For large electrical motors designed for VFD operation we often install insulated bearings in order to avoid bearings being destructed by currents circulating through the shaft and body of the motor.

We often assume that these currents are caused by harmonics in the supply current, but what can be the mechanism behind creating these currents?

Is it simply some noise, or do the harmonics cause some parasitic Homo-polar or Uni-polar generation effects due to geometric differences in the magnetic circuit at the two ends of the machine, or are there other explanations?