1: Rotor due to moisture and insulation resistance reduced to below the allowable value, such as the generator out of operation for a long time, the environment is humid and other reasons caused by insulation resistance reduced.

2: Rotor due to long service life, or running due to various reasons to make the rotor overheating caused by aging and deterioration of the coil insulation material.

3: There is carbon brush powder or oil accumulation under the slip ring, so that the rotor lead wire insulation damage.

4: Due to poor sealing of the generator's cooling system or due to its shaft tile leakage so that the rotor coil end accumulation of ash, oil or carbon powder, resulting in reduced insulation performance. This cause is influenced by the centrifugal force of the rotor.

5: Due to the effect of ventilation and thermal expansion in operation, the rotor groove at the groove lining protection layer aging, fracture or even fall off, so that the groove at the groove lining of the mica gradually peeled off, fracture by the wind blowing off plus the groove ash accumulation and other factors caused.

6: rotor slot insulation fracture caused by rotor insulation resistance is too low or metallic grounding.