Stray Current = Ruined Bearings

[iwire]

I was at a facility today aiding in some testing.

The facility has a group of four - 500 HP motors driven from VFDs. The problem is that the motor bearings are going bad very quickly.

GE took a motor back pulled it apart and determined that the bearings are 'fluted' which they say is an indication of current flowing through them. What has been found so far is 8 amps flowing through the EGC out to the motor.

I will try to describe the set up.

There is a dedicated 2000 KVA transformer feeding a 2500 amp 480 volt switch board, from the switchboard four feeders run to four Delta auto transformers then to the four VFDs finally out to the four motors.

The motors are outside and each motor has a 4/0 bare connecting it into a common grounding grid.

When an amp clamp is placed on any of these bare 4/0s and any single motor is run up we see 8 amps.

When I lifted the EGC from the motor that runs back to the VFD we lose the 8 amps.

Anyone have some ideas?

We are thinking induction as all the feeders on both the line and load side of the VFDs share a cable try system.

Any and all comments welcome.

 

[hardworkingstiff]

Re: Stray Current = Ruined Bearings

How are the VFD's grounded?

 

[pierre]

Re: Stray Current = Ruined Bearings

Hello Bob


"When I lifted the EGC from the motor that runs back to the VFD we lose the 8 amps."

1. Did you lift this when the power was on? - if so did you see an arc?

"The motors are outside and each motor has a 4/0 bare connecting it into a common grounding grid."

2.Is this supplementary grounding? Is it bonded to the electrical system?

If not, maybe there is a parallel path that the current is flowing on.

why don't you try to leave the equipment grounds intact, and lift the 1/0 GECs - see if that has any result as to the 8 amp current flow.


Just thinking out loud.

 

[iwire]

Re: Stray Current = Ruined Bearings

Originally posted by hardworkingstiff:
How are the VFD's grounded?

Each feeder has three 1/0 CU EGCs

From the MCC to the VFDs are 3 sets of 600 kcmils each with 1/0 EGCs.

Same running out to the motors from the VFDs.

 

[iwire]

Re: Stray Current = Ruined Bearings

Originally posted by pierre:
Hello Bob


"When I lifted the EGC from the motor that runs back to the VFD we lose the 8 amps."

1. Did you lift this when the power was on? - if so did you see an arc?

Yes it was done with power (and gloves) on.

No arc seen however it was bright sunlight.

Originally posted by pierre:
"The motors are outside and each motor has a 4/0 bare connecting it into a common grounding grid."

2.Is this supplementary grounding? Is it bonded to the electrical system?

Yes it is an elaborate grounding grid and the ties into the 2000 KVA transformer.

 

[tom baker]

Re: Stray Current = Ruined Bearings

Last year I was at a class given by a large HVAC contractor. They install a lot of VFD,s and were having problems with bearing failures due to circulating currents. They found a retrofit device that prevents bearing damage, however I don't recall the name or manufacture

 

[hardworkingstiff]

Re: Stray Current = Ruined Bearings

It sounds like you are saying the current is coming from the VFD's. Is that correct?

 

[ccjersey]

Re: Stray Current = Ruined Bearings

see this link
http://www.eng-tips.com/viewthread.cfm?qid=102205

pretty much beat it to dust over there

Jim

 

[iwire]

Re: Stray Current = Ruined Bearings

Originally posted by tom baker:
Last year I was at a class given by a large HVAC contractor. They install a lot of VFD,s and were having problems with bearing failures due to circulating currents. They found a retrofit device that prevents bearing damage, however I don't recall the name or manufacture

I don't know if this is the same device but one possibility is installing ceramic (non-conductive) bearings.

However the EEs feel this is a not a fix but a band aid. They want to find the source of this current.

We had to stop today as the testing companies recording equipment quit working.

 

[hardworkingstiff]

Re: Stray Current = Ruined Bearings

I just did a quick web search, and this appears to be a recurring problem.

 

[iwire]

Re: Stray Current = Ruined Bearings

Yeah after looking at ccjersey's link it seems we are not alone.

The EEs also talked about grounding the shaft.

 

[pierre]

Re: Stray Current = Ruined Bearings

ccjersy's link to the forum has a lot of interesting discussion.

one thing that was mentioned was using ceramic bearings - but that could divert the current from the motor bearings to the pump bearings.

They do mention other remediation - did you read any of it yet?


BTW - that link can really bring a person to reality as to how complicated this industry can really be

 

[iwire]

Re: Stray Current = Ruined Bearings

Originally posted by pierre:
BTW - that link can really bring a person to reality as to how complicated this industry can really be

Yes and I found that true as the least educated of the group working together today.

These motors are spinning fans, smoke exhaust fans actually.

The fan shaft is isolated from the motor shaft by the rubber in the coupling.

 

[hardworkingstiff]

Re: Stray Current = Ruined Bearings

Originally posted by pierre:
BTW - that link can really bring a person to reality as to how complicated this industry can really be

Yea,

The more I learn, the dumber I feel.

 

[iwire]

Re: Stray Current = Ruined Bearings

For what it's worth a 500 HP motor spinning a very large (8' 9') fan does not sound good running on fluted bearings.

 

[oliver100]

Re: Stray Current = Ruined Bearings

I had similar problem in a power plant. The generators (12 MGW) had sleeve bearing and the oil was loosing its lubrication capabilities due to a stray current (looping the rotor shaft, the bearings and the metal foundation). We solved the problem by installing insulation plate under the foundation of one of the bearing (the loop was broken).

In the case of 500 HP I would suggest similar measures - non conductive cover gasket and not conductive bolt sleeves and washers to break the loop.

 

[benaround]

Re: Stray Current = Ruined Bearings

Bob,

I've got to assume this is a 3 phase 3 wire ungrounded system (no grounded supply conductor).If this is true,then you can at least eliminate ground loops. On the other hand if it is a 3ph.4wire with some L-N loads I think loops would be present.But why the bearings???

 

[jim dungar]

Re: Stray Current = Ruined Bearings

Circulating currents in motor bearings is a known problem with variable frequency drives. It is rare but it does occur. Contact the drive manufacturer as well as the motor manufacturer. Be ready for lots of finger pointing. Remeber, normally, it is not "someone's" fault as much as it is just bad luck.

 

[tony_psuee]

Re: Stray Current = Ruined Bearings

Bob,

I agree with Jim about contacting the manufacturer of the VFD. Prior to doing that though I would check the installation wiring for compliance with the recommended wiring instructions for the VFD's. Most of the VFD's I have dealt with require the motor ground to be landed on the VFD first, before going to any other grounding point. Also, are all of the VFD line and load wiring in a common cable tray, or are they line and load for each VFD in a cable tray separate from the others? Again, almost all VFD's I have dealt with will state not run wires from multiple VFD's in the same wiring system.

Tony

 

[iwire]

Re: Stray Current = Ruined Bearings

Thanks everyone for their suggestions.

I am just a small part of the group that is working on this problem. Actually my job is to perform the lock out tag out and open and close electrical enclosures so the 'brains' can have a look. with a quick solution.

After reading the replies and the links it sounds like this is a problem with no quick solution.

The EEs involved want the source of the current to be identified and eliminated. From what I gather that is not possible and that the best idea is to simply isolate the bearings and live with the current flow.

Again thanks to all and if anyone has more info I would be glad to hear it.

By the way the 'finger pointing' started along time ago, I think maybe the goal of this exercise may be to determine where the finger should actually point. :roll: