# three phase transformer question

• posted

I have a three phase transformer, with which I want to do something interesting to me. It is shaped like a letter E, with three separate transformers, one on each leg.

Its output is essentially six leads, two from each leg.

Let's say that they are

A1 B1 C1

A2 B2 C2

A, B and C are three transformers on separate legs of 3 phase (separate horizontal pieces of letter E).

I want to feed this into a "three phase rectifier", which takes just three incoming wires.

I think that I have to tie together A1 to B2 => X, B1 to C2 => Y, and C1 to A2 => Z.

Tie points X, Y and Z is what is fed into a 3 phase rectifier.

Is that correct?

i
• posted

Unless there is another set of 3 windings, this is not a 3-phase transformer.

• posted

In fact, yes, each of these secondary sets is in fact two windings. They are parallel.

If I call them Atop and Abottom, Btop and Bbottom, Ctop and Cbottom, there are six leads coming out of the secondaries.

I can draw a picture on paper.

i
• posted

Don, here's the schematic of this transformer:

• posted

It depends on how much voltage you need out of it. It depends on the three phase rectifier. Full wave or Half wave.

Full Wave would have six rectifiers Half Wave would have three rectifiers.

If you have the correct taps, you can get twelve phase power out of three phase inputs, for really smoth, low ripple output. You could also do this with two transformers, one connected delta, and one connected WYE, giving you six phase output power.

Yes, with proper transformer connections, you can convert from three phase input to any type of : 2,3,4,5,6,7,8,9,10,11,12 etc. phase outputs. Basically any polyphase system is convertable to another polyphase system.

The original generation system at Niagara Falls, used two phase generators that were transformed into three phase output power.

There are many ways to connect your transformers up to some kind of rectifier circuit.

Pete

• posted

OK. You can probably assume that the windings are all polarized the same, with "start" on the left and "finish" on the right.

There are still many ways to connect it, including

primary delta, secondary delta primary delta, secondary wye primary wye, secondary delta primary wye, secondary wye

several variants using each winding as an autotransformer for that phase

etc

The choice will determine the transformation ratio and therefore the output voltage and current capability.

• posted

That is a 3 phase. Now you need to decide to wire the secondary (which set) to the rectifiers - by wiring the secondary in a WYE.

The same leg - phasing counts - so often like you drew - a leg is drawn to the right or the left - e.g. wire all right pointed ends together and that is a common. Then the other three are those that go to the rectifiers and come out as a + or a -. (assume three diodes for the rectifier).

The primary can be Wye or delta as wanted or needed.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member

Ignoramus8495 wrote:

• posted

That's interesting. Maybe I should start another thread.

I have a 3 phase CC welder that is now configured as a six phase rectifier, similar to this schematic:

I acquired (many thanks to wonderful people at PCTI) a PCTI three phase firing system. It can control both current as well as voltage.

What I want is to convert this welder to both CC as well as CV so that I could MIG weld as well as TIG.

I have two alternatives:

1) Try to somehow make this six phase rectifier to be controlled by the PCTI firing board. The current configuration, where the gates face the common positive bus, is not compatible with the PCTI board, since it needs to get synchronization signals on K2, K4, and K6.

If I invert the SCRs so that the bus is negative instead, maybe it will work, I am not sure.

2) Reconvert this transformer:

to output three phase 60VAC power, on three leads, to be rectified by a setup similar to schematic of PCTI.

i
• posted

Thanks. This is for my welder. I have a 3 phase firing system from PCTI that could convert it to CC/CV welder. The voltage across each half of each secondary (there are six of those) is 66 volts.

That is the voltage needed for welding.

Right now the current rectifier is similar to this schematic:

The schematic for the PCTI firing system is different:

So, to get full output at 66 volts, as I would need for welding, I would probably just need to rewire it, as follows. Note that the picture now has all leads numbered A1...C4:

1) Would this be 3 phase delta?

A1, A3, C2, C4 ==> X A2, A4, B1, B3 ==> Y B2, B4, C1, C3 ==> Z

2) Would this be 3 phase Wye?

A1, A3, B1, B3, C1, C3 ===> O A2, A4 ===> X B2, B4 ===> Y C2, C4 ===> Y

Both of these, I hope, would give me 3 phase output at voltage suitable for arc welding.

i
• posted

Probably, if polarities are as assumed.

• posted

Ignoramus,

You don't have to assume the polarities of your secondaries. It's easy to check them. Your transformer most likely has a primary coil for each pair of secondaries. Let's call the primaries for your A pair of secondaries H1 and H2 and for your B pair of secondaries H3 and H4 and your C pair of secondaries H5 and H6.

If you connect H1 to A1 and apply a small voltage across H1 and H2 and read the voltage from A2 to H2, it will either boost or buck your voltage depending on the direction your secondary coil is wound. By repeating this with your coil A3 - A4 you can match your two A coils. And repeat this with H3 to B1, apply voltage across H3 H4, and read from B2 to H4. And so on.

Don

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