The crude Microwave Oven Transformer Welder

It's been a long time since I've been thinking to make Welder on my own. But being a hobbyist in the simple electronic project, I did not have enough knowledge on Welding or what so. Its then I started research on welding machine how do they work, how many types are there etc. Anyway, this project is to all my Electrical and Electronics friends out there, who are fond of Electronics and Power and DiY...

RESTORATION

                     

I got to know that one could restore an old microwave oven from any local home shops and convert them to a welding machine. Hence I started my search on old MOT's, unfortunately here in Kerala, we got no or less access to old mid conditioned electronics. After searching for a month I got two MOT from my Dad's friend. After that salvage when I once went to my relative's house I found a local shop where a ton of MOT's where sold in accordance with their weight. I scavenged 2 more from there for ₹200.

Now to the main part. MOT welding machines are one of the simplest projects which one can do to learn more on power electronics and stuff (In a sense, dealing with huge currents, For the first time in my lifetime I was able to see 200 amps)

PRINCIPLE & NEW BIRTH

The principle is quite simple, MUTUAL INDUCTION...TTa DDaaa... Sounds similar right? It is the same principle which you might have studied in you 12th standards. Basically, you got 2 to 3 microwave oven which usually outputs about 2000V in its secondary each. Our goal is high CURRENT !!!. So we need to rewire the secondaries of the transformer with lesser number of turns so to get lower voltage with higher current in the secondary.

 From the salvaged transformers, 3 of them were good(overall condition of transformer, rust, terminal quality etc were my concern). So I took apart the transformers I & E sections by using an angle grinder, you just cut along the sleeve and to take it apart with a chisel. Then I used 6sq.mm wire for the secondary. The challenging part was, of course, the winding, sometimes its too loose that you might think of adding one more turn which would make it tight as hell. Be sure to make a firm winding.

        With a bit of math I measured the thicknesses of two windings and hence found out primary to secondary ratio is about 1:10. Hence for 240v input I may get about 2400v on the secondary. With 6 turns on secondary winding in first transformer I got over 6v in secondary. We need about 25+ voltage and 120+ ampere for a proper welding. For this secondary must be 10 to 14 turns each transformer, which adds to 28v maximum with 28 turns on both. Voltage is to establish proper arcing between the metal and electrode and ampere is to make thing hot enough to melt

 

6sq.mm wire                   New secondadry                      'E' section

                            After all the windings were completed, I sealed the top with a stone just for experimenting. You must actually seal the 'I' section with 2 part epoxy glue for a sturdy connection so as to reduce any vibrations.

                            At last, the TEST, with mom beside me (praying to god, oh god! may nothing happen to my boy). With the blessings of my mom and presence of Mr. Faraday, I heard the noise from the transformer. It was the moment where all those Mutual Induction and Mr. Faraday came into action. Glad to see those!

I used just a screwdriver tied to a metal clamp to hold the electrode and just touched one of the broken 'E' section, and whola.. success, I witnessed the spark and spots of the weld (2.5mm rod). with a maximum current of 147A recorded by my sweet clamp meter (By the way, my reserve bank for all the online purchasing is my brother) and a maximum voltage of 29V, spots of welds were seen.

Although the project is delayed due to my exams, I'll make a follow up where I would add another transformer so as to see a blue arc (35+ volts) and the overall casing, ventilation and proper welding.

DISCLAIMER: MOT welding is not the best way of welding, but as of Do it YourSelf projects, estimating around a ₹1000 ($20) its a worth, still the 6sq.mm wires can become a bit hot on prolonged usage and the transformers too can build up heat quickly. So if you are making one on yourself, be aware of these facts.