Genarc Tig set

[Alzax3]

I bought one of these a while back, the chap who sold it to me said he used to run it off the DC output from his Mig welder. (It's not self powered, but needs a +ve and -ve supply plugged in the back.) I had nothing to power it, and have since been told it was designed to be run from an engine powered welding set. I've also been told that if I plug a DC supply from my rectifier Murex unit, the back feed from the Tig's High Frequency circuits will damage it. Any comments? I know I can scratch start tig from the Murex anyway, but having the posibility of HF start to hand makes me want to use it!

 

[Alzax3]

So nobody knows anything about Genset Tig units?

 

[bill_gj]

Seems not.

I acquired one a couple of years back and haven't tried to use it yet......
The plan is to run it from my old arc welder plus a rectifier that I've been collecting parts for.

I did all sorts of web searches for these Genset add-ons and just couldn't find anything, just references to the generator powered mobile welders that carry the same name.

 

[Alzax3]

Oh well, we seem to be in the same boat, then! Pity, it looks like quite a good and well designed unit......

 

[bill_gj]

I'll tell you what Alzax3, if/as/when I get my rectifier finished and the Genset box connected, I'll phone you from my hospital bed and let you know what not to do.

Seriously, I'll keep you in mind and try and let you know how it goes. Can't promise anything just yet.

 

[Alzax3]

That would be brilliant! (the sharing information bit - not the getting it excitingly wrong!) So your idea is to rectify the output of an AC arc welder to DC with an add on box? Somehow I'd rather do that than risk the possibility of HF feedback spoiling my shiny new DC set........unless that was hogwash from my welding parts dealer!

 

[shenion]

A HF unit should be ok on a decent transformer setup that does not use electronic control.

My Airco uses no electronics, just a saturation transformer for control. I added electronic control and saw about 180V of the HF bleed back through the control circuit. Took some filtering to get it to work.

Adding some capacitors across the welder terminals will help. The HF units that both the work and electrode lead pass through are better as they will be able to filter the HF from feeding back into the unit.

 

[bill_gj]

Here's a badly-drawn pic of what I'm doing to the old AC welder:


The diodes will go onto heatsinks and I suppose I should put a fan in to keep stuff cool.
I got the diodes via ebay from a dealer in the USA. Not bad price, about £15 for the four by the time I paid postage. They'd cost a lot more than that from the UK, if you can find them.

The inductor/choke was acquired for me by my son - it was in the scrap skip where he works. Apparently it was part of some heavy duty motor control gear that was dismantled. It's made out of windings of flat copper strip that looks like it will take the current.

The capacitors are ex-computer power supplies, I think. Mainframe stuff, not puny PCs. They are about 3" diameter and about 6" long - can't remember the values (I'm at work) but it's around 40,000 microfarad each, I think.

 

[shenion]

You don't want the second cap on the output side of the inductor. It will lessen the effectiveness of the inductor to maintain a stable arc.

You can put both after the rectifier. A small AC cap on the input will help dampen radiated noise. Motor run caps work well.

Here is a thread on scarecrow's conversion.

I have 2 30,00uf on my Miller MIG. They are about the same size.

 

[bill_gj]

Many thanks for the advice Scott, it's much appreciated.

Time for a little sycophancy - I've read most, if not all, of your technical posts on here and on your web site and think it's great stuff. Your electronics skills are well beyond mine - I just tinker and try not to blow too many fuses. I've also been collecting junk for one of those car alternator-powered TIG welders, but I don't expect to get the time to fool around with that for a while yet.

It's people with true knowledge, experience and skill that really make this forum, unlike others, which are often full of negativity and abuse. Thanks to all the positive contributors.

I'll try to get some pics of my capacitor collection and the jumbo inductor.

 

[Alzax3]

A HF unit should be ok on a decent transformer setup that does not use electronic control.

So does that mean my Murex rectifier DC unit would be happy with HF feedback - or does that count as electronic control? - I don't want to break it as I've only just bought it!

 

[shenion]

So does that mean my Murex rectifier DC unit would be happy with HF feedback - or does that count as electronic control? - I don't want to break it as I've only just bought it!

Don't know.

My Airco has absolutely no electronics. Just huge transformers, resistors and caps (only "electronic" part is 4 huge 500A diodes.)

I woudl neeed to see a schematic to see if it is ok.

 

[Alzax3]

Don't have schematics, and haven't found a source online, guess I'll have to lift the lid and find out what's lurking inside!

 

[bill_gj]

Scott - IF I have "too much" capacitance between the rectifier and the inductor, is there a chance of blowing diodes at switch-on due to the inrush current? Can I avoid that?
I wasn't anticipating using high welding currents (just for sheet metal) so I didn't go for jumbo diodes.

 

[shenion]

Yes, I was worried about blowing the breaker when the contactor turned on. Was going to add a zero-cross SCR setup to prevent it.

The welder will limit the current before the diodes. Probably about 2x what the set current is. Diodes are rated for about 3-4X their rated current for one half cycle.

What is the current rating on the diodes? Remember, each only runs on a half cycle, so 100A diodes will work at 200A in a full-wave rectifier (and probably 400 peak on startup.)

80kuf is a lot, you will probably trip your breaker before damaging decent diodes.

BTW, the output DC will be 1.4x the OCV of the welder so you will need caps rated at least that voltage. My Airco is 90V so I would need 150V caps.

 

[bill_gj]

My diodes were 120A I think, so by your advice, they'll be plenty for my needs. The AC welder was a cheapy I bought about 26 years ago and was rated at 140A max, so I dare say that that was optimistic. I don't know what the OCV is as I never needed to measure it, but I will, just to be sure.

I had a read through the scarecrow thread - very interesting.

If you don't mind, I'll draw my revised circuit plan, using the info from here and the scarecrow thread, and put it up here for your scrutiny and comment.