Arc stabilizing in a AC welder  - Page 1
Forum section: electronics / beginners

author: rodrigopires
author_rank: Regular Contributor
timestamp: November 03, 2014, 02:59:28 pm
content: 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!

author: richard.cs
author_rank: Super Contributor
timestamp: November 03, 2014, 03:14:45 pm
content: 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 shunts2) 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.

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2014, 06:01:08 pm
content: 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.

author: fcb
author_rank: Super Contributor
timestamp: November 03, 2014, 06:05:12 pm
content: Don't they tend to have an HF generator for starting the arc and stabilizing it when running.

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2014, 06:11:39 pm
content: Quote from: fcb on November 03, 2014, 06:05:12 pmNo, it is only used for TIG aluminium welding.

author: ivaylo
author_rank: Frequent Contributor
timestamp: November 04, 2014, 06:19:25 am
content: 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...

author: Yago
author_rank: Frequent Contributor
timestamp: November 04, 2014, 06:36:02 pm
content: Quote from: richard.cs on November 03, 2014, 03:14:45 pmI had read about transformer switches in aircaft, never thought it through far enough to see the use you posted.Cheers for thatWhat was done to the batteries to modify them, "flooded"?

author: rodrigopires
author_rank: Regular Contributor
timestamp: November 04, 2014, 07:06:43 pm
content: 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!

author: oldway
author_rank: Super Contributor
timestamp: November 04, 2014, 07:52:01 pm
content: 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.jpgWith a 2.5mm rod, welding current should be choosen between 50 and 100A.

author: rodrigopires
author_rank: Regular Contributor
timestamp: November 04, 2014, 08:07:19 pm
content: 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!

author: oldway
author_rank: Super Contributor
timestamp: November 04, 2014, 08:58:04 pm
content: Quote from: rodrigopires on November 04, 2014, 08:07:19 pmhttp://www.thierry-lequeu.fr/data/AN308.pdfSee page 8/11 figure 11

author: richard.cs
author_rank: Super Contributor
timestamp: November 04, 2014, 10:00:59 pm
content: Quote from: Yago on November 04, 2014, 06:36:02 pmFlooded 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.

author: G7PSK
author_rank: Super Contributor
timestamp: November 05, 2014, 10:18:15 am
content: 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.

author: Chris Wilson
author_rank: Super Contributor
timestamp: November 06, 2014, 04:45:39 pm
content: Quote from: oldway on November 03, 2014, 06:11:39 pmSurely an AC welder is only going to be mainly useful *FOR* welding aluminium alloys?

author: oldway
author_rank: Super Contributor
timestamp: November 06, 2014, 05:34:57 pm
content: 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.

author: Chris Wilson
author_rank: Super Contributor
timestamp: November 06, 2014, 05:45:36 pm
content: 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?

author: BradC
author_rank: Super Contributor
timestamp: November 09, 2014, 03:33:02 pm
content: Quote from: oldway on November 06, 2014, 05:34:57 pmI 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.

author: rodrigopires
author_rank: Regular Contributor
timestamp: November 11, 2014, 03:10:01 pm
content: Quote from: Chris Wilson on November 06, 2014, 05:45:36 pmHi, 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).