There are several types of motor stator failures. These include:
- Turn to turn short due to voltage spikes
- Turn to turn short due to defective insulation
- Phase to phase fault due to excessive temperature
- Insulation breakdown due to corona or partial discharge
- Shorted terminal leads due to excessive temperature
Spikes may be due to lightning, problems with the utility, or the drive or the cables.
Over-temperature may be due to overloading the motor, or a problem with the drive or its cabling.
An excellent article describing motor stator faults including photos is in IEEE Industry Applications, Vol.18 No. 6, "Root Cause Methodology For Induction Motors" by Bonnet, Austin H.