Step Up Transformer Build 240v to 415v 3 Phase. REVERSE FEEDING A TRANSFORMER HELP!

sapwood · Jan 8, 2018

So im on the hunt for a suitable transformer that i can use to wind my own step up transformer for a 240v single to 415v 3 phase converter system that will be used within the home workshop where we havent a dedicated 3 phase supply.
So im looking on the usual sources for a transformer and i find one that in theory looks like exactly what i need, though this one would need feeding in reverse to give me what im wating output wise.....
Its a 6kva 415v 3 phase input to 220v single phase output.. So i was figuring what the effects would be if I flipped this transformer and feed the secondary with 240v and what I would get out of the primary 415v 3 phase side as an output?
heres some pics of what i was looking at..

adama · Jan 8, 2018

Yep thats it, just change the socket for a plug and the plug for a socket, but there may need to be some more too it than that, thats a 110V plug on the front, (purple low voltage DC, yellow 110Volts here, blue 240V and red 415V three phase) hence you probably need to change its internal tapping, as based on the plugs on that its setup as 415V three phase in and 110V single phase out! other wise if you apply 240V to thoes, you may well find you get 830V out if the insulation holds out which is not as likely!

There is also the question about neutral or not on the three phase side, Neutral needs a star transformer output here in the uk, as it needs to be 240V to neutral on all three phases 415V phase to phase.

Hence that may or may not be the transformer your looking for, you need to delve deeper!

jhovel · Jan 8, 2018

You cannot make three phase power from singe phase input with a tansformer. Sorry.
However, my suspicion is that the input of this transformer is from a 3-phase PLUG, but connecting only 2 phases to the transformer to have 415V SINGLE PHASE at the primary. In other words, it may well be a single phase transformer to isolate the secondary 240V from the primary. So it may be EXACTLY what you are looking for!
The transformer doesn't know or care which end produces it's core magnetic field - it just produces electricity in the other coil. THere is no problem with feeding 240V in one end and getting 415 at the other - but it will NOT be 3-phase. For that you will need a 415V VFD that will work (or can be fooled to work) on 415V single phase. It will then produce 415V 3-phase power for you at a variable frequency.

I've got a 1kW transformer just like this and it's running a 900W 415V 2-speed 3-phase motor for me just fine - with a 415V 2.2kW VFD.

metalmagpie · Jan 8, 2018

Using a step-down transformer backwards is commonly done. But you do sacrifice a bit of efficiency. I say go ahead IF 6 kVA is big enough.

metalmagpie

northernsinger · Jan 8, 2018

Yep, I ran a transformer backwards for my 440, it worked just fine.

JST · Jan 8, 2018

OSHA will never be through there to see, so no chance of a complaint about it.

You will not get 3 phase out, of course, You need a converter for that, as you know.

I can;t read the label stamping clearly, it appears to be 3 phase wound, and if so any pair of windings will be only 2 kVA , 1/3 of the total 6 kVA it handles at 3 phase. (each pair handle 2 kVA, total 6 at 3 phase)

Mark Rand · Jan 8, 2018

Absolutely ideal for the job. I use a single phase transformer to feed my 415V inverter, but that one will happily go on the output of a 240V converter. Price is ok too

When you wire it up, earth the star point on the output. That gives you a nice 'neutral' that's referenced to earth.

9100 · Jan 8, 2018

You can make a single phase output from a three phase transformer by seriesing all three secondary windings. If you get the polarities right, you will be drawing power from all three legs of the input. Reversing it won't work. This appears to be a custom unit since the numbers seem to be scratched in. Without getting into it and seeing how it is connected, there is no telling what it is.

Bill

Mark Rand · Jan 8, 2018

It's a three phase transformer. The box on the side is a three phase MCB for the output. The 110V single phase socket is almost certainly tapped from a single winding.

sapwood · Jan 8, 2018

thats what i found strange seeing that yellow plug, but it does say on the plate which i included the picture that its deff a 3phase to 220v single phase step down transformer....... i shall delve deeper though

sapwood · Jan 8, 2018

jhovel said:
You cannot make three phase power from singe phase input with a tansformer. Sorry.
Oh i know and your completely right. but i have plans from unique 3 phase and i was going to follow those plans to make my own single to three phase supply. im just at the Find A Suitable Transformer Stage, which is why i am query this one above.

sapwood · Jan 8, 2018

my main question was really "would the laminations of this be a good suitable donor for me to wind my own transformer? ive got plans for this from Unique 3 Phase and ive not looked at them too deeply just yet im just lo for a suitable transformer to do the project

Garwood · Jan 8, 2018

I haven't had very good luck boosting voltage with small transformers. I burned up a big pile of control transformers trying to make a machine work once. However, I have done it with several 30-45KVA Delta transformers supplying 50% of less of their capacity no problem at all.

I just mention it because 6KVA is a pretty small little guy.

What is your actual phase-phase voltage? What is the load you are needing to power?

My 240V is 255 actual.

Peter. · Jan 8, 2018

Something screwy going on there. Why even need a transformer for 415-240 when all you need is one phase and neutral? Yellow plug is for 110v but it might just have been a replacement for a broken blue one. Poor practice but I've seen it done.

EPAIII · Jan 8, 2018

There is no transformer that, by itself, will convert three phase to single phase or single phase to three phase. A three phase transformer is simply three single phase transformers, perhaps in the same enclosure.

Now I also can not read the phase rating in your photo of the nameplate, but if your unit is really three phase then it is actually three single phase transformers. If you use just one of them to convert from 220V to 415V, then it is not going to be rated at that full 6 KVA rating. You will be at half that or even less. You could try to parallel the separate transformers to get the full rating, but do be very careful when making the connections because if you get one of them backwards, you will have a very spectacular sound and light show and probably a puddle of melted copper. In any case, you do need to know exactly what you have before you try any new wiring schemes.

jhovel · Jan 9, 2018

Mark Rand said:
It's a three phase transformer. The box on the side is a three phase MCB for the output. The 110V single phase socket is almost certainly tapped from a single winding.

Oh, I hadn't recognised that - They don't look like that that in this continent.

Sapwood: As EPAIII said, if you find that is correct (i.e. there are three phases on the output), then you have effectively 3 x 2kVA transformers on the same core. Each one can be used as a single phase 240V in to 415V out transformer. However, ALL UNUSED COIL END WILL BE LIVE! Don't ignore them!

jhovel · Jan 9, 2018

9100 said:
You can make a single phase output from a three phase transformer by seriesing all three secondary windings. If you get the polarities right, you will be drawing power from all three legs of the input. Reversing it won't work. This appears to be a custom unit since the numbers seem to be scratched in. Without getting into it and seeing how it is connected, there is no telling what it is.

Bill

Bill, would you explain that statement please? The way I see it, if you put all three phase windings in series, you get zero magnetism in the core. Each phase is 120 deg out from the next, so you would get the sum of every pair of phases cancel out the magnetism created from the third one..... The only way you can single phase from 3-phase is to use just one pair (or one phase and neutral) as a primary. The magnetism of the core will then follow the current flowing in the pair and generate power in the secondary at whatever voltage it's wound for.

or am I missing something?

9100 · Jan 9, 2018

Reverse one winding and they add vectorialy.

Bill

9100 · Jan 9, 2018

Here are the vector diagrams of a standard delta and single phase configurations. Note that the B and C windings must be made twice the turns of A to balance the load on all three phases. The power factors on B and C will also be goofy.

Ignore the fuzzy nature of the lines. I have to go through several programs to get the drawing into a JPEG and one of them adds the fuzz.

Bill

Monarchist · Jan 9, 2018

EPAIII said:
There is no ^^^ static ^^^ transformer that, by itself, will convert <redacted> single phase to three phase.

Fixed that for yah.

The idler in a Rotary Phase Converter is being used as a ROTATING transformer [1].

Dynamic relationships, rather than fixed IOW. Or one could say forever chasing its own tail, relative phase-angles-wise.

3-P to 1-P OTOH, is as simple as CONNECTING TO but one of the three phases, whether tapped for any specific voltage or otherwise.

[1] And yes, one CAN build an all electromagnetic Oscillator to derive multiple phases from but one, and "static", eg: with no gross-motion parts.

They just don't happen to be as efficient - most especially as to space, mass or cost of materials, nor as stable under load - too easily knocked off their carefully tuned perch - as an ignorant RPC is inherently.

Step Up Transformer Build 240v to 415v 3 Phase. REVERSE FEEDING A TRANSFORMER HELP!

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