INDUCTION MOTOR

89

efficiency at load, that is, curves starting from (1) and rising up
to (2).

Hereto belong: The synchronous motor at constant excita-
tion, marked by S.

Concatenation to a commutating machine,
CC.

Induction motor with condenser in secondary
circuit, C.

These three curves are very similar, the points calculated for
the three different motor types falling within the narrow range
between the two limit curves drawn in Fig. 28.

Regarding the speed characteristics, two types exist: the motors
So, S, CSo and CS are synchronous, the motors /, CC and C are
asynchronous.

In their efficiencies, there is little difference between the
different motors, as is to be expected, and the efficiency curves
are almost the same up to the overloads where the motor begins
to drop out of step, and the efficiency thus decreases.

Induction Motor with Commutator
58. Let, in an induction motor, the turns of the secondary
winding be brought out to a commutator. Then by means of
brushes be.aring on this commutator, currents can be sent into
the secondary winding from an outside source of voltage.
Let then, in Fig. 34, the full-frequency three-phase currents
supplied to the three commutator brushes of such a motor be
shown as A. The current in a secondary coil of the motor,
supplied from the currents, A, through the commutator, then is
shown asB. Fig. 34 corresponds to a slip, s = J£- As seen from
Fig. 34, the commutated three-phase current, B, gives a resultant
effect, which is a low-frequency wave, shown dotted in Fig. 34
B, and which has the frequency of slip, s, or, in other words, the
commutated current, B, can be resolved into a current of fre-
quency, s, and a higher harmonic of irregular wave shape.
Thus, the effect of low-frequency currents, of the frequency
of slip, can be produced in the induction-motor secondary by
impressing full frequency upon it through commutator and
brushes.
The secondary circuit, through commutator and brushes, can
be connected to the supply source either in series to the primary,