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.