Mechanism of electrical fluting

SKF's take on fluting, not necessarily the best as far as the question.

http://www.skf.com/portal/skf/...&newlink=5_15_020700

In layman's terms..... Welding SS and raising carbon. Welding with carbon electrodes or burning with them. Great path for electricity without being consumed as rapidly as the consumed material.

In this instance the path to ground raises an instant of heat raising carbon. This newly created bump in the road provides a source of least resistance for the electrical path thus removing the random path causing recurring path to ground at the same location hence fluting.

This is how I've viewed it.......... naturally, I could be wrong ;-P

It's a good question. I don't know the answer. I think someone knows the answer.

Attached is excerpt from section 6.4 of "Solving Tribology Problems in Rotating Machines".

You can see the discussion wanders from electrical to thermal to metalurgical (tempering) to chemical (corrosion) and beyond. In the end, it is apparently understood so well that the pitch and width of the corrugations can be predicted from bearing parameters using equations given.

Does the discussion make sense? Hmmm. Let's just say it's an electro-chemical-metalurgical-tribological phenomenon.

"electro-chemical-metalurgical-tribological phenomenon"

Betcha can't say that five times fast!

quote:

"electro-chemical-metalurgical-tribological phenomenon"

Right up there with the catch-all diagnosis of "mechanical looseness" that was so commonly seen as a potential cause of virtually every vibration problem; well, at least by one vendor who shall remain nameless.

Does anyone know someone in the civil/road construction field? They might have the answer. For any of us that regularly travel unsealed dirt roads we will often come across corrugations on the road surface. there could be a link that has nothing to do with electrical arcing, but more to do with the raceway/road surface becoming fluid on a microscopic scale, compaction from the bearing roller/car tyres, etc
Just thinking out loud.......

Nok

Washboarding of roads has to do with the natural frequency of cars suspension systems being fairly consistant among the various types.

I'll check out the articles you have all been so kind to supply but I liked Sam's simple explanation

Now I sound like most of my clients. We want a non-technical answer to a technical question.

As for roads, asphalt roads seem to have a subtle washboard effect even when new. I've always wondered if the vibration of the large steel drum rollers (compactors) has something to do with this.

Maybe fluting works something like the resonant rubblizer shown in these videos..... probably not, but these things are so cool, and since we got off onto roads....

Rusty,

I recall seeing a video on the subject of washboarding some time ago and it is the same thing with asphalt roads and dirt roads. It gets worse at stop lights where the load from breaking goes up.

Or it could be a prolific prankster with a rubblizer.

Gentlemen,
Past examinations of fluted bearings do indicate one thing:
If the bearing came from a machine operating at just 60 Hz (low frequency circulating currents), the fluting pattern is very distinct and clear.
If the bearing came from a machine operating on a vfd, the pattern is blurred. I have attached one example of each.
The third picture is the beginning of the pattern. As you can see, it looks random, but it is obviously happening at given intervals that have to do with the charge up time of the rotor/inner race/ball and the speed of the rotor.
Enjoy!

Hi everyone ,
there are many EDM cases about VFD motors,the attached is one of them .

Although that is a motor with VFD, whether could that bearing be damaged by low frequency shaft current ?

I appreciate your comments !

sb,
Definitely EDM. Good pics. The method that charges up the rotor (high freq. current) and the circulating path (low freq. current) are very different, however, the discharge path physics are the same. You can only distinguish which one it is by reading the shaft current with a Rogowski Coil.
Besides, this is like asking,'I have a motor with a very bad bearing, but I don't know if it's the DE or NDE.' What's the differnce, you should change both bearings anyway.
Same here, fix both problems at once. VFD cable for the high freq. current and NDE insulated bearing for the low freq. current.
Ron

quote:

t it is obviously happening at given intervals that have to do with the charge up time of the rotor/inner race/ball and the speed of the rotor.

How did you conclude that electrical discharge time controls the pattern rather than other mechanisms?

ep,
I hate to wander over into your side of the technical pond. I may have overstated the obvious for my case. It is the electrical discharge that caused the high amp arc that burns off the metal.
I wasn't talking about the time of the discharge, but the time between them.
Ron

Thanks Ron.

I did realize you were talking about charging time rather than discharge... not sure why I wrote discharge.

Good pictures and good thought to consider what we might learn from difference in fluting appearance on various machines.

Have you (or anyone) ever seen a fluting pattern on non-vfd induction machine? I know from the books large machines are susceptible to bearing current resulting in some kind of bearing damage if insulation breaks down, but I sure have never seen any fluting pattern myself on our large non-vfd induction motors with rolling element bearings.

quote:

Have you (or anyone) ever seen a fluting pattern on non-vfd induction machine?

Machine train with 2500-hp motor driving 3-gearboxes and 4-blowers with a total of 7-couplings. Motor was not VFD, and it had sleeve bearings. One of the gearboxes adjacent to motor (metal grid coupling) had AF bearings with fluting damage. Vibrations were so high (over 70-g)that accelerometer with flat rare earth magnet was skiding down the face of the gearbox case!

Walt

I've seen it in plenty of dc motors but I think Pete might have been asking specifically about ac induction motors with across the line starters.

Titleist had two (200-ish +/- HP maybe?) DC motor driven mixers to mix and knead the yellow golf ball rubber. In color it was Not unlike the "egg" I get in my McMuffin. I can't speak for the taste or texture.

One day one after running several years one of the motors decided to start fluting one or the other of its ball bearings. Our PdM program caught it a few times. It may have gone away but I do not know why.

quote:

I think Pete might have been asking specifically about ac induction motors with across the line starters.

Yep, that 2500-hp was AC induction with across the line starter!

Walt

EP,
Check the motor repair specification for you facility. If it doesn't call for a check of the NDE bearing insulation during recondition, it should.
Regards,
Ron Brook

Yes, of course our spec requires that. We also have some non-vfd induction motors with both bearings insulated and we check insulation resistance of those ourselves on-site (where only the outboard/top bearing is insulated, we can’t check without removing the uninsulated inboard/bottom bearing).

I have not seen this fluting pattern among our motors (the ones I work with are all non-vfd large induction motors).

I can form a plausible explanation for why this pattern would be uncommon for large non-vfd induction motors. It requires two independent “problems”: 1 - breakdown of insulation; 2- magnetic asymmetry.

But plausible doesn't necessarily mean it's the correct. I could argue the other side:
1 - The operating condition of the bearing insulation is not perfect. We have found more motors below one megaohm than over. And I have seen two motors whose bearing insulation resistance looked like a dead short using a megger. (never checked with Fluke multimeter, but that would have been interesting).
2 - Large motors tend to have a built-in asymmnetry when they have segmented laminations. Small differences in the inevitable gap between segments creates unbalance.
3 – There tend to be many more large non-vfd induction motors than vfd motors (at least that’s the case in the power generation industry, not as much in other industries). If the pattern is the same for these non-vfd large motors (when bearing insulation breaks down and magnetic asymmetry is present), then you’d think there would be some more common appearance of this pattern. Until Walt’s message, I don’t recall ever hearing this pattern mentioned for induction motors.
4 - Even where we have the unfavorable conditions of malfunctioning insulation and magnetic asymmetry which creates potentially destructive bearing current, are we sure it would look the same on induction motors as on vfd motors? Or maybe it would just look like pits on induction motor bearings (those I have seen... in fact saw some small pits on one of those same motors that had bad bearing insulation).

So to help settle my question, I asked for observations. Differences in occurrence and pattern may gives some clues to the mechanism as you suggested initially. It is also of interest to me to know if our motors are susceptible, regardless of what it tells us about the mechanism for fluting. So far in this thread, I have heard one sync generator (Ron) and one induction motor (Walt) with this pattern. I guess we might conclude it’s possible on those machines, but rare. Just as a further data point for my own curiosity, are there any other non-vfd induction or ac machines where someone has seen fluting ?

I didn’t think to mention dc motors. Those are not my responsibility. If the appearance is different on those, might also be some clue about the mechanism.

One other point:
Very common in large DC machines and usually occurs after reconditioning, where either the NDE bearing insulation was not re-established or a non-insulated bearing RTD or tach was re-installed.
EP- I am talking about non-VFD motors for the inclusion of the bearing insulation test.
I agree, the power industry is not a good database for vfd applications. I will also try to rack my brain for non-vfd fluted bearings.
Ron

Yes, our motor repair spec requires test of bearing insulation for all large motors (doesn’t matter if vfd or not). I don’t understand why it is even a question. I have never heard of a large motor repair spec that didn’t require bearing insulation resistance test:

• EPRI 100987 = Medium Voltage Motor Repair Spec, Rev 0 (section 10.2.4 addresses bearing insulation resistance test).
  
• EPRI 1016679 = Medium Voltage Motor Repair Spec, Rev 1 (section 6.5 addresses bearing insulation resistance test).
  
• EASA AR100-2006 – Recomended Practice for Repair of Rotating Electrical Apparatus (Section 4.2.9 addresses bearing insulation resistance test).

EP,
Well then why do you think that is part of the spec? I am missing something about why you think EDM in induction motors is not an issue.
Regards,
Ron

The motivation for my question was explained in some detail above. You’ll note I never said anything resembling your characterization that I “think EDM in induction motors is not an issue.” In fact I have mentioned that I saw pits in non-vfd induction motor bearing that I believed may have been related to our observation of poor bearing insulation resistance on the same motor. But I have not seen the distinctive pattern called "fluting"(much different than pitting) on our many large non-vfd induction motors, or mentioned for similar applications on the forum (other than Walt one example), and that is a source of curiosity for me.

rack your brain no longer Ron
Fluted bearing from Non VFD AC motor. ID fan in fossil fuel power gen application

EP,
Uneven pitting on a bearing race can be the result of weldling without grounding to the shaft.
The 'fluting' pattern is going to be the end game in an anti friction bearing after time. In your case, nothing says success like success! Be glad you are not bit by this problem. Evidently the motor repair vendors you use are doing the job right.
noknroll,
Thanks. Do you have a pic of the race? Picky, ain't I?!
Ron Brook

Thanks noknroll and Ron and Walt.

I'm starting to believe.

Yes, maybe we are lucky and others have even worse bearing insulation than us.

Noknroll - which end of motor was that bearing on?

Just looking at examples posted on the forum over the years, it seems like the majority of cases of fluting are on roller bearings (particularly crb), rather than ball bearings.

Agreed?
Any idea why that is?

Pete,

Not in my perception. I've seen plenty of 62xx and 63xx bearings.

EP,
Again, you are fortunate, or blessed, or your company is just plain prepared.
I have to agree with Danny (OUCH!!!!). Fluting is not limited to roller bearings. Heck, it isn't even limited to anti-friction bearings. Sleeve bearings will 'frost' from EDM.

Ron,
did you scroll down? 2nd page of attach shows inner race
Pete
It was from NDE
This was replaced in situ, then the EDM migrated? to the D.end. Motor then replaced with a supposedly "properly insulated" and rebuilt motor.
Now, 12months later, the erosion at D.end has again propagated enough to be very loud and amplitudes are heading skyward

Hi Noknrol,

In the roller shot just the middle roller of the 3 appears to have a subtle fluting pattern.

Are the parts still around, to see if the roller fluting is confined to a few rollers?

Hey Dan,
The parts are long gone but I remember distinctly only that one roller had fluting marks

Hello everyone ,

There are a few of motors with VFD damaged by EDM , during operation ,I measured the voltage of these motors by multimeter between motor shaft and motor grounding terminal , found that the voltage value of every motor is less than 1 voltage . about measuring point ,pls see the attached photo .what is the possible reason ?
Can’t the high frequency voltage be detected by multimeter ? or high frequency voltage has been released by circulation current ?

I appreciate your comments !