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What to do if the motor bearing is corroded by shaft current, is there any solution?

wallpapers Industry 2020-10-26
 
Recently, the high-voltage motor that once had a good reputation in the market has suffered from a strange lack of oil in the bearings. And along with the local melting and unevenness of the bearing raceways and rolling elements, without exception, it is an obvious case of bearing corrosion by shaft current!
What is bearing electric corrosion?
When the motor is running, the potential difference between the two ends of the shaft or between the shaft and the shaft is called the shaft voltage. If the two ends of the shaft constitute a loop through the motor frame, etc., 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 the motor, bearing, stator frame or auxiliary device. Because the shaft voltage is low under normal conditions, the lubricating oil film in the bearing can play the role of insulation and suppress the shaft current generation; but when the shaft voltage If the oil film is not formed stably at the moment when the motor is started, the shaft voltage will cause the lubricant film to discharge and break down to form a path to generate shaft current.
The high temperature generated by the release of partial discharge energy of shaft current can melt many small areas on the inner ring, outer ring or balls of the bearing, and form grooves, resulting in noise and vibration. If the treatment cannot be found in time, it will lead to bearing failure, which will bring damage to production. Great influence. The most significant feature of the galvanic corrosion of the high-frequency shaft current to the bearing in the frequency conversion speed regulation system is the production of "washboard" dense groove stripes on the inner and outer rings and balls of the motor bearing. What are the ways to fundamentally eliminate the shaft current of the motor? One is to add a bleeder device, that is, to install a bypass brush, and the shaft current will bypass the bearing to leak; the second is to use an insulated bearing to block the path of the shaft current through the bearing. In order to block the path of the shaft current, the motor must have at least one end Use electrically insulated bearings.
Overcurrents often occur in wheel bearings and traction motors (rail vehicles), DC and AC motors (power transmission systems), and generators (wind power). Overcurrent may cause damage to raceways and rolling elements and deterioration of lubricants, which may cause premature failures of motors or generators, increase maintenance costs, and bring losses such as shutdowns and production shutdowns. Rolling bearings used in some related equipment such as motors and generators may be affected by the passage of current during use, which will not only damage the rolling elements and raceways in the bearing, but also affect the grease performance.
Bearing damage caused by current leakage will affect the abnormal vibration of the motor and increase the noise. The motor is not working properly.
Generally speaking, it is very difficult to completely eliminate the potential difference inside the bearing, but it is possible to prevent or greatly reduce the current passing through the bearing to prevent electrical corrosion of the bearing. The induced voltage along the shaft creates a current loop that also passes through the bearing housing. One reason for the shaft voltage is the uneven distribution of magnetic flux inside the motor, especially in some motors with a small number of pole pairs. In this case, as long as one end of the bearing is insulated, the current loop can be completely cut off, and the non-drive end bearing is usually insulated. If a voltage occurs between the shaft and the bearing housing, the current will pass through each bearing in the same direction. In this case, the bearings at both ends of the motor should be insulated.
The decisive factor in choosing insulation is the time characteristic of the current. If it is DC voltage or low-frequency AC voltage, the insulation effect depends on the pure resistance value of the insulation layer; if it is high-frequency AC voltage (common in equipment using inverters), it depends on the capacitive reactance value of the insulation layer. The best solution to these problems is to use electrically insulated bearings. Bearings with ceramic coating on the inner ring or outer ring are called insulated bearings. The ceramic coating can prevent the passage of current and has the ability to insulate. The rolling elements of hybrid bearings are made of ceramics, and the rolling elements prevent the passage of electric current, so they also have insulating ability.
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Tag: shaft current