Bearing Insulation Protection 2

[smrodriguez4]

Hi all,

I have a very generic question and I was hoping you guys could help me. Today we just changed the bearings out of a 1375 HP Squirrel cage motor and noticed that none of the journal bearings had any sort of insulation protection. I am more mechanically inclined than electrical but all the motor's I have worked before had some sort of layer that worked as an insulation protection; either fiberglass looking or a sort of teflon tape.

Looking at the fit between the housing and the bearing, it looks kind of hard to get insulation protection in there without machining the bearing or the housing. I guess my question is, is there any rule out there that dictates if your bearings need insulation protection or not?

Thank you!

 

[macmckim]

smrodriguez4

There are number of factors why your 1375HP does not have any sleeve bearing insulation
- age of motor, if over 10 years old, was not important at that time
- if motor is operated from utility power, would not be required, unless your company has requirement in company specifications.

How is motor powered? Utility power or VFD powered?

As for having motor overhauled and installing bearing insulation - you really need to discuss with motor manufacture.
Other question - are the bearing being damaged by shaft currents? If no damage, why modify bearings?

Mac

 

[smrodriguez4]

Mac,
- This is a Louis Allis Armor Line motor, I don't know the exact manufacturing date but it is definitely more than 10 years old. I keep thinking about the issue, would it be that the oil film in the bearing would work as an insulator?
The motor is not operated through a VFD May I ask, why would insulation protection be needed for a motor running out of a VFD and not on utility power? I am assuming that a VFD causes some sort of current in the shaft that utility power wont but I am just a mechanical guy speaking non-sense.

The bearings we replaced did not show any damage by shaft currents,.I was just asking to inform myself about why most motors I have seen have it and why some don't.

Could we install a grounding brush just to be safe?

Thanks for taking the time address my questions.

 

[itsmoked]

The high frequencies generated by VFDs generate currents that can EDM (Electrical Discharge Machine) the bearings as the current uses them as conductors out of the rotor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

There are few other possibilities, depending on the motor construction and what you are trying to protect against. One technique used on large machines is to completely isolate one bearing pedestal from the frame to eliminate axial shaft currents, typically with a glass-resin composite spacer. A shaft-riding brush is often used on large synchronous machines to prevent windage-induced static charge from building up and eventually punching through the oil film. Capacitive current at high frequency resulting from the use of variable speed drives requires slightly different mitigation because the current is continuous and usually of a higher magnitude than for static drains.

 

[electricpete]

Motors driven from vfd's certainly require bearing insulation as discussed.

But also bearing insulation was required long before vfd's for large enough motors. The cutoff in horsepower is not clear but if the motor is large enough to require segmented laminations (cannot be cut out of a single sheet) then it will almost certainly need bearing insulation because variations in gaps of segmented laminations will inevitably introduce asymmetries that lead to circulationg currents.

Why didn't you see it? At least three possibilities:
[ul]
[li]The motor may be small enough to not required segmented laminations (not sure)[/li]
[li]It may be only at one end (the non-drive end).[/li]
[li]As Scotty said there are multiple ways to provide bearing insulation[/li]
[/ul]

=====================================
(2B)+(2B)' ?

 

[jraef]

Our member Gunnar (skogsgura) is kind of a resident expert on that issue. He used to publish an excellent presentation on it, but last time I looked for it on his website, I didn't find it. Hopefully he sees this in his travels and hooks you up with it, his description of the phenomenon is one of the best I have seen.

One of my favorite parts in that presentation is his use of the word "stochiastically" correctly in a sentence. For a non-native English speaker, he exhibits an impressive command of our sometimes difficult language.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[macmckim]

Hi smrodriguez4

An Louis Allis 1375 HP, Armor Series - it will be more than 20+ years old. Louis Allis Milwaukee Co went of of business about 1998 or so,
great products, high quality, rotor designs ahead of their time. I have some old catalogues in my paper files.

As noted by ElectricPete - if motor was a segmented there is possibility of stray shaft currents be created.
Motor of this size has single lamination for stator and rotor (double strike lam's die)
If you have done a recent over-haul, repaired the sleeve bearing (sounds like you had spares brg shells for this motor)
leave it and it will go for another 20+ years. Its a tank, one tough machine.

Mac