Comprehensive Investigation and Evaluation of Cogging Torque Suppression Techniques of Flux-Switching Permanent Magnet Machines

To obtain the best comprehensive performance of the flux-switching permanent magnet (FSPM) machines, six cogging torque suppression techniques based on rotor-side including skewing, step-skewing, notching, pairing, right-angle chamfering, and cos-chamfering rotors are investigated in this article. First, suppression mechanisms of the rotor techniques are classified into two categories and analyzed through general air-gap field modulation theory (AFMT). Then, based on finite element analysis (FEA), cogging torque waveforms of the FSPM machine by each technique are predicted to evaluate cogging torque suppression effectiveness, and the reasons and differences of cogging torque reduction are explained from the perspective of air gap magnetic field harmonics. Next, electromagnetic performances are calculated and compared, including saliency ratio ( $R_{\mathrm{ sa}}$ ), back electromotive force (EMF), torque, and rotor robustness. Furthermore, a system-level theoretical analysis for cogging torque suppression of FSPM machines is conducted to disclose the common nature of different techniques. According to six key factors, namely, cogging torque suppression rate ( $R_{\mathrm {ct}}$ ), per unit saliency ratio [ $R_{\mathrm {sa}}$ (p.u.)], fundamental proportion ( $P_{f}$ ) of back EMF, torque retention rate ( $R_{\mathrm {tr}}$ ), torque ripple suppression rate ( $R_{\mathrm {trs}}$ ), and torque ripple robustness [ $R_{\text {ob}}(T_{\mathrm {rip}})$ ], a more comprehensive rotor design idea (or general design guideline) is provided for machine researchers and designers. Finally, prototype experiments concerning a 12s/10p FSPM machine with four rotor techniques are conducted. The results can provide a significant reference for cogging torque suppression techniques of FSPM machines.