Comparisons of Rare-Earth and Rare-Earth-Free External Rotor Permanent Magnet Assisted Synchronous Reluctance Motors

This paper presents the comparisons of five-phase external rotor permanent magnet assisted synchronous reluctance motors (PMa-SynRMs) with neodymium (Nd) and ferrite (Fe) based PMs. Five-phase PMa-SynRMs, in general, have been proposed as an alternative to conventional three-phase motors utilized in automobile applications. However, with increased attention towards higher torque density and lower torque pulsations in electric motor designs, five-phase external rotor PMa-SynRMs can be highly attractive. In addition to increased fault tolerance and reliability of five-phase PMa-SynRMs, an adaptation of external rotor design can significantly increase the average torque developed with reduced torque pulsations and back-electromotive force (EMF) harmonics. However, with the design complexity involved in such high performance five-phase PMa-SynRM designs, the shape of flux barrier and choice of PMs have been one of the main challenges. In this study, novel five-phase external rotor PMa-SynRM designs with optimal flux barrier number and shape with Nd and Fe PMs are presented. Finite element simulations are conducted on the 2-D designs to compare their performance characteristics such as torque developed, torque pulsations, structural stability, thermal heat flow, demagnetization effects, and back-EMF harmonics. Optimal 3.8 kW Nd-based and 3.7 kW Fe-based five-phase external rotor PMa-SynRM designs are fabricated, and experimental tests are conducted on the prototypes to compare the results with a benchmark internal rotor PMa-SynRM.