8

ELECTRICAL APPARATUS

speed, however, the hysteresis device reduces the slip, while the
temperature device leaves it unchanged.

The foremost disadvantage of the use of the hysteresis device.
is the impairment of the power-factor, as seen in Fig. 3 as p.

The introduction of the effective resistance representing the
hysteresis of necessity introduces a reactance, which is higher
than the resistance, and thereby impairs the motor characteristics.

Comparing Fig. 3 with Fig. 176, page 319 of " Theoretical

Z

INDUCTION MOTOR

.2j; Z0=.05-f.15 j, 00=100

SPEED CONTROL BY HYSTERESIS
SPEED CURVES

6 1,0.1.6 2,0 25 3,0 3.5 4,0 4,5 5.0 5

56065 7 07J5

140
130
120
110
100
90
80
70
60
CO
40
30
20
10
0

FIG. 3.—Load curves of induction motor with hysteresis starting device.
Elements of Electrical Engineering/7 which gives the load curves
of Tf of Fig. 2, it is seen that the hysteresis starting device reduced
the maximum power-factor, p, from 91 per cent, to 84 per cent.,
and the apparent efficiency, 7, correspondingly. •
This seriously limits the usefulness of the device.
C. Eddy-current Starting Device
6. Assuming that, instead of using a well-laminated magnetic
circuit, and utilizing hysteresis to give the increase of effective
resistance with increasing slip, we use a magnetic circuit having
very high eddy-current losses: very thick laminations or solid
iron, or we directly provide a closed high-resistance secondary
winding around the magnetic circuit, which is inserted into the
induction-motor secondary for increasing the starting torque.