Chapter XLIX: 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.

~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.

~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.

~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.

~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.

~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

/# 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.

~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.

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.

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.

~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.

~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.

~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

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.

~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.

~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.

~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.

~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.

~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.

~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.

_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.

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.

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.

~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.

~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.

~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.

~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.

~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.

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.

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.

~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.

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.

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.

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.

~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.

~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.

~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.

~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

/# 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.

~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.

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.

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.

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.

~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.

~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.

~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.

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.

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.

~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.

~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.

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.

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.

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.

~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

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.

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.

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.