Recently, interior permanent magnet motors have been widely employed in various commercial electric vehicles. To expand the cruising range of EVs, the analysis and improvement of efficiency for the IPM motors have attracted increasing attention 1. For the IPM motors, a large current is often required to weaken the PM magnetic field to increase the speed range, which inevitably increases the copper and iron losses of the motor, resulting in a decrease in efficiency under high speed 2. Consequently, it is necessary and full of challenges to research and expands the high-efficiency region of the IPM motor, especially at high speed. In this paper, by introducing the new magnetic field variable effect into an interior PM motor, the variable magnetic field can be obtained under different operation conditions, which not only helps to reduce the copper and core losses under different operation conditions but also widens high-efficiency region.3 Fig 1 shows the topology of the proposed VMF motor and the structure of the corresponding variable magnetic field. To explore the performance advantage of the proposed VMF motor, three operation points are selected to investigated, which can be classified into two operation conditions, as shown in Fig. 2. By the purposeful design of variable magnetic field structure, the increased efficiency of high-speed operation point and the expanded high-efficiency region can be realized. Fig 3 shows the comparison of the performance of the VMF and IPM motor, including no-load flux density and back-EMF. Fig 4 shows the flux density distributions of the VMF motor under different operation condition. Furthermore, the loss distributions of the IPM and VMF motor under different conditions are shown in Fig 5. As can be seen from the Table I and Fig6, the efficiency of the motor has been improved and broadened, especially at point C, the efficiency has increased to 96.5%. Consequently, it is noted that the efficiency of the VMF motor is improved and the high-efficiency region is widened, which verifies the effectiveness of the VMF motor design proposed in this paper. More detailed analysis and experimental verification will be presented in the full paper.

References 1 W. Jiang, S. Feng and Z. Zhang, IEEE Transactions on Magnetics., vol. 54, no. 11, pp. 1-5, Nov. 2018, Art no. 8108005.
2 H. Hua and Z. Q. Zhu, IEEE Trans. Energy Convers, vol. 32, no. 2, pp. 495-504, June 2017.
3 A. Athavale, T. Fukushige and T. Kato, IEEE Trans. Ind. Appl., vol. 52, no. 1, pp. 234-241, Jan.-Feb. 2016.