Investigation of Postdemagnetization Unbalanced Magnetic Force in PM Machines Considering Short-Circuit Faults

This article investigates postdemagnetization unbalanced magnetic force (UMF) in single-three-phase permanent magnet (STPPM) and dual-three-phase PM (DTPPM) machines considering three-phase short circuit (3PSC). First, when eccentricities are not considered, for rotationally symmetrical STPPM machines and DTPPM machines with each winding group being rotationally symmetrical, demagnetization distribution is symmetrical. Therefore, there is still no UMF after demagnetization. For rotationally asymmetric STPPM machines and DTPPM machines with each winding group being rotationally asymmetric, the first-order harmonic is added to the UMF
$x$
and
$y$
components. Then, the influence of eccentricity on 3PSC current and demagnetization is analyzed, and postdemagnetization UMF for STPPM machines with eccentricities is investigated. It shows that eccentricities almost have no influence on 3PSC currents but gather demagnetization around PMs facing larger airgap length. As a result, under static eccentricity, the original UMF components decrease, while the first-order harmonic is added after demagnetization. Under rotating eccentricity, the original UMF components decrease in asymmetric STPPM machines, whereas they increase in symmetrical ones. The postdemagnetization UMF for DTPPM machines considering eccentricities is similar to that for symmetrical STPPM machines. In addition, the variations of UMF harmonics increase with the eccentricity ratio. Finally, experiments are conducted to validate the theoretical analysis.