# VFD Ground Issue?



## j30rndmag (Sep 1, 2010)

Ok so I ran across something today that I have never heard of and wanted to throw out there to see what you all thought about it. 

About a year and a half ago I installed (2) 300 hp Allen Bradley powerflex 700 drives at a plant that I am contracted out for maintenece and projects. They are each controlling a seperate 250 hp, 290 FLA, 480v 3 phase Toshiba motor which is direct coupled to a blower for plant process air. I ran (3) 500 mcm copper conductors and (1) #3 copper ground to each motor. Also installed a line reactor between the source and the drive. It ran fine for a year with no problem then the vibration analysis contractor noticed some strange readings and has been digging to find out why. Today the contractor brought a drive technician and he found a reading of 2.5v ac between the drive circuit board ground and the main ground bar inside the drive cabinet. He later told me that the drives nowadays need the ground to be 2-3 sizes smaller than the current carrying conductors and said I need to run another conduit to install a 4/0 ground to each motor. I have never heard of this so decided to poke around online for a bit tonight. I did find on the Allen Bradley's site (www.rockwellautomation.com) a document IN001 that was titled 
*"Wiring and Grounding Guidelines **for Pulse Width Modulated (PWM) AC Drives" *​
On page 11 under "number of conductors" the last paragraph states "The ground conductor(s) should be rated for full drive ampacity." 
So I am at a loss at the moment. I am thinking about temporarily running a 4/0 ground to the motor and see if the voltage dissapears. What do you guys think?
Thanks for any info to bring me up to speed on this subject.

J30rndmag


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## John Valdes (May 17, 2007)

I have heard this before. I am still not sold on this principle. But I have had drive manufacturers in the past ask for full sized grounding conductors. To use the same size ground as the CCC's.
I have not been asked to provide a larger grounding conductor, larger than the CCC's.
And I am still unsure as to the advantage of this practice.


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## Jlarson (Jun 28, 2009)

j30rndmag said:


> said I need to run another conduit to install a 4/0 ground to each motor.


That's not a good idea and could very well crate a code violation, the EGC needs to be grouped with the phase conductors. You could create a noise issue doing that too. 

Make sure all your ground terminations are tight and solid and all control shields are bonded well. 

Did the vibration contractor check the alignment of the motor and blower and the mechanics of the blower to try and find the source of their "strange readings"?


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## Wirenuting (Sep 12, 2010)

I just size the egc as if I had no drive. Most of our drives have a bypass feature on them. 
I'm the only one who pulls a PM on them and there are to many. So they fry faster then I can drink coffee.


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## JRaef (Mar 23, 2009)

The section you are reading is saying that the ground conductor(s) need to be capable of full current, not that EACH one needs to be rated for that, in other words it's perfectly acceptable for you to have multiple ground conductors that total to enough.

With VFD grounding, there are two issues of concern: the safety issue normally addressed by NEC requirements, but also you want a low impedence / resistance path to ground for common mode noise current that is created in / by the drive. Too small of a ground conductor MIGHT affect the circuit resistance and allow that common mode noise potential to get too high. But to be quite honest, I have NEVER seen that happen. Still, if need be, run another #3 ground in with each phase conductor.

By the way you said:


> He later told me that the drives nowadays need the ground to be *2-3 sizes smaller* than the current carrying conductors *and said I need to run another conduit to install a 4/0* ground to each motor.


Something is contradictory in that statement. Makes no sense to run a SMALLER ground conductor.

Read the rest of that document, it's the holy bible on VFD grounding practices. The size of the conductors is mentioned but in my experience it's not that important, especially in a 4 wire system. But you didn't say what your system is. If it's 3 wire ungrounded delta, then this issue might loom very large.


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## j30rndmag (Sep 1, 2010)

Thank you all for your input! I have still been digging and its so wierd that there is not much info on the web on this subject. Jlarson, Good call on the code violation possibillity. That is just what the drive tech suggested I do and that is the reason I decided to do my homework and read deeper into this. That side of the plant is going to be shutdown next week so I will check all the connections and also from what I have been told the vibration analysis contractor has checked everything mechanical in the 6 months he has seen these "strange" readings. JRaef, I may try and talk the plant into trying to pull another #3 in the conduit and see if that helps. Sorry it sounded like a contradicting statement. That was exactly what the drive tech told me. He said the ground needs to be a minimum of 2-3 sizes smaller than the feed. This is all new to me and still have yet to talk to anyone I have worked with that has ever heard of this issue. I am thinking of talking to allen bradley directly and get hold of a drive engineer to clear things up. Also, this is a 3 wire system.

Again, thank you all for your input and comments! I will keep ya posted as this tale unfolds.

*Also wanted to note that the drive tech also told me that if this problem is left unaddressed, It could leave to motor failure as it will damage the rotor. Could someone please clear this up because I don't see how this could be.


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## JRaef (Mar 23, 2009)

j30rndmag said:


> ... Sorry it sounded like a contradicting statement. That was exactly what the drive tech told me. He said the ground needs to be a minimum of 2-3 sizes smaller than the feed. This is all new to me and still have yet to talk to anyone I have worked with that has ever heard of this issue. I am thinking of talking to allen bradley directly and get hold of a drive engineer to clear things up. Also, this is a 3 wire system.
> 
> Again, thank you all for your input and comments! I will keep ya posted as this tale unfolds.
> 
> *Also wanted to note that the drive tech also told me that if this problem is left unaddressed, It could leave to motor failure as it will damage the rotor. Could someone please clear this up because I don't see how this could be.


Your "drive tech" sounds like he is making it up as he goes along. It's damned near impossible to damage a rotor unless it's from too much torque or a bearing fries and the rotor crashes into the stator. You can have bearing damage (called "fluting") from unaddressed grounding issues. But that has more to do with SHAFT grounding, nothing at all to do with ground conductor sizing as far as I know. And again, telling you to use a SMALLER ground conductor is just plain idiotic and likely violates the NEC requirements.

Call A-B's CSM (services) group and tell them you want to talk to their best drives guy in your area. My guy out here in Califonia is the guru of VFD grounding, I'll run it by him too if I see him in the office today or tomorrow (he never answers phone calls from me now ever since I stiffed him on helping to swap out bus bars in an MCC at a WWTP one day last year, but I was stuck in traffic, honest!).


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## Jlarson (Jun 28, 2009)

Break the rotor? :laughing: 

Biggest thing I would be worried about is a potential for damage to the boards from surges or faults since you have a potential difference and maybe the fact you could have a grounding conductor coming loose somewhere.


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## j30rndmag (Sep 1, 2010)

ok guys I did some more poking around today and some of the things the "drive tech" told me just don't pan out. First I went back to the drives to actually read the potential voltage on the other drive next to the one he "checked" out. I found 0v in the second one so went back to the first where I found 0v there also. Continuity also showed they were bonded well so I started scratching my head as to where he found this voltage. 3 terminals over from where he told me to check I found 2.5v however this was NOT a ground and was an option to wire to a plc in case of a drive fault. It was a n.o., n.c., and a common. These are not used in this application but are wired back to the board so I didn't see an issue here. Also I contacted an Allen Bradley drive engineer and spoke with him for a while to bring him up to speed. He has never heard of an issue with the ground like I was told to upsize, but is going to do some checking with another engineer and then get back with me. So my question is was the tech on the wrong terminal or is he just trying to drum up work? What do you guys think?


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## Jlarson (Jun 28, 2009)

So he measured across an open and he was going with whatever random number the meter was putting up. Oops. :laughing:

Normally when I get an odd reading like that I re check myself a few times not just tell the customer to throw more copper at the problem.


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## j30rndmag (Sep 1, 2010)

Exactly! Thats why I think there is something strange with his "findings".


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## JRaef (Mar 23, 2009)

As I said, sounds to me like he is making things up as he goes. Poking around to find a terminal somewhere that reads something unusual and saying that it is proof of his BS story is a classic scam artist ploy if you ask me.

By the way as I thought about this earlier today when looking for my Field Tech, it dawned on me that *if* you DO have a shaft voltage issue, you might be seeing the beginnings of a bearing fluting problem in your vibration monitor. It starts off really slow but gets worse with time. Here's a link for a company that I recommend a lot for shaft grounding bushings, Aegis. They have a lot of good information on their website, worth browsing around in there.
Aegis Shaft Grounding link.

Here is a presentation on it from a friend of mine in Sweden (it's in English, but look up the word "stochastically" before reading it, he likes unusual words).

GKE presentation on EDM bearing damage.


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## j30rndmag (Sep 1, 2010)

Great info thank you! Hopefully the drive engineer will contact me tomorrow so we can nail down this issue. I will also look more into the vibration analysis findings to find out what he has been seeing. I will keep you all posted.


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