322

ELECTRICAL APPARATUS

I!

motor, but depends upon the brush angle, 8, and the ratio, c, of
rotor -r- stator impressed voltage.

At this no-load speed, So, the armature current, /i, of the
polyphase shunt motor is in general not equal to zero, as it is
in the polyphase induction motor.

Two cases are therefore of special interest:

1. Armature current, Ii = o, at no-load, that is, at slip, SQ.

2. No-load speed equals synchronism, s0 = o,
1. The armature or rotor current (79):

becomes zero, if:

° sZZQ

c = — as

Z+

or, since Z\ is small compared with Z, approximately:

c = — as — — -s (cos 8 — j sin 8);
hence, resolved:

c = — 5 cos
o = s sin 0;

hence:

(85)

That is, the rotor current can become zero only if the brushes
are set in line with the stator circuit or without shift, and in this
case the.rotor current, and therewith the output of the motor,
becomes zero at the slip, s = — c.
Hence such a motor gives a characteristic curve very similar
to that of the polyphase induction motor, except that the stator
tends not toward synchronism but toward a definite speed equal
to (1 + c) times synchronism.
The speed of such a polyphase motor with commutator can,
therefore, be varied from synchronism by the insertion of-an
e.m.f. in the rotor circuit, and the percentage of variation is the
same as the ratio of the impressed rotor e.m.f. to the impressed
stator e.m.f. A rotor e.m.f., in opposition to the stator e.m.f.
reduces, in phase with the stator e.m.f., increases the free-run-
ning speed of the motor. In the former case the rotor impressed
e.m.f. is in opposition to the rotor current, that is, the rotor
returns power to the system in the proportion in which the speed