사이클로이드 곡선을 적용한 EPS용 SPMSM의 자석 형상 설계 및 검증

Abstract

The depletion of fossil and oil supply combined with global warming, pollution and greenhouse gas emissions worldwide have resulted in a growing demand for efficient and eco-friendly vehicles such as EV and HEV. The fuel economy regulations will also apply to existing internal combustion engine vehicles. To obtain these goals, many hydraulics systems have been removed and replaced with electric systems. A representative example of this is the EPS (Electric Power Steering) system. EPS system demands are continuously increasing because it has the superior fuel efficiency compared to other electric actuator systems. Reflecting this trend, the quality of EPS system is becoming more strict. In particular, to improve the steering feel and the stability of the vehicle at high speed, it is required to reduce the cogging torque and the torque ripple characteristics of the motor, which is one of the main parts of the EPS system, and many researchers have studied the design methods therefore. In 2008, Evans proposed a design method applying the inverse cosine curve to the SPMSM magnet and the IPM rotor core shape. And in 2012, Wu and other many researchers proposed the design method to reduce cogging torque and torque ripple by applying the eccentric circles. Though several design methods have been proposed to reduce cogging torque and torque ripple, these design methods did not consider the following points. First, the motors that strictly restrict the motor performances such as the cogging torque and the torque ripple in EPS motor, are so sensitive to manufacturing tolerances that it is necessary to verify the performance variation according to the manufacturing tolerance of magnet shape. Second, since the cogging torque and the torque ripple are affected by various manufacturing tolerances in addition to the manufacturing tolerance of magnet shape, it is not known whether the proposed method is insensitive to performance variation in consideration of manufacturing tolerances. Third, many design methods have been proposed to improve the output and efficiency of the motor. However, a design method that is insensitive to manufacturing tolerance will be also needed to improve productivity. In this paper, cycloid CET (Curtate Eptirochoid) and PET (Prolate Epitrochoid) among cycloid curves applicable to the magnet shape were selected and the magnet shape design method of SPMSM was proposed for them. In order to compare and evaluate the cycloid curve with the magnet shape having the conventional curve, the length of air gap on the axis , which is the electrical axis of the magnet, was proposed and applied to it in the same way as conventional curves. By using the analysis method of tolerance sensitivity, the evaluation index TIR was used to evaluate the tolerance insensitiveness for motor performance variation. TIR means the variation rate of motor performance for the manufacturing tolerance. From the FEA results, it was confirmed that the performance variation such as cogging torque, average torque and torque ripple of the motor with the cycloid was insensitive to the manufacturing tolerance. The prototypes were manufactured to verify the FEA results. The performances of the prototype with the cycloid curve were improved over the prototype with the eccentric curve through the test results of cogging torque, average torque and torque ripple. The magnet shape design with the cycloid curve can also be applied to the rotor pole shape of synchronous motor such as IPMSM and WFSM. The proposed magnet shape design will be useful for the tolerance insensitive design considering the improvement of the motor productivity in the future.