VFD Shaft Voltage and Bearing Current
Bearing failure in motors, fans or pumps can put a building out of commission for days or weeks depending on the severity of the failure and the availability of replacement parts. Performing periodic dynamic balancing and vibration testing can greatly reduce bearing failure, but two phenomena that can cause shaft voltage are on the rise due to the increase in the use of variable frequency drives, or VFDs.
Bearing currents and shaft voltage occur in VFDs when voltage pulses couple from a motor’s stator to its rotor. When this voltage exceeds the insulating capacity of the bearing grease, voltage arcs or electrical discharge machining can occur, damaging bearings and causing premature motor failure. Voltage-damaged bearings will show frosting, fluting or pitting of the bearing race surfaces. Shaft voltage can cause damage to bearings in as little as one week.
Shaft voltage discharge can be measured using a shaft voltage probe and a shaft voltage tester. One of the best ways to reduce VFD-induced shaft voltage is to install a shaft grounding ring, which conducts voltage discharge currents away from the bearings to ground. A grounding brush is another economical way to provide a low-impedance path to ground, but these devices are subject to a number of drawbacks including excessive wear, contamination and oxidation buildup. Nonconductive ceramic bearings are fairly effective but costly. Insulated bearings are costly, as well, and may not prevent bearing damage in all cases.
Omni Instrumentation & Electrical Services, Inc. is a premier instrumentation and electrical contractor, providing superior total care solutions since 1986. Services include instrument installation, tubing installation, instrument calibration, control loop testing, startup and commissioning, power and lighting, process control wiring, BMS wiring, telecommunications and data wiring, fire alarm wiring, security wiring, process network wiring, and control panel fabrication. Omni Instrumentation & Electrical Services, Inc., is certified in Foundation Fieldbus, Profibus, DeviceNet and ASI-Bus installation.