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Magnetic gears have advantages such as having low acoustic noise and being maintenance-free in comparison with widely-used mechanical gears. This paper deals with a magnetic planetary gear which has inner and outer surface permanent-magnet rotors, and has ferromagnetic-stator pole pieces arranged between both rotors. The magnetic planetary gear has higher transmission torque than other types of magnetic gears since all rotor magnets continuously contribute to generating torque. However, it has not been put into practical use yet. To achieve future practical use, this paper describes a method of improving the efficiency of magnetic planetary gears.

Magnetic gears can transmit torque without any mechanical contact. Hence, they have low vibration, no wear, and fatigue, which ensure maintenance-free operation. There are various types of magnetic gears. Among them, a planetary type magnetic gear, which consists of coaxial inner and outer rotors with surface-mounted permanent magnet and ferromagnetic stationary parts called pole-pieces, has recently attracted interest since it offers higher torque than other type magnetic gears. This paper presents a comprehensive investigation of the influence of geometry and position of the pole-pieces on torque characteristic based on finite element analysis and experiment. Surveyed parameters of the pole-pieces include lengths in the radial and axial directions and position in the radial direction. Finally, it is demonstrated that the maximum torque of the improved prototype magnetic gear is increased by 45% and the maximum efficiency achieves up to about 99%, which is equal to or more than a conventional planetary type mechanical gear.