Hobart mega-arc 300

That should be a pretty good machine, but he's asking a lot of money for it. If the HF is the old fashioned sort, a cleaning of the spark gap or replacement of the transformer should be all that's required, I think any ordinary furnace lighting transformer would work fine.

Offer him $200, see what happens.

A 10 hp convertor should power this machine well.

John

No, it is supposed to be the "new sort", with more electronics and lighter. Also comes with 200 feet of welding cable and tig gun and stinger.

My calculation, based on 300 amps at 40V, gives me energy consumption at least 12 kW. That's a little bit too much for a 10 HP RPC. Am I mistaken?

I doubt that I will need 300A very often though, although I have no idea.

thanks John...

Can't be sure, I was thinking Ignoramus meant that it did not have the HF system at all, rather than it was not working. Anyway, you really WANT HF. Now that I have my Lincoln Square-wave TIG 300, I can't imagine using anything without HF. And, you will have a horrible time doing Aluminum without HF. Since it is a 3-phase machine, it either has to be an inverter square-wave system, or DC only. You can do practically anything BUT aluminum with DC. The whole reason I got a square-wave machine was to have aluminum capability. (And, my whole system of acquiring tools was to get only ONE of any type machine, and always try to get the most versatile machines I could.)

Jon

Correct. To be honest, I have a separate ESAB/Airco HF arc stabilizer that I got from the military recently, but I have it on sale on ebay, ending today, at a good price, so I will not cancel the auction. In any case, since I do not have AC, I cannot weld aluminum, so I am not sure if I need an arc stabilizer.

so, what could I do with the welder in question? It is a DC CC welder that can do stick and tig welding. What kinds of materials can I weld?

Probably a good machine but parts are almost none existent for these old Hobart welders. If it has square wave AC capability then it would probably make a pretty decent TIG welder. If not then aluminum capability is going to be ok but not as good as the newer welders.

No you're not mistaken. Mine takes 80 amps single phase to run full out (though I've only got it on a 60 amp breaker).

Not likely.

Wayne Cook Shamrock, TX

It is from mid 90s and does not have AC capability.

Yep.

So, Wayne, what can I weld with such a welder?

The last statement is the question; how much are you going to ask from the machine? Try it out, it'll either suit you or it won't, no big deal either way.. I'm guessing you'll find it's more than adequate.

HF *should* be easy to repair, regardless of the age of the machine, but there's sure no guarantees. Go over and ask on sci.eng.welding, I think there's some guys there who don't hang out here, one of them may have dealt with that very machine.

John

Ok. Still hard to get parts for since Hobart got split up in the buy out.

Steel, stainless steel, actually in one form or another it would be capable of most metals even aluminum. Just not in the most efficient way.

Without HF you would be limited to scratch start TIG (if you'd kept the arc stabilizer it would of fixed this problem). The real problem with this is tungsten contamination. However it's been done this way for years and is certainly doable.

Aluminum is most efficiently TIG welded with a square wave machine. The good ones allow adjustment of frequency of the AC, and percentage of cleaning verses heating polarity in the square wave (I know not proper terminology but it's the clearest way of describing the actual end product that I can think of). Next would be sine wave AC with HF. It was the only method available for years and does work but requires some more skill. However TIG welding aluminum started with reverse polarity DC. The drawback here is that it puts so much heat into the tungsten that you need a large torch and tungsten to do any real thickness of aluminum. In truth sometimes reverse polarity DC is better than AC. It has a much higher cleaning action for really dirty aluminum. I've used it in the past a few times on troublesome castings. But it really blasts the torch and tungsten to do it.

Most other metals will DC TIG just fine on straight polarity so the only thing you'll have to watch out for is contaminating the tungsten when you start.

This welder will likely make a superior stick welding power source. Depending on the controls it has may well be one of the best stick welding power sources out there in terms of wide range capability. I can make my old CyberTIG 300 weld thin metal with small rods better and easier than with any other power source I've ever used. Yet it's also able to weld with large enough rods for 99% of anything I need and will certainly be large enough for your needs.

Wayne Cook Shamrock, TX

Hm, thanks.

That's very nice to know. Let's say, for instance, it should be easy for a skilled welder to weld up a trailer frame for a serious trailer. I do not consider myself a skilled welder and will not undertake such a high responsibility project, but it is kind of a test of a welder's capability for me.

Sounds very nice.

Wayne, I am a little confused. With a bunch of 30 A transistors like these

I could relatively easily convert DC into square wave AC. I mean, really, there is not much needed for this other than properly mount the transistors (to cool them), add a blower/cooling fan and apply AC voltage to their signal inputs, so that either half would be producing either + voltage or - voltage.

Am I wrong here?

Very nice to know. Sounds like it could be done, with a big enough tungsten electrode.

I got it. Thanks!

Sounds great. I feel a little better about this project. I will ask about square wave generation in a separate thread.

Very true.

Thanks. Like I said, the machine has no HF.

This is not a good idea!

For my nickel, a DC only machine without HF and 3 phase input that is hard to get parts for? I would offer him a case of beer and hauling it out of his shop. $500 will get you lots more useful machines if you have interest in alumuinum or TIG welding.

As for building your own inverter, Don is right. That is a serious project and by the time it is working well, it would have been cheaper to buy a Miller Dynasty!

Save your money, Bob

thanks Bob... I will reconsider...

Stick is a really good method for building a trailer as long as you're not in a hurry. It's more likely to produce sound welds in heavier metal than MIG. MIG is faster and in skilled hands a big enough MIG is fully capable of welding on a trailer. However in unskilled hands it can be a problem especially with the smaller machines that most home shop welders will buy. The problem there is that it's possible to get a weld that looks perfect but isn't stuck to the metal.

Unfortunately I think you are. Everything I've heard from the people who build power supplies like this leads me to believe that it's almost more of a art than a science to get one that doesn't blow up components. You're talking about switching serious amperage here and paralleling components like that is asking for problems with one of them carrying more current than it's capable of while the others carry less. This can lead to a cascade melt down where as each component fails the rest take on more than they can handle.

Generally big welders like this use large high current SCR's and the like which can be rather expensive. Mine has two SCR's rated at 200 amps in it and then there's two ultra fast blow fuses to protect them. The catch here is that the fuses cost more than the SCR's (found that out the hard way). All of this is just to control the reactor which in turn controls the weld current. In other word they aren't even seeing the full weld current.

It can. The rule of thumb that I've heard is the tungsten needs to be as big as the aluminum is thick.

As stated above that's a rather big kettle of worms that you're about to open. :-)

Wayne Cook Shamrock, TX

I haven't replaces a HF transformer with one from a furnace, but I've opened up a few old welders to blow them out and the transformer looked for all the world like the furnace ignitor type. I think I remember two Hobarts that had those transformers.. If there's a fundamental difference between the two I'd certainly be interested in knowing it, the more I know the better off I am.. I think...

John