Geometric optimization of variable flux reluctance machines for full electric vehicles

As one of the crucial components for electric vehicles, the electric machine has been widely examined from the perspective of machine topology, torque and power density, material usage, and efficiency. In this study, a relatively novel type of electrical machines without permanent magnet material, namely the variable flux reluctance machine (VFRM) is optimized towards a high drive cycle efficiency. In particular, a 12-stator/l0-rotor pole (12/10) VFRM is designed to match the torque and power level of a BMW i3 electric motor. The aim of this design study is to verify whether or not the VFRM is a viable, low-cost alternative for the main traction motor. In addition, rotor skewing has been investigated to successfully reduce the torque ripple. Finally, several down-scaled versions of the VFRM design are analyzed using Finite Element simulations to determine their efficiency maps. These maps are then compared to linearly scaled efficiency maps of the original VFRM design. The results of this comparison show that the efficiency maps change shape and magnitude and are therefore not directly comparable.