Optimisation of Torque Performance of Less-rare-earth PMSM for Electric Vehicles

In order to improve the torque performance of the permanent magnet synchronous motor(PMSM) for electric vehicles and reduce the usage of rare earth materials in permanent magnets, a rotor topology of a less-rare-earth PMSM and a method of constructing asymmetric magnetic poles to weaken the torque ripple of the PMSM are proposed. Firstly, the expression of the electromagnetic torque of the PMSM is derived. Finite element models of three types of PMSMs, namely the less-rare-earth PMSM, the V-type rare-earth PMSM, and the rare-earth PMSM with a magnetic isolation bridge, are respectively established. The electromagnetic characteristics of these three PMSMs and the influence of the inductance difference between the d-axis and q-axis on the electromagnetic torque are analyzed. Secondly, an equivalent magnetic circuit model of the less-rare-earth PMSM is established, and the mechanism of weakening the torque ripple by the asymmetric magnetic poles is analyzed. Taking the angles of the asymmetric magnetic poles as the optimization parameters, and the average output torque and torque ripple as the optimization objectives, the optimal solution of the angles of the asymmetric magnetic poles is obtained through the multi-objective optimization method and the technique for order preference by similarity to ideal solution(TOPSIS). Finally, a prototype is manufactured and tested, which verifies the reliability of the finite element simulation and the effectiveness of the theoretical analysis. The results show that the proposed less-rare-earth PMSM has a larger reluctance torque, and ensures the torque output performance of the PMSM while reducing the usage of rare earth materials in permanent magnets. The construction of asymmetric magnetic poles can weaken the torque ripple while increasing the electromagnetic torque. After optimization, the electromagnetic torque of the PMSM is increased by 7.24%, and the torque ripple is reduced by 35.72%.