CONCENTRIC MAGNETIC GEAR TECHNOLOGIES HOLD PROMISE FOR TORQUE DENSE ELECTRICAL MACHINES

Mechanical gears can suffer from wear and overheating, and they can be damaged in over-torque situations. They also require regular lubrication. Contactless magnetic gear technology can potentially overcome these issues, and much of their development to date has focused on matching the torque performance of mechanical gears and on reducing manufacturing costs and complexity.

In magnetic gears (MGs), torque is produced by the interaction between two magnetic fields generated by multi-pole permanent magnets (PM) instead of the teeth meshing mechanism in a mechanical gear. The magnitude of the torque produced is proportional to the product of the field strength of the two fields.

Research and Development

Early development of magnetic gears can be traced back to 1901, and since then it has been shown that a magnetic gear can be designed for any mechanical topology, including rack pinion gears, worm gears, bevel gears, spur gears and planetary gears. However, their torque densities have generally not been able to compete with their mechanical equivalents.

In the late 1960s, a topology consisting of three concentric parts was introduced. This consisted of an inner magnet rotor, an outer magnet rotor and a flux modulator in between. This topology enables nearly all the PMs to be involved in torque transmission, resulting overall in a high torque density.

Research effort has recently accelerated with the development of high-performance concentric MG topologies that can utilise relatively low-cost PMs.

“The basic operation principle of a concentric MG is that the ferromagnetic pole-pieces interposed between the inner and outer PM rotors modulate the magnetic field such that each rotor ‘sees’ a working space harmonic corresponding to its own number of poles,” explains a review paper co-authored by Wang and published by the Institute of Electrical and Electronics Engineers.

Pole slipping can occur in MGs when they are overloaded, and this can decouple the rotors. Therefore, they need to be designed with a reasonable safety factor or have automatic pole-slip detection.

Additionally, they can suffer from undesirable oscillations during rapid load change as a result of their lower torsional stiffness compared to mechanical gears. This can potentially be alleviated by adding damper windings on the high-speed rotor, but it decreases efficiency. An alternative approach is to use feedback sensors to monitor the speed of both rotors and apply stabilizing control algorithms to stabilise the system.

Recent research into concentric MG topologies has led to higher torque density, better torque quality and greater efficiency, says Wang. Work has been undertaken on the mitigation of imbalanced magnetic forces and vibrations, improvement of structural integrity on flux modulation rings and life-span estimation. To improve cost effectiveness, effort has also been put into reducing the amount of rare-earth materials used and using alternative materials such as ferrite magnets. Flux modulators are key to the operation of concentric MGs, and research has shown that an internally connected modulator has the least impact on torque.

Magnetically Geared Electrical Machines

Concentric MG topologies are relatively simple, and their geometry offers significant advantages for integration into a new class of electrical machines, magnetically geared machines (MGMs), where the MG is integrated with a PM machine.

They can be configured with an inner stator or an outer stator. For an inner-stator MGM, the high-speed inner-rotor is shared by the MG and PM machine. An outer-stator arrangement reduces mechanical complexity because the outer magnets are attached directly to the inner surface of the stator, resulting in two rather than three air gaps. Since the MG and the PM machine occupy a single volume, a dedicated design procedure is needed to ensure a well-matched capability between the MG and machine components.

According to Wang, the main benefits of concentric MG/MGMs are high torque density (they are lighter and more compact), contactless torque/power transmission (so there is no friction or wear and tear), maintenance free operations (no lubricant required), high reliability and inherent overload protection. “In terms of the efficiency, concentric MG can compete with the best-in-class commercial mechanical gearbox,” he says. “For both inner-stator and outer-stator MGMs, they can be more efficient and compact than conventional direct-drive synchronous machines.”

However, there are also attempts to look into other high gear ratio topologies such as magnetic harmonic and planetary gears. Their mechanical complexity and construction challenges are still the fundamental deterring factors, says Wang.

The research conducted by Wang’s group has demonstrated that an inner-stator MGM featuring a full magnetic gear integrated with a PM motor (with three air gaps) shows the highest torque per volume and torque per mass density. This is followed by the outer-stator MGM topology (with two air gaps, also known as pseudo direct drive) and then the MGM similar to a conventional PM machine with a single air gap (PM Vernier machine).

The PM Vernier machine has the same mechanical simplicity as a conventional direct-drive PM machine, but its torque density is higher and PM usage is less. Its main drawback is its poor power factor. To address this issue, alternative PM Vernier machine designs with double-stator or double-rotor could be considered at a cost of compromising structural simplicity and manufacturability, says Wang. So, among the three types of flux modulating PM machines, the outer-stator pseudo-direct-drive design still looks to be a good option due to its moderate structural complexity, high torque density and high power factor, says Wang.

For both torque dense MGMs and MGs, one common problem is that they suffer from severe end-effects. To overcome this and further improve their efficiency, the flux modulation ring needs to be made of special ferromagnetic material, such as soft magnetic composites, that can handle 3D magnetic flux. A new type of laminated PMs could further minimise eddy losses in PMs.

“Their structural complexity implies high manufacturing costs and construction challenges,” says Wang. “With the new manufacturing technologies such as additive manufacturing and 3D printing techniques, it will be possible to manufacture these machines in a more cost-effective manner.”

Potential Marine Applications

A concentric MG has two mechanical ports, so when the flux modulator ring is kept still, the outer and inner rotors rotate in an opposite direction. There has been some research looking into applying this topology to contra-rotating propeller applications.

For large ship propulsion power trains, there are geared and gearless design options, says Wang. A multi-stage concentric MG with a high gear ratio combined with a high-speed synchronous motor or a single-stage concentric MG with a medium gear ratio combined with a medium speed synchronous motor could be a good alternative to a conventional mechanically geared motor system.

In gearless designs, the torque dense inner-stator and outer-stator of a pseudo direct drive could be attractive, says Wang. “Although the inner-stator type MGMs are structurally more complex, with the up-scaling for large power, some of the mechanical challenges we experienced in building small power prototypes could be somewhat eased. Considering the reduced mechanical complexity of the outer-stator (pseudo direct drive) MGMs, they appear to be the better choice.”

Magnomatics has already started developing a 2.5MW pseudo direct drive (outer-stator MGM) marine propulsion motor, which Wang says will provide a good indication of the viability of MGM technology for marine propulsion applications.

In October this year, Magnomatics was awarded over £300,000 in grant funding to develop and supply power electronics, machines and drives for the Winder project, aimed at de-risking the technology required for the manufacture of large generators for offshore wind to the UK. While the grant is intended to encourage the development of efficient and reliable generators, Magnomatics has also identified podded propulsion, winch gears and main engines in the marine segment as potential target markets for its solutions.

Further research

Cogging torque of an electrical machine is a pulsating torque resulting from the interaction between the permanent magnets of the rotor and the stator slots. For concentric MGs and MGMs with multiple concentric components, their cogging torque calculation is more complex. Wang’s research group has conducted theoretical research into the calculation of cogging torque which he says can pose serious challenges for speed increaser applications such as wind power generators. “For speed reducer applications such as ship propulsion, the problem is less severe. It can be significantly reduced with judicious selection of a pole-slot combinations.”

Wang’s research will continue into the development of high integrity and fault tolerant flux modulated machine designs.