Welder made with microwave oven transformer

This is borderline as far as electronic is concerned but here it is anyway.

Making it is very easy, just a matter of finding 2 old microwave ovens (the older the better), removing the secondary winding of each transfomer and replacing it with 20 turns of gage 10 stranded wires, wiring these 2 transformers in serie to make the base for the welder. It is very simple.

I plug each transformer in different circuits in the house. Even a strong 20 Amp. circuit is not enough to carry the load of the 2 transformers together.

As for any other welder, using a suitable welding rod is crucial. Preheating the work make the starting easier.

A lot more about MOT welders can be found on YouTube including the warnings about discharging safely the high voltage capacitor inside the Microwave before doing anything else.

More Pictures!

I am not a good welder. This is a skill that has to be learned by practice.

Hi,


Very nice pics, and very interesting subject too. I've known others who made welders out of these transformers but i think they used multiple transformers like 5 or more to make one welder.

You get a good strong arc?

Yes, I get a good and strong arc. But I consider this thing to be good for small works only. I would not trust it if I had to build a trailer.

I just tested it and the output voltage is 41 volts. It is also outputting 152 Amps when shorted. I don't know how much when it is welding. Probably much less.

In fact I was thinking that the output could be near 40A when welding but maybe it is more then that. I read on the net that it sould be around 70A. I was very surprise to see 152A on my clamp meter when I shorted the 2 leads.

I built this machine last summer. The picture show a work that I have done then: Replacing a steel plate that was rusted. I will have to replace the other one too.

I've built one of these a couple of years back using 6 microwave transformers, it wasn't very powerful and had to be powered from the 32A cooker ring main. I hacksawed the 2kV secondary windings off and threaded 8 gauge wire through for about 15 turns (as much as will fit). On a second attempt I tried cutting the transformer apart along the weld and trying to fit more turns more compactly in the space. I then welded the transformer back together using the first arc welder, but this buzzed a LOT and was way less powerful. I think I might have ruined the magnetic field by cutting it apart. It was quite hard to start an arc, and could only weld up to 5mm steel sheet with 2.5mm rod. It worked fine for welding bike frames together, which is what I built it for.

Jules

Hi,

Im surprised they weld them at all because the steel used for transformers is grain oriented and it could loose that property if heated to near melting point. It must not affect enough of the steel to matter too much but would probably work better if it was not welded but rather done with a more normal technique.

I've experimented with mot welders too, and added a tig welding function.
My results were better when I removed the magnetic shunt from the tranny, if fits between the primary and secondary, looks like you pulled it out too, its there to improve the coupling between the magnetron and the supply.

You can improve welding by making up an inductance from another mot, mine just has as much cable wrapped around it as would fit, the inductance does 2 things, it evens out the welding current making the rod burn smoother and constant instead of spluttering and also when you short and release the rod the back emf causes a voltage spike which helps maintain the arc.

A HF start for tig can be put together from a car ignition coil, a light dimmer and a motor start capacitor if you wanted tig capacity.

Your construction looks very tidy.

Hey there dr peppers,


I havent looked at these transformers super closely (although i've removed a few myself too he he) but i would guess that the shunt is there to obtain resonance. It could also be used for regulation though. If i could see a pic of the transformer i would be able to tell much better. So if this was going to be used as a 'regular' step down transformer the shunt would not be needed at all.

I have to wonder if there is some optimal turns ratio for this kind of thing, assuming you do use the extra inductor. The extra inductor would help start the arc too right so a lower voltage (and hence more current) might work better. I'd like to hear about some experiments with this if you've done any yet or intend to do some in the future.

As someone who worked as a welder service tech there are a few things you need to know that relate to making a welder good or bad.

One is open circuit voltage. Typical good AC and DC stick welders have a 75 - 90 volt open circuit voltage. Anything below that limits the arc strength and starting ability.

Two the small shunts in the transformer cores are to limit the short circuit current and peak power the transformer has to carry. They also have a stabilizing effect on the arc and limit the amount of HF arc feedback noise that goes back up the power lines.

Third when properly done the output amps should be fairly uniform from half the open circuit voltage down to a dead short.

Fourth if setting up a DC output the rectifiers should be rated for 2x the peak amps and at least 10x the OC voltage and the capacitor bank should be around 20,000 - 30,000 uf per 100 amps output at 2x the OC voltage. A second smaller transformer rewound to work as a high current inductive reactor will greatly improve the DC arc welding smoothness and working capacity.

Fifth for any welder working off of a limited power source adding the proper amount of power factor correction on the primary side via a common motor run type AC capacitor can greatly improve the peak power you may be able to get out of a very limited power source. Freeing up another 2 - 3 amps on the primary side can make all the difference in some limited power welding situations.

Last wire guage is everything. More is always better and you do not want your wiring to be the current limiting device in the system on any part of the primary and/or secondary circuits, thats what the shunts in the transformer are there for.

Heres a protoype inductor I tried, it made a big diffo to the weld quality, even though its only a few uH.

I wound the finished one with intemp, this one got pretty hot pretty quick.

This isnt gonna make a fantastic welder however it does make something good enough to snot bits together to keep stuff running, I take mine out on site jobs where I might need to weld a nut on, or weld a bolt to a bearing to pull it out.

You can control current with either a simple phase angle thyristor circuit, or you can make up a variable choke, I tried the latter but couldnt get it to work, the weight of the paperwork (and the knowledge) didnt equal the weight of the equipment.


View attachment 63437

Hi,

Yes another reason for shunts is current limiting, which that kind of transformer design can do.

Another thing to think about is the wire size to use or not to use. I'd have to look a couple things up here, but bigger is not always better, and i think number 8 AWG should be the largest size wire used around 60Hz. If we get too big on the wire size skin effect will play a part in the losses and thus heating of the wire which would cause the transformer or inductor to overheat. Two number 8 gauge wires can be wound to make a larger size wire, and it's easier to handle too. Just being easier to bend makes it attractive.

I've worked with high current inductors that weight in at over 100 pounds easy.

Sounds like you've been there, theres some fairly big stuff knocking about here too, on power factor machines.
Constructive argument (not a shun): if you used stranded cable wouldnt that get over the skin effect issue as you'd be using a pile of stands (were you thinking of solid copper wire - that'd be a sod to pull round the core).
And skin effect as I've seen in the past plays a part at 100 khz up, is there an effect at lower frequency?

Hi dr peppers,

Yes skin effect is in everything that has to pass AC current unless maybe a superconductor. The skin depth is the key point and it's not as deep as we might think at first even for frequencies as low as 50 or 60Hz. Yes at 100kHz for that kind of transformer design we have to restrict to low wire gauges like 22 or even lower, but at 60Hz there is a limit too. Offhand maybe a tenth of an inch, but i'd have to look that up. Im sure there is a table on the web for some common frequencies. Another key point is that the conduction is less the closer we get to the exact center of the wire, and is considerably reduced at the distance of what is commonly called the "skin depth".

Strands do help if they are insulated from each other (ie Litz or Lutz wire). If the strands are not insulated then that could still cause problems. You're right about winding THROUGH the laminations, im so used to seeing them done before they are put together. If you cant get the laminations apart then yes it is harder to do. The problem with regular wire is the insulation layer is so thick that it starts to take up too much room in the core window. I guess it really depends how many turns you want on the core though, but you could definitely get more with wire made for transformer winding than regular wire.

The weld on that transformer looks pretty good.

Right then, if a tenth of an inch is the approx depth at 50hz then thats going to be an issue for power guys not so much electronics guys, as your talking tens of amps rather than miiliamps, interesting non the less I didnt know that.

I'd a suspicion that multi strand didnt work unless they were insulated, sounds almost like a gyroscopic where electrons 'want' to travel down the outer of the conductor.

I can tig weld to a reasonable standard, I have to say though not as straight as the pic, that was done by the manufactirer, probably on an automated machine, welding laminations is hard in my experience I think conductivity to ground varies with the laminations as you pass over them, wouldnt be surprised if they laser weld them, or if the tranny isnt wound maybe induction weld, I had to maintain an induction welder at my last place, it was a scary thing.

I wouldn't worry about the skin effect at 50 - 60 hz with anything the average DIY person could wind being at 60 hz its roughly 3/8 of an inch! https://en.wikipedia.org/wiki/Skin_effect

If you are going to wind a transformer properly I would seriously suggest finding your local electric motor winding shop and buying the correct enameled wire from them. They have many different sizes to choose from and most often just sell lengths or weights off their bulk spools which make it very cheap!

Try 3/32" diameter 7014 rods.

Put an Amp Clamp on your welding cables and get a friend to read it next time your welding. I am guessing you amps will fluxuate between 70 and 90 amps. When the rod is closer to the metal amps will be higher, when the rod is farther away from the metal amps will be lower.

If you build a secondary coil with taps you can use the taps as your amp adjustment. You have 20 turns per transformer = 40 turns total. Put 16 turns on one transformer then tap at 16, 18 and 20. This will give you 3 current ranges.

It's a nice project, but it's EXTREMELY dangerous! Professional welders often run at low voltage and high current. Microwave oven transformers run at high voltage and high current. If you touched the output of a real welder, you might feel a jolt, but it probably won't kill you. One touch from a microwave oven transformer will kill you on the spot.

I hate to be "That doomsayer guy", but that's a death machine if you're not careful. Please take necessary precautions when using it, if you use it at all. I can't even begin to tell you how much damage it could do to you if you slip and touch it wrong

It's a nice project, but it's EXTREMELY dangerous! Professional welders often run at low voltage and high current. Microwave oven transformers run at high voltage and high current. If you touched the output of a real welder, you might feel a jolt, but it probably won't kill you. One touch from a microwave oven transformer will kill you on the spot.

I hate to be "That doomsayer guy", but that's a death machine if you're not careful. Please take necessary precautions when using it, if you use it at all. I can't even begin to tell you how much damage it could do to you if you slip and touch it wrong

Click to expand...

You are aware he took the HV windings out and replaced them with 20 turns of 10 ga which now only put out 41 volts open circuit right?

I think as long as he doesn't stick his tongue on it he may be okay!

With all the informations available on the web on this subject, it is very surprising that DerStrom8 could state such an uninformed opinion.

The dangers of using this welder are the same that the ones of using any other stick welder. No more! It is a tool and tools are dangerous.

But there could be a lethal danger while removing the transformer from the microwave oven. Here is what I said about that on my first post:

"A lot more about MOT welders can be found on YouTube including the warnings about discharging safely the high voltage capacitor inside the Microwave before doing anything else."

Personnaly, I salvaged 9 microwave ovens and on all of them the capacitor was already discharged.

On edit: There is one possible danger that I see. That is if someone forget to switch off the welder after use. Maybe the transformers would get so hot that it could melt something and possibly start a fire. I don't know. These transformers do get pretty hot when leaved on for a while. It would be a good idea to use the thermal fuse that can be found inside the microwave oven. I should have kept them.