Arc stabilizing in a AC welder

[rodrigopires]

I have done my homemade AC welder, but the arc is a bit unstable when welding. I used two microwave transformers and did a new secondary, they are in series. It delivers about 36V and up to 200A, with 230VAC input. My question is how can i stabilize the arc and how can i make a regulator for the amperage? Should i turn it into dc?

Thanks!

[richard.cs]

36V open-circuit is a bit low, double that would be perfect. Older AC welders had 60-70V O/C, newer ones keep it below 50V as this is considered the maximum safe touch voltage but that is pretty marginal for welding with. How are you regulating the current now? Did you leave the magnetic shunts in? Your options for current regulation are:
1) Movable magnetic shunts
2) A seperate limiting choke (on primary or secondary), with either an adjustable gap, multiple taps or a DC control winding.
3) Series resistance somewhere, see below.
4) Fancy electronics.

DC welding can be done on lower voltages, I found 24V unusable for any but the smallest rods and 36V to be about right :-). To convert to DC you'll need a high current rectifier (easy/cheap) and a *big* smoothing inductor (much less so). I used 3*12V flooded lead acid batteries, current limiting was a length of very heavy fence wire and a sliding croc clip. The whole thing was packaged in a large wooden crate with a homemade 230V-36V charger and it allowed me to do big welding on a very limited 2 kW supply so long as the duty cycle was low enough to keep the batteries topped up.

[oldway]

Arc voltage stay between 20 and 25V, 36V is not that low.
Such low open voltage could make arc starting more difficult but does'nt have great influence on arc stability.
I think you problem is that you have too high or too low current for the electrode size you use.
The first thing to do is to measure the welding current with an AC clamp meter.

With a full wave rectifier (bridge), there will be a great improvement of the arc stability.
Smoothing inductor is not needed.

For current ajust, you could use a primary voltage control (triac dimmer) but the load is highly inductive and that is not that easy to do.

[fcb]

Don't they tend to have an HF generator for starting the arc and stabilizing it when running.

[oldway]

Don't they tend to have an HF generator for starting the arc and stabilizing it when running.

No, it is only used for TIG aluminium welding.

[ivaylo]

Also make sure your electrodes are:
 - dry/clean,
 - make good contact (good ground connection on the other end too),
 - sized for your current (well, current sized to them),
 - and not like 10 years old...

[Yago]

36V open-circuit is a bit low, double that would be perfect. Older AC welders had 60-70V O/C, newer ones keep it below 50V as this is considered the maximum safe touch voltage but that is pretty marginal for welding with. How are you regulating the current now? Did you leave the magnetic shunts in? Your options for current regulation are:
1) Movable magnetic shunts
2) A seperate limiting choke (on primary or secondary), with either an adjustable gap, multiple taps or a DC control winding.
3) Series resistance somewhere, see below.
4) Fancy electronics.

I had read about transformer switches in aircaft, never thought it through far enough to see the use you posted.
Cheers for that

What was done to the batteries to modify them, "flooded"?

[rodrigopires]

Hi, i have taken some new measurements and it has 30VAC, and a maximum of about 300A (278). The transformers don't have magnetic shunts. I'm using the wires straight from the transformers, don't have any circuitry.
I regularly use 2.5mm rods (a little less than 1/10th of an inch) and it pulls about 140A when welding, which i think its a bit too much. My hope is to make a amperage output regulator and stabilizer for the arc (its not too bad, but a little stabilizing would be nice).
My first idea was to turn it into dc and use some caps and a regulator circuitry, but that takes a bit of work and expensive parts, which is not viable for a homemade project.
Some suggestions?

Thanks!

[oldway]

You need 1/3 more turns in the secondary of both transformers to reach 40V open voltage.
To reduce current, you must increase the short circuit voltage using magnetic shunts. (see picture)

http://www.qsl.net/kh6grt/page4/xfmr/pictures/transshunts.jpg

With a 2.5mm rod, welding current should be choosen between 50 and 100A.

[rodrigopires]

What if i add in a third transformer and magnetic shunts on the three? Would it be better? But still, is there a way to regulate the amperage using AC voltage? Does a thing like a light dimmer (with proper amperage) work?

Thanks!

[oldway]

What if i add in a third transformer and magnetic shunts on the three? Would it be better? Yes if there is no room enough for more turns on the transformers. But still, is there a way to regulate the amperage using AC voltage? Does a thing like a light dimmer (with proper amperage) work? Yes, there are welding transformers on the market using it.

Thanks!

http://www.thierry-lequeu.fr/data/AN308.pdf
See page 8/11 figure 11

[richard.cs]

What was done to the batteries to modify them, "flooded"?

Flooded as in with a wet electrolyte, not gel-cells or absorbed glass matt (AGM). So just bog-standard car starting batteries. Good for high peak currents so long as they're not discharged very deeply.

[G7PSK]

You could try a power factor correction capacitor on the input or a capacitor bank on the output. 2.5 mm rods should be run at around 70 amps.

[Chris Wilson]

Don't they tend to have an HF generator for starting the arc and stabilizing it when running.

No, it is only used for TIG aluminium welding.

Surely an AC welder is only going to be mainly useful *FOR* welding aluminium alloys?

[oldway]

The subject is not to analyze the various types of welding generators, nor the various welding process.
With a simple transformer without current control, we can't do nothing other than MMA (manual metal arc) welding.

I do not know any commercial MMA welding transformer with a high voltage / high frequency generator.
It's only used to start the arc in some TIG welding equipments.

This kind of generator is not working all the time: it is used only to start the arc, not to stabilize it.

Tig is a far better process than MMA to weld aluminium alloys.

[Chris Wilson]

I was assuming he had built a (perhaps semi) square wave output welder, possibly incorrectly. So is it just for stick welding steel Rodrigo? A simple AC output at mains frequency?

[BradC]

I do not know any commercial MMA welding transformer with a high voltage / high frequency generator.
It's only used to start the arc in some TIG welding equipments.

I have one of those Chinese 200A MMA/TIG/Plasma machines and I quite often use TIG mode with MMA electrodes. As a beginner with little experience on MMA I find using the HF start rather than a scratch or peck, and being able to control the welding current with the foot pedal is a lot more forgiving of my sloppy stick control. It was a happy accident discovering the machine would do that. Conversely leaving the arc force control turned up while tigging is a recipe for disaster.

Phase angle control of standard transformer welders works well, but you want to make sure your triac is *big* and well heat-sunk. Trailing edge control is nice to have too (it makes the transformer buzz less).

Nice job on the DIY welder. A great achievement.

[rodrigopires]

I was assuming he had built a (perhaps semi) square wave output welder, possibly incorrectly. So is it just for stick welding steel Rodrigo? A simple AC output at mains frequency?

Hi, i dont know very much about electronics, just enough to able me to make some DIY projects. I use mains input for the two transformers, it's 230VAC 50Hz. I only use it to stick, for tig and mig it needs more voltage (i think).