Patent ID: 7308192

Claim:
A drive system for a multi-phase brushless motor having a plurality of phase windings, the drive system comprising: a drive circuit including switch means for varying the electric potential applied to each of the phase windings by switching said drive circuit between a plurality of states including a plurality of conducting states, said drive circuit operable to be in each of said states for a respective state time; a current sensor operable to sense an instantaneous total current flowing through the phase windings, said current sensor requiring a time τ min in one of said conducting states to sense said total current; and a control means operable to provide pulse width modulated drive signals to control said drive circuit switch means to control said switch means thereby to control the time that said drive circuit switches between said states in each of a series of pulse width modulation periods of length τ p , said control means also operable to control the switching times of said switch means whereby in some pulse width modulation periods a sum of the two longest state times for conducting states exceeds τ p −τ min .