OT -- DC welders

[grumble]

I've never seen a question go unanswered with this group, no matter how far off topic from cast boolits. So, here's hoping someone will have an answer to this one.

At an auction, I bought a Lorain 200A, 50V rectifier, intended for a 204V 3-phase power supply (I paid $2.50 for it; it weighs about 800 lbs, and probably cost a couple thou when first purchased). And, I've been using an AC "crackerbox" welder, wishing I had a nice DC welder. So, I'm wondering if I can use the diodes from the rectifier, assuming they're still good, to convert my crackerbox to a DC welder. I can make the full wave rectifier easily enough, but I don't know if I'm going to need any of the filtering and smoothing circuitry.

Have any of you ever looked inside a DC welder? Are there lots of transformers and capacitors inside? Anyone have any idea where I can find a schematic of a DC welder? I can make the LC filter from the parts inside the rectifier if need be, but it all won't fit inside the case of my crackerbox.

Any help will be appreciated!

[Scrounger]

Grumble, go here:
http://www.yodermfg.com/Tube_Mills/Main_TM_SqWave.htm
or here:http://www.motherearthnews.com/library/1980_November_December/Build_a__20_Portable_DC_Arc_Welder
or here:http://www.motherearthnews.com/top_articles/1980_November_December/Build_a__20_Portable_DC_Arc_Welder

[grumble]

Good links, Scrounger! Many thanks.

From what I've found so far, no filtering is needed, but an inductor at the output lead can be used to reduce ripple.

If those diodes are good, I may have a DC welder soon!

[Linstrum]

Hey, there, Grumble, it has been awhile since I said howdy to you! Howdy!

Get yerself a cup of coffee, sit down, and take yer shoes off, this is kinda loooong.

Find out what the peak inverse voltage (PIV) rating is on your diodes, sometimes loosely referred to as the maximum puncture resistance voltage. Welders run at an open circuit voltage of about 85 volts AC root mean square, which is an actual peak voltage of 85 volts times the square root of 2, or 85 x 1.414, which gives you 120 volts peak to peak operating voltage for single phase. For three phase the actual peak-to-peak voltage is 85 volts times the square root of 3, or 85 times 1.73, which is 147 volts peak to peak. For welders running on 85 volts open circuit they recommend a PIV rating of 400 volts minimum for the rectifier diodes and most commercial welders use diodes with 1000 volts PIV rating. The output voltage on arc welders is right around 30 volts closed circuit (weld in progress). Some run 35 volts, some run 28 volts, but it is always right around 30 volts for flux covered stick electrodes. On TIG-MIG welders some are running up around 100 volts on some rigs, but they usually avoid having too high voltage for safety reasons. Electrocution in a dry work environment is not likely below 30 volts, which is considered the upper limit for safety where bare skin comes in contact with bare metal, and is why you MUST keep your welding gloves, clothes, and shoes dry from sweat! Also, if the voltage strays too far from 30 volts closed circuit then the amperage ratings for the welding electrodes gets skewed, since the actual rating for producing a successful weldment with a given electrode (like E6011, E6013, E7018, etc.) is watts, not amps. For example, if the welder put out 1 volt and the stick electrode is supposed to be used at 90 amps, this is only 90 watts of work available. If the welder puts out 30 volts and the electrode is supposed to be used at 90 amps, then there is 2700 watts available to do the weld. The difference is 30 times the energy, and it is the amount of energy that melts the metal, not the amps. Welder outputs are actually rated at volts times the amps, not amps alone. The welding machine manufacturers never mention volts time the amps (which is watts) or the 30-volt universal standard, they make the assumption that you know that all stick electrode electric arc welders are 30 volts, and to keep things simple they just mention the amps. Anyway, enough said on that. Check the rating of your diodes. The only other thing you need is a big inductor, also called a choke. I don't recall how many Henries a welder choke has to be for 30 volts with a welding range of 40 amps to 225 amps, but it is a BUNCH. About Henries and inductors, to help you understand what a Henry is, if you recall from electronics, a Henry of inductance is the equivalent of a Farad of capacitance where the rate of change of electron flow into or out of the inductor choke is the same as the rate of change of flow into or out of a capacitor at the same voltage. Basically, one Farad and one Henry are both where the rate of flow changes one amp per second at a constant voltage of one volt into or out of the energy storage device. Inductors are used for two purposes, one is to smooth out the rectifier ripple and the other is to absorb the humongous voltage spike created when you break the arc, which keeps the rectifier diodes from puncturing. The voltage spike occurs when there is a collapse of the magnetic field created by the flow of welding current through the cables and pieces of metal being welded on (like a car body, which can become magnetized and turn into one huge magnet!), and the energy stored up in the huge magnetic field appears as a sharp momentary rise in the open circuit voltage. Without the choke, the diodes would get their PIV workout, to the tune of a couple of hundred volts "kick"! It can "kick" you, too! By the way, most arc welding work is best done with AC machines because the AC eliminates "arc blow" when you come up to an edge, corner, or point where you are making a weld. On DC there are some places where the arc blow will totally prevent the arc from going where you want it to, or it will constantly go out. On AC this doesn't happen. Where DC really shines is welding thin sheet metal and in "out of position" locations, like across the ceiling, or up, down, and horizontally along walls. For places like those, DC can't be beat. Good luck, that is an ambitious but worthwhile project!

[grumble]

Great reply, Linstrum! Many thanks! An excellent Power Factor tutorial as well.

All the parts are coming from a huge commercial rectifier, rated at 200A @ 50V. Even though it's supposed to be powerd by 207VAC 3-phase, a little bit of wire swapping could make it into a single phase unit.

So far, I've scavenged the 4 diodes (they all seem to test ok with an ohmmeter) mounted in a heat sink, and a filter transformer, or choke. Since it's going from 3-phase to single phase, I had to cut the heat sink into 3 pieces to make the full-wave rectifier bridge. I have them "breadboarded," but haven't done a smoke test yet. I think the choke that was in the unit should be OK since it's also rated at 200A.

When I first posted, I was wondering about the need for a more complex filter, and whether I'd have to use some of the big condensers that were in the unit to make an LC filter. From what I've been able to learn so far, that would be overkill for what I'm doing, and just the choke alone should do the trick.

I origionally planned to try to mount everything on my crackerbox welder, but have since decided to make the rectifier a separate box, with welding cables coming out of it, and powered by the output of the crackerbox (a little home-sized unit, 150A).

Now I need to figure out what useful purpose I can put the rest of the big rectifier to. It has two HUGE stepdown transformers (60~, 207V to 50V) that probably weigh 200 lbs each, and they seem to be functional. It seems almost criminal to just haul them off to a junkyard for their copper content. There are also a couple smaller power supplies used for the control circuitry, and I think I might be able to use them for other hobby work, but I haven't checked them out yet. All-in-all, I think I got my $2.50's worth out the thing. <GGG>