I want to make an arc welder from this stripped transformer. As far as I know it is a 3 phase transformer that could transform 380V to 12,24,40. I know how a regular 2 coil transformer works, but I don't understand the need of these many wires.

The 4 on the back were cut, the top 3 on the front go into the voltage switcher, bottom 4 on the front go into another switch(which I guess is for amps). There are lots of loose wires and missing parts. I want to make something useful out of this. Do I have to rewire the transformer to be useful for me?

I was thinking to make the 2 outside coils the primary and hook it up to 230V. The middle to be the secondary and use less winding. I don't know if this would work at all.

Any help?

Update: So I mesaured the 3*4 wires on the back side each 4 are linked which I guess means 3 phase + neutral and that should mean star topology. The problem is I only mesaured 1.6 ohm on these wires that's very low isn't it? The 3*10 wires mesaured 0.9-1.3 ohm, but all of them are linked when I tested it with continuity and resistance. Does that mean that all of the wires are melted/corroded? I have also noticed the remains of a blue and red wire and 3 white, the 3 white colored wires are much thinner.

enter image description here enter image description here

  • 1
    \$\begingroup\$ Power transformers dont have an air gap to regulate current. \$\endgroup\$ May 19, 2018 at 18:58
  • \$\begingroup\$ Transformers can have a lot of winding taps to compensate for higher or lower source voltage. There could also be additional windings for various reasons. Do you know what the transformer was originally used for. There is really no way to know very much from the information given. \$\endgroup\$
    – user80875
    May 19, 2018 at 20:35
  • \$\begingroup\$ I have noticed the remains of a blue and red wire and 3 white, the 3 white colored wires are much thinner.(so i guess it's transformed 380v 3 phase to 12-24-40v dc hence the wiring colors on the back ) So maybe it was used for charging batteries, my grandmother says that it was a transformer for radio equipment, no clue. \$\endgroup\$
    – Attila
    May 20, 2018 at 20:06
  • \$\begingroup\$ Take a look at this 12 wire generator wiring diagram sheet. power-tronics.com/help/wiring-diagrams/Terminal-Chart.pdf \$\endgroup\$
    – user57037
    May 20, 2018 at 22:50

2 Answers 2


Transformers are often manufactured to handle a variety of input and output conditions so they can be marketed to people with different sources and needs.

Most likely, the side with 4 wires is going to be your primary. The use of 4 wires instead of 2 allows it to be configured either in series or in parallel depending on the input voltage.

An example from the datasheet for the first power transformer that appeared on digikey can help illustrate the probable situation. In this case, 230V in series or 115V in parallel both put 115V across each half of the primary windings to produce the same desired output voltages on the secondaries.

enter image description here

Your secondary may have been built with similar variety of options and numbers of outputs in mind.

For testing:

Never trust markings that aren't permanent and from the manufacturer. You will have to start by doing continuity testing with all the possible connections to establish a winding pinout. Keep in mind that center tapped windings may exist, so don't stop when you find continuity on a single wire. Test for continuity to the frame as well in case you have any damaged windings. Ideally, a high voltage insulation resistance tester would be a good idea to verify the winding integrity.

Unfortunately, resistance measurements alone aren't reliable to determine the winding ratios. I highly recommend doing your winding ratio testing with an isolated low voltage signal generator. Testing unknown transformers can be extremely dangerous, so always assume you could be dealing with lethal levels of high voltage and start with the lowest test voltage that gives you a dependable measurement.

  • \$\begingroup\$ This is a three phase transformer, not single phase. OP's four input wires are maybe three phase and neutral, but it's more common for the primary side to be delta-connected. Unless one is a chassis earth? \$\endgroup\$ May 20, 2018 at 9:43
  • \$\begingroup\$ I'm going to make mesaurments when I will have the chance next time. Is there a difference between delta and star windings when I mesaure mith a multimeter? I know how they should be wired, but I can't see it without cuting into the protective shell which I don't want to do. If all the 4 wires are connected that should mean 3 phase + neutral which should mean star connection right? \$\endgroup\$
    – Attila
    May 20, 2018 at 14:47
  • \$\begingroup\$ @SomeoneSomewhere, look at the picture. There are 4 wires coming from each of the three phases. 12 wires total. I believe this is a totally uncommited three-phase primary. So it could be wired delta or star and grounded whichever way you desire, also. And each phase could be set up as low voltage (windings in parallel) or high voltage (windings in series). Of course I could be wrong, too. \$\endgroup\$
    – user57037
    May 20, 2018 at 20:22
  • \$\begingroup\$ I'll admit some ignorance and clarify that I'm definitely not a 3phase transformer expert. My loose understand and assumption was that this is effectively 3 single phase transformers that can be wired to handle a 3phase input. It seemed unlikely that a 3phase input would be used for a homemade welder, so sticking with single phase transformer diagnostics made sense. If anyone can clarify why that's not a safe assumption, I'll gladly edit or delete the answer. \$\endgroup\$
    – Phil C
    May 21, 2018 at 4:28
  • \$\begingroup\$ I think it is a good answer. It applies to each phase of the 3 phase transformer. I think that there are basically two windings on each phase, hence 4 wires per phase. This provides maximum flexibility in wiring the transformer. I suspect that the secondary has very many taps to allow for a wide variety of different voltages. \$\endgroup\$
    – user57037
    May 22, 2018 at 5:05

What it looks like to me is that each of the three phases has two windings. So you have two windings, each with two wires. That is four wires per phase. Depending on how you deal with those windings, you can operate in high or low voltage. Series for high, parallel for low.

Then, you can connect the separate phases together either in delta or star. And you have a variety of ways to connect neutral as well.

Here is a screen cap from this document: https://www.power-tronics.com/help/wiring-diagrams/Terminal-Chart.pdf

enter image description here

As you can see, there are 12 total wires giving a variety of configurations.

As far as the low voltage side goes, my take on that is that there are a wide variety of taps on the secondaries so that different output voltages can be achieved. I believe this is fairly standard with transformer-based welders. Some newer welders may use AC-DC conversion techniques instead of a line frequency transformer.

I think you basically have three separate transformers. So you can try stimulating one of the primaries with a small voltage until you figure out what is what. Obviously this can be quite dangerous. You accept all risk (don't blame me if you get shocked or electrocuted).

I can't tell from the picture whether some of the various primary wires are connected together using jumpers on the far side of the terminal block. I apologize if this is too obvious, but when measuring resistance on the primary wires, you need to be sure they are not connected together at the terminal block. I recommend you disconnect the wires altogether from the terminal block if you are not sure. Label them first so you can get them back in the same order when you are done. My expectation is that you will have two pairs of wires that Ohm out to a low value. But the pairs are high impedance to each other.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.