Electric Motors and Drives, 2nd Edition

240 Electric Motors and Drives

with the speed, as shown in Figure 7.5. This region is 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. Operation at still

higher