Comparative Assessment of Single Piece and Fir-Tree-Based Spoke Type Rotor Designs for Low-Cost Electric Vehicle Application

Permanent magnet motors with ferrite magnets may be applicable to demanding electric vehicle applications, provided that designs with high power density and efficiency, as well as high resistance to demagnetization are developed. One rotor topology that allows meeting those challenges is a design with a so-called spoke arrangement of the magnet poles. Although, a spoke type rotor topology is widely known, the influences of the rotor mechanical design on the electromagnetic performance have largely been overlooked, while a comparison of different feasible design solutions has not been provided to date. In this paper, a novel spoke type traction motor using ferrite magnets and based on a single piece rotor topology has been presented. The design is based on a coupled structural-electromagnetic optimization, where the key design features influencing the structural stress and the electromagnetic torque density have been explained. This design is, further, compared against a previously disclosed fir-tree spoke type rotor solution, which has been designed for the same set of geometrical and output requirements. Through the comparisons, and for the first time, it is shown that the two alternative spoke type designs show significant differences in performance, including torque density, efficiency, and demagnetization.