A comparative study between inner and outer rotor variable flux reluctance machine topologies for heavy-duty electric vehicles

Abstract

This article presents a comprehensive comparative study between two variable flux reluctance machine topologies with inner and outer rotors. Electrical and geometrical parameters of both topologies are optimized for maximum torque density, minimum torque ripple, and maximum efficiency to be used in a heavy-duty electric vehicle which requires 500 Nm electromagnetic torque at 1200 rpm base speed. Then, both optimal designs are compared using the 2-D and 3-D finite element and analytical models. The 2-D electromagnetic finite element model coupled with a 3-D analytical thermal model shows that the outer rotor VFRM is able to reach higher torque density with 21 Nm/L than inner rotor topology when the water cooling is also included in the volume of the electrical machine. Although the optimal variable flux reluctance machine with the inner rotor exhibits slightly lower efficiency and higher torque ripple than the optimal design with the outer rotor, it is possible to apply higher current densities with the outer rotor topology due to its improved thermal characteristics. However, the 3-D mechanical finite element model coupled with the electromagnetic model shows that the maximum von Mises stress and deformation in the electrical steel is more than one order of magnitude higher in the outer rotor topology than the inner rotor topology.