ALTERNATING-CURRENT MOTORS

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(1) is the synchronous motor of the electrical industry. (2)
and (3) are used occasionally to produce synchronous rotation
without direct-current excitation, and of very great steadiness
of the rate of rotation, where weight efficiency and power-
factor are of secondary importance. (4) is used to some extent
as frequency converter or alternating-current generator.
(2) and (3) are occasionally observed in induction machines,
and in the starting of synchronous motors, as a tendency to
lock at some intermediate, occasionally low, speed. That is,
in starting, the motor does not accelerate up to full speed, but
the acceleration stops .at some intermediate speed, frequently
half speed, and to carry the motor beyond this speed, the im-
pressed voltage may have to be raised or even external power
applied. The appearance of such "dead points77 in the speed
curve is due to a mechanical defect—as eccentricity of the
rotor—or faulty electrical design: an improper distribution of
primary and secondary windings causes a periodic variation of
the mutual inductive reactance and so of the effective primary
inductive reactance, (2) or the use of sharply defined and im-
properly arranged teeth in both elements causes a periodic
magnetic lock (opening and closing of the magnetic circuit, (3)
and so a tendency to synchronize at the speed corresponding to
this cycle.
Synchronous machines have been discussed elsewhere. Here
shall be considered only that type of motor in which the electric
and magnetic relations between the stator and rotor do not vary
with their relative positions, and the torque is, therefore, not
limited to a definite synchronous speed. This requires that the
rotor when connected to the outside circuit be connected through
a commutator, and when closed upon itself, several closed cir-
cuits exist, displaced in position from each other so as to offer a
resultant closed circuit in any direction.
The main types of these motors are:
1. One member supplied with polyphase or single-phase alter-
nating voltage, the other containing several circuits closed upon
themselves—polyphase and single-phase induction machines.
2. One member supplied with polyphase or single-phase alter-
nating voltage, the other connected by a commutator to an
alternating voltage—compensated induction motors, commutator
motors with shunt-motor characteristic.
3. Both members connected, through a commutator, directly