HomeFAQHigh frequency ground current issues in VFD applications

High frequency ground current issues in VFD applications

Motor Vibration Analysts that I have worked with swear that shaft/ground brushes are ineffective in addressing high frequency ground currents because they try to address motor bearing currents after these currents have already gone through the motor bearings. If the high frequency ground currents are being generated by the IGBT in the variable frequency drive, how can shaft/ground brushes (mounted on the motor) address these issues. I have heard that there are motor manufacturers who will not warranty motors that have ground / shaft brushes added.

They appear to work when retrofitted to existing problem applications (according to at least one motor shop I deal with).  The issue is rotor current, trying to find a route to discharge. And thus builds up to a point where it will arc through the bearings, to the motor frame (ground).

Where they have been fitted, both as a commercially available kit and as a kludged-together DC brush rack just bolted to the frame and riding on the shaft, there seems to be a significant improvement in bearing life.

The problem can be reduced as well with some filtering between the VFD and motor, be it a reactor, or a more significant filter.

VFD manufacturers and Engineers have stated the best way to address these high frequency ground current issues is by providing a high frequency low impedance path which these currents will always follow. This method will not only address the motor bearing current issues, but will also address sensitive device (interference) and arcing/sparking (safety), which brushes cannot.

As these pricey brushes wear, the microfiber brushes leave metallic deposits inside the motor which cannot help the life of the motor. Even if the brushes "appear" to work based on a single motor shop statement, users have claimed when they use brushes and the bearings still fail, most of these suppliers will provide you a new set of bearings. This does not "prevent" unplanned downtime which is the objective. No motor manufacturer warranties bearings. There is talk that some motor manufacturers discontinue their warranty when these brushes are added to their motors. Brushes do not seem to be the ideal way to address these high frequency ground issues.

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