Re: AC genset to DC welder


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Posted by D. Sherman on Tuesday, February 12, 2008 at 15:46:15 :

In Reply to: AC genset to DC welder posted by David N. Lundstrom on Tuesday, February 12, 2008 at 14:28:12 :

I have a similar unit that somebody gave me because it was "worn out". With a lot of work and $1000 worth of parts (a piston alone, from Onan, was $300) I got it working and it's been my workhorse genset at my cabin ever since. Being an 1800 rpm diesel engine with a real oil pump, I expect it will last a long time. There's a 5 kW military genset that's basically the same machine, just derated for altitude.

As far as converting it to a welder, I'm not so sure how trivial it is, especially if you want to do it right. The genset wants to put out 240 volts, split phase. A welder is usually only putting 20-50 volts into the arc. The trick with a welder is to provide a way to limit the current, since an arc acts like a negative resistance. With a line-powered welder, the usual method is a "reactor", which is basically a transformer with a variable air gap in the magnetic circuit (the iron core). An inductor in series with a constant voltage AC source (the power line) makes a rough approximation of a constant-current source. By varying the air gap, you vary the current. The transformer turns ratio steps the voltage down from line voltage to welding voltage, while raising the current up. With a generator set, it's actually easier to provide current limiting because all you need to do is disconnect the voltage regulator and use a big rheostat to manually vary the field current. Without a voltage regulator, a generator acts like a constant current source whose output current is proportional to the field current. The problem I see is that the windings are made with the size of wire that it takes to provide 6kW at 240 volts, which works out to 25 amps. If you disconnect the regulator and adjust the field current to get a good welding voltage out of it, you'll still only be able to safely draw 25 amps without burning up the generator. The heat produced in the generator windings is proportional to the current you draw out of it, not the voltage, which means 25 amps at 20 volts will make it just as hot as 25 amps at 240 volts. I think it would be better to use the generator to power a conventional 240 volt AC buzz-box welder. Then you'd get a full 6 kW of welding power, rather than something in the .5 to 1 kW range.

I also know there are guys who "weld" with cables attached directly to a car battery, or straight onto a 240 volt power source. The welds I've seen done like that are pretty atrocious, but they clearly were able to melt some steel and splatter it around. Pretty much anything that will kick out some juice will make an arc and melt steel, but whether that's something that you can actually use to do decent quality work is another matter.



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