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

Irreversible demagnetization of permanent magnets (PM) in PM synchronous motors (PMSM) most commonly occurs when reverse magnetomotive force (MMF) is applied to PMs operating at high temperature. This causes local demagnetization on the trailing edge (TE) of the PMs with respect to the direction of rotation. The non-uniform distribution of flux in the PMs of all the rotor poles due to TE demagnetization shifts the magnetic d-axis, which alters the maximum torque per ampere (MTPA) torque trajectories and degrades control performance. Therefore, TE demagnetization must be detected and distinguished from other types of demagnetization to reconfigure the control to maintain optimal control performance. However, it is a challenge to detect this fault with existing test methods, as it does not produce asymmetry between poles. In this paper, a new off-line test method to detect and classify the type of PM demagnetization is proposed. The amplitude and angular distribution of equivalent resistance and reactance acquired from high frequency pulsating field injection at PMSM standstill are used as indicators for detecting and distinguishing TE and other types of demagnetization. The proposed method is verified experimentally on 1.8 kW, 8 pole surface PMSMs.