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Motor design considerations and test results of an interior permanent magnet synchronous motor for electric vehicles

This paper describes the high performance motor design of an interior permanent magnet synchronous motor (IPM motor) for electric vehicles. The authors examined the differences in motor characteristics based on how the magnets were embedded. For this comparison, they set conditions so that the volume of magnets remained constant, and they used both computer simulation and experiments with a prototype motor. As a result, they were able to develop a double layer IPM synchronous motor, which has two layers of magnets embedded lengthwise in the radial direction in the rotor. The q-axis flux path can be expanded by using an IPM rotor with magnets divided into two layers with the separation lengthwise at the rotor radius. An evaluation of prototype motors confirmed that a double-layer IPM motor produces a 10 percent or more increase in the torque generated compared to a single-layer IPM motor using the same current. Also, the high efficiency operating region (min. 90%) was a minimum of 10% wider than the single layer IPM motor. However, reluctance torque of a double layer IPM motor with concentrated winding cannot be designed as high as that of a similar motor with distributed winding. This is because the inductance difference between the d and q-axes cannot be sufficiently increased in this former. Here, it was learned that a concentrated winding is inferior to a distributed winding both in terms of generated torque and the constant power region size.