Comprehensive Investigation and Evaluation of Cogging Torque Suppression Techniques of Flux-Switching Permanent Magnet Machines
Zhiheng Zhang, Peixin Wang, Wei Hua, Tianxiang Zhang, Guangchen Wang, Mingjin Hu
Abstract
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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\mathrm{ sa}}$ </tex-math></inline-formula> ), 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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\mathrm {ct}}$ </tex-math></inline-formula> ), per unit saliency ratio [ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\mathrm {sa}}$ </tex-math></inline-formula> (p.u.)], fundamental proportion ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P_{f}$ </tex-math></inline-formula> ) of back EMF, torque retention rate ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\mathrm {tr}}$ </tex-math></inline-formula> ), torque ripple suppression rate ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\mathrm {trs}}$ </tex-math></inline-formula> ), and torque ripple robustness [ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\text {ob}}(T_{\mathrm {rip}})$ </tex-math></inline-formula> ], 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.