Homemade Arc Welder

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.
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There are two basic power sources for welding one is constant current used by stick welding and TIG. MIG welders, often called wire welders, are constant voltage. You are basically trying to make a stick welder without having a constant current supply although you are correct in saying the coiled extension core probably added enough impedance to give you some sort of a constant current characteristic. Constant current transformers employ a magnetic shunt, often variable to adjust the current, to create the constant current. Since you don't have that you need to resort to some other method to limit current which makes me suggest you go back to the original article you found on using MOTs.
Regular stick welders often have open-circuit voltages of 55-80V to enable them to strike an arc easily. Once the arc is struck the voltage between the rod and the workpiece drops to somewhere in the 20V range and the current is limited by the constant current supply setting. The current does the heating, more current more heat.
Billh
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Hmm, I wondered about those shunts when I was taking apart the transformers. I managed to save the shunts and put them back in place on one MOT, but the others got trashed in the process. So what you're saying, is that is if all three MOT's have the proper shunts, it should provide a constant enough current to allow for smooth welding, and stop blowing circuit breakers? I did notice, when I was using the coiled cord, that the arc seemed much more steady.....

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I doubt the original shunts would be of much use since they are designed for proper operation of the magnetron which is high voltage and relatively low current. I assume they are there to limit current if it arcs. Billh

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(clip) I suspect the first extension cord, in its coiled state, mayhave acted as an inductive ballast, limiting the current available to my welder.(clip) ^^^^^^^^^^^^^ (clip) you are correct in saying the coiled extension core probably added enough impedance to give you some sort of a constant current characteristic. (clip) ^^^^^^^^^^^^^^ Sorry--not correct. An extension cord carries current in both directions in a closely bundled pair, so its inductance is pretty low. Coiling it up does not increase the inductance, because the wires are still closely paired and opposing, all the way through the coil.
Secondly, inductive voltage drop is 90 degrees out of phase with the voltage, so there is no power loss--so no heat generated. The fact that your cord was hot suggests that it was simply too small to handle the load.
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characteristic.
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does
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Well, if you consider impedance to be resistance plus reactance but I have to admit I was considering the reactance in my statement without thinking about paired wires. Billh
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IR drop - the light weight power lead dropped voltage and ran the transformers at a lower input voltage.
The resistance in the wire times the current = voltage drop in the wire.
I*R = E
Good go! - I did something like this years ago getting a Thyratron up and going - needed a very high current 2.5V current - Used a trashed out Electric soldering gun. The soldering tip is running around 100 amps on the good ones - and that is 1 turn. Low wattage (core size limits this ) so it wasn't up to the job on a continuous basis.
I found a 5 Volt 25 A unit once I realized the cathodes were isolated.
Martin
Chris F. wrote:

--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
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...

Hi Chris. If you deviate from a known design, you really have to know the rules before you break them. Just winging it usually produces the standard results.
...

Not enough.

What kind? This does make a difference.

Not a bad idea, at least for testing. Don't forget your shortcut as time goes on. It is pretty easy.
...

No.
This is expensive. I got some quotes for this kind of activity, and all of a sudden, that Miller Maxstar starts looking inexpensive. Plus, you can take it with you.

Both are needed. High voltage will sustain an arc, but it will not be in the right mode to be useful. Think of a low pressure mercury vapor arc (the one that occurs in a fluorescent tube).
It seems that you are trying to do things with too many variables changed. Why don't you try taking some baby steps, and get a few success experiences under your belt before starting an electrical wiring fire? One way to start is by increasing the power that you have to work with. This can be done by either:
o using a 220 V outlet and grouping the MOT's in series pairs; this would require 4 of them, but you have plenty
o acquiring or borrowing a 120V inverter welder, which will make better use of the limited circuit that you are working with
o bypassing the MOT approach and moving up to an alternator welder, which also makes better use of the limited circuit
For explanations of "better use" review the concepts of inductive current limiting, power factor, and alternator field control. A good website to look at is www.diywelder.com
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. .8 lines deleted .

First thing to do: go to a good library and look through the AWS _Welding_Handbook_. I believe volume 2 is a good place to start.

You want constant current, and this is a tough way to get it. Also, the voltage is too low. When you strike the arc, the voltage will drop, both due to IR losses and increased magnetic losses (primarily hysteresis) in the transformer. If the primary is being run at it's design voltage/turns/frequency, you shouldn't be saturating the transformer core. For most stick electrodes, you will need at least 35 to 40V OCV, many want as high as 80, and a sustainable arc voltage of 20 to 30V, and again some want more.
Typical older transformer based welders run control the current through a combination of saturable core transformers and reactors, series inductance, and sometimes series resistance. At least one magnetic element is typically run in saturation, sometimes with a DC bias winding used to push the core toward saturation (such as in a mgnetic amplifyer) to control the effective load saturation current.
The other (more modern way with a transformer) is to use SCR's (or other active swithching elements) to control the primary side excitation using phase controlled switching, like a dimmer for an incandescent bulb. The output inductor actually contros the current. This is NOT an inverter as it happens at line frequency (actually twice line frequency) and the AC input isn't converted to DC and reinverted before the transformer--inverters are small as they switch the transformer at a frequency higher than the line

good guess

. .25 line deleted .

Nope. The extension cord wouldn't act as much of a ballast because there is no net circulating current.

Short answer: sort of. Voltage sustains the arc and drives the current through the arc plasma as through any resistance, the current provides the heat OF THE ARC, resistive rod heating being a minor, but not insignificant, player.
Long answer (which will probably get a lot of criticism for incompleteness, but hopefully not much for correctness)
For AC welding current, the arc breaks at every zero crossing of the current, and must be re-lit. The series inductor insures that the current is fairly close to 90 degrees out of phase with the voltage, so when the arc breaks, the voltage will be near maximum. Higher votage restrikes the arc easier. This is helped by a) the ionized gas from the just extinguised arc b) flux additaves that increase the ion life at arc termination, c) flux additaves the reduce the votage needed to restrike the arc.
For both AC and DC, note that the arc plasma has resistance, and therefore a voltage across it when the arc is lit. One primary heat source (actually, arguably all of the heat at the arc) comes from the voltage drop times current thru the arc. The voltage is needed to sustain the current through the resistance for several reasons: The electrons need to be pulled from their parent atoms (ionization to form the plasma) and need to be accellerated in the direction of electron current flow; the ionized metal atoms need to be accelerated in the opposite direction (this is a minor part of the metal transfer, and in DCEN welding doesn't contribute at all); and, since the plasma isn't a perfect conductor, there must be a voltage across it for current to flow. The current, on the other hand, has several effects: preheat the welding rod, the electron current transfers most of the heat kineticly, so the higher the current, the more eectrons, and the more heat. But wait!! In DCEP, that means the heat goes into the rod, not the work! Yup. Melts the rod and the melted metal sputters to the workpiece. Then how does DCEN welding work? The heat of the arc and the resistive heating of the rod is more important. Note that the flux is a major player for both polarities, as it contributes to the plasma in a big way, and can greatly alter the trnafer properties, by enhancing electron effect in heating, enhancing the effect of the positive ions in heating, enhancing the sputtering effect at the rod tip, and a ton of other things.
In some forms of welding (Such as TIG) polarity is a tremendously obvious parameter, as the heat goes mostly where the electrons go. They carry much of the kinetic energy in the plasma. The radiant heat from the arc is distributed more evenly, but is still not perfectly balanced, due to the effect of polarity on arc shape. The electrode emits electrons more readily than the workpiece in most situations, which will also produce polarity based effects.
Ok... enough of my oral diherria...

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