Rotor structure in 50 kW spoke-type interior permanent magnet synchronous motor with ferrite permanent magnets for automotive applications

In light of global warming and the rapidly increasing prices of rare earths, electric vehicles with motors free of rare earths are needed. We are therefore developing an interior permanent magnet synchronous motor (IPMSM) that utilizes ferrite permanent magnets. Because the residual flux density of a ferrite magnet is lower than that of a rare earth magnet, we have adopted a spoke-type IPMSM with radially arranged ferrite magnets. This paper proposes two rotor structure designs for spoke-type IPMSMs with ferrite magnets, and presents the results of two-dimensional finite element analysis (FEA) for the two designs. The results indicate that the performance loss from using ferrite magnets can be moderated by effectively utilizing reluctance torque in a spoke-type IPMSM. Finally, a rotor structure that has sufficient performance to be usable in a traction motor free of rare earths is examined through experiments on a prototype and three-dimensional FEA.