High-performance magnetic gears
Introduction
It is usually more cost and weight effective to employ a high-speed electrical machine together with a mechanical gearbox to achieve the required load torque and speed. However, although, high system torque densities can then be achieved, gear lubrication and cooling are often required, whilst noise, vibration and reliability can be significant issues. Magnetic gears are an alternative technology which may offer significant advantages, such as, reduced maintenance and improved reliability, inherent overload protection, and physical isolation between input and output shafts. However, despite these advantages magnetic gearing has received relatively little attention to date, probably because of the relative complexity and poor torque density of the magnetic circuits which have been proposed [1], [2]. Therefore, for applications in which mechanical gearing cannot be accommodated, direct electrical drives are employed.
The paper describes a magnetic gear topology, Fig. 1, which combines a significantly higher torque transmission capability, and a very high efficiency.
Section snippets
Flux density waveforms
Fundamental to the operation of the magnetic gear is the modulation of the magnetic fields produced by either the high- or low-speed rotor permanent magnets, by the steel pole pieces, which results in space harmonics having the same number of poles as the other rotor permanent magnets [3]. It can be shown that the number of pole pairs in the space harmonic flux density distribution produced by either the high- or low-speed rotor permanent magnets, is given bywhere ,
Torque transmission
Fig. 4 shows the variation of the maximum torque on low-speed rotor with the radial thickness of the steel pole pieces, when the diameter of the airgap adjacent to the low-speed rotor and the magnet volumes of high- and low-speed rotors are kept constant. It can be seen that an optimum radial thickness of stationary pole pieces exists.
Fig. 5 shows the prototype 5.75:1 magnetic gear, and the measured variation of its efficiency with transmitted torque (low-speed rotor). It can be seen that the
Conclusion
A high-performance magnetic gear has been presented. It has been shown that, by employing rare-earth magnets, a high torque density can be achieved, with efficiences in excess of 97% for transmitted torque values higher than 75% of the pull-out torque.
References (3)
- D.E. Hesmondhalgh, D. Tipping, A multielement magnetic gear, IEE Proc. 127 (Part B) (1980)...