Detection of Trailing Edge PM Demagnetization in Surface PM Synchronous Motors

Irreversible demagnetization of permanent magnets (PM) in PM synchronous motors (PMSM) degrades the performance and efficiency of the machine and drive system. Demagnetizing MMF applied to PMs operating at high temperature is known as one of the leading root causes of irreversible PM demagnetization. This is most likely to cause of demagnetization on the trailing edge of the PMs in all poles, as witnessed by prior research and industrial case studies presented in this work. However, most of the work on detecting PM demagnetization focus on uniform or partial (local) PM demagnetization. In this paper, a comparative evaluation of existing test methods on the detectability of trailing edge PM demagnetization, is given. It is shown that it is difficult to detect this type of fault with back-EMF, stator current/voltage, vibration, or leakage flux-based analysis, since trailing edge demagnetization does not produce asymmetry between PMs. An experimental study on a 380 V, 8 pole, 1.8 kW surface PMSM with trailing edge PM demagnetization shows that airgap flux-based detection is the most reliable means of detecting this fault.