Magnetomotive force harmonic reduction techniques for fractional-slot non-overlapping winding configurations in permanent-magnet synchronous machines

Compared to conventional distributed winding configurations, the fractional-slot non-overlapping (concentrated) windings exhibit advantages such as short end-winding length, high copper packing factor (particularly with segmented stator structure), low cogging torque, good field weakening capability owing to relatively large d-axis inductance, and better fault tolerant capability due to low mutual inductance. However, one of the key problems of employing concentrated windings in Permanent-magnet Synchronous Machines(PMSMs) is the high eddy-current losses in rotor magnets and/or rotor iron due to the presence of a large number of lower and higher order space harmonics in the stator Magneto-Motive Force(MMF). These MMF harmonics also result in other undesirable effects, such as acoustic noise and vibrations, and localized core saturation which tend to reduce reluctance torque. This paper reviews the current state-of-the-art of the MMF harmonic reduction techniques for concentrated winding configurations in PMSMs, including winding split and shift, delta-star connected windings, multiple 3-phase windings, multilayer windings, uneven turn numbers, and stator flux barriers. Their concepts, advantages and disadvantages are presented and assessed.