Method & apparatus for reducing cogging torque in an electric motor

Abstract

Certain permanent-magnet-rotor electric motors exhibit an irregularity in their torque which is known as 'cogging.' The present invention creates at least one supplemental reluctance torque, and superimposes this supplemental reluctance torque on the cogging torque, in order to reduce or eliminate the effect of the cogging torque. Preferably, in the rotor, each boundary between north and south poles is subdivided into a plurality of parallel segments, some of which are angularly displaced with respect to others. Four different embodiments have different respective patterns of this displacement. These structures each result in production of a plurality of respective reluctance torques, which superpose and thereby substantially cancel each other. To minimize variation in the magnetically effective air gap, slot closures of ferromagnetic material may be provided. The dimensions of the air gap and of the auxiliary slots can be adjusted by grinding down or otherwise changing the air-gap-adjacent surface of the stator which faces the rotor, until a predetermined desired torque curve is achieved.