INDUCTION-MOTOR REGULATION

135

accelerates only to speed &i, and keeps revolving at this low
speed (and a correspondingly very large current). If, howevei'j
the load is taken off and the motor allowed to run up to syn-
chronism or near to it, and the load then put on, the motor slows
down only to speed 63, and carries the load at this high speed;
hence, the motor can revolve continuously at two different speeds,
bi and 63, and either of these speeds is stable; that is, a momen-
tary increase of speed decreases the motor torque below that

FIG. 52.—Speed torque characteristics of induction motor and load for
determination of the stability point.
required by the load, and thus limits itself, and inversely a de-
crease of motor speed increases its torque beyond that correspond-
ing to the load, and thus restores the speed. At the intermediary
speed, 62, the conditions are unstable, and a momentary increase
of speed causes the motor to accelerate up to speed 63, a momen-
tary decrease of speed from &2 causes the motor to slow down to
speed &i, where it becomes stable again. In the speed range
between &2 and 63 the motor thus accelerates up to b$, in the
speed range between Z>2 and 6j it slows down to 61.
For this character of load, the induction-motor speed curve,
D, thus has two stable branches, a lower one, from standstill, t,
to the point n, and an upper one, from point m to synchronism,