EMF Voltage Distortion Mitigation in Fractional-Slot Permanent Magnet Machines by Varying Coil Distribution and Turn Ratio

Electromagnetic force (EMF) voltage distortion, caused by armature reaction, not only deteriorates torque ripple in constant torque region but also reduces the flux weakening operation region, especially for fractional slot permanent magnet (PM) machines. This article mitigates voltage distortion by varying coil distribution and turn ratio with employment of triple layer (TL) and quadruple layer (QL) windings with varied coil turns. Coil turn ratios of TL and QL windings are optimized to minimize voltage distortion ratio. Reduction mechanisms are exhaustively analyzed. It is found that, for the TL winding with optimal turn ratio, the armature leakage flux and PM leakage flux in tooth tips will not reverse polarities simultaneously, resulting in much lower voltage distortion. On the other hand, for the QL winding, voltage distortion reduction is realized by lowering the amplitude of armature tooth tips leakage flux. Compared with conventional double or single layer (SL) winding machines, torque ripple can be reduced and flux weakening capability is greatly enhanced. The 12-slot/10-pole prototypes with both SL and optimum TL windings are fabricated and tested to verify the analysis.