480V 3 phase to 120/240V single phase Xfmr

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petersonra said:
You cannot produce single phase from 3 phase. What you can do is put a single phase transformer on 2 lines of the 3 phase source and that is very common.

I have heard talk that such a service might exist, but how common it is, or whether it is currently available is something else.

If you need 120/240V single phase, why not just get a 120/240 V single phase service?
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smarteng said:
Hi Winnie, thank you for the response. Yes its a fairly large building. The total load is approximately 500amp@240V / 1000 amp@120V. Mostly load is 1 phase 120v, except the AC unit, which is 240V. I am getting enormous cable like like 4 parallel of 500MCM if I use 240V. So clients wants to have a step down xfmr near building and a run of 480V. And one more thing, all panels in the building are 120/240 so 3 phase 120/208 is not really an option ( is it an option? ) that being said we need appx 250 amp@480V and the next standard service size is 400amp.
Avoiding MV since client does not really have qualified people to maintain.
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Your service is 480v 3 phases, you should run 3 phases 480v 1500ft to you bldg, put the service disconnect before then run to your bldg w/o neutral instead run egc smaller conductor. At bldg, use 3-single phase 480-240/120v Xformer. If want little more fancy, put 3 phases Xformer w tap changer then 3-single Xformer. 3 phases power is required to balance the load, otherwise it may trip cb or flow fuse.
 
Engser18 said:
Your service is 480v 3 phases, you should run 3 phases 480v 1500ft to you bldg, put the service disconnect before then run to your bldg w/o neutral instead run egc smaller conductor. At bldg, use 3-single phase 480-240/120v Xformer. If want little more fancy, put 3 phases Xformer w tap changer then 3-single Xformer. 3 phases power is required to balance the load, otherwise it may trip cb or flow fuse.
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Your service grounded conductor doesn't necessarily need be same size as ungrounded conductors, can't be any smaller than minimum required SSBJ in all cases though. Possible the EGC after service disconnect could end up a size or two smaller though.
 
wwhitney said:
Can you expand on that?

Say we have a transformer wound as in the diagram below, with 480V delta primary, and 120/240V wye/hexa(hemi)phase secondary. Say windings 1-3 are the primary windings, with 4 times the turns ratio as windings 4-9. And windings 4 and 7 are connected in series to provide a 120V/240V center-tapped secondary phase; likewise with windings 5 and 8 and with windings 6 and 9.

If the windings are rated for the current, what difference does it make if the secondary load on say windings 4 and 7 are full current on 4 only (120V load), full current on 7 only (120V load), or full current on 4 and 7 (240V load or balanced 120V loads)?

Thanks, Wayne

View attachment 2561455
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I am (obviously) no transformer expert, but to my inexperienced eye it seems like the flux from the three sets of windings on the same core would cancel each other out.
 
ggunn said:
I am (obviously) no transformer expert, but to my inexperienced eye it seems like the flux from the three sets of windings on the same core would cancel each other out.
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The single core 3-phase transformer has a core constructed with three independent legs of core, with one winding on each. (Example would be to draw a rectangle (normal core) and add an additional core metal link from the top to bottom center of the rectangle. Three windings on a single bar or square core would not work.
So the windings are technically one the same core, but the flux from each winding is on a separate part of that core.
 
petersonra said:
You cannot produce single phase from 3 phase. What you can do is put a single phase transformer on 2 lines of the 3 phase source and that is very common.

I have heard talk that such a service might exist, but how common it is, or whether it is currently available is something else.

If you need 120/240V single phase, why not just get a 120/240 V single phase service?
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Single Phase can be produced from 3-phase. Delta - T. Its an old way of doing it. :)
 
xformer said:
Single Phase can be produced from 3-phase. Delta - T. Its an old way of doing it. :)
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Perhaps it would be more accurate to say that you cannot convert a single phase load into a balanced three phase load without some sort of energy storage.

You can certainly get single phase power from a three phase source, but that three phase source will not see _balanced_ loading.

A Scott-T transformer converts three phase power to _two_ phase power, and you could tap one of those two phases for a single phase load. It would still not be _balanced_ load on the three phase source.

There is also a connection (Leyton???) where you split open a delta secondary to get something that looks like _/\. This connection fully loads all three coils of the transformer, but it still places only single phase loading on the three phase source.

-Jon
 
winnie said:
There is also a connection (Leyton???) where you split open a delta secondary to get something that looks like _/\. This connection fully loads all three coils of the transformer, but it still places only single phase loading on the three phase source.

-Jon
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Forty years ago I was told by an industry expert at Sorgel, div of Square D, that winding configuration was not practical in an inexpensive dry type transformer, although it would work in a generator.
 
GoldDigger said:
The single core 3-phase transformer has a core constructed with three independent legs of core, with one winding on each. (Example would be to draw a rectangle (normal core) and add an additional core metal link from the top to bottom center of the rectangle. Three windings on a single bar or square core would not work.
So the windings are technically one the same core, but the flux from each winding is on a separate part of that core.
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The way it was drawn it appeared to me that all three phases had windings on the same core, hence my confusion.
 
ggunn said:
The way it was drawn it appeared to me that all three phases had windings on the same core, hence my confusion.
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If you're referring to the image in my post #34, it is a single core, just one with 3 legs, each phase on a separate leg. Such a core imposes (absent leakage flux?) a constraint on the fluxes through the 3 legs, that they sum to zero at each point in time. I assume that's a magnetic analogue of Kirchoff's Current Law, that the sum of the currents going into a node must zero.

Cheers, Wayne
 
wwhitney said:
If you're referring to the image in my post #34, it is a single core, just one with 3 legs, each phase on a separate leg. Such a core imposes (absent leakage flux?) a constraint on the fluxes through the 3 legs, that they sum to zero at each point in time. I assume that's a magnetic analogue of Kirchoff's Current Law, that the sum of the currents going into a node must zero.

Cheers, Wayne
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Yeah, I totally missed the very large "Phase 1", "Phase 2", and "Phase 3" notations across the top. Duhhh...
 
winnie said:
A Scott-T transformer converts three phase power to _two_ phase power, and you could tap one of those two phases for a single phase load. It would still not be _balanced_ load on the three phase source.

-Jon
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My understanding is with the Scott-T you can have two 120/240 single phase panels that present a fully balanced 3 phase load on the 480 side.

What is Scott-T Transformer Connection?

What is Scott-T Transformer Connection? - The Scott-T connection is a method of connecting two 1-phase transformers to perform the 3- phase to 2-phase conversi ...
www.tutorialspoint.com www.tutorialspoint.com
 
tortuga said:
My understanding is with the Scott-T you can have two 120/240 single phase panels that present a fully balanced 3 phase load on the 480 side.
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Power accounting alone says the the 3 phase side could be balanced If the 120/240 loads are fully balanced. E.g. (4) identical resistive loads, supplied by the (4) different L-N legs of a 5-wire 2-phase system, is a constant instantaneous power load.

I'm not so familiar with the Scott-T, does it successfully transform that load into a balanced 3-phase load? I guess power accounting again says it must, but I'm not sufficiently confident in my transformer theory to be sure.

Cheers, Wayne
 
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Going by the OP's numbers he has approximately a 240kVA load:
smarteng said:
Hi Winnie, thank you for the response. Yes its a fairly large building. The total load is approximately 500amp@240V / 1000 amp@120V. Mostly load is 1 phase 120v, except the AC unit, which is 240V.
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And he said
smarteng said:
And one more thing, all panels in the building are 120/240 so 3 phase 120/208 is not really an option
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The advantage of the scott-T would be you could have two 120kVA feeders instead of 3 separate 80kva single phase transformers
 
tortuga said:
I was taking it as [1000Amps of 120 Volt load] plus [500 amps of 240 volt load] = 240kva.
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Pretty sure the / means "or" (after transformation), not "and". Since the next sentence says "most of the load is 120V single phase". Which would not be true if there were equal amounts of 120V and 240V loads.

Cheers, Wayne
 
wwhitney said:
Pretty sure the / means "or" (after transformation), not "and". Since the next sentence says "most of the load is 120V single phase". Which would not be true if there were equal amounts of 120V and 240V loads.

Cheers, Wayne
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Well either way you can power bigger single phase loads with a scott-T and my understanding is as long as the two single phase loads are balanced the three phase side is as well.
So if the load is 120kVA then he's limited to 3 40kVA separate single phase systems (166A each)
But with a Scott-T two 60 kVA or two 250A feeders.
 
tortuga said:
My understanding is with the Scott-T you can have two 120/240 single phase panels that present a fully balanced 3 phase load on the 480 side.
Click to expand...

If you have two balanced single phase loads, connected to a Scott-T, then that is essentially a two phase load being converted to a 3 phase load, and could be balanced.

But if only one of those single phase panels was actually loaded, then the 3 phase would not be balanced loading.

Jon
 
jim dungar said:
Forty years ago I was told by an industry expert at Sorgel, div of Square D, that winding configuration was not practical in an inexpensive dry type transformer, although it would work in a generator.
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Generators were where I have seen that connection. One of many that can be used with a 12 lead machine, and it does load 3 windings when supplying single phase loads.
 
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