"Dimmer" for 3kVA welder transformer - Page 1
Forum section: electronics / projects

author: Evangelopoulos Panagiotis
author_rank: Contributor
timestamp: November 23, 2021, 04:48:34 pm
content: Greetings,I have an old 3kVA welder that uses a 5 position selector and a separate ballast transformer to limit the amperage.I was thinking of removing these two components and replace them with a high power "dimmer" that controls the transformer's input.I have a BTA40-700A isolated triac and I was thinking of creating my own controller.The problem is I don't know what circuit to use for inductive loads.Can anyone please provide me with a circuit that can drive such a load? Ideally, I would like a second potensiometer to set the lower working point of the transformer.Thank you in advance,P.E.

author: TheMG
author_rank: Frequent Contributor
timestamp: November 23, 2021, 09:32:32 pm
content: I don't think that would work at least not very well. First of all, this arrangement wouldn't limit current, it would reduce power to the "load" by controlling the conduction phase angle.The magnetic ballast, on the other hand, allows the transformer to reach maximum open circuit voltage, while limiting the current once the arc is struck.So your circuit would require some form of current feedback to achieve proper current limiting.Also, the very dynamic and unpredictable characteristics of the "load" (the arc) would make achieving stable control of the TRIAC phase angle somewhat difficult, as it will present a inductive load, resistive load, and everything else in between.The there's also EMC to consider. An arc welder already emits pretty strong EMI, I can only imagine how much worst adding phase angle control to that, without adequate filtering, could make the problem much worst.TBH, I would save myself the headache and keep using the welder as originally designed, or go purchase an 'inverter' type of welder if you need finer current control (they're much smaller and lighter too).

author: Benta
author_rank: Super Contributor
timestamp: November 23, 2021, 09:45:42 pm
content: Worst idea I've ever seen.

author: Evangelopoulos Panagiotis
author_rank: Contributor
timestamp: November 23, 2021, 09:55:17 pm
content: I was afraid of answers like these... I was hoping there would be a solution but I was skeptical about it too.I've seen DIYers doing this modification to their welders but the method always felt a bit off.I'll stick to the magnetic ballast and if I ever feel brave/stupid enough I'll do a test with a few different schematics.The only mods I'll make to it is a soft starter because the permanent magnetization of the core lowers the reactance and it trips the breaker on start-up.Thank you for the help.

author: Benta
author_rank: Super Contributor
timestamp: November 23, 2021, 10:12:22 pm
content: Sorry for my curt answer.Welding transformers are different from others in that primary and secondary are relatively loosely coupled. This is also known as "high stray inductance". The reason is, that you want a "soft" voltage/current relationship at the load (which is your spark).This makes them very difficult to regulate, and it's actually unwanted, your molten metal and the current through it is the regulator. Controlled by the welder through choice of approximate welding current and electrode type and size.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 23, 2021, 10:42:35 pm
content: I think I've heard of something along those lines being done for DIY TIG conversions to give pedal control, back before low cost imported inverter welders were a thing. Not worth messing with given that development, especially if you're just using it as a stick welder.

author: Zero999
author_rank: Super Contributor
timestamp: November 24, 2021, 10:28:12 am
content: Quote from: Evangelopoulos Panagiotis on November 23, 2021, 09:55:17 pmWould you rather have no replies? I did think about responding with: it's a bad idea, but thought I'd leave it.Why do you want to do this? Is it to get a continuously variable output? You could try a variac, but there will be a lot of deadband, at lower voltages, which won't work. Perhaps a 0 to 120V variac and 230V to 120V autotransformer could be used to give 120V to 240V.

author: CaptDon
author_rank: Super Contributor
timestamp: November 25, 2021, 03:14:16 am
content: I used some high current inductors with my Lincoln 225 buzz box. I can keep the higher open circuit voltage for getting a good arc started, even on really dirty metal, but at the same time limiting the current in the 'working' arc. What I never liked about simple buzz boxes is that the open circuit voltage goes down with lower amperage selections. There are times I may want a long arc, but with a slightly lower than normal current.