Induction motor drive circuits

DETAILED DESCRIPTION I claim: 1.
A drive circuit for a three-phase induction motor, the circuit comprising first and second switching means associated with each phase of the motor, connection means for connecting all the first switching means to a common supply rail of one polarity and for connecting all the second switching means to a common supply rail of the other polarity, speed control means for cyclically switching the first switching means to connect each phase in turn to the supply rail of said one polairity and for cyclically switching the second switching sense to connect each phase in turn to the supply rail of said other polarity so as to supply alternating current to each phase of the motor at a fundamental frequency which may be varied in accordance with the required motor speed, the speed control means being operative to render each switching means conductive for no more than a third of a cycle and such that, at any instant, no more than one of the first switching means associated with one phase and one of the second switching means associated with another phase are rendered conductive together, sensing means for sensing the current supplied to the motor by the connection means which corresponds to the current through a single phase of the motor at any instant, and current limiting means operative, at least during low speed operation of the motor, to switch at least one of the switching means which are conductive at any instant on and off at high frequency relative to the fundamental switching frequency and to switch said at least one switching means off when the current sensed by the sensing means exceeds a limit value.
2.
A drive circuit according to claim 1, wherein the current limiting means is arranged to switch said at least one switching means on when the sensed current falls below a minimum value after a minimum interval of time has passed since switching off of the switching means.
3.
A drive circuit according to claim 1, wherein the current limiting means incorporates a comparator for comparing a value proportional to the sensed current with a preset limit value and for supplying a control output in dependence on the result of that comparison