Rewiring transformer from 3 phase to single phase

I have a three phase delta-delta 480-240 transformer I would like to repurpose as a single phase 480-240V. Is this possible? It is rated at 30KVA, I would be happy with 20KVA single phase.

About all you can do is use one set of windings and ignore the rest. That would give you 1/3 of the original capacity. The windings on the other two legs will have voltage on them but it will be half of normal. You can use these voltages but you will not gain much in capacity because it will be limited by the primary winding on the first leg.

Bill

What if I reconfigure the windings so that all the primary windings are all in parallel, and all the secondary windings are in parallel? obviously I would have to be careful to get the directions of each winding correct.

thermite said:

Suspect you'd also have to 'be careful' in sawing the core laminations into three separate sections, space them well apart and/or turn them at right angles in three dimensions to ALSO not have the magnetic fields going insubordinate to your plan.

The physical layout works WITH the timing of 3-phase, against it for single phase, which ends up 'in-step' only once, 'out of step' twice.

Try to tap the sibling coils, and you could see waveform distortion.

That was what I was afraid of. What if I use only the two ouside coils and wire them in the opposite direction so that the flux from each outside leg is going in the same as it passes through the centre leg.

See threads on running 3-P DC welders with the Hass-Kamp circuit where that is used as an asset rather than a liability... so long as one is rectifying the AC anyway. Otherwise, NOT.

Bill

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That is what I was afraid of. What if I use only the two outside coils and wire them in the opposite direction so that the flux from each of them is going in the same direction as it passes through the middle leg.

SBAER said:

That is what I was afraid of. What if I use only the two outside coils and wire them in the opposite direction so that the flux from each of them is going in the same direction as it passes through the middle leg.

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You run out of middle leg. A typical single phase transformer with three legs has a middle one that is twice the area. You can run one with windings on the outer legs just as well but that uses up more wire for the same output. If you reduce the middle leg to the same area as the outer ones, it will saturate at half the voltage on each outer winding because each will be feeding flux to the center.

Bill

Edit: I think that if you connect the outer windings so the flux is going around in a circle through the outer legs, the paths through the middle one cancel so it might as well not be there.

The basic issue is that flux from any one coil will split, but not quite evenly, through the other two legs. (one path is longer/higher reluctance).

However, imagine that you are in the 3 phase situation where two wires are at 86% of max voltage..... in that case, you have current in two coils, and zero in the other. The transformer "works" for that case, and what you propose is essentially ALWAYS being in that case. the third coil would get NO flux then, it will always cancel.

if you draw it out as angular phases, they no longer rotate, they "pulse" in amplitude. Never tried it, but no really good reason (before I have my coffee) why it won't work.

9100 said:

Edit: I think that if you connect the outer windings so the flux is going around in a circle through the outer legs, the paths through the middle one cancel so it might as well not be there.

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That's essentially the case I was suggesting.

It absolutely works for the case where the 3rd phase wire is at zero... If the circuit is "single phased", it does not necessarily instantly go wonky....

What you would do is to wire it as if the unit were being wired for 3 phase. But you 'forget to connect" the 3rd phase (which does not exist). Now it is operating as a "single phased" 3 phase line, at reduced capacity.

There is the tiny detail that each "phase" can be a pair of wires.....considered as delta. Delta and wye are indistinguishable from "outside the box", as far as the line currents.

The relative voltage between any pair of wires, can have only ONE possible phase relation.... one more positive than the other at any given time. *Which* is more positive, alternates between them.

And, the three phase as a whole is simply the superimposition of three separate circuits, commonly and loosely called "phases", which happen to have a certain phase relation between them. However, while the line currents in the B-C and A-C "phases" do add up with the A-B phase, the system is "linear", there is no interaction, each is independent.

The core flux due to each is independent of the others, although they add up.

if voltage is only supplied to the A-B phase, the missing B-C and A-C phases affect line current, but not voltage... and there will be an A-B phase on the secondary

That being the case, the one pair of wires can accept single phase power in, and the energized secondaries will then supply power to a load....

And because the system is generally linear, a missing phase or phases do not affect the ones still present.

Bottom line is that yes, there will be output. And it will be the same voltage as if all phases are present. Current may be different, and only the output which is due to the remaining phase will be present.

If you hook electric heaters up in delta to the secondary, and turn off two, you have the same basic situation as far as currents are concerned. The sky will not fall, and physics is not flouted.

thermite said:

They are not interested in stepping-up from carrying a one-third share of the nameplate kVA rating each to carrying a 1/2 share each without stubborn protest and rude behaviour.

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Eh.........?

Not the point.

FOR SURE the power is reduced..... to about 58% of nameplate or so, assuming you power, say, the A-B phase. Who suggests that the thing will run full power at the same frequency/voltage with only 2 instead of three windings? Not this boy....

Nope, point is that the superposition principle says that it will pass power just like it did before, as much as the windings *in use* would have done to begin with.

The fact that it is a 3 phase transformer does not mean it won't pass single phase power. What it does mean is that there is a lot of unused iron and copper when you do that.

That unused copper etc is one really good reason that nobody wastes label space on a single phase rating......

The three legged core in a three phase transformer has another advantage. Three single phase cores need return legs while that function is performed by the other two legs, sort of time sharing with the other phases. Sawing the core up just leaves you with left over copper and iron you can't do anything with. Exciting two windings just uses more wire to get the core up to circa 15,000 Gauss instead of doing it with only one winding. In the end, you will only have 1/3 capacity with two wasted windings. Work out the voltages and flux densities for other configurations and you will see that all you may gain is a slight reduction in core losses. If you were closer, I would make you a good deal on a 25 KVA single phase transformer, but shipping would be a deal killer.

Bill

I am in Canada a 480/240 single phase transformer is a bit of a rare animal, I have not been able to find a transformer locally at a price that makes sense for this application. The transformer I have has no value here in Canada. I am going to do some experimentation later this week, I will borrow a scope from work to look at the wave form. One problem I will have is that I will not be able to test this under load. The application is to power a 480V 25Hp VFD with single phase. The end user will use the machine intermittently and probably never at full power. Whatever happens I will post the results.

I use a 20 kva 240/480 Delta/ wye dry trans to up my shop voltage.

Three VFD's on it. Line in to the two outer coils.

Seems to work fine, at least it has for the past five years. I leave it power on continuously and have not noticed a cost on my power bill.

I would pull the front panel and take a photo, but there is a Carver lab press on top, and I'm afraid if I pull the panel, without support, the entire chassis will fall over. ;-)
I'm not man enough to lift that 20 tom hydraulic press off in one piece, and dissasembly is work!

At any rate, 3ph x-formers can be configured to transform single phase power. At least this one can.

Cheers

Cal

I've hooked up the two outer coils in parallel, tried it in two different directions, one made a lot of noise so I did it the other way. With the transformer unloaded, the current draw on the primary (240V) is 7A, I am, hoping this is mostly reactive power, it sounds pretty normal. The output voltage according to my true RMS Fluke multimeter is exactly twice the primary.

I should have mentioned, that when first attempting to "reconfigure" this 3ph x-former to single phase bost duty, I made use of a Variac and a pair of amp clamps along with a pair of VOMs. I did get some nasty growling with a couple of my wire selections. She purrs like a kitten when the electrons all flow the right way.

Sounds like you have it!

Good on ya"!