field weakening
Operation of the motors above rated nameplate speed (base speed) is
possible, but is limited to conditions that do not require more power
than the nameplate rating of the motor. This is sometimes called "field
weakening" and, for AC motors, means operating at less than rated V/Hz
and above rated nameplate speed. Permanent magnet synchronous motors have quite limited field weakening speed range due to the constant magnet flux linkage. Wound rotor synchronous motors and induction motors have much wider speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would
be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100%
power. At higher speeds the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus rated power can be typically produced only up to
130...150% of the rated nameplate speed. Wound rotor synchronous motors
can be run at even higher speeds. In rolling mill drives often
200...300% of the base speed is used. The mechanical strength of the
rotor limits the maximum speed of the motor.
