Design of axial-flux permanent magnet motors with high torque density and low thermal raise for electric motorcycle

Abstract

Axial-flux permanent magnet synchronous motor (AFPMSM) is a compact and high-torque solution for direct-drive e-bikes. However, its complex 3-D flux path and thermal management challenges hinder its efficiency. To address these issues, an AFPMSM with Halbach PM array (H-AFPMSM) is proposed. Leveraging self-shielding magnetization, the H-AFPMSM demonstrates higher magnetic loading and significantly reduces heat dissipation while maintaining torque density. The performance of the AFPMSM is first analyzed to guide topology selection and a rapid three-stage optimization method is proposed thereby. Comparative analysis with surface-mounted PM AFPMSM (S-AFPMSM) and radial-flux PMSM (RFPMSM) via finite element analysis and 5-kW full-loading prototype testing shows a 30% increase in torque density for H-AFPMSM compared to RFPMSM and a 40_C decrease in coil temperatures compared to S-AFPMSM due to 25% loss reduction. Moreover, the proposed H-AFPMSM exhibits a 5%∼10% improvement in efficiency across the entire operating speed range compared to S-AFPMSM. Via rapid simulation and performance enhancement techniques, the H-AFPMSM is developed as an ideal fit for e-bike scenario.