Damp winding reduces insulation resistance; Long term overload operation of motor; Harmful gas corrosion; Metal foreign matters intrude into the winding and damage the insulation; When rewinding the stator winding, the insulation is damaged and touches the iron core; The winding end touches the end cover base; Insulation burn caused by stator and rotor friction; The insulation of outgoing line is damaged and collides with the shell; Overvoltage (such as lightning strike) causes insulation breakdown.

3. Inspection method

(1) Observation method. Observe whether there is any damage and blackening trace through visual inspection of the insulation at the end of the winding and in the trunking. If so, it is the grounding point.

(2) Inspection method. Check with the low resistance gear of the multimeter. If the reading is very small, it is grounded.

(3) Different megohmmeters are selected according to different grades to measure the insulation resistance of each group of resistance. If the reading is zero, it means that the winding is grounded. However, for motor insulation damp or breakdown due to accident, it needs to be judged according to experience. Generally speaking, when the pointer swings at "0", it can be considered to have a certain resistance value.

(4) Test lamp method. If the test lamp is on, it indicates that the winding is grounded. If there is spark or smoke somewhere, it is the winding grounding fault point. If the lamp is slightly on, there is grounding breakdown in the insulation. If the lamp is not on, but there is spark when the test rod is grounded, it indicates that the winding has not been broken down, but is seriously damp. You can also tap it gently at the edge of the stop of the shell with hardwood, knock it to a certain place and wait for it to go out When it is on, it means that the current is on and off, and this is the grounding point.