Mark:

I had the opposite situation of having all of them grounded but one. It was at the exciter. This was the atypical arrangement for any m-g set coupled with "hard" or rigid couplings (in deference to couplings using non-metallic elements). Only one bearing or stand in the configuration is insulated. The reason, of course, is to reduce "circulating currents" which build up on the machine shafts and then Discharge (capacitance effect) between the shafts and bearings or seals, causing
Deterioration of the babbit (sleeve), roller or ball (anti-friction) surfaces.

Only one bearing or stand is insulated, because this prevents the currents from building up enough voltage to discharge or "spark" between the shafts and bearings. The lower level is then conducted safely to ground through the grounding brush assembly. Insulating more than one point would be counter-productive because then the currents are permitted to circulate and build up a larger charge which either finds its way to an uninsulated or poorly insulated point and discharges.

Remember that the (all) metal couplings make a common shaft for the entire assembly. Oil and grease are not normally conductive to electricity, but the high discharge levels ionize a momentary path through the thin layer of lubricant to reach the metal beyond.

Individual motors usually insulate the OPE (Opposite Shaft End) bearing only. Even if no accessory grounding is used, the lower discharge levels in 480vac induction motors erode the bearings at a very slow rate. At medium AC voltages, and (surprisingly) DC voltages above 240vdc,some accessory grounding must be used.

This is from a discussion I had with Jim Norris from Joliet Equipment and Thomas Sohre from Sohre Turbomachinery Inc.

Carl


Carl Lee Tolbert
Technical Support
AC Drives
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