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 that require high power/torque density and efficiency, such as electric transportation. NdFeB is popular because of its exceptionally high remanant flux density ($B_{\mathrm{r}}$) and large maximum energy product ($BH_{max}$), 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 promising alternative to ferrites, with magnetic properties superior to ferrites but inferior to NdFeB at ambient temperatures. MnBi is relatively new compared to commercial PMs and shows a unique trend of increasing coercivity with increasing temperature, so an opportunity remains for further optimization of MnBi-based PMSMs with particular emphasis on realistic operational conditions. This paper presents design tradeoffs of using MnBi in PMSMs relative to commercial PMs and presents exemplary designs for IPMSMs using MnBi.