INDUCTION MOTOR

91

by varying the field excitation. Good constants can be secured,
if in addition to the energy component of impressed voltage, used
for speed control, a suitable anti-inductive wattless component
is used.
However, this type of motor in reality is not an induction
motor any more, but a shunt motor or series motor, and is more
fully discussed in Chapter XIX, on " General Alternating-current
Motors."
59. Suppose, however, that in addition to the secondary wind-
ing connected to commutator and brushes, a short-circuited
squirrel-cage winding is used on the secondary. Instead of
this, the commutator segments may be shunted by resistance,
which gives the same effect, or merely a squirrel-cage winding
used, and on one side an end ring of very high resistance em-
ployed, and the brushes bear on this end ring, which thus acts
as commutator.
In either case, the motor is an induction motor, and has the
essential characteristics of the induction motor, that is, a slip, s,
from synchronism, which increases with the load; however,
through the commutator an exciting current can be fed into the
motor from a full-frequency voltage supply, and in this case, the
current supplied over the commutator does not meet the full-
frequency reactance, Xi, of the secondary, but only the low-fre-
quency reactance, sxi, especially if the commutated winding is in
the same slots with the squirrel-cage winding: the short-circuited
squirrel-cage winding acts as a short-circuited secondary to the
high-frequency pulsation of the commutated current, and there-
fore makes the circuit non-inductive for these high-frequency
pulsations, or practically so. That is, in the short-circuited con-
ductors, local currents are induced equal and opposite to the
high-frequency component of the commutated current, and the
total resultant of the currents in each slot thus is only the low-
frequency current.
Such short-circuited squirrel cage in addition to the commu-
tated winding, makes the use of a Commutator practicable for
power-factor control in the induction motor. It forbids, how-
ever, the use of the commutator for speed control, as due to the
short-circuited winding, the motor must run at the slip, s, corre-
sponding to the load as induction motor. The voltage impressed
upon the commutator, and its phase relation, or the brush posi-
tion, thus must be chosen so as to give only magnetizing, but