Patent ID: 7545110

Claim:
A drive system for a multi-phase brushless motor comprising: a drive circuit including switch means for varying the electric potential applied to each of the phases by switching the drive circuit between a plurality of states; a current sensor connected such that it can sense the instantaneous total current flowing through the windings, and; control means arranged to provide pulse width modulated drive signals to control the switch means so as to control the time that the drive circuit switches between said states in each of a series of pulse width modulation periods, said control means operative to: determine a demanded voltage parameter set, identify pulse width modulation periods during which the demanded voltage parameter set is such that neither two nominal corresponding state times, nor a higher number of equivalent state times producing the same net voltage, in a single pulse width modulation period, can allow a predetermined minimum time to be spent in a predetermined number of active states sufficient for the current in each of the phases to be determined by means of the current sensor, and for such pulse width modulation periods, to calculate modified state times that do allow sufficient time to be spent in a sufficient number of active states for the current in each of the phases to be determined by means of the current sensor.