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ssommers

Anyone up for a resistance welder mystery?

37 posts in this topic

The drive does not do the welding. The drive powers a motor that turns the material for the welder. Many welders are doing pipe, valves and other ROUND objects that by having them rotate past the welder's stick, the job goes faster, smoother and productivity is increased. Most of these machines have the ability to run in either direction and the speed can be set by a hand pot or foot pedal. I have set up a couple of large ball valve welders with VFDs and black max motors so they can go 2 X base speed past the opening to speed things up.

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Ok, that makes a lot more sense. I was confusing this with a motor-driven welding power source.

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This is correct. The Focus 2 drive drives a DC motor which moves the Generator which generates the welding voltage sent to the electrode wheel that rolls along the tube seam. It's a square wave welder made by Yoder that's over 30 yrs old and normally does a good job. The problem I'm seeing with long sections of unwelded tube is infrequent, but dangerous for the operators if the unwelded section gets water into the downstream processes. The other key is that this is the only tube mill that has this problem and there are 3 other mills with the same style of welder, beadwinder motors & trim tool solenoids. In tube welding, square wave welders used to be the best, but induction welding has overtaken it because there's no contact required between the induction coil & the tube. No more moving parts means no more downtime for daily electrode wheel maintenance. This changeover is going to happen on this mill sometime in the next couple of years. It should have already happened, but we can't make enough tube ahead to allow for the 2 months of downtime. The curse of having only 1 mill that makes our best-selling product... So I'm back to needing to find out what I can do to fix this problem for now. Last weekend, I had the electrician isolate the beadwinder motors & trim tool solenoid to their own control box & power supply per management's request. They still think there's a noise component involved, but my charts didn't show anything beyond the normal noise level. I don't think this will do anything for or against it, but I'll have to wait a few weeks to see. I got the new chart recorder hooked up on Tuesday afternoon, but was out with a sick kid yesterday & found that the gremlins had it unplugged again this morning. So I found a new plug further away & hid the extension cord better. Hopefully, the union contract will be approved this Sunday.

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Now that you have given more detail, it would seem you may have more issues with the generator side than the motor side. If the focus and DC motor are used to control a generator's output power, and this generator causes welding to happen, take a real good look at the generator. The focus due to having a non regulated field, will give you 5% speed regulation. I do not see this as a major issue with the speed of the generator. 30 years ago, this was high tech. Unless you put a modern processor control on the generator IMO the driving force is not the issue. You mention recording. What type of recorder are you using? How is the generator excited? Is the generator a permanent magnet and the voltage is controlled by the speed? Do you have brushes associated with the generator? I guess you do. The brush tension and commutator/slip ring state are critical too.

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The recorder is an Astro-Med Dash 8n. Nice little unit that lets me record as fast as I want to. I've got it set for 2ms intervals which is much faster than anything really changes on this beast. You are dead on with the 5% speed variation. Every operator is knows that the normal variation is about 5V on the welder at any given speed setting and I know this is noise. I can see when a major noise spike comes in that it passes right through the SCR. The weld problem though doesn't seem to be related to the MG set. See post #7 for the chart. The output of the Focus 2 board that drives the DC motor drops first - so the motor slows down, then the generator voltage drops. They're simply following directions. When the Focus board output comes back up, the others come back. I'm fairly convinced this has to be something to do with the Start signal to the board, but I have to wait until it happens again. Thanks!

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That is one SWEET recorder. I have all of the info on it and have recommended it numerous times to customers that want to record events. The hamlin relay I mentioned earlier is the mfgr of the CRR relay in your fo cus. A noticable drop in speed is usually due to line variations and/or excessive noise. I recommend you monitor the input line voltage, field output voltage, control logic input and the speed pot. That is assuming all connections have been inspected and no corrosion is present and everything is tight. Unfortunately, if you look closely at the drive prints, the armature + is also the PCB zero volt point. Ideally, any noise on the drive will be ignored as the O volt point floats up and down referenced to the noise. Reality wise, that aint so. You can do a lot for your speed pot voltage noise issues with a 0.1 uf 100 volt mylar cap across the common and wiper. They are great for noise spikes and have NO appreciable effect on accel/decel. Scope the 10 vdc and make sure it is smooth. If it is not, then the filter caps on the 24 volt supply are shot. You might try and replace all of the electrolytic caps on the focus board. I have fixed stability problems too many times to remember doing just that. Do you have a board level print for your fo cus? I can make one appear if you do not.

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Mystery solved!!!! After several months of recording, on April 16th I finally got the machine to have 2 long sections of unwelded tube within a minute of each other. The operator got me right away and I was able to see that the start signal went down from 24VDC to about 12 VDC and then back up again. It wasn't enough to disturb the potentiometer signal, but the welder thought it should be shutting down. My best guess at that point became corroding contacts & grime - 15 years of it. We had 2 weeks of planned shutdown for the machine right after this happened, so I took advantage of it and replaced every relay & pushbutton, had the wires restripped & the terminal blocks replaced. My electrician showed me a relay that had nasty, dirty contacts and lots of corroded wire ends. He put it back together and I monitored it for another 3 weeks. No more problems! Normal startups look smoother too. Crossing my fingers that it's gone for good... especially since I had to return the recorder. Thanks Everyone!

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Glad you figured it out Susan. Problems like that can be awful to solve. Congratulations

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A clean machine is a happy machine. It is funny after all the great troubleshooting suggestions it comes down to good old PM and cleaning. Good job Susan. Thanks for letting all of us know what the final solution was. Bud

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Impressive Susan. In my experence that is one of the harder problems to solve and it is real common with welding processes. Taking down a 24 VDC power supply is not as diffcult as you would think. One suggestion for your tool box. I use a Fluke 189 DMM on problems like this. It logs similar to the chart recorder you used but does not need a extension cord. I will usually put it in the cabinet setup to monitor the event and walk away and wait. Usually after a couple of dozen wrong guesses I can latch onto the correct problem.

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The Fluke 189 looks like a sweet little tiny datalogger and is going on my Christmas wish list!

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It will spoil you. Only suggestion I have is when you buy it you buy it with Fluke View Forms. It adds to the price but you come out in the long run cheaper. They throw in a REAL handy magnet strap when you buy it as a kit. I used to have a 789 Process Calibrator at the last place I worked and I like this one alot better. Can do more with it.

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