An Ultra-Lightweight PM Electrical Motor for High-Altitude Platform Applications

This paper describes key aspects of the development process of an ultra-lightweight electrical motor for solar-powered high-altitude (20 km) platform applications (HAPSs). A radial-flux permanent magnet (PM) machine topology with outer rotor and integrated direct air-cooling is considered here. The mission profile of the analysed solar-powered aircraft calls for a high-torque overload (×2.5) at take-off and a high-efficiency (>95%) at high-altitude cruise operation. Consequently, a fine balance between both operating points needs to be seen when sizing the motor. Effective thermal management of the motor at take-off is critical for achieving a compact motor design. Further to these, the motor design should enable a simple and cost-effective manufacturing and sassembly process, which makes use of the lightweight composite materials for all structural elements of the motor assembly. In this work, a careful integration between both active and structural subassemblies of the motor resulted in a significant weight reduction, with contribution of all structural components less than 5% of the overall motor weight. The theoretical and experimental results discussed in the paper confirm the key design targets of the motor are met, with a motor torque density of 10Nm/kg (take-off operation) and 94% efficiency (cruise operation). Further to the motor development process, a comprehensive insight into the existing state of the art HAPS solar-powered aircrafts with focus on the commercial electrical motor technology and motor to aircraft integration is provided to highlight some of the application driven engineering challenges.