Effect of Partial Demagnetization on Speed in Permanent Magnet Synchronous Machine

Permanent magnet synchronous machines (PMSMs) have been widely used in many industrial applications due to their superior performance. Under normal operating conditions, the permanent magnets (PM) used in these machines get partially demagnetized depending on the electrical loading and return to their initial state once it is removed. However, if any fault is causing a very high current or the temperature increases beyond a certain threshold, the permanent magnets may suffer from irreversible demagnetization when the operating point falls below the knee point. The resulting demagnetization fault will cause a disturbance in the airgap flux density which increases the torque ripple. The pulsating torque reflects in the motor speed, particularly at low operating speeds. This paper aims to show the effect of demagnetization fault on the motor speed by analytically assessing the motor torque under the magnet fault conditions. The theoretical analysis has been verified in the experiment.