Hello, What causes electrical fluting in motor bearings? Is this an indication of weak insulation and or is it more of a magnetic flux issue? Also why does it only seem to effect only one bearing and not the other?
I know this is probably a pretty broad subject, but is there a simple explanation for this issue? Thanx Bean...

bean,
Depends. VFD involved? If so, you will get bearing fluting from high frequency transients capacitively coupling across the airgap and charging up the rotor. This is usually due to incorrect power leads between the motor and the drive. All VFD applications should have a cable with three power leads, symmetrically positioned grounds (3), all surrounded by either a braided shield or a copper film. This outer shield or film is the most important component and must be connected to the motor frame inside the motor terminal box AND the drive PE ground. This shield is reponsible for carrying the high frequency current back to the drive. Stranded copper DOES NOT work!!!!
Now, that being said, you can have circulating currents on a 50Hz or 60 Hz machine if the frame is over NEMA framesize, or it is one of the new 'long' low profile motors (in that case, over a 444 frame size). This motor size has the potential of a voltage potential difference from end to end on the winding, creating a current path. Since it is low frequency, insulating one bearing will correct this problem. Be careful. If one bearing has been insulated from the bearing bracket and then you put in a non-insulating bearing RTD, you have re-established the path! The same can be said for non-insulated tachs, etc. This problem can happen is AC as well as DC machines.