Recently, high-voltage motors that once had a good reputation on the market have been plagued by a strange lack of oil in their bearings. And accompanied by local melting and non-uniformity of bearing raceway and rolling element, without exception, this is the obvious case of bearing corrosion by shaft current!
What is bearing electrical corrosion?
When the motor is running, the potential difference between two ends of the shaft or between shafts is called shaft voltage. If the two ends of the shaft form a loop through the motor frame, etc., the shaft current will be generated under the action of the shaft voltage. Shaft current is the shaft voltage generated by a closed circuit formed by a motor, bearing, stator frame or auxiliary device. Because the shaft current is normally low, the lubricating oil film in the bearing can play the role of insulation and inhibition of shaft current generation; However, when the shaft voltage is started by the motor, if the oil film is not formed stably, the shaft voltage will cause the lubricating oil film to discharge and breakdown, forming a path to produce shaft current.
The high temperature generated by the partial discharge energy release of the shaft current can melt many small areas on the inner ring, outer ring or ball of the bearing, and form grooves, causing noise and vibration. If it cannot be found and treated in time, it will lead to bearing failure and bring damage to production. Big impact. The most significant characteristic of the galvanic corrosion of bearings by high frequency shaft current in variable frequency speed regulation system is that the dense grooves of "washboard" are produced on the inner and outer rings and balls of motor bearings. What are the methods to eliminate motor shaft current fundamentally? One is to increase the discharge device, that is, install the side current brush, shaft current will bypass the bearing leakage; The second is to use insulated bearings to prevent shaft current from passing through the bearing's path. To block the path of shaft current, at least one end of the motor must use electrically insulated bearings.
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