Patent ID: 7141949

Claim:
A method of electronically commutating current to a permanent magnet rotor brushless dc motor having three phase stator windings for producing rotating magnetic flux, a three phase bridge inverter including a pair of series connected switching devices per phase and wherein motor torque is controlled by pulse width modulation of said motor current comprising the steps of: controlling said switching devices to commutate current through successive combinations of three of said windings such that magnetic flux produced by said stator windings rotates in a desired direction by 60° per commutation at a speed inversely proportional to the period between commutations, controlling motor torque and speed by pulse width modulating the operation of that switching device which has been switched from an off state to an on state on each new commutation, applying voltage to one out of said three windings on each new commutation for an active period less than the period between commutations, after expiry of said active period sensing in said one of said windings the periodic back EMF induced by rotation of the permanent magnet rotor, digitising said sensed back EMF signal by detecting the zero-crossings of said back EMF signal, and causing each said commutation to occur at times which are substantially defined by or derived from each logic transition in said digitised signal due to zero crossings.