Investigation of post-demagnetization torque ripple in integer-slot surface-mounted PM wind power generators after short-circuit faults

Abstract

This article investigates the post-demagnetization torque ripple of integer-slot surface-mounted PM (SPM) wind power generators after three-phase short-circuit (3PSC) faults. Firstly, the demagnetization distribution pattern is analyzed by analytically modelling demagnetizing MMF from 3PSC currents. It is revealed that there exists a severest demagnetization line, which shifts from the PM pole axis in the rotating direction. The demagnetization level gradually decreases in the radial direction away from the airgap and along both sides in the circumferential direction. Thereafter, the article investigates the post-demagnetization torque ripple by analyzing cogging torque and back-EMF harmonics, respectively. It is found that, under low PM temperature, the cogging torque decreases when demagnetization deteriorates. With higher PM temperature, the cogging torque first decreases and then increases. Additionally, both back-EMF harmonic and the resulting torque ripple decrease with demagnetization aggravating. Although the harmonic torque ripple and cogging torque amplitude exhibit opposite phases, the former has a smaller amplitude. Therefore, the total torque ripple follows the variation trend of cogging torque with demagnetization level. Most importantly, the torque ripple is more likely to exceed the pre-demagnetization value when PM temperature increases. Furthermore, the influence of 3PSC start rotor position is investigated. It is found demagnetization is the least severe when 3PSC starts at 7π/6+kπ/3 (k = 1, 2, 3... ), yet with the largest torque ripple. Finally, experiments are conducted to validate the theoretical analysis.