Design of a Forced Air-Cooling System for an Automotive Axial Flux Traction Motor using Computational Fluid Dynamics

The 'Net Zero' emissions strategy has driven the automotive industry into electrification. Ambitious roadmaps outlining challenging machine performance requirements have motivated the exploration of more revolutionary technologies. Axial flux permanent magnet machines are a prime focus, as they can provide high power-densities with significant material and cost reduction. The trend of high-speed automotive traction machines creates a thermal challenge for the rotors, resulting in higher component and windage losses. Air-cooling is an effective technique for removing heat from the rotor. This can be further enhanced by utilising forced air-cooling methods, particularly in the airgap region where machine losses are concentrated. However, this increased heat transfer comes at the consequence of a higher windage loss. The trade-off between these two characteristics is essential, as poorly implemented cooling topologies can negatively impact machine performance. This study investigates the heat transfer and windage characteristics of forced air-cooling for the rotor of an axial flux permanent magnet machine. Computational fluid dynamics is utilised to simulate an inflow of ambient air through the airgap of the machine. The flow rates were varied alongside the rotational speed, which was executed at regular intervals up to 15,000rpm. The results were compared with existing rotating disc correlations. Implementing higher flow rates enabled a magnet temperature reduction of 50°C at maximum speed.