INDUCTION-MOTOR REGULATION

133

so its torque further decreases, and it slows down still more,
loses more torque, etc., until it comes to a standstill. Inversely,
a momentary increase of speed increases the motor torque, Z>,
beyond the torque, L, consumed by the load, and thereby causes
an acceleration, that is, an increase of speed. This increase of
speed, however, increases the motor torque and thereby the
speed still further, and so on, and the motor increases in speed
up to the point, c, where the motor torque, D, again becomes

0 0.1 02 0,3 Of4

SPEED

0|5 0,6

07

0,8 0T9 l.|0

FIG. 51.—Speed-torque characteristics of induction motor and load for
determination of the stability point.

equal to the torque consumed by the load. A momentary in-
crease of speed beyond c decreases the motor torque, D, and thus
limits itself, and inversely a momentary decrease of speed below
c increases the motor torque, D, beyond L, thus accelerates and
recovers the speed; that is, at c the motor speed is stable.
With a load, requiring constant torque the induction motor
thus is unstable at speeds below that of the maximum torque
point, but stable above it; that is, the motor curve consists of
two branches, an unstable branch, from standstill, t, to the maxi-