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newvibe

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Reply with quote  #1 
Hi guys,

As motor's bearings damage due to electrical fluting is getting more common these days. What is your prefered solution(s) for a system that is known with electrical fluting?

Our current solution is insulated bearings at both ends of the motor. Pros: bearings can be replaced and equipment back online in one day. Cons: expensive. Also, the shaft current has no path to ground.
electricpete

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Reply with quote  #2 
The thin coating of insulation on insulated rolling element bearings may not help if the motor is fed from a vfd.  That high frequency components are easily coupled across the capacitance of that insulation. 

If bearing insulation is the preferred solution and you're dealing with a vfd, you have to insulate both bearings (insulating one does not solve the problem).

If you are just dealing with a large motor fed from grid frequency and trying to interrupt the circulating 60hz current, then insulating the outboard / ODE only is sufficient and we have many like that in our plant.  BUT you'll never be able to test that insulation (because you'll always be grounded through the other bearing). So even for large motors fed from grid frequency I prefer both insulated instead of ODE-only insulated. 

Of course shaft grounding is a whole 'nother option. 

There is a lot that can be said about the subject and I'm sure others will add on.  It would help focus the discussion if you mention the particulars of your installation. 


OLi

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Reply with quote  #3 
If you buy a VFD rated motor from the big Blue motor supplier (n Sweden) and big expense. Basically the only you get that I was able to figure out was a isolated bearing in NDE side..... but may be wrong but that is what I see so far IRL. 
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newvibe

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Reply with quote  #4 
Most of our motors are fed from VFDs.

Yes, large motor companies often supply motors with an insulated bearing at the NDE. As Oli said, I found having only one insulated bearing/shaft grounding device at the NDE will only protect that bearing. The other bearing will still get damaged and that is why once I found the motor bearings are damaged due to electrical fluting, I will replace both bearings with insulated bearings.

Some companies recommend shaft grounding at one end and insulated bearing at the other end. Some use a common mode filter/choke.

I have never tried a common mode filter/choke. Has anyone used this before? any success story? Does it work on its own or you still need to add insulated bearings/shaft grounding along with it?

What I am trying to find out is if you encounter a motor with electrical fluting from being fed from a VFD, what would be your recommendation(s) to minimise the chances of those bearings getting damaged by electrical fluting again? 
jvoitl

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Reply with quote  #5 
Depending on the type of coupling used if you use only insulated bearings to protect the motor the problem may move to the bearings in the driven machine.  That voltage on the shaft wants to go to ground somewhere.
Alex

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Reply with quote  #6 
Quote:
Originally Posted by jvoitl
Depending on the type of coupling used if you use only insulated bearings to protect the motor the problem may move to the bearings in the driven machine.  That voltage on the shaft wants to go to ground somewhere.


I have doubts about that because as I see the purpose of isolated bearings is to prevent (reduce) shaft voltage
Big Al

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Reply with quote  #7 
We had a DC motor many years ago experiencing fluting. Insulated bearings were fitted and the current passed through the grid type coupling into the gearbox, causing fluting on the flanks of the gears.
fburgos

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Reply with quote  #8 
insulated couplings are enough right?.... elastomeric/plastic or plastic elements? grounding shaft at driven machine is a common thing?

in our plant we have many VFD but most motors have coated isolated bearings or ground brushes... on a 1500hp pump the end shield had insulation and brushes with regular bearings.
OLi

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Reply with quote  #9 
I had a case tracing isolation all the way from 1 motor bearing to 2 isolated motor bearings to the gbx bearing via the steel coupling so yes, electrons are lazy as I am and follow the path of least resistance, no isolating bearings do not reduce the shaft voltage just isolate so it maybe will dissipate slow and not arc but I don't know. I have seen this myself and followed the brg damage and done the measurements and made the brg swaps to be done. 
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Dave Reynolds

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Reply with quote  #10 
This product reduces voltage spikes that drive the circulating currents at the source. It is not necessarily the fix all, but this will also protect wiring to the motor and the motor bearings.
It takes several tests to determine what is driving your fluting issues and it may also take several technologies to eliminate the problem 


http://www.coolblue-mhw.com/

http://www.coolblue-mhw.com/downloads/

http://www.coolblue-mhw.com/installation/

Dave
OLi

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Reply with quote  #11 
I have not read the details, but  if  the VFD is CE compliant it give no spikes in VFD end it gives a HF noise, this in my book charges shaft as a capacitor until it arcs thru the bearing load zone giving a spark machine machining of the bearing surface... so you can isolate it, filter it, ground it etc even conductive grease and lower resistance spark gap but you cant take away sparks in the wrong end of the system that does not exist in my presumption, may understood wrong about the principle, excuse me in that case but this is how the process is described in my book..... If VFD is none CE compliant it is another story and all bets are off.
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jvoitl

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Reply with quote  #12 
I have doubts about that because as I see the purpose of isolated bearings is to prevent (reduce) shaft voltage

The purpose of insulated bearings is to prevent shaft voltage from finding ground thru the bearings and thus creating the fluting, not to prevent shaft voltage which is going to be there regardless.
OLi

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Reply with quote  #13 
In this case I see the shaft system as a capacitor you charge and the isolation of whatever kind, I even saw a optimist using paper.... (didn't work) as another added dielectric in the capacitor and depending it may work, it may be scratched and not isolate etc etc it may be expensive composite ball bearings that give extreme hi level of isolation at a cost... Still the hi voltage charged in the shaft system will remain until discharged, leaked or grounded out. If you go the other way with grounding brush, conductive grease with carbon in it etc etc. If it discharge thru the bearing depend on if there is a path that way and the way of least resistance, electrons are as lazy as I am and may go thru the grounding system if it's good. Only my 1 SEK.
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Alex

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Reply with quote  #14 
I have found something interesting here:

https://sixthelementaustralia.com/


OLi

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Reply with quote  #15 
I have seen the movie........ I am soo Green and yes it's Friday now, sorry could not help myself.
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