Anyone up for a resistance welder mystery?
Dear Susan: Yes I made it thru but I am sorry I am not going to be much help. I have little background with welding, but if I get this right the 0 to 10 DC input controls the 230 AC out which runs the generator which creates the actual welding voltage. Is this correct? Did I also read that your data logger shows no change in the 0 to 10 DC but the 230 output from the control board does drop. THis means to me that either your board is experiencing a thermal condition or the inputs are telling it to cut the voltage back. My only input would be on your search for a constant AC transformer. Controlled Power company in Troy MI makes units for NASA and has a great product at a reasonable price. And no I don't work for them, jsut used their units before.
I spec'd out the Sola Hevi-Duty 7.5 KVA to be able to handle 3HP at full load. It's approx $5000 which is just the level where capital requests kick in and the general manager has to scrutinize the expense to death. The plant manager realizes that it should improve weld quality, but the GM doesn't believe it will solve the problem of the welder dropping out for those 4 seconds. I have to admit I have my doubts too, but it would be one more layer of noise insulation.
I haven't seen any spikes on the power line due to the beadwinders going on or off (they're on 480), but I'll check it on the datalogger this morning. I doubt the line supervisor will let me try to induce the fault, but he might if I catch the mill starting up for a new tube size. I know there's a bunch of HF noise on the 480VAC power line. I have induction welders & annealers in the plant (including right behind this resistance welder) so noise is a constant concern. The Focus 2 has a 480/240 transformer without any line conditioning in front of it and all the power noise goes straight through it to the output to the DC motor. That alone shouldn't make the output drop so drastically. I'll put up more info when I get to work.
The potentiometer itself has been changed. In fact, I think every piece in this part of the mill - beadwinder motors, contactors, solenoids, switches, pots, Focus drives, and wires - has been changed at least twice in the last 6 months. The electricians & supervisors were taking a "shotgun" approach to this problem long before I arrived here almost year ago. I just had enough time this morning to add monitoring for the trim tool solenoid & the beadwinder motor voltages. I'll add another data point for the potentiometer supply voltage next time the machine is down for a change over. Question: Could the pot voltage going down be a result of something else happening inside the drive? The pot supply voltage is generated from inside the Focus 2 drive which has many, many op-amps & resistor-capacitor networks. I tried doing some analog feedback analysis from the schematic on p.53 in the drive book and managed to make my head hurt alot. I keep thinking that there's some kind of cascade going on in the control board that has something to do with the SCR output to the DC motor, but I can't figure out what starts the whole thing off. It has to be an impulse type of thing. I'm sorry if I sound frustrated, but I am. Thanks for hanging in with me!
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I have some experience with resistance welding, but only with spot and projection welding. Never did any continuous seam welds. Nonetheless though, I feel compelled to chime in :) The first thing that strikes me as odd is this motorized generator you're using. Is that common in your industry, or is it just very, very old? I've never seen anything like that in an auto plant.. The only thing I've seen that comes close is an old Lincoln arc welder at the community college I attend. It has a huge DC generator as the power source. It was from the mid-60s. Would it be feasable for your company to install a modern mid-frequency DC inverter welding control? That is what is considered commonplace in the auto industry, and would probably eliminate all your problems. (again though, I'm not familiar with seam welding.. not sure if MFDC offers the same advantages to your process) Here's some information on what I'm talking about: http://www.medar.com/welding_concepts/controls_mfdc.html Of course, this would be a major redesign of your process, so it may not be practical or possible for your company to do it. Just a thought though!
While you're at it adding measurement points, can you also check the GND of the control board. Many of the problems in that area I encountered were due to bad grounding.... I have learned to check this the hard way.
This is just very, very old - mid 70's approximately. There are 3 resistance welders in the plant with 100HP (or bigger) MG sets. The rest of the tube mills are induction welders. The company is heading toward induction welders - cleaner non-contact welds, less daily maintenance, etc - but it will take some serious shoehorning to get a new induction welder into this spot. I've been working on the planning the changeover for 6 months and it may be another 2 years before it actually happens. Until then, I have to keep plugging on this. Beegee - Thanks. I've already checked the control board GND. I'm a stickler for shielding & grounding so I know that's not it. But it's a good thought.
caveat : I'm no welding expert. If I understand correctly, the control board regulates the speed of a DC motor that is driving a DC generator that supplies the welding current. I've studied the schematic and the plot. If the scaling is correct, you seem to be operating at the low extreme of the speed pot - approx. 140 millivolts in a 10 volt range. The dip is approx 80 millivolts. Perhaps other gain settings on the board are too high, giving you no span to work with. It would appear from the plot that the motor is attempting to stop (control o/p goes to zero). The generator output doesn't reach zero (coasting perhaps?). With the "power cube" supplying the motor in parallel with the control board's supply, any load disturbance at the motor that strains the 230V supply transformer will be seen by the controls. The reference for the speed pot comes from a simple zener regulator and unregulated 24V. If installing a CVT, I would be inclined to only supply the control board from it, therefore it would be small.
I can personally vouch for removing every last one of those constant voltage transformers from my plant. All you are buying is an autotransformer. So it fails to provide even the small protection that an isolation transformer gives you since one leg is tied directly to the primary side. In theory it will help cut down on some sags and the collapsing field MAY help it muddle through mild brownouts that last for less than a cycle. That's about it. I can demonstrate case after case after case of strange electrical failures and voltage sags that I've tracked down to CVT's. However, it will do a wonderful job of overheating anything around it, including itself. It also does an excellent job of failing on a regular basis, requiring replacement. You may as well pay for some cheap line reactors or something like that because it's going to give you just as much of an improvement as those overpriced heaters that Sola sells.
Mybe some ideas. If you can measeure voltag on potenciometer bitvin pin 8 and 10. It mast bee steade at 10V. How old is drive thet is replaced, ther is some problem with elektrolit if it's older then 10 year.If so tray to replace it with new, I have some result with this.
Do your 10/24VDC stay ok during the drop of speed or haven'tv you been able to log that?
the Hamlin relays used in focus drives start logic on the main pcb, can be a bit of a pain. I have replaced too many. If you are looking for a better solution, look seriously at a 3 hp VFD and a marathon black max motor. I recommend the V7 or 3G3 MV Yaskawa/Omron for your situation. At the 3 hp level you should be able to get both for around a grand. The drive can have external controls and speed pot or use the built in operator if so equipped. Using a 3% reactor on the VFD input helps with noise both ways. I have seen many single phase 5 hp and less DC drives on welders and on the ones converted to VFD, they run much steadier and smoother too. The DC bus in a VFD helps with line voltage variations. A DC drive with no tach like the Focus 2, have 5% speed regulation characteristics. The VFD has about a 0.5% or better speed regulation characteristic. If you do not want to convert to a VFD, I have seen using a regualr single phase transformer, rated for at least 50% bigger than the drive/motor help out quite a bit. The xfmr helps smooth out noise and eliminates line grounds from getting to the drive and motor. It also helps when the motor grounds or drive has issues by limiting what gets put back to the line.
That is why I said V7 as it is a vector drive and has excellent low speed torque especially when wired to a Black Max motor. Typically these welders do not require tremendous starting torque as they tend to have a large gear reduction. ssommers, Glad to hear some of your "headaches" have been cured. BTW, by going to an inverter, you might be able to eliminate the MG set. Unless it powers multiple units. Even at that, you will me SSSSSOOOO far ahead to dump the MG set and get some newer equipment in there. Cost to operate alone by going to the VFD's drops dramatically.
I guess I'm still curious about the whole using-a-motor-for-welding concept. Is that still the state of the art for this type of weld? It seems to me that if we're talking inverters, we should be directly converting line voltage to welding current and leaving the mechanical stuff out of it! That's how all the spot, projection, and MIG welds are done in all the plants I've been to.