Some of you more experienced guys will probably laugh when I describe this
project, but here goes.....
A few weeks ago I got the idea to build my own arc welder, as I have
several old lawn mowers that needed welding and I was too cheap to either
take them to a pro or buy a decent welder. Also, I have a fair knowledge of
electronics and I've always wanted to learn how to weld. I found some plans
on the net describing a homemade unit, made from 8 microwave oven
transformers (MOT's), and using a high-power SCR module to adjust the power
output. I have plenty (literally dozens) of MOT's to work with, but I wanted
to keep the weight under 80 lbs so that the unit could be somewhat portable.
So, I decided to use three MOT's instead of 8.
For those of you unfamiliar with MOT's, these transformers weigh anywhere
from 8 to 16 lbs, the primary has about 150-200 turns of ~18AWG and the
secondary has several thousand turns of ~28AWG. With an input of 115VAC, the
output is typically around 2000-2300V. I selected three large transformers,
and with the use of some power tools I managed to get them apart, remove the
secondary windings, and wind new secondaries. Initially, I wound the
secondaries from #4 AWG cable - there was only enough space for about 4
windings on each transformer. The total voltage output of all three
transformers (in series), with no load, was about 12 volts. It's hard to
estimate the current potential but it was probably around 100 amps or more.
I decided to give the machine a try at this point (before adding the
cooling fans/thermal protection/other gadgets) so I wouldn't have to
backtrack too far if there was a problem. By now I had obtained a proper
ground clamp, electrode holder, welding mask ($5 at a flea market) and a box
of 5/64" rods. I took the rig outside and plugged it in via what appeared to
be a heavy extension cord. For my first try, I decided to attempt to cut a
thin (less than 1mm thick) piece of steel pipe. I tried to strike an arc
many times, using a scraping motion, but to no avail. It would not sustain
an arc, and the rod would just keep sticking to the pipe, sometimes melting
the rod. At this point I assumed that the voltage output was not sufficient
to create an arc, and I set about making new secondary windings. Since I
couldn't fit anymore #4 AWG on the transformer cores, I removed it and
rewound the secondaries with #10 (solid) wire. Yeah I know it's rather thin,
but this unit is only intended for light to medium-duty use, and it will
have forced-air cooling and thermal overload protection when done. Anyway, I
wound about 8 windings on each transformer, for a total voltage output of
about 24 volts. I tried it again, and this time it almost managed to start
an arc. So I added a few more transformer windings (voltage now up to about
35V) and this time I was finally able to strike an arc. Quite thrilled that
I had succeeded so far, I continued cutting up the pieces of steel and
trying to get the hang of this new hobby - this was the first time I'd ever
welded, after all! But remember that extension cord I mentioned earlier? It
was a 100' retractable cord, and I had only pulled out about 10 feet to
reach outside the house. The rest was left rolled up on the spool, next to
the wall where it was plugged in. A few minutes later, the welder dies and
when the fuse on the machine checks good, I decide to check the extension
cord. I guess I shouldn't have been surprised to see smoke rolling off the
coiled cord. I took it outside, and in an attempt to keep it from melting
together, pulled it all out and allowed it to cool. Closer inspection of the
wire revealed it was very light - only 16AWG, but I suspect that wasn't the
only problem.....
Once I located a heavier extension cord, I fired up the welder again and
to my surprise, it was much hotter than before and would actually blow the
breaker box in the house whenever I struck an arc. Based on my electronics
knowledge, I suspect the first extension cord, in its coiled state, may have
acted as an inductive ballast, limiting the current available to my welder.
Would I be right?
So that's how this project has gone so far. Closer inspection of my house
wiring revealed that two entire floors, in addition to the outside plug I
was using, are all wired on the same 15-amp breaker. I'll probably have an
electrician install a 20 or 25-amp breaker, as well as perhaps bring the
wiring up to date.
At this point I'm not sure whether my transformer design is OK or not.
Which brings me to my last question - what effect do current and voltage
have on the arc? My uneducated guess is that the voltage is needed to
sustain an arc, and the current is needed to heat the rod.
I still have a lot to learn about this hobby, but if I can at least get
this machine to function reliably that will be a good start.
Thanks for any advice.