SPEED CONTROL

23

for the larger number of poles, about inverse proportional to the
square root of the number of poles:

e'o = -—s*

since the output is proportional to the square of the voltage.

The same is accomplished by changing connection from multiple
connection at higher speeds to series connection at lower speeds,
or from delta connection at higher speeds, to Y at lowei speeds.

If, then, the voltage per turn is chosen so as to make the actual
torque proportional to the synchronous torque at all speeds, that

MULTISPEED INDUCTION MOTOR
4 POLES 18OO REV.

FIG. 9.—-Load curves for multispeed induction motor, highest speed, four

poles.

is, approximately equal, then the magnetic flux per pole and the
density in the primary core decreases with increasing number of
poles, while that in the teeth increases, but less than at constant
impressed voltage.

The change of constants, by changing the number of poles by
the ratio:

n'

— = a
n

thus is:
from:

60,

i to

and the characteristic constant is changed from # to a2$.
17. As numerical instance may be considered a 60-cycle 100-
volt motor, of the constants :