Improving Constant Power Flux Weakening in MnBi Interior Permanent Magnet Traction Motors

Interior permanent magnet synchronous motors (IPMSMs) using Neodymium Iron Boron (NdFeB) permanent magnets (PMs) dominate in applications, like in electric vehicles (EVs), which require high torque density, high power density, and high efficiency while maintaining reasonable cost and manufacturability. However, extraordinary price volatility of rare-earth (RE) elements in NdFeB PMs has led to an effort to develop low cost, RE-free PMs that outperform ferrites (the best existing alternative). Because of its high coercivity (Hc) at high temperatures and relatively low cost with reduced price volatility, manganese bismuth (MnBi) based PMs have attracted attention despite a remanent flux density (B
r
) closer to those of ferrites. Lower Br reduces short circuit current thus reducing constant power speed ratio (CPSR) when directly replacing NdFeB in reference designs with MnBi. This paper reviews why CPSR lowers when doing so but then shows that redesigning the rotor using a segmented PM-assisted synchronous reluctance motor (PMASynRM) topology with MnBi can improve CPSR.