90

ELECTRICAL APPARATUS

or in shunt thereto, and thus gives series-motor characteristics,
or shunt-motor characteristics.

In either case, two independent variables exist, the value of
the voltage impressed upon the commutator, and its phase,
and the phase of the voltage supplied to the secondary circuit
may be varied, either by varying the phase of the impressed
voltage by a suitable transformer, or by shifting the brushes on
the commutator and thereby the relative position of the brushes
with regards to the stator, which has the same effect.

However, with such a commutator motor, while the resultant
magnetic effect of the secondary currents is of the low frequency

FIG. 34.—Commutated full-frequency current in induction motor
secondary.
of slip, the actual current in each secondary coil is of full fre-
quency, as a section or piece of a full-frequency wave, and thus
it meets in the secondary the full-frequency reactance. That is,
the secondary reactance at slip, s, is not: Z8 = TI + jsxi, but is:
Z8 = Ti + jxij in other words is very much larger than in the
motor with short-circuited secondary.
Therefore, such motors with commutator always require
power-factor compensation, by shifting the brushes or choosing
the impressed voltage so as to be anti-inductive.
Of the voltage supplied to the secondary through commutator
and brushes, a component in phase with the induced voltage
lowers the speed, a component in opposition raises the speed,
and by varying the commutator supply voltage, speed control
of such an induction motor can be produced in the same manner
of the same character, as produced in a direct-current motor