Improvement of torque density of variable reluctance vernier machine for hybrid electric vehicle

The objective of this study was to determine a design that can improve torque density and can suppress torque pulsation in a variable reluctance vernier machine (VRVM). Thus, we targeted the torque specification of a traction machine mounted on a Honda Insight, which employs a parallel hybrid system. We first examined a machine as a basic model that was based on the winding configuration of the draft model constructed by Lee, the inventor of VRVMs. However, its torque characteristics achieved neither the required maximum torque nor torque density (torque per machine volume). In a previous study we conducted, a machine designed for an increase in torque had problems with torque pulsation. Therefore, in the present study, we have derived theoretical expressions for VRVMs that utilize FEA, and experimented on a machine that can be obtained higher torque and lower torque pulsation by using the expressions. As a result, a machine with four poles, 15 stator slots and 11 rotor salient poles was found to, theoretically, obtain 1.13 times more torque than a previous machine and was able to suppress torque pulsation. In conclusion, our novel machine obtained 1.10 times more torque than a previous machine when a low magnetomotive force was applied, and its ratio was similar to the calculated ratio that was determined by using the theoretical expressions. In addition, the maximum torque pulsation rate was significantly reduced from 44.5% to 13.9%. Furthermore, our novel machine obtained a maximum efficiency of 95.9%, which is a simulation result. The novel machine had 0.99 times higher torque than a previous machine when a high magnetomotive force was applied. Moreover, our machine required a motor volume that was 1.35 times larger than the traction machine of a Honda Insight in order to achieve the target maximum torque. In the future, we will examine further improvements of torque density to achieve the same motor volume as the traction machine of the Honda Insight.