Effects of Maximum Speed on Output Characteristics of IPMSMs in EV Applications: A Comparative Study

Increasing the speed of an electric machine is a means of increasing its power density. However, increasing the speed is challenging due to the rotor’s mechanical stability issues. In interior permanent magnet synchronous machines (IPMSMs), the radial ribs are required to reduce stress concentrations and improve the rotor’s durability. The thickness of radial ribs is of great importance in minimizing the rotor leakage flux. Thus, radial rib thickness must be adjusted as a function of geometrical variables, maximum rotational speed, and material characteristics during machine optimizations. This research proposes a new semi-numerical approach for computing the radial rib thickness of a V-shaped IPMSM rotor based on the rotor geometry. The calculated rib thickness is included in the rotor model for detailed optimizations where the effect of the rotor leakage flux is accounted for. In the second part of the study, this model is used for a comparative study that examines how varying the maximum speed affects the machine’s electromagnetic performance. According to the findings, when the stator’s outer dimensions and copper loss are kept the same, there is a particular maximum speed and current density in which the machine shows an optimum balance between electromagnetic and mechanical performances. This is because the thick radial rib produces significant leakage flux as maximum speed increases. On the other hand, lower speeds require a larger stator volume to match high-speed performance. The approach presented in this study can also be used to determine the suitable stator outer dimensions for a given maximum rotational speed.