Last year, the plant upgraded a large, critical motor system by adding a variable frequency drive (VFD). The main impetus for this decision was to stop the damage from across the line starts, an outcome that has since been verified.
Unfortunately, the motor spun its thrust bearing a few months after installing the VFD. In the post-mortem, the motor shop found severe pitting in what was left of the bearings. Grounding the motor housing didn’t prevent a recurrence. What should you do?
Grounding the motor housing serves no electrical purpose; bonding it and all other nearby metallic objects is your first line of defense, however.
Consider installing “grounding” (bonding) rings to drain undesired current from the motor shaft. Consult with the manufacturer for the correct sizing methodology; This is critical to achieving the desired result. Consider a split ring system, which you can install with the motor in place.
Put a power analyzer on the VFD supply and look at the power quality. If you find an especially bad waveform, contact the VFD manufacturer for a solution.
While the pitting indicates the flow of undesired current through the bearings, lubrication deficiencies might also be shortening bearing life. The grease can melt out if there is too much of it, or if dirt contaminates it.
