# VFDs and ungrounded systems



## micromind (Aug 11, 2007)

A company I do a lot of work for builds mostly prototype equipment, and has recently moved to a new shop. The new shop has 120/208Y only, no 480. 

Just about everything they build has at least one VFD, most stuff has a bunch of them. All are 480. 

Of course, the proper solution is to get a transformer with a 208∆ primary and a 277/480Y secondary, and of equal course, they weren't too thrilled about buying one. Especially when they have about a dozen or so 480∆ - 120/208Y ones sitting around. 

VFDs are none too keen on being supplied with an ungrounded system, even less so with a grounded B system. Some VFDs can be modified for such systems, but just about every brand there is rolls through here. 

One possible solution I've come up with is to backfeed one of the 480∆ - 120/208Y transformers (yes, XO will be isolated.......) and connect 3 single phase transformers to the 480 side in a zig-zag fashion, and ground the derived neutral. 

My question here is; will this system provide a solid enough neutral to not burn up the MOVs in the VFDs?

I believe it will; since not much more than a reference is needed, no actual current is involved. 

Of course, in the event of an actual ground-fault, the 480 breaker may or may not trip, and regardless, the VFD would very likely be reduced to a smoking pile of burned semiconductors.......

Has anyone ever seen this done, or is there a better way?

P.S. The zig-zag transformers are small and far less costly than a 208∆ - 480Y transformer.


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## Introyble (Jul 10, 2010)

Well to all hell I just deleted my response lol so this is what you are going to get. Are you describing a HRG or NGR system? Most of the time VFD's in a HRG will not trip to ground fault.


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## Introyble (Jul 10, 2010)

I read this once for some reason, took me a while to find it again lol, I still like my deleted response :laughing:

http://www.littelfuse.com/~/media/F...Littelfuse_White_Paper_Ground_Fault_EL731.pdf


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## micromind (Aug 11, 2007)

I'm leaning more toward creating a solidly grounded wye connection to the input of the VFDs. 

The main reason (as I understand it) is that VFDs need a center of the wye ground reference in order to utilize the built-in MOVs for transient voltage protection. If the system is not held to ground at the center of the wye, the MOVs will clamp whenever the phase to ground voltage goes higher than about 350 or so. This varies with different manufacturers, but most VFDs will see 480 to ground as a problem.


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## Introyble (Jul 10, 2010)

Yeah, I dislike that paper also. He eludes to several difficult and interesting subjects but fails to follow through with inclusive hypothesis. By nature, your entire inquiry seems to be a fallacy. Why are you in this situation to begin with?


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## micromind (Aug 11, 2007)

Introyble said:


> Why are you in this situation to begin with?


The building service is 120/208, and I need to use VFDs that are 480.


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## frenchelectrican (Mar 15, 2007)

micromind said:


> I'm leaning more toward creating a solidly grounded wye connection to the input of the VFDs.
> 
> The main reason (as I understand it) is that VFDs need a center of the wye ground reference in order to utilize the built-in MOVs for transient voltage protection. If the system is not held to ground at the center of the wye, the MOVs will clamp whenever the phase to ground voltage goes higher than about 350 or so. This varies with different manufacturers, but most VFDs will see 480 to ground as a problem.


 
Bonjour Micro.,

I know you did bring up a good subject about the VSD's and unforetally most VSD's are set up as wye connection due the MOV's protection and If I did recall one of our junk ( made in chine stuff ) were the worst they can not even run on Delta system at all without blowing it up as soon you lit it up.

Our European specs VSD's it is very common to see in wye format so it is not a issue with our 415 v wye system but not too freindly with 415 volt delta ( floating or grounded verison)

I know you came up a idea of reversed transfomer then add a zig-zag tranfomer but one issue is with reversed transfomer as far we do know we useally can't float it but I know it can be done in floated mode but very limited mode and add a zig-zag can change the format and use the centre point for grounding connecton and I think it should work allright as long it is legit per NEC code ( the only question is between the transfomers )

But the biggest question is the zig zag sizing that part I havent done any figureing for a quite while. ( I will do some figureing for while then may end up updated with more info once I get some answer from my mind )

Merci,
Marc


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## Introyble (Jul 10, 2010)

micromind said:


> My question here is; will this system provide a solid enough neutral to not burn up the MOVs in the VFDs?
> 
> I believe it will; since not much more than a reference is needed, no actual current is involved.


Nevermind I see what you posted :laughing:


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## JRaef (Mar 23, 2009)

micromind said:


> I'm leaning more toward creating a solidly grounded wye connection to the input of the VFDs.
> 
> The main reason (as I understand it) is that VFDs need a center of the wye ground reference in order to utilize the built-in MOVs for transient voltage protection. If the system is not held to ground at the center of the wye, the MOVs will clamp whenever the phase to ground voltage goes higher than about 350 or so. This varies with different manufacturers, but most VFDs will see 480 to ground as a problem.


That should be fine. The issue really is, most drives are designed for IEC power systems that are Wye, not Delta, so the MOVs are connected in Wye as well, and the neutral point is connected to ground. When you hook that drive up to a delta or NRG system, the MOVs become an easier Neutral point for any fault and noise current. They fail rapidly, then they are no longer there at all and the drive front-end fails in short order. Some drives that are designed primarily for the North American market provide a simple means of just removing the ground bonding jumper for the MOVs to avoid this. A-B drives are like this for example. But if you are testing different brands of drives and don't want to mess with this issue, doing what you described will suffice.

Read page 45 of this white paper, it explains it in detail. But this only addresses the fault issue, having a ground path for common mode noise is also another benefit of doing what you plan.


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