Design of Three-Phase Flux-Reversal Machines With Fractional-Slot Windings

This paper proposes several slot-pole combinations of flux-reversal machines (FRMs) with fractional-slot windings (FSWs) to achieve a larger torque density and a lower pulsating torque. First, the general rules for the determinations of the winding pole pair and slot per pole per phase (SPP) are introduced, and then the feasible slot-pole combinations of three-phase FRMs with FSWs are given. Second, the analytical expressions for the back electromotive force (EMF) and torque of fractional-slot FRMs are derived and expressed in terms of machine dimensions. Based on the analytical equations, the effects of rotor pole number, split ratio, slot opening ratio, permanent magnet (PM) thickness, and rotor pole arc on back EMF are investigated and analyzed, which gives a prediction for maximal achievable back EMF and power density of the FRM with FSWs. Third, the proposed 12-slot/14-pole FRM is compared to a conventional 12-slot/16-pole FRM 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. Moreover, the rated torque of the former is also 4% higher than the latter. Finally, the analysis results are verified by the experiments on a 12-slot/14-pole FRM prototype with FSWs.