Design and Validation of SPMSM with Step-Skew Rotor for EPS System using Cycloid Curve

This study deals with robust design methodology to reduce cogging torque in EPS (Electric Power Steering) of automotive system. Cogging torque reduction is the key design factor to enhance steering feeling and drive stability in EPS system. In this reason, SPMSM (Surface Permanent Magnet Synchronous Motor) type has been widely used as drive motor in EPS system. Furthermore, two design methods, which are magnet shape design and step-skew design for rotor assembly, have been mainly used to reduce cogging torque in SPMSM. In this paper, SPMSM is selected as drive motor and propose a robust design methodology to reduce cogging torque in EPS system. First, cycloid curve on the magnet shape is used to reduce cogging torque. An evaluation index q is also used to compare the conventional magnet shape design. Second, based on the results of the magnet shape design with the cycloid curve, the step-skew design for rotor assembly is also applied to e cogging torque. In order to verify the effectiveness of the robust design for the cycloid curve and conventional magnet shape with rotor step-skew, the analysis with FEM (Finite Element Method) results and prototype tests are compared. The cycloid curve magnet shape model with rotor step-skew was verified to reduce the cogging torque and enhance the robustness for cogging torque variation through the analysis and protype test results. The verified results for the proposed model will be extended to meet the required cogging torque variation for the various application driven by SPMSM with the robust design model.