Contents I. Introduction
Crude oil prices are expected to rise rapidly in the long term. Emissions from gasoline-driven automobiles are one of the main causes of global warming and environmental pollution. Hybrid electric vehicle (HEV) technology is one of the best solutions to alleviate these environmental problems. For HEV drives, in-wheel electric motors, which are mounted in the rear wheel axles, have many merits in that the front wheels and rear wheels can form a series-parallel drive combination without special mechanical coupling. Since the dimension of electric machines is inversely proportional to its running speed, and the wheels of vehicles need to run at low speed, it is obvious that direct drives in vehicle applications are bulky and, hence, very expensive. Normally, a mechanical gear is needed to reduce the speed. The use of a mechanical gear reduces the motor size, but additional space is needed for the gear. The mechanical gear also reduces the energy transmission efficiency. Recently, magnetic gears (MGs) are proposed to compete with mechanical gears in terms of torque transmission capability and efficiency [1]. Compared with their mechanical counterparts, MGs have a highly competitive torque transmission capability with very high efficiency. The MG can be directly combined with a conventional permanent-magnet (PM) motor inside one frame [2]–[4]. The system torque density can be significantly improved. However, this system has two rotating parts. Its mechanical structure is complex and it runs noisily.
