Question about old arc welder component - Page 1
Forum section: electronics / projects

author: laejf
author_rank: Regular Contributor
timestamp: August 02, 2021, 08:04:35 pm
content: I opened up our old 2-phase arc welder which basically only consists of a transformer. Although, there is a component that I don't understand how it works. It looks like some sort of switch and has one of the phases going through it. It has two connectors that are making contact that are attached to thin pieces of metal which makes a spring-action such that the contact can easily be broken.Anyone has any idea of what it is?

author: james_s
author_rank: Super Contributor
timestamp: August 02, 2021, 08:10:19 pm
content: Looks like it's a thermal cutout to protect the transformer from overheating. It might also be used to generate pulses to aid starting the arc, acted upon by the magnetic field from the transformer. Does it do anything when you strike an arc?

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 02, 2021, 08:12:15 pm
content: Its a thermal breaker, contacts open when transformer gets hot!Last time I saw anything like this was in the jungle in the Philippines (long track with 7&1/2 Kv single wire feed).Just found a picture of it

author: floobydust
author_rank: Super Contributor
timestamp: August 02, 2021, 08:14:09 pm
content: It does look like the transformer has seen some very hot days, see the varnish discolouration.It looks like a thermal circuit breaker, bimetallic strip that opens the contacts when the transformer secondary gets too hot - but... bypassed due to nuisance trips? The adjustment screw is all in.It should be fan-cooled and please ensure the grounding is decent.

author: james_s
author_rank: Super Contributor
timestamp: August 02, 2021, 08:18:55 pm
content: The adjustment screw being all the way in looks like it would provide maximum sensitivity, assuming the bimetal strip is the one in the back and it bends out to push the contacts open.

author: floobydust
author_rank: Super Contributor
timestamp: August 02, 2021, 08:34:39 pm
content: Not sure if it's bent, I figure the bimetallic gets longer with heat so the upper contact should snap up to disconnect, and it should be on the screw when cold. It might also sense overcurrent for the primary. How old is this, I would check if the insulating washer is lookin like asbestos.

author: james_s
author_rank: Super Contributor
timestamp: August 02, 2021, 09:56:25 pm
content: Who cares if it's asbestos? Don't eat it, and don't crush it up and inhale the dust.

author: floobydust
author_rank: Super Contributor
timestamp: August 02, 2021, 10:10:58 pm
content: My old man died from that stuff, so I mention it.Arc welders I've repaired all have massive output inductors to do (arc) current limiting, this beast looks like there's nothing but the secondary resistance?You could replace the old thermal cutout with a Klixon or KSD opens around 90°C the only hassle is mounting it on top of the windings.

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2021, 03:28:21 am
content: Was he a miner or ship breaker or something or was he exposed to asbestos around the home? There's a big difference between touching a small piece of it and daily occupational exposure.The arc welders I've worked on have a loosely coupled transformer, usually with a sliding core that is cranked in or out to adjust the current. I've never seen one with a separate inductor but I have only been inside maybe 3 or 4 arc welders so not exactly a massive sample size.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 03, 2021, 03:49:55 am
content: Quote from: floobydust on August 02, 2021, 08:34:39 pmIf you mean the square insulator it is almost certainly porcelain, the fluffy stuff visible is from the cord tied around it.

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 03, 2021, 04:16:26 am
content: Quote from: james_s on August 03, 2021, 03:28:21 amIIRC old school transformer based DC stick welders have an inductor on the output to aid in scratch starting the arc and provide a touch of smoothing to help keep the arc lit. AFAIK they all set current with the transformer design, be it the adjustable core type or the tapped transformer type.

author: floobydust
author_rank: Super Contributor
timestamp: August 03, 2021, 04:51:03 am
content: Quote from: vk6zgo on August 03, 2021, 03:49:55 amI meant the flat insulating spacer between the two switch leafs, not sure what it is made of but it ain't mica.Visually I'd wonder if this thermal overload works properly, portions of the windings are kinda dark. The adjustment screw is all in so that seems wrong? usually means someone mangled it to get the welder to work again. Just speculation. If it goes bad, put in a Klixon and a cooling fan lol.I'm not sure where the old man got exposed, he was a WWII vet worked in rough industries, its use was prevalent. Docs figured from breathing/swallowing saliva with it, got in his stomach. Terrible way to go. I'm not trying to be paranoid but if something looks like it I tend to be careful not to rough it up or inhale around it.

author: Circlotron
author_rank: Super Contributor
timestamp: August 03, 2021, 05:03:25 am
content: Quote from: floobydust on August 02, 2021, 10:10:58 pmThis one with the primary and secondary separated means it will have relatively large leakage inductance which functions more or less the same as if it had an inductor in series with the output. Likely both windings are made of aluminium for a bit of intentional resistance. The ones you have worked on with the inductor, do you remember how the tranny was wound? Was the secondary right over the top of the primary, or was it spaced away like in the picture?

author: floobydust
author_rank: Super Contributor
timestamp: August 03, 2021, 05:25:00 am
content: It was spaced, construction just like OP's. I also see that construction used for VFD output inductors. The arc welder I fixed ended up having a shorted inductor (turn-turn) which I did not expect on such a large part and hard to gauge what mH's look like.I was curious what is keeping current flat in this welder. Leakage inductance - or poor coupling.

author: laejf
author_rank: Regular Contributor
timestamp: August 03, 2021, 09:30:11 am
content: Quote from: james_s on August 02, 2021, 08:10:19 pmIt seems like this is correct. I have not tried to run it without the cover but I guess I could. Any idea to why one of the phases seems to go straight to the secondary winding?

author: laejf
author_rank: Regular Contributor
timestamp: August 03, 2021, 09:33:56 am
content: Quote from: fourtytwo42 on August 02, 2021, 08:12:15 pmSo if the current to the primary gets to high it opens? How do the screw adjust the sensitivity?Wow, well it works! But its difficult to start with a fresh stick, easier when it's half gone and hot.

author: laejf
author_rank: Regular Contributor
timestamp: August 03, 2021, 09:38:12 am
content: Quote from: floobydust on August 02, 2021, 08:14:09 pmYeah it's been living at a farm all its life so it's been rough.So it has not to do with the current to the primary? It's pretty loosely tied to the secondary. How do you mean it is bypassed?How does the screw adjustment work?

author: laejf
author_rank: Regular Contributor
timestamp: August 03, 2021, 09:41:49 am
content: Quote from: floobydust on August 02, 2021, 08:34:39 pmI guess I could blow some hot air on it and see if it moves. I couldn't find any info on it but it's very old.Edit: Actually, it says on it that it's from 69!

author: laejf
author_rank: Regular Contributor
timestamp: August 03, 2021, 09:48:03 am
content: Quote from: BrokenYugo on August 03, 2021, 04:16:26 amThis one seems to be all AC with the current being set with a rotary switch, switching between the different taps. I opened it up because switching above 55A (it goes up to 175A) trips the beaker. But if I start it in with a current selected it works. My guess is that the 'break before make' in the switch does not work. Any guesses?

author: laejf
author_rank: Regular Contributor
timestamp: August 03, 2021, 09:57:00 am
content: The main reason I opened this welder up was that, if the rotary switch that sets the current is rotated past 55A it trips the breaker that is 16A. Switching from 0A to 55A works. But if I start the welder (by resetting the breaker) at a current above 55A it works. Should I suspect the 'make before break' function in the switch or has it to do with the transformer inrush current when switched between taps at higher current?Edit: Also, the phase that doesn't go through the heat breaker seems to go to the secondary windig. Does anybody know why?

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2021, 09:48:42 pm
content: Quote from: laejf on August 03, 2021, 09:48:03 amThat sounds like a likely hypothesis. You're going to have to test the switch, possibly open it up and see what's happening.