ratings of machines. This table represents usual practice
NUMBER OF POLES AND USUAL SPEED LIMITS OF D.-C. GENERATORS
Output in kw.
Number of poles
Speed in rpm,
maximum and minimum
2400 to 600
10 to 50
1300 to 350
50 to 100
4 or 6
1100 to 230
100 to 300
6 or 8
700 to 160
300 to 600
6 or 10
500 to 120
600 to 1000
8 or 12
400 to 100
1000 to 3000
10 or 20
200 to 70
Safe Armature Speeds. The safe speed of an armature
varies with the armature construction. Direct-current ma-
chines are built with a peripheral speed of 2,500 to 3,500 feet
per minute. It is not advisable to exceed an armature periph-
eral speed of 6,000 feet per minute in machines which are
not designed to take care of the mechanical stresses incident
to the higher speeds.
1 Principles of Electrical Design, page 81.
ALTERNATING -CURRENT WINDINGS
In the main, the windings for alternating-current motors
and generators are alike. For this reason details of the wind-
ings that can be used for either machines will be given. It
should be noted at this point that direct-current and alter-
nating-current windings differ essentially by the former being
of the closed-circuit type while most alternating-current wind-
ings are of the open-circuit type. Either open- or closed-
circuit types of windings may however be employed in alter-
nating-current machines, but the open type is in most common
use. By a closed winding is meant one which has a continuous
path through the armature and re-enters itself to form a
closed circuit. Such a direct-current winding always has at
least two current paths between brushes. In the case of an
open-circuit alternating -circuit winding, there is a continuous
path through the conductors of the coils of each phase of the
winding, with the ends of this path forming two free ends.
Such a winding does not close on itself.
In the closed-circuit windings of direct-current machines,
the bars of the commutator are simply connected at equally
distant points around the winding. In the open-circuit wind-
ing of an alternating-current generator with revolving arma-
ture, the terminals of the completed winding are connected to
collector rings and the winding is open-circuited until closed
by the connections between the brushes. Closed-circuit wind-
ings are only used for special alternating-current machines.
The conditions which usually call for a closed-circuit wind-
ing are the following:
1. An alternating-current machine which must deliver a large
current at low voltage. In such a case the winding usually
consists of several similar paths connected in parallel to the
ALTERNATING-CURRENT WINDINGS 27
terminals of the armature, thus forming one or more closed cir-
cuits within the armature winding.
2. In designs of machines that must handle direct and alternating
current, as in double current generators and in rotary converters.
In general it may be said that a direct -current winding may
be changed so that the machine can be used as an alternator,
but an alternating-current winding cannot be used for a direct
current generator since it is not reentrant.
Types of A.-C. Windings. With reference to the arrange-
ments of coils used in an alternating-current armature, wind-
ings may be divided into two general classes as follows:
I. Distributed Windings.
1. Spiral or chain.
II. Concentrated Windings.
Distributed Windings. An armature winding which has
its inductors of any one phase undigr a single pole placed in
several slots, is said to be distributed. When these inductors
are Dunched together in one slot per pole, per phase, the
winding is called concentrated. It is usual in a distributed
winding to distribute the series inductors in any phase of the
winding among two or more slots under each pole. This
tends to diminish armature reactance and gives a better emf .
wave, besides offering a better distribution of the heating due
to armature copper loss than in concentrated windings.
Concentrated Windings. The uni-slot or concentrated
winding gives the largest possible emf from a given number of
inductors in the winding. That is for a definite fixed speed
and field strength in an alternator, the concentrated winding
requires a less number of inductors than a distributed winding,
but increases the number of turns per coil.
Spiral or Chain Winding. In this winding as shown in
Fig. 14 there is only one coil side in a slot. An odd or even
number of inductors per slot may be used but several shapes
of coils are required since the coils enclose each other and must
ARMATURE WINDING AND MOTOR REPAIR
have special end shapes to clear each other. This arrange-
ment, however, makes possible good insulation of end con-
nections through adequate separating air spaces in high vol-
tage machines. The number of coils required in this winding
FIG. 14. A spiral or chain winding for a 2-phase, 4-pole machine with
the coils of one phase in place and connected. The coils of the other phase
go in the slots shown by the full lines inside the coils.
is also small compared with other windings. This type of
winding is mainly used in alternating-current generators.
Lap and Wave Windings.- Both distributed and concen-
trated windings make use of lap and wave connections. These
FIG. 15. Single-phase lap winding for the same conditions as the wave
winding in Fig. 16.
FTG. 16. Single-phase distributed wave winding with two slots per pole per
phase and one coil side per slot.
arrangements are in principle the same as used in direct-current
windings. (See Chapter I, pages 9 and 16.) The diagrams of
Figs. 15 and 16 show single-phase distributed lap and wave
windings for a four-pole armature, having two slots per pole
with one inductor per slot.
In a double-layer lap or wave winding for an alternating-
current armature, the coils are usually of the same general
shape as used in direct-current windings. Instead of con-
necting the terminals of the coils to a commutator, they are
connected together in a definite order (see Chapter XI, page
288) for each phase. The phase windings are then connected
together in star or delta as shown in Fig. 22. In that diagram
a three-phase wave winding is shown for a four-pole machine.
In the double-layer winding, the number of conductors per
FIG. 17. At the left is shown the appearance of one formed coil in a 2-
layer winding. One side of the coil lies in the top of the slot and the other
side in the bottom of the slot. The top side bears an odd number and the
bottom side an even number. The coil pitch in this case is 6 slots or 13
winding spaces. At the right the method of connecting coils for a 2-layer
winding using form coils is shown. The finish (F) of one coil is joined
to the start (S) of the next.
slot must be a multiple of two. It lends itself to a variety
of connections, particularly to a fractional pitch lap winding
where the two sides of a coil are not similarly placed in respect
to the center lines of the poles. The double-layer winding
is not, however, as well suited for high voltages as the single-
layer winding. For this reason many water-wheel types of
generators have been built with a single-layer winding, which
in its most common form, is known as the spiral or chain
When laying out or changing a double-layer winding, it
is usual to assign odd numbers to the sides of coils in the top
of the slots and even numbers to the sides in the bottom of
the slots. This is important when the pitch of the armature.
30 ARMATURE WINDING AND MOTOR REPAIR
coils is expressed in terms of coil sides (winding spaces) in-
stead of slots.
Whole-coiled and Half-coiled Windings. When the coils
of an alternating current winding are connected so that there
are as many coils per phase as there are poles, the winding is
called "whole-coiled." When the coils are connected so that
there is only one coil per phase per pair of poles, the winding is
called " half-coiled." The main difference between these two
connections is in the method of making the end connections
A Whole-coiled winding B Half-coiled winding
FIG. 18. A 6-pole stator with whole-coiled and half-coiled windings.
The whole-coiled winding, A, has as many coils per phase as there are poles.
The half-coiled winding B has only one coil per phase per pair of poles.
for the coils. In the " whole-coiled " winding each slot con-
tains two coil sides. It is not, however, strictly a double-
layer winding, as the coil sides are placed side by side and
not one above the other. In the " half -coiled" winding, how-
ever, each coil may have twice the number of turns of a " whole-
coiled" winding or the two coils under a north or south pole
of the latter type may be connected in series and taped to-
gether to form one coil in case of a change in connections.
The " half -coiled " winding has the advantage that, when
used with large generators the armature frame may be split
into two sections for shipment or repair, without disturbing
many of the end connections.
Single-phase and Polyphase Windings. The winding of
a single-phase motor or generator has only one group of induc-
tors per pole, placed in one slot or several slots depending
upon whether or not the winding is concentrated or distrib-
FIG. 19. A simple single-phase winding.
FIG. 20. Simple 2-phase winding.
FIG. 21. Simple 3-phase winding.
FIG. 22. A 3-phase winding showing how it may be connected in delta
32 ARMATURE WINDING AND MOTOR REPAIR
uted. Such a single-phase concentrated wave winding for
a four-pole armature is shown in Fig. 19.
Two-phase and three-phase windings may be considered
as made up of single-phase windings properly placed on the
same armature. For the two-phase windings two separate
single-phase windings are used spaced 90 electrical degrees
apart. This is shown in Fig. 20. For the three-phase wind-
ing, three single-phase windings are used, spaced 120 degrees
apart, as illustrated in Fig. 21. Although the single-phase
windings are independent of each other, their terminals are
connected in star or delta as shown in Fig. 22.
Coil Pitch. In the case of a two-phase winding, the total
number of slots should be just divisible by two so that each
phase will have the same number of winding elements or
coils per pole. In the same way, for a three-phase winding
the total number of coil sides or the total number of slots
should be just divisible by three (the number of phases)
and sometimes by the number of poles. This will result in
a full pitch winding, that is, a winding in which a coil spans
exactly the distance between the centers of adjacent poles.
If the coil spans less than this distance, so that its two sides
are not exactly under the centers of adjacent poles at the same
time, it is said to have a fractional-pitch. When a fractional-
pitch is used in alternators on account of the electrical factors
of the design, such as to secure as nearly as possible a sine
wave shape of emf, the total number of slots per phase must
be a whole number. A fractional-pitch is also widely used
in induction motors.
Coil pitch is expressed as a fraction of the pole pitch, in
slots, in electrical degrees or in winding spaces (coil sides).
In the case of a six-pole machine having 72 stator slots, and
a double-layer winding, the pole pitch would be 12 slots. If
the coil pitch were given as %, this would be 120 degrees
or eight slots or 13 winding spaces (coil sides). A full coil
pitch for this winding would be 180 degrees, 12 slots or 21
Phase Spread of Windings. The spread or space occupied
by each single-phase winding is known as the phase spread
of the winding. For a two-phase winding the phase spread
ALTERNATING-CURRENT WINDINGS 33
is (180 -r- 2) or 90 degrees. For a three-phase winding, it is
(180 -T- 3) or 60 degrees. In a single-phase winding, the phase
spread is theoretically 180 degrees. Prof. Alfred Still points
out, however, in his book on " Principles of Electrical Design, "
that nothing is gained by winding all the slots on the armature
surface of a single-phase machine. After a certain width of
winding has been reached the filling of additional slots merely
increases the resistance and inductance of the winding with-
out any appreciable gain in the developed voltage. In prac-
tice only about 75 per cent, of the available slot space is
utilized making the phase spread for a single-phase winding
about 135 electrical degrees.
The fact that, in polyphase machines, the whole of the arma-
ture surface is available for the winding, while only a portion
is utilized in a single-phase alternator, accounts for the fact
that the output of the latter is less than that of the polyphase
machine using the same size of frame. In a three-phase
machine it is only necessary to omit one of the phase windings
entirely and connect the two remaining phases in series to
obtain a single-phase generator. Such a modified generator
will give about two-thirds of the output of the polyphase
connection. A three-phase star connected induction motor
can also be used as a single-phase motor by properly con-
necting two phases of it (see Chapter XI).
Two-phase from Four -phase Windings. In many cases the
two-phase induction motor is designed as a four-phase machine
with the connections between conductors of the winding
arranged so as to permit operation on a two-phase supply
circuit. As shown in Fig. 23, a two-phase winding maybe
secured from a four-phase grouping of coils by connecting the
first and third groups in series and the second and fourth
groups in series.
Three-phase from Six-phase Windings. Few strictly
three-phase induction motors are built. The design may be
more properly called a six-phase winding with the three phases
' spaced 120 electrical degrees and the connections of coils
such as to permit the motor to be operated on a three-phase
circuit. As shown in Fig. 24 in a six-phase winding the coils
of the six-phases are spaced 60 electrical degrees. For three-
ARMATURE WINDING AND MOTOR REPAIR
phase operation the coils of phases one and four, three and
six, and five and two are connected in series. The terminals
of phases two, four and six are connected to a common point.
FIG. 23. Connections for four windings spaced 90 electrical degrees apart
to secure a 2-phase motor. There are really four phases shown, the first and
third and the second and fourth are connected in series. The full lines are
front connections of coils and dotted lines the back connections. $ and F
indicate the start and finish of the different groups of coils.
FIG. 24. Connections for a 6-phase winding design so that a 3-phase
winding is secured. The six windings are spaced 60 electrical degrees
apart. In this diagram phases one and four, three and six, and five and two
are connected in series and the terminals F 2 , F4 and Fe jointed to a common
point. The 3-phase leads are Si, S 3 , and SB. The full lines indicate
front connections and dotted lines back connections. S and F indicate the
start and finish of the different groups of coils.
Wire, Strap and Bar Wound Coils. For the coils used in
small motors round insulated wire is most employed. These
ALTERNATING-CURRENT WINDINGS 35
coils are either wound in the slots by hand or assembled by
use of specially formed coils wound in forms and insulated
before being placed in the slots. Such formed coils are usu-
ally used except in cases where the slots are closed or nearly
closed. For further information on formed coils see page 4,
Chapter I, and page 141, Chapter VI.
For large motors and generators where the amperes to be
carried in each armature circuit is a large value, copper straps
are frequently employed for making up the armature coils.
In very large machines a copper bar is used instead of the
copper straps. In such a case one bar serves as the inductor
of a coil having one turn per slot. A two-layer bar winding
made up of two bars per coil and four bars per slot is also
used. The copper bars are connected to the end connections
of the coils by brazing, welding or bolting. In all cases,
whatever the construction of the coil used, the slots must be
properly insulated with fullerboard, mica, fish paper or other
suitable insulating material. For data on slot insulation see
The current density in alternating-current windings is
about 2500 amp. per square inch of armature conductor in
small machines, 2000 amp. in medium sizes and 1500 amp. in
high-voltage designs. Except in high-speed machines it is
not safe to use the maximum limit owing to damage to wind-
ings from over heating.
METHODS FOR LAYING OUT AND CONNECTING ALTERNATING-
In rewinding an alternating-current machine, the number of
slots on the stator, the operating voltage, speed, phase and
frequency of the supply circuit are points that must be con-
sidered in laying out a new winding or reconnecting an existing
one. The fundamental requirements of windings and the ways
in which they can be fulfilled as outlined by M. W. Bartmess
(Electric Journal, Vol. VIII, No. 5) are given in what follows.
Group Windings. Group winding may be defined as that
class wherein the total winding is divided into separate parts,
composed of adjacent coils or conductors. The grouping is,
in the case of lap and wave windings, an arbitrary one, the
ARMATURE WINDING AND MOTOR REPAIR
coils being all similar and divided into groups solely by their
connections. The number of coils per group may equal the
number per pair of poles divided by the number of phases, or
the number per pole divided by the number phases. The latter
method of grouping is generally used on modern machines.
In the case of a six-pole, three-phase winding of 36 slots, the
number of coils per group is 36 -j- (6 X 3) or 2. When the
number of slots is not evenly divisible by the product of poles
FIG. 25. Methods of connecting pole-phase groups shown in (a), (6) and (c).
(a) Four-pole winding with alternately positive and negative pole-phase groups,
(fc) Four-pole winding of the consequent pole type, (c) Two-pole winding obtained
from (b) by reconnecting the pole-phase groups alternately positive and negative.
and phases, dissimilar groups must be employed. In such
cases it is advisable to arrange the grouping so that all the
phases have an equal number of coils, and if possible the group-
ing should be arranged symmetrically with respect to the core
itself. To prevent local currents, which may prove injurious,
all circuits which are in parallel must have an equal number of
coils and should be symmetrically arranged with respect to
each other and to the other phases.
Although one turn coils only are shown in Figs. 27 to 29,
the same connections are applicable to windings having
ALTERNATING-CURRENT WINDINGS 37
any number of conductors per coil. These conductors may
all be in series, in which case there is one lead at each end of the
coil or the conductors may be divided into any number of equal
parallels, in which case there are as many leads at the ends of
the coils as there are parallel circuits. The leads at the
beginning and end of the coils are connected in the same man-
ner as indicated for the one turn per coil winding. For the
sake of simplicity the number of coils per group and hence the
total number of coils in the diagrams has been kept lower than
is generally found in commercial machines.
Full and Fractional Pitch Windings. The number of
slots in the core, divided by the number of poles gives a value
of the pole arc expressed in terms of the slots. A full pitch
winding is one in which the effective span of the coils is equal
to the pole arc, and a fractional pitch winding is one in which
the effective span of the coils is not equal to the pole arc. For
a two coil per slot, lap or wave winding, the effective span of the
coil is equal to the actual span of the coil. In this case the
full pitch winding is one where the coil throw is equal to
/total number of slots , A ^
I r e~ r~ ~ plus 1 ) . For a one coil per slot lap
\ number of poles /
winding the effective span of the coil may be greater or less than
its actual span. In Fig. 26 (a) and (6) show two different coils,
in each of which the effective span is the full pitch of 12 slots
while the actual span in (a) is only 11 slots and that in (6)
is 13 slots. Needless to say, (a) is more generally used on
account of the saving in copper and space for end connections.
A coil with a span either less or greater than that shown
would result in a fractional pitch, as in (c) and (d) .
Representative cases of concentric group windings are
shown in Fig. 26, (e) and (f), (e) representing a three-bank
winding, in which the number of coils per group equals the
total number of coils per phase divided by the number of poles,
while (f) represents a two-bank winding of the consequent
pole type in which the number of coils per group equals the
total number of coils per phase divided by the number of pairs
of poles. Neither of these types can be conveniently wound
with a fractional pitch, especially with formed coils. Where
dissimilar groups are employed, that is where the number of
ARMATURE WINDING AND MOTOR REPAIR
slots is not evenly divisible by the product of phases and poles,
the full pitch is frequently not a unit and hence a fractional
pitch is necessary. In the case of a three-phase, four pole
winding with 30 slots, 30 coils, two coils per slot, full pitch
covers a span of 7.5 slots and the nearest lower even pitch gives
a throw of 1-8.
In general, fractional pitch affects the performance of the
apparatus similarly to a reduced number of turns in the wind-
+ + + +O OOO 4 -1-4.4.
FIG. 26. Possible pitch for one coil per slot windings.
(a) Full pitch, effective space 12, actual space 11, throw 1 to 12. (6) Full pitch,
effective space 12, actual space 13, throw 1 to 14. (c) Fractional pitch, effective
space 10, actual space 9, throw 1 to 10. (d) Fractional pitch, effective space 10,
actual space 15, throw 1 to 16, (e) Concentric group, full pitch, effective space 12,
actual space 9 and 11, throw 1 to 11. (/) Concentric group, full pitch, consequent
poles, effective space 12, actual space 9, 11, 13, and 15, throw 1 to 13.
ing, but not in the same proportion. In a generator this re-
duces the voltage of the machine. In an induction motor, the
maximum available torque is increased but the densities in the
magnetic circuit are also increased with a resulting reduction of
power-factor. For either motor or generator, considerable
copper may thus be saved in the coil ends and a standard
frame may frequently be used for special purposes.
Simple Winding Diagram. It is evident that for all com-
binations the number of diagrams necessary would be unlimited.
A simplified diagram may be employed which will not only
reduce the required number of such diagrams but will also
minimize the labor in tracing out the connections. Thus it will
be seen that the diagram in Fig. 27, will satisfy many require-
ments for connections of groups. In addition to this it will
apply for any similarly connected three-phase, four-pole, series
star lap-winding, irrespective of the number of coils per
group (provided the groups are regular) or of the throw of the
coils, that is, whether the winding is full or fractional pitch.
This information for the throw of the coils and the number of
coils per group may be carried
on the same specification with
the remaining winding con-
stants. The groups are formed
by connecting the required num-
ber of coils together, the end of c
the first coil to the beginning of
the second, etc., the beginning
of the first coil and the end of
the last coil in the group forming
the beginning and end of the
group. Such diagrams may be
made for any number of phases,
poles or possible parallel circuits,
and for any desired method of
connection of the groups. In