Trade Study for Rare-Earth-Free Interior Permanent Magnet Synchronous Machine Using MnBi Permanent Magnets

Permanent magnet synchronous machines (PMSM) with neodymium iron boron (NdFeB) permanent magnets (PM) are typically used in applications, such as electric vehicles (EVs), that require high power/torque density and high efficiency yet low cost and easy manufacturability. NdFeB is popular because of its exceptionally high remanent flux density (B-r) and large maximum energy product (BHmax), but supply chain limitations of rare-earth elements (REE), such as neodymium (Nd) and dysprosium (Dy), have recently caused significant price fluctuations for this material. As a result, researchers are searching for ways to reduce or eliminate the use of NdFeB in PMSM designs. In the past, ferrite PMs were the next best option, but recently, manganese bismuth (MnBi) has emerged as one possible alternative, with magnetic properties superior to ferrites but inferior to NdFeB at ambient temperatures. This paper extends upon previous work to present a detailed trade study for using MnBi in motors rated for EVs. This is done by designing a MnBi IPMSM with similar ratings and topology to an exemplary EV motor that uses NdFeB PMs: the motor in the 2011 and 2012 Nissan Leaf. MnBi PMs are not yet commercially available but show a unique trend of significantly increasing coercivity with increasing temperature, complicating the motor design procedure. Therefore, to address weaknesses in initial studies, this paper emphasizes the impact of magnet posts on electromagnetic performance, low temperature irreversible demagnetization, and maximum stress high speeds.