88

ELECTRICAL APPARATUS

reactance, 32, gives a greater slip, s,-that is, lower operating
speed, and inversely, as was discussed in Chapter I.

57. It is interesting to compare, in Fig. 28, the various methods
of secondary excitation of the induction motor, in their effect in
improving the power-factor and thus the apparent efficiency of
a motor of high exciting current and thus low power-factor, such
as a slow-speed motor.

The apparent efficiency characteristics fall into three groups:

LOW SPEED INDUCTION MOTOR

WITH CONDENSER IN SECONDARY CIRCUIT

6Q = 500 Z0 = .1 4- .3j

Y0=.02 -.Qj Z!=.1 +.3j

Z2 - -.012*
ASYNCHRONOUS

10 20 30 40 50 60 70 80 9JO 100 110 120 130 140 IgO 160 1?0

J.O.

FIG. 33.—Load curves of high-excitation induction motor with, condensers in

secondary circuits.

1. Low apparent efficiency at all loads: the straight slow-
speed induction motor, marked by /.
2. High apparent efficiency at all loads:
The synchronous motor with unity power-factor excitation, SQ.
Concatenation to synchronous motor with unity power-factor
excitation, CSQ.
Concatenation to synchronous motor with constant excitation,
CS.
These three curves are practically identical, except at great
overloads.
3. Low apparent efficiency at light-loads, high apparent