Investigation of Single-Phase Asymmetric Short Circuit Faults in Permanent Magnet Machines Considering Gravitational Lift Torque Load

Single-phase asymmetric short circuit (ASC) is a critical fault in permanent magnet (PM) machines, which can cause demagnetization and torque ripple. In this article, combining the equivalent circuit of single-phase ASC and motion equation of PM machine, a single-phase ASC model is built through Matlab/Simulink, which can consider the influence of gravity lift torque (GLT) load. Based on this model, the single-phase ASC response of PM machines is firstly investigated. The results show that there is a critical torque when the GLT load is considered. When the load torque is lower than the critical value, the speed and current during short circuit tend to be stable. Conversely, if the load torque exceeds the critical value, the PM machine accelerates in the negative direction. In addition, when the load torque is less than the critical value, the peak currents under different load torque and inertia are always less than that at constant speed. On the contrary, if the load torque exceeds the critical value, the peak currents under different load torque and inertia are almost the same, which is far more than that at constant speed. It means that excessive short circuit current and demagnetization can be mitigated by keeping load torque under critical value. Thereafter, the effect of machine parameters on the critical torque is investigated, including total inertia, phase resistance, inductance, PM flux linkage, and diode forward voltage. Moreover, this article also compares the results calculated by the proposed model and FE simulation method, which verifies the accuracy of the proposed method.