# two transformers, three phase?



## Dennis Alwon (May 9, 2009)

Sounds like a high leg delta. I have never seen that at that voltage but I have seen them at 120/240 3 phase systems.


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## wildleg (Apr 12, 2009)

check the autorange function of your meter. are you sure you weren't reading near zero (0.46v) on the B phase ?


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## oldtimer (Jun 10, 2010)

Dennis Alwon said:


> Sounds like a high leg delta. I have never seen that at that voltage but I have seen them at 120/240 3 phase systems.


 Dennis! 

I think a Delta system requires 3 transformers, Open Delta, only requires 2.

This is interesting, please let us continue this thread.

The term High Leg Delta..... does it mean the same as Open Delta?


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## jeremievc (Dec 12, 2011)

wildleg said:


> check the autorange function of your meter. are you sure you weren't reading near zero (0.46v) on the B phase ?


No, range was good, shouldn't there be three transformers on the service pole? Looks as though they are supplying two legs with respective transformers and combining the two for the third leg to get the 480 on the third leg? I appreciate any help or suggestions.


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## alpha3236 (May 30, 2010)

I see this quite often on smaller (< 30hp) 480V irrigation pumps. it is an open delta setup using both windings in series for the high leg. In my area everything for agriculture is open delta with a high leg apparently due to POCO line length.


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## Big John (May 23, 2010)

If you've got two pigs on the pole, you've gotta have an open delta.

I've never seen it but apparently you can actually center-tap one of those and you will have a high-leg open delta.

But your voltages don't add up for that: I would expect one phase to be 1.73 times higher to ground than your other phase-to-ground voltages, which would put you at 406 not 460.

-John


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## jeremievc (Dec 12, 2011)

How would this affect a motor wired 480 Y? Wouldn't this destitute the leg wired to the high leg?


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## electroman (Apr 10, 2009)

The motor will hook up fine. Just don't use a single phase load on this service. I see this a lot in glass plants where all of the loads are 480 3phase. I learned the hard way once when we hooked up a bunch of 277v fixtures to one leg and the "Supposedly" neutral bar. Smoked every fixture.


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## Big John (May 23, 2010)

jeremievc said:


> How would this affect a motor wired 480 Y? Wouldn't this destitute the leg wired to the high leg?


 The motor doesn't care about phase-to-ground voltages. As long as you've got balanced voltages P-P and they're somewhere around 480, you're good to go.


electroman said:


> ...Just don't use a single phase load on this service...I learned the hard way once when we hooked up a bunch of 277v fixtures to one leg and the "Supposedly" neutral bar. Smoked every fixture.


 I don't follow? If he actually does have an open delta high leg, it's designed specifically so he can run line-to-neutral on two phases.

Or do you mean, don't accidentally connect high leg to neutral?

-John


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## jeremievc (Dec 12, 2011)

I will check all of my voltages again. "Destitute" was supposed to read " destroy" by the way. Thanks to everyone for your helpful input. Please continue with anymore suggestions.


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## electroman (Apr 10, 2009)

The Open Delta 480v system I'm referring to didn't have a tapped neutral. Corner or center. Phase to ground voltages were all over. Come to think of it, not a great system for ensuring proper ground faulting.

It just hit me, 277/480 is usually a wye system. So in the poster's instance, being delta with a center tapped neutral, the voltages look good. Even for 240v loads.


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## SHOCKnAWE (Dec 25, 2011)

It certainly sounds like an open-delta, but your voltages don't seem right. If it's to be a 480 open-delta, one phase is typically grounded (corner grounded). These are widow-makers if you don't understand what you're looking for or how to properly test for it. You may have lost your corner bond/ground. phase-phase-phase should equal 460-480, as well as two phases to ground should equal that as well; one phase to ground needs to be zero for a corner-grounded delta; no high leg on this delta connection.


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## micromind (Aug 11, 2007)

This is indeed a 3ø 4 wire 240/480 open delta.

It'll run any 3ø load as well as a wye or a closed delta. 

It'll also run any 240 single phase load, provided it's 240 and not 120/240.

120 doesn't exist at this service. If needed, it is derived from a transformer.

The theoretical voltages would be 240,416,240 phase to neutral/ground, and 480 from any phase to any other phase.


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## SHOCKnAWE (Dec 25, 2011)

I didn't read it close enough, but I still have a question as to why B phase to ground is 460 and not closer to the 410 range based on A to ground and C to ground. B would be the high leg, but shouldn't be as high as phase to phase

As far as running the motor, the motor won't know the difference, theoretically. I have had a case of two identical 20 hp dryer motors, one served a 480-wye transformer and one by a 120/240 open delta. Both were sized for their appropriate voltage, the 480 wye would allow the air dampers to be wide open for maximum air flow, whereas the 240 delta had to have the air flow restricted or it would overload. All I can blame that on is voltage imbalance characteristics of the delta system.

There is nothing wrong with an open-delta if it's sized right. I prefer them. A case I had was where a primary line fuse, not transformer fuse, blew on a segment of line that was serving several "closed" (three tubs) delta banks. On the line that went down, there were several single phase customers and they all experienced brown outs due to the banks backfeeding through the line that had the bad fuse.

With an open delta, if a line is lost or transformer goes down, you know you have a problem because you lose the third phase. If a "closed" delta looses a line or fuse the third phase is still there.


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## Big John (May 23, 2010)

_Jeremievc_, did you ever re-confirm what that high phase-to-ground voltage actually was? You sure it was 460V?

-John


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## jeremievc (Dec 12, 2011)

I did check it again, it was about 420. Thank you and everyone else for your help. I am thankful for a group of people willing to share their in their respective areas of expertise.


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## micromind (Aug 11, 2007)

Just for info, potential transformers (PTs) in medium-voltage switchgear are usually open delta. 

Grounded B, 120 volt 3ø. 120 phase to phase, 120 to ground on A and C, 0 to ground on B. 

This set-up is used for instrumentation, not power. 

Most of the gear I work with has fuses on the secondary side of the PTs. Usually a 3 pole fuse block, and the center fuseholder has a solid slug in it, not a fuse.


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