Three Transformers Changing 3-phase to Single Phase?

TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
This must be my week for single to 3-phase and visa versa.

Got a different customer that has special 65kw 480V 3-ph gennys that need to provide 240/120 power. They are moving stuff around and are asking for a quote. When told that at another site they had problems with using just 2-legs of the genny because it would trip on load imbalance. (yep, seen that before). So they had three separate transformers installed and wired together that produce 240/120V and load the genny evenly (or even enough). My first thought was a Scott transformer setup with genuine 2 ph power, using only 2 legs. But no, they have 3 identical 25kw wired up to provide 240/120! Sooo ... we switched to the other genny and I traced the connections, best I could. (There were some phase tape errors ...)

Came home, drew it out from the sketches I made, compared to pics, and realized there must be a black jumper separate from the black output wire. (Arrrgghh!) But which one? This looks familiar to a 12 wire 3-ph genny configured for single phase. Z something, Ill have a look see on the big G.

So does my attached drawing look right? And btw, how does this work ???

EDIT: And can this be done with a single poly-transformer instead of 3 separate ones?

1751406821587.jpeg
 
I don't know of any three phase transformer that would provide available center taps on each of its windings.

Someone might make a triplex core style unit, but this is far from a common transformer configuration.
I proposed something like this once to Jerome Frank, from Sorgel div of Square D, and he told me it would not be possible on a single core.
 
I don't see you getting 120/240 out of that, if anything maybe will release some smoke?

You have all three 240 volt sides that are 120 degrees out of phase with one another in a series connection. Not exactly certain what voltages will have at each output if you don't have situation of high circulating current, which since the top corner isn't closed you might not.

Still think three separate 120/240 volt systems is best you can do unless you have an individual load to supply that is more than 1/3 the capacity of the source.
 
jim dungar said:
Generators can do this, single core transformers cannot.
Click to expand...
Well, the set up I saw did provide single phase from 3-phase. The only essential difference I see in the wiring I between what I drew and the generator drawing is the polarity of T3. I may have missed another error in the phase taping.

But why do you say it wouldn't work?
 
kwired said:
I don't see you getting 120/240 out of that, if anything maybe will release some smoke?

You have all three 240 volt sides that are 120 degrees out of phase with one another in a series connection. Not exactly certain what voltages will have at each output if you don't have situation of high circulating current, which since the top corner isn't closed you might not.

Still think three separate 120/240 volt systems is best you can do unless you have an individual load to supply that is more than 1/3 the capacity of the source.
Click to expand...
Yes, I remember when I talked to a guy at Marathon a few years ago he said it needed to be derated. (We didn't do the work, if it was done at all.) And I may have gotten something wrong with the polarity of T3 due to more phase tape errors in the field. I understand the installer had to change things around to make it work. The company guy that showed me the setup mentioned that the transformers were wired in series, which seems to hold true.

The way I see it working is there is an open delta which manufactures the third side at 120V. In the genny drawing this is created by the open delta of Coils 2 and 3. Then coil 3 has it's polarity reversed to provide the other side of the 120V with the junction of coil 2 and 3 being the neutral. The two hot leads go to the ends of the Coil 3 and 1. The junction of coils 3 and 2 is not used.
 
TwoBlocked said:
Well, the set up I saw did provide single phase from 3-phase. The only essential difference I see in the wiring I between what I drew and the generator drawing is the polarity of T3. I may have missed another error in the phase taping.

But why do you say it wouldn't work?
Click to expand...
Draw out the magnetic flux path in the standard three legged core.

There is a reason that this is an industry accepted standard wiring diagram for generators but is not contained in any manufacturer's literature for transformers.
 
TwoBlocked said:
Yes, I remember when I talked to a guy at Marathon a few years ago he said it needed to be derated. (We didn't do the work, if it was done at all.) And I may have gotten something wrong with the polarity of T3 due to more phase tape errors in the field. I understand the installer had to change things around to make it work. The company guy that showed me the setup mentioned that the transformers were wired in series, which seems to hold true.

The way I see it working is there is an open delta which manufactures the third side at 120V. In the genny drawing this is created by the open delta of Coils 2 and 3. Then coil 3 has it's polarity reversed to provide the other side of the 120V with the junction of coil 2 and 3 being the neutral. The two hot leads go to the ends of the Coil 3 and 1. The junction of coils 3 and 2 is not used.
Click to expand...
Probably needs derated because you are bucking voltage with portions of the windings? Possibly will be less cost to go with three single phase transformers and feed three single phase feeders? First you would need a transformer with all six secondary coil ends accessible to change the connections I don't recall that being all that common on stocked items. Again unless you have an individual load that is more than 1/3 the overall rating of the source I still would think the three systems is the way to go if 120/240 single phase is the desired output. Or sell the generator and get one with the desired output voltage/phases. If it is a 12 lead unit it may be able to be reconfigured to do what is wanted with the zig zag connection but I believe the generator also needs derated if zig zag connection is utilized.
 
This is the 'Leyton' connection. If you want to go down the rabbit hole I followed last decade (and can fix the broken links to old forum threads)...

Leyton transformer connections

Recently a discussion of the 'Leyton' transformer connection came up: http://forums.mikeholt.com/showthread.php?t=182511 The 'Leyton' connection is purported to convert a single phase load into a balanced three phase load. Hugh Leyton had previously joined the MH forum, but has not been active...
forums.mikeholt.com

Here is a more recent thread on the topic, with a diagram:

Transformer

Straight 240 with no neutral but you can get the idea. The neutral would connect where the SBJ is located. On the picture seems like red and black wires are secondary conductors. I dont see EGC and neutral going to secondary conduit. Should not the secondary 240/120V single phase have 2 phase...
forums.mikeholt.com
 
This drawing will get you 208 on one leg, 120 on the other, and 240 phase-phase, and odd phase angles. I have re-sketched it to simplify.

All you need to do is reverse the last winding that goes to the red. Then you will get, what we call in the generator business, low dogleg output.

You will still have the same current flow inside the generator copper as if you had re-wired the generator to make this configuration natively. That would be my first choice rather than a transformer arrangement.

You haven't given any detail on the generator.

View attachment img20250702_06385614.jpg

TwoBlocked said:
This must be my week for single to 3-phase and visa versa.

Got a different customer that has special 65kw 480V 3-ph gennys that need to provide 240/120 power. They are moving stuff around and are asking for a quote. When told that at another site they had problems with using just 2-legs of the genny because it would trip on load imbalance. (yep, seen that before). So they had three separate transformers installed and wired together that produce 240/120V and load the genny evenly (or even enough). My first thought was a Scott transformer setup with genuine 2 ph power, using only 2 legs. But no, they have 3 identical 25kw wired up to provide 240/120! Sooo ... we switched to the other genny and I traced the connections, best I could. (There were some phase tape errors ...)

Came home, drew it out from the sketches I made, compared to pics, and realized there must be a black jumper separate from the black output wire. (Arrrgghh!) But which one? This looks familiar to a 12 wire 3-ph genny configured for single phase. Z something, Ill have a look see on the big G.

So does my attached drawing look right? And btw, how does this work ???

EDIT: And can this be done with a single poly-transformer instead of 3 separate ones?

View attachment 2578537
Click to expand...
 
jim dungar said:
Draw out the magnetic flux path in the standard three legged core.
Click to expand...
Per the title of this thread, this was done with 3 single phase transformers, rather than a single 3 phase transformer.

But if you could get a 3 phase transformer with a delta primary and 3 (or 6) secondary coils with all ends brought out, I don't see why you couldn't wire the secondary as a low zig-zag. Maybe your point is that such transformers are not available off the shelf, and the standard secondary configurations will all have a fixed connection that precludes a low zig-zag?

BTW, how does this parallel low zig-zag compare with a "double delta"? The latter is more symmetric.

Cheers, Wayne
 
wwhitney said:
BTW, how does this parallel low zig-zag compare with a "double delta"? The latter is more symmetric.
Click to expand...

In DD 2/3 of the current is flowing on the in-phase winding and 1/3 is flowing on the out-of-phase windings in series. Sometimes this puts the in-phase winding current slightly over spec for that particular piece of copper.

However, total ohmic heat dissipation for the whole group of copper is lower in DD than in dogleg.

Cummins and Stamford liked DD and everybody else liked dogleg. Dogleg is much simpler to wrap your brain around if you are flying by the seat of your pants.

If you are not running 100% maximum current, it should really not matter at all, and probably doesn't matter even at 100%.
 
Birken Vogt said:
This drawing will get you 208 on one leg, 120 on the other, and 240 phase-phase, and odd phase angles. I have re-sketched it to simplify.

All you need to do is reverse the last winding that goes to the red. Then you will get, what we call in the generator business, low dogleg output.

You will still have the same current flow inside the generator copper as if you had re-wired the generator to make this configuration natively. That would be my first choice rather than a transformer arrangement.

You haven't given any detail on the generator.

View attachment 2578549
Click to expand...
Thanks very, very much Birken. The generator is a gas turbine 65kw 480V 3-ph. I called the contact at the manufacturer given me and it was obvious he did not want to give me any advice whatsoever on the transformer configuration, but I believe he knew much about it. All I could get out of him was the genny is rated at 125 amps.

The low zig-zag makes perfect sense to me for providing 240/120V single phase from 3-phase voltage. Will it create a balance load? Dunno, I'm not that good. As I have said, I am uncertain if I got the diagram of what was installed correct, as there were phase tape errors. I may not have caught them all, and I did not realize at the time that some black wires were jumpers, not output power connections. Regardless, the output was 240/120V.

Looks like we will build a skid at the shop, and I can energize it momentarily with 208 3-ph on the primary and see what we get on the secondary.
 
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kwired said:
Probably needs derated because you are bucking voltage with portions of the windings? Possibly will be less cost to go with three single phase transformers and feed three single phase feeders? First you would need a transformer with all six secondary coil ends accessible to change the connections I don't recall that being all that common on stocked items. Again unless you have an individual load that is more than 1/3 the overall rating of the source I still would think the three systems is the way to go if 120/240 single phase is the desired output. Or sell the generator and get one with the desired output voltage/phases. If it is a 12 lead unit it may be able to be reconfigured to do what is wanted with the zig zag connection but I believe the generator also needs derated if zig zag connection is utilized.
Click to expand...
For a single transformer to do what is required, It would have to be a delta/delta 480V/120V transformer. Don't know if I want to embarrass myself by asking if there is such a unicorn. Besides, I have seen a three transformer bank work, but am uncertain if a single transformer would.

I see derating might be needed due to the open delta portion, which requires derating. But in this case, I am not concerned. The single phase will be going to a 100 amp manual transfer switch, so we'll fuse the secondary down to that.
 
TwoBlocked said:
The low zig-zag makes perfect sense to me for providing 240/120V single phase from 3-phase voltage. Will it create a balance load?
Click to expand...
It cannot create a balanced 3-phase load. A balanced 3-phase load of constant current has constant power delivery over once cycle. While a single phase load has time varying power delivery over a cycle, with zero power delivery twice per cycle at the voltage zero crossing.

TwoBlocked said:
For a single transformer to do what is required, It would have to be a delta/delta 480V/120V transformer.
Click to expand...
To do a low zig-zag, you'd need such a transformer with a secondary that has each end of each coil brought out, rather than factory connected in a delta pattern.

Cheers, Wayne
 
TwoBlocked said:
For a single transformer to do what is required, It would have to be a delta/delta 480V/120V transformer. Don't know if I want to embarrass myself by asking if there is such a unicorn. Besides, I have seen a three transformer bank work, but am uncertain if a single transformer would.

I see derating might be needed due to the open delta portion, which requires derating. But in this case, I am not concerned. The single phase will be going to a 100 amp manual transfer switch, so we'll fuse the secondary down to that.
Click to expand...
Do you only have 120 volt loads? If so what is wrong with more readily available 480 delta x 208/120 wye transformers?
 
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