CHAPTER XIX
ALTERNATING-CURRENT MOTORS IN GENERAL
171. The starting point of the theory of the polyphase and
single-phase induction motor usually is the general alternating-
current transformer. Coming, however, to the commutator
motors, this method becomes less suitable, and the following
more general method preferable.
In its general form the alternating-current motor consists of
one or more stationary electric circuits magnetically related to
one or more rotating electric circuits. These circuits can be
excited by alternating currents, or some by alternating, others
by direct current, or closed upon themselves, etc., and connec-
tion can be made to the rotating member either by collector
rings—that is, to fixed points of the windings—or by commutator
—that is, to fixed points in space.
The alternating-current motors can be subdivided into two
classes—those in which the electric and magnetic relations
between stationary and moving members do not vary with their
relative positions, and those in which they vary with the relative
positions of stator and rotor. In the latter a cycle of rotation
exists, and therefrom the tendency of the motor results to lock at
a speed giving a definite ratio between the frequency of rotation
and the frequency of impressed e.m.f. Such motors, therefore,
are synchronous motors.
The main types of synchronous motors are as follows:
1. One member supplied with alternating and the other with
direct current—polyphase or single-phase synchronous motors.
2. One member excited by alternating current, the other con-
taining a single circuit closed upon itself—synchronous induction
motors.
3. One member excited by alternating current, the other of
different magnetic reluctance in different directions (as polar
construction)—reaction motors.
4. One member excited by alternating current, the other by
alternating current of different frequency or different direction
of rotation—general alternating-current transformer or fre-
quency converter and synchronous-induction generator.