Complementary Series Magnetic Circuit Based Stator-Yokeless Dual Rotor PM Vernier Reluctance Machine for In-wheel Propulsion

With a relatively high gear ratio and consequently a long magnetic path in the Vernier reluctance machine, its yoke is always thicker than that of conventional PMSM machines, which deteriorates the slot area for the windings under the same peripheral diameter constraints. To more efficiently utilize the inner space so as to further improve the torque density of the machine, a dual-rotor permanent magnets (PMs) Vernier machine with a stator yokeless structure is proposed in this paper. The key is to artificially utilize radial-magnetized PM located at the slot opening and inner/outer rotors to construct a complementary series magnetic circuit for the working harmonic and eliminate the stator yoke to expand the slot area. Furthermore, the distributed winding configuration is adopted to enhance the winding factor. By comparing the proposed machine with one featuring a parallel magnetic circuit design, it is shown that the proposed machine could improve the back-EMF by 37% and torque density by 38% respectively. In addition, the machine with 12 slots and 10 poles exhibits better efficiency and overload capability than its peers, and is selected for the prototype. The experimental results verify the validity of the proposed design.