Making a welder from an alternator and engine

I read on this page that you can make a pretty decent TIG welder with an alternator and electric motor. My question is, can I make an Arc/Stick welder with an alternator and a gas engine? Does arc welding put a higher load on the electronics (alternator) because it is more of a direct short? Will it burn out the alternator, or will it be okay? The engine I have is an old 3HP Briggs horizontal shaft lawnmower engine. How much of an alternator (how many amps) could I run off of that? The above linked page recommends a 5HP electric motor for a 130amp alternator, but electric motors and gas engines have different torque characteristics.

If this will work, how much do you think an old alternator will cost and where should I get it (U pick parts maybe)?


Uh isn’t an alternator also known as an electric motor? I would take a look at Automative alternators and look at their output but i know their is a dirrect corelation between the power a motor outputs and the power it can make. Im pretty sure that if you ground it, other then the speed wearing down the part or being too stressfull, the motor shouldn’t burn it out. Ehhh not as much experience here as i would like but im interested in knowing info about this too!

You could use an alternator as an electric motor, but it wouldn’t be too efficient or all that great of a motor. It was designed to be used more as a generator rather than a motor. Really, both of them are reversible. A motor could be a generator and a generator can be a motor.

Most autoshops have alternators, but they can get quite pricey. This one antique auto shop near my home had a chromed alternator for $100, but I’m not sure on how many amps it would generate. You could try Ebay, I saw some alternators that could generate around 160 amps, but they’re also quite pricey. Some at $160, it’s like $1/amp.

Good luck, and I think I saw that article on Hackaday. haha.

I think you guys are sort of missing what I am asking.

no no i get it. sorry to get sidetracked.
Heres the first thing that i came up with at first it looks off topic but it should help answer some of your questions. Also here is a good welding altenator site that seems to work in conjunction with gas engines. . At least they would be good people to ask.

Well it all depends on what you want to do. 3 hp isnt that much power. its about 2250 watts. The welders I have are about… 240 KW. I’m serious. 240 volts, 100 amps. Ok and then the small welder I have, a cheap $2000 miller is like, 5 KW. But still, thats twice the power of your engine, and I doubt it still gives 3hp, as its probably old. You can weld steel sure, but it’ll be slow. I wouldn’t use that motor, unless you seriously dont care how good a welder you get out of it, and just want to do it for the point of making a welder out of a gas engine. I always appreciate those kinds of efforts.

As for the alternator question, generally welding runs on low voltage high current, so I would go with something around at least 100 amps for the 3HP engine, but less than maybe 160 amps. Also remember that the alternator gives 100 amps at its nominal speed, meaning you have to gear it off the lawnmower. Then I would get some nice cables for that thing, you dont want to blow anything (or anyone) up with that much current. I wish you the best, and good luck.

You think I’d be able to do 1/8" wall 1" box tubing with it? See, I thought it would be cool to run an alternator off this engine to make a welder. Then weld up a gokart frame with it, and then use the engine to power the gokart.

If I can find an alternator for cheap (my truck can live without it right? :smiley: j/k), I’d like to try this. Do you think I should gear it 1:1 or gear it up or geat it down. I think the peak HP rating on the welder is at about 3300 RPMs (I don’t remember exactly but it was around there).

Sanddrag, do you mean peak HP for the engine is 3300 rpm? If so, you have one of your numbers down. Then you just need to gear the alternator to the engine, gearing the alternator to go faster than its normal speed. This way you get the maximum power out of the alternator, since you are running it closer to what a car revving hard would be. However if you gear too high, the alternator provides too much resistance to the engine, so your engine runs at below max power. Its tricky work. To tell you the truth I’m not entire sure what to tell you, since I dont know the power curves of the engine and alternator.

Sanddrag, your best bet to get an alternator cheap is a salvage yard. See if you can locate one that has retired police cars. With all the electronics they require part of the “police package” from the factory was a heavy duty electrical system and larger alternator than a standard production vehicle. Caprice, Crown Vic…doesn’t matter. If it’s still installed on the car get the mounting brackets also which may help you out when you go to assemble your welder.

For the Crown Vic, the police package included a 135 amp vs. a 95 amp or 100 amp (depending on year) for a regular production vehicle. Caprice would be similar. You should be able to locate a used alternator for $25-$50 at a salvage yard.

Sorry I wasn’t able to check the boards yesterday.
Alternators are not motors. What the original article suggests is that you couple the 130 amp Ford alternator to a constantly running compressor motor such that the alternator is running at 6k-7k RPM. The output (voltage and current) of an alternator is a function of the field current and input speed. As the article suggests, you need to connect a battery starter such that the field is supplied current to start and a second circuit (supplying field current) to continue generating current. Please note the mention of the addition of a current limit. The article was written to auto repair specialists who have an understanding of the way alternators function so some things are expected to be understood and were left out of the discussion.
As to the alternator, you should be able to get one at a junkyard. Be sure to get a high current rated alternator if you can’t get the one specified in the article. Please also note that the article suggests you can weld up to 1/4" with this setup but 1/8" is a better bet. The unspoken caveat here is that this will work for small fix it jobs, not continuos welding like when assembling a robot base from square tubing. The diodes can’t take that kind of abuse for more than a few minutes at a time.

just a side note for people who don’t know.

when welding aluminum you use AC current which is produced by the alternator but there are diodes in the circuit which basically turn it into DC. while this will work for steel welding don’t think you are going to get any good aluminum beads with it

Just to add onto that, even if you remove the diodes to get AC, you wont be doing aluminum either. You need high-frequency AC to break through the oxide coating that builds on the puddle and will actually rectify they current.