Article 110

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tony_psuee

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I had a discussion recently with a GC regarding performance of our equipment with a field installed VFD. While discussing the installation I found out the electrician used the RMC as the EGC. That differs from the grounding instructions from the manufacturer of the VFD. I stated to the GC it appears not following the manufacturers installation instructions was most likely the problem and additionally this is a violation of Article 110.3.B. Until the VFD wiring is corrected we will not be involved on a field service basis. He stated the job passed final so he doesn't care what I think, we have an equipment problem. The EC is pleading ignorance that no one told him it required special grounding and they wired it to what they bid and Article 250 and will change it for $$$. To my knowledge that area adopted NEC2002 completely. Any feedback would be greatly appreciated.

Tony
 
Re: Article 110

In my opinion, I doubt that the "special grounding" is a result of a listing requirement. With that in mind, I don't think 110.3(B) applies. It's just like some of the IT equipment you see that requires its own ground rod...the manufacturer might think its a nifty idea, but doubt that UL (or other) would make it a requirement of the listing. Also, from what I have seen, GRC is many times a better fault clearing path than even an insulated EGC.

I see no problems from a code realted standpoint.
 
Re: Article 110

most vfds come with explicit instructions on grounding that really do need to be followed for optimal operation of the vfd/motor combination.

that does not mean that the installation does not meet code.

If someone told him to "look in the manual and install it that way", or gave him drawings that showed what was wanted, it seems to me the EC ought to make it right.

If someone told him "here's a VFD and a motor, go wire it up however you see fit" then the nitwit that told him that is responsible.

IMO those are the only two choices - you either give the EC explicit instructions on what you want done or you take what they give you (assuming its to code).
 
Re: Article 110

If the manufactures instructions call for an EGC thats what you got, per NEC 250.118 GRC is an EGC. If the instructions call for an isolated ground conductor you did not get one.
Just what does the instructions call for, the EC met code (if installed properly).

[ January 06, 2005, 09:55 AM: Message edited by: guesseral ]
 
Re: Article 110

Just curious what type of performance problem you are having with the VFD??

Usually the "special grounding" is to help eliminate noise caused by the IGBT output section of the drive. I have seen the noise cause nuisance trips of a drive (or other equipment). One of the "lower cost" drives our plant was using seems to have a hard time dealing with 230V Delta High leg and the only solution that seemed to work was installing a reactor on the load side of the drive. These can be purchased relatively cheap for lower horsepower drives and it beats the heck out of revisiting the wiring method...

Could you post a reply that includes more specifics about the performance problems you are having??

Mike
 
Re: Article 110

Ryan,

If I am understanding your response correctly, the grounding requirements would have to be part of the UL listing for it to be required to meet Article 110.3.B rather than the recommended method by the manufacturer in their installation/instruction manual.

guesseral,

The VFD manufacture's instructions call for an isolated ground wire. Specifically stating not to use conduit as the EGC. We did not supply nor specify the VFD or how it was required to be installed. That was done by either the mechanical, electrical, independent sales rep, consulting engineer, or end user. :confused:

chevy1969pu,

The stated symptom is a grinding noise in the motor in addition to high amp draw on all three phases, about 5% over nameplate. We have warranty replaced a motor and gearbox, both of which were evaluated by their companies and claimed to be 100% operational, free of defects, when returned for inspection. The problem continues with the replacement equipment.

I appreciate all responses.

Thanks,

Tony
 
Re: Article 110

I have no idea what a VFD is but I can't restrain myself from asking this.

How would a ground connection be related to a grinding noise in a gear and motor assembly?

Editted in the word is

[ January 06, 2005, 05:58 PM: Message edited by: physis ]
 
Re: Article 110

Tony,

I am not sure exactly what kind of grinding noise you are experiencing, but it sounds like you may have a motor/drive combination that is not compatible with the load you are coupling it to...

If this is a quality drive (like an A/B Power Flex), there should have been some commissioning done to tune the motor to the load. This is an important step in getting the desired performance from the drive.

I suggest you consult the OEM documentation and consult with their Tech Support to resolve the issue...

Mike
 
Re: Article 110

physis,

One of the concerns with improper grounding of the variable frequency drive(VFD) is the possibility of getting bearing currents in the motor. This will lead to premature bearing failure. The other is creating interference with the input giving the drive the speed reference signal.

chevy1969pu,

As I stated in my previous post, we did not supply the drive. What is on the job is a typical commodity type. I have been informed that a "factory certified" start up was performed and of course they stated the problem is not in the drive as they have installed "hundreds of these on this type of equipment". Unfortunately all the information I am receiving is second and third hand and from a source that I believe is less than objective in this situation.


My goal was to see if I was correct in the installation being in compliance and it appears that unless the manufacture of the VFD specifically called out the grounding requirement in the listing it is.

Thanks to all,

Tony
 
Re: Article 110

Tony,
I don't think that the EGC itself has anything to do with the elimination of bearing currents.
Don
 
Re: Article 110

__________________________________________________
The VFD manufacture's instructions call for an isolated ground wire. Specifically stating not to use conduit as the EGC. We did not supply nor specify the VFD or how it was required to be installed. That was done by either the mechanical, electrical, independent
__________________________________________________As I stated in my previous post, we did not supply the drive. What is on the job is a typical commodity type. I have been informed that a "factory certified" start up was performed and of course they stated the problem is not in the drive as they have installed "hundreds of these on this type of equipment". Unfortunately all the information I am receiving is second and third hand and from a source that I believe is less than objective in this situation.
__________________________________________________

These contradict! But in the second one if factory started and the statement was made on the grounding BY THE FACTORY START-UP TECH then I guess they don't need the ISO ground.

"I have no idea what a VFD is"
A VFD is a variable frequency drive!
 
Re: Article 110

Don,

I agree that the EGC will not completely eliminate bearing motor currents. However, in discussing VFD's and grounding with a couple of factory types they have at least indicated the need to reduce. Unless it is just sales BS.

guesseral,

I personally don't put a lot of weight behind a "factory certified" start up. I have seen that to be anything from a factory field engineer onsite for a several hour check out and start up, to a local sales guy standing over someone's shoulder telling them what button to push to change the menu. I was at a job where the EC ran the wiring for two VFD's in a single conduit using a single EGC. The VFD installation instructions stated 1 VFD per conduit and isolated EGC. The VFD manfacturer had a field engineer on site who said, "no problem with the wiring has be the equipment." Guess what happened when I ran a separate wire to single VFD and isolated ground? Problem with the equipment virtually disappeared. Yet he continued to state that not following his companies installation instructions was not a problem. Those VFD's were later replaced with another brand and the problem with the equipment went away. I am not a VFD or grounding expert, just someone that figures if it was important enough to print specifics about how it should be done than maybe it is important to follow those specifics. OK I'm off my soap box.

Tony
 
Re: Article 110

Have you considered a reactor on the output of the VFD? These are usually reccomended by the manufacturer if the leads between the VFD and the motor are very long.

Does the grinding noise happen at all speeds? Some VFD's have "skip" frequencies that can be programmed if it causes a resonance in the motor. And I think some VFD's can have the fundamental "chopper" frequency changed. That also might be worth a try.
 
Re: Article 110

Also, if possible, the first thing I would do is bypass the VFD and see if the motor still makes the noise when running across the line. Disconnecting the gearbox or the load might also give some insight into where the problem is.

Steve
 
Re: Article 110

VFD= Variable Frequency Drive. Electronics are used to change the motor speed by changing the frequency of the electrical power supplied to the motor.
VFD's, because their electrical load is not linear, are notorious for creating harmonics; that is, the neutral may end up carrying more current 'out' than went 'in' to begin with. One result is electronic noise; another is overheated neutrals.
These problems are minimised by oversizing the neutral, and by paying particular attention to grounding. It is not unusual to have to run a green wire all the way back to the panel, and isolate the drive from the building ground.

Of course, the noise might also be from a mis-wired motor. Check your leads for correct marking, as well as connection.
 
Re: Article 110

Physis,
A VFD rectifier section changes the 60 HZ input into DC, and the inverter section changes the DC back into AC at a frequency determined by the switching rate of the inverter transistors/SCRs.
A complex control section determines the switching rate and other variables in response to user input.

A block diagram and basic schematic of the power circuits below.

More info here -
Basic VFD Tutorial

Ed

vfd3.gif
 
Re: Article 110

Without telling my age, Around 40 years ago, I worked on a project to control a washing machine with electronics. Took a specially designed 18-pole motor. Needless to say, the cost was astronomical. We didn't know to call if a VFD then.
 
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