Design optimizations of outer-rotor permanent magnet synchronous machines with fractional-slot and concentrated-winding configurations in lightweight electric vehicles

This paper concerns the design optimization of an outer-rotor permanent magnet synchronous machine (PMSM) with fractional-slot and concentrated-winding (FSCW) configuration for a lightweight direct-drive electric vehicle. Based on the rigid space envelope limits and performance specifications required by the vehicle, four candidate versions of a FSCW PMSM have been determined by initial machine sizing analysis. All versions share the same rotor outer diameter, rotor back iron thickness, permanent magnet thickness, air gap length, and magnet pole arc width ratio. A window-zoom-in response surface method based on two-dimensional parametric finite element analysis is proposed for comprehensive electromagnetic optimization for maximum torque density. Additionally, the machine losses at rated torque output including copper joule loss, rotor eddy current loss, and stator core loss maintain the same in all the candidates. It is found that significant correlation exists between stator slot depth and active axial length in the optimal outer-rotor PMSMs. Finally, the performance characteristics of the four optimal machines are comprehensively evaluated and compared to confirm the best candidate, and its utility for the vehicle's mission.