View Full Version : Resistance welder troubleshooting

02-28-2010, 12:03 AM
I've been troublshooting a resistance welding machine (spot welder) for hours now and aren't getting anywhere. It is a press-style resistance welder (air cyclinder pushes the electrodes together) and has some basic controls for the close time, weld time, weld heat percent, etc.

The machine no longer is making welds. When I step on the pedal, the machine goes through its cycle (electrodes close, weld attempt, etc.) however does not successfully weld. Actually, if you keep trying, maybe one in 20 cycles now does generate a spark at least, however even then it is still too weak to make a weld.

We have already gone through all the basics of troubleshooting: incoming voltage is good, fuses are good, connections have been checked and cleaned of oxidation, weld parameters are right, nothing appears to be shorted.

Normally, when it is performing the actual weld portion of its cycle (when current flows), the transformer hums. I noticed that now, the transformer only hums sometimes. The times when the transformer does not hum, there is a very faint hum which appears to be coming from the electrical cutoff switch cabinet which houses the fuses. Actually, I think the sound might be coming directly from one of the two fuses. This is definitely not normal.

I have done some troubleshooting of the transformer. It appears to be normal. There are no shorts in the primary or secondary windings.

I have also tested the SCR (a solid state switch which I think is responsible for turning on and off the current through the transformer). I think it is working properly, but there are more tests that I could do to be really sure.

So I'm thinking that the problem is either with the SCR or with the controls.

Has anybody experienced anything of this sort?

03-08-2010, 10:25 PM
I figured out the problem and fixed it. I learned how to test the SCR, which tested out fine, so I determined that the controls were the culprit. Being that they were old and from a company that is out of business, and being that they were made up of proprietary circuit boards, I replaced the controls with a newer unit I found on ebay.

After several hours of wiring, I'm back up and running!

In my searches to troubleshoot the problem, the best information resource I found was this article called "Cures for a sick spot welding machine (http://www.unitrol-electronics.com/pdf/diagnostics1.pdf)" . There's really not much information available online on resistance welding machines, so that article was a lifesaver.

05-16-2012, 01:14 PM
Check your incoming air pressure

05-16-2012, 01:44 PM
An old post so it is a moot point now it is running, but the majority of problems similar to that that I have seen are a poor connection on the secondary somewhere.
It only takes a little as the voltage is extremely low, and a small resistance is enough to prevent 100a-200a flowing.
Low air pressure usually is seen by an arc flash when the electrodes meet.