Weight optimisation of coreless axial-flux permanent magnet machines

This study explores the upper limits in power-to-weight and torque-to-weight ratios of coreless axial-flux machines with permanent magnets. Moreover, it provides a comprehensive multifunctional optimisation procedure that is utilised for obtaining these limits. The procedure encompasses analytical analysis of electro-magnetic, thermal and structural (mechanical) aspects of axial-flux machines. Obtaining global minima is ensured by considering the whole machine design space, and mapping it into the performance space, where a Pareto front can be easily identified. From it, an optimal motor/generator for airborne wind turbines is identified. The design has a power-to-weight ratio of 6.4 kW/kg (19 Nm/kg at 3200 rpm) including structural (purely mechanical) parts, at an efficiency of 95%. This is a significantly higher ratio than the one in modern commercial machines or designs reported in the literature. Therefore, the resulting machine is manufactured and experimentally tested in order to verify the claimed limits and the optimisation methodology.