68

ELECTRICAL APPARATUS

unity power-factor, as shown by curve e in Fig. 26, while still
considerable, is very much less in this poor induction motor,
than it was in the good induction motor Figs. 22 to 25.

The constant-excitation load curves, Fig. 27, give character-
istics, which are very much superior to those of the motor as in-
duction motor. The efficiency is not materially changed, as was
to be expected, but the power-factor, p, is very greatly improved
at all loads, is 96 per cent, at full-load, rises to unity above full-

LOW SPEED INDUCTION MOTOR
CONSTANT DIRECT CURRENT EXCITATION

.

Yo=.02~.6j Zt-.H
(Z-.H-.2j)

SYNCHRONOUS

PIG. 27.—Load curve of low-speed high-excitation induction motor con-
verted to synchronous motor, at constant field excitation.
load (assumed as 75 kw.) and is given at quarter-load already
higher than the maximum reached by this machine as straight
induction motor.
For comparison, in Fig. 28 are shown the curves of apparent
efficiency, with the power output as abscissae, of this slow-speed
motor, as:
I as induction motor (from Fig. 20);
So as synchronous motor with the field excitation varying to
maintain unity power-factor (from Fig. 26);
S as synchronous motor with constant field excitation (from
Fig. 27).