136

ELECTRICAL APPARATUS

where m and n are the points of contact of the tangents from the
required starting torque, p, on to the motor curve, D; these two
stable branches are separated by the unstable branch, from n to
m, on which the motor can not operate.

84. The question of stability of motor speed thus is a func-
tion not only of the motor-speed curve but also of the character
of the load in its relation to the motor-speed curve, and if the
change of motor torque with the change of speed is less than the
change of the torque required by the load, the condition is stable,

otherwise it is unstable; that is, it must be TO- < j« to give
stability, where L is the torque required by the load at speed, S.

01 02 03 04

SPEED

0!5

7

06 07 0.8

0,9

FIG. 53.—Speed-torque characteristic of single-phase induction motor.
Occasionally on polyphase induction motors on a load as repre-
sented in Fig. 52 this phenomenon is observed in the form
that the motor can start the load but can not bring it up to
speed. More frequently, however, it is observed on single-
phase induction motors in which the maximum torque is nearer
to synchronism, with some forms of starting devices which de-
crease in their effect with increasing speed and thus give motor-
speed characteristics of forms similar to Fig. 53. With a
torque-speed curve as shown in Fig. 53, even at a load requiring
constant torque, three speed points may exist of which the
middle one is unstable. In polyphase synchronous motors and
converters, when starting by alternating current, that is, as