Abstract: An AC motor drive using current control instead of the more usual voltage control. The instantaneous inverter phase currents are monitored by sensors 16, 17, 18 and used as feedback to enable the inverter phases to be switched so that each phase current conforms in frequency and amplitude to a phase reference sinusoid produced by a sine wave generator 6. The reference sinusoids are isolated from the feedback comparators 7, 8, 9 with the phase current sensors and comparators having a common reference potential to overcome problems inherent in isolating the sensors.Motor fluxing is optimised at all working frequencies and motor loading by controlling the magnitude of each phase current I, such thatI=I.sub.q .times.f (.phi.)/sin.phi.where I.sub.q is the motor magnetising current, .phi. is the phase angle between the motor phase voltage and current, and f(.phi.) is a function of .phi. which varies between a minimum value and 1. Means for continuously measuring .phi.

Abstract: Switching waveform generator for pulse width modulation triggering of static inverters producing variable frequency output. The number of chops per half-cycle of output fundamental frequency is decreased as fundamental frequency is increased so as to keep the switching frequency of the inverter within the capabilities of the power switching devices used while obtaining maximum harmonic suppression. The chop number is changed between sequential frequency ranges. In a preferred form the chop width is continuously varied as output frequency is varied to decrease output amplitude with decreasing frequency to enable constant torque operation of an induction motor load.