Development of a magnetic gearbox for high speed applications

The research subject for this PhD study is the development of a magnetic gearbox for high speed applications. A particularly interesting application for high-speed magnetic gearboxes is flywheel energy storage systems. Modern designs have been prototyped that operate at very high speeds in the region of 60,000 RPM. The system-level advantage of operating at such a speed is that very high energy densities can be achieved. This does however pose significant mechanical problems. Both PM rotors spin at high speeds and thus have to be mechanically contained. As well as being the slower of the two rotors, the outer rotor is inherently contained by the gearbox casing. However, the higher-speed inner rotor experiences significant centrifugal effects and additional containment is required. Traditionally this containment has been non-magnetic. This is because highly magnetic materials will act to 'short-circuit' the magnetic flux. However, non-magnetic materials effectively increase the airgap in the magnetic circuit. Previous studies at the University of Bristol and elsewhere have demonstrated methods where low-permeability materials can be used to construct containment sleeves to improve the magnetic loading for a given thickness compared to magnetically inert materials such as carbon fibre. More recent work has looked at the development of multi-functional composites for the same purpose. Another challenge is the containment of the ferrous pole pieces in the central ring. The conventional approach to manufacturing this subassembly is to construct a non-magnetic carrier that incorporates pockets to hold the ferromagnetic poles. It is expected that the application of emerging manufacturing techniques such as multi-material sintering processes and additive manufacturing could yield significant improvements in the magnetic performance of this aspect of the gearbox. This is especially interesting when this structure forms the boundary of a vacuum. The continued evolution of magnetic gearboxes requires a multidisciplinary approach, bringing together knowledge of materials, manufacturing processes and electromagnetic modelling. The research that will be undertaken will involve electromagnetic and mechanical material characterisation leading to electromagnetic and mechanical Finite Element Analysis of system designs as well as experimental validation though dynamometer testing of prototype systems.