External rotor permanent magnet-less electric motors for traction application: a review

Abstract

Electric vehicles (EVs) demand variable speed drives with high starting torque, high power density, and appropriate speed-torque characteristics. Due to this, the EV industry widely considers permanent magnet (PM) motors for traction in powertrains. However, the harmful environmental impacts of the PM open the doors to magnet-less machines such as induction motors, synchronous reluctance motors, switched reluctance motors and flux switching motors as alternatives. Direct-drive in-wheel external rotor motors are an appropriate choice in traction applications to mitigate transmission losses. This paper has thoroughly overviewed the magnet-less machines with in-wheel external rotor topology. The discussions are based on magnetic structure variations, winding topologies, parameter optimisation, control algorithms, and mechanical aspects such as thermal and structural aspects involved in these machines. Further, other applications with external rotor motors are also discussed at the end.