n

SPEED CONTROL

25

Four poles, 1800 rev.: ZQ = r0 + jxQ == 0.1 + 0.3 j\

Zi = n +jxl = 0.1 + 0.3 j; Fo = g - jb = 0.01 - 0.05 j.

Six poles, "1200 rev.: ZQ = r0 + jxQ = 0.15 + 0.45 j;

Zl = n + jx1 = 0.15 + 0.45 j] 7o = g -jb = 0.0067 - 0.0667 j.

Eight poles, 900 rev.:Z0 = r0 + jxQ = 0.2 + 0.6 j;

Zi = rx + jo?i = 0.2 + 0.6 j; YQ = g - jb = 0.005 - 0.1;.

Figs. 9, 10 and 11 show the load curves of the motor, at the
three different speeds. Fig. 12 shows the load curves once more,

MULTISPEED INDUCTION MOTOR
4-6-8 POLES 1800-1200-900 REV

100 200 300 400 500 600 700 800 900100011001200130014001500160017001800
FIG. 13.-—Speed torque curves of three-speed induction motor.
with all three motors plotted on the same sheet, but with the
torque in synchronous watts (referred to full speed or four-
polar synchronism) as abscissae, to give a better comparison.
S denotes the speed, / the current, p the power-factor and 7 the
apparent efficiency. Obviously, carrying the same load, that
is, giving the same torque at lower speed, represents less power
output, and in a multispeed motor the maximum power output
should be approximately proportional to the speed, to operate
at all speeds at the same part of the motor characteristic. There-
fore, a comparison of the different speed curves by the power
output does not show the performance as well as a comparison
on the basis of torque, as given in Fig. 12.