CONSTRUCTION OF ALTERNATORS
The construction of alternators follows much the same lines as dynamos,
especially in the case of machines of the revolving armature type.
Usually, however, more poles are provided than on direct current
machines, in order to obtain the required frequency without being
driven at excessive speed.
The essential parts of an alternator are:
1. Field magnets;
2. Armature;
3. Collector rings;
and in actual construction, in order that these necessary parts may be
retained in proper co-relation, and the machine operate properly there
must also be included:
4. Frame;
5. Bed plate;
6. Pulley.
~Field Magnets.~--The early forms of alternator were built with
permanently magnetized steel magnets, but these were later discarded
for electromagnets.
Alternators are built with three kinds of electromagnets, classed
according to the manner in which they are excited, the machines being
known as,
1. Self-excited;
2. Separately excited;
3. Compositely excited.
[Illustration: FIGS. 1,448 and 1,449.--Westinghouse laminated hub
and laminated pole piece for revolving field having squirrel cage
winding. Thin steel is used for the laminations of both hub and pole
piece; these are assembled and firmly riveted together under hydraulic
pressure. The laminations are of the same thickness in both hub and
spider.]
[Illustration: FIGS. 1,450 to 1,452.--Views of Triumph pole pieces.
These consist of laminated punchings securely clamped between two cast
steel end plates. The laminations are shaped with polar horns or shoes
as shown, and which serve to keep the field coils securely wedged in
position. In some designs the horns are separate. The two holes in
each pole piece are for through bolts which secure the pole piece and
coil to the spider run. Dovetail joints are sometimes used instead of
through bolts, as in figs. 1,448 and 1,449.]
~Ques. What is a self-excited alternator?~
Ans. One in which the field magnets are excited by current from one
or more of the armature coils, or from a separate winding (small in
comparison with the main winding), the current being transformed into
direct current by passing it through a commutator.
[Illustration: FIG. 1,453.--Fort Wayne armature for self-excited
alternator. There are two independent windings, one for the main
current, and one for the exciting current. The winding for the latter
current occupies a very small amount of space, and is placed in the
slots on top the main winding. The commutator to which the exciter
winding is connected, is located between the collector rings and the
core. It is of standard construction with end clamps holding the bars
in place on the insulated commutator drum. The armature coils are form
wound and the core is built of sheet steel laminations, annealed and
japanned to prevent hysteresis and eddy current losses. Ventilated
openings are provided to allow a free circulation of air both around
the ends of the windings and through ducts in the laminated core. The
core is clamped by bolts between the flanges of the armature spider
which is keyed to the shaft. These flanges have cylindrical extensions
with ribbed surfaces, which form a support for the ends of the armature
coils. The ribbed surfaces form air passages from the core outward
around the ends of the coils, thus ventilating both core and coils.]
Fig. 1,453 shows an armature of a self-excited machine, the
exciting current being generated in a separate winding and
passed through a commutator.
~Ques. For what class of service are self-exciting alternators used?~
[Illustration: FIG. 1,454.--Allis-Chalmers three bearing type
alternator with exciter direct connected. The bearing pedestals are
bolted to a substantial cast iron base having, in the large sizes,
sufficient length to permit shifting frame sideways along the base
to give access to the field and armature coils. The field coils are
designed for 120 volt excitation, and are wound edgewise with copper
strip. There is a liberal margin of field excitation to take care
of overloads or for operation on loads of low power factor. The
regulating qualities are as good as can be obtained without making
the machine unnecessarily large and expensive. By regulation is meant
the percentage rise in voltage when full load is thrown off, field
excitation and speed being held constant; the percentage is referred to
normal full load voltage. An alternator with poor regulation will show
large variations in voltage with changes in load, the pressure falling
whenever a load is thrown on and rising when it is thrown off. These
changes will be especially pronounced if the load be inductive. A badly
designed alternator might show very fair regulation on non-inductive
load and yet be unable to give full voltage on inductive load.]
Ans. They are employed in small power plants and isolated lighting
plants where inductive loads are encountered.
~Ques. What is a separately excited alternator?~
Ans. One in which the field magnets are excited from a small dynamo
independently driven or driven by the alternator shaft, either direct
connected or by belt as shown in fig. 1,455.
[Illustration: FIG. 1,455.--Diagram of separately excited alternator.
The field winding is supplied with direct current, usually at 125 volts
pressure by a small dynamo called the "exciter." The latter may be
driven by independent power, or by belt connection with the main shaft,
and in some cases the exciter is directly connected to the alternator
shaft.]
~Ques. What is a compositely excited alternator?~
Ans. A composite alternator is similar to a compound wound dynamo in
that it has two field windings. In addition to the regular field coils
which carry the main magnetizing current from the exciter, there is a
second winding upon two or upon all of the pole pieces, carrying a
rectified current from the alternator which strengthens the field to
balance the losses in the machine, and also if so desired, the losses
on the line as shown in fig. 1,456.
[Illustration: FIG. 1,456.--Diagram of compositely excited alternator.
The current for exciting the field magnets is obtained, partly from
an exciter and partly from the windings of the alternator, being
transformed into direct current by the rectifier. The connections are
as shown. One end of the armature winding is connected to one of the
collector rings; the other end, to the light part of the rectifier,
as shown, the solid black part of the rectifier being connected to
the other collector ring. Two brushes bear on adjacent teeth of the
rectifier and are connected to the compensating winding circuit across
which is a shunt. These connections are shown more clearly in fig.
1,457. In operation the separately excited coils set up the magnetism
necessary for the generation of the voltage at no load. The main
current coming from the armature is shunted, part going through the
shunts and the remainder around the compensating winding, furnishing
the additional magnetism necessary to supply the voltage to overcome
the armature impedance. This composite method of field excitation is
very similar to that used on a compound wound dynamo. As shown, both
field windings encircle every pole, but in some machines the rectified
current will traverse a few poles only, the current from the exciter
traversing the remainder.]
~Ques. What is a magneto?~
Ans. A special form of alternator having permanent magnets for its
field, and used chiefly to furnish current for gas engine ignition and
for telephone call bells.
Details of construction and operation are shown in figs. 1,458
to 1,461.
[Illustration: FIG. 1,457.--Diagram showing construction of rectifier
and connections of compositely excited alternator. The rectifier
consists of two castings M and S with teeth which fit together as
shown, being insulated so they do not come in contact with each other.
Every alternate tooth being of the same casting is connected together,
the same as though joined by a conducting wire. There are as many teeth
as there are poles. One end of the armature winding is connected direct
to one of the collector rings, while the other is connected to M of the
rectifier, the circuit being through brushes P and Q, the shunt, and
compensating winding to the other collector ring. The brushes P and
Q contact with adjacent teeth, when one is in contact with the solid
black casting the other touches the light casting. The principle of
action is the same as a commutator, briefly: to reverse the connections
terminating at the brushes P and Q in synchronism with the reversals of
the alternating current induced in the armature winding, thus obtaining
direct current for the compensating field winding. The shunt resistance
placed across the compensating winding circuit permits adjusting the
compounding of the machine to the circuit on which it is to work, since
by varying the resistance the percentage of the total current passing
through the compensating winding can be changed. It will be seen by
tracing the path of the current for each direction in the armature
winding that while the rectifier causes the current to flow in the
same direction in the compensating field winding, it still remains
alternating in the external circuit.]
[Illustration: FIG. 1,458.--Connecticut magneto; view showing permanent
magnets in dotted lines. It consists of three permanent U shape
magnets, between the poles of which is a shuttle type armature. The
latter is geared to a hand crank in sufficient velocity ratio to give
the desired speed without too rapid turning of the crank. This type of
magneto is used to generate current for operation of telephone call
bells.]
[Illustration: FIGS. 1,459 to 1,461.--Diagram illustrating the
operation of a magneto. The shuttle shaped armature is wound from
end to end with insulated wire, so that when rotated, a powerful
alternating current is produced in the windings by cutting the magnetic
lines, whose varying strength is shown by the shaded portions in the
two views. When in the position shown in the first diagram, the lines
of force mostly converge at the top and bottom, finding a direct path
through the metal end flanges of the shuttle. When in the position
shown in the second diagram, the lines are converged so as to pass
through the armature core. Fig. 1,460 shows detail of the armature
core.]
~Ques. What are the two principal types of field magnet?~
Ans. Stationary and revolving.
~Ques. What is the usual construction of stationary field magnets?~
Ans. Laminated pole pieces are used, each pole being made up of a
number of steel stampings riveted together and bolted or preferably
cast into the frame of the machine. The field coils are machine wound
and carefully insulated. After winding they are taped to protect them
from mechanical injury. Each coil is then dipped in an insulating
compound and afterwards baked to render it impervious to moisture.
[Illustration: FIG. 1,462.--Stationary field of Fort Wayne multiphase
revolving armature alternator; view showing brass girds on pole pieces
for synchronous motor operation. When designed for this use the machine
is provided with amortisseur winding on the poles. As shown in the
illustration this winding consists of a brass collar around the pole
tip with a cross rib integral with the collar, fitting in a slot in the
pole face parallel to the shaft. This construction assists in bringing
the machine up to synchronous speed as an induction motor, ordinarily
checks any tendency toward hunting and does not in any way affect the
operation of the machine as an alternator. The main field winding
should be connected through switches on the field frame in order that
the field circuit may be broken up to eliminate any danger that might
arise from induced voltage. It is not advisable to throw on a full
rated voltage and a compensator should, therefore, be provided to
reduce the pressure.]
[Illustration: FIG. 1,463.--Triumph 36 pole fly wheel type revolving
field. The spider has the form of a fly wheel having spokes and rim
to which the field magnets are attached by through bolts. The field
coils are of copper strap bent on end, the kind generally used on large
machines. The series connection of the coils is plainly shown, also the
two cables leading via one of the spokes to the slip rings.]
[Illustration: FIG. 1,464.--Wagner cast steel hub with dovetail grooves
for attaching the revolving field magnets. Such construction is
generally used on machines of small and medium size.]
[Illustration: FIG. 1,465.--Wagner laminated pole piece with horns
stamped in one piece. The laminations are held together between two end
pieces by through rivets, as shown.]
~Ques. Describe the construction of a revolving field.~
Ans. The entire structure or rotor consists of a shaft, hub or spider,
field magnets and slip rings. The magnet poles consist of laminated
iron stampings clamped in place by means of through bolts which, acting
through the agency of steel end plates, force the laminated stampings
into a uniform, rigid mass. This mass is magnetically subdivided into
so many small parts that the heating effect of eddy currents is reduced
to a minimum. The cores are mounted upon a hub or spider either by
dovetail construction or by means of through bolts, according to the
centrifugal force which they must withstand in operation, either method
permitting the easy removal of any particular field pole if necessary.
The field coils are secured upon the pole pieces either by horns in one
piece with the laminations, or separate and bolted. All the coils are
connected in series, cable leads connecting them to slip rings placed
on the shaft.
[Illustration: FIG. 1,466.--Wagner revolving field of 300 kilowatt
alternator during construction, illustrating the method of attaching
the field magnets to the hub by dovetail joints. After the notched ends
of the pole pieces are slid into the grooves in the hub, tapered keys,
which are plainly seen, are driven in, thus making a tight joint which
will not shake loose.]
~Ques. What are slip rings?~
Ans. Insulated rings mounted upon the alternator shaft to receive
direct current for the revolving field, as distinguished from collector
rings which collect the alternating currents generated in an alternator
of the revolving armature type.
In construction provision is made for attaching the field
winding leads. The rings are usually made of cast iron and are
supported mechanically upon the shaft, but are insulated from
it and from one another.
The current is introduced by means of brushes as with a
commutator. Carbon brushes are generally used.
A good design of slip ring should provide for air circulation
underneath and between the rings.
[Illustration: FIG. 1,467.--General Electric field coil, showing one
method of winding. In the smaller machines the wire is wound on spools
which are slipped over the pole pieces, which are built of sheet iron,
spreading at the pole face so as to secure not only a wide polar arc
for the proper distribution of the magnetic flux, but also to hold the
field windings in place.]
~Ques. What form of spider is used on large alternators?~
Ans. It is practically the same form as a fly wheel, consisting of hub,
spokes, and rim to which the magnets are bolted.
[Illustration: FIG. 1,468.--General Electric field coil showing another
method of winding. The field coils on the larger machines consist of
a single strip of flat copper, wound on edge as shown, so that the
surface of every turn is exposed to the air for cooling. The flat sides
of the copper strip rest against each other and the entire coil forms a
structure of great solidity which can be easily removed for inspection
and repair.] [Illustration: FIG. 1,469.--Allis-Chalmers 60 kva. belted
two bearing alternator on base arranged so the armature can be shifted
sideways as shown, to give access to the field and armature coils.] On
alternators of the fly wheel type the spider rim is made of sufficient
weight to obtain full fly wheel effect, thus making a separate fly
wheel unnecessary.
[Illustration: FIG. 1,470.--Revolving field of Fort Wayne 10 pole
alternator. In construction, the cores of the field poles are built up
from punchings of laminated steel, and assembled under considerable
pressure between malleable iron or steel end plates and riveted
together. Substantial insulation is placed on the pole cores and over
this is wound the field coils of cotton covered wire. After the wire
is in place, the completed poles are baked to expel any moisture and
are then treated with insulating varnish. They are then assembled on
a laminated spider, being held in place by dovetail joints made tight
by the use of taper keys. Special casting plates are finally fastened
in place over the dove tails effectually closing them. The assembly of
the field is completed by the insertion of the shaft into the field
spider under heavy hydraulic pressure. All the coils are connected in
series, cable leads connecting them to slip rings placed on the shaft.
Each slip ring is provided with a double type brush holder, making it
possible to clean brushes while the alternator is in service, by simply
removing one brush at a time.]
[Illustration: FIG. 1,471.--General Electric slip rings; view showing
construction and attachment of cable leads to field winding. They are
so designed that all surfaces of the rings have easy access to the
air, in order to obtain good ventilation. _Slip rings_, through which
current is transmitted to a revolving field, are to be distinguished
from _collector rings_ whose function it is to "collect" or transmit
the alternating currents induced in the armature to the brushes.]
~Armatures.~--In construction, armatures for alternators are similar
to those employed on dynamos; they are in most cases simpler than
direct current armatures due to the smaller number of coils, absence of
commutator with its multi-connections, etc. Alternator armatures may be
classified in several ways:
1. With respect to operation, as
_a._ Revolving;
_b._ Stationary.
[Illustration: FIG. 1,472.--Allis-Chalmers brush holder and slip rings.
The latter are made of cast copper, which the builders claim to be more
satisfactory than cast iron. On some of the large low speed machines
the collector rings are split, but on the majority of alternators they
are in one piece. Current is led into the rings by means of carbon
brushes, the number of brushes being such that the current density
at the rubbing contact is kept within conservative limits. At least
two brushes per ring are provided, so that one can be removed for
inspection without interrupting the exciting current. In large machines
the brush holder studs are mounted on a stand supported from the base;
on small alternators they are usually fastened to the cap of one of the
bearing pedestals.]
[Illustration: FIG. 1,473.--Fort Wayne multiphase revolving armature
alternator, designed for use in small power plants and isolated
lighting plants where inductive loads are encountered. Built for
pressures of 120, 240, 480, and 600 volts. These voltages have been
recommended by the American Institute of Electrical Engineers, and will
cover the needs of any set of conditions ordinarily met with. These
standard voltages not only permit economical distribution, but they are
such that no transformers are necessary to reduce the line pressure
for ordinary cases. For transmitting power relatively long distances,
600 volts is usually employed. Where there is a demand for 480 volt
service, a 480 volt alternator should be selected and if lower voltages
are also desired an auto-transformer may be furnished by means of which
240 volts can be obtained. When 120 volt circuits are necessary for
lighting, etc., the 240 volt pressure can be still further reduced to
120 volts by means of another auto-transformer. However, this double
reduction will rarely be found necessary.]
[Illustration: FIG. 1,474.--Western Electric stationary armature. In
this type of armature, the core upon which the winding is placed, is
built into the frame as shown, the core teeth projecting inwardly like
internal gear teeth, forming a cylindrical chamber for the revolving
field. The core is built up of iron, laminated and japanned to prevent
eddy currents and hysteresis losses. The laminations are rigidly bolted
between two heavy end plates. The armature coils are of copper bar
impregnated with insulating compound. They are held in the slots by
wedges which allow their ready removal for inspection or repairs.]
[Illustration: FIGS. 1,474 to 1,477.--Various types of armature;
fig. 1,474 ring armature; fig. 1,475 disc armature; fig. 1,476 drum
armature. The latter type is now almost universally used, the others
being practically obsolete. A Gramme ring wound and connected to
collector rings as in fig. 1,474, will yield an alternating current.
In a multipolar field, the ring will need multipolar connections
alternated at points corresponding to the pitch of the poles. Fig.
1,475 illustrates the so-called "Siemens" disc armature. The armature
coils are arranged around the periphery of a thin disc. The field
magnets consist of two crowns of fixed coils, with iron cores arranged
so that their free poles are opposite one another. This type was
created in 1878 by Herr von Hefner, engineer to Messrs. Siemens and
Halske. Fig, 1,476 shows a modern drum armature of a three phase
machine. It is similar in appearance to a direct current armature
except for the absence of the commutator and its connections. The drum
armature is the prevailing type.]
2. With respect to the core, as
_a._ Ring;
_b._ Disc;
_c._ Drum.
Ring and disc armatures are practically obsolete and need not
be further considered. A ring armature has the inherent defect
that the copper inside the ring is inactive.
Disc armatures were employed by Pacinotti in 1878, and
afterwards adopted by Brush in his arc lighting dynamos.
The design failed for mechanical reasons, but electrically it
is, in a sense, an improvement upon the Gramme ring, in that
inductors on both sides of the ring are active, these being
connected together by circumferential connectors from pole to
pole, thus, corresponding to the end connectors on modern drum
armatures.
3. With respect to the core surface, as
_a._ Smooth core;
_b._ Slotted core.
In early dynamos the armature windings were placed upon an
iron core with a smooth surface. A chief disadvantage of this
arrangement is that the magnetic drag comes upon the inductors
and tends to displace them around the armature. To prevent
[Illustration: FIG. 1,478.--A style of disc largely used for
armature cores. The teeth are provided with dovetail grooves near
the circumference. After the coil is inserted in a groove, a wooden
wedge is driven in the groove which encloses the coil and secures it
firmly in position. This obviates the necessity of bands to resist the
centrifugal force acting on the inductors.]
[Illustration: FIG. 1,479.--Large revolving armature construction with
segmental discs dovetailed to spider spokes.]
[Illustration: FIG. 1,480.--Construction of large stationary armature;
view showing section of core and frame. The core discs are in segments
and are attached to the frame by dovetail joints as shown. The joints
are staggered in building up the core, that is, they are overlapped
so as not to unduly increase the reluctance of the magnetic circuit.
Dovetail joints obviate the use of through bolts which, if not
insulated, are liable to give rise to eddy currents by short circuiting
the discs.] /# this, projecting metal pieces called _driving horns_
were fixed into the core so as to take the pressure, but they proved
unsatisfactory. This defect together with the long air gap necessary
in smooth core construction resulted in the type being displaced by
slotted core armatures.
A slotted core is one whose surface is provided with slots or teeth
which carry the inductors, as shown in the accompanying illustrations,
and is the type almost universally used. The inductors are laid in the
slots, the sides and bottoms of which are first carefully insulated by
troughs of mica-canvas, micanite or other suitable insulating material.
~Ques. What are the advantages of slotted core armatures?~
Ans. The teeth protect the inductors, retain them in place against the
electrical drag and centrifugal force, and the construction permits a
reduction of air gap to a minimum, thus reducing the amount of copper
required for the field.
[Illustration: FIG. 1,481.--General Electric revolving field and
exciter armature. This is an example of direct connected exciter
construction. In this arrangement the armature of the exciter is
carried on the alternator shaft at the end farthest from the pulley. In
the smaller sizes the magnet frame is bolted to the bearing bracket,
but in the larger sizes special construction is used depending upon
the conditions to be met. On all alternators of standard design,
the field is built for 125 volts excitation and on account of the
increased danger from induced voltage, in case the machine is used
as a synchronous motor, the builders consider any higher voltage
undesirable.]
[Illustration: FIG. 1,482.--Section of General Electric Alternator
showing method of dovetailing core laminations to frame. The latter
is made in two general styles, known as the _box type_ and _skeleton
type_. ~The box type~ consists of a single casting for the smaller
sizes, but for large capacity alternators the frame castings are
usually divided into upper and lower sections. ~The skeleton type~
consists of two side castings between which substantial spacing rods
are set at regular intervals. The core consists of the usual sheet
iron lamination slotted and assembled; they are mounted on the inner
periphery of the frame, making lap joints (that is "staggered" as in
fig. 1,480), each section being dovetailed to the frame. Heavy clamping
rings or end plates are mounted on both sides of the core by means of
bolts, and supporting fingers extend along the slot projections. The
design is such as to provide for air circulation as shown in figs.
1,483 and 1,484.]
~Armature Windings.~--In general, the schemes for armature windings
for alternators are simpler than those for direct current machines,
as in the majority of cases the inductors are an even multiple of the
number of poles, and the groupings are usually symmetrical with respect
to each pole or each pair of poles. Furthermore, as a general rule,
all the inductors of any one phase are in series with one another;
therefore, there is only one circuit per phase, and this is as it
should be, since alternators are usually required to generate high
voltages. These general principles establish the rule, that in the
circuit in a single phase armature, and in the individual circuits
in a polyphase armature, the winding is never re-entrant, but the
circuits have definite endings and beginnings. In exceptional cases,
as those of polyphase converters, re-entrant circuits are employed,
and the armature windings are so constructed that a commutator can
be connected to them exactly as in direct current machines. These
armatures are usually of the lap wound drum type.
[Illustration: FIG. 1,483.--Section of General Electric alternator
frame showing air ducts and supporting fingers extending along the slot
projections. The air circulation is provided for by means of ducts
formed by suitable spacing blocks inserted at intervals between the
laminations, as shown here and in fig. 1,484. The armature coils are
form wound and designed so they can be readily replaced in case of
injury. They are taped and treated with an impregnating compound, in
the usual way, then inserted in the armature slots in an armour of horn
fibre and retaining wedges of wood are dovetailed into the slot walls.]
Alternator windings are usually described in terms of the number of
slots per phase per pole. For instance, if the armature of a 20 pole
three phase machine have 300 slots, it has 15 slots per pole or 5
slots per each phase per pole, and will be described as a five slot
winding. Therefore, in order to trace the connections of a winding,
it is necessary to consider the number of slots per pole for any one
phase on one of the following assumptions: 1, that each slot holds one
inductor; 2, that there is one side of a coil in each slot; and 3, that
one side of a coil is subdivided so as to permit of its distribution in
two or more adjacent slots.
The voltage depends upon the number of inductors in a slot, but the
breadth coefficient and wave form are influenced by the number of slots
per pole, and not by the number of inductors within the slots.
[Illustration: FIG. 1,484.--Section of General Electric stationary
armature showing method of assembling the coils. These are form
wound and are held in the slots by suitable wedges, the open slot
construction permitting the use of form wound coils that can be
easily removed and replaced in case of damage. Where heavy windings
project beyond the laminations, an additional support is provided
by means of an insulated metal ring, to which the outer ends of the
coils are fastened; the coils are thereby protected from mechanical
displacement, or distortion due to the magnetic disturbances caused
by violent fluctuations of the load or short circuits. The figure
shows a section of a supporting ring of this type and indicates the
method of connecting the coils to it. In order to admit of the prompt
replacement of damaged coils, sufficient space is usually provided
between the alternator bearings to allow ample movement of the armature
to permit of ready access to both armature and field coils. Where space
necessitates the use of a short shaft, access to the windings may be
had by disconnecting some of the coils and lifting the upper half of
the armature.]
~Classification of Windings.~--The fact that alternators are built
in so many different types, gives rise to numerous kinds of armature
winding to meet the varied conditions of operation. In dividing these
forms of winding into distinctive groups, they may be classified,
according to several points of view, as follows:
1. With respect to the form of the armature, as:
_a._ Revolving;
_b._ Stationary.
2. With respect to the mode of progression, as:
_a._ Lap winding;
_b._ Wave winding.
3. With respect to the relation between number of poles and number of
coils, as:
_a._ Half coil winding;
_b._ Whole coil winding.
4. With respect to the number of slots, as:
_a._ Concentrated or uni-coil winding;
_b._ Distributed or multi-coil winding.
Partially distributed;
Fully distributed.
5. With respect to the form of the inductors, as:
_a._ Wire winding;
_b._ Strap winding;
_c._ Bar winding.
6. With respect to the number of coils per phase per pole, as:
_a._ One slot winding;
_b._ Two slot winding;
etc.
7. With respect to the kind of current delivered, as:
_a_. Single phase winding;
_b_. Two phase winding;
_c_. Three phase winding.
[Illustration: FIG. 1,485.--Section of Western Electric stationary
armature core showing laminations clamped in place, and ventilating
ducts. The stator or stationary armature consists of soft iron
laminations assembled in the magnet frame with stator coils embedded
in the core slots. The laminations are punched separately and then
carefully annealed to reduce hysteresis losses. After annealing, a coat
of japan is applied, effectively preventing the flow of eddy currents
in the assembled core. The frame is cast iron and of the box type
construction. The frames of the smaller sizes are cast in one piece,
while frames of the larger sizes are split to facilitate installation.
Large openings are provided in the box type frame, in order to improve
the ventilation. The laminations are securely held in place in the
frame by heavy end rings and by steel clamping fingers which are firmly
bolted to the frame. The outer circumference of the core is dovetailed
to the frame, and the inner circumference is slotted to receive the
windings. The alignment of the slots is insured by means of metal
wedges, and no filing is done on the slots, so that each lamination is
always insulated from the next one. Numerous ventilating ducts allow
the free circulation of cool air through and around the coils. The open
slot construction is employed and the coils are fitted into insulating
troughs which offer excellent mechanical and electric protection. The
coils are held in place by suitable wedges.]
8. With respect to the shape of the coil ends, as:
_a_. Single range;
_b_. Two range;
etc.
In addition to these several classes of winding, there are a number of
miscellaneous windings of which the following might be mentioned:
_a._ Chain or basket winding;
_b._ Skew coil winding;
_c._ Fed-in winding;
_d._ Imbricated winding;
_e._ Mummified winding;
_f._ Spiral winding;
_g._ Shuttle winding;
_h._ Creeping winding;
_i._ Turbine alternator winding.
[Illustration: FIG. 1,486.--Method of assembling form wound coils. The
picture shows a section of a General Electric armature with part of the
coils in place. A layer of insulating material is first placed in the
slots, before inserting the coils as seen at the left. When the coils
are in place and surrounded by this layer of insulating material the
retaining wedges are inserted in the notches, thus closing the slots
and protecting the coils from mechanical injury. A few wedges are seen
in position at the right.]
~Ques. Define a revolving and a stationary winding.~
Ans. The words are self-defining; a winding is said to be revolving or
stationary according as the armature forms the rotor or stator of the
machine.
~Ques. What is the significance of the terms lap and wave as applied to
alternator windings?~
Ans. They have the same meaning as they do when applied to dynamo
windings.
[Illustration: FIG. 1,487.--Section of General Electric stationary
armature ventilating ducts and winding in position.]
These are described in detail in Chapter XVIII. Briefly a lap
winding is one composed of lap coils; a wave winding is one
which roughly resembles in its diagram, a section of waves.
~Half Coil and Whole Coil Windings.~--The distinction as to whether the
adjacent sides of consecutive coils are placed together under one pole
or whether they are separated a distance equal to the pole pitch, gives
rise to what is known as half coil and whole coil windings.
A half coil or hemitropic winding is _one in which the coils in any
phase are situated opposite every other pole_, that is, _a winding in
which there is only one coil per phase_ ~per pair of poles~, as in fig.
1,488.
_A whole coil winding is one in which there is one coil per phase_
~per pole~, as in fig. 1,489, the whole (every one) of the poles being
subtended by coils.
[Illustration: FIGS. 1,488 and 1,489.--Elementary bipolar alternators
with _half coil_ and _whole coil_ windings. In a half coil winding
there is one coil per phase _per pair of poles_; in a whole coil
winding there is one coil per phase _per pole_.]
~Concentrated or Uni-Coil Winding.~--Fig. 1,492 shows the simplest type
of single phase winding. It is a one slot winding and is sometimes
called "monotooth" or "uni-coil" winding. The surface of the armature
is considered as divided into a series of large teeth, one tooth to
each pole, and each tooth is wound with one coil, of one or more turns
per pole. Since all the turns of the coil are placed in single slots,
the winding is called "concentrated."
[Illustration: FIG. 1,490.--Multi-polar revolving armature alternator
with half coil winding, shown in radially developed diagram to clearly
indicate the path of the winding. A half coil or hemitropic winding has
a slightly higher reactance than a winding in which two distinct coils
are used in the same slot, one going forward and the other backward.
The most usual three phase windings are of the half coil type as the
three sets of coils are equispaced over a pair of poles.]
[Illustration: FIG. 1,491.--Multi-polar revolving armature alternator
with whole coil winding shown in radially developed diagram to clearly
indicate the path of the winding.]
~Ques. What are the features of concentrated windings?~
Ans. Cheap construction, maximum voltage for a given number of
inductors. Concentrated windings have greater armature reaction and
inductance than other types hence the terminal voltage of an alternator
with concentrated winding falls off more than with distributed winding
when the current output is increased. An alternator, therefore,
does not have as good regulation with concentrated winding as with
distributed winding.
[Illustration: FIGS. 1,492 and 1,493.--Concentrated windings. A
concentrated winding is one in which the armature has only _one tooth
per phase per pole_, that is, the number of teeth equals the number of
poles. A concentrated winding of the half coil type has only one side
of a coil in each slot as in fig. 1,492. In the whole coil variety,
each slot contains neighboring sides of adjacent coils, as in fig.
1,493. In construction, wedges are generally used for retaining the
half coils, and with whole coils the teeth have projecting horns for
this purpose.]
~Ques. What should be noted with respect to concentrated windings?~
Ans. A concentrated winding, though giving higher voltage than the
distributed type with no load, may give a lower voltage than the latter
at full load.
[Illustration: FIG. 1,494.--Laminated core with two coils in position;
type of punchings used on some machines having concentrated whole coil
windings. The manner of assembling the coils is shown in fig. 1,495.]
~Ques. What is the wave form with a concentrated winding?~
Ans. The pressure curve rises suddenly in value as the armature slots
pass under the pole pieces, and falls suddenly as the armature slots
recede from under the pole pieces.
[Illustration: FIG. 1,495.--Westinghouse single phase concentrated coil
armature; view showing method of placing coils. The coils are machine
wound on formers and after being taped, varnished and baked, are spread
out slightly so as to pass over the teeth and are then forced into
place in the deep slots by means of wooden wedges, being securely held
in place by retaining wedges, as shown in fig. 1,494.]
~Distributed or Multi-Coil Windings.~--Instead of winding an armature
so it will occupy only one slot per phase per pole, it may be spread
out so as to fill _several slots per phase per pole_. This arrangement
is called a distributed winding.
To illustrate, fig. 1,496 represents a coil of say fifteen
turns. This could be placed on an armature just as it is, in
which case only one slot would be required for each side, that
is, two in all. In place of this thick coil, the wire could be
divided into several coils of a lesser number of turns each,
arranged as in fig. 1,497; it is then said to be _partially
distributed_, or it could be arranged as in fig. 1,498, when it
is said to be _fully distributed_.
[Illustration: FIGS. 1,496 to 1,498.--Alternator coils, showing
difference between the concentrated, partially distributed, and fully
distributed forms. Fig. 1,496 shows a concentrated coil in which all
the wire is wound in one large coil; in the partially distributed type
fig. 1,497, the wire of fig. 1,496, is wound in two or more coils or
"sections" connected as shown, leaving some space inside not taken up
by the subdivisions. In fig. 1,498 the wire of fig. 1,496 is _fully
distributed_, being wound in a series of coils, so that all the
interior space is taken up by the wire, that is to say, the spaces not
occupied by the wire (the teeth when placed on the armature) are of
equal size.]
A partially distributed winding, then, is one, as in fig. 1,499,
in which the coil slots do not occupy all the circumference of the
armature; that is, the core teeth are not continuous.
A fully distributed winding is one in which the entire surface of the
core is taken up with slots, as in fig. 1,500.
~Ques. In a distributed coil what is understood by the breadth of the
coil?~
Ans. The distance between the two outer sides, as B in figs. 1,497 and
1,498.
[Illustration: FIG. 1,499.--Partially distributed winding. Each coil
unit is here divided into two concentric coils of different dimensions
and connected in series, as shown in detail in fig. 1,497. This being
a "whole coil" winding the several units are so connected that the
winding of adjacent units proceeds in opposite directions, that is, one
coil is wound clockwise, and the next counter clockwise, etc., so that
the induced currents flow in a common direction as indicated by the
arrows for the position shown.]
[Illustration: FIG. 1,500.--Fully distributed winding. In this type
of winding each coil consists of so many sub-coils that the winding
occupies the entire surface of the armature core; that is, there are no
extensive spaces unoccupied, the spacing being uniform as shown.]
~Ques. How far is it advisable to spread distributed coils of a single
phase alternator?~
Ans. There is not much advantage in reducing the interior breadth
much below that of the breadth of the pole faces, nor is there much
advantage in making the exterior breadth greater than the pole pitch.
Undue spreading of distributed coils lowers the value of the
Kapp coefficient (later explained) by reducing the breadth
coefficient and makes necessary a larger number of inductors to
obtain the same voltage.
The increase in the number of inductors causes more armature
self-induction. From this point of view, it would be preferable
to concentrate the winding in fewer slots that were closer
together. This, however, would accentuate the distorting and
demagnetizing reactions of the armature. Accordingly, between
these two disadvantages a compromise is made, as to the extent
of distributing the coils and spacing of the teeth, the
proportions assigned being those which experience shows best
suited to the conditions of operation for which the machine is
designed.
[Illustration: FIG. 1,501.--Developed diagram of single phase
concentrated whole coil winding in two slot stamping for six pole
alternator. If the sides of adjacent whole coils be slightly separated
by placing the winding in a two slot stamping the electrical result
will not differ materially from the monotooth whole coil winding, but
if the winding be hemitropic, as in fig. 1,502, and has coils of two
sizes as shown, it will be suitable for high voltages.]
~The Kapp Coefficient.~--A volt or unit of electric pressure is defined
as the pressure induced by the cutting of 100,000,000 or 10⁸ lines
of force per second. In the operation of an alternator the maximum
pressure generated may be expressed by the following equation:
π_f_ZN
Eₘₐₓ = ------ (1)
10⁸
in which
_f_ = frequency;
Z = number of inductors in series in any one magnetic circuit;
N = magnetic flux, or total number of magnetic lines in one
pole or in one magnetic circuit.
The maximum value of the pressure, as expressed in equation (1), occurs
when θ = 90°.
[Illustration: FIG. 1,502.--Developed diagram of single phase partially
distributed half coil winding for six pole alternator in two slot
stamping, same as in fig. 1,501. In this arrangement the direction of
rotation is not reversed. It is a question as to how far the coils of a
single-phase armature may be spread with advantage. There is not much
advantage in reducing the interior breadth of the coils below that of
the pole face, nor in widening the exterior breadth beyond that of the
pole pitch.]
The virtual value of the volts is equal to the maximum value divided by
√2̅, or multiplied by ½ √2̅, hence,
½ √2̅ × π_f_ZN 2.22_f_ZN
Eᵥᵢᵣₜ = -------------- = --------- (2)
10⁸ 10⁸
This is usually taken as the fundamental equation in designing
alternators. It is, however, deduced on the assumptions that the
distribution of the magnetic flux follows a sine law, and that the
whole of the loops of active inductors in the armature circuit acts
simultaneously, that is to say, the winding is concentrated.
[Illustration: FIG. 1,503.--Developed diagram of single phase winding
with fully distributed coils. As explained, excessive spreading lowers
the value of the "Kapp" coefficient, and consequently the voltage; also
the use of a larger number of inductors to obtain the same voltage
results in an increase of armature self-induction. On the other hand,
if the winding were concentrated in fewer slots and these slots were
closer together, the result will be an increase in distorting and
demagnetizing reactions of the armature. Therefore, a compromise
between these two disadvantages must be made. The common practice is to
wind in two or three slots per pole per phase.]
[Illustration: FIG. 1,504.--Allis-Chalmers lap wound coils forming a
three slot distributed coil unit. In construction, after the coils have
been covered with insulating materials and treated with insulating
compound, the parts that lie in the slots are pressed to exact size
in steam-heated moulds. This runs the insulating material into all
the small spaces in the coil, excluding moisture and rendering the
insulation firm and solid. The ends of the coils, where they project
beyond the slots, are heavily taped.]
[Illustration: FIG. 1505.--Allis-Chalmers armature construction; view
showing section of frame and two layer winding.]
In practice, the coils are often more or less distributed, that is,
they do not always subtend an exact pole pitch; moreover, the flux
distribution, which depends on the shaping and breadth of the poles, is
often quite different from a sine distribution. Hence, the coefficient
2.22 in equation (2) is often departed from, and in the general case
equation (2) may be written
~_kf_ZN
Eᵥᵢᵣₜ = ------- (3)
10⁸~
where _k_ is a number which may have different values, according to the
construction of the alternator. This number _k_ is called the _Kapp
coefficient_ because its significance was first pointed out by Prof.
Gisbert Kapp.
[Illustration: FIGS. 1,506 and 1,507.--Effect of breadth of coils in
distributed windings. In the section of the alternator shown in fig.
1,506 the directions of the pressures induced as the armature rotates
clockwise are represented by dots for those which act towards the
reader, and by crosses for those which act from the reader (the dots
and crosses representing respectively the heads and tails of arrows).
Since the field is not uniform but maximum at the center and gradually
weakening towards the extremities, it is obvious that the maximum
pressure is induced in any inductor as it passes the center of the
pole, this variation being indicated by the heavier dots and crosses
toward the center. Now if a number of these inductors be connected up
to form a distributed coil as in fig. 1,507, the pressures induced in
each will be added, but all the maximum pressure will not be induced
in all at the same time, hence the total pressure induced in the
distributed coil is less than it would be if the coil were concentrated
as in fig. 1,509.]
[Illustration: FIG. 1,508.--Diagram of distributed coil whose
inner breadth is less than the breadth of the pole face, showing
the disadvantage of such arrangement. The pressures induced in the
inner windings of such a coil are opposing each other at the instant
depicted, that is, while the inductors are under the pole face, such
action of course being objectionable.]
The value of _k_ is further influenced by a "breadth coefficient" or
"winding factor."
The effect of breadth in distributed windings is illustrated in figs.
1,506 to 1,508.
~Wire, Strap, and Bar Windings.~--In the construction of alternators,
the windings may be of either wire, strap, or bar, according to which
is best suited for the conditions to be met.
~Ques. What conditions principally govern the type of inductor?~
Ans. It depends chiefly upon the current to be carried and the space in
which the inductor is to be placed.
[Illustration: FIG. 1,509.--Simple form of alternator coil, consisting
of numerous turns of insulated wire wound around a form, then covered
with a tape winding, varnished and baked.]
~Ques. What kind of inductors are used on machines intended for high
voltage and moderate current?~
Ans. The winding is composed of what is called _magnet wire_, with
double or triple cotton insulation.
~Ques. Where considerable cross section is required how is a wire
inductor arranged?~
Ans. In order that the coil may be flexible several small wires in
multiple are used instead of a single large wire.
~Ques. How is the insulation arranged on inductors of this kind?~
Ans. Bare wire is used for the wires in parallel, insulation being
wrapped around them as in fig. 1,510.
This construction reduces the space occupied by the wires, and
the insulation serves to hold them in place.
[Illustration: FIGS. 1,510 and 1,511.--Multi-wire inductors. When the
cross section of inductor necessary to carry the current is large, the
use of a single wire would present difficulties in winding on account
of its stiffness. Accordingly two or more smaller wires are used in
parallel to secure the required cross section. Bare wire is used and
the several sections encased in insulation as shown, the combination
being more flexible than an equivalent single wire.]
[Illustration: FIG. 1,512.--Two coil slot for whole coil winding. The
slot has two recesses A and B for the reception of separate coils. In
assembling the winding, the inner wedge is first placed in position and
then the slot line with the insulating material. This usually consists
of alternate layers of mica and pressboard. The coils composed of
several turns of wire or copper strip are wound in place, and after
covering with a layer of insulation, the outer wedge is pushed in place
to retain the inductors in position.]
~Ques. What precaution is taken in insulating a wire wound coil
containing a large number of turns?~
Ans. On account of the considerable difference of pressure between
layers, it is necessary to insulate each layer of turns as well as the
outside of the coil, as shown in fig. 1,513.
[Illustration: FIG. 1,513.--Method of winding a coil containing a large
number of turns, when there is considerable difference of pressure
between the layers. In such cases to guard against short circuits or
breakdown of the insulation, each layer of turns is insulated from the
next layer by the insulating strips A, B, C, in addition to the regular
insulation around each wire. After the coil is made up it is wound with
insulating tape, varnished and baked.]
~Ques. Do distributed coils require insulation between the separate
layers?~
Ans. Since they are subdivided into several coils insulation between
layers is usually not necessary.
~Ques. How is a coil covered?~
Ans. It is wound with a more or less heavy wrapping of tape depending
upon the voltage.
[Illustration: FIGS. 1,514 and 1,515.--Single and double layer
multi-wire inductors and methods of placing them on the core. Here the
term layer means unit, in fact each unit is made up of several "layers"
of wires. In fig. 1,514, where so many wires are bunched together in
one unit, each layer of turns is separated from those adjacent by
insulating strips on account of the considerable difference of pressure
between layers. This insulation is not necessary in fig. 1,515 where
there are two units or so called layers. In both cases the inductors
are held in place by wedges driven into dovetail grooves.]
Linen tape of good quality, treated with linseed oil, forms a
desirable covering. Where extra high insulation is required the
tape may be interleaved with sheet mica.
~Ques. Is the insulation placed around the coils all that is necessary?~
Ans. The slots into which the coils are placed, are also insulated.
[Illustration: FIG. 1,516.--Copper strap or ribbon with insulation.
These are generally from ¹/₃₂ to ¹/₁₆ inch thick with rounded edges as
shown to avoid cutting the insulation.]
[Illustration: FIG. 1,517.--Bar inductor. Its shape enables putting the
maximum cross section of copper into the slot and is used to advantage
on machines which generate large currents.]
[Illustration: FIG. 1,518.--Style of armature core stamping used with
bar wound machines. This construction, since there are no indentations
in the teeth for wedges, makes it necessary to provide bands to hold
the bars in place.]
~Ques. How are bar windings sometimes arranged?~
Ans. In two layers, as in fig. 1,523.
~Single and Multi-Slot Windings.~--These classifications correspond to
_concentrated_ and _distributed windings_, previously described. In
usual modern practice, only two-thirds of the total number of slots
(assuming the spacing to be uniform)
[Illustration: FIGS. 1,519 and 1,520.--Bent bar inductor and method of
connection with soldered joint. Fig. 1,519 shows one bar and shape of
bent ends. The portion from C to D is placed in the slot; B to C and D
to E, bent or connector sections; A to B and E to F, ends bent parallel
to slot for soldering. Fig. 1,520 shows two bar inductors connected.]
[Illustration: FIGS. 1,521 and 1,522.--Method of avoiding a soldered
joint at one end of a bar inductor by using a bar of twice the length
shown in fig. 1,519, and bending it into a long U form, as in fig.
1,521, after which it is spread out forming two inductors, as in
fig. 1,522.] of a single phase armature are wound with coils. The
reason for this may be explained by aid of fig. 1,524, which shows an
armature with six slots per pole, four of which are wound. Owing to the
different positions of, say, coils A and B, there will be a difference
in phase between the pressure generated in them and consequently the
resultant pressure of the two coils joined in series will be less than
the sum of the pressure in each coil.
[Illustration: FIG. 1,523.--Arrangement in slot of two layer bar
winding. With bar inductors, as must be evident from the illustration,
the maximum cross section of copper can be placed in a slot of given
dimension, hence a bar winding is used to advantage for alternators
designed to carry a large current. Bar inductors, on account of the
shape of their ends, must be placed in the slots from the top, because
the bent ends do not admit of pushing them in. Straight slots are
therefore necessary, the inductors being held in place by wooden strips
and tie bands as shown.]
Fig. 1,525 shows the pressure plotted out as vector quantities, and the
table which follows gives the relative effectiveness of windings with
various numbers of slots wound in series.
The figures in the last column of the table show that a large increase
in the weight of active material is required if the inductors in a
single phase machine are to be distributed over more than two-thirds
the pole pitch. Again, if much less than two-thirds of the surface be
wound, it is more difficult to provide a sine wave of pressure.
[Illustration: FIG. 1,524.--Diagram of single phase multi-coil or
distributed winding to show characteristic differences in action and
construction from single coil or concentrated winding.]
[Illustration: FIG. 1,525.--Vector diagram of pressures induced in the
single phase multi-coil or distributed winding shown in diagram in fig.
1,524.]
~TABLE OF RELATIVE EFFECTIVENESS OF WINDINGS~
----------------------------------------------------------------
| | | | |
|Slots wound | Pressure across | Winding | Quantity of |
| in series | coils | coefficient | copper to produce |
| | | | same pressure |
+------------+-----------------+-------------+-------------------+
| | | | |
| 1 | 1 | 1 | 1 |
| 2 | 1.93 | .97 | 1.03 |
| 3 | 2.73 | .91 | 1.10 |
| 4 | 3.34 | .84 | 1.19 |
| 5 | 3.72 | .74 | 1.35 |
| 6 | 3.86 | .64 | 1.56 |
+------------+-----------------+-------------+-------------------+
~Ques. What other advantage besides obtaining a sine wave is secured by
distributing a coil?~
Ans. There is less heating because of the better ventilation.
[Illustration: FIG. 1,526.--Developed diagram of a single phase
monotooth or one slot bar winding; it is suitable only for operation at
low voltage.]
~Single Phase Windings.~--There are various kinds of single phase
winding, such as, concentrated, distributed, hemitropic, etc. Fig.
1,527 shows the simple type of single phase winding. It is a "one slot"
winding, that is, concentrated coils are used.
The armature has the same number of teeth as there are poles, the
concentrated coils being arranged as shown. In designing such a
winding, the machine, for example, may be required to generate, say,
3,000 volts, frequency 45, revolutions 900 per minute.
These conditions require 720 inductors in series in the
armature circuit, and as the armature is divided into six slots
corresponding to the six poles, there will be 120 inductors
per slot, and the coil surrounding each of the six teeth on
the surface of the armature will consist of 60 turns. The
connections must be such as to give alternate clockwise and
counter-clockwise winding proceeding around the armature.
[Illustration: FIG. 1,527.--Diagram of six pole single phase revolving
armature alternator, with monotooth or concentrated whole coil
winding. For 3,000 volts at 900 revolutions per minute, 120 inductors
are required. And in the case of a concentrated or monotooth winding
they may be arranged in "whole coils" as above or in "half coils"
(hemitropic) as in fig. 1,528.]
~Ques. In what other way could the inductors be arranged in
concentrated coils?~
Ans. They could be grouped in three coils of 120 turns each, as shown
in fig. 1,528.
When thus grouped the arrangement is called a hemitropic
winding, as previously explained.
[Illustration: FIG. 1,528.--Diagram of six pole single phase alternator
with concentrated half coil or hemitropic winding of same capacity as
in fig. 1,527. There are an equal number of inductors, but in this case
arranged in three instead of six coils. In this winding the direction
of winding is alternately reversed so that the induced pressures do not
oppose one another.]
~Ques. What is the advantage, if any, of a half coil winding?~
Ans. In single phase machines a half coil winding is equivalent,
electrically, to a monotooth winding, and, therefore, is not of any
particular advantage; but in three phase machines, it has a decided
advantage, as in such, a concentrated winding yields a higher pressure
than a distributed winding.
[Illustration: FIG. 1,529.--Two phase concentrated whole coil winding.
In this style winding the total number of slots is twice the number
of poles, or one slot per pole per phase. It comprises two windings
identical with fig. 1,527, being spaced 90 polar degrees as shown. The
two circuits are independent, the windings terminating at the four
collector rings.]
[Illustration: FIG. 1,530.--Two phase winding in two slots per pole
per phase. This stamping distributes the coils of each phase into two
sections, as A and B. The coils are of the "whole" type and with six
poles the total number of slots is 4 × 6 = 24, uniformly spaced as
shown.]
~Two Phase Armature Windings.~--This type of winding can be made from
any single phase winding by providing another set of slots displaced
along the surface of the armature to the extent of one-half the pole
pitch, placing therein a duplicate winding.
[Illustration: FIGS. 1,531 and 1,532.--Developed diagram of the single
phase monotooth windings shown in figs. 1,527 and 1,528.]
For instance: If the six pole monotooth, single phase winding,
shown in fig. 1,527, be thus duplicated, the result will be the
one slot two-phase winding shown in fig. 1,529, which will have
twelve slots, and will require four slip rings, or two rings
for each phase.
By connecting up the two windings in series, the machines could
be used as a single phase, with an increase of voltage in the
ratio of 1.41 to 1.
[Illustration: FIG. 1,533.--Two phase winding in three slots per pole
per phase. The coils of each phase are of the partially distributed
type, each coil being made up of three sections as shown. The direction
of winding is alternately reversed.]
[Illustration: FIG. 1,534.--Section of two phase winding showing
shaping of the coil ends. Every other coil is flat, while the
alternates have their ends bent down as shown. With respect to the
shaping of the coil ends, it is called a _two range winding_.]
~Ques. How must the coils be constructed for two phase windings?~
Ans. They must be made of two different shapes, one bent up out of the
way of the other, as in fig. 1,534.
There are numerous kinds of two phase windings; the coils may
be concentrated or distributed, half coil or whole coil, etc.
Fig. 1,530 shows a two phase winding with four slots per pole,
and fig. 1,533 one with six slots per pole.
[Illustration: FIG. 1,535.--Section of Triumph armature showing method
of arranging the three phase winding.]
~Three Phase Armature Windings.~--On the same general principle
applicable to two phase windings, a three phase winding can be made
from any single phase winding, by placing three identical single phase
windings spaced out successively along the surface of the armature
at intervals _equal to one-third and two-thirds, respectively, of
the double pole pitch_, the unit in terms of which the spacing is
expressed, being that pitch, which corresponds to one whole period.
[Illustration: FIG. 1,536.--Three phase winding with distributed
coils--wound in four slots per pole per phase; diagram showing
placement of the coils.]
[Illustration: FIG. 1,537.--Treatment of coil ends in two phase, two
range windings. In this arrangement _straight out_ (B) and _bent up_
(A) coils are used which are placed on the armature as is clearly shown
in the illustration.]
[Illustration: FIG. 1,538.--Three phase, 10 pole, 30 slot winding in
two ranges. In this winding perfect symmetry occurs after every four
poles. Accordingly in the case of an odd number of pairs of pole, one
of the coils must necessarily be askew going from the inner to the
outer range as at M.]
Each of the three individual windings must be concentrated into narrow
belts so as to leave sufficient space for the other windings between
them. This limits the breadth or space occupied by the winding of any
one phase to one-third of the pole pitch.
[Illustration: FIG. 1,539.--Three phase 10 pole 30 slot winding in
three ranges. The coils of each phase are alike, those of the A phase
being all in the straight out range, those in the B phase, in a bent up
range, and those in the C phase in a bent down range. This arrangement
has the disadvantage, that by reason of the third range, the field
magnet cannot be withdrawn. This treatment of the coil ends is more
clearly shown in fig. 1,540.]
~Ques. How are three phase coil ends treated?~
Ans. They may be arranged in two ranges, as in fig. 1,538, or in three
ranges, as in fig. 1,539.
~Ques. What kind of coil must be used for three phase windings in order
that the ends may be arranged in only two ranges?~
Ans. Hemitropic or half coils; that is, the number of coil per phase
must be equal to one-half the number of pole.
[Illustration: FIGS. 1,540 and 1,541.--Treatment of coil ends in three
phase, three range windings. Fig. 1,540, inadmissible arrangement in
which the field magnet cannot be withdrawn; fig. 1,541, admissible
arrangement in which the armature segments can be divided. This enables
the top half of armature to be removed by disconnection without
unwinding any coil.]
~Grouping of Phases.~--In the preceding diagrams, the general
arrangement of the coils on the armature surface are shown for the
numerous classes of winding. In polyphase alternators the separate
windings of the various phases may be grouped in two ways:
1. Star connection;
2. Mesh connection.
[Illustration: FIG. 1,542.--Three phase winding with half coils. The
advantage of employing half coils is that the ends may be arranged in
two ranges as shown. There is one slot per phase per pole, that is,
total number of slots = 3 × number of poles.]
[Illustration: FIG. 1,543.--Three phase winding with whole coils.
Two sides of adjacent coils come in one slot. Number of coils per
phase = number of poles per phase. Total number of slots = 3 multiplied
by number of poles per phase. Whole coils require the ends arranged in
three ranges as indicated. The coils are concentrated.]
~Ques. Describe the two phase star connection.~
Ans. In this method of grouping, the middle points of each of the
two phases are united to a common junction M, and the four ends are
brought out to four terminals _a_, _a'_, _b_, _b'_, as shown in fig.
1,544, or in the case of revolving armatures, to four slip rings.
[Illustration: FIG. 1,544.--Diagram of two phase star grouping.]
~Ques. What does this arrangement give?~
Ans. It is practically equivalent to a four phase system.
[Illustration: FIG. 1,545.--Diagram of two phase mesh grouping.]
~Ques. How is the two phase mesh connection arranged?~
Ans. In this style of grouping, the two phases are divided into two
parts, and the four parts are connected up in cyclic order, the end
of one to the beginning of the next, so as to form a square, the four
corners of which are connected to the four terminals _a_, _b_, _a'_,
_b'_, as shown in fig. 1,545, or in the case of revolving armatures, to
four slip rings.
~Ques. Describe a three phase star connection?~
Ans. In three phase star grouping, one end of each of the three
circuits is brought to a common junction M, usually insulated, and the
three other ends are connected to three terminals _a_, _b_, _c_, as
shown in fig. 1,546, or in the case of revolving armatures to three
slip rings.
[Illustration: FIG. 1,546.--Diagram of three phase star grouping,
commonly called ~Y~ grouping owing to its resemblance of the letter
~Y~. The current in each main is obviously equal to the current in
each phase winding, but the terminal pressure is the vector sum of the
pressures in the component phase windings, that is, √3̅ multiplied by
the pressure in one phase.]
~Ques. What other name is given to this connection, and why?~
Ans. It is commonly called a ~Y~ connection or grouping owing to the
resemblance of its diagrammatic representation to the letter ~Y~.
[Illustration: FIG. 1,547.--Radial diagram of three phase, one slot
winding with ~Y~ connection.]
[Illustration: FIG. 1,548.--Radial diagram of three phase one slot
winding with delta connection.]
~Ques. How is a three phase mesh connection arranged?~
Ans. The three circuits are connected up together in the form of a
triangle, the three corners are connected to the three terminals, _a_,
_b_, _c_, as shown in fig. 1,549, or in the case of revolving armatures
to three slip rings.
[Illustration: FIG. 1,549.--Diagram of three phase mesh grouping,
commonly called delta grouping owing to its resemblance to the Greek
letter Δ. The voltage at the terminals is equal to the voltage in one
phase, and the current in each line is equal to the vector sum of the
currents in two phases, that is, it is equal to √3̅ multiplied by the
current in one phase.]
~Ques. What other name is given to this style of connection, and why?~
Ans. It is commonly called a _delta_ grouping on account of the
resemblance of its diagrammatic representation to the Greek letter Δ.
[Illustration: FIG. 1,550.--Three phase winding with short coils. The
use of short coils as here shown, in which the coil breadth = ⅔ pole
pitch, avoids the necessity of overlapping.]
In polyphase working, it is evident that by the use of four
equal independent windings on the armature, connected to eight
terminals or slip rings, a two phase alternator can be built to
supply currents of equal voltage to four independent circuits.
Likewise, by the use of three equal independent windings,
connected to six terminals or slip rings, a three phase
alternator can be made to supply three independent circuits.
This is not the usual method employed in either case, however,
as the star grouping or mesh grouping methods of connection
not only gives the same results, but also, in star grouping,
a greater plurality of voltages for the same machine, and a
higher voltage between its main terminals.
Radial diagrams of the arrangement and connections of ~Y~
grouping of lap windings and wave windings for three phase
alternators are shown by figs. 1,551 and 1,552.
[Illustration: FIG. 1,551.--Radial diagram of three phase _lap_ winding
with star connection.]
~Ques. In three phase star grouping, what is the point where the phases
join, called?~
Ans. The star point.
~Ques. In a three phase star connected alternator what is the voltage
between any two collector rings?~
Ans. _It is equal to the voltage generated per phase multiplied by √3̅
or 1.732._
[Illustration: FIG. 1,552.--Radial diagram of three phase _wave_
winding with star connection.]
~Ques. In a three phase star connected alternator what is the value of
the current in each line?~
Ans. The same as the current in each phase winding.
[Illustration: FIGS. 1,553 and 1,554.--Gramme ring armatures showing
three phase star and mesh connections, respectively, with direction of
currents in the coils. In the figures, the coils A, B, C, are spaced at
equidistant positions on the ring core. The arrow heads represent the
directions of the induced pressures or currents for the position shown,
the rotation being clockwise. In coil A the pressure is increasing, in
coil B it is diminishing, but is in the same direction as in A, whereas
in coil C it is also diminishing, but is in the opposite direction to
what it is in coils A and B. As the rings rotate the three coils have
similar alternations of pressure induced in them, but differ in phase.
If _a_, _b_ and _c_ be joined to collector rings three phase currents
can be supplied to the outer circuits. In fig. 1,553 at the instant
represented _a_ and _b_ are giving their current to their lines, while
_c_ is receiving from its line a current equal to the sum of _a_ and
_b_. In fig. 1,554, at the instant represented, the currents sent out
from _a_ will be equal to the sum of the currents in _x_ and _y_, and
intermediate between them in phase. The current from _b_ will be equal
to the difference of the currents in _z_ and _y_, and of intermediate
phase, while similarly the current received by _c_ will be equal to the
sum of the currents in _x_ and _z_.]
~Ques. What is the value of the total output in watts of a star
connected alternator?~
Ans. It is equal to the sum of the outputs of each of the three phases.
When working on a non-inductive load, the total output of a star
connected alternator is equal to √3̅ multiplied by the product of the
line current and line voltage.
~Ques. What is the value of the line voltage in a three phase delta
connected alternator?~
Ans. It is equal to the voltage generated in each phase.
~Ques. What is the value of the line current in a three phase delta
connected alternator?~
Ans. It is equal to the current in each phase multiplied by √3̅.
~Ques. What is the total output of a three phase delta connected
alternator working on a non-inductive load?~
Ans. The total watts is equal to √3̅ multiplied by the product of the
line current and the line voltage.
[Illustration: FIGS. 1,555 to 1,557.--Separate coils, and section
of Allis-Chalmers alternator with coils in place. Numerous openings
are provided in the frame through which air currents, set up by the
revolving field, can pass freely and carry off heat. Shields are
provided to protect the armature coils where they project beyond the
core. In assembling the core spacing segments are placed at intervals
to form ventilating ducts. After the coils have been covered with
insulating materials and treated with insulating compound, the parts
that are to lie in the slots are pressed to exact size in steam heated
moulds. This runs the insulating material into all the small spaces in
the coil so as to exclude moisture, it also makes the coil structure
firm and solid. The projecting ends of the coils are heavily taped,
suitable supports being provided for the coil connections so that they
cannot become displaced on account of stresses due to short circuits
or other causes. On high pressure machines the armature terminals are
arranged so that it is impossible for an attendant to make accidental
contact with them.]
~Ques. What are the features of the star connection?~
Ans. It gives a higher line voltage than the delta connection for the
same pressure generated per phase, hence it is suited for machines of
high voltage and moderate current.
The delta connection gives a lower line voltage than the
star[5] connection for the pressure generated per phase, and
cuts down the current in the inductors; since the inductors, on
this account, may be reduced in size, the delta connection is
adapted to machines of large current output.
[5] NOTE.--In the star connected armature the proper ends to connect
to the common terminal or star point are determined as follows: Assume
that the inductor opposite the middle of a pole is carrying the maximum
current, and mark its direction by an arrow. Then the current in the
inductors on either side of and adjacent to it will be in the same
direction. As the maximum current must be _coming from_ the common
terminal, the end toward which the arrow points must be connected
to one of the rings, while the other end is connected to the common
terminal. The current in the two adjacent inductors evidently must
be flowing into the common terminal, hence the ends toward which the
arrows point must be connected to the common terminal, while their
other ends are connected to the remaining two rings.
[Illustration: FIG. 1,558.--Diagram of Westinghouse two phase composite
wound alternator, showing connections between two phase armature and
a single phase rectified and composite field winding. The arrangement
makes use of a series transformer, mounted on the spokes of the
armature. By means of this series transformer the voltage delivered
to the rectifying commutator and the fields is much less than that
generated by the machine. The armature of this machine is of the closed
coil single winding type, all the armature inductors being connected
with each other to form a closed circuit which resembles to a certain
extent the ordinary drum winding of a multipolar direct current
machine. This winding is tapped out at two points per pole just as is
the continuous winding of a two phase rotary converter, these taps
running to collector rings through which the currents are delivered to
the outside circuits. On account of this connection of both phases to
one winding there is a definite voltage set up between the inductors
of phase A, and of phase B, this voltage being shown by the figures
given in the diagram. The arrangement is adapted for two phase work
by fitting the series transformer for the auxiliary field excitation
with two primaries connected respectively in one leg of each of the
two phases; thus the transformer is excited by two currents normally ¼
period out of phase with each other. The result upon the secondary is
a combination of the effects of the two primary currents, the voltage
delivered by the secondary being intermediate in phase between those
pressures which would be separately set up by the two primaries. This
combination effect is shown in the small diagram in the upper right
hand corner of the illustration. If OA be the effect set up in the
secondary of the series transformer by the primary current of phase
A, and OB be the effect set up by the primary current of phase B, OC
represents in magnitude and phase relation the resultant effect upon
the secondary. It is readily seen that this resultant is not equal to
the arithmetical sum of the two components since, to a certain extent,
they work at cross purposes. However, if either one of them increase
the resultant effect increases, although not in exact proportion. If
the load remain balanced, the two components remaining equal to each
other, the resultant OC varies in exact proportion to any changes in
the components. If the load become unbalanced, the resultant swings
around more nearly into phase with the larger load; thus if OB become
greater, OA remaining the same, OC swings around, becoming more nearly
horizontal. This requires a readjustment of the position of the brushes
on the commutator to set them properly for minimum sparking, an
adjustment exactly similar to that required when the power factor of
the load changes.]
[Illustration: FIG. 1,559.--Diagram of Westinghouse three phase
composite wound alternator. The armature inductors are of the closed
coil or delta connected type, but are tapped at three points per
pair or poles to the three collector rings. All three connections
between the armature coils and the collector rings run through primary
circuits of the series transformer within the armature, these three
primaries each giving their own effect upon the secondary. Since the
resultant of three equal alternating electromotive forces 120° apart
is zero, so that some special arrangement must be adopted to make
these electromotive forces act with instead of against each other. The
arrangement is a reversal of the connections of one of the primaries
of the series transformer. This is shown in the case of the lowest
primary indicated in the diagram. The combination of the effects of
the three primaries is again indicated in the small vector diagram in
the upper right hand corner. Here OA is the effect of one primary,
OB that of another ⅓ of a period displaced from the former in phase,
and OC that which the third would exert were it not reversed, but the
reversal brings the effect of this third coil into the phase relation
OD, so that the three are only 60° apart. The combination of OA and OB
is equal to OC, which combined again with OD gives a resultant effect,
OE. In this case, as in the other, the effect upon the series field
does not remain exactly proportional to the load unless the latter is
balanced; in fact, an increased current through the one leg represented
by OD, affects the series field as much as an equal increase in each
of the other legs put together. Practically, however, any increase of
the load--distributed as it must be in two legs at least--increases the
field excitation so that proper regulation is secured.]
~Ques. How is the path and value of currents in a delta connected
armature determined?~
Ans. Starting with the inductors of one phase opposite the middle of
the poles, assume the maximum current to be induced at this moment;
then but one-half of the same value of current will be induced at the
same moment in the other two phases, and its path and value will best
be shown by aid of fig. 1,560, in which X may be taken as the middle
collector ring, and the maximum current to be flowing from X toward Z.
It will be seen that no current is coming in through the line Y, but
part of the current at Z will have been induced in the branches _b_ and
_c_.
[Illustration: FIG. 1,560.--Diagram showing determination of path and
value of current flowing in delta connected armature.]
~Ques. Since most three phase windings can be connected either Y or
delta, what should be noted as to the effects produced?~
Ans. With the same winding, the delta connection will stand 1.732 as
much current as the ~Y~ connection, but will give only 1 ÷ 1.732
or .577 as much voltage.
[Illustration: FIG. 1,561.--Triumph brushes and brush holder. The
holder is of the box type provided with an adjustable tension spring,
making the brushes self-feeding. Each holder is carried on insulated
studs attached to a cast iron yoke which is mounted on the bearing.]
[Illustration: FIG. 1,562.--Diagram of ~Y~ connection with a common
return wire. When the three lines leading from _a_, _b_ and _c_ are
equal in resistance and reactance, or in other words when the system
is _balanced_, the currents of the three phases are equal and are
120° apart in phase (each current lagging behind its pressure by the
same amount as the others) and their sum is at each instant equal to
zero. In this case the resultant current being equal to zero there is
no need of a common return wire. However, in some cases, where power
is distributed from transformers or three wire systems, the different
branches are liable to become unbalanced. Under such circumstances the
common return wire is sometimes used, being made large enough to take
care of the maximum unbalancing that may occur in operation. The return
wire is used sometimes on alternators that furnish current mostly for
lighting work.]
~Chain or Basket Winding.~--One disadvantage in ordinary two-range
windings is that two or three separate shapes of coil are required.
The cost of making, winding, and supplying spares would be less if
one shape of coil could be made to do for all phases. One way of
accomplishing this is by the method of chain winding, in which the two
sides of each coil are made of different lengths, as shown in fig.
1,563, and bent so that they can lie behind one another.
[Illustration: FIG. 1,563.--Diagram showing chain winding. In this
method of winding the coils are all similar with long and short sides.
It obviates the extra cost of making coils of several different shapes.
The diagram represents a winding for one slot per pole per phase.]
[Illustration: FIG. 1,564.--General Electric terminal board showing
cables leading to three phase winding.]
In the case of open slots the coils may be former wound and afterwards
wedged into their places.
In chain winding the adjacent coils link one another as in a chain
(hence, the name); the winding is similar to a skew coil winding. This
plan of winding is supposed to have some advantage in keeping coils of
different phases further separated than the two range plan.
[Illustration: FIG. 1,565.--Section of armature winding of
Allis-Chalmers 500 kw. three phase water wheel alternator. The coils
are of the concentrated "half" type. Each coil is completely insulated
before being placed on the core and no insulation is placed in the
slot itself. The ends of the coils where they project beyond the slots
are heavily taped. Where necessary suitable supports are provided for
the coil connections so that they cannot become displaced on account
of stresses due to short circuits or other causes. The winding is of
the "chain" type. This is shown by the way the coils are connected
together at the right. The armature terminals are either provided with
insulated connectors or are led to a marble terminal board on which
the terminals are so mounted and protected that it is impossible for
an attendant to make accidental contact with them. The position of the
illustration would indicate a horizontal alternator but the machine is
of the vertical type; the lug on the right shows this, being used for
adjusting the alternator on the foundation.]
~Skew Coil Winding.~--In this type of winding the object is to shape
the coils so that all may be of one pattern. This is accomplished by
making the ends skew shape as shown in figs. 1,566 to 1,568.
[Illustration: FIGS. 1,566 to 1,568.--Views of a section of skew coil
winding; so called on account of the skew shape given to the coil ends
in order that all the coils may be of one shape.]
~Fed-in Winding.~--This name is given to a type of winding possible
with open or only partially closed slots, in which coils previously
formed are introduced, only a few inductors at a time if necessary.
They are inserted into the slots from the top, the slot being provided
with a lining of horn fibre or other suitable material, which is
finally closed over and secured in place by means of a wedge, or by
some other suitable means. An example of a fed-in winding is shown in
figs. 1,566 and 1,568.
~Imbricated Winding.~--This is a species of spiral coil winding in
which the end connections are built up one above the other, either in a
radial, or in a horizontal direction.
The winding is used especially on the armatures of turbine alternators
and dynamos.
~Spiral Winding.~--This is a winding in which "spiral" coils, as shown
in fig. 1,569, are used. The spiral form of coil is very extensively
used for armature windings of alternators.
[Illustration: FIG. 1,569.--Diagram showing a spiral coil. This type
of coil is one in which each successive turn lies entirely within
the previous turn, starting with the outermost turn of the coil. The
successive turns of a spiral coil are thus not of the same size, and
are not overlapping as in a "lap" coil.]
~Mummified Winding.~--The word _mummified_ as applied to a winding is
used to express the treatment the coils of the winding receive in the
making; that is, when a winding, after being covered with tape or other
absorbent material, is saturated in an insulating compound and baked
until the whole is solidified, it is said to be mummified.
~Shuttle Winding.~--This type of winding consists of a single coil
having a large number of turns, wound in two slots spaced 180° apart.
It was originally used on Siemens' armature and is now used on
magnetos, as shown in figs. 1,459 to 1,461.
[Illustration: FIG. 1,570.--Frame and armature winding of Westinghouse
pedestal bearing alternator. Armature frames are of cast iron and
ventilated. Interior transverse ribs strengthen the frame and support
the core laminations. The armature core is built up of annealed and
japanned punched laminations. Armature slots are open. Armature coils
are form wound, impregnated, and interchangeable; they are held in
place with fiber wedges. Ventilating spaces are provided at intervals
in the armature core and also between all coil ends.]
~Creeping Winding.~--Another species of winding, known as a creeping
winding is applicable to particular cases.
If three adjacent coils, each having a pitch of 120 electrical
degrees, be set side by side, they will occupy the same breadth
as 4 poles, and, by repetition, will serve for any machine
having a multiple of 4 poles, but cannot be used for machines
with 6, 10 or 14 poles. Fig. 1,571 shows this example.
[Illustration: FIGS. 1,571 and 1,572.--Diagram of creeping windings.
Fig. 1,571, three coils subtending four poles; fig. 1,572, nine coils
subtending eight poles.]
In the same way 9 coils, each of 160 electrical degrees, will
occupy the same angular breadth as 8 poles.
Further, 9 coils of 200 electrical degrees will occupy the same
angular breadth as 10 poles.
Now of these 9 coils, any three contiguous ones are nearly in
phase, if wound alternately clockwise and counter-clockwise.
For the 8 pole machine, the phase difference between adjacent
coils is 20 degrees.
For the 10 pole machine, the phase difference is also 20
degrees.
The cosine of 20 degrees is .9397, consequently, if 3 adjacent
coils be united in series, their joint pressure will be 2.897
multiplied by that of the middle one of the three.
The 9 coils may therefore be joined up in three groups of 3
adjacent coils, for the three phases.
By repetition, the same grouping will suit for any machine
having a multiple of 8 or of 10 poles. These two cases are
illustrated in figs. 1,572 and 1,573. In the figures, the coils
are represented as occupying two slots each, but they might be
further distributed.
[Illustration: FIG. 1,573.--Developed diagram of creeping winding: nine
coils subtending ten poles.]
[Illustration: FIG. 1,574.--Triumph pedestal and brush rigging for
large revolving field alternators. Carbon brushes are used, carried
in box type brush holders. The stand or pedestal here shown is the
kind used with the engine and fly wheel types of alternator. The brush
studs are mounted on the stand in such a manner that the brushes are
easily accessible. The latter carry only the low voltage direct current
necessary for exciting the field.]
~Turbine Alternator Winding.~--For the reason that steam turbines
run at so much higher speed than steam engines, the construction of
armatures and windings for alternators intended to be direct connected
to turbines must be quite different from those driven by steam engines.
Accordingly, in order that the frequency be not too high, turbine
driven alternators must have very few poles--usually two or four, but
rarely six.
[Illustration: FIGS. 1,575 and 1,576.--Westinghouse turbine alternator
armature construction. Fig. 1,575. View showing dovetail grooves in
armature casting; fig. 1,576, laminæ assembled in dovetail grooves of
armature casting.]
The following table will show the relation between the revolutions and
frequencies for the numbers of poles just designated.
~TABLE OF FREQUENCY AND REVOLUTIONS~
--------------------------------------
| | |
| | REVOLUTIONS |
| Frequency +--------------------------+
| | | | |
| | 2 pole | 4 pole | 6 pole |
+-----------+--------+--------+--------|
| | | | |
| 25 | 1,500 | 750 | 500 |
| 60 | 3,600 | 1,800 | 1,200 |
| 100 | 6,000 | 3,000 | 2,000 |
+-----------+--------+--------+--------|
[Illustration: FIG. 1,577.--Armature of Westinghouse turbine alternator
with end bells removed showing method of bracing the coil ends.]
[Illustration: FIG. 1,578.--Stationary armature of Westinghouse turbine
alternator with part of the winding in place. Because of the small
number of coils in a turbine alternator as compared with a slow speed
machine of the same kva. rating, each coil carries a great amount of
power on large load, particularly at times of short circuits or grounds
on the external circuit. The "throw" of the coils is large, leaving a
considerable part of the winding in the end turns unsupported by the
armature core. For these reasons great stresses, which are dangerous,
if effective means be not adopted to withstand them, may exist between
the coils. The inductors are of such cross section that they can be
made rigid and insulated satisfactorily. The end turns are given a fan
like form as shown, affording ventilation and effective bracing as
shown in fig. 1,577. Cord lashings are, except in the smallest frames,
used only for holding in the small spacing blocks between the coils.
They are not depended on to support the coils. Malleable iron braces,
hard maple blocks, and brass or steel bolts with brass washers are used
to withstand the mechanical stresses imposed on the armature coils by
external short circuits.]
From the table, it is evident that a large number of poles is not
permissible, considering the high speed at which the turbine must be
run.
[Illustration: FIG. 1,579.--Two pole radial slot field. Radial slot
fields are used on very small and very large alternators. The field
diameters are so small that the end turns of the winding can be
effectively bound into place, such binding being necessary with a
radial slot machine. The shaft and disc are a one piece forging of
steel.]
~Ques. How is the high voltage obtained with so few poles?~
Ans. There must be either numerous inductors per slot or numerous slots
per pole.
[Illustration: FIGS. 1,580 to 1,582.--Westinghouse two pole parallel
slot field with ends removed showing construction. The parallel slot
design of field construction is used in Westinghouse machines up to
10,000 kva. capacity. In fig. 1,581, the large holes at the end near
the circumference of the cylinder are for the accommodation of the
bolts that hold the bronze end discs and stub shafts. In winding,
the cylinder is mounted on a horizontal turntable that rotates in a
horizontal plane. The copper strap field coil winding is wound turn
by turn under pressure and strip insulation is wound in between. When
completed the turns are held rigidly in position with heavy brass
wedges. An end disc made of bronze holds the stub shaft and is bolted
to each end of the steel center. When the leads are attached to the
collector rings the field is complete.]
~Ques. What form of armature is generally used?~
Ans. A stationary armature.
~Ques. What difficulty is experienced with revolving armatures?~
Ans. The centrifugal force being considerable on account of the
high speed, requires specially strong construction to resist it,
consequently closed or nearly closed slots must be used.
~Ques. How is the design of the rotor modified so as to reduce the
centrifugal force?~
Ans. It is made long and of small diameter.
Some examples of revolving fields are shown in figs. 1,579 to
1,584. Figs. 1,577 and 1,578 show some construction details of
a stationary armature of turbine alternator.
[Illustration: FIG. 1,583.--Westinghouse two pole parallel slot turbine
alternator field.]
[Illustration: FIG. 1,584.--Westinghouse four pole parallel slot
turbine alternator field.]
HAWKINS PRACTICAL LIBRARY OF
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~ELECTRICAL GUIDE, NO. 1~
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.
~ELECTRICAL GUIDE, NO. 2~
The construction of Dynamos, Motors, Armatures, Armature
Windings, Installing of Dynamos.
~ELECTRICAL GUIDE, NO. 3~
Electrical Instruments, Testing, Practical Management of
Dynamos and Motors.
~ELECTRICAL GUIDE, NO. 4~
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,
Storage Batteries.
~ELECTRICAL GUIDE, NO. 5~
Principles of Alternating Currents and Alternators.
~ELECTRICAL GUIDE, NO. 6~
Alternating Current Motors, Transformers, Converters,
Rectifiers.
~ELECTRICAL GUIDE, NO. 7~
Alternating Current Systems, Circuit Breakers, Measuring
Instruments.
~ELECTRICAL GUIDE, NO. 8~
Alternating Current Switch Boards, Wiring, Power Stations,
Installation and Operation.
~ELECTRICAL GUIDE, NO. 9~
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
~ELECTRICAL GUIDE, NO. 10~
Modern Practical Applications of Electricity and Ready
Reference Index of the 10 Numbers.
~Theo. Audel & Co., Publishers. 72 FIFTH AVENUE, NEW YORK~
TRANSCRIBER'S NOTES
Silently corrected simple spelling, grammar, and typographical
errors.
Retained anachronistic and non-standard spellings as printed.
Enclosed italics markup in _underscores_.
Enclosed bold markup in ~tildes~.
End of Project Gutenberg's Hawkins Electrical Guide v. 5 (of 10), by Hawkins