VFD's and bearing "Fluting" | Practical ...

Monarchist has a point. There is going to be current unless you can shield the coils.* What you want is to make sure the current is NOT through the bearings, but goes somewhere else.You send the current elsewhere by insulating the bearings, and then providing a drain for any charges on the rotor.With that, you will see current regardless, all you are doing is "steering it".As for the Rogowski coils, the usual power current pickup goes, if it is a good one, no higher than 10 or 20 kHz. I have a 150A Fluke clamp-on that does that, and a meter to match bandwidth. So even 5 or 10 MHz is very good by comparison. And a good high current 60 MHz clamp probe is both expensive, and limited in "window area". I have a 60 MHz pickup, but it is only good to an amp, with a very small window for wire.A Rogowski coil would be easier to use, but not lacking in its own problems.No turns ratio is involved. There is no end-to-end current involved, at least that is not the main issue. I'd have to think about it to see what it would take to provide a driver for a circulating current, it would need to be a transverse field, going in a circumferential path. It's probably possible, although the existing steel will tend to "short it out". Might be more of an issue with a larger salient pole machine. Coil imbalances maybe could drive such a field. Driving voltage should be quite low, where the capacitive currents have a very high voltage driving them.main issue is just capacitance from coils to rotor. Circuit is completed through the bearings.to ground, and back to the VFD.* Emerson, or maybe Nidec now, has a patent on one way of shielding, invented by a guy who was my boss at one point. It uses metal wedges in the slots, over insulation, of course. There are other ways, and the slot covers do not take care of the end turns, which also have capacitance.