What causes shaft currents in VFD driven motor?

There are about 6 different causes of shaft currents in Variable Frequency Drive driven motors. I can't remember them all but some are:- high DV/Dt, phase unbalance - shift in theoretical neutral point (poor inverter and lack of filtering), capacitive coupling. Many manufacturers recommend insulating the non-drive end bearing and installing carbon brush on the drive-end. If this option is to be used you need to ensure that the brush contact resistance is maintained at all times and there is no conductive path through the drive coupling. From years of experience I have found that the secret is to use high quality drives (more expensive ones with inbuilt filtering) and standard bearings with no brushes (lower long term maintenance cost). It can also make a difference to set the switching frequency to the low setting. This may not be possible in an office air conditioning system but in an industrial situation there is that much other noise that a little more does not matter. Increasing the switching frequency does not remove the noise, it just shifts it out of the range of the human ear.

Some year ago I was given a very good technical paper on this subject by a Danfoss engineer. However I have misplaced it at the moment.
If you take the option on using insulated bearings then make sure you have a spare motor set up that can't be used in other standard applications. Don't forget couplings to gearboxes and pumps or you will just shift the bearing issue to the gearbox or pump.