Generator bearings Insulation 1

[NCTHAI]

It is noted that Generator bearings (Only one) is insulated.

- Why generator bearings are insulated?
- What is minimum acceptable insulation resistance for operation?
- Can the bearin IR measured on line? (when generator is running)
- If yes, is it possible that IR value may differ for on line and off line conditions?
- Can someone suggest papers, technical information for bearing insulation fndamentals?
- What is maintenance practice for bearing IR?

 

[NCTHAI]

This question is for hydrogen cooled mid size generators.

 

[byrdj]

the inboard bearing should have a grounding brush/strap.
the outboard is insulated. this is to prevent surface voltages on the forging (journals) from circulate (current) and arc through the thin oil film. Besides the bearing, the H2 seals and oil deflectors should be insulated too.

I should remeber, but can't now, but thinking 50K was minimun for operation, normally 5M during assembly. If value is low, measuring shaft voltage for the outboard can be monitored (assuming you know what this voltage was with good insulation)

For the manufactor I worked for, the larger (more expensive) units had double insluation. that was a floating ring between inner / outer ring of insulation. This ring could be monitored. there were two termenals on the end bell, one for seals other for bearing ( I like to add one for the outer deflector).

The smaller units only had one layer of insulation, so to check during assembly, the rotor had to be seperated from the dry seal rings and lower bearing babbit. Only when in service could the insulation be measured. even with good insulation, wet lube oil could then cause a false low value.

there should be something in the instruction book, but I don't remeber how detailed that info was either.

as far as maintenance, I could be available for the inspection . care must be given to the cleaning process and any fluids used in the process (alcohal absorbs water and can be bad). I also like using a 2 part epoxy paint that covered the metal surfaces so oil contaimanted couldn't bridge the inslulation sheet

That should be enough for now

 

[petronila]

Hello NCTHAI,

Some Winding Generator designs could produce shaft circulating currents due to assymetries in the magnetic designs and problems with the eddy fields formed in the slots. So the bearings most be insulated in order to prevent this shaft current circulations-

The sleeve bearings are insulated with insulated strap and in some small generators you coul find ceramic or hibrid bearings.

Regards

Petronila

 

[RRaghunath]

byrdj,

Good explanation. Star for you.

Coming to IR value limits - In India, the practice is 80kohms - Alarm and 5kohms - Trip