# eliminating a high leg?



## powerpig (Oct 22, 2015)

Brand new to the forum, so I'll start with a "howdy" to all.
Some time ago, we installed, what I think was a 1:1 3 phase transformer to get rid of the high leg of an open delta system, for a particular machine.
Has anyone else encountered this scenario? If so, how did you address it?

Any help is much appreciated.
Ted


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## Big John (May 23, 2010)

If the machine can't tolerate >120V to ground, putting in a ∆∆ transformer is about the only solution I can think of.


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## powerpig (Oct 22, 2015)

Hi John,
The problem is not he 120v to ground, its the 208v to ground that becomes an issue


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## JW Splicer (Mar 15, 2014)

powerpig said:


> Brand new to the forum, so I'll start with a "howdy" to all.
> Some time ago, we installed, what I think was a 1:1 3 phase transformer to get rid of the high leg of an open delta system, for a particular machine.
> Has anyone else encountered this scenario? If so, how did you address it?
> 
> ...


High leg of an open delta? that's actually kind of cool! You'd have to pretty much do 1:1 isolation xfmr delta delta right? I'm not sure you could go delta-wye without that 3rd coil? Was it 2 single phase bugs in open delta?


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## JW Splicer (Mar 15, 2014)

Actually the easiest way might be an auto transformer on the high leg.


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## Big John (May 23, 2010)

JW Splicer said:


> ...I'm not sure you could go delta-wye without that 3rd coil...?


 It would work: The primary only cares about proper phase voltage. The only thing you lose in an open-∆ is a 42% capacity loss because kVA-wise you're running single phase.

And I agree a bucking transformer would work like a champ, but I think it would be a code violation.


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## telsa (May 22, 2015)

powerpig said:


> Brand new to the forum, so I'll start with a "howdy" to all.
> Some time ago, we installed, what I think was a 1:1 3 phase transformer to get rid of the high leg of an open delta system, for a particular [ 3-phase] machine.
> Has anyone else encountered this scenario? If so, how did you address it?
> 
> ...


Well, that was money well wasted. :thumbsup:

You realize that ANY such device has to have the ability to withstand the 240 VAC -- hot to hot to hot.

Further, that a center-tapped delta provides the single most popular control voltage: 120 VAC -- grounded.

The windings of your delta transformer are - mathematically - identical to the field windings of the 3-phase motor. [ H1-H2-H3 are assumed delta. ]

This fundamental reality is brought up by the second paragraph of any engineering text covering the matter.

( Fink & Beaty, Standard Handbook for Electrical Engineers, McGraw-Hill)

BTW, you didn't happen to establish an ungrounded SDS ?


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## JW Splicer (Mar 15, 2014)

Hmmm I didn't know that! Thanks! What about a T-T connection? 3ø to 3ø?


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## telsa (May 22, 2015)

JW Splicer said:


> Hmmm I didn't know that! Thanks! What about a T-T connection? 3ø to 3ø?


Mathematically, that equates to a motor-generator set.

Transformers are solid state 'motors' that only operate via magnetic effects.

Way, way, way back when, Edison did, in fact use motor-generator sets to get his voltages up for long distance transmission.

This proved to be wholly impractical almost from the first.

Just right there, his DC current was gutted by Tesla's AC current.

It was costing Edison thirty to fifty times as much money to transform voltages as it did Westinghouse.

Further, Edison was not at all prepared for other effects that only became apparent when dealing with high voltage DC current. 

Like, how do you turn the dang thing off ? :laughing:

Without commutation, yikes !


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## Big John (May 23, 2010)

JW Splicer said:


> Hmmm I didn't know that! Thanks! What about a T-T connection? 3ø to 3ø?


 I'd have to look it up. Only time I've ever seen Scott connections was 2 phase: I don't know what value they have in three phase.


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## telsa (May 22, 2015)

Big John said:


> I'd have to look it up. Only time I've ever seen Scott connections was 2 phase: I don't know what value they have in three phase.


He's not changing the phase count.

A *Scott-T transformer*[1] (also called a *Scott connection*) is a type of circuit used to derive two-phase electric power (2-φ, 90-degree phase rotation)[2] from a three-phase (3-φ, 120-degree phase rotation) source, or vice versa.

wiki

The need for a Scott T transform is _passé._

IIRC, it's heyday was when Tesla's AC drove Edison's DC out of the market. His scheme left legacy systems all over that were wired for merely two legs of power.


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## JW Splicer (Mar 15, 2014)

You can hook up a T-T connection for 3 phase to 3 phase though. It's just using 2 coils on each side instead of 3. But I don't know how that affects kva ratings or anything.


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## micromind (Aug 11, 2007)

My first question here is why must the high leg be eliminated? 

If it truly must be gone, a transformer with a 240∆ primary and a 120/208Y secondary will work. 

The fact that it's an open ∆ is irrelevant, the load cannot tell the difference.


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## JW Splicer (Mar 15, 2014)

I'm only asking from a hypothetical stand point. I know a Scott T has seen its days. I know that a 3 phase load won't care, if you really want it gone you can lift the center tap and install ground fault detection. I don't know why you'd need to eliminate a high leg honestly but I'm not sure how a T-T works and am just wondering.


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## powerpig (Oct 22, 2015)

*Thanks to all!*

Thanks to all for your help! Actually micromind had the info I needed.
A simple 240 volt delta to a 208 volt wye connection is what I was looking for.
I was on the phone with tech support for a half hour the other day in regards to this machine and the "highleg". Apparently this very big, and very expensive printer, will not tolerate the higher voltage to ground. Im not sure if this tech. even knows what his talking about, but I will not be the one to apply a highleg to a machine( when instructed not to) that cost twice what my house is worth!
Thanks again for all input! Much appreciated.

Pig


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## micromind (Aug 11, 2007)

powerpig said:


> Thanks to all for your help! Actually micromind had the info I needed.
> A simple 240 volt delta to a 208 volt wye connection is what I was looking for.
> I was on the phone with tech support for a half hour the other day in regards to this machine and the "highleg". Apparently this very big, and very expensive printer, will not tolerate the higher voltage to ground. Im not sure if this tech. even knows what his talking about, but I will not be the one to apply a highleg to a machine( when instructed not to) that cost twice what my house is worth!
> Thanks again for all input! Much appreciated.
> ...


The factory tech may very well be right......

If the machine has VFDs and they use MOVs for spike protection (most do), then they'll see the high leg as a spike and when they cannot clamp the voltage down, they'll blow up. 

As much as I like high-leg systems, they have their disadvantages.


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## telsa (May 22, 2015)

This would've been a VERY short thread if the OP had included that teeny-tiny detail.

'Getting rid of the high leg' reads quite a bit differently than: " My machine HAS to HAVE 208Y120 power to feed its VFD and complex controls."

:laughing::laughing::laughing::laughing:

Which would stop all here from assuming that the load was delta 240 VAC.... a voltage that is still as common as dust.

We expect that the existence of a VFD would be made evident -- straight off.

Sometimes the OP is just too terse. :whistling2:


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## JRaef (Mar 23, 2009)

micromind said:


> The factory tech may very well be right......
> 
> If the machine has VFDs and they use MOVs for spike protection (most do), then they'll see the high leg as a spike and when they cannot clamp the voltage down, they'll blow up.
> 
> As much as I like high-leg systems, they have their disadvantages.


Yeah, It's not just that VFD can't handle that high leg, it can't handle ANY kind of delta input. So the Delta to Wye transformer would be the only viable solution. It's not really the fact that it's a "high" leg, the problem is that the drive can't deal with the LACK of a ground reference, even though on a high leg system, it's only that one phase that has no ground reference. The MOVs on the drive are referenced to ground in a Wye inside of the VFD, as are any EMI/RFI filters. With a Delta supply, the MOVs attempt to become the ground reference for the entire supply and it pops them the first time there is an issue on the line , then they are not there any more and you don't know it, so eventually the drive front-end gets damaged.

Some drives however allow you to remove the ground reference for the MOVs. It solves the symptom, but not necessarily the problem.


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