Efficiency Optimisation of Axial Flux Traction Motors with High Frequency Litz Wire as a Substitution of Parallel Enamelled Single Wire

Electric mobility brings new challenges for electric traction motors in terms of energy efficiency and power density. Classic winding technologies for electric traction motors use round single enamelled copper wire as standard conductor material for stator coil winding. Several enamelled wires in parallel add up to the required conductor cross sections. In comparison to that, high frequency (HF) litz wire as an alternative conductor material consists of several enamelled wires, but all strands twisted in bundles or sub-bundles to reduce eddy currents. Converter technologies have relied on HF litz wires for coil winding since decades, but developers consider it recently for modern traction motors, since speeds and inverter frequencies are rising. This paper analyses HF litz wire in comparison to parallel enamelled single wires in terms of reduction of eddy current losses in the conductors, also known as alternating current losses. For this purpose, two state of the art axial flux motors with an assembly of these different wire technologies are tested and analysed on a dynamometer. The goal is to quantify differences of both wire technologies in efficiencies, power dissipation and temperature behaviour. Finally, a review displays findings relating to advantages in energy efficiency, power density and continuous power.