewer in number. Each brush set is
made up of numerous narrow brushes rather than two abnormally wide ones.
An example of brush and brush gear designed to meet such
conditions is shown in fig. 328.
In large machines for electrolytic work, it is not unusual to
find the current divided between two wide commutators, one at
each end of the armature, thus giving a longer axial bearing
surface for the brushes without inconveniently lengthening the
pins upon which the separate brushes are threaded.
=Multipolar Brush Gear.=--The brush gear which includes the
holders and carrier arm or ring, becomes more complicated as the number
of poles and magnitude of the current is increased.
In the early days of multipolar machines, schemes of armature
winding were devised such that all the necessary cross
connections were made inside the machine, and the number of
brush holders reduced to two and placed at an angular distance
apart depending upon the number of poles. Such windings,
though possible, are not used much, chiefly on account of
their complexity, which not only increases the danger of error
in construction, but also makes repairs costly. In modern
multipolar machines, such complicated windings are avoided,
and the several sets of brushes are connected together in two
groups, positive and negative. These connections are carefully
designed as part of the brush gear.
=Ques. How are the brushes held in large multipolar dynamos?=
Ans. They are held at the proper points of commutation by arms offset
from a cast iron rocker ring, which is itself supported by brackets
projecting from the magnet yoke as shown in fig. 346.
=Ques. What provision is made for shifting the ring to adjust the
lead?=
=Ans.= The ring is rotated by means of a worm gear and hand wheel.
CHAPTER XXII
ARMATURE CONSTRUCTION
The armature of a dynamo has been defined as: _a collection of coils of
wire wound around an iron core, and so arranged that electric currents
are induced in the wire when the armature is rotated in a magnetic
field._
From the mechanical point of view the armature may be said to be made
up of the following parts:
1. Shaft;
2. Core;
3. Spider
(in large machines);
4. Winding;
5. Commutator.
(broadly speaking).
Of the two types of armature, ring and drum, the latter is almost
universally used, hence the examples of construction which follow will
be confined chiefly to this type.
=Shaft.=--A typical armature shaft is shown in fig. 349. It is
made of steel and, except in the smaller machines, is thicker in the
middle than at the ends for stiffness to withstand the strong magnetic
side pull on the core when the latter is slightly, nearer one pole
piece than the other.
=Ques. What is the object of providing shoulders on the shaft as in
fig. 349?=
Ans. They serve to keep the armature in the proper position with
respect to the bearings.
=Ques. How is the shaft proportioned?=
Ans. If it be proportioned to secure the proper stiffness, it will be
found of ample size to resist the twisting strain.
[Illustration: FIG. 349.--Typical shaft for an armature. The
illustration shows the keyways for pulley armature and commutator. In
the smaller sizes, there is usually a flange at A, and threads at B and
C for retaining nuts.]
The shaft is subject also to bending by the weight of the
armature, by the magnetic drag on its core, and in belt driven
machines, by the lateral drag of the pulley. When running, it
is also subjected to bending stresses if the armature be not
properly balanced. If the bearings do not give, it is evident
that all such actions tend to bend the shaft at definite points.
=Core.=--In the small and medium size dynamos, the core is
attached direct to the shaft. There are two kinds of core:
1. Smooth;
2. Slotted.
=Ques. What may be said of the smooth type of core?=
Ans. It has become obsolete, except in special cases, as for machines
used for electrolytic work where a large current at low voltage is
required.
=Ques. What is necessary with a smooth core?=
Ans. Driving horns as later described.
[Illustration: FIG. 350.--Laminated smooth core armature
partly assembled. It consists of numerous discs of thin sheet iron
threaded on the shaft and pressed together by end plates. The object of
this construction is to prevent eddy currents.]
=Ques. What is a slotted core?=
Ans. One having a series of parallel slots, similar to the spaces
between the teeth of a gear wheel, and in which the inductors are laid.
=Ques. What provision is made to avoid eddy current in cores?=
Ans. They are laminated.
=Ques. Describe this method of construction.=
Ans. The core is made of stampings of thin wrought iron or mild steel.
The numerous discs stamped from the sheet metal are threaded on the
shaft as in fig. 350, forming a practically solid metal mass.
[Illustration: FIG. 351.--Sectional view of laminated smooth
core armature showing end plates, flange and retaining nut. A key is
provided to prevent rotation of the core with respect to the shaft.]
=Ques. How thick are the discs?=
Ans. The thickness ranges from .014 inch to .025 inch, corresponding to
27 and 22 B and S gauge respectively, 27 gauge being mostly used.
=Ques. How are the discs held in place?=
Ans. By two end plates pressed together either by large nuts screwed
directly on the shaft as in fig. 351, or by bolts passing through the
core from end to end, as in fig. 352, holes being punched in the discs
for the purpose.
=Ques. What precaution is taken with respect to the core bolts?=
Ans. They are insulated from the core by tubes and washers of mica or
other insulating material.
[Illustration: FIG. 352.--Laminated armature core with through
retaining bolts. In the larger sizes, these bolts are used instead of a
nut threaded on the shaft on account of the large size of the latter.]
=Ques. What is the construction of the core end plates, and why?=
Ans. The rims are beveled quite thin to avoid eddy currents.
=Ques. How is the core connected to the shaft?=
Ans. Since the core has the full torque exerted upon it by the drag of
the inductors, it must be firmly connected to the shaft by means of a
key, as shown, so that it may be positively driven.
Core discs are stamped in one piece up to about 30 inches in
diameter, and for larger sizes they are built up from sections
as later described.
Figs. 353 and 354 show two forms of disc stamped in one
piece. The first illustrates a solid disc, and the second
a _ventilated_ disc in which more or less of the metal is
cut away near the center, thus providing passages for the
circulation of air which carries away some of the heat
generated in the armature.
[Illustration: FIGS. 353 and 354.--Solid and ventilated core
discs. In fig. 353, the metal cut away near the center reduces the
weight and provides passages for air circulation. In some instances
a forced circulation is secured by means of a fan attached to the
armature, as shown in fig. 366.]
=Insulation of Core Discs.=--When the discs are stamped from
very thin metal, the mere existence of a film of oxide is sufficient
insulation. It is usual, however, to apply a quick drying varnish
that will give a hard tough coat and not soften with heat or become
brittle and crumble under vibration. The varnish may be applied either
by dipping or with a japanning machine; it must be very thin, and the
solvent employed should be a very volatile spirit.
=Forms of Armature Teeth.=--The teeth stamped in the core discs
are made in various shapes, depending largely on the method of
securing the inductors in the slots against electromagnetic drag and
centrifugal force. The teeth may be cut with their sides:
1. Inclined;
2. Projecting;
3. Notched.
=Ques. What may be said of teeth with inclined sides?=
Ans. A tooth of this type is shown in fig. 356, being slightly narrower
at the root than at the top, the resulting slot having parallel sides.
[Illustration: FIG. 355.--Western Electric slotted armature
core. The laminations are of sheet steel, annealed and japanned. They
are mounted directly on the shaft, (except in the large sizes) and held
in place by substantial end plates.]
=Ques. What are the features of the projecting type of tooth?=
Ans. The projecting type is shown in figs. 357 and 358 in which the
tops project; this gives a larger core area around the circumference of
the armature which reduces the reluctance of the air gap, and provides
projecting surfaces for retaining the inductors in the slots by the
insertion of wedges.
=Ques. What is the object of cutting notches in teeth?=
Ans. They are provided for the insertion of retaining wedges, as in
fig. 361; this results in less area at the top of the teeth.
=Ques. How should teeth be proportioned to secure most efficient
operation?=
Ans. The width of the tooth should be about equal to the width of the
slot minus twice the thickness of the slot insulation; that is, the
cross sectional area of the teeth should be equal to that of the slots.
[Illustration: FIGS. 356 to 359.--Various forms of armature
teeth; fig. 356 inclined type forming a slot with parallel sides; figs.
357 and 358 projecting type which provides a support for the retaining
wedges; fig. 359 enclosed type which forms "tunnels" for the inductors.]
=Advantages and Defects of Slotted Armatures.=--The slotted
armature, sometimes called the Pacinotti armature, after its inventor,
has the following advantages over the smooth type:
1. The inductors are held more firmly in place to resist stresses due
to electromagnetic drag and centrifugal force;
2. The inductors are protected by the teeth against mechanical injury;
3. Less reluctance of the air gap;
4. The intermittent induction due to the presence of the teeth prevents
the formation of eddy currents.
5. When the teeth are saturated they oppose the shifting of the lines
due to armature reaction.
[Illustration: FIGS. 360 and 361.--Projecting and notched
teeth; cross sections showing inductors and retaining wedges in place.]
The disadvantages of slotted armatures compared with the smooth type
are:
1. Greater hysteresis loss, caused by denser flux in the teeth;
2. Generation of eddy currents in the polar faces when the latter are
not of laminated construction;
3. Greater self-induction in the armature coils;
4. Construction more expensive;
5. Leakage of magnetic lines through core, exterior to winding.
The generation of eddy currents in the polar faces may be
overcome by making the air gap at least 50 per cent. of the
distance between the teeth, so that the magnetic lines can
spread from the corners of the teeth, and become nearly
uniformly distributed over the polar faces. Magnetic leakage
through the core may be reduced by making the amount of metal
above the inductors very small.
=Slotted Cores; Built Up Construction.=--In the case of large
dynamos, the core discs are built up in order to reduce the cost of
construction; the following parts are used:
1. Spider;
2. Core rings split into sections.
[Illustration: FIGS. 362 and 363.--Side and end view of built
up armature core. The sheet metal ring sections containing the teeth
are fastened into dovetail notches in the spider as shown. The layers
of ring sections are placed so as to break joints and are held by end
clamps and through bolts B. Distance pieces are inserted at intervals
to provide ventilating spaces D, D, D.]
=Ques. What is the approved method of core construction in large
armatures?=
Ans. The core should be of the built up construction to avoid waste of
material in the stampings.
=Ques. Describe the construction of a built up core.=
Ans. Ring sections stamped, from sheet metal are fastened to a central
support or _spider_, which consists of an iron hub with radiating
spokes and a rim with provision for fastening the rings. The rim of
the spider is provided with dovetail notches into which fit similarly
shaped internal projections on the core segments. These features are
shown in figs. 362 to 364. Each layer of core sections is placed on the
spider so as to break joints and the core thus formed is firmly held in
place by end clamps as shown. The manner of fastening the rings to the
spider is an important point, for it must be done without reducing the
effective cross section of the core in order not to choke the magnetic
flux.
[Illustration: FIG. 364.--Built up core with four spoke
spider, each spoke carrying two dovetail notches In this construction a
little more air space is obtained for ventilation than where a separate
spoke is provided for each notch.]
In order to secure a better fit and reduce the machine work,
the spider hub in large machines is sometimes cored with
enlarged section between the outer bearing surfaces, and it
is not unusual to find these surfaces turned to two different
sizes as in fig. 365, to admit of easier erecting.
To avoid any trouble that may arise by unequal expansion,
the rim of the spider is not made continuous, but in several
sections as shown in fig. 364. The rim here consists of four
sections each of which has two dovetail notches. By thus
dividing the rim into sections, its weight is somewhat reduced
and the ventilating spaces between the sections increased.
=Ventilation.=--In the operation of a dynamo more or less heat is
generated, depending on the load; hence it is desirable that provision
be made to carry off some of this heat to prevent excessive rise of
temperature.
[Illustration: FIG. 365.--Hub and shaft design on large
machines to reduce the machine work and facilitate erecting.]
=Ques. Why do armature cores heat?=
Ans. They heat from these causes: eddy currents, hysteresis, and heat
generated in the inductors.
=Ques. How is adequate ventilation secured?=
Ans. The spider is constructed with as much open space as possible
through which air currents may circulate. The core is divided into
several sections with intervening air spaces D as shown in fig. 363,
the discs being kept apart at these points by distance pieces. These
openings between the discs are called _ventilating ducts_; they are
usually spaced from 2 to 4 inches apart.
[Illustration: FIG. 366.--Western Electric barrel wound
armature, having a fan attached at one end to induce a circulation of
air for ventilation.]
=Ques. What other provision is sometimes made to secure
ventilation?=
Ans. In some machines a forced circulation of air is secured by means
of a fan attached to one end of the armature as shown in fig. 366.
=Insulation of Core.=--Before the winding is assembled on the
core, the latter should be thoroughly insulated. Japan or enamel
insulation is not sufficient because it is liable to have bubbles or
minute holes in it, or be pierced by particles of metal or by the rough
edges of the core discs. Two or more layers of strong paper, fibre,
canvas or mica, should be applied to the core before placing the
inductors in position. The ends of the core should be insulated with
thicker material, since the strain upon it is greater, especially at
the edges.
[Illustration: FIG. 367.--Holzer Cabot partially wound barrel
wound armature showing arrangement of coils. The core is built up of
thin discs of soft annealed steel, which are slotted to allow the
wire to sink below the surface, this being sometimes called _iron
clad construction_. The discs are held by end plates, clamped without
through bolts. The coils are machine formed of round ribbon or bar
copper depending on the size and purpose of the machine, being without
joint except at the commutator. They lie in insulated troughs, the
upper layers being insulated from the lower layers by fibre.]
=Armature Windings.=--The subject of windings has been fully
treated from the theoretical point of view in chapter XVIII. It remains
then to explain the different methods employed in the shop and the
mechanical devices used to construct the scheme of winding adopted.
=Ques. What is the construction of the inductors?=
Ans. They are made of copper; the ordinary form consists of simple
copper wire, insulated with a double or triple covering of cotton, and
in some cases copper bars are used for large current machines.
=Ques. What is the objection to copper bars?=
Ans. They are liable to have eddy currents set up in them as
illustrated in fig. 291.
[Illustration: FIG. 368.--Holzer Cabot iron clad band wound
armature complete; view showing openings for ventilation. The advantage
of the form of winding adopted, is the ease with which a coil may be
replaced in case of injury and the additional cooling surface. The
coils are held in place by maple wedges secured by binding wires which
are soldered throughout their length.]
=Ques. What may be said with respect to the sizes of wire used for
inductors?=
Ans. Wire larger than about number 8 B and S gauge (.1285 inch
diameter) is not easily handled, hence for large inductors, two or more
wires may be wound together in parallel.
According to the mechanical features and manner of assembling on the
core, drum windings may be divided into several classes, as follows:
1. Hand winding;
2. Evolute or butterfly winding;
3. Barrel winding;
4. Bastard winding;
5. =Former= winding.
=Hand Winding.=--The first windings were put on by hand and proved
objectionable on account of the clumsy overlapping of the wires at the
ends of the armature, which stops ventilation and hinders repairs,
while the outer layers overlying those first wound, bring into close
proximity inductors of widely varying voltage. The method is still used
in special cases and for small machines. Such a winding has rarely, if
ever, been made with one continuous wire.
[Illustration: FIGS. 369 and 370.--Evolute and "straight
out" connectors. In small machines the connectors must be curved as
in fig. 369, but in large machines, especially where the teeth are
wide, they may be straight as in fig. 370. These connectors may take
either of the following forms: 1. _involute or evolute connectors_--An
involute is the curve drawn by the extremity of a piece of string which
is unwound from a cylinder; 2. _spiral connectors_--These consist of
double spirals, the commutator being usually connected to the junction
of the two spirals. These connectors are also known as "butterfly"
connectors.]
=Evolute or Butterfly Winding.=--This mode of winding, was
introduced by Siemens for electroplating dynamos to overcome the
objections to hand winding. It takes its name from the method of
uniting the inductors by means of spiral end connectors as shown in
fig. 374, also in figs. 369 and 370, which show more modern forms.
[Illustration: FIG. 371.--Holzer Cabot armature; rear view
showing back head and coil guard. The construction of core and winding
is described in fig. 367. The shaft is of crucible steel ground to
gauge. The commutator segments are of drop forged copper in the smaller
and hard-drawn copper in the larger sizes. The insulating material
between the segments is mica. On the larger sizes, the commutator shell
is fitted with a thread and mounted on a spider. This construction
provides openings between the commutator and shaft for ventilation.]
=Ques. What are evolute connectors?=
Ans. The fork shaped strips used to connect bars at different positions
on the armature, as shown in fig. 369.
In large machines, especially where the teeth are wide, these
connections may be straight, but in small cc machines they must
be curved in the manner shown in the upper part of the figure,
as the room available may diminish by as much as half, as the
lowest point is reached, and the room occupied by the strip
is the width of a horizontal section at various points. This
width, in the case of the straight connections, is constant.
In place of the wooden block, used in early machines, for
fastening the middle part of the connectors, they may be
anchored to an insulated clamping device built up like a
commutator and for that reason called a _false commutator_.
[Illustration: FIGS. 372 and 373.--_Barrel_ and _evolute_
windings; end views showing placement of coils. When all the coils are
wound on the former, the placing of them on the armature is a simple
matter. After insulating the slots, the winder begins at any convenient
slot, and inserts the coils as shown. Before he can fill all the slots,
some of the first coils must be raised and the last ones inserted
underneath. There is not much difference between barrel and evolute
winding and one style may be used at one end of the armature and the
other at the =opposite end=.]
=Ques. How are the inductors arranged in evolute winding?=
Ans. In fig. 373, it will be seen that the ends of the evolute
connectors lie in two planes, hence the inductors must project to
different distances beyond the core. Accordingly, one long and one
short bar may be conveniently placed in each slot, side by side. In
large machines, especially where the teeth are wide, the connectors
may be straight as in fig, 370. Evolute connectors may be used for
either lap or wave windings.
[Illustration: FIG. 374.--Siemens' bar armature; end view.
Each inductor in the form of a bar is connected to the next by means
of two evolute spiral copper strips, one bending inwardly, the other
outwardly, their junction being in some cases secured to a block of
wood upon the shaft. Their outer ends are attached to the bars by
rivets or silver solder.]
=Barrel Winding.=--This is a form of drum winding _in which the
inductors are arranged in two layers and carried out obliquely on an
extension of the cylindrical surface of the drum to meet and connect
with radial risers_.
[Illustration: FIGS. 375 and 376.--Single layer and double
layer barrel winding. Barrel winding is a method of arranging the ends
of armature coils as they pass from one pole to the next, in which,
instead of using _involute_ or _butterfly_ connections, V-shaped end
connections are used which lie on a cylindrical surface, which is a
continuation of the armature surface. The coil _ends_ must of necessity
be arranged in two layers, but the method may be used for either one
or two coils per slot, the difference in arrangement for these is here
illustrated.]
Barrel winding has been very widely adopted. Although it
involves an increased length of armature, this gives additional
cooling surface and provides for good ventilation.
In barrel winding, the coil ends must of necessity be arranged
in two layers but the method may be used for either one or two
coils per slot, the difference in arrangement for these two
cases being shown in figs. 375 and 376. In the single layer
barrel winding, fig. 375, each slot is occupied by but one side
of one coil. In the double layer barrel winding, fig. 376, the
opposite sides of two separate coils occupy space in the same
slot. The coils, on emerging from the slots bend in opposite
directions, and if one side of a coil occupy the bottom portion
of a slot, its other side usually occupies the top portion of a
slot distant from the first slot by the polar pitch.
[Illustration: FIG. 377.--Westinghouse barrel wound armature.
The coils are former wound from copper strap and are interchangeable.
In the larger size machine they are of the single type. The
illustration shows plainly the characteristic feature of barrel
winding, namely the oblique end connectors carried out on the extended
drum.]
=Bastard Winding.=--In this type of winding, the end connectors
project from the inductors in straight lines parallel to the shaft and
then are bent inward. It has the effect of being somewhat shorter than
the barrel winding. In order to secure better ventilation, it is usual
to combine a bastard winding at the rear end of the armature with a
barrel winding at the commutator end. This class of winding is used
only with bar armatures.
[Illustration: FIG. 378.--Rear end of Westinghouse wave-barrel
wound armature; view showing ventilation.]
=Former Winding.=--This relates to a method of winding coils,
and not to any particular type; that is, mechanical winding as
distinguished from hand winding. While hand winding is necessary for
ring armatures, a drum armature is wound better and more easily by the
aid of machinery.
=Ques. What is a "former" coil?=
Ans. A former coil, as its name suggests, is one that is wound complete
upon a former before being placed upon the armature.
=Ques. What is the advantage of this method of winding coils?=
Ans. By the use of formers much time is saved, thus reducing the cost,
and also by their use all the coils are symmetrical which improves the
appearance of the finished winding.
[Illustration: FIGS. 379 and 380.--Diagrams illustrating lap
and wave barrel windings.]
=Ques. How is the required shape of the template or former for
winding the coils determined?=
Ans. By winding one coil on the armature in order to ascertain its
dimensions and shape; it is then removed from the armature and used as
a pattern in constructing the former.
=Types of Former Coil.=--Of the numerous shapes of former coil,
mention should be made of:
1. Evolute coils;
2. Straight out coil
=Ques. Describe the evolute type of former coil.=
Ans. The evolute coil is wound around eight pins inserted in a board as
shown in fig. 381. The required number of turns are taken around these
pins and their ends G and H left projecting. The coil thus formed is
now covered with tape and after removal from the board, is put into a
clamp at C and F, and opened up as shown in fig. 382, which is the form
required for insertion in the proper slots of the armature.
[Illustration: Fig. 381.--Method of winding evolute coils. In preparing
the former, it is necessary to know the dimensions of the coil, hence,
a pattern coil must first be made, from which the spacing of the pins
can be taken so that the completed coil will fit into the slots for
which it is intended. After the pins have been properly spaced on the
board, the wire is wound around them as indicated, as many turns being
taken as decided on for each coil. When the coil is thus completely
wound, it is taken from the pins, and the lower ends, C and F, placed
in a suitable clamp. The two halves of the coil are then spread apart,
the coil assuming the shape illustrated in fig. 382.
Fig. 382.--Appearance of an evolute former wound coil opened out. The
points A, B, C, etc., correspond to similar points in fig. 381.]
=Ques. What is the peculiarity of the evolute coil?=
Ans. The two sides of the evolute coil have unequal dimensions. The
part marked AB, in fig. 381 which is an upper layer inductor is longer
than the part DE, which constitutes a lower layer inductor. The
portions DC and EF act as parts of an inner layer of evolutes, and
the portions AF and BC as parts of an outer layer of evolutes. These
features are shown in fig. 382.
=Ques. How are evolute coils placed on the core?=
Ans. They are placed in position as shown in figs. 372 and 373,
continuing around the core until all the slots are filled. To complete
the operation it is necessary to raise some of the first laid coils and
insert the last ones below them. The winding is thus completed and is
symmetrical.
[Illustration: FIG. 383.--Westinghouse combination bastard and
barrel winding. A bastard winding at the rear end is combined with a
barrel winding at the commutator end, as shown in the illustration, to
secure better ventilation.]
=Ques. Describe the method of winding the "straight out" type of
former coil.=
Ans. The straight out coil may be wound on a former such as shown in
fig. 384. This consists of a board having four upright pins, A, B, D,
E, properly spaced and two horizontal pins C, F, attached to extensions
at each end of the board. A coil of the required number of turns is
wound around these pins and then opened out as in fig. 385. After
varnishing and baking it is ready to be placed on the armature.
=Ques. For what class of winding are straight out former coils
suitable?=
Ans. For barrel winding.
[Illustration: FIG. 384.--Method of winding "straight out"
coils. There are several ways of making these coils. A former may be
prepared, as shown in the figure, with a board having inserted four
pins, and having attached two blocks at the ends carrying horizontal
pins as shown. Around the several pins, the coil is wound to the
required number of turns and taped. This coil differs from the evolute
coil in that the two halves are of equal size, the parts which act
respectively as upper and under inductor being of equal length. The
coil as shown is suitable for wave winding.]
[Illustration: FIG. 385.--Appearance of straight out coil
after being opened out. In opening out the coil, the ends C and F are
put into a clamp and twisted at right angles to the plane of the coil.
The letters correspond to the points indicated in fig. 384.]
=Ques. How are straight out coils placed on the core?=
Ans. In the same manner as described for evolute coils; when in
position straight out coils appear as in fig. 372.
=Ques. What is the approved method of putting tape on a coil?=
Ans. Considerable time is saved by the use of a machine designed for
the purpose, such as shown in fig. 387.
[Illustration: FIG. 386. Another and simpler method of winding
a "straight out" coil. A board with only two pins is employed as shown;
this plan, however, gives more trouble in the subsequent opening out of
the coil.]
The construction of these machines is such that a roll of tape
placed on a split metal ring is revolved around the coil to
be taped, the coil being gradually moved until it is entirely
covered.
=Coil Retaining Devices.=--In the operation of a dynamo there are
two forces which tend to throw the inductors out of position:
1. Armature drag;
2. Centrifugal force.
Both of these forces are present with smooth core armatures, but only
centrifugal force with slotted armatures. The devices used to hold the
inductors in position against these forces are:
1. Driving horns;
2. Binding ribbons;
3. Retaining wedges.
=Ques. What are driving horns?=
Ans. They are simply pins or strips projecting from the surface of a
smooth core as shown in fig. 251.
[Illustration: FIG. 387.--Armature coil taping machine.
Numerous machines have been invented for taping armature coils. They
consist essentially of a device which revolves a roll of tape around
the coil, in such a direction that the tape is unwound from the roll
and rewound on the coil. The speed at which the coil is fed through the
machine will determine the overlapping of the tape.]
=Ques. What other kinds of retainer are used on smooth core
armatures?=
Ans. They require several binding ribbons or brass bands placed around
the winding to prevent the inductors being thrown off the core by
centrifugal force.
=Ques. With slotted armatures what provision must be made for
retaining the inductors in position?=
Ans. Retaining wedges must be inserted into the notches or between the
projecting tops of the teeth.
[Illustration: FIG. 388.--Front view of large armature for
direct connected dynamo, built by the General Electric Co.]
=Ques. How are the wedges made?=
Ans. They are usually made of well baked hard wood, such as hornbeam,
or hard white vulcanized fibre. Sometimes a springy strip of German
silver is used.
CHAPTER XXIII
MOTORS
An electric motor is just the reverse of a dynamo; it is _a machine for
converting electrical energy into mechanical energy_.
The electrical energy delivered by the dynamo must be obtained from
a steam engine, gas engine, or other power; the mechanical energy
obtained from the motor comes from the energy of the current flowing
through its armature.
=Ques. What is the construction of a motor?=
Ans. It is constructed in the same manner as a dynamo.
Any machine that can be used as a dynamo will, when supplied
with electrical power, run as a motor, and conversely, a motor
when driven by mechanical power, will supply electrical energy
to the circuit connected to it. Dynamos and motors, therefore,
are convertible machines, and the differences that are found
in practice are largely mechanical; they arise chiefly from
the conditions under which the motor must work. Hence, the
study of the motor begins with a knowledge of the dynamo, and
accordingly the student should understand thoroughly all the
fundamental principles of the dynamo, as already given, before
proceeding further with the study of the motor.
=Principles of the Motor.=--All the early attempts to introduce
motors failed, chiefly because the law of the conservation of energy
was not fully recognized. This law states that _energy can neither be
created nor destroyed_.
Early experimenters discovered, by placing a galvanometer in a circuit
with a motor and battery, that, _when the motor was running, the
battery was unable to force through the wires so strong a current as
that which flowed when the motor was standing still_. Moreover, the
faster the motor ran, the weaker did the current become.
[Illustration: FIG. 389.--Conductor, lying in a magnetic
field and carrying no current; the field is not distorted whether the
conductor be at rest or in motion.]
=Ques. Why does less current flow when the motor is running than when
standing still?=
Ans. Because the motor, on account of its rotation acts as a dynamo and
thus tends to set up in the circuit a _reverse electromotive force_,
that is, an electromotive force in opposite direction to the current
which is driving the motor.
=Ques. What is the real driving force which causes the armature of a
motor to rotate?=
Ans. _The propelling drag_, that is, the drag which the magnetic field
exerts upon the armature wires through which the current is flowing, or
in the case of deeply toothed cores, upon the protruding teeth.
=The Propelling Drag.=--In fig. 389 is shown the condition which
prevails when a conductor carrying no current is placed in a uniform
magnetic field. The magnetic lines pass straight from one pole to the
other. The field is not distorted whether the conductor be at rest or
in motion, so long as there is no flow of current. This represents
the condition in the air gap of a motor or dynamo, when no current is
flowing in the armature.
=Ques. What happens when a current flows in the conductor of fig.
389.=
Ans. It sets up a magnetic field of its own as shown in fig. 390.
=Ques. What is the effect of this magnetic field?=
Ans. It distorts the original field (fig. 389) in which the conductor
lies, making the magnetic lines denser on one side and less dense on
the other as in fig. 390.
=Ques. What is the nature of these distorted magnetic lines?=
Ans. They tend to shorten themselves to their original form of straight
lines.
=Ques. What effect has this on the conductor?=
Ans. It produces a force on the conductor tending to push it in the
direction indicated by the arrow, fig. 390.
[Illustration: FIG. 390.--Conductor carrying a current in a
magnetic field. The current flowing in the conductor sets up a magnetic
field which distorts the original field as shown, making the magnetic
lines denser on one side and less dense on the other. This results in a
force upon the wire, which, in the case of a dynamo (fig. 391) opposes
its movement, and which forms the _propelling drag_ in the case of a
motor (fig. 392).]
The distorted magnetic lines may be regarded as so many rubber
bands tending to straighten themselves; The result then is
clearly to force the conductor in the direction indicated.
According to Lenz' law, the direction of the current in the
armature of a dynamo is such as to oppose the motion producing
it. When the armature of a dynamo is rotated, the bending of
the lines of force of the main magnetic field due to armature
reaction acts as a drag against the motion of the armature.
Armature reaction increases with the increase of the armature
current. Therefore, the effect of the drag increases with the
increase of load and requires an additional expenditure of
power to drive the armature.
In a motor, the direction of the actuating current is
the reverse of that of the armature current of a dynamo,
consequently, the armature reaction which constitutes a drag,
acting against rotation of the armature of a dynamo, becomes a
pull in the direction of rotation of the armature of a motor
and constitutes its real turning effect or _torque_ which is
used at the pulley to do mechanical work. The greater the load
applied to the motor, the greater will be the amount of current
taken from the supply mains, and consequently, the greater the
_torque_.
[Illustration: FIGS. 391 and 392.--Action of the magnetic
force in a dynamo and motor. In the first instance, according to
Lenz' law, the direction of the current induced in the wire is such
as to oppose the motion producing it. In the operation of a motor,
the current supplied in flowing through the armature winding distorts
the field and thus produces rotation. In the figures, the direction
of the force is clearly indicated by remembering that the distorted
lines of force act like rubber bands tending to straighten and shorten
themselves.]
=Ques. What are the essential requirements of construction in a
motor?=
Ans. They are: 1, a magnetic field, 2, conductors placed perpendicular
to the field, and 3, provision for motion, of the conductors across the
field in a direction perpendicular to both themselves and the field.
=The Reverse Electromotive Force.=--When an electric current flows
through some portion of a circuit in which there is an electromotive
force, the current will there either receive or give up energy,
according to whether the electromotive force acts _with_ or _against_
the current.
[Illustration: FIG. 393.--Force exerted on a current carrying
conductor placed across a magnetic field. Let N, S, be the pole pieces
of an ordinary electromagnet, having their faces flat and with only a
narrow air gap between. In this gap is stretched the vertical copper
wire A B, kept taut by a strong spring at A; current can be passed into
the wire from the leads C and D. Attached to the wire in the middle of
the gap is a horizontal cord passing over a pulley P and kept taut by
a weight W; the pulley carries a pointer F which moves in front of a
scale _s s_. If the electromagnet be now excited and have the polarity
indicated, it will be found that on passing a strong current _down_
the wire, the index F moves toward the _right_, showing a similar
movement in the wire. The index returns to zero when the current in
the wire ceases, and moves in the opposite direction if the current
in the wire be reversed and sent _up_ instead of down. The experiment
can be further varied by reversing the magnetizing current of the
electromagnet.]
This is illustrated in fig. 395, which represents a circuit
in which there is a dynamo and a motor. Each is rotating
clockwise, and accordingly, each generates an electromotive
force tending upward from the lower to the upper brush.
In both cases the upper brush is positive. In the dynamo,
however, where energy is being supplied to the circuit, the
electromotive force is in the same direction as the current,
and in the motor, where work is being done, the electromotive
force is in the reverse direction to that of the dynamo.
[Illustration: FIG. 394.--Showing relative directions of
armature current and reverse electromotive force of a motor. When a
motor is in operation, the wires around the periphery of its armature
"cut" the magnetic lines of force produced by the field magnet exactly
as in the case of the dynamo. Consequently, an electromotive force
is induced in each wire, as in the dynamo armature. This induced
electromotive force is in opposition to the flow of current due to
the electromotive force of the supply circuit, and tends, therefore,
to keep down the flow of current. The figure shows a single loop
of wire, on the armature core connected directly to the source of
electricity. With current flowing in the loop in the direction
indicated by the arrows marked _c_, a magnetic field is set up in the
direction indicated by the large arrow marked "direction of armature
flux." With the field magnet energized so as to produce a field in the
direction indicated by the large arrow F, the reaction between the
two fields will turn the armature core in the direction indicated by
the arrow R. As the core turns, the upper wire of the loop will cut
the flux under the south pole of the field magnet, and the other side
of the loop will cut the flux under the north pole. The result will
be the induction of a reverse electromotive force in the loop, the
direction being indicated by the small arrows marked _e_. The actual
flow of current in the armature is that due to the difference between
the impressed and reverse voltage; the latter is proportional to the
speed of the armature, the number of armature wires and the strength
of the magnetic field in the air gaps between the armature and the
pole faces. The speed of a motor supplied with current at constant
voltage varies directly with the reverse electromotive force, also with
other conditions fixed, the stronger the field, the slower the speed.
Weakening the field will increase the speed up to the point where the
increase in reverse electromotive force due to the increased speed
cuts down the armature current below the value necessary to give the
requisite pull at the armature periphery. When this point is reached,
any weakening of the field will reduce the speed of the armature. The
pull or _torque_ of a motor armature is directly proportional to the
strength of the magnetic field, and to the strength of the armature
current, the number of armature inductors being fixed. In a field of
constant strength, therefore, the pull of the armature depends on the
amount of current passing through the winding. The torque must be just
sufficient to overcome the load; if in excess, the speed will increase
until the increase of the reverse electromotive force reduces the
current and the increase of speed increases the load to the point of
equilibrium between load and torque. If the torque be insufficient for
the load, the speed will diminish until equilibrium is established,
assuming the motor is running on constant voltage circuit.]
=Ques. Describe similar conditions which prevail in the operation of
a dynamo.=
Ans. When no current is being generated by the dynamo, little power
is required to drive it, but when the external circuit is closed and
current is forced through it against more or less resistance, work is
being done, hence more power is required. In other words, there is
an opposition to the mechanical force applied at the pulley which is
proportional to the electric power delivered by the dynamo. An opposing
reaction or _reverse force_ then is set up in a dynamo when it does
work.
[Illustration: FIG. 395.--Circuit with generator and motor.
Whenever current flows through some portion of a circuit in which
there is an electromotive force, the current will there either receive
or give up energy according to whether the electromotive force acts
_with_ the current or _against_ it. In the figure, the generator and
motor are rotating clockwise, and hence each generates an electromotive
force tending upwards from the lower brush to the higher. In each case
the upper brush is the positive one. In the dynamo, where energy is
being supplied to the circuit, the electromotive force is in the same
direction as the current, while in the motor where work is being done
and energy is leaving the circuit, the electromotive force is in a
direction which opposes the current.]
=Ques. In the operation of a motor what is the nature of the reverse
electromotive force?=
Ans. It is proportional to the velocity of rotation, the strength of
the magnets, and to the number and arrangement of the wires on the
armature, that is, the reverse voltage depends on the _rate_ at which
the lines of force are cut.
[Illustration: FIGS. 396 and 397.--Water and electric
circuits. Diagrams showing comparison between water motor and electric
motor.]
In the diagrams:
The pump corresponds to the dynamo.
The high level pipe " " " positive conductor.
The low level pipe " " " negative conductor.
The valve " " " switch.
The water motor " " " electric motor.
The water pressure (called head) " " " electric pressure
(called voltage).
The flow in gallons per minute " " " amperes.
The size of pipe " " " size of conductor.
The foot pounds " " " watts.
The greater the difference between the height of the two pipes
the higher the pressure, and the greater the difference between
the pressures of the two conductors the higher the voltage.
The larger the diameter of the pipes the less resistance is
offered to the flow of water, and the larger the diameter of
the conductors the less resistance is offered to the flow of
electricity. The more water required by the water wheel, the
more power is required to drive the pump. The more electricity
required by the motor the more power is required to drive the
generator.
[Illustration: FIG. 398.--Fairbanks-Morse standard TR type
motor. This type is built in the smaller sizes and the design is such
that the motor can be installed upon the floor, wall or ceiling, the
bearing yokes being attached to the frame by four equally spaced bolts
so that they can be turned to provide for proper operation of the
oiling devices in either position. A substantial base is provided with
a thrust screw for adjusting the belt tension. This base has clamping
bolts which permit adjusting the position of the motor while suspended.
There is a cast ring type frame having steel side pieces which press
firmly together, the steel laminations making up the pole pieces. The
field coils armature, and armature coils are illustrated in detail in
figs. 399 to 401. The commutator bars are of drawn copper, insulated
with mica. The lugs which extend outward from the bars to receive the
lead wires from the armature windings are formed in one piece with
the bars, and are of the full width of the bars with the insulator
extending outward between them, so that when assembled a solid flange
is formed to receive the armature connections. Self-oiling bearings
are provided and the location of the bearing sleeves in the housing
is adjustable so that the armature may be centered in the magnetic
field. The brush rigging is carried on a skeleton rocker supported in
a groove, turned in the edge of the frame. The brush holders are of
the box type with independently adjustable tension spring for each.
Standard shunt windings are for 115, 230 and 550 volts. The compound
wound motors operate at approximately the same full load speeds as the
shunt wound, but the no load speeds will be about 20 per cent. higher
than the full load speeds. They have, however, the ability to exert a
more powerful starting effort than shunt motors without drawing such a
heavy current from the line, and are, therefore, especially adapted for
driving apparatus that has to be frequently started and stopped under
load and where close speed regulation is not required.]
=Ques. Describe an experiment which shows the existence of a reverse
electromotive force in a motor.=
Ans. The apparatus required consists of a small motor, battery, and
ammeter. They should be connected in one circuit and the deflection of
the ammeter observed when the armature is held stationary, and when it
rotates with various loads.
In an experiment of this kind made on a motor with separately
excited magnets, the following figures were obtained:
Revolutions per minute 0 50 100 160 180 195
Amperes 20 16.2 12.2 7.8 6.1 5.1
Apparently, if the motor had been helped on to run at 261½
revolutions per minute, the current would have been reduced to
zero. In the last result obtained, the current of 5.1 amperes
was absorbed in driving the armature against its own friction
at the speed of 195 revolutions per minute.
[Illustration: FIG. 399.--Fairbanks-Morse field coil and pole
piece. The field coils are wound upon iron forms, each layer treated
with insulating compound. Afterward they are removed from the forms and
baked hard and dry and finally wrapped with insulating materials; all
but the three smaller sizes are wrapped with a protecting cord. The
series and shunt coils of the compound winding here shown are wound
separately, the smaller one being the series coil and the larger the
shunt coil.]
=Ques. Explain the action of the current supplied to a motor for its
operation.=
Ans. The motor current passing through the field magnets polarizes them
and establishes a magnetic field, and entering the armature, polarizes
its core in such a way that the positive pole of the core is away from
the negative pole of the magnetic field, and the negative pole is away
from the positive pole of the magnetic field. The magnetic repulsions
and attractions thus created cause the armature to rotate in a position
of magnetic equilibrium or so as to bring its positive and negative
poles opposite the negative and positive poles respectively of the
magnetic field. It is evident that unless suitable means were provided
to reverse the polarity of the armature core at the instant it reached
the position of the magnetic equilibrium, the armature would not rotate
any further. The construction is such that the polarity of the armature
core, or the direction of the current in the armature coils is reversed
at the proper instant automatically by the commutator, thus giving
continuous rotation.
[Illustration: FIG. 400.--Fairbanks-Morse armature for 7½
H.P., 1300 R.P.M., TR type motor. The armature core is built up of thin
sheet steel laminations with notches in the circumference, which, when
the discs are placed together, form grooves or slots to receive the
armature coils. The armature cores for the larger machines are mounted
on a cast iron spider, which also carries the commutator, making
the two parts entirely self-contained, and with this construction,
it is possible to remove the armature shaft, without disturbing the
core, commutator or windings. Cores of all sizes are provided with
ventilating spaces, running from the surface to the central opening of
the core, so that air is drawn through the core and blown out over the
windings by the revolution of the armature.]
=Direction of Rotation of Motors.=--In the case of either a motor,
or a dynamo used as a motor, the direction in which the armature will
rotate is easily found by the left hand rule, as illustrated in fig.
411, when the polarity of the field magnets and the direction of
currents through the armature are known.
=Ques. How may the rotation of a motor be reversed?=
Ans. By reversing either the current through the fields, or the current
through the armature.
=Ques. What will happen if both currents be reversed?=
Ans. The motor will run in the same direction as before.
[Illustration: FIG. 401.--Fairbanks-Morse wire wound armature
coils. These coils are form wound and are thoroughly insulated and
baked before assembling in the slots. Material of great mechanical
strength as well as high insulating value is used, and the coils
are subjected to dippings in insulating compound and to bakings,
thus driving out all moisture and making a coil which is practically
waterproof and which will withstand rough handling. These coils, when
completed, are placed in the slots, where they are retained by bands on
the three smaller sizes and by hardwood wedges on the larger sizes.]
=Ques. What is the effect of supplying current to a series dynamo?=
Ans. It will run in a direction opposite to its motion as a dynamo.
=Ques. What is the result of reversing the direction of current at
the terminals of a series motor?=
Ans. It will not change its direction of rotation, since the current
still flows through the armature in the same direction as through the
field.
[Illustration: FIGS. 402 to 410.--Diagrams showing relative
direction of rotation of motors and dynamos. From figs. 391 and 392, it
is seen that the direction of the current in a motor armature must be
such as will increase, by the flux it produces, the intensity at the
_leading polar edge_ and decrease the intensity at the _trailing polar
edge_. In a dynamo, the armature has to be moved by mechanical force,
against a magnetic force, hence the leading polar edge is weakened,
while the trailing edge is strengthened. The magnetomotive force in a
motor armature is, therefore, opposed to the direction of that in a
generator armature, when the direction of rotation and the direction of
the field magnetomotive force are the same. Upon this depends all the
relations existing between the direction of rotation of a machine when
acting as a motor or as a dynamo.]
=Ques. What is the behavior of a shunt dynamo when used as a
motor?=
Ans. Its direction of rotation remains unchanged.
=Ques. Why is this?=
Ans. Because if the connections be such that the current supplied will
flow through the armature in the same direction as when the machine is
used as a dynamo, the current through the field will be reversed, since
the field windings are in parallel with the brushes.
[Illustration: FIG. 411.--The "left hand rule" for direction
of motion in motors. _Place the left hand_, as shown, _so that the
thumb points in the direction of the current, the 3rd, 4th and 5th
fingers in the direction of the lines of force, then will the 2nd or
forefinger, at right angles to the others, point in the direction in
which the conductor is urged_.]
=Armature Reaction in Motors.=--In the operation of a motor the
reaction between the armature and field magnets distorts the field in a
similar manner as in the operation of a dynamo. A current supplied from
an outside source magnetizes the armature of a motor and transforms it
into an electromagnet, whose poles would lie nearly at right angles
to the line joining the pole pieces, were it not for the fact that
_negative_ lead must be given to the brushes.
[Illustration: FIG. 412.--Principle of the electric motor
as illustrated by experiment showing effect of a magnetic field on a
wire carrying an electric current. Let a vertical wire _ab_ be rigidly
attached to a horizontal wire _gh_, and let the latter be supported
by a ring or other metallic support as shown, so that _ab_ is free
to oscillate about _gh_ as an axis. Let the lower end of _ab_ dip
into a trough of mercury. When a magnet is held in the position shown
and a current from a cell is sent through the wire as indicated, the
wire will move in the direction shown by the arrow _f_, that is, at
right angles to the direction of the lines of magnetic force. Let the
direction of the current in the wire be reversed, then the direction
of the force acting on the wire will be found to be reversed also. The
conclusion is that _a wire carrying a current in a magnetic field tends
to move in a direction at right angles both to the direction of the
field and to the direction of the current_. The relation between the
direction of the magnetic lines, the direction of the current, and the
direction of the force, is often remembered by means of the following
rule, known as the _motor rule_, and which differs from the dynamo rule
only in that it is applied to the fingers of the _left hand_ instead
of to those of the right. _Let the forefinger of the left hand point
in the direction of the magnetic lines of force and the middle finger
in the direction of the current sent through the wire, then will the
thumb, at right angles to the other two fingers, point in the direction
in which the wire is urged._]
Negative lead is the amount of backward advance of the brushes
against the direction of the rotation of the armature, measured
in degrees from the neutral plane.
If the brushes be given positive lead, that is, placed in
advance of the neutral plane _in the direction of rotation_,
the cross magnetizing force is converted into one that tends
to increase that of the field magnet, while if they be given
negative lead, it tends to demagnetize the field magnet.
Since with positive lead the armature polarity strengthens that
of the field magnet, it is possible, disregarding sparking,
to operate a motor without any other means being taken to
magnetize the field magnets, because the armature will induce a
pole in the field magnet and then attract itself towards this
induced pole.
=Ques. What effect has the cross magnetizing force on the field?=
Ans. It tends to shift the field around in a direction opposite to that
of the rotation.
[Illustration: FIG. 413.--Current commutation in a motor.
Considering the coil W which is ascending, current is flowing through
it from the top brush, while it is itself the seat of an electromotive
force that tends to stop or reverse its current. The condition for
sparkless commutation requires that during the interval the coil is
short circuited by the brush, the coil should be passing through a
field that is not only sufficiently strong but one that tends to
reverse the direction of its current. The coil is already in such a
field, hence, commutation must take place _before_ it passes put of
this field. To accomplish this the brushes must be shifted backward,
that is, given _negative lead_, to overcome sparking. In other words,
the _commutating plane_ must be shifted _back_ of the neutral plane in
a motor instead of being placed _in advance_ as in a dynamo.]
=Ques. What are the conditions of minimum sparking?=
Ans. The same conditions must obtain as in a dynamo, that is, the
current in the coil undergoing commutation must be brought to rest and
started again in the opposite direction. This involves that while the
coil is short circuited by the brush, it should be passing through a
field that tends to reverse the direction of the current. Since the
coil is already in such a field, the act of commutation must take
place before it passes out of this field. Accordingly, a negative lead
must be given the brushes.
[Illustration: FIG. 414.--Railway motor. This type of motor,
since it must operate under cars, has taken on the peculiar form under
which it is most familiar. As illustrated, the case is of such shape
that compactness and water proofing are secured, and the means of
attachment to the car axle and support from the axle and truck frame
are provided.]
=Method of Starting a Motor.=--Although motors and dynamos are
practically similar in general construction and either one of them
will act as the other when suitably traversed by an electric current,
there are certain differences between the connections and accessories
of a machine operated as generator and one employed as a motor. For
instance, when a machine is operated as a dynamo, it is first driven up
to speed until it has excited itself to the right pressure, and then
it is connected to the circuit; but when a machine is used as a motor
it will not start until it has been connected to the circuit, and this
must not be done until the proper precautions have been taken to ensure
that the current, which will pass through it when so connected, will
not be excessive and thereby result in serious injury to the motor.
For this reason a rheostat or variable resistance, commonly called a
starting box is usually inserted in the armature circuit of a motor to
prevent an undue rush of current before the motor attains its speed,
and subsequently the speed is regulated by the cutting in or out of the
circuit of certain extra resistances which constitute the controller
used on a series motor requiring variable torque at variable speed, as
in the case of elevator or electric traction service.
[Illustration: FIG. 415.--View of railway motor, open. The
frame is of cast steel for lightness, and which serves as magnetic
circuit and protecting case. It is circular or octagonal in form except
in very large motors. Four short magnets project from the case. The
armature is large in order to secure the required torque. It is always
series wound, requiring two brushes. The brush holders are mounted upon
a frame of insulating material which is attached to the upper half of
the case. The brushes are adjustable radially, but usually it is not
necessary to provide for shifting as they remain in the neutral plane.
In motors which receive so little attention as these, special attention
must be given to the design of devices for keeping oil and grease
out of the case. These would injure the insulation of the coils and
produce sparking at the commutator. Oil rings are, therefore, placed
on the shaft, and these discharge into chambers connected to the oil
wells or allow the oil to overflow on the track. The bearings are made
self-oiling or self-greasing by means of rings or wicks and will run
for weeks without attention.]
=Classes of Motor.=--Motors are classified in the same manner as
dynamos. The fields may be either bipolar or multi-polar, and with
respect to the type of armature winding employed, motors are classed as:
1. Series wound;
2. Shunt wound;
3. Compound wound.
[Illustration: FIG. 416.--Series motor connections. A series
motor on a constant voltage circuit does not have a constant field
strength, and does not run at uniform speed. If the load be taken
off it will run at excessive speed. To start the motor, the circuit
is completed through a variable resistance or rheostat by moving the
switch S so that the resistances R, R_{1}, R_{2}, R_{3}, are gradually
cut out of the circuit. To stop, the switch S is moved back to its
"off" position.]
=Series Motors.=--A series motor is one in which the field magnet
coils, consisting of a few turns of thick wire, are connected in series
with the armature so that the whole current supplied to the motor
passes through the field coils as well as the armature. Fig. 416 is a
diagram of a series motor showing the connections and rheostat.
=Ques. What are the characteristics of a series motor?=
Ans. The field strength increases with the current, since the latter
flows through the magnet coils. If the motor be run on a constant
voltage circuit, with light load, it will run at a very high speed;
again, if the motor be loaded heavily, the speed will be much less than
before.
[Illustration: FIG. 417.--General Electric type CL-B motor
for slow and moderate speeds. It is of multipolar construction, having
six pole pieces. The advantages of slow speed machinery are generally
understood, and in motors the additional outlay to secure slow speeds
is warranted, inasmuch as it results in diminished wear and friction
losses in gearing, belting, bearings, and commutators, and decreased
brush renewals. The comparatively slow speeds of these motors are of
importance in that they permit belting or gearing the motors directly
to ordinary slow speed line shafting without employing intermediate
counter shafting. When motors are geared to heavy duty machines, it is
considered better practice to supply an outboard bearing to take up
the additional strain that would otherwise be put on the gearing and
bearing.]
=Ques. For what kinds of service are series motors unsuited?=
Ans. Series motors should not be employed where the load may be
entirely removed because they would attain a dangerous speed. They
should not be used for driving by means of belts, because a sudden
release of the load due to a mishap to the belt would cause the motor
to "run away."
Very small series motors may be used with belts since their
comparatively large frictional resistance represents an
appreciable load, restraining the motor from reaching a
dangerous speed.
=Ques. For what service are series motors adapted?=
Ans. For gear drive.
In the case of a sudden release of the load the gears provide
some load on account of the frictional resistance of the gear
teeth.
[Illustration: FIG. 418.--Shunt motor connections. A shunt
motor runs at constant speed on a constant voltage circuit. In
connecting the motor in circuit, the field coils must be placed in
circuit first, so that there is a certain amount of field strength to
produce rotation of the armature and thus prevent excessive current
through the armature. If the field magnets were not put in the circuit
first, the armature, at rest on receiving current, would probably burn
out, because it is of low resistance, and would take practically all
the current supplied, especially since no reverse voltage is generated
in the armature at rest. The method of starting is shown in the
illustration. To start, the switch is closed, and the rheostat lever
pushed over so as to make contact with A and B, thus _first_ exciting
the magnets. On further movement of the lever, the rheostat resistances
R, R_{1}, R_{2}, R_{3}, etc., are gradually cut out as the speed
increases, until finally all the resistance coils are cut out. To stop,
the lever is brought back to its original position.]
=Ques. What advantage is obtained with series motors with respect to
the connections?=
Ans. A single wire only proceeds from the rheostat to the motor, so
that, with the return wire, only two wires are required.
=Ques. For what service are series motors specially adapted?=
Ans. Series motors are used principally for electric railways,
trolleys, and electric vehicles, and similar purposes where an
attendant is always at hand to regulate or control the speed. They
are also used on series arc light circuits in which the current is of
constant strength. Very small motors are generally provided with series
windings.
[Illustration: FIG. 419.--Speed regulation of a shunt motor.
The speed of a motor depends on the voltage of the current supplied and
the field strength. The motor tends to rotate so fast as to produce
a _reverse voltage_ nearly equal to that supplied to the brushes;
hence, the speed varies with the voltage supplied. By decreasing this
voltage then, the speed is decreased. Accordingly, the speed may be
reduced by inserting, by means of a rheostat, a resistance in series
with the motor. By inserting this resistance in the field circuit,
the voltage at the terminals of the motor is lowered, thus giving the
condition necessary to reduce the speed. The arrangement for speed
regulation shown in the figure includes a starting regulator and a
shunt regulator.]
=Shunt Motors.=--A shunt motor may be defined as one in which
the field coils are wound with many turns of comparatively fine wire,
connected in parallel with the brushes. The current then is offered
two paths: one through the armature, and one through the field coils.
[Illustration: FIGS. 420 to 422.--Reversing the direction of
rotation of a series motor. Fig. 420 shows the connections for counter
clockwise rotation. The motor may be reversed: 1, by allowing the
current to flow in its original direction (from D to C) in the field
magnet coils, and altering the direction of the armature current by
changing the two connections on the brushes A and B, thus connecting
C to A and B to the return wire as in fig. 421, or 2, by leaving the
direction of the current in the armature in its original direction,
and reversing that of the field current, as in fig. 422. If the wires
leading to the rheostat and motor directly, were reversed there would
be no reversal of the motor, because by so doing, both the armature and
field magnet currents would be reversed.]
[Illustration: FIGS. 423 to 425.--Reversing the direction of
rotation of a shunt motor. Fig. 423 shows the connections for counter
clockwise rotation. The motor may be reversed: 1, by allowing the
current to flow in its original direction through the field magnet
coils (from D to C), and reversing its direction through the armature
(from A to B) as in fig. 424, or 2, by allowing the armature current to
flow in its original direction (from B to A) and reversing the current
through the field coils (from C to D) as in fig. 425.]
=Ques. What may be said with respect to the speed of a shunt
motor?=
Ans. It is practically constant with varying loads.
The variation of speed ranges from 1/10 to 5 per cent., except
in the case of small motors, in which the variation may be much
greater.
=Ques. How should a shunt motor be started?=
Ans. To properly start the machine, the field coils must be fully
excited.
It is, therefore, necessary to switch the magnet coils
immediately on to the voltage of supply, while a variable
resistance must be provided for the armature circuit. To get
both connections at the same time, rheostats for shunt motors
are arranged as shown in fig. 418.
=Influence of Brush Position on Speed.=--In the case of a shunt
motor supplied with current at constant pressure, the speed is a
minimum when the brushes are in the neutral plane, and the effect of
giving the brushes either positive or negative lead is to increase the
speed, especially with little or no load.
=Ques. Why does the speed increase?=
Ans. When the brushes are shifted from the neutral plane, the reverse
voltage between the brushes is decreased, speed remaining unchanged.
Accordingly, the pressure in the supply mains forces an increased
current through the armature thus producing an increased armature pull
which causes the speed to increase until the reverse voltage reaches a
value sufficiently large to reduce the current to the value required to
supply the necessary driving torque.
=Compound Motors.=--This type of motor has to a certain extent,
the merits of the series motor without its disadvantages, and is
adapted to a variety of service. If the current flow in the same
direction through both of the field windings, then the effect of the
series coil strengthens that of the shunt coil; this strengthening is
greater, the larger the armature current.
[Illustration: FIG. 426.--Compound motor connections for
starting from a distant point. A compound winding may be used on motors
for many different purposes. If the current flow in the same direction
through both windings, then the effect of the series coil strengthens
that of the shunt coil. This strengthening increases with the load.
Thus the motor gets, at increasing load, a stronger magnetic field, and
will therefore, if the voltage remain constant run slower than before.
Accordingly, for a given current, the starting power will be greater
than that of a shunt motor. With a decreasing load the motor will run
faster. The compound motor has, to a certain extent, the merits of the
series motor without its disadvantages. By means of compound motors
the starting at a distance with only two mains may be effected, just
as in the case of the series motor. The connections are shown in the
diagram. If the motor be regarded as being without the shunt coil, then
it is connected up exactly as the series motor in fig. 416. The current
coming from the starter enters the series coil at F, flows through the
series coil and leaves it at E, flowing from there to the armature
brush B, through the armature to brush A, and from there through the
second main back to the generator. The shunt winding is connected
directly with the armature brushes A and B, and gets at starting,
therefore, only a very small voltage, hence its field is nearly
ineffective. But on account of the series winding, the motor starts
as a series motor. Obviously such a motor will not develop a very
large starting power like a real series motor, for, on account of the
large space occupied by the shunt coils, there is less space available
for the series coils than with a series motor. A compound motor may,
however, even with this arrangement, be easily started, provided the
load on starting be not too heavy. When once running the armature will
produce a reverse voltage and the shunt coil will be supplied with
nearly the full terminal voltage.]
=Ques. Mention some characteristics of the compound motor.=
Ans. Since it is a combination of the shunt and series types, it
partakes of the properties of both. The series winding gives it strong
torque at starting (though not as strong as in the series motor), while
the presence of the shunt winding prevents excessive speed. The speed
is practically constant under all loads within the capacity of the
machine.
=Ques. Describe the connections for starting a compound motor at a
distance.=
Ans. Control at a distance can be effected with only two wires, just
as in the case of a series motor. In the diagram fig. 426, the current
coming from the rheostat enters the series coil at F, and leaves it
at E, thence it flows to the armature brush B, through armature to
brush A, and from here back to the dynamo. The shunt winding, which is
connected across the brushes, gets a very small voltage at starting
and is accordingly very ineffective. The motor then starts as a series
motor. The starting effect is smaller than in a series motor because
of the fewer turns in the series winding, most of the available space
being occupied by the shunt coils.
=Power of a Motor.=--The word "power" is defined as _the rate
at which work is done_, and is expressed as the quotient of the work
divided by the time in which it is done, thus:
work
power = ------
time
The difference between power and work should be clearly understood.
_Work is the overcoming of resistance through a certain distance._ It
is measured by the product of the resistance into the space through
which it is overcome, thus:
work = distance × space
For instance, in lifting a body from the earth against the
attraction of gravity, the resistance is the weight of the
body, and the space, the height to which the body is raised,
the product of the two being the work done.
The unit of work is the _foot pound_, which is _the amount of work done
in overcoming a pressure or weight equal to one pound through one foot
of space_.
The unit of power is the _horse power_ which is equal to 33,000 _foot
pounds of work per minute, that is_:
foot pounds per minute
horse power = ------------------------
33,000
The unit of power was established by James Watt as the power
of a strong London draught horse to do work during a short
interval, and used by him to measure the power of his steam
engines.
In order to measure the mechanical power of a motor, it is necessary
to first determine the following three factors upon which the power
developed depends:
1. Pull of the armature, in pounds;
2. Distance in feet at which the pull acts from the center of the shaft;
3. Revolutions per minute.
EXAMPLE.--If the armature pull of a motor having a two
foot pulley, be such that a weight of 500 lbs. attached to the
rim, is just balanced, and the speed be 1,000 revolutions per
minute, what is the horse power?
Here, the distance that the pull acts from the center of the
shaft is one foot, hence for each revolution the resistance
of 500 pounds is overcome through a distance equal to the
circumference of the pulley or
π × diameter = 3.1416 × 2 = 6.2832 feet.
[Illustration: FIG. 427.--General Electric type CQ Motor.
These motors range in capacity from 1/6 to 20 horse power. The
small sizes are bipolar, and the larger sizes have four poles. For
installations where the motor is exposed to dust, mechanical injury or
moisture, it may be partially or entirely enclosed by means of hand
hole covers. The standard voltages are 115, 230 and 550.]
The _work done_ in one minute is expressed by the following equation:
{ work } {weight} {circumference} {revolutions}
{ per } = { in } × { of pulley } × { per } = foot pounds
{minute} { lbs. } { in feet } { minute }
= 500 × 6.2832 × 1,000 = 3,141,600.
Hence, the power developed is
3,141,600 ÷ 33,000 = 95.2 horse power.
=Ques. What is "brake" horse power?=
Ans. The net horse power developed by a machine at its shaft or pulley;
so called because a form of brake is applied to the pulley to determine
the power.
=Ques. Describe the apparatus used in making a brake test.=
[Illustration: Fig. 428.--Prony brake for determining brake horse
power. It consists of a friction band ring which may be placed around a
pulley or fly wheel, and attached to a lever bearing upon the platform
of a weighing scale in such a manner that the friction between the
surfaces in contact will tend to rotate the arm in the direction in
which the shaft revolves. This thrust is resisted and measured _in
pounds_ by the scale. In setting up the brake the distance between the
center of the shaft and point of contact (knife edge) with the scales
must be accurately measured, _the knife edge being placed at the same
elevation as the center of the shaft_. An internal channel permits
the circulation of water around the interior of the rim as shown, to
prevent overheating.]
Ans. Tests of this kind are usually made with a Prony brake as shown in
fig. 428. It consists of a band of rope or strip iron--the latter is
the arrangement shown--to which are fastened a number of wooden blocks,
several carrying shoulders to prevent the contrivance from slipping
off the wheel rim. The brake band is drawn tight, as shown, so that
the blocks press against the surface all around. The brake thus formed
is restrained from revolving with the pulley by two arms attached near
the top and bottom centers of the wheels, and joined at the opposite
ends to form a lever which bears upon an ordinary platform scale, a
suitable leg or block being arranged to keep its end level with the
center of the shaft. By this arrangement the amount of friction between
the brake band and the revolving wheel is weighed upon the scales.
Since the brake fits tightly enough to be carried around by the wheel,
but for the arms bearing upon the scale, the amount of frictional
power exerted by the wheel in turning free within the blocks may be
transmitted and measured, just as would be the case were a machinery
load attached, instead of a friction brake.
=Ques. Why must the point of contact of the brake with the scales be
level with the center of the shaft?=
Ans. In order to determine the force acting at right angles to the line
joining the point of contact and center of the shaft.
=Ques. What is the distance between the center of the shaft and point
of contact with the scales called?=
Ans. The lever arm.
=Ques. What three quantities must be determined in a test in order to
calculate the brake horse power?=
Ans. The lever arm, the force exerted on the scales, and the
revolutions per minute.
=Ques. How is brake horse power calculated?=
Ans. From the following formula:
B. H. P. = 2 π L N W
-----------
33,000
in which
B. H. P. = brake horse power;
L = lever arm, _in feet_;
N = number of revolutions per minute;
W = force _in pounds_ at end of lever arm as measured by
scales.
EXAMPLE--In making a brake test on a motor, the lever
arm of the brake is 3 ft., and the reading of the scales is 30
lbs. When the motor is running 1,000 revolutions per minute,
what is the brake horse power?
Substituting the given values in the formula,
2 π × 3 × 1,000 × 30
B. H. P. = --------------------- = 17.1
33,000
Now, if the voltmeter and ammeter readings be 220 and 65 respectively,
what is the efficiency of the motor at this load?
The amount of power absorbed by the motor, or in other words, the
_input_ is
220 × 65
E. H. P. = -------- = 19.16
746
and since the output is 17.1 horse power,
output brake horse power 17.1
efficiency = ------ = ---------------------- = ----- = 89%.
input electrical horse power 19.16
=Speed of a Motor.=--The normal speed at which any motor will run
is such that the sum of the reverse electromotive force and the drop in
the armature will be exactly equal to the electromotive force applied
at the brushes. The drop in the armature is the difference between the
applied voltage and the reverse voltage.
=Mutual Relations of Motor Torque and Speed.=--The character of
the work to be done not only determines the condition of the motor
torque and speed required, but also the suitability of a particular
type of motor for a given service. There are three general classes of
work performed by motors, and these require the following conditions of
torque and speed:
1. Constant torque at variable speed;
Suitable for driving cranes, hoists, and elevators, etc., where
the load is constant and has to be moved at varying rates of
speed.
[Illustration: FIG. 429.--Two path method of speed regulation
of series motor. A rheostat is connected in shunt to the field coils as
shown. The current passing from _a_ to _b_ divides between the magnet
coils and the rheostat coils; the higher the resistance of the rheostat
the less current passes through it, and the more through the magnet
coils, hence the stronger the field magnet.]
2. Variable torque at constant speed;
Suitable for driving line shafting in machine shops, which must
run at constant speed regardless of variations of torque due to
variations in the number of machines in operation at a time, or
the character of work being performed.
3. Variable torque at variable speed.
Suitable for electric railway work. For example: when a car is
started, the torque is at its maximum value and the speed zero,
but as the car gains headway, the torque decreases and the
speed increases.
=Speed Regulation of Motors.=--The speed of motors connected to
constant voltage circuits is usually regulated by the two following
methods:
[Illustration: FIG. 430.--Variable field method of speed
regulation of series motor. The field winding is divided into a number
of sections with leads connecting with switch contact points as
illustrated. The speed then is regulated by cutting in or out of the
circuit sections of the field winding thus varying the strength of the
field.]
1. By inserting resistances in the armature circuit of a shunt wound
motor;
2. By varying the strength of the field of a series motor.
The first method is sufficiently explained under fig. 418 and the
second method is illustrated in fig. 430. The controller switch S is so
arranged that a greater or lesser number of field coils can be inserted
in the field circuit. When the switch arm is on point 1, the motor
current will flow through all the field windings, and the strength
of the field will be at its maximum. When the switch arm is moved so
as to successively occupy positions 2, 3, and 4, thus cutting out of
circuit a greater and greater number of field coils the strength of the
field will be gradually decreased until practically all of the motor
current is led or wired through the armature. Under these conditions,
when the field of a motor is at its maximum strength, the motor torque
will be at a maximum for any given strength of current, and the reverse
electromotive force will also be at a maximum for any given speed,
therefore, when the field strength is increased the speed will decrease
and _vice versa_.
=Ques. What results are obtained by this method of regulation?=
Ans. The speed of a series motor may be nearly doubled, that is, if the
lowest permissible speed of the motor be 250 revolutions per minute it
can be readily increased to 500 revolutions per minute by changing the
field coil connections from series to parallel. It is on this account,
as much as on their powerful starting torque, that series motors have
been until recently almost exclusively employed for electric traction
purposes.
=Series Parallel Controller.=--When two motors are used in
electric railway work, their armatures are connected in series with
each other and an extra resistance which prevents the passage of an
excessive current through the armature before the motor starts. As
the speed of the car increases, the extra resistance is gradually cut
out of circuit and the field winding connections changed from series
to parallel by means of a series parallel controller, which finally
connects each motor directly across the supply mains, or between the
trolley line and the track or ground return.
=Efficiency of a Motor.=--The commercial efficiency of a motor is
the ratio of the output to the input. As a rule, the power developed
by a motor increases as the reverse voltage generated by it decreases,
until this voltage equals one half of the voltage applied at the
brushes. After this point is reached, the power developed by the motor
decreases with the decrease of the reverse voltage. Therefore, a motor
performs the largest amount of work when its reverse voltage is equal
to one half the impressed voltage.
[Illustration: FIG. 431.--Double throw, double pole switch for
reversing direction of rotation of a motor. The direction of rotation
can be reversed by changing the direction of current in either the
armature or the field coils. It is preferable, however, to reverse the
direction of rotation by changing the direction of current through
the armature. The switch is wired as shown, means of reversal being
provided by running the wires as indicated by the dotted lines.]
The efficiency of a motor as just stated is the ratio of the output to
the input; this is equivalent to saying that the efficiency of a motor
is equal to the brake horse power divided by the electrical horse power.
The electrical horse power is easily obtained by multiplying the
readings taken from volt meter and ammeter, which gives the watts, and
dividing the product by 746, the number of watts per horse power. That
is:
volts × amperes watts
Electrical horse power = --------------- = -----
746 746
[Illustration: FIG. 432.--Wiring diagram, showing electrical
connections between the armature, field, and interpoles of an interpole
motor. As the name implies, an interpole motor has in addition to the
main poles, a series of interpoles which are placed between the main
poles, and whose function is to assist in the reversal of the current
under the brushes. They provide a separate commutating field of a
correct value at all loads and speeds, and their windings are for this
purpose connected in series with the armature. The proper functioning
of the interpoles is independent of the direction of rotation of the
armature, also of the load carried over the whole speed range. In
an ordinary motor without interpoles, commutation is assisted by a
magnetic fringe emanating from the main poles, but as the value of this
fringe is altered by the load of the motor and by rheostatic field
weakening, if higher speeds be desired from such a machine, commutation
becomes imperfect and sparking results, making a readjustment of the
brushes necessary.]
=Interpole Motors.=--An interpole motor has in addition to the
main poles, a series of interpoles, placed between the main poles. The
object of these poles is to provide an auxiliary flux or "commutating"
field at the point where the armature coils are short circuited by the
brush.
[Illustration: FIGS. 433 to 437.--Parts of the type S
interpole motor built by Electro-Dynamic Co. They are as follows: 1.
yoke--commutator view; 2. interpole coil; 3. top R.H. main coil; 4.
bottom R.H. main coil; 5. main pole; 6. interpole; 7. armature shaft,
R.O. bearing; 8. commutator; 9. armature wedge; 10. armature coil;
11. brush ring; 12. brush carrier insulation; 13. brush carrier; 14.
brush guard; 15. carbon brush; 16. brush holder; 17. cross connecting
cable; 18. oil ring; 19. commutator end bearing bushing; 20. pulley end
bearing bushing.]
=Ques. What is the object of the commutating field produced by the
interpoles?=
Ans. Its object is to assist commutation, that is, to help reverse
the current in each coil while short circuited by the brush, and thus
reduce sparking.
[Illustration: FIG. 438.--Interpole motor as built by the
Electro Dynamic Co. This type of motor is devised to prevent sparking
at all loads by the use of interpole magnets, that is, small magnets
placed between the field magnets. The interpoles set up a field in a
direction to stop and reverse the current in the armature coils while
they are short circuited by the brushes.]
=Ques. What is the nature of the commutating field?=
Ans. The excitation of the interpoles being produced by series turns,
the field will vary with the load, and will, if once adjusted to give
good commutation at any one load, keep the same proportion for any
other load, provided the iron parts of the circuit be not too highly
saturated.
=Ques. State briefly how sparking is reduced or prevented by the
action of the interpoles.=
Ans. Sparking is due to self induction in the coil undergoing
commutation, which impedes the proper reversal of the current. The
action of the interpoles corrects this in that they set up a field in
a direction that causes a reversal of the current in the coil while it
is short circuited. Thus, the coil at the instant it leaves the brush,
is not an idle coil, but has a current flowing in it in the right
direction to prevent sparking.
=Ques. Mention some of the claims made for interpole motors.=
Ans. Constant or adjustable speed, and momentary overloads without
sparking; constant brush position; operation at adjustable speeds on
standard supply circuits of 110, 220, and 500 volts; constant speed
with variable load; reversal without changing the position of the
brushes.
CHAPTER XXIV
SELECTION AND INSTALLATION OF DYNAMOS AND MOTORS
=General Conditions Governing Selection.=--In any particular
case, the voltage, current capacity, and type of dynamo selected will
depend upon the system of transmission or distribution to which it is
to be connected, and the character of the work which it is required to
perform. The suitability of the different types of dynamo for various
kinds of work has already been considered to some extent, but there are
certain general conditions which are applicable to almost all cases,
such as:
1. Construction;
2. Operation;
3. Cost;
4. Number and size of units.
=Construction.=--This should be as _simple_ as possible and of the
most solid character. All parts should be interchangeable, and have a
good finish. All machines should be provided with eye bolts or other
means by which they can be lifted or moved, as a whole or in parts,
easily and without injury. These features are so carefully attended to
and guaranteed by the manufacturers as to leave little choice in this
direction.
=Operation.=--The considerations relating to the operation of a
machine involve an examination of the details of its construction,
in order to determine the amount of attention it will require, the
character of its regulating device, its _capacity_, _form_, and
_weight_.
=Ques. What may be said regarding capacity?=
Ans. Dynamos and motors should not be overloaded, because the
efficiency is greater when the working load does not exceed the rated
capacity of the machine.
=Form.=--As a rule, there is not much choice in the matter of form
between standard machines, as they are uniformly symmetrical, well
proportioned and compact. It is a mistake, however, to select a light
machine for stationary use, as the weight of a machine increases its
strength, stability and durability.
=Cost.=--In some cases, the matter of first cost is important
and deserves careful consideration. It should be remembered, however,
that high grade electric machinery cannot be built out of low grade
materials and with poor workmen; therefore, when necessity compels
the selection of a cheap machine, it should not be expected that its
service will be as satisfactory as that of a first class machine.
=Number and Size of Units.=--The best number and size of units
for an electrical plant is usually governed by the requirements of
the driving engines. As a rule, dynamos and motors are not much less
efficient at quarter load than at full load, and the smaller dynamos
are fully equal to the larger machines in this respect, therefore, a
generating plant can be subdivided, and if so desired, without any
detrimental results except those to a multiplicity of units.
=Ques. What is the important consideration with respect to
efficiency?=
Ans. Efficiency at maximum load is not so important as efficiency at
average load.
For instance, in the diagram, fig. 439, the rated efficiency of
one dynamo as shown by the curve A, is 95 per cent., and that
of another, as shown by curve B, is 91 per cent., but it will
be observed that the average efficiency of B is much higher,
being 75 per cent. at quarter-load, 89 per cent. at half-load,
and 91 per cent. at three-quarter load, to 55, 77 and 89 per
cent. of A, at the corresponding loads. In this case, A is
higher than B only at full load, and as full load is a limit
which should not be reached except in special cases, and then
only for short intervals of time, the service rendered by B
would be much more satisfactory in the long run. In order to
avoid the difficulties possible under these conditions, a
guarantee to carry 25 per cent. overload for two hours without
injury should be required, and either this or the rated load be
taken, as the full load, so as to give a factor of safety of 25
per cent.
[Illustration: FIG 439.--Efficiency curves for 100 K. W.
dynamos. The efficiency of a dynamo at maximum load is not so important
as at average load. For instance, if in the figure the curve O B C
represent the efficiency of a 100 K. W. dynamo and O A D, that of
another machine, it would be in accordance with common practice to
compare them at rated load, at which the efficiency of the first is
only 91%, while the other is 93%. The first machine, however, is far
better than the second, since its average efficiency is much higher,
being nearly 91% between half load and 25% overload. It should be noted
that full load is a limit which should be but occasionally reached, and
then only for short periods of time.]
=Ques. Upon what does the choice of field winding of a dynamo
depend?=
Ans. The different classes of field winding have already been
discussed, but in general the conditions governing selection are as
follows: The series dynamo is used where a constant current at variable
voltage is desired, as in series arc lamp circuits. A shunt dynamo is
used on constant voltage circuits, where the distance from the machine
to the load is not great, that is, where there is small line loss. With
a compound dynamo there is compensation for line loss, that is, it can
be constructed so that the voltage at its terminals, or at the load can
be maintained constant or allowed to increase or decrease with a change
in load. It can thus operate lamps at constant voltage though they be
located at some distance, or the voltage at the end of the line can be
made to increase with an increase of load, as is frequently the case in
railway work.
[Illustration: FIG. 440.--Holzer-Cabot performance curves of
standard 20 H. P. motor, showing efficiency, speed regulation, and
amperes input.]
=Ques. For what conditions of service are series motors adapted?=
Ans. They are used on constant current circuits, and also on constant
voltage circuits as in railway work and similar purposes where an
attendant is always at hand to regulate the speed.
=Ques. Name some advantages and disadvantages of series motors.=
Ans. They are easily started even under heavy loads, the winding is
cheaper than the other types and the speed is nearer constant than
shunt motors when operated on constant current circuits. When used
on constant pressure circuits, such as is employed for incandescent
lighting, the speed will depend on the load.
=Ques. What kind of circuit is suitable for shunt motors?=
Ans. They are used on constant voltage circuits.
=Ques. What are the advantages of shunt motors?=
Ans. The speed remains nearly constant for variable load.
=Ques. State the disadvantages.=
Ans. They start less easily under a heavy load than do series motors,
and the speed cannot be varied through any wide range without
considerable loss. The shunt motor requires more attention than the
series type and is more liable to be burnt out.
=Location.=--The place chosen for the dynamo or motor should be
dry, free from dust, and preferably where a cool current of air can be
had. It should allow sufficient room for a belt of proper length when a
belt drive is used.
=Foundations.=--It is most important to secure a good foundation
for every dynamo, and great care should be taken to have them entirely
separate from those of the walls of the building in which the machine
is installed, and if the dynamo be directly driven, but not on the same
bed plate as the engine, a foundation large enough for both together
should be laid down. Stone or concrete may be used, or brick built with
cement, having a large thick stone bedded at the top.
For small machines the holding down bolts may be set with lead or
sulphur in holes in the stone top, but for large machines the bolts
should be long enough to pass down to the bottom, where they should be
anchored with iron plates.
=Setting up of Dynamos and Motors.=--In unpacking the machines
care should be taken to avoid injury to any part, and in putting the
parts together, each part should be carefully cleaned, and all the
parts put together in exactly the right way. The shafts, bearings,
magnetic joints, and electrical connection should receive especial
attention and be thoroughly cleaned of every particle of dirt, grit,
dust, metal clippings, etc.
=Ques. Who should preferably assemble the machines?=
Ans. Whenever possible, they should be assembled by someone thoroughly
familiar with the construction; but if the services of such a person
cannot be had, no one should attempt to put a machine together unless
he has a drawing or photograph of the same for a general guide.
=Ques. What precaution should be taken with the armature?=
Ans. It should be handled carefully to avoid any injury to the wires of
the winding and their insulation.
If it become necessary to lay the armature on the ground it
should be laid on clean paper or cloth, but it is better to
support it by the shaft on two wooden horses or other supports,
and thus avoid any strain on the armature body or commutator.
[Illustration: FIG. 441.--Foundation. It may be made either
of concrete, stone or brick. The machinery is held firmly in place
on the foundation by anchor bolts built into it; the proper position
for the bolts are determined by a wooden template suspended above the
foundation as shown. The bolts are surrounded by iron pipe that fixes
them vertically but permits a little side play to allow for any slight
errors in locating the centers on the template.]
=Connecting Up Dynamos.=--The manner in which the connections
of the field magnet coils, brushes, and terminals, are connected to
one another depends entirely upon the type of machine. The field
magnet shunt coils of shunt and compound wound dynamos, are invariably
arranged in series with one another, and then connected as a shunt to
the brushes or terminals of the machine. The series coils of series and
compound wound machines are arranged either in series or in parallel
with one another, according to conditions of operation, and then
connected in series to the armature and external circuit.
=Coupling Up Field Magnet Coils.=--In coupling up the coils
of either salient or consequent pole field magnets, assume each of
the pole pieces to have a certain polarity (in bipolar dynamos two
poles only, a north and south pole respectively, are required; in
multipolar dynamos the poles must be arranged in alternate order
around the armature, the number of N and S poles being equal), then
apply Flemming's rule as given under fig. 132, to each of the coils,
and ascertain the direction in which the magnetizing current must
flow in each in order to produce the assumed polarity in each of the
pole pieces. Having marked these directions on the coils, they can
be coupled up in either series or parallel connection according to
requirements, so that the current flows in the proper direction in each.
[Illustration: FIG. 442.--Comparison of space occupied by
direct and belt connected dynamos. In office buildings space is of
value and the room required by belt connected dynamos can always be
put to profitable use. For this reason the direct connected unit has
become generally adopted in the best type of office buildings. In large
factories the direct connected unit is generally adopted also to save
space. Where these conditions do not obtain, belted type of dynamo can
be used to advantage as a given output can be obtained with a smaller
size machine than where it is direct connected to the engine. This is
due to the limited rotative speeds at which engines can be run. The
illustration shows the relative space required by the two types.]
=The Drive.=--Various means are employed to connect the engine or
other prime mover with the dynamo, or the motor with the machinery to
driver. Among these may be mentioned the following:
1. Direct drive;
2. Belt drive;
3. Rope drive;
4. Gear drive;
5. Friction drive.
[Illustration: FIG. 443.--General Electric type M P, marine
generating set with tandem compound engine. The requirements of such
units are compactness, light weight, simplicity, freedom from vibration
and noise at high speed, perfect regulation and durability. By adopting
a short stroke for the engines and a special armature winding for the
dynamos, the height and length of the sets have been reduced. The
bed is carried out to the full width of the dynamo frame, making an
ample base surface for foundation without increasing the floor space
required. While the construction gives a massive appearance, the bed
has been cored out and the various parts so designed that the complete
sets have an approximate capacity of 3½ watts per pound. All of
the moving parts are enclosed by the engine column, excluding dust
and reducing wear and attention to a minimum. The bearing are oiled
automatically under pressure. These sets are made in sizes from 25 K.
W. to 75 K. W., the cylinder dimensions for the smallest size being
6½ and 10½ by 5, and for the largest size 10½ and 18 by 8.
Single cylinder sets are made in sizes ranging from 2½ K. W. to 50
K. W., the cylinder dimensions ranging from 3½ × 3 to 12 × 11. See
fig. 730.]
=Ques. What is a direct drive?=
Ans. One in which the driving member is connected direct to the driven
member, without any interposed gearing.
Fig. 443 shows a direct connected unit, which is an example of
direct drive.
=Ques. What may be said with respect to direct drive?=
Ans. It is the simplest method and the space required is less than with
belt drive. With direct drive the engine and dynamo must run at the
same speed; this is a disadvantage because the desirable speeds of the
two machines may not agree.
Since the usual engine speeds are slower than dynamo speeds,
direct drive involves the use of a larger dynamo for a given
output than would be necessary with belt connection, and
involves a corresponding increase in cost and greater friction
loss due to the rotation of larger and heavier parts.
[Illustration: FIG. 444.--Belt clamp for stretching belt and
holding the ends while making joint. It consists of a _stretching
frame_, the two ends of which are coupled by screwed bars; used for
pulling the ends of a belt together with the proper tension, when
lacing or joining the ends.]
=Ques. Mention some of the features of belt drive.=
Ans. Greater flexibility in the original design of a plant is possible
and new arrangements of old apparatus can be made at any time. It gives
conveniently any desired speed ratio and permits the use of high speed
dynamos and motors.
=Ques. State some of the disadvantages of belt drive.=
Ans. Considerable space is required and the action is not positive.
Belts exert a side pull on the bearings which results in wear, also
loss of power by friction.
[Illustration: FIGS. 445 and 446.--Two methods of lacing a
belt. In fig. 445 two rows of oval holes should be made with a punch,
as indicated. The nearest hole should be ¾ inch from the side, and
the first row 7/8 inch from the end, and the second row 1¾ inches
from the end of the belt. In large belts these distances should be
a little greater. A regular belt lacing (a strong, pliable strip
of leather) should be used, beginning at hole No. 1, and passing
consecutively through all the holes as numbered. In fig. 446 the holes
are all made in a row. This method has the advantage of making the
lacers lie parallel with the motion on the pulley side. The lacing is
doubled to find its middle, and the two ends are passed through the
two holes marked "1" and "1_a_" precisely as in lacing a shoe. The two
ends are then passed successively through the two series of holes in
the order in which they are numbered, 2, 3, 4, etc., and 2_a_, 3_a_,
4_a_, etc., finishing at 13 and 13_a_, which are additional holes for
fastening the ends of the lacer.]
=Ques. Give a rule for determining the proper size of belt.=
Ans. _A single belt travelling 1,000 feet per minute will transmit one
horse power per inch of width; a double belt will transmit twice this
amount._
EXAMPLE.--What size of double belt is required to transmit 50
horse power at 4,000 ft. speed, and what diameter pulley must
be used for 954 revolutions per minute at 4,000 ft. speed of
belt?
[Illustration: FIG. 447.--Wrong way to run a belt. The pull
should _not_ come on the top side, because, with slack at bottom there
is a tendency to slip.]
The horse power transmitted per inch is
4,000
----- × 2 = 8
1,000
accordingly, the width of belt required to transmit 50 horse
power is
50 ÷ 8 = 6.25, say 6".
For 4,000 ft. per minute belt speed, the distance _in inches_
travelled by the belt _per revolution_ of the pulley.
4,000 × 12
---------- = 50.31 inches
954
This is equal to the circumference of the pulley, and the
corresponding diameter is
50.31
---------- = 16.1, say 16 inches.
π
=Ques. What is the proper speed for a belt?=
Ans. From 3,000 to 5,000 feet per minute, depending on conditions.
Points Relating to Belts.
1. The amount of power that a belt of given size can transmit
is not a very definite quantity. The rule just given is
conservative and will give an amply large belt for ordinary
conditions.
2. A belt should make a straight run through the air and over
the pulleys without wabbling; it should maintain an even and
perfect contact with that part of the pulley with which it
comes in contact. In order to do this it should be kept soft,
pliable, and have no abrasions or rough places.
[Illustration: FIG. 448.--Right way to run a belt. The pull should come
on the lower side bringing the slack on top.]
3. When belt fasteners give way there is too much strain upon
belt. The greatest amount of slack in a belt is found where
it leaves the driving pulley, hence the tightener should be
near the driving pulley, as it takes up the slack, prevents
vibration and diminishes strain on belts and bearings. More
than 100 degrees of heat is injurious to belts.
4. Double belts should always run with the splices, and not
against them. Quarter turn belts should be made of two ply
leather, so as to diminish the side strain.
5. Friction is greatest when the pulleys are covered with
leather. Friction depends upon pressure, but adhesion depends
upon surface contact; the more a belt adheres to pulley surface
without straining, through too much tightening, the better the
driving power. Slipping occurs on wet days because the leather
absorbs dampness.
6. A leather covered pulley will produce more resistance than
polished or rough iron ones. A good belt dressing makes a
smooth, resisting surface, and as it contains no oils which
create a slippery surface to belts, it increases belt adhesion.
The friction of leather upon leather is five times greater than
leather upon iron.
7. Moisture and water distend the fibres, change the properties
of the tanner's grease and softening compounds. Repeated
saturation and drying will soon destroy leather. Leather well
filled with tanner's grease or animal oil, if allowed to hang
in a warm room for several months without handling, will dry
out, become harsh, and will readily crack.
8. A running belt is stretched and relaxed at different times
and unless there be perfect elasticity in all its parts there
will not be uniform distension.
9. There should be 25 per cent. margin allowed for adhesion
before a belt begins to slip.
[Illustration: FIGS. 449 to 451.--Method of aligning engine
and dynamo. In fig. 449, a line is stretched from A to E and the dynamo
shifted until the line contacts with points A, D, I, and E. In a small
dynamo, the pulley may be loosened and set back on the shaft as in fig.
450, while lining up the faces, and then moved back to its original
position as in fig. 451. When the pulley is not easily shifted the
distances at A and D (fig. 449) may be measured.]
=Rules for Calculating Speed and Sizes of Pulley.=--When two
pulleys are working together connected by a belt, the one which
communicates the motion is called the _driver_ and the other which
receives it, the _driven pulley_.
=To Find the Size of the Driving Pulley:= Multiply the
diameter of the driven pulley by its required number of
revolutions, and divide the product by the revolutions of the
driver. The quotient will be the diameter of the driver.
=To Find the Number of Revolutions of the Driven Pulley:=
Multiply the diameter of the driver by its number of
revolutions, and divide by diameter of driven. The quotient
will be the number of revolutions of the driven.
=To Find the Diameter of the Driven that shall Make a Given
Number of Revolutions, the Diameter and Revolutions of the
Driver Being Given:= Multiply the diameter of the driver by
its number of revolutions, and divide the product by the number
of revolutions of the driven pulley. The quotient will be the
diameter of the driven pulley.
=Rope Drive.=--In this method of power transmission, rope is run
in V-shaped grooves in the rims of the pulleys; this form of drive, in
some cases, is more desirable than others.
[Illustration: FIG. 452.--General Electric C Q back geared
motor driving Hamilton sensitive drill. When slowly moving machines
are to be driven, or where, for any reason, very moderate belt speeds
are required, the back geared motor is desirable. Two ratios of gear
reduction have been adopted as standard; they are:--4 to 1 and 8 to 1.]
=Ques. What are some of the advantages of rope drive?=
Ans. More power can be transmitted with a given diameter and width of
pulley, on account of the increased grip in the grooves. Rope drive can
be employed for long or short distances by reason of its lightness and
the action of the grooves.
=Gear Drive.=--This method is used where a positive drive is
desired, as for elevator or railway motors. It admits of any degree of
speed reduction without attending difficulties as would be encountered
with belt drive.
Thus, with the worm type of gear as used on elevator motors a
great reduction in velocity can be made without incurring the
expense of countershaft as with a belt.
[Illustration: FIG. 453.--Watson vertical motor designed
to operate a vertical shaft, either through belt connection, or by
direct drive. Hess-Bright ball bearings are used, taking the downward
thrust due to the weight of the armature. For mounting on the floor or
ceiling, a tripod base (as shown) is furnished, the standard sliding
base being used on a side wall. The armature shaft may be extended for
pulley or coupling either above or below the motor.]
=Friction Drive.=--This is a very simple mode of transmitting
power and has the advantages of simplicity and compactness. In
operation, the driving wheel is pressed against the wheel to be driven,
transmitting motion to the latter by the frictional grip. The drive is
thrown out of gear by slightly moving the machine on its sliding base.
In construction, the friction may be increased by making one wheel of
the pair of wood, compressed paper, or leather.
=Electrical Connections.=--Circuits for dynamos and motors should
be carefully planned so as to secure the simplest arrangement, and to
avoid unnecessary expense and delay, the wiring should be installed in
accordance with the requirements of the National Electrical Code.
[Illustration: FIG. 454.--Sling for handling armatures. In
raising an armature it should be supported by the shaft to avoid any
strain on the armature body or commutator.]
=Ques. What may be said with respect to exposed and concealed
wiring?=
Ans. Exposed wiring is cheap and accessible; a short circuit or ground
is easily located and repaired. Concealed wiring, especially when
placed under the floor, has the advantage of being out of the way, and
thus protected from injury.
=Ques. In wiring a dynamo what are the considerations with respect to
size of wire?=
Ans. All conductors, including those connecting the machine with the
switchboard, as well as the bus bars on the latter, should be of ample
size to be free from overheating and excessive loss of voltage. The
drop between the generator and switchboard should not exceed one-half
per cent. at full load, because it interferes with proper regulation
and adds to the less easily avoided drop on the distribution system.
CHAPTER XXV
AUXILIARY APPARATUS
There are numerous devices that must be used in connection with dynamos
and motors for proper control and safe operation. Among these may be
mentioned:
1. Switches;
2. Fuses;
3. Circuit breakers;
4. Rheostats;
5. Switchboards.
=Switches.=--A switch is a device by means of which an electric
circuit may be opened or closed. There are numerous types of switch;
they may be either single or multi-pole, single or double throw and
either of the "snap" or knife form.
=Ques. What is the difference between a single and double pole
switch?=
Ans. A single pole switch controls only one of the wires of the
circuit, while a double pole switch controls both.
=Ques. What is the difference between a single break and a double
break switch?=
Ans. The distinction is that the one breaks the circuit at one point
only, while the other breaks it at two points.
=Ques. What is the advantage of a double break?=
Ans. If the circuit be opened at two points in series at the same
instant, the electromotive force is divided between the two breaks and
the length to which the current will maintain an arc at either break is
reduced to one-half; thus there is less chance of burning the metal of
the switch. Another reason for providing two breaks is to avoid using
the blade pivot as a conductor, the contact at this point being too
poor for good conductivity.
[Illustration: FIGS. 455 to 457.--Adam's single throw knife
switches without fuse connections. Fig. 455, single pole switch; fig.
456, double-pole switch; fig. 457, three-pole switch.]
[Illustration: FIGS. 458 to 460.--Adam's single throw knife
switches with fuse connections at the handle end. Fig. 458, single pole
switch; fig. 459, double pole switch; fig. 460, three pole switch.]
=Ques. When should a knife switch be used?=
Ans. When the capacity of the circuit in which it is to be placed
exceeds 10 amperes.
=Ques. Describe a knife switch.=
Ans. Fig. 461 illustrates a knife switch of the double pole, single
throw type. It consists of the following parts: base, hinges, blades,
contact jaws, insulating cross bar, and handle, as shown.
=Ques. How should knife switches be installed?=
Ans. They should be placed so that _gravity tends to open them_.
Otherwise if the hinges become loose, the weight of the blades
and handle would tend to close the switch, thus closing the
circuit and possibly resulting in considerable damage.
[Illustration: FIG. 461.--A single throw, two pole knife
switch. As usually constructed it is made of hard drawn copper with
cast terminal lugs and fibre cross bar.]
=Ques. How should switches be proportioned?=
Ans. The minimum area of the contact surfaces should not be less than
.01 square inch per ampere, and in those used on arc lighting or other
high voltage circuits where the current is usually small, the area of
the contact surfaces are usually from .02 to .05 inch per ampere.
Since dirt or oxidation would prevent good contact under a simple
pressure between the contact surfaces, the mechanism of a switch
provides a sliding contact.
In the general design of switches, all parts which carry
current are given a cross sectional area of at least one square
inch per 1,000 amperes if they be made of copper, and about
three times as much if made of brass, as the conductivity of
the latter is only one-third that of the former. Furthermore,
the current should never be permitted to pass through springs,
as the heat generated will destroy their elasticity.
[Illustration: FIG. 462.--Triple pole, double break double
throw knife switch for very heavy current. The blades are made up of
numerous strips to give adequate contact area. A double throw switch
is used when it is desirable to open one circuit and immediately close
another, or to transfer one or more connections from one circuit
to another in the least practical interval of time, also, when one
connection is to be broken and another closed and it is undesirable to
allow both to be closed at the same time.]
=Ques. What difficulty is experienced in opening a circuit in which a
heavy current is flowing?=
Ans. It is impossible to instantly stop the current by opening the
switch, consequently the current continues to flow and momentarily
jumps the air gap, resulting in a more or less intense arc which tends
to burn the metal of the switch.
=Ques. How is this remedied to some extent?=
Ans. The contact pieces are so shaped that they open along their whole
length at the same time, so as to prevent the concentration of the arc
at the last point of contact. This feature is clearly shown in fig. 461.
[Illustration: FIG. 463.--A "quick break" knife switch of the
single throw, single break, one pole, type. The contact blade is held
between the jaws by their clamping friction until the handle compresses
the spring sufficiently to force the blade out. As soon as it breaks
contact with the jaws, the spring expands and drives the blade away
from the jaws with greater rapidity than could be done by hand. The
object of this action is to break the arc as quickly as possible to
prevent burning the metal of the switch.]
[Illustration: FIGS. 464 and 465.--Snap switch; views showing
switch with cover on, and exposed to show mechanism. The switch is
provided with indicating dial which registers "on" and "off" positions.
FIG. 466.--Gas Engine snap switch. The first snap makes
connection so that igniter is run from storage battery; second snap
connections are changed so that igniter is supplied from dynamo; third
snap makes connections so that dynamo supplies igniter and charges
storage battery; fourth snap, all off.]
=Ques. For what service are "snap" switches suitable?=
Ans. They are used on circuits containing lamps in comparatively small
groups, and other light duty service.
=Ques. What is a quick break switch?=
Ans. A form of switch in which the contact pieces are snapped apart
by the action of the springs, as shown in fig. 463, so as to make the
duration of the arc as short as possible.
The current allowed in each branch circuit of an electric
lighting system is limited by the insurance rules to a maximum
of 660 watts equivalent to 12 lamps of 16 c.p. each at 110
volts. They are also employed to control lamps in groups in
theatres and other places where many lamps are turned on or off
at about the same time.
[Illustration: FIG. 467.--Spool of fuse wire. The wire is
usually made of an alloy of tin and lead, such as half and half solder.
Bismuth is frequently added to the alloy to lower the melting point For
half and half solder the melting point is 370° Fahr. The quickness with
which a fuse will melt after the current has reached the limit depends
upon the specific heat and latent heat of the metal. The current
required to "blow" a fuse increases somewhat with the age of the fuse
owing to oxidation and molecular changes. Fuses are sometimes rated
according to the number of amperes to be taken normally by the circuit
they are to protect. Thus, a 10 ampere fuse is supposed to protect a
circuit whose regular current should not exceed 10 amperes, and to blow
if the current rise to say 12 amperes. The Underwriters' rule requires
that the rating be about 80% of the maximum current it can carry
indefinitely, thus allowing about 25% overload before the fuse melts.
The fusing current varies considerably according to circumstances. The
temperature of the surrounding air or other substances affects the
melting current greatly, because the rate at which heat from the fuse
will be transferred to the surroundings depends upon the difference of
temperance between them and the fuse. Hence a fuse in a warm place will
be melted by a smaller current than a similar fuse in a cold place. For
a similar reason, a fuse in an enclosed place where there is little
chance for the heat to be dissipated, will melt with a smaller current
than the same in an open place. If the current increase gradually to
that which would ordinarily melt the fuse, the high temperature makes
the fuse wire oxidize rapidly; this sometimes makes a sort of tube of
oxide which will not break even after the fuse wire inside has melted,
and so the fuse carries more than its rated current. Open fuses are so
unreliable that circuit breakers are preferable for large currents;
when fuses are used, the enclosed type as shown in figs. 468 to 470, is
usually the more desirable.]
=Fuses.=--All circuits subject to abnormal increase of current
which might overheat the system, should be protected by fuses which
will melt and thus open the circuit. A fuse is simply a strip of
fusible metal, often consisting of lead with a small percentage of tin,
connected in series in the circuit.
Experiments have shown that for large fuses, a multiple fuse is
more sensitive than a single one. A one hundred ampere fuse may
be made by taking four wires of twenty-five amperes capacity.
A fuse block may be overloaded, not because the metal of the
terminals is not of sufficient cross section to carry the
current, but because of insufficient area of, contact, or loose
contact of fuse and wires; the overload thus caused results in
heating and frequently melts the fuse.
[Illustration: FIGS. 468 to 470.--D & W, enclosed "cartridge"
fuses. Fig. 468, type for 3 to 60 amperes; fig. 469, type for 61 to 100
amperes; fig. 470, type for 101 to 1,000 amperes.]
=Ques. Where should fuses be placed?=
Ans. They should be inserted wherever the size of wire changes or
wherever there is a branch of smaller size wire connected, unless the
next fuse on the main or larger wire is small enough to protect the
branch or small wire.
[Illustration: FIGS. 471 to 478.--Interior construction of
D. & W. fuses. In the manufacture of these fuses, four types of fuse
link are used according to capacity of fuse, and classified as: 1,
air drum link; 2, flat link; 3, multiple link; 4, cylinder link. In
the air drum link, figs. 471 and 472, a capsule provides an air space
about the center of the link, the rate of heat conduction through the
confined air being very slow, the temperature of that portion of the
link rises rapidly with increasing current, rendering the blowing point
practically constant; fig. 473 shows a section through the complete
fuse. In the flat link, fig. 474, the section is reduced in the center,
cutting down as far as possible the volume of metal to be fused.
Figs. 475 to 478 show various form of multiple link construction. By
sub-dividing the metal, increased radiating surface is obtained which
permits a reduction in the volume of fusible metal necessary, and
the metal vapor formed when the fuse blows on heavy overload is more
readily dissipated. Figs. 477 and 478 show two forms of the cylinder
link, the plain cylinder fig. 477, being used for low voltage and large
current, and fig. 478, for certain high tension service. The corrugated
cylinder presents more surface to the fuse filling than the plain type
and secures a maximum radiating surface with resulting minimum volume
of metal for a given current.]
=Ques. How should fuses be mounted?=
Ans. They should be placed on a base of slate, porcelain, marble, or
other incombustible material.
=Ques. What is the objection to copper fuses?=
Ans. They heat perceptibly soon after their rated capacity is passed.
The melting temperature is higher than lead alloy.
=Ques. Upon what consideration does the choice between switches and
circuit breakers depend?=
Ans. Simple knife switches are suitable for use when the circuit is not
liable to be opened while carrying large current. A circuit breaker,
operated automatically or by hand should be used for interrupting heavy
currents.
[Illustration: FIGS. 479 and 480.--D & W fuse indicator. The
operation is illustrated in the figures which show appearance of the
label before the blowing of the fuse, fig. 479, and the same fuse
blown, as indicated by the appearance of the black spot within the
circle fig. 480.]
=Circuit Breakers.=--A circuit breaker is a switch which is opened
automatically when the current or the pressure exceeds or falls below a
certain limit, or which can be tripped by hand.
=Ques. What is the construction of a circuit breaker?=
Ans. It is composed of a switch and a solenoid in the main circuit.
When the current, flowing through the circuit, exceeds a certain value,
the core of the solenoid is drawn in and trips a trigger which allows
the switch to fly open under the action of a spring.
[Illustration: FIGS. 481 to 486.--Various open fuses. Fig.
481, fuse for main and branch blocks; fig. 482, standard railway
fuse; fig. 483, Edison main style; fig. 484, W.U. pattern; fig. 485,
Bell telephone style; fig. 486, sneak current fuse. When an open fuse
"blows" as a result of overloading, the rupture is accompanied by a
flash, and by spattering of the fused material. With large currents
this phenomenon is a source of danger, and the use of enclosed fuses
is accordingly recommended whenever the rating of the fuse exceeds 25
amperes. Various types of enclosed fuse are shown in figs. 468 to 470.]
There are numerous kinds of circuit breaker to meet the varied
conditions of service of which may be mentioned the following:
1. Maximum circuit breaker;
2. Minimum circuit breaker;
3. Reverse current circuit breaker;
4. Maximum and reverse circuit breaker;
5. No voltage circuit breaker.
[Illustration: FIGS. 487 and 488.--Reverse current circuit
breaker; fig. 488, view looking at end of coils of cut out, showing
direction of current. A to + bus bar; B, resistance lamp; C, brush of
cut out; D, shunt coil; E, series coil; F, core that trips cut out; G,
to - bus bar; H, to + pole of dynamo.]
Of these the maximum, reverse, and maximum and reverse types
are the more important.
A maximum circuit breaker is equivalent to a fuse, but has the
advantage that it can be at once reset, whereas a fuse must be
replaced.
A reverse breaker is used in connection with dynamos in
parallel, to automatically cut out a machine if it take more
than say, 10 per cent. motor current.
Maximum and reverse circuit breakers are frequently used on
dynamo panels.
[Illustration: FIGS. 489 and 490.--Front and top views of
I-T-E automatic overload circuit breaker. In fig. 489 the current in
the circuit enters at A, passes through the solenoid coil B (which
in its iron jacket becomes a powerful magnet), through the copper
terminal C, to the contact blades D, across the bridge at E to the
contact blades F, and out into the line at G. The path of the current
as indicated above is more clearly indicated in the top view fig. 490.
When the current in the solenoid coil produces sufficient magnetism
to overcome the weight of the plunger, the latter is drawn up with
constantly increasing velocity until it strikes a restraining latch
or trigger which forces the arm out of the switch, thus automatically
opening the circuit. The device is so constructed that in opening the
circuit the arc is broken on the carbon contacts instead of the copper
contacts.]
=Ques. Describe a reverse current circuit breaker or discriminating
cut out.=
Ans. This type of circuit breaker is arranged to open a circuit in the
event of current flowing in the circuit in a direction reverse to the
normal. This is sometimes effected by winding the electromagnet of the
circuit breaker with two coils, one connected as a shunt across the
main circuit and the other in series with the main circuit, the two
coils being so arranged that when the main current flows in the normal
direction their effects assist one another, whereas, when the main
current reverses, the effects of the coils are neutralized and the
breaker opens.
[Illustration: FIG. 491.--Roller-Smith "S.E." plain overload
circuit breaker. In operation, current entering through the lower
studs flows through the laminated strap windings C, from this into
the arm D, through the contact plate E, into the stationary brush F,
and finally out through the upper stud Q. In its passage through the
laminated windings C, the square core A is of course magnetized to
a degree dependent on the current strength. When this magnetization
reaches a pre-determined value, the attraction exerted on the ends
K of the pivoted armature causes the same to rise with great and
increasing velocity, finally bringing the finger D which forms part of
the armature into violent contact with the face R of the corresponding
projection on the housing which carries the handle and the roller H.
This heavy blow causes H, in its rotation about the shaft J, to go over
the center and consequently allows the strong outward pressure of the
brush F and the resilient coil C to throw the arm outward with a high
velocity and so break the circuit, first between the brush fingers
and the contact plate and finally between the carbons S and F, the
one of which is rigidly secured to the arm and the other of which is
resiliently mounted on its supporting spring. To reset the breaker,
the handle, which the act of opening has raised, is pulled down, thus
bringing roller H into engagement with roller G once more and in that
way forcing the arm back into its initial position.]
[Illustration: FIG. 492.--Roller Smith "S.E." combination
overload and underload circuit breaker. Attached to the supporting
frame B is the extension Z, which like B, is of non-magnetic material
and carries a rectangular magnetic core around which there are wrapped
laminated copper conductors. Hinged at U is a heavy cup-shaped mass
of magnetic material, and hinged at V is a flat lever X which bears
against the extension Y secured to the housing which carries the
operating handle. The circuit through the breaker conveys the current
around the windings of this underload coil carried by frame Z and
passes from it to the regular overload winding C from which it pursues
the same course and exercises the same function as in a plain overload
breaker. The core of Z being thus magnetized, the cup-shaped member W
is held in firm contact therewith and the lever X hangs free. Should,
however, the current fall below the minimum value, W is no longer
sustained by the magnetic attraction but drops away, swinging on its
hinge U until the projection on the heel thereof strikes the lever X,
which blow is transmitted through Y to the handle and thus trips the
breaker. When closing to reset the breaker, the handle is manipulated
just as in the case of a plain overload breaker, that is, it is pulled
down, thus not only closing and locking the breaker as before but
through the pressure exerted by Y on X and by X on W, putting the
latter into contact with its rectangular core to which it will adhere
if the necessary current be present.]
=Ques. State some disadvantages of a discriminating cut out.=
Ans. If one current reverse very rapidly, and soon reach a large value
in the opposite direction, it is possible the cut out may not open at
the desired instant, and thereafter the effect of the heavy reverse
current will be so great that the breaker will be held in more and more
strongly; a second disadvantage is that should the supply fail, the
breaker will open in any case, and have to be reset before the supply
can be resumed, though in certain cases, as, for instance where there
is a motor load, this feature is an advantage and not a disadvantage,
since the breaker acts as a no-voltage cut out as well as a reverse
current cut out.
Reverse breakers, however, can be made positive in their
action; that is, they can be so arranged that a reverse current
exerts a positive pull on the tripping gear, so that the
greater the reverse current, the greater the tripping effect.
=Ques. What are time limit attachments?=
Ans. Devices which are fitted to circuit breakers and which act as
dampers and prevent the too sudden operation of the breakers on what
may be only a temporary overload or reverse current.
By having different time limits on feeder and dynamo breakers
it can be ensured that the former operate before the latter,
and suitably in other cases where it is desired that one
breaker shall operate before another.
=Ques. Describe a time limit attachment.=
Ans. There are numerous types. It may consist of a clockwork device,
a weight acting on a small drum or pulley, a modified dash pot
arrangement, or a device operating by the expansion of a conductor due
to the heat generated by a current passing through it.
=Ques. How should a time limit device be arranged?=
Ans. It should be so arranged that the heavier the overload the quicker
the device acts, until with a short circuit the device is almost
instantaneous in its action.
[Illustration: FIG. 493.--Diagram showing connections of a
rheostat. The various resistance coils are connected to brass buttons
or "contacts." The rheostat is connected in series in the circuit that
it is to control. In operation when the lever is on contact 1, the
current is opposed by all the resistance of the rheostat so that the
flow is very small. As the lever is moved over contacts 1, 2, 3, etc.,
the coils are successively cut out, thus diminishing the resistance,
and when contact 11 is reached all the resistance is short circuited
allowing the full current to flow. In some types of rheostat the wire
is wound around an iron frame-work which has been previously dipped
into a fireproof insulating enamel. The advantage of this construction
is that the heat from the wire is dissipated much more rapidly, so that
a much smaller wire can be used to carry a given current. The size of
such an enameled rheostat required for absorbing a given amount of
energy is much smaller than one made of coils of wire stretched between
an iron supporting framework.]
=Rheostats.=--These devices consist of conductors inserted into
a circuit for the purpose of diminishing, either constantly or in a
variable degree, the amount of current flowing, or to develop heat by
the passage of a current through them. Rheostats designed to be used in
starting electric motors are frequently called "starting boxes."
=Ques. Describe the construction of a rheostat.=
Ans. In fig. 493, resistance coils, A, B, C, etc., are mounted in a
frame or box, and are connected at intervals to the contacts 1, 2, 3,
etc. The rheostat arm or lever L is pivoted at S, and when moved over
the contacts, inserts more or less of the resistance in the circuit
thus regulating the flow of the current. One terminal M of the rheostat
is connected to the first contact and the other terminal O, to the
lever at S.
[Illustration: FIG. 494.--Starter with no voltage release for
a series motor. A helical spring coiled around the lever pivot P, and
acting on the lever A, tends to keep it in the off position against the
stop S. This lever carries a soft iron armature I, which is held by
the poles of the electromagnet E, when, in starting the motor, the arm
has been gradually forced over as far as it will go. Should anything
happen to interrupt the current while the motor M is running, E will
lose its magnetism and A will be released, and will fly over to the off
position. E is usually shunted by a small resistance R, so that only a
portion of the main current flows through it. This device constitutes
the _no voltage release_, and ensures that all the resistance is in
circuit every time the motor is started.]
=Ques. How is a starting box connected to a motor?=
Ans. In series.
=Ques. Why should a starting box be used with a motor?=
Ans. If the line voltage should be applied directly to the terminals
of the armature when not running, an excessive flow of current will
result, on account of the low resistance. Accordingly, to prevent
injury to the winding, a variable resistance or starting box is
inserted between one supply terminal and the armature so that the
pressure may be applied gradually while the motor is coming up to speed.
[Illustration: FIG. 495.--Starter with no voltage release
for a shunt motor. The terminals of the motor are at M, M', _m_, and
those of the starter at S, S', _s_. The lever SA is shown in the "on"
position. The current enters the motor at the terminal M, and there
divides, part going through the field coil F, and the main current
through the motor armature A. The armature current enters the starter
at the terminal S', and traversing the lever SA, leaves by the terminal
S. The field current enters the starter at the terminal _s_, traverses
the coil of the magnet E (which holds up the armature _a_ linked to
the lever) and thence completes its journey through the whole of
the resistance R, and through the lever SA, to the terminal S. When
the supply is cut off by opening SW, or should the field circuit be
accidentally broken, the magnet E will release _a_ and the lever,
which will thereupon fly to the "off" stop O. It should be noticed
that when SA is off, A and F form a closed circuit with the resistance
R and magnet E. The inductance of F has consequently no chance of
causing destructive sparking when the current is shut off. In starting
the motor, SW is first closed, and then, as the lever is slowly
moved, the resistance R, which at first is all in circuit with A, is
gradually transferred from A to F. The resistance of R is too small to
affect appreciably the current in F, which necessarily consists of a
comparatively large number of turns of fine wire. The arrangement is
adopted to render the breaking of the shunt circuit unnecessary and is
rendered clearer by the diagram fig. 496. It should be noted that E may
be provided with a short circuiting key or push if required.]
[Illustration: FIG. 496.--Simplified diagram of the
connections of fig. 495.]
[Illustration: FIG. 497.--Starter with no voltage release
and overload release connected to a compound motor. With a shunt
motor, the only difference in the diagram would be that the series
winding SE would be absent, and the armature A would then be
connected straight across between the main terminals M and M'. When
switch SW is closed, the current will enter the starter at
its terminal S, and pass through the magnet coil _m'_ of the overload
release to the switch lever L, which is shown in the off position. As
soon as L is moved up to make contact with the first contact S the
current divides; part going through the resistance R and the terminals
S' and M' to the series coil SE (if a compound motor) and
armature A; and part through the no voltage magnet E to the shunt
winding SH. As the lever L is moved up toward E, the effect
is to take R out of the armature circuit and put it into the shunt
circuit. When the iron armature _a_, fixed on the switch lever, comes
against the poles of E, the laminated copper brush C bears against the
blocks B, B, and so affords a better path for the current than through
the spindle _s_. Should the supply voltage fail, either temporarily or
permanently, E will release _a_, and L will fly off under the tension
of a helical spring coiled round _s_. If there should be an overload
on the motor, tending to pull it up and cause an excess of current to
flow through the armature; this excess current, passing through _m'_,
will make it attract its armature, so bringing two contacts together
at K which will short circuit E, and allow the switch to fly off. The
connections between E and _m'_ are not shown in the figure, but they
are indicated at C in fig. 498, which is a simplification of fig. 497,
and which should be carefully compared therewith. When only the normal
current is flowing, the attraction between _m'_ and its armature is not
sufficient to pull the latter up. The actual forms and arrangement of
parts on the starters are well shown in some of the figures.]
=Ques. What attachments should be provided on a starting box?=
Ans. An overload release, and a no voltage release.
=Ques. Describe these devices.=
Ans. The overload release is an electromagnetic circuit breaker that
opens the circuit if the motor become greatly overloaded. A no voltage
release may consist of an electromagnet in series with the shunt field
circuit; it holds the rheostat arm in the operating position as long
as current flows through the shunt field from the line. If the line
switch be opened or the shunt field circuit accidentally broken, the
device becomes demagnetized and releases the arm, which returns to its
starting position by the action of a spring.
[Illustration: FIG. 498.--Simplified diagram of the
connections of starter connected to compound motor as shown in fig.
497.]
The general arrangement of switches, cut outs and starting boxes should
be in accordance with the requirements of the National Electrical Code
as follows:
"Each motor and starting box must be protected by a cut out and
controlled by a switch, said switch plainly indicating whether
'on' or 'off.' The switch and rheostat must be located within
sight of the motor, except in cases where special permission
to locate them elsewhere is given, in writing by the inspection
department having jurisdiction.
"Where the circuit breaking device on the motor starting
rheostat discs disconnects all wires of the circuit, this
switch may be omitted.
"Overload release devices on motor starting rheostats will not
be considered to take the place of the cut out required if they
be inoperative during the starting of the motor.
"The switch is necessary for entirely disconnecting the motor
when not in use, and the cut out to protect the motor from
excessive currents, or careless handling when starting. An
automatic circuit breaker disconnecting all wires of the
circuit, may, however, serve as both switch and cut out."
[Illustration: FIG. 499.--View showing general arrangement
of a switchboard. The wires are shown to illustrate the various
connections, but in actual construction these wires are connected on
the back of the switchboard.]
=Switchboards.=--A switchboard consists of a panel or series of
panels of slate, marble, soapstone or brick tile erected in an electric
plant for the purpose of mounting in a convenient group the instruments
for controlling and distributing the current and safeguarding the
system. Switchboards may be divided according to operation into two
classes:
1. Direct control;
2. Remote control.
A direct control switchboard has all its apparatus mounted directly
on the board and controlled by hand, while in the remote control
type, the main current carrying parts are at some distance from the
operating board, the control being effected by mechanical devices or
by electric motors or solenoids. When the control system of a plant is
very extensive, it sometimes occupies a separate building known as the
_switch house_.
=Ques. What may be said with respect to the material for
switchboards?=
Ans. In order to avoid danger of fire from short circuits, the panel
should be made of some non-combustible material, such as marble,
slate, glass plates or earthenware tiles. If slate be used, care
should be taken to have it free from conducting veins, or it should be
marbleized, that is, subjected to a treatment that will fill up the
pores of the veins and thus prevent the absorption of moisture.
Wood is seldom used, except in cases where the switches, fuse
blocks, wire supports, etc., are all mounted on porcelain or
other incombustible material.
=Ques. How should the instruments and connections be arranged on a
switchboard?=
Ans. They should be arranged so as to provide the shortest possible
path for the current, and preferably always in the same direction, that
is, from right to left or from top to bottom, the connecting wires
being brought in on one side and out on the other, and the crossing of
wires avoided as far as possible.
All wires and current carrying parts should be kept far
enough apart at all points to prevent accidental contact or
the jumping across of the current where there is a great
difference of voltage. Such wires should be also kept at a
sufficient distance from screw heads, metal brackets, gas
pipes, water pipes, and other conducting bodies, in order to
prevent accidental grounds or short circuits.
All instruments and switches should be placed so as to be
conveniently accessible for observation and operation, and
sufficiently out of reach of accidental contact by persons;
otherwise they should be protected by some form of insulating
shield.
[Illustration: FIG. 500.--Small switchboard suitable for
two dynamos; view showing ammeters and voltmeters, switches, circuit
breakers, etc.]
=Ques. What type of switch is used on switchboards?=
Ans. The "knife" switch.
=Ques. Describe a small switchboard.=
Ans. Fig. 500 shows one suitable for two dynamos. At the top is a
voltmeter and two ammeters. Immediately below is a row of feeder
switches serving to connect and disconnect the various feeders with
and from the bus bars which are mounted behind the board. Below are two
rheostat handwheels, and two large switches connecting the dynamos with
the bus bars. VS is a voltmeter switch connecting the voltmeter with
various parts of the system. Below the voltmeter switch is a double
throw switch to transfer the bus bars from connection with the dynamo
switches to one with some other source of current such as a street
circuit, in the event of a breakdown. At the bottom are two circuit
breakers.
[Illustration: FIG. 501.--Diagram showing various connections
of voltmeter switch of the small switchboard shown in fig. 500.]
=Ques. Describe the voltmeter switch.=
Ans. Fig. 501 shows the connections, from which it can be seen that
the voltmeter can be connected with the terminals of either dynamo or
with the bus bars, or with either a central or remote part in the lamp
circuits.
Under ordinary conditions it remains connected to the circuit
at the central point of distribution. When one dynamo is
already in circuit, however, and it becomes necessary to
connect up the other one, the voltage of the latter must be
the same as that at the bus bars. Accordingly, connections
are provided to the voltmeter switch such that the attendant
can compare the voltages at the dynamo terminals and bus bars
before closing the dynamo switch. All the positive connections
are on one side of the circle swept by the switch and all the
negative connections on the other side.
[Illustration: FIG. 502.--Roller-Smith, single pole, plain
overload circuit breaker. As its name indicates, the function of the
plain overload circuit breaker is to automatically interrupt the
circuit in which it is placed when the flow of current through it
exceeds the predetermined limit for which the apparatus is set. It is
the most common of all of the types and is utilized for the protection
of dynamos and motors and all other electrical apparatus which, by
reason of the conditions of operation, may become subject to loads
in excess of the normal. The single pole type may be used separately
for the protection of a single wire of a given circuit or grouped to
protect the two or more wires of one circuit, becoming in the latter
case the so called independent arm multipole apparatus. The action of
this type of circuit breaker is fully explained in fig. 491.]
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
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=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 Note:
Inconsistent spelling and hyphenation are as in the original.]
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