Microwave oven transformers and welding...?

Question

I've looked around on the internet quite some time, and noticed there are a few different designs based off of MOT's into welders. I've got 4 mots, now, (ones a little smallish...so I don't think I'll use that one?) 

All of them are in good shape. I know in order to weld with them, I need to rewind them to act as step down transformers. Now this is where my questions come in; 

Most of the designs have two mots. However, I've seen designs that have as many as 4, and the primaries are all connected in either series or parallel, as this apparently increases output current/ output voltage, and thus gives a hotter arc, or better welding characteristics. I've got 4 mots, as said before, and I don't know how I would wire them up. I know MOT's run in saturation, so would connecting them in series prevent this? 

And when connecting them to the mains, do I have to use two separate breakers, or just one? I have 15 and 20 amp circuits in my house. (there's 30, 40, and 50 amp ones, but they're all used by large appliances)

As far as connecting the secondaries up, if I run them in parallel, they need to be near the same voltage, or at the same voltage, correct? And if in series, how many of them should I put in series to get an effective arc? I understand arc welding (with a 15 to 20 amp circuits, at least, this is what people say) works best at around 70 - 90 amps, at around 35 - 50 volts. I'm not sure that's going to work so well on my breakers. 

And, at last, what about PFC? Since they're technically inductive loads, I'm pretty sure I'm going to need some sort of PFC capacitor. How do I calculate the value, and what kind of capacitor should I look for? (I know not electrolytic. :) ) 


I really really appreciate the help guys!

WOMBAT, Manliness Expert · Accepted Answer

Jared, MOT's are NOT suitable for welding purposes. I'm not going to insult your intelligence by explaining exactly why this is a bad idea in excruciating detail. Turn your critical-thinking skills back on, and use you imagination. You seem to know enough about transformers and the relationship between voltage, power, resistance, and current to figure this out yourself.

The people spreading such ill-advised information around the web are being extremely irresponsible, by claiming what they are doing is safe.

Both welding machines and microwave require the use of potentially lethal amounts of electricity.

Steve C · Answer

probably easier to make your own transformer. Get some plastic pipe, and coil the wire onto a length of plastic tube that works as a former- helps to use a lathe, or even an electric drill to do this- one of those hand held tally counters can be adapted to increment each time the coil former has been rotated 360 deg, telling you how many turns have been added. Make a primary and a secondary winding. Then take lots of thin mild steel rod and bend it in to a "C" shape. Place the C shaped lengths of rod through each former, and space out the gaps a bit, and you've got a core with air gaps in it. -The air gaps seem to be iportant for welding transformers, and I think play a large part in improving PF. 

You may find it useful to rectify the output to DC- sometime it makes arc welding easier. homemade borax rectifiers might be adaptable to deal with the large currents.  as might using a mag amp circuit to control the output power see http://sparkbangbuzz.com/mag-amp/mag-amp.htm .

lare · Answer

the magnetron in a MW oven operates at about 1 amp, 1000 volts. messing around with this kind of transformer will get you electrocuted.  what you need for welding is a voltage step-down transformer, not a step-up kind.  look at the secondary winding on a MW transformer, does it look like it can carry 90 amps? not hardly, the internet is full of lies and deceptions.

Bobo · Answer

For all your trouble, you can buy a 110 volt stick welder for about $100. By the time you BUY welding cables, ground clamp, and electrode holder for your homemade welder, you'll be paying the same or more if you bought a new one.

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Microwave oven transformers and welding...?

4 Answers