# bearing fluting



## JRaef (Mar 23, 2009)

A grounding brush SHOULD have helped, but it didn't? That's bad news. It could be an indicator that you have a high impedance to your frame ground connections, you might want to double check that. What happens a lot is that someone thought it was a good idea to use rubber isoltors to dampen vibration, then you are left with the ground wire in the connection box as your only path to ground. But that is in the same conduit as your power conductors, so any common mode noise is still there. You may have attached your brush to the motor frame, but if the frame is not well grounded itself, it did no good.

The VFD cable might help a little, but unless you need it for OTHER reasons*, a better use of your money would be a dV/dt filter on the output of the VFD, or upgrade to a sine wave filter if it's more than 300ft of cable run length from drive to motor. Slowing down the rise time of the PWM pulses helps to prevent the capacitive coupling of the stator and rotor that causes the bearing discharge in the first place.

Also, Allen Bradley and some models of ABB large drives both use Common Mode Filters , a toroidal transformer on the output leads (internal) that help to mitigate this. I suspect that the Telemecanique drive does not.

Read this, it's from a friend of mine who is an expert in this field.
http://www.gke.org/presentationer/files/EDM%20Physics%20and%20Reality%201.pdf

*If you are using STEEL conduit between the drive and motor and everything is properly grounded and bonded, you probably do not need the VFD cable too. If however you used PVC conduit, open tray, concrete ducts etc. then you should have ALWAYS had shielded cable, so upgrade now regardless.


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## John Valdes (May 17, 2007)

They do make insulated bearings just for this purpose.
A last resort in my opinion, as getting to the bottom of this issue and correcting it, is by far the better option.

With this size motor, let a motor shop install the bearings. This way they can check all the machine tolerances, balance and do a vibration report for you to use as a base line.

Whats the carrier frequency set at?


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## m1004 (May 27, 2015)

we had a motor company install insulated bearings but that did not help either.we have a drive spe******t come out and he mentioned static from the belts.i don't know off hand what the carrier freq is.he the drive guy seems to think that the vfd cable to the motor will fix the prob.i don't know this is the first time I had to deal with issues like this.in my opinion its the drive because we have six other machines with the exact same system but abb drives.i'm at a loss


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## KennyW (Aug 31, 2013)

So has someone measured the shaft voltage at various operating conditions with a scope? 

I agree output dv/dt or sine filter is the best bet. 

Static from the belts would still suggest some kind of fundamental bonding/grounding issues....


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## John Valdes (May 17, 2007)

m1004 said:


> we had a motor company install insulated bearings but that did not help either.we have a drive spe******t come out and he mentioned static from the belts.i don't know off hand what the carrier freq is.he the drive guy seems to think that the vfd cable to the motor will fix the prob.i don't know this is the first time I had to deal with issues like this.in my opinion its the drive because we have six other machines with the exact same system but abb drives.i'm at a loss


It is the drive. But that does not mean the drive is bad.
I have seen this several times and in some cases the customer replaced the drive, used insulated bearings (I don't see how this did not correct the problem since this will insulate the rotor from the frame) or it miraculously corrected itself. But it was corrected later then sooner. It was a headache.

From the times I have had this experience, it was a cheap drive or a certain manufacturer. 
Got a picture of the pitted bearing?

We had no special drive cables back when I was in this business. So it may be the correction you need. I have no experience with special drive cabling.


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## JRaef (Mar 23, 2009)

John Valdes said:


> It is the drive. But that does not mean the drive is bad.
> I have seen this several times and in some cases the customer replaced the drive, used insulated bearings (I don't see how this did not correct the problem since this will insulate the rotor from the frame) or it miraculously corrected itself. But it was corrected later then sooner. It was a headache.
> 
> From the times I have had this experience, it was a cheap drive or a certain manufacturer.
> ...


I agree. No way it was "static from the belts", totally bogus concept that shows he had no idea what he was talking about. Any static would build up on the shaft and sheave, but if the motor had properly installed insulated bearings, it would have no path to the frame and ground. It's more likely that your bearings were not done right. The biggest offense along those lines is something I see all the time, that people ASSUME it is going to solve the problem by using insulated bearings on ONLY the Non-Drive End of the motor only. I see this in several older pieces of literature, and the problem is, that was a solution for when the source was OUTSIDE of the motor, i.e. static or welding being done on a machine. When the source is from WITHIN the motor, insulating only one side just concentrates the discharge to the other side.

John Valdes is right, it needs to be done by a motor shop, but it also has to be someone KNOWLEDGEABLE about the issues at hand to make sure it is done _right_. That's why I tell people that as a mitigation strategy, insulated bearings are best left to being done when the motor is MADE, or at least done by a motor shop that can demonstrate that they know what they are doing.


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## m1004 (May 27, 2015)

just changed out the motor cable to vfd rated cable I was told would fix the prob but did not. upon further inspection there is no ground conductor pulled with the feeds. the conduit is ground. do you think this could be a issue also. the motor company that changed the bearings recommended using only one insulated bearing because the motor would become more of a capacitor than it already is. what are your opinions


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## JRaef (Mar 23, 2009)

The problem with using the conduit as the only ground path is that even the slightest amount of corrosion begins to interfere with the ground conductivity and make this issue even worse. Plus, using the VFD shielded cable means grounding it at both ends so that common mode (ground) noise is not impressed on the shield drain itself. The concept of only grounding one end is for signal conductors, not power. Power conductors must have the shield grounded at both ends. If the only ground path is the conduit, it totally defeats the purpose. 

Ground the motor frame directly to the building ground, then ground your VFD cable shield drain to the motor frame as well.


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