Analysis of Optimal Rotors Skew to Improve the Total Harmonic Distortion of Back Electromotive Force in MG-PMSM for Traction

This paper presents optimal rotor permanent magnet (PM) and rotor pole-piece skew to improve the total harmonic distortion (THD) of the back electromotive force (Back-EMF) in magnetic geared permanent magnet synchronous motor (MG-PMSM) for traction. MG-PMSM is a system that integrates a magnetic gear and a PMSM which can minimize the size of the driving system and reduce power loss because there is no mechanical friction unlike the existing motor. For this reason, it is more advantageous in terms of output density and efficiency. However, MG-PMSM has a pole-piece rotor for magnetic flux modulation, which can produce additional spatial harmonics in back electromotive force (back-EMF). In addition, this motor has inevitable to have a salient permanent magnet structure for the stator due to magnet loss, and in the case of winding methods, fractional slot concentrated winding (FSCW) is mainly used, which also causes additional harmonics. THD in back-EMF adversely affects torque ripple and make accurate position control difficult. Therefore, in this paper, THD in back-EMF is reduced by focusing on the optimal rotors skew. Considering the characteristics of MG-PMSM, skew is applied to each rotor, and then compared and analyzed with the proposed model.