Detection of Trailing Edge Demagnetization in Surface PM Synchronous Machines based on High Frequency Equivalent Impedance

The most common type of irreversible demagnetization in PMSMs occurs on the trailing edge (TE) of PMs due reverse MMF applied to PMs operating at high temperature. This causes non-uniform distribution of flux in the PMs of all the rotor poles, and shifts the magnetic d-axis, which alters the MTPA torque trajectories and degrades control performance. Therefore, TE demagnetization must be detected and distinguished from other PM defects to reconfigure the control for optimal performance. TE demagnetization is very difficult to detect with existing methods, as it does not produce asymmetry between poles. In this paper, a new method for detecting TE demagnetization and distinguishing it with uniform demagnetization is proposed. The magnitude and angular distribution of the equivalent impedance acquired from high frequency pulsating field injection are used as indicators for detection and classification of demagnetization. The proposed method is verified experimentally on 1.8 kW, 8 pole surface PMSMs.