Review on Development of Permanent Magnet In-Wheel Motors

Due to the advantages of small size, high power density, and high efficiency, the permanent magnet (PMSM) motor can be widely applied in electrical vehicles, new energy power generation, flywheel energy storage, machine tool equipment, and aerospace. This paper compares the characteristics and performance indicators of in-wheel motor products from domestic and international manufacturers. Then, the basic structure, characteristics, technology, and materials of the PMSM are dissected. Different schemes and structures are explored, with an emphasis on high efficiency, power density, reliability, and wide speed regulation. Finally, the common scientific problems of the PMSM motor are summarized, and the technical paths to improve the efficiency and power density are explored, providing a reference for future research and application of in-wheel motors. Two primary integration forms for in-wheel motor drive systems, namely deceleration drive and direct drive, are discussed. The emergence of compact planetary reducers is highlighted, particularly for deceleration-driven hub motor systems, presenting great competitive advantages, especially in heavy-duty vehicle applications. In the 21st century, in-wheel motor technology has been widely developed, with Foreign technologies leading the way, particularly in Europe. The main research and development enterprises are Schaeffler, Protean, Elaphe, NTN, TM4, and Michelin. The products of Schaeffler are all inner rotor structures. The products of Protean are outer rotor direct drive structures. Domestic technology is also continuously developing. Representatives are THIM Tianhai, Edes, Tate, and Shanghai Electric Drive. Shanghai Electric Drive and Shanghai University jointly developed an in-wheel motor with the same overall power density as the Protean's products. According to the direction of the motor's magnetic field, a permanent magnet in-wheel motor can be divided into a radial flux motor, axial flux motor, and transverse flux motor. Radial flux motors remain mainstream in the market due to their low cost and mature technology. Axial flux motors exhibit the characteristics of axial compactness and high-power density, but the process is complex. The transverse flux motors have the characteristics of electromagnetic load decoupling, high power density, and high design freedom. However, their power factor is low with magnetic flux leakage and a complex structure. New materials are critical in overcoming development bottlenecks of in-wheel motors. Achieving lightweight design, efficient heat dissipation capacity, and copper content are identified as key objectives to reduce losses, enhance torque movement, and improve operation efficiency. By leveraging electromagnetic characteristics, these innovations contribute to increased power/torque density, expanded speed operation range, and reduced NVH and motor cost. © 2024 China Machine Press. All rights reserved.