Pitting of a bearing race wall at regular intervals leads to a phenomenon called fluting. |
While a number of shaft-grounding technologies exist, they vary in effectiveness. One new system is designed to last the life of the motor. Engineered with special conductive microfibers, the Aegis™ shaft grounding ring from Electro Static Technology redirects shaft currents from shaft to frame (ground), bypassing the motor's bearings.
Bearing Damage
Serious electrical bearing damage can be attributed to high peak voltages and extremely fast voltage rise times associated with the high switching frequencies of modern VFDs. The higher the carrier frequency, the higher the discharge rate.
From its first minute of operation, a VFD induces voltages that build up on the motor shaft until they find discharge paths to the frame (ground). In most cases, the motor bearings present the path of least resistance. Once voltage is sufficient to overcome the resistance of the oil film layer in the bearing, shaft current discharges, causing electrical discharge machining (EDM) pits and fusion craters in the race wall and ball bearings.
Discharges through bearings can be so frequent that before long the entire bearing race wall becomes riddled with fusion craters known as frosting.
Since many of today's motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in VFD-controlled AC motors.
In the phenomenon known as fluting, the operational frequency of the VFD causes concentrated pitting at regular intervals along the race wall, forming washboard-like ridges. Fluting can cause excessive noise and vibration, and by that time, bearing failure is imminent.
Short of dismantling the motor, there are two main ways to check for bearing damage –– measuring vibration and measuring voltage. Neither method is foolproof. By the time vibration tests confirm bearing damage, it is usually far advanced. Likewise, the main benefit of voltage tests may be the relief they provide if the results indicate no bearing damage. If a baseline voltage measurement is taken right after a VFD has been installed, subsequent monitoring may provide early warning of harmful current loops, but there are many variables.
