Robust Design of Interior Permanent Magnet Synchronous Motor for Reduction of Cogging Torque Considering Manufacturing Tolerance Effects

Abstract

Electric motors have been employed as a driving source for various applications and motor applications were expanded to vehicle and mechanical systems, recently. In particular, on account of high efficiency and torque density, a permanent magnet motor is mainly applied to electric power steering (EPS) systems. EPS systems have increasingly been favored as alternatives to hydraulic power steering (HPS) owing to the advances in electrical machines, sensors, and control electronics. The EPS systems have many advantages when compared to HPS in terms of fuel efficiency and sizing. However, disadvantages also exist: steering feel, stability, price and so on. In particular, many studies have been performed to improve the steering feel caused by using EPS system. The cause of unpleasant steering feel is fluctuation of torque ripple or cogging torque. Particularly, at high speed, the steering control ability decreases in driving and the steering feel gets worse. Therefore, to improve the steering control ability, the cogging torque needs to be reduced. The methods of reducing the cogging torque are varied: increased air gap length, the use of fractional slots/poles, skewing stators or rotors, and the use of a large number of slots/poles. Among them, skewing is the most useful method. However, if the magnetic distribution of the rotor or stator shape is asymmetric, the cogging torque cannot be eliminated by applying a skew. These magnetic unbalance is frequently generated in the manufacturing: asymmetry of magnet dimension, location, magnetization level, and the shape of stator or rotor. For reducing the cogging torque due to manufacturing tolerances, a robust design method is necessary. In this thesis, the effect of the asymmetry of residual induction is studied by using the finite element method (FEM). The Taguchi robust design is performed to reduce the cogging torque taking into consideration the asymmetry of the residual induction. The influence of these manufacturing tolerances was identified in this paper and reduced with design. The response was satisfied by the design to reduce cogging torque of EPS motor with Taguchi design, and by calculating individual SN ratios, and the ANOVA is conducted. And the prototype model and optimum model are compared, and the optimum model is verified by analysis according to design parameter considering manufacturing tolerance.