The paper focusses on the analysis of the demagnetisation process of permanent magnets in line-start synchronous motors in dynamic states related to start-up and resynchronisation. A field-circuit model of electromagnetic phenomena was used to analyse the demagnetisation process, taking into account the influence of temperature on the properties of permanent magnets and their resistance to demagnetisation. The results of the conducted research have shown, among other things, that the process of resynchronisation of the motor is much more dangerous from the standpoint of the risk of demagnetisation than the start-up itself.

This paper deals with the finite element analysis of the demagnetization process of the line start permanent magnet synchronous motor. Special attention has been paid to demagnetization risk assessment after resynchronization during a short-term supply power outage. The current and torque waveforms have been determined assuming the difference depending initial rotor position angle. It has been demonstrated that the highest demagnetization risk occurs when resynchronization (motor reclosing) is performed whe induced electromotive forces are in anti-phase to the supply voltage waveforms. The effect of cage winding resistance on the risk of demagnetization is examined and discussed.