Can I Build a DIY Welding Transformer From Scratch?

[ferball]

I built a welder using Microwave oven transformers, but I keep melting the secondary coil, and they are a pain to rewrap. So I was thinking about building a transformer from scratch using available materials. My first obstacle is the core material, I know laminated core material is recommended, but I think I will have an easier time finding stuff for a solid core. Bad does a solid core effect performance? Is it just a heat issue, or is output seriously compromised? My original thought was to use steel/iron pipe with about a 1 inch diameter, would that work, or does the pipe make it more of an air core?

And lastly how do I calculate output? I know I will have a standard 110v input where are my amps coming from?

 

[sophiecentaur]

I don't think it's a matter of "just a heat issue". If the core conducts then won't you have, effectively, a shorted turn in the transformer, made of a thick piece of iron? I know that Faraday's ring was just a loop of iron but he was only using pulses of current. If you could get hold of lots of iron wire, you could make a ring of it with many turns and that should provide an iron core with no eddy currents. I heard once that old car ignition coils used a core made up of many strands of iron wire.
btw, I think that iron and not steel is essential because of the hysteresis of steel.

 

[sophiecentaur]

Oh yes, and the primary current will be whatever the output current is divided by the turns ratio. (inverse ratio to the voltage ratio) But you need some mechanism to limit the output current because an arc is a pretty good dead short. You'd need to read up on this as I think it's a bit of specialist engineering, to get it right. I think that could have been the trouble with your other attempts with the transformers you had.

 

[ferball]

I am thinking of possibly using "garden wire" which is an insulated steel wire for the core. This should reduce the eddy currents, but I do realize that steel will have some loss due to hysteris. Also my understanding is that a shunt should limit the output current, and a movable shunt will give me some adjustablity for current. Any thoughts? and what kind of math am I looking at to figure the shunt size, or is trial and error doable if I am not looking for super precise numbers?

 

[sophiecentaur]

To be honest, I reckon this project is a bit ambitious to start your transformer building career on. You are short of experience of materials and design and, also, it involves fairly hazardous quantities of volts and power!

I have actually wound straightforward, low power, step down transformers (using a kit of laminations and a formers) where the spec was fairly non-critical and they did their job ok. This is a serious quantum leap, though. One problem is that a welding transformer should not be what you'd normally call a 'good transformer'. It needs significant leakage inductance. Do you have test equipment to measure its performance before you plug it in? It worries me that you could be making a very effective smoke-making machine. That would be a significant loss of your time and money investment.
If you really need a welding transformer, why not look for a s/h one?
I know that's not what you want to hear but, as an Engineer with a fair amount of experience here and there, I have seen my share of projects that failed because of over-opimistic starts. (Government software projects, for instance!)

 

[Averagesupernova]

A good transformer is made with what I believe is commonly called electrical steel. It has a higher than normal content of silicon. I have toyed around with the idea of building a plasma cutter but decided a better option was to just buy one and that is what I did. But, transformer design still somewhat interests me. For current limiting I would use a saturable core reactor. Do a bit of googling on that as there are some experimenters out there and a person can learn a lot from their experiences.

 

[Carl Pugh]

Why don't you use good quality film insulated copper wire for the secondary?
Some of the film insulated wire is rated for 220 deg C operation.
Another option would be to use copper tubing for secondary, insulate the tubing with tape and then run water through the copper tubing.

 

[ferball]

Ambitious...maybe but I don't think so, Arc welding technology is a hundred years old, and I am not looking to make a super deluxe ultra efficient transformer, just something that will take a 110v ac current on a 15 amp breaker and get maybe 60 amps and 20 volts out without popping the breaker.

Electrical steel would be nice, but part of the aim of this project is to make it out of available/low cost materials I don't think electrical steel counts. I looked at saturable core reactors, sounds like something out of Iron Man, and it will bear further experiments, but I think the DC circuit makes a saturable core an unrealistic option as the added complexity and engineering to get the appropriate values would put this into the "ambitious" category.

The secondary winding on my first attempt would most likely hold up if I switched my 12 wire to 10 wire, but limited space in the core makes it hard to get the appropriate amount of turns, and I think the idea of building from scratch would be educational, plus I can't find a lot of info on it out there so it would be nice to get some "practical" information about a project like this out there and circulating.

 

[ferball]

Here is another thought I had today regarding my DIY welder and it might be easier to do from a design perspective. What if I made like six small transformers then wired them to together, it would allow scalability and some adjustment, by adding or removing a transformer from the circuit. What is the math for combining transformers in series/paralell? And what about the secondary winding series/paralell? and can different transformer sizes be wired together, or do they all need to be fairly close in size? My understanding is that the primary coils need to be fairly equal, but secondaries can vary with no problem. Any electrical engineers want to shed some light?

 

[Averagesupernova]

Multiple transformers in parallel is not a good idea. The transformers need to be virtually identical. If they are not, when under load one transformer can end up supplying almost all the current. Kind of defeats the purpose doesn't it?

 

[sophiecentaur]

If you want to have multiple transformers then I could suggest that, rather than using parallel connections, connecting several Low Voltage transformers in series might be a better suggestion. Relatively few secondary turns on each transformer could be really fat and could handle plenty of current. Series connection could provide whatever voltage you needed.
Any lack of uniformity in the designs would only result in slightly different voltages across each transformer.

 

[ferball]

Now is that series wiring for both the secondary and primary windings?

 

[sophiecentaur]

Only for the secondary windings. Isn't that obvious?
The primaries would be in parallel across the mains. There would be no possible advantage in sharing the mains voltage amongst all the primary windings in series. The point is that you could be getting a high current out of low voltage secondaries and their voltages would add up, in series, to a few tens of volts, necessary for the arc.

 

[ferball]

Only for the secondary windings. Isn't that obvious?
The primaries would be in parallel across the mains. There would be no possible advantage in sharing the mains voltage amongst all the primary windings in series. The point is that you could be getting a high current out of low voltage secondaries and their voltages would add up, in series, to a few tens of volts, necessary for the arc.

Was an obvious one, just checking myself and showing my ignorance.

 

[sophiecentaur]

Thinking about it again, I think there may be no difference, actually, assuming the primaries have the appropriate number of turns. I think turns ratios and ampere turns would make the sums come out the same, however the primaries are connected. What I said about the secondary connections must still be true, though.

But, of course, it would be better (and probably cheaper) to use standard (thinner) primary windings - they could even be already on a transformer you could cannibalise. But partly dismantling transformers can be a bit fraught.

 

[ferball]

I built a toroidal coil by wrapping galvanized electric fencing around a soup can. It comes out to about a 3 inch circle that is about 5/8 of an inch thick by a little less than an inch wide. I put about a thousand turns of 30gauge magnet wire for the primary coil, it gets a little warm, but it does not blow the 15 amp breaker. So far so good. I may add a few more turns before messing around with the secondary. I don't know if it will actually work for welding, but it is kinda cool making a transformer from scratch.

 

[Bob S]

Superior Electric used to make a 1000-watt (actually 1000VA) toroidal transformer with a 120-turn primary and a 2-inch diameter open hole in the center for winding your own secondary, about 1 volt per turn. It was called a TP1000 Flexiformer. They occasionally appear on eBay or in surplus lists (NOT the TP150 Flexiformer).

Bob S

 

[sophiecentaur]

I built a toroidal coil by wrapping galvanized electric fencing around a soup can. It comes out to about a 3 inch circle that is about 5/8 of an inch thick by a little less than an inch wide. I put about a thousand turns of 30gauge magnet wire for the primary coil, it gets a little warm, but it does not blow the 15 amp breaker. So far so good. I may add a few more turns before messing around with the secondary. I don't know if it will actually work for welding, but it is kinda cool making a transformer from scratch.

Absolutely. You can't beat actually making something work. Beats those pesky simulations any day.