Bearing Stands Grounding

[pennsy]

This question has been bothering me for 30 years. This was a Motor room. There was one 1000 hp synchronous motor, a 500kw generator(for main mill drive) and 2 250 kw generators one for each of the reel on a rolling mill. In 1972 we replaced the MOR's for Reliance drive paks, far cry from what they are now.
Any way we had to remove all the generators and the 1000 Hp sync. motor. and also the bearing stands there were 5 stands each stand was insulated from ground all except the last one.
I assumed at the time that it was a path for stray currents and/or eddy current/ noise to ground.
I have asked many people even Reliance Electric in Cleveland and even they had nt a clue why it was done this way.
the original Mill, Mcc and motor room was installed in the late 50's.
Any help would be appreciated to this old question that I have never found an adequate answer.

TIA
Wm.Colt

 

[BJC]

You can now specify motors with insulated bearings. I don't think you can with larger motors. When that plant was built that was probably the only way to effectively insulate the shaft and eliminate currents through the bearings. Do a search here or on google on insulated bearings and/or shaft grounding you'll find plenty of information.
Who ever built that plant were probably amont the first to run into stray ground currents in rotating parts, sounds like they were quite inovatative in solving it.

 

[pennsy]

Thanks BJC, for the reply. This is what I suspected but could never get a deifinitive answer. Years later I worked with the company who installed it, Reliance Electric and as I said no-body had a clue as to why it was done that way.
As you said it was a well built system, it certainly was. The only problem I remember with that system was a brush problem with the generators, someone had ordered the wrong brushes but once that problem was rectified it ran great and still is as far as I know.

Thanks Again
Wm.Colt