A drive system for a multi-phase brushless motor comprising a plurality of phases, the system comprising a drive circuit including switch means (16,18) 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 (16,18) so as to control the time that the drive circuit switches between said states in each of a series of pulse width modulation periods, and in which the control means is arranged in use to: determine a demanded voltage parameter set, and characterised by being further arranged in use to: identify PWM 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 PWM period, would 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 due to one nominal state time being greater than a predetermined maximum state time, for such PWM 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 by ensuring that no modified state time is above the predetermined maximum state time. determine a demanded voltage parameter set, and characterised by being further arranged in use to: identify PWM 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 PWM period, would 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 due to one nominal state time being greater than a predetermined maximum state time, for such PWM 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 by ensuring that no modified state time is above the predetermined maximum state time. identify PWM 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 PWM period, would 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 due to one nominal state time being greater than a predetermined maximum state time, for such PWM 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 by ensuring that no modified state time is above the predetermined maximum state time.