Design of three-phase flux reversal machines with fractional-slot windings

The conventional three-phase flux reversal machines (FRMs) are with integer slot windings (ISWs) and the most significant drawback is large cogging torque. This paper proposes several slot-pole combinations of FRMs with fractional slot windings (FSWs), taking advantage of the FSW with low cogging torque. First, the general rules for the determination of the FSW pole pair are introduced, and the feasible slot-pole combinations of three-phase FRMs with FSWs are predicted and analyzed. Second, the winding connections of the proposed combinations are obtained with the employment of star of slots method. Third, based on finite element analyses (FEA), the effects of rotor pole number, split ratio, slot opening ratio, PM thickness and rotor pole arc on cogging torque and rated torque are investigated and analyzed, which gives a prediction for optimum achievable torque waveforms of FRM with FSWs. Finally, a proposed 12-slot/14-pole FRM with FSWs is compared with a conventional 12-slot/16-pole FRM with ISWs in terms of cogging torque, flux linkage, back-EMF, average torque, torque ripple and efficiency. It is found that the cogging torque and torque ripple of the proposed FRM are 55% and 48% lower than that of the conventional one, respectively.