Disc Motors for Automotive Applications

The importance of electric machines in hybrid and electric power trains is often overlooked due to a focus on energy storage and system controls. Yet there is significant scope for improving motor and generator performance, size, weight and cost, thus improving hybrid and electric vehicle performance as a whole. This paper describes a new motor/generator design based on axial flux topology, developed and commercialised by UK based EVO Electric Ltd. Axial Flux machines offer significant improvements over both high-speed induction machines and conventional permanent magnet machines. This is due to a number of factors, including increased cooling area made possible by the disc shape of these machines. This leads to large increases in torque and power densities. For example, power and torque densities of 1.8 kW/kg and 5 Nm/kg (continuous, 3600 rpm) have been demonstrated by EVO. Integrated liquid cooling allows for a very compact design with high efficiency (>96% at 3,600 rpm). Further increases up to 2 kW/kg and 6 Nm/kg are expected in the near future. The paper discusses the technology behind the EVO machines, and compares the machines to conventional types to show why the axial flux design is beneficial in terms of weight, size, efficiency, battery loading and cost. The materials and manufacturing innovations that have enabled EVO to build robust and low-cost machines, thus overcoming the historical barriers to axial flux machine commercialisation, are also discussed. The paper then explores further possibilities for vehicle cost and performance improvement, such as gear- less power trains made possible by the high torque of EVO machines.