Spot welder

Alan Case · May 10, 2005

I have a very ancient spot welder that I wish to automate. (automate the on/off control only)It looks like the ouput side of the tranformer is switched by 2 transistors (I assume transistors) 1 for the positive cycle of the wave and one for the negative.
I am thinking along the lines of trying to switch these transistors on and off at the zero crossing point of the wave. Is this a better method than just random switching and if so what is there in the market to detect when the cycle is at zero.
I intend to use a SSR to trigger the transistors.
Regards Alan

PS 1 strange looking welder. The transistors are actually water cooled.

ELake20 · May 10, 2005

You could use palm buttons and a palm button relay or you could use a small PLC...its all personal preference..I'd put a PLC in there and do what the palm button relay does with a couple rungs of logic
and pull up your SSR.
water cooled transistors? cool!
get some pictures!!

Alan Case · May 10, 2005

I intend to use a PLC but how do I know the exact instant to fire the SSR, ie when the voltage is at zero.
Regards Alan

colerick · May 10, 2005

Might check out Intertron, I have multiple older 50kva rocker arm spotwelders, and am planning on using 100b weld controller w/600 amp scr's. About 1600$ ea.

Regards,

Andy

TConnolly · May 10, 2005

Alan Case said:
strange looking welder. The transistors are actually water cooled.

Alan, I suspect those are actually SCRs - one forward and one reverse. Water cooled SCRs are common. The SCRs should be gated by a trigger board that uses comparators to determine where the ZCP is. A PLC won't be fast enough to give you fine resolution control on when to gate the SCR.

S7Guy · May 10, 2005

I agree. Those are definitely SCRs. I've seen them in a lot of welders.

And I can't see how you can do this with a PLC. Every automated welder I ever saw has a welder controller that the PLC talks to. Even the ones that had motion controllers for precision location didn't do the actual welding.

Rod · May 10, 2005

Alaric and S7guy are correct.

I've repaired those machines; the 'firing angle' is critical. PLC is not what you need.

I know there is a ZCS chip out there, I've used them, but have slept since then.

If you're going to 'roll-your-own', I'd search DigiKey or Allied.

I worked on a machine that used a BUNCH of those SCR hocky-pucks. One was out of phase and blew the transformer off the pole. The neighbors didn't like that.

And I normally don't carry a clean set of underwear.

I REALLY don't like working on machines that use a curtain of chains and a 1" poly-carbonate screen - Whine

Your welder is a cake walk - Google for them and see if you can get a Schematic

Rod (The CNC dude)

Alan Case · May 10, 2005

I have always intended to use the PLC to say "I need to fire now" then whatever else I put in the circuit will sit there and wait for the zero crossing point and when it sees that point the SCRS will be triggered. (Hope that makes sense). Sorry I did not give enough info. The PLC is a CLX running motion control cards. It looks like I am not the only one trying to do this.
Regards Alan

rsdoran · May 10, 2005

I have worked with automated welders that used plcs to tell it when to fire. To fire the pucks (could be a stud type also) you need a firing board. This site may help visualize what the water cooled type looks like look like.
http://www.darrahelectric.com/scrcontactors_water.htm

Here are pictures of different size/types of SCRs:
http://www.jptechinc.com/rpmain.htm

This site has info, drawings, wiring diagrams etc on firing boards:
http://www.enerpro-inc.com/html/firing.html
The firing board should handle the HOW when given a signal to fire.

Alan Case · May 11, 2005

Thanks Ron. Where do you find all this stuff or are you really the owner of google and have all the servers in your basement. Regards Alan

rsdoran · May 11, 2005

Alan Case said:
Thanks Ron. Where do you find all this stuff or are you really the owner of google and have all the servers in your basement. Regards Alan

I have said I work with about everything and in many cases I already have links on the subject. In this case it just so happens I had to replace some water cooled SCRs for a welder about a year ago.

Funny part is that I dont use Google to search, got in the habit of using Search Assistant and it does fine.

walters · May 12, 2005

what kind of search assistant is that ron?

and where can i find it

rsdoran · May 12, 2005

Its the one built into IE. If you click Search in the menu above a frame will open on the left, at the top is Customize, click that then checkmark Search Assistant

Spot welder

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