Motor drive control with a single current sensor using space vector technique

A drive system for a multi-phase brushless motor comprising a plurality of phase windings, the 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;
• a current sensor operable to sense the instantaneous total current flowing through the windings, and
• control means operable to provide pulse width modulated drive signals to control said switch means so as to control the time that said drive circuit switches between said states in each of a series of pulse width modulation periods as a function of a demanded voltage which has one of a plurality of space vector modulation sectors associated with it;
• wherein the control means is arranged to inhibit a change, between two adjacent periods, of the order in which said switch means are switched within said periods resulting from a change between said two adjacent periods of said space vector modulation sector of the demanded voltage, when said demanded voltage is low, but to allow such change of order when said demanded voltage is high.