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.