REVIEW 469
a series or shunt motor; if it is closed upon itself, directly or
through another circuit, the motor is called a repulsion motor.
A combination of both gives the series repulsion motor.
Single-phase commutator motors of series characteristic are
used for alternating-current railroading, of shunt characteristic
as stationary motors, as for instance the induction repulsion
motor, either as constant-speed high-starting-torque motors, or
as adjustable-speed motors.
Lagging the field magnetism, as by shunted resistance, pro-
duces a lead of the armature current. This can be used for
power-factor compensation, and single-phase commutator motors
thereby built with very high power-factors. Or the machine,
with lagging-quadrature field excitation, can be used as effective
capacity. The single-phase commutator motor is the only type
which, with series field excitation, gives a varying-speed motor
of series-motor characteristics, and with shunt excitation or its
equivalent, give speed variation and adjustment like that of the
direct-current motor with field control, and is therefore exten-
sively used. Its disadvantage, however, is the difficulty and
limitation in design, resulting from the e.m.f. induced in the short-
circuited coils under the brush, by the alternation of the main
field, which tends toward sparking at the commutator.
Single-phase Generation.—135.
Speed Control of Polyphase Induction Motor.—
By resistance in the secondary, 8. Gives a speed varying
with the load.
By pyro-electric resistance in the secondary, 10. Gives good
speed regulation at any speed, but such pyro-electric conductors
tend toward instability.
By condenser in the secondary, 11. Gives good speed regula-
tion, but rather poor power-factor, and usually requires an un-
econornically large amount of capacity.
By commutator, 58. Gives good speed regulation and per-
mits power-factor control, but has the disadvantage and com-
plication of an alternating-current commutator.
By concatenation with a low-frequency commutating machine
as exciter, 52. Has the disadvantage of complication.
Stanley Induction Generator.—117. Induction machine with
low-frequency exciter. (See "Concatenation (2).")
Stanley Inductor Alternator.—159. Inductor machine with
two armatures and inductors, and a concentric field coil between
the same. (See " Inductor Machine.")