SPEED CONTROL 5

by going to temperature of about 350°C. in the rotor conductors
—which naturally would require fireproof construction—it be-
comes possible to convert curve A into (7, or A1 into B, in Fig. 1.
Probably, the high temperature would be permissible only in
the end connections, or the squirrel-cage end ring, but then, iron
could be used as resistance material, which has a materially
higher temperature coefficient, and the required temperature
rise thus would probably be no higher.

B. Hysteresis Starting Device

4. Instead of increasing the secondary resistance with increas-
ing slip, to get high torque at low speeds, the same result can be
produced by the use of an effective resistance, such as the effect-
ive or equivalent resistance of hysteresis, or of eddy currents.

As the frequency of the secondary current varies, a magnetic
circuit energized by the secondary current operates at the varying
frequency of the slip, s.

At a given current, ii, the voltage required to send the current
through the magnetic circuit is proportional to the frequency,
that is, to s. Hence, the susceptance is inverse proportional
to s:

V = \ (5)

The angle of hysteretic advance of phase, a, and the power-
factor, in a closed magnetic circuit, are independent of the
frequency, and vary relatively little with the magnetic density
and thus the current, over a wide range,1 thus may approxi-
mately be assumed as constant. That is, the hysteretic con-'
ductance is proportional to the susceptance:

gf — V tan a.

(6)

Thus, the exciting admittance, of a closed magnetic circuit
of negligible resistance and negligible eddy-current losses, at the
frequency of slip, s, is given by:

Y' = g'-jV =

(7)

1" Theory and Calculation of Alternating-current Phenomena,"
Chapter XII.