
222 Electric Motors and Drives
Torque
Figure 7.5 Constant torque, constant power and high-speed motoring
regions
therefore known as the 'constant power' region. There is of
course a close parallel with the d.c. drive here, both systems
operating with reduced or weak field in the constant power
region. The region of constant power normally extends to
somewhere around twice base speed, and the motor is
allowed to operate with higher slips than below base speed.
The rotor currents are not excessive, however, because the
flux is reduced.
At the upper limit of the constant power region, the
current limit coincides with the pull-out torque limit