Generator bearing resistance

[401608]

Hello,
we are about to specify a 6,25MW 6-pole induction generator for a DFIG drive system. I know that shaft voltages and resulting bearing failures have been a great deal sometimes. Also I know that effective shaft grounding at both drive ends helps, but how shall the bearing insulation be done? Is it suffficient to state e.g. a 10MOhm bearing resistance, shall both bearings be insulated or better just one?

Who has experience or can probably recommend any expert in this field?

Thanks for any input and regards, Pat

 

[edison123]

At that size, I assume it's white-metal bearings. The question is whether they are mounted on the generator frame or they are supported independently on separate bearing pedestals.

In the former case, the bearing manufacturer would insulate the bearing housing (normally Teflon coated) so that bearing is isolated.

In the latter case, an fiber glass insulation pad of about 10 mm thick is placed beneath the pedestals and the fixing bolts are insulated along with fiber glass washers. Also, any external oil lines have to be isolated from the bearing reservoir to by way of insulated bolts and washers.

A 10 Megohm resistance is ample for such insulation.

As for insulating one end or bot ends, hopefully Gunnar (Skosgurra) would chip in since he is deeply involved in EDM in bearings.

 

[401608]

Dear edison123,
thanks a lot for the information. The bearings are integrated inside the generator case (no pedestals). My feeling is that insulating both bearings is the method of choice: If you let one bearing be conductive you'll never know if a current loop could build with shaft external equipment (coupling, GBX or slip ring unit, encoder etc.). However I can not backup this view with facts.

Best regards, Pat

 

[sundep]

What is effective shaft grounding?
We have used carbon brushes and slip rings on VFD motor shafts with limited success. I suspect that the problem here is that the carbon brush will not conduct until there is a 0.5 volt differential accross the brush and the slip ring, but the grease in the bearing will start to conduct at 0.3 volts. I don't have facts and data to support these assumptions.

 

[Zogzog]

The manufacturer should have published data on the minimum bearing insulation resistance.

 

[edison123]

Hi Pat

I have seen ABB motors with frame mounted sleeve bearings with both side bearings insulated. They were ring-oil lubricated with no external oil supply. They also had a grounding brush at the coupling end.

I think Gunnar is enjoying his holidays and hopefully he will chip in soon.