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VFD parasitic capacitance and bearing currents

Reducing variable frequency drive parasitic capacitance and bearing currents is a complicated problem with a lot of variables. If you use insulated bearings and don't have a shaft grounding brush things may get worse. The vast majority of 500 hp and smaller motors running on VFDs have no bearing problems. It is more likely to have bearing problems when driving a centrifugal pump where there is a poor ground through the impeller or fluid being pumped.

There is a lot of discussion and research being done on this and so far no one has a definite solution. Shaft grounding with brushes seems to be effective but can be hard to implement. Part of the problem is that rotors are not identical from manufacturer to manufacturer so the frequency that produces a high parasitic capacitance changes. A solution on one motor might not work on another of the same hp.

My advice would be to read the various white papers and in depth discussions on sites. A lot of reading but very informative.  A good start to the bearing problem on a retro fit drive is to ensure good return earthing with flat copper braid or strip between motor and drive.

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