Alfred Still.

Principles of electrical design; d. c. and a. c. generators online

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21. Introductory. The object of this chapter is to explain
the essential points which the designer must keep in mind when
determining the number of slots, the space taken up by insula-
tion, the cross-section of the copper windings, and the method
of connecting the individual conductors so as to produce a finished
armature suitable for the duty it has to perform. It is assumed
that the reader is familiar with the appearance of a D.C. machine
and understands generally the function of the commutator.
For this reason it is proposed to omit such elementary descrip-
tive matter as may be found in every textbook treating of elec-
trical machinery. On the other hand, the practical details of
manufacture and much of the nomenclature used in the design
room and shops of .manufacturers will also be omitted, because
space does not permit of the subject being treated exhaustively;
but if the reader will exercise his judgment and rely upon his
common sense, he will be able to design a practical armature
winding to fulfil any specified conditions.

The direction of the generated e.m.f. will depend upon the
direction of the flux through which the individual conductor is
moving, and it is therefore a simple matter so to connect the
armature coils that the e.m.fs. shall be additive. It does not
matter whether the machine is bipolar or multipolar, ring- or
drum-wound, it is always possible to count the number of con-
ductors in series between any pair of brushes and thus make
sure that the desired voltage will be obtained.

Closed-coil windings will alone be considered, because the
open-circuit windings as used in the early THOMSON-HOUSTON
machines and other generators for series arc lighting systems
are now practically obsolete. Another type of machine, known
as the homopolar or acyclic D.C. generator, although actually
used and built at the present time, has a limited application
and will not be considered here. The absence of the com-
mutator is the feature which distinguishes this machine from the



more common types; but it is not suitable for high voltages,
and the friction and PR losses in the brushes are very large. 1

22. Ring- and Drum-wound Armatures. The GRAMME ring
winding is now practically obsolete. In this type of machine
the coils form a continuous winding around the armature core
which is in the form of a ring with a sufficient opening to allow
of the wires passing down the inside of the core parallel to the
axis of rotation. The objections to this winding are the high
resistance and reactance of the armature coils due to the large
proportion of the " inactive" material per turn. This has a
bearing not only on the cost and efficiency of the machine but
also on commutation, because the high inductance of the wind-
ings is liable to cause sparking at the brushes.

In the drum winding all the conductors are on the outside
surface of the armature, and although space is usually provided,
even in small machines, between the inside of the core and the
shaft, this space is used for ventilating purposes only and is not
occupied by the armature coils. The drum winding is simply
the result of so arranging the end connections that the e.m.fs.
generated in the various face conductors shall assist each other
in providing the total e.m.f. between brushes. Both windings,
whether of the ring or drum type, are continuous and closed
upon themselves, and if the brushes are lifted off the commutator
there will be no circulating currents because the system of
conductors as a whole is cutting exactly the same amount of
positive as of negative flux.

23. Multiple and Series Windings. Nearly all modern
continuous-current generators are provided with former-wound
coils. These coils are made up of the required number of turns,
and pressed into the proper shape before being assembled in
the slots of the armature core. Smooth-core armatures are very
rarely used, and the slotted armature alone will be considered.
In all but two-pole machines (which are rarely made except for
small outputs or exceptionally high speeds) there is practically
only one type of coil in general use. This is generally of the

1 For information on the homopolar type of machine see " Acyclic
(Homopolar) Dynamos," by J. E. NOEGGERATH, Trans. A. I. E. E., vol.
XXIV (1905), pp. 1 to 18, and the article by the same writer, "Acyclic
Generators," in the Electrical World, Sept. 12, 1908, p. 575. Also "Homo-
polar Generators, " by E. W. Moss and J. MOULD, Journal Inst. E. E., vol.
49 (1912), pp. 804 to 816.


shape shown in Fig. 26. The finished coil may consist of any
number of turns, but as there are two coil-sides in each slot,
there will be an even number of inductors per slot. The portion
of the armature periphery spanned by each coil is approximately
equal to the pole pitch T, because it is necessary that one coil
side shall be cutting positive flux while the other coil side is
cutting negative flux. If the two coil sides lie in slots exactly


FIG. 26. Form-wound armature coil.

one pole pitch apart, the winding is said to be full pitch. If
the width of coil is less than r, the winding is said to be short
pitch or chorded. The shortening of the pitch slightly reduces
the length of inactive copper in the end connections. Some
designers claim that it gives better commutation; but, on the
other hand, it reduces the effective width of the zone of commu-
tation, and it is doubtful if either winding has a distinct
advantage over the other.

There are two entirely different methods of joining together



the individual coils. Thus, if the two ends of the coil shown in
Fig. 26 are connected to neighboring commutator bars, as in
Fig. 27a, we obtain a multiple or lap winding, while, if the ends
are taken to commutator bars approximately a pole pitch apart,
(Fig. 276), we obtain the series or wave winding. The important
distinction between these two styles of winding is the fact that,
with the multiple winding, there are as many sets of brushes and



[< r ( Approximately ) >-J

FIG. 27. Multiple and series armature coil connections.

as many parallel paths through the armature as there are poles
while, with the series winding, there are only two electrical paths
in parallel through the armature, and only two sets of brushes are
necessary, although a greater number of brush sets may be used. 1

1 What are known as simplex windings are here referred to. Multiplex
windings may be used on lap-wound machines when the current is large and
it is desired to have two or more separate circuits connected in parallel by
sufficiently wide brushes. This tends to improve commutation. In series-
wound machines the multiple winding has the advantage that it enables
the designer to obtain more than two circuits in parallel, but not as many
as there are poles. In this manner it is possible to provide for a number of
parallel paths somewhere between the limits set by the simplex wave and
lap windings respectively. These windings are, however, rarely met with.
Again, a duplex winding may be singly re-entrant or doubly re-entrant.
In the former case, the winding would close on itself only after passing twice
around the armature, while, in the latter case, there would be two inde-
pendent windings. It is suggested that the reader need not concern
himself with these distinctions, which have no bearing on the principles of
armature design. More complete information can be found in many text-
books and in the handbooks for electrical engineers.


The two kinds of winding are shown diagrammatically in
Figs. 28 and 29. The former shows a simplex lap-wound
multipolar drum armature, while the latter represents in a
similar diagrammatic manner a simplex wave-wound multipolar
drum armature. In practice there would ordinarily be a greater
number of coils and commutator bars, and the conductors would
be in slots. This is indicated by the grouping of the conductors

FIG. 28. Diagram of simplex multiple winding.

in pairs, the even-numbered inductors being (say) in the bottom
of the slot, with the odd-numbered inductors immediately
above them in the top of the slot.

It should particularly be noted that the lap or multiple wind-
ing provides as many electrical circuits in parallel as there are
poles, and the number of brush sets required is the same as the
number of poles. Thus if 7 is the current in the external circuit,
plus the small component required for the shunt field excitation,
the current in the armature windings is I/p and the current


collected by each set of brushes is




In the case of the wave, or series, winding there will be two
paths in parallel through ^he armature, whatever may be the
number of poles. Two sets of brushes will, therefore, suffice
to collect the current; but more sets may be used if desired, in
order to reduce the necessary length of the commutator. In
this case the brushes would be placed one pole pitch apart
around the commutator, and all brush sets of the same polarity
would be joined in parallel. This does not, however, increase

FIG. 29. Diagram of simplex series winding.

the number of electrical paths in parallel in the armature, but
merely facilitates the collection of the total current, as will be
understood by carefully studying the winding diagrams. The
series winding is usually adopted when the voltage is high and
the current correspondingly reduced; it is, therefore, rarely neces-
sary to provide more than two sets of brushes. When a greater
number of brush sets is provided on a series-wound machine,
it is not easy to ensure that the current will be equally divided
between the various sets of brushes. What is known as selective


commutation then occurs, each brush set collecting current in
proportion to the conductance of the brush contact. This leads
to sparking troubles unless ample brush surface is provided.

The fact that there may be only two sets of brushes on a multi-
polar dynamo does not necessarily indicate a wave-wound
armature. The commutators of lap-wound machines are some-
times provided with an internal system of cross-connections
whereby all commutator bars of the same potential are joined


(a) (6)

FIG. 30. Appearance of lap and wave windings.

together. This allows of only two sets of brushes being used;
but the length of the commutator must, of course, be increased
to provide the brush-contact surface necessary for the proper
collection of the current. The external appearance of the
parallel and series windings respectively is indicated by sketches
(a) and (6) of Fig. 30. The observer is supposed to be looking
down on the cylindrical surface of the finished armature.

If there are two coil sides in each slot, the number of commu-
tator bars will be the same as the number of armature slots,
whether the coils are connected to form a multiple or a series
winding. It is, however, by no means necessary to limit the


number of commutator bars to the number of slots, as the total
number of inductors in each slot may be subdivided, and a
correspondingly greater number of commutator bars can be
used. This point will be again referred to when treating of the
slot insulation.

With a series-wound armature, the number of commutator
bars cannot be a multiple of the number of poles, because this
would lead to a closed winding after stepping once around the
armature periphery. The winding must advance or retrogress
by one commutator bar when it has been once around the arma-
ture, and this leads to the rule that a series-wound machine must
have a number of commutator segments such as to fulfil the
condition :

Number of commutator segments 1 ,p

in wave-wound machine j 2 "

where k is any whole number. 1

24 % Equalizing Connections for Multiple -wound Armatures.
If the magnetic circuits of the various parallel paths in the lap-
wound dynamo are not of equal reluctance, there will be an un-
balancing of the generated e.m.fs. producing circulating currents
through the brushes. The inequality of the magnetic perme-
ances is usually due to excentricity of the armature relatively
to the bore of the poles, and even when the unbalancing effect is
small in a new machine, it is liable to increase owing to wear of
the bearings.

Fig. 31 is a developed view of a four-pole winding. Sup-
pose that the section A of the armature winding is nearer to the
pole face than the section C. The voltage generated in the con-
ductors occupying the latter position will be less than in the

1 Although an armature may be provided with an odd number of slots, it
does not follow that it will accommodate a wave winding suitable for all
voltages, without modification. Thus, if a six-pole machine has 73 armature
slots, it may be necessary to have two coils (or four coil-sides) per slot in
order to obtain the necessary voltage and avoid too great a difference of
potential between adjacent commutator bars. This means that the number
of coils and of commutator segments would be 146, which would ^not be
suitable for a wave winding. By having one dummy or "dead" coil, the
total number of coils (and commutator segments) will be 145, which being
equal to (48 X %) +1 will give a wave winding. The "dead" coil is put
in for appearance and to balance the armature, but it is not connected up.
The use of dead coils should be avoided as it adds to the difficulties of


conductors moving through the region A. The result will be
a tendency for a current to circulate in the path AECF as
indicated by the dotted arrows. The net result will be a strength-
ening of the current leaving the machine at one set of brushes
and a corresponding weakening of the current at the other set
of brushes. This unbalancing of the current may lead to serious
sparking troubles. To prevent the inequality of voltage in the
different sections of the windings it is necessary to go to the root
of the trouble and correct the differences in the reluctance of the
various magnetic paths. This cannot, however, always be ac-
complished perfectly or in a lasting manner; but, by providing


h -r



- -rl^-^-J






FIG. 31. Equalizer connections.

easy paths for the out-of-balance current components, it is
possible to equalize the differences of pressure before the current
reaches the brushes. This is done by connecting together points
on the armature winding which should be at the same potential.
In practice a number of insulated copper rings are provided and
connected to equipotential points on the commutator. The
dotted lines in Fig. 31 show six equalizing rings. Actually,
from six to eight points per pair of poles would probably be cross-

It should be clearly understood that the equalizing connections
of lap-wound armatures do not prevent the unbalancing of
currents; but, by providing a short-circuit to the paths through
brushes and connecting leads of the same sign, they tend to
maintain the equality of currents through the various brush
sets. In the simplex wave winding, with only two armature
paths in parallel, equalizing connections are not necessary.

26. Insulation of Armature Windings. No great amount of
insulation is necessary on each wire or conductor of an armature


winding because, even in machines of large output, the voltage
generated per turn of wire is comparatively small. The diff-
erence of potential between the winding as a whole and the
armature core may, however, be very great on high-voltage
machines, and the slot lining must be designed to withstand
this pressure with a reasonable factor of safety. About the
same amount of insulation as will be necessary for the slot
linings will also have to be provided between the upper and
lower coil-sides in each slot, because the potential difference
between the two sets of conductors in the slot is the same as that
between the terminals of the machine. The space occupied by
insulation relatively to the total space available for the winding
will depend not only upon the voltage of the dynamo, but also
upon such factors as the number of slots and their cross-section
and proportions. Even if the total slot area remains constant,
the larger number of slots will naturally require the greater
amount of insulation, and thus reduce the space available for
copper. Again, a wide slot, by reducing the tooth width, may
be the cause of unduly high densities in the teeth, while a deep
slot is undesirable on account of increased inductance of the
windings, and because it may lead to an appreciably reduced
iron section at the root of the tooth in armatures of small

With the ordinary double-layer winding, the square coil
section would give the best winding space factor. Thus if each
of the two coil-sides were made of square cross-section, the total
depth of slot including space for binding wires or wedge would
be from two and one-fourth times to two and one-half times the
width. In practice the slot depth is frequently three times the
width, but this ratio should not exceed 3J^ because the design
would be uneconomical, and the high inductance of the winding
might lead to commutation difficulties.

Although the calculation of flux densities in the teeth will be
dealt with later, it may be stated that it is usual to design the
slot with parallel sides and make the slot width from 0.4 to 0.6
times the slot pitch. It is very common to make slot and tooth
width the same (i.e., one-half the pitch) on the armature surface,
especially in small machines. In large machines the ratio
tooth width .

asTridth- 1S

What has been referred to as the slot pitch may be denned as


the ratio, armature surface periphery divided by the total number
of slots.

26. Number of Teeth on Armature. It is obvious that a small
number of teeth would lead to a reduction of space taken up by
insulation and, generally speaking, would lead also to a saving
in the cost of manufacture. Other considerations, however,
show that there are many points in favor of a large number of
teeth. Unless the air gap is large relatively to the slot pitch,
there will be appreciable eddy-current loss in the pole pieces on
account of the tufting of the flux lines at the tooth top. Again,
pulsations of flux in the magnetic circuit are more liable to be of
appreciable magnitude with few than with many teeth, and when
the tooth pitch is wide in relation to the space between pole tips,
commutation becomes difficult because of the variation of air-gap
reluctance in the zone of the commutating field. A good practi-
cal rule is that the number of slots per pole shall not be less than
10, and that there shall be at least three and one-half slots in the
space between pole tips. In high-speed machines with large pole
pitch, from 14 to 18 slots per pole would usually be provided.
With the exception of small generators (machines with armatures
of small diameter), the cross-section of the slot is about constant,
and approximately equal to 1 sq. in. This corresponds to about
1,000 amp. conductors per slot for machines up to 600 volts,
on the basis of the current densities to be discussed later.

27. Number of Commutator Segments Potential Difference
between Segments. Machines may be built with a number of
commutator bars equal to the number of slots in the armature
core. In this case there will be one coil per slot, i.e., two coil-
sides in each slot. There is, however, no reason why the number
of coils 1 should not be greater than the number of slots. The
usual number of commutator segments per slot is two or three
in low-voltage machines, with a maximum of four or five in low-
speed dynamos for high voltages. The number of commutator
bars may therefore be a multiple of the number of slots. A

1 The word coil as here used denotes the number of turns included between
the tappings taken to commutator bars. In practice one-half the number of
conductors in a slot might be taped up together and handled as a single coil,
but if tappings are taken from the ends so as to divide the complete coil in
two or more sections electrically, we may speak of four, six, or more coil-
sides in a single slot, notwithstanding the fact that these "may be bunched
together and treated as a unit when placing the finished coils in position.


large number of bars improves commutation, but increases the
cost of the machine; a large diameter of commutator is necessary
in order that the individual sector shall not be too thin. The
copper bars are insulated from each other by mica, usually
about ^2 m - thick, increasing to J^o m - f r machines of 1,000
volts and upward. It follows that a commutator with a very
large number of segments is less easily assembled and less satis-
factory from the mechanical standpoint than one with fewer

The best way to determine the proper number of commutator
bars for a particular design of dynamo is to consider the voltage
between neighboring bars. This voltage is variable, and depends
upon the distribution of the magnetic flux over the armature
surface, and upon the position of the armature coil under con-
sideration. The maximum potential difference between adjacent
commutator bars rarely exceeds 40 volts, and the average voltage
should be considerably lower than this. The average voltage
between bars may be defined as the potential difference between
+ and - - brush sets divided by the number of commutator
segments counted between the brushes of opposite sign. As a
rough guide, it may be stated that the value of 15 volts (average)
between segments should not be exceeded in machines without
interpoles. About* double this value is permissible as an upper
limit on machines with commutating interpoles, especially if
they are provided with compensating pole-face windings which
prevent the distortion of flux distribution under load. In
practice the allowable average voltage between commutator
bars is based upon the machine voltage and, to some extent,
upon the kilowatt output, although few designers appear to
pay much attention to the influence of the current in determining
the number of commutator segments. As an aid to design, the
following values may be used for the purpose of deciding upon
a suitable number of coils and commutator bars.

Machine voltage Volts between commutator segments

110 1 to 6

220 2.5 to 10

600 5 to 18

1,200 9 to 25

28. Nature and Thickness of Slot Insulation. Since the
average voltage between the terminals of an armature coil does
not exceed 25 volts, it follows that the potential difference be-


tween the conductors in one coil cannot be very high. The
copper conductors are usually insulated with cotton spun upon
the wire in two layers. Cotton braiding is sometimes used on
large conductors of rectangular section; and a silk covering is
used on very small wires where a saving of space may be effected
and an economical design obtained notwithstanding the high
price of the silk covering. A triple cotton covering is occasionally
used when the potential difference between turns exceeds 20
volts. Conductors of large cross-section may be insulated by a
covering of cotton tape put on when the coil is being wound.

In addition to the comparatively small amount of insulation
on the wires, a substantial thickness of insulation must be pro-
vided between the armature core and the winding as a whole.
The materials used for slot lining are:

1. Vulcanized Fiber; Leatheroid or Fish Paper; Manilla Paper;
Pressboard; Presspahn; Horn Fiber; etc.; all of which, being
tough and strong, are used mainly as a mechanical protection
because they are more or less hygroscopic and cannot be relied
upon as high-pressure insulators, especially when moisture is

2. Mica; Micanite; Mica Paper or Cloth. These materials
are good insulators, and the pure mica or the micanite sheet will
withstand high temperatures. Sheet micanite is built up of
small pieces of mica split thin and cemented together by varnish.
The finished sheet is subjected to great pressure at high tem-

Online LibraryAlfred StillPrinciples of electrical design; d. c. and a. c. generators → online text (page 8 of 30)