Getting Rare-Earth Magnets Out of EV Traction Machines: A review of the many approaches being pursued to minimize or eliminate rare-earth magnets from future EV drivetrains

Abstract

There are now more than 1.5 million plug-in hybrid electric vehicles (HEVs) and battery electric vehicles (BEVs) that have been commercially produced in the world during the past 11 years, and it is likely that the number produced during 2016 exceeded 700,000, a new record. A large percentage of those electrified vehicles use ac permanent magnet (PM) synchronous machines for the traction machines used in their drivetrains. Even more specifically, nearly all of these PM synchronous machines fall into the class of interior PM (IPM) synchronous machines. The origin of this name becomes apparent by inspecting the cross section of a typical IPM machine in Figure 1 that has been simplified to highlight the key features. This figure shows that the magnets are buried in cavities inside the rotor, and the cavities are carefully shaped, often in V configurations, to concentrate more of the PM magnetic flux into the machine's air gap.