ALTERNATORS
~Use of Alternators.~--The great increase in the application of
electricity for supplying power and for lighting purposes in
industry, commerce, and in the home, is due chiefly to the economy of
distribution of alternating current.
Direct current may be used to advantage in densely populated districts,
but where the load is scattered, it requires, on account of its low
voltage, too great an investment in distributing lines. In such cases
the alternator is used to advantage, for while commutators can be built
for collecting direct current up to 1,000 volts, alternators can be
built up to 12,000 volts or more, and this voltage increased, by step
up transformers of high economy, up to 75,000 or 100,000 volts. Since
the copper cost is inversely as the square of the voltage, the great
advantage of alternating current systems is clearly apparent.
The use of alternating current thus permits a large amount of energy to
be economically distributed over a wide area from a single station, not
only reducing the cost of the wiring, but securing greater economy by
the use of one large station, instead of several small stations.
The higher voltages generated by alternators enables the transmission
of electrical energy to vastly greater distances than possible by a
direct current system, so that the energy from many waterfalls that
otherwise would go to waste may be utilized.
~Classes of Alternator.~--There are various ways of classifying
alternators. They may be divided into groups, according to: 1, the
nature of the current produced; 2, type of drive; 3, method of
construction; 4, field excitation; 5, service requirements, etc.
From these several points of view, alternators then may be classified:
1. With respect to the current, as:
_a._ Single phase;
_b._ Polyphase.
2. With respect to the type of drive, as:
_a._ Belt or chain driven;
_b._ Direct connected.
3. With respect to construction, as:
_a._ Revolving armature;
_b._ Revolving field;
_c._ Inductor.
Homopolar and heteropolar.
4. With respect to mode of field excitation, as:
_a._ Self-exciting;
_b._ Separately excited;
Exciter direct connected, or gear driven.
_c._ Compositely excited.
5. With respect to service requirements, as:
_a._ Slow speed;
_b._ Fly wheel;
_c._ High speed;
_d._ Water wheel type;
_e._ Turbine driven.
~Single Phase Alternators.~--As a general rule, when alternators
are employed for lighting circuits, the single phase machines are
preferable, as they are simpler in construction and do not generate the
unbalancing voltages often occurring in polyphase work.
[Illustration: FIG. 1,370.--Elementary four pole single phase
alternator. It has four "inductors" whose pitch is the same as the pole
pitch. They are connected in series and terminate at the two collector
rings as shown. The poles being alternate N and S, it is evident that
there will be two cycles of the current per revolution of the armature.
For any number of poles then the number of cycles equals the number of
poles divided by two. Applying Fleming's rule for induced currents,
the direction of the current induced in the inductors is easily found
as indicated by the arrows. The field magnets are excited by coils
supplied with direct current, usually furnished from an external
source; for simplicity this is not shown. The magnets may be considered
as of the permanent type.]
~Ques. What are the essential features of a single phase alternator?~
Ans. Fig. 1,370 shows an elementary single phase alternator. It
consists of an armature, with single phase winding, field magnets, and
two collector rings and brushes through which the current generated in
the armature passes to the external circuit.
~Ques. In what respect do commercial machines differ mostly from the
elementary alternator shown in fig. 1,370, and why?~
Ans. They have a large number of poles and inductors in order to obtain
the desired frequency, without excessive speed, and electromagnets
instead of permanent magnets.
[Illustration: FIG. 1,371.--Developed view of elementary single phase
four pole alternator and sine curve showing the alternating current or
pressure generated during one revolution. The armature is here shown as
a flat surface upon which a complete view of the winding is seen. If M
be any position of an inductor, by projecting up to the curve gives N,
the corresponding value of the current or pressure. Magnetic lines are
shown at the poles representing a field decreasing in intensity from
a maximum at the center to zero at points half way between the poles,
this being the field condition corresponding to the sine form of wave.
In actual machines the variation from the sine curve is considerable in
some alternators. See figs. 1,247 and 1,248.]
~Ques. In actual machines, why must the magnet cores be spaced out
around the armature with considerable distance between them?~
Ans. In order to get the necessary field winding on the cores, and also
to prevent undue magnetic leakage taking place, laterally from one limb
to the next of opposite sign.
~Ques. Is there any gain in making the width of the armature coils any
greater than the pole pitch, and why?~
Ans. No, because any additional width will not produce more voltage,
but on the contrary will increase the resistance and inductance of the
armature.
[Illustration: FIG. 1,372.--Elementary four pole two phase alternator.
The winding consists of one inductor per phase per pole, that is, four
inductors per phase, the inductors of each phase being connected in
series by the "connectors" and terminating at the collector rings.
This arrangement requires four collector rings, giving two independent
circuits. The pitch of the inductors of each phase is equal to the
pole pitch, and the phase difference is equal to one-half the pole
pitch, that is, phase B winding begins at B, a point half-way between
inductors A and A' of phase A winding. Hence when the current or
pressure in phase A is at a maximum, in the ideal case, when inductor A
for instance is under the center of a pole, the current or pressure in
B is zero, because B is then half-way between the poles.]
~Polyphase Alternators.~--A multiphase or polyphase alternator is one
which delivers two or more alternating currents differing in phase by a
definite amount.
For example, if two armatures of the same number of turns each
be connected to a shaft at 90 degrees from each other and
revolved in a bipolar field, and each terminal be connected to
a collector ring, two separate alternating currents, differing
in phase by 90 degrees, will be delivered to the external
circuit. Thus a two phase alternator will deliver two currents
differing in phase by one-quarter of a cycle, and similarly
a three phase alternator (the three armatures of which are
set 120 degrees from each other) will deliver three currents
differing in phase by one-third of a cycle.
In practice, instead of separate armatures for each phase, the
several windings are all placed on one armature and in such
sequence that the currents are generated with the desired phase
difference between them as shown in the elementary diagrams
1,372 and 1,373 for two phase current, and figs. 1,374 and
1,375 for three phase current.
[Illustration: FIG. 1,373.--Developed view of elementary two phase four
pole alternator and sine curves, showing the alternating current or
pressure generated during one revolution of the armature. The complete
winding for the three phases are here visible, the field magnets being
represented as transparent so that all of the inductors may be seen. By
applying Fleming's rule, as the inductors progress under the poles, the
directions and reversals of current are easily determined, as indicated
by the sine curves. It will be seen from the curves that four poles
give two cycles per revolution. Inductors A, and B are lettered to
correspond with fig. 1,372, with which they should be compared.]
~Ques. What use is made of two and three phase current?~
Ans. They are employed rather for power purposes than for lighting, but
such systems are often installed for both services.
~Ques. How are they employed in each case?~
Ans. For lighting purposes the phases are isolated in separate
circuits, that is, each is used as a single phase current. For driving
motors the circuits are combined.
[Illustration: FIG. 1,374.--Elementary four pole three phase
alternator. There are three sets of inductors, each set connected in
series and spaced on the drum with respect to each other two-thirds
pole pitch apart. As shown, six collector rings are used, but on actual
three phase machines only three rings are employed, as previously
explained. The inductors have distinctive coverings for the different
phases. The arrows indicate the direction in which the induced
pressures tend to cause currents.]
~Ques. Why are they combined for power purposes?~
Ans. On account of the difficulty encountered in starting a motor with
single phase current.
Ferarris, of Italy, in 1888 discovered the important principle
of the production of a rotating magnetic field by means of two
or more
[Illustration: FIG. 1,375.--Elementary four pole three-phase alternator
and sine curves showing current or pressure conditions for one
revolution. Six collector rings are shown giving three independent
circuits. The pitch of the inductors for each phase is the same as the
pole pitch, and the phase difference is equal to two-thirds of the pole
pitch, giving the sequence of current or pressure waves as indicated by
the sine curves. The waves follow each other at ⅓ period, that is, the
phase difference is 120 degrees. Inductors A, B, and C, the beginning
of each phase winding, are lettered to correspond with fig. 1,374, with
which they should be compared.] /# alternating currents displaced
in phase from one another, and he thus made possible by means of the
induction motor, the use of polyphase currents for power purposes.
~Ques. What is the difficulty encountered in starting a motor with
single phase current?~
Ans. A single phase current requires either a synchronous motor to
develop mechanical power from it, or a specially constructed motor of
dual type, the idea of which is to provide a method of getting rotation
by foreign means and then to throw in the single phase current for
power.
[Illustration: FIG. 1,376.--Diagram of six phase winding with star
grouping, being equivalent to a three phase winding in which the three
phases are disconnected from each other and their middle points united
at a common junction.]
[Illustration: FIG. 1,377.--Diagram of six phase winding with mesh
grouping.]
~Six Phase and Twelve Phase Windings.~--These are required for the
operation of rotary converters. The phase difference in a six phase
winding is 60 degrees and in a twelve phase winding 30 degrees. A six
phase winding can be made out of a three phase winding by disconnecting
the three phases from each other, uniting their middle points at a
common junction, as shown by diagram fig. 1,376. This will give a star
grouping with six terminals.
In the case of a mesh grouping, each of the three phases must be cut
into two parts and then reconnected as shown in fig. 1,377.
[Illustration: FIG. 1,378.--Diagram of twelve phase winding star
grouping.]
[Illustration: FIG. 1,379.--Diagram of six phase winding consisting of
combination of mesh and star grouping.]
As the phase difference of a twelve phase winding is one-half that of a
six phase winding, the twelve phases may be regarded as a star grouping
of six pairs crossed at the middle point of each pair as shown in fig.
1,378, or in mesh grouping for converters they may be arranged as a
twelve pointed polygon. They may also be grouped as a combination of
mesh and star as shown in fig. 1,379, which, however, is not of general
interest.
~Belt or Chain Driven Alternators.~--The mode in which power is
transmitted to an alternator for the generation of current is governed
chiefly by conditions met with where the machine is to be installed.
In many small power stations and isolated plants the use of a belt
drive is unavoidable. In some cases the prime mover is already
installed and cannot be conveniently arranged for direct connection,
in others the advantage to be gained by an increase in speed more than
compensates for the loss involved in belt transmission.
[Illustration: FIG. 1,380.--Belt-driven alternator. By use of a belt,
any desired speed ratio is obtained, enabling the use of a high speed
alternator which, being smaller than one of slow speed, is cheaper. It
affords means of drive for line shaft and has other advantages, but
requires considerable space and is not a "positive" drive. Belting
exerts a side pull which results in friction and wear of bearings.
Means for tightening the belt as shown in fig. 1,381, or equivalent,
must be provided.]
There are many places where belted machines may be used advantageously
and economically. They are easily connected to an existing source of
power, as, for instance, a line shaft used for driving other machinery,
and for comparatively small installations they are lower in first cost
than direct connected machines. Moreover, when connected to line shaft
they are run by the main engine which as a rule is more efficient than
a small engine direct connected.
Where there is sufficient room between pulley centers, a belt is a
satisfactory medium for power transmission, and one that is largely
used. It is important that there be liberal distance between centers,
especially in the case of generators or motors belted to a medium or
slow speed engine, because, owing to the high speed of rotation of the
electric machines, there is considerable difference in their pulley
diameters and the drive pulley diameter; hence, if they were close
together, the arc of contact of the belt with the smaller pulley would
be appreciably reduced, thus diminishing the tractive power of the belt.
[Illustration: FIG. 1,381.--Sub-base and ratchet device for moving
alternator to tighten belt. A ratchet A, operated by lever B, works the
block C by screw connection, causing it to move the block. The latter,
engaging with the frame, causes it to move, thus providing adjustment
for belt. After tightening belt, the bolts D, which pass through the
slots in the sub-base, are tightened, thus securing the machine firmly
in position.]
~Ques. What provision should be made in the design of an alternator to
adapt it to belt drive?~
Ans. Provision should be made for tightening the belt.
[Illustration: FIG. 1,382.--Allis-Chalmers pedestal type, belted
alternator. The bearings are of the ring oiling form with large oil
reservoirs. The bearings have spherical seats and are self aligning.]
~Ques. How is this done?~
Ans. Sometimes by an idler pulley, but usually by mounting the machine
on a sub-base provided with slide rails, as in fig. 1,381, the belt
being tightened by use of a ratchet screw which moves the machine along
the base.
[Illustration: FIG. 1,383.--Diagram illustrating rule for horse power
transmitted by belts. _A single belt travelling at a speed of 1,000
feet per minute will transmit one horse power; a double belt will
transmit twice that amount_, assuming that the thickness of a double
belt is twice that of a single belt. This is conservative practice, and
a belt so proportioned will do the work in practically all cases. The
above rule corresponds to a pull of 33 lbs. per inch of width. Many
designers proportion single belts for a pull of 45 lbs. For double
belts of average thickness, some writers say that the transmitting
efficiency is to that of single belts as 10 is to 7. This should not
be applied to the above rule for single belts, as it will give an
unnecessarily large belt.]
~Ques. Give a rule for obtaining the proper size of belt to deliver a
given horse power.~
Ans. _A single belt travelling at a speed of one thousand feet per
minute will transmit one horse power; a double belt will transmit twice
that amount._
This corresponds to a working strain of 33 lbs. per inch of
width for single belt, or 66 lbs. for double belt.
Many writers give as safe practice for single belts in good
condition a working tension of 45 lbs. per inch of width.
~Ques. What is the best speed for maximum belt economy?~
Ans. From 4,000 to 4,500 feet per minute.
EXAMPLE.--What is the proper size of double belt for an
alternator having a 16 inch pulley, and which requires 50 horse
power to drive it at 1,000 revolutions per minute full load?
The velocity of the belt is
circumference in feet × revolutions = feet per minute
16
-- × 3.1416 × 1,000 = 4,188.
12
Horse power transmitted per inch width of double belt at 4,188
feet speed
4,188
2 × ----- = 8.38.
1,000
[Illustration: FIG. 1,384.--Fort Wayne revolving field belt driven
alternator. It is designed for belted exciter, having a shaft extension
at the collector ring end for exciter driving pulley.]
Width of double belt for 50 horse power
50 ÷ 8.38 = 5.97, say 6 inch.
~Ques. What are the advantages of chain drive?~
Ans. The space required is much less than with belt drive, as the
distance between centers may be reduced to a minimum. It is a positive
drive, that is, there can be no slip. Less liability of becoming
detached, and, because it is not dependent on frictional contact, the
diameters of the sprockets may be much less than pulley diameter for
belt drive.
[Illustration: FIGS. 1,385 and 1,386.--Diagram showing the distinction
between _direct connected_ and _direct coupled_ units. In a direct
connected unit, fig. 1,385, the engine and generator are permanently
connected on one shaft, there being one bed plate upon which both are
mounted. An engine and generator are said to be direct coupled when
each is independent, as in fig. 1,386 being connected solely by a jaw
or friction clutch or equivalent at times when it is desired to run the
generator. At other times the generator may be disconnected and the
engine run to supply power for other purposes.]
~Ques. What are some objections?~
Ans. A lubricant is required for satisfactory operation, which causes
more or less dirt to collect on the chain, requiring frequent
cleaning; climbing of teeth when links and teeth become worn; noise and
friction.
[Illustration: FIG. 1,387.--Engberg direct connected, or "engine type"
alternator. In many places direct connected units are used, owing to
the great saving in floor space, convenience of operation, and absence
of belts.]
~Direct Connected Alternators.~--There are a large number of cases
where economy of space is of prime importance, and to meet this
condition the alternator and engine are direct connected, meaning, that
there is no intermediate gearing such as belt, chain, etc., between
engine and alternator.
One difficulty encountered in the direct connection of engine
and alternator is the fact that the most desirable rotative
speed of the engine is less than that of the alternator.
Accordingly a compromise is made by raising the engine speed
and lowering the alternator speed.
The insistent demand for direct connected units in the small
and medium sizes, especially for direct current units, was the
chief cause resulting in the rapid and high development of what
is known as the "high speed automatic engine."
Increasing the engine speed means that more horse power is
developed for any given cylinder dimensions, while reducing
the speed of the generator involves that the machine must be
larger for a given output, and in the case of an alternator
more poles are required to obtain a given frequency, resulting
in increased cost.
The compactness of the unit as a whole, simplicity, and general
advantages are usually so great as to more than offset any
additional cost of the generator.
[Illustration: FIG. 1,388.--Crocker-Wheeler 2,000 kva. 2,400 volt
coupled type alternator. The coupled type of alternator is desirable
for use with steam, gas, and oil engines, and water wheels where it
is inconvenient to mount the alternator on the engine shaft or to
extend the engine base to accommodate a bearing. This type consists
of alternator complete with shaft and bearings similar to belt type
machines, but with bearings not necessary designed for the side pull of
belts.]
~Ques. What is the difference between a direct connected and a direct
coupled unit?~
Ans. A direct connected unit comprises an engine and generator
permanently connected; direct coupling signifies that engine and
generator are each complete in itself, that is, having two bearings,
and are connected by some device such as friction clutch, jaw clutch,
or shaft coupling.
~Revolving Armature Alternators.~--This type of alternator is one
which has its parts arranged in a manner similar to a dynamo, that is,
the armature is mounted on a shaft so it can revolve while the field
magnets are attached to a circular frame and arranged radially around
the armature, as shown in fig. 1,389. It may be single or polyphase,
belt driven, or direct connected.
[Illustration: FIG. 1,389.--Revolving armature alternator. Revolving
armatures are suitable for machines generating current at comparatively
low pressure, as no difficulty is experienced in collecting such
current. Revolving armature alternators are also suitable for small
power plants, isolated lighting plants, where medium or small size
machines are required.]
~Ques. When is the revolving type of armature used and why?~
Ans. It is used on machines of small size because the pressure
generated is comparatively low and the current transmitted by the
brushes small, no difficulty being experienced in collecting such a
current.
[Illustration: FIG. 1,390.--Ring wound dynamo arranged as alternator by
replacing commutator with collector rings connected to the winding at
points 180° apart.]
~Ques. Could a dynamo be converted into an alternator?~
Ans. Yes.
~Ques. How can this be done?~
Ans. By placing two collector rings on one end of the armature and
connecting these two rings to points in the armature winding 180°
apart, as shown in fig. 1,390.
~Ques. Would such arrangement as shown in fig. 1,390 make a desirable
alternator?~
Ans. No.
Alternating current windings are usually different from those
used for direct currents. One distinction is the fact that
a simple open coil winding may be, and often is, employed,
but the chief difference is the intermittent action of the
inductors.
In a direct current Gramme ring winding a certain number of
coils are always active, while those in the space between the
pole pieces are not generating. In this way a practically
steady pressure is produced by a large fraction of the coils.
In the case of an alternator all of the coils are either active
or inactive at one time. Hence, the winding need cover only as
much of the armature as is covered by the pole pieces.
[Illustration: FIG. 1,391.--Engberg alternating current generating set;
shown also in cross section in fig. 1,387. The set comprises a vertical
engine and alternator, direct connected and placed on one base. The
lubrication system comprises an oil pump situated in the base of the
engine, pumping the oil from an oil reservoir up into a sight feed
oil cup which leads to a distributing oil trough on the inside of the
engine frame, from here oil pipes lead to all movable bearings, which
are grooved to insure proper distribution of oil. The oil is drained
from bearings into the base, filtered and re-pumped. A water shed
partition is provided in the engine frame, preventing any water passing
from the cylinder down into the engine base and mixing with the oil,
consequently leaving good, clean oil in the oil reservoir at all times.
The details of the lubrication system are shown in fig. 1,387.]
~Revolving Field Alternators.~--In generating an electric current by
causing an inductor to cut magnetic lines, it makes no difference
whether the cutting of the magnetic lines is effected by moving an
inductor across a magnetic field or moving the magnetic field across
the inductor.
[Illustration: FIG. 1,392.--Allis-Chalmers revolving field
self-contained belted type alternator.]
_Motion is purely a relative matter_, that is, an object is said to
move when it changes its position with some other object regarded as
stationary; it may be moving with respect to a second object, and at
the same time be at rest with respect to a third object. Thus, a dory
has a speed of four miles per hour in still water; if it be run up
stream against a current flowing four miles per hour it would move at
that speed with respect to the water, yet remain at rest with respect
to the earth.
It must be evident then that motion, as stated, being a purely relative
matter, it makes no difference whether the armature of a generator
move with respect to the field magnets, or the field magnets move with
respect to the armature, so far as inducing an electric current is
concerned.
[Illustration: FIG. 1,393.--Marine view, showing that motion is purely
a relative matter. In order that there may be motion something must be
regarded as being stationary. In the above illustration a catboat is
shown at anchor in a stream which is flowing at a rate of four miles
per hour in the direction of the arrow. The small dory running at a
speed of four miles per hour against the current is _moving_ at that
velocity _relative_ to the current, yet is at a standstill relative to
the catboat. In this instance both catboat and dory are moving with
respect to the water if the latter be regarded as stationary. Again if
the earth be regarded as being stationary, the two boats are at rest
and the water is moving relative to the earth.]
For alternators of medium and large size there are several reasons why
the armature should be stationary and the field magnets revolve, as
follows:
1. By making the armature stationary, superior insulation methods may
be employed, enabling the generation of current at very much higher
voltage than in the revolving armature type.
2. Because the difficulty of taking current at very high pressures from
collector rings is avoided.
The field current only passes through the collector rings.
Since the field current is of low voltage and small in
comparison with the main current, small brushes are sufficient
and sparking troubles are avoided.
[Illustration: FIG. 1,394.--Diagram showing essential parts of
a revolving field alternator and method of joining the parts in
assembling.]
3. Only two collector rings are required.
4. The armature terminals, being stationary, may be enclosed
permanently so that no one can come in contact with them.
~Ques. What names are usually applied to the armature and field magnets
with respect to which moves?~
Ans. The "stator" and the "rotor."
The terms armature and field magnets are to be preferred to
such expressions. An armature is an armature, no matter whether
it move or be fixed, and the same applies to the field magnets.
There is no good reason to apply other terms which do not
define the parts.
~Ques. Explain the essential features of a revolving field alternator.~
Ans. The construction of such alternators is indicated in the diagram,
fig. 1,394. Attached to the shaft is a field core, which carries the
latter, consisting of field coils fitted on pole pieces which are
dovetailed to the field core. The armature is built into the frame and
surrounds the magnets as shown. The field current, which is transmitted
to the magnets by slip rings and brushes, consists of direct current of
comparatively low pressure, obtained from some external source.
[Illustration: FIG. 1,395.--Western Electric stationary armature and
frame of engine driven alternator. It is of cast iron and surrounds the
laminated iron core in which the armature windings are embedded. Heavy
steel clamping fingers hold the core punchings in place and numerous
ventilating ducts are provided in the core at frequent intervals to
allow free circulation of cool air. The armature coils are form wound,
insulated, and retained in the core slots by means of wedges.]
~Inductor Alternators.~--In this class of alternator both armature and
field magnets are stationary, a current being induced in the armature
winding by the action of a so called inductor in moving through the
magnetic field so as to periodically vary its intensity.
[Illustration: FIGS. 1,396 and 1,397.--Elementary inductor alternator;
diagram showing principle of operation. It consists of a field magnet,
at the polar extremities of which is an armature winding both being
stationary as shown. Inductors consisting of iron discs are arranged
on a shaft to rotate through the air gap of the magnet poles. Now in
the rotation of the inductors, when any one of them passes through
the air gap as in fig. 1,396, the reluctance or magnetic resistance
of the air gap is greatly reduced, which causes a corresponding
increase in the number of magnetic lines passing through the armature
winding. Again as an inductor passes out of the air gap as in fig.
1,397, the number of magnetic lines is greatly reduced; that is, when
an inductor is in the air gap, the magnetic field is dense, and when
no inductor is in the gap, the field is weak; a variable flux is thus
made to pass through the armature winding, inducing current therein.
The essential feature of the inductor alternator is that iron only is
revolving, and as the design is usually homopolar, the magnetic flux
in its field coils is not alternating, but undulating in character.
Thus, with a given maximum flux through each polar mass, the total
number of armature turns required to produce a given voltage is just
twice that which is required in an alternator having an alternating
instead of an undulating flux through its field windings. The above
and the one shown in figs. 1,398 and 1,399 are examples of real
inductor alternators, those shown in the other cuts are simply so
called inductor alternators, the distinction being that, as above, the
inductor constitutes no part of the field magnet.]
~Ques. What influence have the inductors on the field flux?~
Ans. They cause it to undulate; that is, the flux rises to a maximum
and falls to a minimum value, but does not reverse.
~Ques. How does this affect the design of the machine as compared with
other types of alternator?~
Ans. With a given maximum magnetic flux through each polar mass, the
total number of armature turns necessary to produce a given pressure
is twice that which is required in an alternator having an alternating
flux through its armature windings.
[Illustration: FIGS. 1,398 and 1,399.--A low tension ignition system
with an inductor magneto of the oscillating type. The inductor E is
rotated to and fro by means of a link R, one end of which is attached
to the inductor crank, and the other to the igniter cam C. Two views
are shown: immediately before and after sparking. S is the grounded
electrode of the igniter; T an adjustable hammer which is secured in
position by a lock nut N.]
~Ques. Is the disadvantage due to the necessity of doubling the number
of armature turns compensated in any way?~
Ans. Yes, the magnetic flux is not reversed or entirely changed in
each cycle through the whole mass of iron in the armature, the abrupt
changes being largely confined to the projections on the armature
surface between the coils.
~Ques. What benefit results from this peculiarity?~
Ans. It enables the use of a very high magnetic flux density in the
armature without excessive core loss, and also the use of a large flux
without an excessive increase in the amount of magnetic iron.
The use of a large flux permits a reduction in the number
of armature turns, thus compensating, more or less, for the
disadvantage due to the operation of only one-half of the
armature coils at a time.
[Illustration: FIGS. 1,400 and 1,401.--One form of inductor alternator.
As shown, the frame carries the stationary armature, which is of
the slotted type. Inside of the armature is the revolving inductor,
provided with the projections built up of wrought iron or steel
laminations. The circular exciting coil is also stationary and
encircles the inductor, thus setting up a magnetic flux around the path
indicated by the dotted line, fig. 1,401. The projecting poles are all,
therefore, of the same polarity, and as they revolve, the magnetic
flux sweeps over the coils. Although this arrangement does away with
collector rings, the machines are not so easily constructed as other
types, especially in the large sizes. The magnetizing coil becomes
large and difficult to support in place, and would be hard to repair
in case of breakdown. Inductor alternators have become practically
obsolete, except in special cases, as inductor magnetos used for
ignition and other purposes requiring a very small size machine. The
reasons for the type being displaced by other forms of alternator are
chiefly because only half as great a pressure is obtained by a flux of
given amount, as would be obtained in the ordinary type of machine. It
is also more expensive to build two armatures, to give the same power,
than to build one armature. This type has still other grave defects,
among which may be mentioned enormous magnetic leakage, heavy eddy
current losses, inferior heat emissivity, and bad regulation.]
~Classes of Inductor Alternator.~--There are two classes into which
inductor alternators may be divided, based on the mode of setting of
their polar projections:
1. Homopolar machines;
2. Heteropolar machines.
~Homopolar Inductor Alternators.~--In this type the positive
polar projections of the inductors are set opposite the
negative polar projections as shown in fig. 1,402. When the
polar projections are set in this manner, the armature coils
must be "staggered" or set displaced along the circumference
with respect to one another at a distance equal to half the
distance from the positive pole to the next positive pole.
[Illustration: FIGS. 1,402 and 1,403.--Homopolar and heteropolar
"inductors". Homopolar inductors have their N and S poles opposite each
other, while in the heteropolar type, they are "staggered" as shown.]
~Heteropolar Inductor Alternators.~--Machines of this class are
those in which the polar projections are themselves staggered,
as shown in fig. 1,403, and therefore, do not require the
staggering of the armature coils. In this case, a single
armature of double width may be used, and the rotating inductor
then acts as a _heteropolar magnet_, or a magnet which presents
alternatively positive and negative poles to the armature,
instead of presenting a series of poles of the same polarity as
in the case of a _homopolar magnet_.
~Use of Inductor Alternators.~--Morday originally designed
and introduced inductor alternators in 1866. They are not the
prevailing type, as their field of application is comparatively
narrow. They have to be very carefully designed with regard to
magnetic leakage in order to prevent them being relatively
too heavy and costly for their output, and too defective with
respect to their pressure regulation, other defects being heavy
eddy current losses and inferior heat conductance.
~Hunting or Singing in Alternators.~--Hunting is a term applied to
the state of two parallel connected alternators running out of step,
or not synchronously, that is, "see sawing." When the current wave of
an alternator is peaked and two machines are operated in parallel it
is very difficult to keep them in step, that is in synchronism. Any
difference in the phase relation which is set up by the alternation
will cause a local or synchronizing current to flow between the
two machines and at times it becomes so great that they must be
disconnected.
[Illustration: FIG. 1,404.--Revolving field of Fort Wayne alternator
equipped with _amortisseur winding_. The object of this winding is to
check any tendency toward _hunting_ when the alternator is to be run
as a synchronous motor, either for rotary condenser or power service.
The amortisseur winding consists of heavy copper bars, placed around
and through the pole faces and short circuited at the ends by heavy
copper rings; it serves as a starting winding to bring the rotor up to
speed as an induction motor, and also serves as a damping device to
neutralize any tendency toward "hunting" caused by variation in speed
of the generator supplying the current.]
Alternators which produce a smooth current wave and are maintained at
uniform speed by properly designed governors, operate fairly well in
parallel, but are not entirely free from hunting, and other means are
provided to overcome the difficulty.
When heavy copper flanges, called dampers, are put over the polar
projections or copper bars laid in grooves on the pole face and short
circuited by connecting rings (called amortisseur winding), the
powerful induced currents which are produced when the alternators get
out of step tend to quickly re-establish the phase relation.
[Illustration: FIG. 1,405.--Westinghouse field with amortisseur or
"damper" winding for 75 kva. and larger belted alternators, which
prevents hunting and reduces eddy currents in the pole pieces. The
copper bars of the amortisseur cage winding are arranged in partially
closed slots in the pole pieces.]
Two examples of a field provided with amortisseur[4] winding are shown
in figs. 1,404 and 1,405.
[4] NOTE.--Amortisseur windings are often erroneously called "squirrel
cage" windings on account of similarity of construction. The latter
term should be reserved for its proper significance as being the name
of the type of armature winding generally used for induction motors,
the name being suggested by the resemblance of the finished armature
to the wheel of a squirrel cage. A comparison of figs. 1,405 and 1,746
will show the distinction. In a squirrel cage winding there is a large
number of bars uniformly spaced; an amortisseur winding consists of
a comparatively small number of bars, usually unevenly spaced, that
is they are divided into groups with considerable space between the
groups, as in fig. 1,405, and less pronounced in fig. 1,404. The bars
are short circuited by rings the same as in squirrel cage winding.
[Illustration: FIG. 1,406.--Diagram of monocyclic system, showing
monocyclic armature and transformer connections. The monocyclic system
is a single phase system primarily intended for the distribution of
lights with an incidental load of motors. The lighting load is entirely
connected to one single phase circuit, and the motors are started and
operated from this circuit with the assistance of the teazer wire.
The long coil indicates the main winding of the armature, which is
similar in its arrangement and size to the ordinary armature winding
of a single phase alternator. The short coil which connects at one
end to the middle point of the coil above mentioned, and at the other
to a third collector ring is called the "teazer" coil. Its use is to
generate a pressure in quadrature with that of the main coil. This
pressure is combined with the main pressure of the alternator by
transformers, so as to give suitable phase relations for operating
induction motors. In the diagram the voltage has been assumed to be
2,080 volts, and the voltages marked to correspond with the generated
pressure. The coils of the alternator armature are connected, as
shown, to two main leads and to a teazer wire. Between each end of the
main coil and the end of the teazer coils, a resultant pressure is
generated. These resultants are about 12 per cent. larger than half the
main pressure. They also have a phase difference.]
~Monocyclic Alternators.~--This type of alternator was designed
prior to the introduction of the polyphase systems, to overcome the
difficulties encountered in the operation of single phase alternators
as motors. A single phase alternator will not start from rest as a
motor, but must first be started and brought up to the proper speed
before being connected with single phase mains. This condition
constituted a serious difficulty in all cases where the motor had to be
stopped and started at comparatively frequent intervals.
[Illustration: FIG. 1,407.--Monocyclic system diagram showing
transformer connections.]
[Illustration: FIG. 1,408.--Diagram showing section of monocyclic
alternator armature illustrating the armature winding. The main coils
are wound on every other tooth, and the teazer coils are placed in
quadrature with them, as shown.]
The monocyclic alternator is a single phase machine provided with
an additional coil, called a _teaser coil_, wound in two phase
relationship with, and connected to the center of the main single
phase coil. It is provided with three collector rings; two for the
single phase coil, and one for the free end of the teaser coil.
[Illustration: FIG. 1,409.--Diagram showing connections of General
Electric Monocyclic alternator. For 2,300 volt machine, connect as
shown by solid lines. For 1,150 volt machine, omit connections A to
B, C to D, E to F, and G to H, and connect as shown by dotted lines.
The armature of a standard monocyclic alternator rotates in a counter
clockwise direction facing the commutator. When the alternator is
loaded, the voltage between the teazer coil and the two terminals of
the main coil may be different; therefore, it is necessary to have the
commutator connected in corresponding ends of the main coil. If the
machine has not been arranged for clockwise rotation, the following
change in the connections on the commutator-collector must be made if
the machine is to be run in parallel with another. Fig. 1,410 shows the
connections of monocyclic alternators. In fig. 1,409, the studs on the
commutator-collector marked 1 and 6 are the terminals of the main coil.
These should be reversed. The numbers are stamped on the ends of the
stud and may be seen with the assistance of a mirror. By reference to
this diagram it is a simple matter to trace out the connections with
a magneto, after the armature leads are disconnected and the brushes
raised.]
By this arrangement ordinary single phase incandescent lighting can be
accomplished by means of a single pair of wires taken from the single
phase coil. Where three phase motors have to be operated, however, a
third wire, called the power wire, which is usually smaller than the
main single phase wires is carried to the point at which the motor is
located, and by the use of two suitably connected transformers three
phase currents are obtained from the combined single phase and power
wires for operating the motors.
[Illustration: FIG. 1,410.--Diagram showing connections of General
Electric monocyclic alternator. The solid lines show standard
connections for counter-clockwise rotation; the broken lines show
connection changed for clockwise rotation.]
Fig. 1,406 shows the connections of the monocyclic system and it is
only necessary to carry the teaser wire into buildings where motors are
to be used.
~Armature Reaction.~--Every conductor carrying a current creates a
magnetic field around itself, whether it be embedded in iron or lie
in air. Armature inductors, therefore, create magnetic fluxes around
themselves, and these fluxes will, in part, interfere with the main
flux from the poles of the field magnet. The effect of these fluxes is:
1. To distort the field, or
2. To weaken the field.
These disturbing fluxes form, in part, stray fluxes linked
around the armature inductors tending to choke the armature
current.
[Illustration: FIGS. 1,411 and 1,412.--Section of armature and field
showing _distorting effect_ of armature reaction on the field. When
a coil is opposite a pole as in fig. 1,411, no current is flowing
(assuming no self induction) and the field is undisturbed, but, as the
inductors pass under a pole face as in fig. 1,412, current is induced
in them, and lines of force are set up as indicated by the dotted
lines. This distorts the main field so that the lines of force are
crowded toward the forward part of the pole face as shown.]
~Ques. Explain how the field becomes distorted by armature reaction.~
Ans. Considering a slotted armature and analyzing the electrical
conditions as the inductors move past a pole piece, it will be
observed: 1, when the coil is in the position shown in fig. 1,411, the
current will be zero, assuming no armature self-induction, consequently
for this position the armature coil has no disturbing effect upon the
field set up by the field magnet; 2, when the inductors have moved
under the pole face, as in fig. 1,412, currents will be induced in
them, and they will tend to set up a magnetic field as indicated by the
dotted lines, and in direction, by the arrow heads. The effect of this
field will be to distort the main field, strengthening one side of the
pole and weakening the other side.
[Illustration: FIG. 1,413.--Section of armature and field showing
_weakening effect_ of armature reaction in the field. Self-induction
being present (as it almost always is), the current lags more or less
behind the pressure, so that when the coil is in the position of zero
induction, as shown, the current has not yet come to rest. Accordingly,
lines of force (indicated by the dotted lines) are set up by the
current flowing through the coils which are in opposition to the field,
thus weakening the latter. The dots and crosses in inductor sections,
have their usual significance in defining the direction of current,
representing respectively the heads and tails of arrows.]
~Ques. Explain how the field becomes weakened by armature reaction.~
Ans. In all armatures there is more or less inductance which causes the
current to lag behind the pressure a corresponding amount. Accordingly,
the current does not stop flowing at the same instant that the pressure
becomes zero, therefore, when the coil is in the position of zero
pressure, as in fig. 1,413, the current is still flowing and sets up a
magnetic field which opposes the main field as indicated by the dotted
arrows, thus weakening the main field.
~Ques. In what kind of armature is this effect especially pronounced?~
Ans. In slotted armatures provided with coils of a large number of
turns.
[Illustration: FIG. 1,414.--Section of armature and field showing
_strengthening effect_ of armature reaction when the current leads the
pressure. If the circuit contain an excess of capacity the current will
lead the pressure, so that when the coil is in the position of zero
induction, as shown, the current will have come to rest and reversed.
Accordingly, lines of force (indicated by the dotted lines) are set
up by the current flowing through the coil and which are in the same
direction as the lines of force of the field, thus strengthening the
latter.]
~Ques. What would be the effect if the current lead the pressure?~
Ans. It would tend to strengthen the field as shown in fig. 1,414.
The value of the armature ampere turns which tend to distort
and to diminish or augment the effect of the ampere turns on
the field magnet is sometimes calculated as follows:
.707 × I × T × P
A = ----------------
_s_
in which
A = armature ampere turns;
I = current per phase;
T = turns per pole per phase;
P = number of phases;
_s_ = product of the distribution and pitch factors of the winding.
This value of ampere turns, combined at the proper phase angle
with the field ampere turns gives the value of the ampere turns
available for producing useful flux.
[Illustration: FIG. 1,415.--Fort Wayne separately excited belt driven
alternator, a form adapted for installation in small plants where low
power factor is to be encountered. This condition exists in a line
where power is supplied to induction motors, transformers or other
inductive apparatus. The type here shown is built in sizes from 37½
kw. to 200 kw., 60 cycles, two or three phases and voltages of 240,
480, 600, 1,150 or 2,300 volts. They may be operated as single phase
alternators by using two of the phases and may then be rated at 70
per cent. of the polyphase rating. The field is excited by direct
current at a pressure of 125 volts. These alternators may be used
as synchronous motors and for this duty are fitted with amortisseur
winding in the pole faces which does not interfere with their use as
alternators.]
~Single Phase Reactions.~--Unlike three phase currents, a
single phase current in an alternator armature produces a
periodic disturbance of the flux through the machine. In
the magnet system this disturbance is of twice the normal
frequency, while in the armature core it is the
[Illustration: FIGS. 1,416 to 1,425.--Diagrams illustrating
superposition of fields. In the figures magnetic curves representing
the effect of the armature currents in several different cases are
superposed upon the magnetic curves assumed to be due to the field
magnet. The uppermost line shows the primary field due to the exciting
coils on the magnet poles. They are shown passing into the armature
teeth in two principal positions, where the middle of a pole is: 1,
opposite a tooth, and 2, opposite a slot. In the second line is shown
the field due to the armature currents assuming no lag, and that the
magnets are not excited. If there be no lag, the places of strongest
current will be opposite the poles. As shown in the right hand figure
when the current in one phase C, is at its maximum, those in the other
phases A and B will be of half strength. In the left hand figure when
the current in one phase B, is at its zero value, those in the other
phases will be of equal value, or 87 per cent. of the maximum. In the
third line is shown the effect of superposing these fields due to the
current upon those due to the magnets as depicted in the first line.
Inspection of this resultant field shows how the armature current
distorts the field without altering the total number of lines per
pole. In the fourth and fifth lines are shown the effects of a lagging
current. A lag of 90° is assumed; and in that case the maximum current
occurs in any inductor one quarter period after the pole has passed,
or at a distance of half a pole pitch behind the middle point of the
pole, as in the fourth line. When these armature fields are superposed
on those of the magnets in the first line the resultant fields are
those depicted in the fifth line. On inspection it will be seen that
in this case there is no distortion, but a diminution of the flux from
each pole, as the lines due to the armature currents, tending to pass
through the pole cores in the sense opposite to those of the primary
magnetism, must be deducted from the total. The twelve lines per pole
are correspondingly reduced to eight; and, of these eight, four go
astray constituting a leakage field. This illustrates the effect of a
lagging current in demagnetizing the field magnets and in increasing
the dispersion.] /# same as the normal frequency. In both cases the
eddy currents which are set up, produce a marked increase in the load
losses, and thus tend to give the machine a higher temperature rise on
single phase loading.
Designers continue to be singularly heedless of these single phase
reactions, resulting in many cases of unsatisfactory single phase
alternators. Single phase reactions distort the wave form of the
machine.
~Three Phase Reactions.~--The action of the three phase currents in
an alternator is to produce a resultant field which is practically
uniform, and which revolves in synchronism with the field system. The
resultant three phase reaction, because of its uniformity, produces no
great increase in the load losses of the machine, the small additional
losses which are present being due to windings not being placed
actually in space at 120°, and to the local leakage in the teeth.
[Illustration: FIG. 1,426.--Diagram showing lateral field between
adjacent poles.]
~Magnetic Leakage.~--In the design of alternators the drop of voltage
on an inductive load is mainly dependent upon the magnetic leakages,
primary and secondary. They increase with the load, and, what is of
more importance, they increase with the fall of the power factor of the
circuit on which they may be working. This is one reason why certain
types of alternator, though satisfactory on a lighting circuit, have
proved themselves unsatisfactory when applied to a load consisting
chiefly of motors.
The designer must know the various causes which contribute to leakage
and make proper allowance.
In general, to keep the leakage small, the pole cores should be short,
and of minimum surface, the pole shoes should not have too wide a span
nor be too thick, nor present needless corners, and the axial length
of the pole face and of the armature core should not be too great in
proportion to the diameter of the working face.
[Illustration: FIGS. 1,427 and 1,428.--Diagram showing respectively
the character of stray field between adjacent straight poles, and
between adjacent poles with shoes. Across the slightly V-shaped spaces
the stray field passes in lines that, save near the outer part, are
nearly straight. Quite straight they would not be, even were the sides
parallel, because the difference of magnetic pressure increases from
the roots towards the pole ends. At the roots, where the cores are
attached to the yoke, the magnetic pressure difference is almost zero.
It would be exactly zero if there were not a perceptible reluctance
offered by the joints and by the metal of the yoke. The reluctance of
the joint causes a few of the lines to take paths through the air by a
leakage which adds to the useful flux. At the tops of the cores there
is a difference of magnetic pressure equal to the sum of the ampere
turns on the two cores, tending to drive magnetic lines across. This
difference of magnetic pressure increases regularly all the way up
the cores from root to top; hence, the average value may be taken as
equal to the ampere turns on one core. The stray field, therefore, will
steadily increase in density from the bottom upwards. In addition to
this stray field between the pole cores there is also a stray field
between the projecting tips or edges of the pole shoes, as shown in
fig. 1,428. In some machines the dispersion due to the pole shoes is
greater than that between the flanks of the cores.]
To keep the increase of leakage between no load and full load from
undue magnitude, it is required that armature reactions shall be
relatively small, that the peripheral density of the armature current
(ampere-conductors per inch) be not too great, and that the pole cores
be not too highly saturated when excited for no load.
[Illustration: FIG. 1,429.--Lincoln revolving field alternator. The
frame has openings for ventilation, the fanning action of the pole
pieces causing a current of air to pass not only over the end of the
windings, as is usual with other designs, but also through ventilating
slots in the windings themselves. The armature core laminations are
annealed after punching and before assembling to guard against the
crystalizing effect of the punching. The armature coils are form
wound and insulated before being placed in the slot. There is also
slot insulation which is put in the slot previous to inserting the
coil. When the winding is completed, it is tested with a pressure of
4 to 10 times the normal voltage of the machine. The bearings are
self-aligning. The machine is normally designed to operate at a power
factor of approximately 70 per cent., which means that at that power
factor, the armature and fields at full load will heat equally. If it
have a higher power factor than 70 per cent. it means that the field
windings will run considerably cooler than the armature windings with
full load. If the power factor be lower than this, it will mean that
the field windings will run hotter than the armature on full load;
however, the machine is designed so that harmful heating does not occur
on full load with greater power factor than 40 per cent.]
The general character of the stray field between adjacent poles is
shown in figs. 1,427 and 1,428 for straight poles and those having
shoes.
~Field Excitation of Alternators.~--The fields of alternators require
a separate source of direct current for their excitation, and this
current should be preferably automatically controlled. In the case of
alternators that are not self exciting, the dynamo which generates the
field current is called the _exciter_.
The excitation of an alternator at its rated overload and .8
power factor would not, in some cases, if controlled by hand,
exceed 125 volts, although, in order to make its armature
voltage respond quickly to changes in the load and speed, the
excitation of its fields may at times be momentarily varied by
an automatic regulator between the limits of 70 and 140 volts.
[Illustration: FIG. 1,430.--Western Electric armature for self-excited
alternator. The main winding is placed at the bottom of the slots, each
coil being surrounded by an armour of horn fibre. The exciter winding
occupies a very small portion of the slot, being placed on top of the
main winding, and connected to the commutator immediately in front of
the core and between core and collector rings as shown.]
The exciter should, in turn, respond at once to this demand
upon its armature, and experience has shown that to do this
its shunt fields must have sufficient margin at full load
to deliver momentarily a range from 25 to 160 volts at its
armature terminals.
It is obvious from the above that an exciter suitable for use
with an automatic regulator must commutate successfully over a
wide range in voltage, and, if properly designed, have liberal
margins in its shunt fields and magnetic circuits.
Alternator fields designed for and operated at unity power
factor have often proved unsatisfactory when the machines were
called upon to deliver their rated kva. at .8 power factor or
lower. This is due to the increased field current required at
the latter condition and results, first, in the overheating of
the fields and, second, in the necessity of raising the direct
current exciting voltage above 125 volts, which often requires
the purchase of new exciters.
~Ques. What is a self-excited alternator?~
Ans. One whose armature has, in addition to the main winding, another
winding connected to a commutator for furnishing direct field exciting
current, as shown in fig. 1,430.
[Illustration: FIG. 1,431.--Frame, bed plate and armature winding for
Westinghouse bracket bearing polyphase alternator.]
~Ques. How is a direct connected exciter arranged?~
Ans. The exciter armature is mounted on the shaft of the alternator
close to the spider hub, or in some cases at a distance sufficient to
permit a pedestal and bearing to be mounted between the exciter and
hub. In other designs the exciter is placed between the bearing and
hub.
Figs. 1,432 and 1,433 are examples of direct connected exciter
alternators, in fig. 1,432 the exciter being placed between the
field hub and bearing, and in fig. 1,433, beyond the bearing.
~Ques. What is the advantage of a direct connected exciter?~
Ans. Economy of space.
This is apparent by comparing figs. 1,432 and 1,433 with fig.
1,434, which shows a belted exciter.
[Illustration: FIG. 1,432.--General Electric alternator with direct
connected exciter mounted on shaft between field hub and bearing. 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. The exciters are capable of furnishing the
desired excitation for low power factors.]
~Ques. What is the disadvantage of a direct connected exciter?~
Ans. It must run at the same speed as the alternator, which is slower
than desirable, hence the exciter must be larger for a given output
than the gear driven type, because the latter can be run at high speed
and accordingly be made proportionally smaller.
~Ques. What form of gear is generally used on gear driven exciters?~
Ans. Belt gear.
[Illustration: FIG. 1,433.--Fort Wayne alternator with direct connected
exciter mounted on the field shaft at such distance as to permit a
pedestal and bearing to be mounted between the exciter and revolving
field. In the view, the bearing is hidden by the exciter, only the foot
of the pedestal being visible.]
~Ques. What are the advantages of gear driven exciters?~
Ans. Being geared to run at high speed, they are smaller and therefore
less costly than direct connected exciters. In large plants containing
a number of alternators one exciter may be used having sufficient
capacity to excite all the alternators, and which can be located at any
convenient place.
~Ques. What is the disadvantage of gear driven exciters?~
Ans. The space occupied by the gear.
[Illustration: FIG. 1,434.--Diagram showing a Westinghouse 50 kva.,
2,400 volt, three phase, 60 cycle revolving field separately excited
alternator direct connected to a steam engine. The exciter is belted to
the alternator shaft, the driving pulley being located outside the main
bearing. The small pulley on the exciter gives an indication of its
high speed as compared with that of the alternator.]
In the case of a chain drive very little space is required, but
for belts, the drive generally used, there must be considerable
distance between centers for satisfactory transmission.
~Slow Speed Alternators.~--By slow speed is here understood relatively
slow speed, such as the usual speeds of reciprocating engines. A slow
speed alternator is one designed to run at a speed slow enough that
it may be direct connected to an engine. Such alternators are of
the revolving field type and a little consideration will show that
they must have a multiplicity of field magnets to attain the required
frequency.
In order that there be room for the magnets, the machine evidently must
be of large size, especially for high frequency.
[Illustration: FIG. 1,435.--Crocker-Wheeler 350 kva., slow speed
alternator direct connected to a Corliss engine. In front is seen a
belted exciter driven from a pulley on the main shaft between the
alternator and the large band wheel. The latter serves to give the
additional fly wheel effect needed for close speed regulation.]
EXAMPLE.--How many field magnets are required on a two
phase alternator direct connected to an engine running 240
revolutions per minute, for a frequency of 60?
An engine running 240 revolutions _per minute_ will turn
240 ÷ 60 = 4 revolutions _per second_.
A frequency of 60 requires
60 ÷ 4 = 15 cycles per phase per revolution, or
15 × 2 = 30 poles per phase. Hence for a two phase alternator
the total number of poles required is
30 × 2 = 60.
It is thus seen that a considerable length of spider rim is
required to attach the numerous poles, the exact size depending
upon their dimensions and clearance.
[Illustration: FIG. 1,436.--Three Crocker-Wheeler 75 kva., slow speed
alternators direct connected to high speed engines. The alternator is
styled slow speed although connected to a high speed engine, because
what is considered high engine speed is slow speed for alternator
operation. The alternators have direct connected exciters which are
plainly seen in the illustration placed on an extension of bearing
pedestal. Direct connected exciters on units of this kind do not, as a
rule, assume too bulky proportions, because of the high engine speed.]
~Fly Wheel Alternators.~--The diameter of the revolving fields on
direct connected alternators of very large sizes becomes so great that
considerable fly wheel effect is obtained, although the revolutions be
low. By giving liberal thickness to the rim of the spider, the rotor
then answers the purpose of a fly wheel, hence no separate fly wheel
is required. In fact, the revolving element resembles very closely an
ordinary fly wheel with magnets mounted on its rim, as illustrated in
fig, 1,437.
[Illustration: FIG. 1,437.--General Electric 48 pole 750 kw., three
phase fly wheel type alternator. It runs at a speed of 150 revolutions
per minute, giving a frequency of 60 cycles per second and a full load
pressure of 2,300 volts. The slip rings and leads to the field winding
are clearly shown in the figure. The field magnets are mounted directly
on the rim of the spider, which resembles very closely a fly wheel, and
which in fact it is--hence the name "fly wheel alternator."]
~High Speed Alternators.~--Since alternators may be run at speeds far
in excess of desirable engine speeds, it must be evident that both size
and cost may be reduced by designing them for high speed operation.
Since the desired velocity ratio or multiplication of speed is so
easily obtained by belt drive, that form of transmission is generally
used for high speed alternators, the chief objection being the
space required. Accordingly where economy of space is not of prime
importance, a high speed alternator is usually installed, except in the
large sizes where the conditions naturally suggest a direct connected
unit.
[Illustration: FIG. 1,438.--Allis-Chalmers high speed belted type
alternator. The small pulley at the right and the angle of the belt
suggest the high speed at which such alternators are run, a 50 kva.
machine turning 1,200 revolutions per minute.]
An example of high speed alternator is shown in fig. 1,438.
Machines of this class run at speeds of 1,200 to 1,800 or more,
according to size.
No one would think of connecting an alternator running at any
such speed direct to an engine, the necessary speed reduction
proper for engine operation being easily obtained by means of a
belt drive.
~Water Wheel Alternators.~--In order to meet most successfully the
requirements of the modern hydro-electric plant, the alternators
must combine those characteristics which result in high electrical
efficiency with a mechanical strength of the moving elements which
will insure uninterrupted service, and an ample factor of safety when
operating at the relatively high speeds often used with this class of
machine.
[Illustration: FIG. 1,439.--Allis-Chalmers 5,000 kva., 450 R. P. M.,
6,600 volt, 60 cycle, 3 phase, horizontal water wheel alternator. The
shaft is extended for the reception of a flange coupling for direct
connection to water wheel. Owing to the wide range in output of the
generating units and also in the speed at which they must operate
to suit varying conditions of head, types of wheels used, and other
features pertaining to water power developments, it has been necessary
to design a very complete line of machines for this work. The bearings
are of the ring oiling type with large oil reservoirs.]
When selecting an alternator for water wheel operation a careful
analysis of the details of construction should be made in order to
determine the relative values which have been assigned by the designers
to the properties of the various materials used. Such analysis will
permit the selection of a type of machine best adapted to the intended
service and which possesses the required characteristics of safety,
durability and efficiency.
[Illustration: FIG. 1,440.--Stator of 500 k.w. Allis-Chalmers
alternator for direct connection to vertical shaft hydraulic turbine.]
The large use of electric power transmitted by means of high pressure
alternating current has led to the development of a large number
of water powers and created a corresponding demand for alternators
suitable for direct connection to water wheels.
~Ques. Name two forms of water wheel alternator.~
Ans. Horizontal and vertical.
Examples of horizontal and vertical forms of water wheel
alternator are shown in figs. 1,439 and 1,440.
~Ques. How should the rotor be designed?~
Ans. It should be of very substantial construction.
~Ques. Why?~
Ans. Because water wheel alternators are frequently required to operate
safely at speeds considerably in excess of normal.
[Illustration: FIG. 1,441.--Allis-Chalmers revolving field for water
wheel alternator. In this type of alternator it is essential that the
rotating part be designed to have a liberal factor of safety not only
at the ordinary operating speed, but also at speeds much in excess of
normal. Frequently machines are required to operate safely at a speed
50 to 75 per cent. in excess of normal, so that there may be no danger
in case the water wheel races. In most machines the field spider is of
steel cast in a single piece for the smaller alternators and in two
or more parts for the larger sizes. For alternators running at high
peripheral speed, the rim is built up of steel laminations supported by
a cast steel spider; the latter serves merely to rotate the rim, which
is in itself able to withstand all stresses due to the high speed.
The field poles are laminated, being built up of steel punchings held
between malleable iron or bronze end plates, the latter being used
on high speed machines. With but very few exceptions the poles are
attached by dovetail projections that fit into corresponding slots.
Steel tapered keys are driven in alongside the dovetails, and the pole
pieces cannot become loose. All field coils, except on a few of the
smallest machines, are of edgewise wound copper strip. This style of
coil is essential for revolving field alternators where the pressure
on the insulation, due to centrifugal force, is so great that cotton
insulation on round wire will not stand. 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 always 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; in small alternators they are usually
fastened to the cap of one of the bearing pedestals.]
[Illustration: FIGS. 1,442 to 1,444.--Diagram of turbine alternator
windings for revolving armature. Fig. 1,442 illustrates a two pole
design in which all overlapping is avoided. It has 72 slots of which
only 48 are filled, giving 8 slots per phase. The projecting claws
from the brass end shield which hold the coils in position are shown
in section. Fig. 1,443 shows a four pole design having 48 slots or 4
slots per phase per pole, the coils being made up of 8 inductors per
slot taped together, the end bends forming two ranges. Fig. 1,444 shows
a two pole design for a two phase armature with 18 slots per pole per
phase. The core discs are spaced out as for 108 slots, but of these,
4 lots of 7 each are not stamped out, and 8 of those stamped are left
empty, so that there are 72 slots filled.]
~Ques. What special provision is made for cooling the bearings?~
Ans. They are in some cases water cooled.
~Turbine Driven Alternators.~--Although the principle of operation of
the steam turbine and that of the reciprocating engine are decidedly
unlike, the principle of operation of the high speed turbine driven
alternator does not differ from that of generators designed for
being driven by other types of engine or by water wheels. There are,
therefore, with the turbine driven alternator no new ideas for the
operator who is familiar with the older forms to acquire.
It must be obvious that the proportions of such extra high speed
machines must be very different from those permissible in generators of
much slower speeds.
~Ques. How does a turbine rotor differ from the ordinary construction?~
Ans. It is made very small in diameter and unusually long.
~Ques. Why?~
Ans. To reduce vibration and centrifugal stresses.
~Ques. What are the two classes of turbine driven alternators?~
Ans. They are classed as vertical or horizontal.
~Ques. How do they compare?~
Ans. The vertical type requires less floor space than the horizontal
design, and while a step bearing is necessary to carry the weight of
the moving element, there is very little friction in the main bearings.
The horizontal machine, while it occupies more space, does not require
a step bearing.
~Ques. Describe a step bearing.~
Ans. It consists of two cylindrical cast iron plates bearing upon each
other and having a central recess between them into which lubricating
oil is forced under considerable pressure by a steam or electrically
driven pump, the oil passing up from beneath.
~Ques. What auxiliary is generally used in connection with a step
bearing?~
Ans. A weighted accumulator is sometimes installed in
[Illustration: FIG. 1,445.--5,000 kw. Curtis turbine alternators
installed for the New York Central R. R. at Yonkers, N. Y. The
illustration shows also the auxiliary apparatus consisting of
condensers, vacuum augmenters, circulating pumps, air pumps, etc. The
augmenter of the first machine is plainly seen between the condenser
and centrifugal circulating pump.] connection with the oil pipe as a
convenient device for governing the step bearing pumps, and also as a
safety device in case the pumps fail.
~Alternators of Exceptional Character.~--There are a few types
of alternator less frequently encountered than those already
described. The essentials of such machines are here briefly
given.
~Asynchronous Alternators.~--In these machines, the rotating
magnet, which, with definite poles, is replaced by a rotor
having closed circuits. In general construction, they are
similar to asynchronous induction motors having short circuited
rotors; for these alternators, when operating as motors, run at
a speed slightly below synchronism and act as generators when
the speed is increased above that of synchronism. Machines of
this class are not self-exciting, but require an alternating or
polyphase current previously supplied to the mains to which the
stationary armature is connected.
Asynchronous alternators may be advantageously used in central
stations that may be required to sustain a very sudden increase
of load. In such cases, one or more asynchronous machines might
be kept in operation as a non-loaded motor at a speed just
below synchronism until its output as a generator is required;
when by merely increasing the speed of the engine it will be
made to act as a generator, thus avoiding the delays usually
occurring before switching in a new alternator.
~Image Current Alternators.~--When the generated frequency of
alternators excited by low frequency currents is either the sum
or the difference of the excitation and rotation frequencies,
any load current flowing through the armature of the machine
is exactly reproduced in its field circuit. These reproduced
currents are characteristic of all types of asynchronous
machines, and are called "image currents," as they are actually
the reflection from the load currents delivered by the armature
circuit.
As the exciter of a machine of this type carries "_image
currents_" proportional to the generated currents, its size
must be proportional to the capacity of the machine multiplied
by the ratio of the excitation and generated frequencies;
therefore, in the commercial machines, the excitation frequency
is reduced to the minimum value possible; from two to five
cycles per second being suitable for convenient employment.
These machines as heretofore constructed are not self-exciting,
but as the principle of image current enables the construction
of self-exciting alternators, it will be of advantage to have a
general understanding of the separately excited machine under
different conditions of excitation.
[Illustration: FIG. 1446.--Diagram of constant pressure image
current alternator connections. The image or reproduced currents are
characteristic of all types of asynchronous machines, and are called
image currents because they are actually the reflection from the load
currents delivered by the armature circuit. The principle of operation
is explained in the accompanying text.]
When the generated frequency of the machine is equal to the
difference of the excitation and rotation frequencies, the
magnetization of the machine is higher under a non-inductive
load than under no load. This is principally due to the ohmic
resistance of the field circuit, which prevents the image
current from entirely neutralizing the magnetomotive force
of the armature current. In other words, the result of the
magnetomotive force of the armature and image currents not only
tends to increase the no load magnetization of the machine at
non-inductive load, but depresses the original magnetization
at inductive load, so that the terminal voltage of the machine
increases with non-inductive load, and decreases with inductive
load.
Again, the generated frequency is equal to the sum of the
excitation and rotation frequencies, the resistance of the
field circuit reacts positively; that is, it tends to decrease
the magnetization, and consequently the terminal voltage of the
machine at both inductive and non-inductive loads.
In the constant pressure machine, the two effects are combined
and opposed to one another.
The connections of two alternators with diphase excitation are
shown by fig. 1,446.
~Extra High Frequency Alternators.~--Alternators generating
currents having a frequency up to 10,000 or 15,000 cycles per
second have been proposed several times for special purposes,
such as high frequency experiments, etc. In 1902 Nikola Tesla
proposed some forms of alternators having a large number of
small poles, which would generate currents up to a frequency of
15,000 cycles per second.
Later, the Westinghouse Company constructed an experimental
machine of the inductor alternator type for generating currents
having a frequency of 10,000 cycles per second. This machine
was designed by Samms. It had 200 polar projections with a pole
pitch of only 0.25 inch, and a peripheral speed of 25,000 feet
per minute. The armature core was built up of steel ribbon 2
inches wide and 3 mils thick. The armature had 400 slots with
one wire per slot, and a bore of about 25 inches. The air gap
was only 0.03125 inch. On constant excitation the voltage
dropped from 150 volts at no-load to 123 volts with an output
of 8 amperes.
~Self-Exciting Image Current Alternators.~--The type of machine
described in the preceding paragraph can be made self-exciting
by connecting each pair of brushes, which collect the current
from the armature, with a field coil so located that the
flux it produces will be displaced by a predetermined angle
depending on the number of phases required, as shown by fig.
1,447. The direction of the residual magnetism of the machine
is shown by the arrows A, A.
[Illustration: FIG. 1,447.--Diagram of connections of self-exciting
image current alternator.]
When the armature is rotated, a pressure will be generated
between the brushes 2 and 4, and a current will flow from
C through the coils XX to B, producing a flux through the
armature at right angles to the residual magnetism and
establishing a resultant magnetic field between D, B, and D,
C. This field will generate a pressure between the brushes 1
and 3, and a current will flow D through XX to E in such a
direction that it will at first be opposed to the residual
magnetism, and afterward reverse the direction of the latter.
At the moment the residual magnetism becomes zero, the only
magnetism left in the machine will be due to the currents
from the brushes 2 and 4, and their field combining with the
vertical reversed field will produce a resultant polar line
between B and E. As these operations are cyclic, they will
recur at periodic intervals, and the phenomena will become
continuous. The negative field thus set up in the air gap of
the machine will cut the conductors of the stator and will
be cut by the conductors of the rotor in such a manner that
the electromotive forces generated between the brushes of
the armature will be equal and opposite to those between the
terminals of the stator.