Increasing the torque density of an in-wheel automotive traction motor

Abstract

In-wheel electric motors have been developed in recent years to provide integrated direct drive traction in electric vehicles. Classical automotive components, including drive shafts, gears and differentials can be eliminated by integrating the drive and the motor in the wheel. Researching in-wheel motors is therefore part of the drive towards efficient electric vehicles. This thesis is concerned with improving the torque performance of an existing radial flux in-wheel electric motor. This work studies alternative motor topologies, which could provide improved torque density as compared with the existing motor design. Firstly three-dimensional flux machines are investigated, specifically the transverse flux machine. Two different configurations are shown to deliver higher torques than the radial flux machine in continuous operation but have failed to deliver the required overload torque. In addition, low power factor and low efficiency is shown to make these machines unsuitable for the application of in-wheel traction motor. Secondly, increase in torque production was researched by investigating the exploitation of saliency in conventional radial flux machines. The use of distributed windings and interior permanent magnets allowed an increase in the salient torque of the machine. However, the available volume is shown to be too small for an interior permanent magnet topology. Salient machines will be better suited for high-power high-speed applications using a mechanical gearbox for torque transfer. The majority of the work relates to the third topology studied which incorporates Halbach arrays. The use of a Halbach array in the rotor allows a reduction in the rotor core back and gives an increase in the torque capability of the machine. Demagnetisation can be solved by reshaping the pole and transition magnets. Moving from a parallel slot to a parallel teeth stator configuration allows a reduction of the stator saturation and thus increases the amount of flux that the machine can handle in an overload condition, improving continuous and transient overload torque capability. In addition, AC copper losses are reduced in the parallel teeth topology. Overall, it is shown that a higher torque density and efficiency can be achieved by adopting a Halbach array rotor with parallel teeth.

A Halbach array rotor was manufactured and tested, validating the simulations. The manufacturing feasibility was studied, using different tests and assembly tools. Compared to the pre-existing design, this manufactured motor is shown to give an increase of 7% torque during continuous operation and 9% on the overload condition. In addition, efficiency at required operational points was increased by up to a 3%.