NRAI01 Processing board

A repair factory undertook the maintenance of a 10kV high-voltage motor. After the test was completed, an alarm occurred during the voltage withstand process.

 However, no abnormalities were found during the insulation resistance test of the motor. Further inspection revealed that the motor malfunction was a typical flashover phenomenon. Ms. has organized some real-life photos to share with everyone, and analyzed the causes of flashover.

1. Fault Case Description

The motor has a compact structure and a relatively small cavity, but from the overall distribution of the stator winding, this problem is not yet apparent. Further inspection revealed that surface blackening marks appeared at a distance along the surface of the winding end. And there are some very fine iron powder below the winding area. After cleaning these iron powder and repairing the burnt areas on the surface, the problem was resolved.

From the appearance of faults and some actual anomalies, it can be basically attributed to typical flashover; Further investigation revealed that during the machining process of the motor base, the blind hole of the fixed end cover was opened, and the fine iron powder that appeared during the tapping and assembly process entered the inner cavity of the motor. During the operation of the motor, the iron powder above the base and in the side end holes vibrated and fell to the position below the base, creating the conditions for flashover to occur.

From the consequences of this case, we would like to remind everyone that the holes in motor components may seem simple, but when they are through holes and blind holes, they should be strictly processed according to the diagram, especially for blind holes, which must have specific functions and principles.

In reality, when the vast majority of large motors start, there are more or less other loads on the transformer, so the amount of other loads also needs to be considered as one of the factors.

Why does starting an electric motor affect the operation of other electrical equipment?

Because the starting current of a cage motor is 5-7 times its rated current, a large current can cause a large voltage drop. If the voltage drop reaches a certain value, other electrical equipment may stop working or cause faults. Therefore, there are regulations that the voltage drop caused by frequent starting of motors should not exceed 10%, and the voltage drop caused by infrequent starting should not exceed 15%.

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