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.