Magnet gear trains push the frontiers

5 min read

Tom Shelley reports on the latest developments in magnetic transmissions both inside and outside electric motors.

Magnetic transmissions with no physical contact between gears have now reached the point of demonstrated motor/gear ratio combinations large enough to function as wheel hub motors in buses and trucks. Furthermore, new harmonic drive variants have been proven with up to 360:1 reduction ratios and also very compact, zero friction CVTs – Continuously Variable Transmissions. The basic idea, originally invented by Dr Kais Atallah and his research group at the University of Sheffield in 2001, is to have an outer ring of permanent magnets which are fixed, an inner ring of a smaller number of permanent magnets, which rotate at high speed, and an intervening ring of a large number of ferromagnetic pole pieces which are made by the machine to rotate at low speed by the interactions between the magnetic fields produced by the permanent magnets in the inner and outer rings. If a rotating magnetic field is imposed on the whole arrangement from external stator coils or windings, the machine functions as a brushless permanent magnet motor driving the inner rotor, with the interaction with the pole piece ring and outer magnet ring functioning as a reduction gearbox, driving the pole piece ring at reduced speed. Dr Richard Clark, research director of spinoff company, Magnomatics told us that a 'PDD' = 'Pseudo Direct Drive' demonstrator unit for a truck or bus wheel hub that they have built has a continuous rated power 0f 65kW and delivers 2000Nm continuous torque at 300 rpm. It has a maximum speed of 750 rpm and can maintain 4000Nm peak torque for hill starts and mounting kerbs. Mounted on a dynamometer test rig in Sweden, it is being driven by a standard Kollmorgen permanent magnet motor drive with a 600 V DC link. The motor will fit into a hub for a 22 inch wheel rim and as Dr Clark observed to us, "No conventional motor can do this, which is why one would normally resort to using a high speed machine with a gearbox. A motor gearbox will achieve this but will not have the advantages of a direct drive". Torque density is about 80 kN/m3 versus 19 kN/m3 for a conventional direct drive. No cooling is required. Other potential applications include aircraft actuators, because of the exceptionally high torque densities possible per unit weight, and military stealth vehicles, warships and submarines, because of the quietness resulting from the lack of gears. It is possible to use the magnetic gear train to speed up as opposed to reduce speed, so that a motor could be made to produce an output speed of 100,000 rpm if required. It is also possible to realise a purely linear machine and magnetic gear solution, although the proposed aircraft actuators employ a lead screw combined with a magnetically geared rotary machine. Similarly, a magnetic speeding up option would do away with the need for mechanical gearboxes on wind turbines, which are presently a major cause of unreliability and have a high cost of failure. The original basic idea has now led onto a combined motor and continuously variable transmission (CVT) which Dr Clark describes as "fully tested and validated". Full details have not been disclosed but a diagram shows it to have three rings of permanent magnets: a larger number in the centre ring, a small number in an intermediate ring, and an intermediate number in the outer ring. The driving electromagnetic coils are outermost as usual, and the driven shaft is attached to a ring of pole pieces between the innermost and intermediate rings of magnets. Variable ratio, Dr Clark says, is achieved by "allowing the third element to rotate and controlling that rotation." He then adds: "The magnetic CVT can act as a power split device in a hybrid vehicle power train allowing the IC engine to operate at its most optimum operating point while the magnetic CVT matches the engine to the road load at the wheels." Another spinout idea, some details of which the company is prepared to disclose, is a high ratio magnetic gear inspired by harmonic drives, with the potential to achieve frictionless gear reduction ratios of from 5:1 to 1000:1. This uses a high-speed rotor which interacts with a ring of permanent magnets which is mounted eccentrically on a bearing within a second ring of permanent magnets with one more pair of pole pieces than those on the ring attached to the inner rotor. The outer ring is fixed. There is in this design, no ring of ferromagnetic pole pieces. As in harmonic drives and other devices working on the same principle, one rotation of the drive shaft walks the inner ring round within the outer ring. The magnets on the two rings maintain their relative positions at their closest points through each rotation, so the end result of a single rotation of the input drive shaft, is to rotate the ring of magnets by one pole pair spacing. The output rotor then undergoes an eccentric motion plus rotation at reduced speed. If however, an extension of the output rotor is then used to drive a second, outer ring of magnets, mounted coaxially with the first outer ring of magnets, and the second outer ring is able to rotate, the effect is to perform two stages of gear reduction, multiplied together, with a final output which is coaxial with the input shaft, so no eccentric take-off is required. A prototype high ratio gearbox built on this principle, 100mm long and 140mm in diameter with a gear reduction ratio of 360:1 has been found able to produce a measured torque output of 115Nm. The company now has 15 families of patents on its different concepts. One big advantage of all the Magnomatics developments over other types of mechanical reduction gearbox is that there is no mechanical gear friction and very low electrical losses. As well as reducing power consumption, this also reduces heat production, doing away with the need for pumped fluid cooling. In the case of the harmonic type gear train, there is no problem with high stresses on gear teeth. If the magnetic drive overloads, it just slips, acting as a magnetic clutch. The only other problem with them is that in order to produce optimal designs, it is essential to do detailed finite element magnetic modelling in 3D. To accomplish this, the company uses 'Opera' 2D and 3D from Cobham Technical Services. Dr Clark says of this software: "At this stage in our company development, short turnaround times on design concepts and ideas are very important to us. Opera allows us to create our own scripts, which greatly shortens the time required to prove the benefits of our magnetic power transmission technology to potential clients. Because our larger designs can have many poles, and because we don't have the symmetry that would allow us to model a segment, simulation speed really matters. Opera's algorithms execute very quickly, enabling us to respond quickly." Magnomatics also designs relatively conventional eddy current dampers, magnetic couplings and high-performance conventional motor generators. Design Pointers • A large integrated permanent motor and magnetic reduction gearbox, suitably sized for fitting into the hub of a truck or bus, is on trial in Sweden • Magnetic continuously variable transmissions have been demonstrated and proven • A magnetic harmonic drive 100mm long and 140mm in diameter with a gear reduction ratio of 360:1 produces a measured torque output of 115Nm. • Magnetic transmissions have zero gear friction, are inherently resistant to shock loads and vibrations, and are much more compact than conventional and mechanical gearbox assemblies