Why to insulate bearings on non driving end of an HT motor?
Stray and unsymmetrical fields in electrical machines can drive currents in a loop from the ground into the rotor, then along the rotor and back to ground. Such currents can enter and leave the rotor at the bearings and they can easily cause substantial damage to the bearings. Sometimes shaft grounding brushes are supplied to take these (and other) currents but it is good practice to insulate the bearing on one end to prevent this form of current flow.
