Here’s an in-depth explanation of the effects shaft current has on bearings. From our good friends from AEGIS
The bearings in all AC electric motors that are operated on PWM IGBT variable frequency controllers are always subjected to shaft current that are inherent to the use of these drives. (L&S Electric provides voltage discharge testing from our Reliability Services team.)
The #ShaftCurrent, as it is related to these voltages, grounds from the shaft through the bearings causing micron-sized welds called frets. These are microscopic pits in the bearing races that create a rough surface for the bearing rolling element to run over. In order for the shaft current to reach the bearing raceway to cause the welds, it must first pass through the lubricant of the bearings. This causes an over temperature of the grease, which leads to a lack of oil to prevent metal on metal contact of the rolling element and the raceway. The welds, combined with the poor lubrication, means the bearings are experiencing higher than acceptable vibration levels, therefore leading to higher temperatures. Electricity causes the mechanical damage in the bearing raceway and lubricant thus leading to premature bearing failure.
Every bearing has a different level of resistance. The amount of electrical resistance in each bearing determines how much shaft current ultimately is required before the discharge event occurs in the bearings. All electric motor radial ball bearings are very similar in their construction, but because they are manufactured within a tolerance range, they do vary substantially relative to electricity’s pursuit of the path of least resistance (OHM’s Law).
The primary component of electrical resistance in a bearing is the amount of grease and the quality of the grease. When radial ball bearings are manufactured (new) they are filled with a 20 to 30% grease fill. The higher the amount of clean grease, the higher the resistance. Other attributes that affect electrical resistance in the bearing are internal clearance in the bearing, amount of load being placed on the bearing, the type of rolling element, and the speed the motor is operating at. These vary in every application.
For more information, contact L&S Electric.