VFDs and Motor Shaft Voltage

Variable frequency drives (VFDs) that are used to control the speed and output of motors can generate harmful electrical currents that flow through the shaft and motor bearings. Because VFDs are being used more and more frequently in HVAC, shaft voltage and bearing currents account for a growing number of motor faults.

VFDs are the most common culprits when bearing currents and shaft voltages occur. The high-frequency switches used in VFDs can cause currents that discharge across motor bearings. Though all motor bearings will always carry some current, excessive levels will lead to failure and decreased life span of the motor. Discharge current damage can also extend to the bearings of other equipment connected to the motor shaft, such as tachometers and gear boxes. When noise and vibration are present and excessive shaft voltage is suspected, a specialized device can be used to measure currents. Failed bearings damaged by shaft voltage will show visible signs of frosting, fluting, or pitting of the bearing race surfaces. Frosting is the usual indicator of damage from currents at low speeds, while bearings damaged at higher frequencies tend to show the more severe effects.

There are a number of prevention measures that can be taken to prevent shaft voltages. Installing a shaft grounding device, which safely channels currents away from bearings to ground, is the best way to reduce VFD-induced shaft voltages. With a shaft grounding device, a normal bearing lifespan, which usually ranges from six to ten years, can be expected. Insulated motor bearings can be installed to stop the flow of discharge current through motor bearings, but they do not prevent damage to other shaft-connected equipment, and in some cases can pose a risk of mild shock when the rotating shaft is touched.