Bearing stands grounding

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wocolt

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Ohio
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 reels 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 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
 
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wocolt

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Location
Ohio
Re: Bearing stands grounding

Thats probably it, I remember when we took everything apart we had to take the roof off the motor room in order to get the Sync. Motor and generators out, and that took a day and a half with a large crane.
The frames of the generators were almost 2 inches thick of cast steel/iron and the field coils were wrapped with flat silver wire.
But what was really amazing was the accuracy of the whole machine, the main control before the new drive paks were installed were the Motorized Rheastats the things looked like a clocks mmounted on a huge slate board and would move when tension or speed control was added.
The whole control was a system of relays and vacuum tubes and it was dead on.
If there was a 'Coble' the whole machine could darn near stop on a dime.
The thing about a coble and stopping the mill was this was steel and when it did stop the steel was folded up raised up to 10-15 feet and it looked like 'ribbon candy'

Just amazing

Thanks for the reply
Wm.Colt
 
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rickcham

Member
Re: Bearing stands grounding

WOCOLT

The reason the motors and generators bearings are "isolated" is for stray currents that will build up on the shaft from the generator excitation system, There also should have been a ground brush to allow this the leak off of any stray currents to ground.
All generators have the isolation on bearings, usually a mica material to prevent the electrical pitting that could take place between the shaft and the bearings.
It is still used even today on all generators
 
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