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I want to make my own DIY spot welder, so I'm trying the instructions I found on several places (for instance, here, here or here) about using an old microwave transformer. I salvaged it from a broken machine and the recipe says to replace the secondary winding by two or three turns of a heavy, thick insulated cable (I got a 16mm2 one).

The problem is that removing the original winding wasn't exactly a walk in the park; after cutting it with a radial jigsaw I had to smash it hard with a chisel until it finally came off. This produced noticeable physical damage to the transformer itself, namely in the core sheets: some of the insulating varnish came off, some of the first sheets got a bit deformed and separated from the rest and finally and what looks more severe, these "separations" also happened in two or three places in the laminated core, opening it in three different blocks that I managed to more or less put back to their original position by, again, heavily smashing them together. Currently there is minimal space between those cracks, I would say less than a mm, but you can still put your nail between the layers by applying some force and, of course, the insulation there (in case it was any, something I'm not sure about) is gone.

I know little about the physics and theory behind transformers, and I'm wondering if I can still attempt to use the damaged transformer and/or if it is a bad idea/hazardous/simply won't work. As far as I understand, the purpose is to get a low voltage but a high current, capable of melting steel, so I don't think "precission" is a concern, but I read about why the core is laminated and, as far as I know, there is a risk that it will overheat due to eddy currents, although I don't know if those couple of cracks and deformations will be THAT significant. Also, I found at least one source (sorry, forgot to bookmark) where they state to be very careful with the core sheets and not to bend or even slightly separate them in any way.

What's your recommendation? One friend, who studied technical secondary school to become an electrician, simple said me to "try it and see what happens", but I don't want to burn my house, destroy all the household appliance or even fry myself.

Two secondary, final questions:

  • Are sheets also isolated between them or just covered on top with varnish after "physically" joining them together "by force"? Should I paint the separated lamines with some epoxy or glue before joining them if I'm going to try the transformer? In that case, would it be dangerous if a little metal part is still in contact with another sheet of metal from the core? Because I cannot assure 100% of the sheet will get covered with the stuff...

  • Is it hazardous and/or could it significantly affect reliability if two turns of the primary coil short between them? I closely inspected it and looks like it was "scratched" in a small area (see 4th picture) and there is a risk it lost its insulation there; I think that chance is very low and I'll try to assure by covering it with epoxy or another type of insulation/varnish, but just in case...

I'm attaching some pictures of how does the transformer currently look; I hope they are useful for making a judgement...

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Thanks a lot for helping!

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  • \$\begingroup\$ I think it is fine unless maybe you plan to run it at full power. But wait until someone who specializes in transformers confirms. I would be more concerned about the insulating varnish and what might happen if the enamel on the winding gets damaged and makes contact with the core. \$\endgroup\$
    – DKNguyen
    Mar 25, 2020 at 17:58
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    \$\begingroup\$ Are you hooking it up to AC? Then no, not safe. You compromised the characteristics of the transformer by splitting the sheets like that. Any shorted sheets would increase eddy currents, but not sure if that would be dangerous. \$\endgroup\$
    – Stiddily
    Mar 25, 2020 at 18:22
  • \$\begingroup\$ The damage to your transformer seems minor to me. A friend of mine made such a welding machine from microwave transformer and I saw how it works. It is important to use a pedal to turn on the power mains so that you have your hands free. Note that the transformer only works from milliseconds to seconds. It is a good idea to use an electronic timer to ensure good repeatability of spot welds. The mechanical construction must be very secure. Don't forget to put on a mains fuse and to wear safety goggles and gloves. \$\endgroup\$
    – Peter MP
    Mar 25, 2020 at 20:35
  • \$\begingroup\$ It's not repair, it's making something new from omething old, but of course the stack having a name as broad as DIY is a bit misleading when it's a lot more narow in the fine print. In any case, I would suggest that the OP treat this one as a learning experience and try again with a less aggressive disassembly technique on the next one, as these transformers are commonly available for free with dead microwaves wrapped around them. \$\endgroup\$
    – Ecnerwal
    Mar 25, 2020 at 21:48
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    \$\begingroup\$ Nope, sorry OP. The nicked primary is a total dealkiller; the thing is scrap. You were far too rough with this transformer, and you need to learn better teardown techniques. This is your first time doing this, you didn't know, you learned, and this is tuition. You already paid the tuition, might as well keep the lesson! \$\endgroup\$ Mar 26, 2020 at 2:07

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The most important thing to check for safety is the insulation on the primary winding wire. Any contact between the wire and the core could cause the core to be at the primary voltage level. Connecting the primary only through a ground fault interrupting (GFI) receptacle or circuit breaker is essential. A connection between one primary winding and another would be a shorted-turns situation. That could cause a fire or damage that would result in a connection between the winding and the core.

Damaged lamination insulation will cause increased heating. The transformer could get hot enough to damage the windings. However, that would probably not happen very fast, so it would probably ok to use the transformer if you never leave it operating unattended. Unless you have a great deal of experience, you should never leave any home made electrical project operating unattended.

Loose laminations will vibrate and make noise, The vibrations might ultimately damage the insulation between laminations.

At welding voltage levels, a damaged secondary winding is of less concern, but a shorted turn would cause the winding to get hot pretty quickly.

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    \$\begingroup\$ +1 Also note that microwave oven transformers are run at very high magnetization levels (to make them cheap and light relative to VA capacity) and get very hot even when operating properly (they are generally fan-cooled and don't run continuously). As you'll probably run a low duty cycle in welding it should not matter, but don't power the primary unnecessarily. \$\endgroup\$ Mar 27, 2020 at 14:52
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Totally agree with Ecnerwal. I made one of those similar to your second video and used a Dremel drill with a cutoff wheel and a regular drill with a 1/2' bit to remove the secondary. The sheets are individually laminated before being assembled and you've messed them up pretty bad. The real deal breaker is your comment about two turns of the primary coil shorting between them. That's dangerous. Get another transformer and take it slower removing the secondary winding.

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  • \$\begingroup\$ Hahaha! It already took me several days to remove the copper from the holes after sawing the secondary, but... Yeah, I'll try to be more careful next time ;) \$\endgroup\$
    – Pere
    Mar 26, 2020 at 21:11
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The laminated plates are there to reduce eddy currents and increase the coupling. the bent plates will not drastically affect the performance of your “new” transformer as long as the movement did not damage the insulation on the winding. Since you said some varnish was damaged you may be able to insulate with motor winding varnish , Make there are no sharp spots that will damage the insulation on the new wire when you rewind. If you are only talking about plate varnish it won’t be a problem they are all welded together to reduce 60hz noise.

I would want to check the coil(s) with a megger and make sure there s not a short prior to winding the second coil and after installing the second coil. A megger is a high voltage ohm meter, not all electricians have them but most that do motors regularly have at least a hand crank model that will show a short (a regular ohm meter won’t show).

I usually want to see 30Meg ohms at 2x the voltage before I will power something like that up, I think my apprentice told me the book value is 100 Meg ohm but I know from experience 30 will work at 2x the voltage.

Other than that I fully agree with Jack , if you can’t locate a megger toss it in the trash and you have learned a lesson. I have repaired a large transformer at work that a big fork lift ran over ((110kva I pounded the damaged plates flat and put new insulators in for the primary that was damaged , got that section of the mill back up and it is still running 4 years later.

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