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Joined: Nov 2001
Posts: 280
motor-T Offline OP
Anyone familar with DC generators ? Years ago I worked for a Mill that had a Rolling Mill. The Mill had two reel motors at 250 HP and a main mill motor at 500 Hp. In the motor room was all the drives.
Set up on bearing stands from left-to-right were the generators in this order-250kw-250kw-1000hp syn, mot- 500kw-, respectively the 250kw generators were for the reel motors the 500 kw for the main-mill motor, the prime-mover was the 1000 Hp syn, motor, 2300volt @ 250 V DC synch. the synch speed was 1800 RPM.
The question is this, in all there were 5 bearing stands to support the MG set, when disassembled we had found that the bearing stands were insulsted from ground except one. Since then I have talked to Reliance Electric and GE who use to be big in Mill MG sets much bigger than these and nobody has a clue why only one bearing stand was grounded,the only thing I can come up with is stray currents, hysteresis losses etc, I am still not sure why.
Has anyone else seen a situation like this and why the bearing stands are set up like this.

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Joined: Aug 2001
Posts: 31

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.


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motor-T Offline OP
Thank you so much for the reply I have been looking for the answer to this for about 29 years now, its sorta what i suspected, but nobody would ever confirm it.
Ours was a Good size MG set in tandem on a common shaft when coupled together and was made by Reliance Electric out of Cleveland, When I first started working there all the control was done by MORs and then was changed to SCR drive paks and solid state, but the MG set never changed, what amazed me was the fields were wound with flat silver wire, we had to replace one, then, and the material 29 years ago was $5500 for one single field coil I can imagine what it would cost today.
Now you mention all grounded save one, now ours was the opposite because I remember replacing the Mica and plastic shims on 4 and the last one used brass and even the mounting bolts had to be insulated and they were 1 1/4 hex bolts, but I suppose both ways make sense.
Just love this field.
Thanks again -Mark-

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