Uptill now, we have seen
torque – slip characteristics of an induction motor. To compare the
performance of induction motor with d.c. shunt and series motors, it is
possible to plot speed-torque curve of an induction motor.
At N = 0, the starting condition, motor produces a torque called starting torque.
|Fig. 1 Speed Torque characteristics|
speeds near the region is stable and the characteristics is straight in
nature. Fall in speed from no load to full load is about 4 to 6 %. The
characteristics is shown in the Fig.1. It can be seen from that the
figure that for the stable region of operation, the characteristics is
similar to that of d.c. shunt motor. Due to this, three phase induction
motor is practically said to be ‘constant speed’ motor as drop in speed
from no load to full load is not significant. The unstable region of
operation is shown dotted in the Fig.1.