Thermal management of electrical machines for propulsion – challenges and future trends

The continuous drive towards electrified propulsion systems has been imposing ever more demanding performance and cost targets for the future power electronics, machines and drives (PEMDs). This is particularly evident when exploring various technology road mapping documents both for automotive and aerospace industries, e.g. Advanced Propulsion Centre (APC) UK, Aerospace Technology Institute (ATI) UK, National Aeronautics and Space Administration (NASA) USA and others. In that context, a significant improvement of the specific performance and cost measures, e.g. power density increase by a factor of 10 or more and/or cost per power unit reduction by 50% or better, is forecasted for the next 5 to 15 years. However, the existing PEMD solutions are already at their technological limits to some degree. Consequently, meeting the performance and cost step change would require a considerable development effort. This paper is focused on electrical machines and their thermal management, which has been recognised as one of key enabling factors for delivering high specific output solutions. The challenges associated with heat removal in electrical machines are discussed in detail, alongside with new concepts of thermal management systems. Several examples from the available literature are presented. These include manufacturing techniques, new materials and novel integrated designs in application to electrical machines.