**A.C GENERATORS AND MOTORS**

The outline of a d.c machine shown earlier in the **previous article** will serve also as a
small alternating-current generator or alternator. Alternators, of course,
have no commutator, and the direct current for energizing the field magnet must
be obtained from a separate source. A small d.c generator termed an exciter is
sometimes mounted on the same shaft for this purpose.

As in the d.c generator, the field-magnet poles, if there
are more than two, are alternately n south, and since a complete cycle of
current is generated every time a conductor passes north and south pole in
succession, the frequency of the current is given by the equation;

FREQUENCY = Revolution per second ×Number of pairs of poles.

The armature carries several groups of conductors, the
groups being spaced around the periphery so as to occupy similar positions in
relation to the field-magnet poles. The current is collected from the armature
by means of slip-rings.

Since all that is necessary for the generation of current is
relative motion between conductors and field, a generator could be made to work
by holding the armature and rotating the field magnet around it. This would be
very inconvenient, but a similar effect can be obtained by placing the
field-magnet system on the central rotating part, and the conductors in which
the current is generated on the surrounding stationary part. As the
field-magnet windings then rotate, they must be connected to the d.c source by
a pair of slip-rings.

Part of an alternator
in which the conductors move and the field magnets are stationary is shown in the diagram below, while the corresponding arrangement, in which the field-magnets
move and the conductors are stationary. In both cases the slots in which the
conductors are housed are indicated, but the conductors themselves are omitted
for the sake of clearness.

In order to avoid confusion, the terms stator and rotor are
used instead of armature and field-magnet. The stator is the stationary part
and the rotor is the rotating part, no matter which of them carries the
armature conductors and with the field-magnet system.

The advantage of the arrangement shown below is that the
slip-rings and moving windings have to deal only with the comparatively low
voltage and small current necessary to produce the field.

As the conductors in which the alternating current is
generated are stationary, their insulation is simplified, while the fact that the connection can be made to them without slip-rings and brushes removes another
difficulty in the generation of high voltages. Alternators of this type are
therefore normal and can be made for much larger outputs than are practicable
in d.c generators.

Note that the right-hand rule for finding out in which direction
induced current flows assumes that the conductor is moving across the field,
and not vice versa. When the conductor is stationary and the field is moving,
the rule must be applied as though the direction of motion were reversed.

**POLYPHASE CURRENTS**

From the generating point of view, it is better to produce a
steady current than one that is continually varying. In the d.c generator we
were able to do this by spacing a number of conductors around the armature, so
that while some were generating their maximum e.m.f., others were generating
their minimum. Something of the same kind can be done in the case of an
alternator, by causing it to generate at the same time two or more currents
differing in phase. The different currents must be taken from the machine over
different circuits, and we are thus led to a polyphase system comprising
several more or less independent supplies of the same frequency, as distinct
from a single-phase system having only one supply.

Polyphase systems also enable the winding space on the
generator to be utilized more efficiently. Moreover, they simplify the design
of alternating-current motors and lead to economies in the amount of copper
required for transmission lines.

Suppose that we provide an alternator with two sets of conductors,
the grouping in relation to the spacing of the rotating field-magnets being
such that one set is producing its maximum. We then have two independent
sources of current, the phase relationship of the voltages being
represented by curves *a *and *b.* This is a two-phase system, and the difference in phase is one-quarter of a
cycle or ninety electrical degrees.

In a similar manner, we can provide a generator with three
independent groups of conductors, thus obtaining three separate sources of
current, the phase relationship of the three voltages being as *a, b and c * in the diagram below. This is a three-phase
system and the difference in phase is one-third of a cycle or 120 electrical
degrees.

Instead of fitting a three-phase generator with six output
terminals, we can connect one end of each winding to a common point, thus using
only four.

The arrangement will then be shown on the diagram below in which the coils represent
the three generator windings, and the center point the fourth terminal. Conductors *a,b and c are connected to the terminal*s
at the free ends of the windings. The common return path to the center point.
Conductors *a, b *and c are known as
the three lines, and the common center point (which is usually earthed) as the
neutral point.

Suppose that three exactly similar loads are connected, one
between each of lines *a, b, and c. *The three currents will then be equal,
and their phase relationship will be the same as that of the voltages. They can
therefore be represented by the curves from which it will be seen that when anyone current is at a maximum, the other two are halfway towards a maximum in the opposite direction and that when anyone is zero, the other two are equal and opposite. Similar conditions apply at
all points on the curves, so that the sum of the three currents at any moment,
taking their directions into account, is zero.

It follows that so long as loads remain the same, there is
no current flowing in either direction in the conductor, which under these
conditions could be omitted. Each of the three lines is then acting in turn as
a return path for the other two. Even if the loads are not the same, the
conductor has to carry only the difference between the current flowing outwards
and that flowing inwards over the three lines at any instant. It can therefore
be smaller in size than the others.

Instead of connecting each load between one of the lines *a, b, c, *and the conductor, we can
connect one load between *a and b*,
one between *b and c, * and one between *c, and a.* the voltage applied to each load is then derived from two
of the generator windings, but as the two voltages do not reach their maximum
at the same time, the joint value is not twice that of one winding, but some
smaller figure. The actual value is √3 or 1.732, times the voltage of one
winding. The current in each line is obviously equal to the current in one
winding.

Generator windings arranged as shown below are said to be *star connected.*

The voltage between any two of the lines a, b, and c * is
called the line voltage, and that between any one of them and the neutral point
the *phase voltage.

Line voltage = phase voltage × 1.732 = 400.

Phase voltage = 400/1.732 = 230.

An alternating arrangement of the generator windings is
shown below, and in this case they are said
to be *delta connected or mesh connected.*

The term “delta” is taken from the Greek capital letter Δ.
There is no tendency for current to circulate around the closed path because the
sum of the voltages at any instant is zero. The line voltage is that produced
by one winding and is therefore equal to the phase voltage. The current in
each line, however, is √3 or 1.732, times the current in one winding.

Note that the two methods of connection (star and delta)
apply not only to the generator windings but also to the loads. Thus, in the
first case we considered, if the loads are connected between each of lines I*a, b, c, *and the conductor, they are
star-connected, but if between *a and b, b and c, and c and a,* they are
delta connected. This can readily be seen by drawing an example.

**A.C MOTORS**

A.C generators, like **d.cgenerators **can be made to run as motors, but only when the frequency of
the supply is in step with the frequency at which the armature conductors pass
the frequency at which the armature conductors pass the pairs of poles. The motor must therefore be rotated by some other means until it is running fast
enough to continue in synchronism with the supply frequency.

Machines designed to operate in this manner are called synchronous
motors. Motors that do not operate in synchronism with the supply frequency
are the most important class of asynchronous motors that depend upon a special property
of polyphase currents which we shall now examine.

**ROTATING FIELDS**

Consider the two pairs of coils shown below. If coils are energized, there will be a magnetic field in line with their axis; let us call this direction north and
south. If coils *b *are energized,
there will be a magnetic field in line with their axis; let us call this
direction east and west.

Suppose now that the coils are connected to a two-phase
supply. We can use a phase diagram to represents the two currents, each curve
corresponding to the similarly lettered coils. Starting on the left-hand side,
current *a *is at a maximum in one direction; let us
assume that this produces a flux in coils *a
*towards the north. At this moment, current *b* is at zero, so there is no flux in coils *b. *the flux at the center may therefore be represented by the arrow
in the sketch below:

Halfway between these positions, curve *a *is still some distance from zero, and curve *b is *some distance from its maximum. This is the point at which the
curves cross, and the two currents are therefore equal. The two small arrows in
sketch 2 represent these conditions, and their combined effect is to produce a
field towards the north-west as shown by the heavy arrow.

This combination of two fields should be noted. It follows
from the fact that the lines of force can not have more than one direction at the
same place and time. An analogy may be helpful. Suppose that we set up two
electric fans at right angles so that one produces a wind towards the north
and the other a wind towards the west. If only the first fan is blowing, a
particle caught in the wind will move north. If only the second fan is blowing,
it will move west. If both fans are blowing, its tendency will be to move
north-west.

As the field from coils *a
*dies away and that from coils *b grows,* the combined field, starting from
sketch 1, passes through all the intermediate positions to sketch 2, and then
through all the intermediate positions to sketch 3. We have, therefore, a
rotating field produced by fixed coils. Moreover, it can be shown mathematically
that the strength of the field does not vary, being always equal to that
produced by one of the coils when the current in the other is at zero.

It is important to note that the rotating field is not a
matter of approximation, or of a sudden jump from north to west, or even from
north to north-west. The rotation of the field is quite smooth and regular,
owing to the gradual dying away of flux in one direction and its equally
gradual building up at right angles.

Continuing the sequence, the current in coil *b* dies away again after reaching its
maximum, and the field towards the west gradually weakens. At the same time,
coil *a* is energized by a growing
current in the direction opposite to that which produced a field towards the
north. This produces a field towards the south, which, in conjunction with the
weakening field towards the west, produces a field passing through the
south-west as shown in the sketch above. when the current in coils *a *has reached a maximum in this
direction and that in coils *b* is at
zero, the field is towards the south. We have now traced the changes for a
complete half-cycle, it will be found that the rotation continues through
south-east, east, and north-east until the field is again towards the north.
The number of revolutions per second made by the field is therefore equal to the
frequency of the current.

We have examined the production of a rotating field by
two-phase current because this is the easiest case to follow without a detailed
mathematical statement. A similar effect can, however, be produced by
three-phase currents. In this case, three sets of coils are needed, and the
field rotates through 120 degrees between the maximum in one set of coils and
the maximum in the next. As this time represents one-third of a cycle, the
field again rotates at the supply frequency.

**INDUCTION MOTORS**

The fact that a rotating field can be produced by stationary
coils has led to the development of the induction motor. In this machine, coils
carried by the stator produce the rotating field and the rotor is provided with
heavy copper conductors accommodated in the usual slots. In many cases, these conductors have no
external connections but are simply joined together at each end of the rotor
by heavy copper rings. The term squirrel-cage is often applied to rotors of
this type.

As the magnetic field rotates, it cuts the rotor conductors
and induces currents in the circuits completed by the copper end rings. These
currents produce a field that reacts
with the rotating field. in accordance with Lenz’s law, the effect is to
produce motion that will tend to prevent the change of flux linkage, i.e., to
make the conductors follow the field.

The rotor therefore turns. It does not, however, actually
reach the speed of the rotating field; if it did there would no longer be any
induced currents, and there would be no power even to overcome the friction of
the motor bearings. The difference between the speed of the rotor and that of
the field is known as the slip, and at full load may amount in practice to,
say, 4% of the speed.

The squirrel-cage rotor without slip-rings is very simple
and robust but is not suitable for starting under heavy loads. Some induction
motors are therefore fitted with wound rotors. The winding is in three
sections and is brought out to three slip-rings. The object of the slip-rings
is to enable starting resistances to be gradually cut out as the motor speeds up until conditions are then similar to those of the squirrel-cage machine.

**MISCELLANEOUS A.C MACHINES**

In addition to the machines we have described, the following
types may be mentioned.

SINGLE-PHASE INDUCTION MOTORS – Although a single-phase
supply cannot produce a rotating magnetic field in the way described for
polyphase currents, it is nevertheless possible to design a single-phase motor
operating on the induction principle. Machines of this kind are not very
efficient, but in small sizes they have come into increasing use in recent
years. They are not naturally self-starting but can be made so at the cost of
some complication.

MAGNETO GENERATORS – These are miniature generators in which
the field is produced by permanent magnets. The armature (rotor) is usually of
the simple two-slot form also known as H-armature shown below.

One cycle of alternating current is generated during each
revolution. Magneto generators have been used for the generation of ringing
current in some telephone systems, and for ignition current in internal combustion engines. In the latter case, provision is made for
interrupting the armature current periodically. The result is to induce a very
high voltage in a second winding, also carried by the armature. The
high-voltage current so made available is led to the sparking-plugs, where it
produces the spark which ignites the explosive mixture in the cylinders.

COMMUTATOR MOTORS – Since in a d.c motor, reversal of both
field and armature connections at the same time does not alter the direction of
rotation, it is possible to run such motors on alternating current. Owing to
the greater tendency to eddy-current losses, however, all the iron, including
the field magnet, should be laminated. Small machines of this kind suitable for
either d.c or a.c supplies are made and are called universal motors.

MINIATURE SYNCHRONOUS MOTORS – For driving electric clocks,
miniature synchronous motors are used. They must be operated from mains on
which the frequency is “controlled” i.e., kept at or near its stated value over
long periods. The motor then runs at a known speed, so that by means of
reduction gearing it can be made to operate the hands of a clock. Some but not
all of these motors are self-starting. Miniature synchronous motors of slightly
larger sizes are commonly used for rotating gramophone turntables.

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