To be precise, it is necessary to know whether the excitation winding is grounded.
1. Test method: Use a multimeter (or megger) to measure the insulation resistance between the field coil and the rotor.
2. Maintenance method: If the magnetic coil is grounded due to slight scratches, it can be simply wrapped and dipped in paint. If an overload causes a short circuit between turns in the winding, rewinding should be considered. Finally, the rotor must also perform dynamic balancing.