Investigation of Post-Demagnetization Torque Ripple in Fractional-Slot Surface-Mounted PM Wind Power Generators After Short Circuit Faults

This article delves into the post-demagnetization torque ripple of fractional-slot surface-mounted PM wind power generators after short circuit faults. Firstly, demagnetizing magnetomotive force from three-phase short circuit (3PSC) currents is modelled and demagnetization distribution pattern is analyzed. Then, this article investigates the post-demagnetization cogging torque. It is shown that the period of cogging torque becomes 360N(p)/N-s electrical degrees, where N-s and N-p are the slot and pole pair numbers of unit generator, respectively. In addition, the amplitude increases when the overall demagnetization level increases with 3PSC rotor speed. Following this, this article analyzes the post-demagnetization back-EMF and load torque ripple. It is found that the demagnetization adds low-order harmonics in the back-EMF and leads to additional torque ripple component under the same stator current, which also increases with demagnetization level. Although the harmonic torque ripple offsets the cogging torque, it has smaller amplitude and thus the total torque ripple still increases with demagnetization level. Most importantly, it is also found that the demagnetization distribution pattern heavily depends on the 3PSC start rotor position. Specifically, when 3PSC starts at the rotor position k pi/3 (k = 0, 1, 2 horizontal ellipsis ), the demagnetization distribution is the most uneven in different PMs, which results in the largest post-demagnetization load torque ripple and the smallest average torque. Conversely, the opposite condition occurs around the 3PSC start rotor position pi/6+k pi/3. In addition, the influence of 3PSC and single-phase short circuit are compared. Finally, the theoretical analysis is verified by experimental evidence.