Changing welder capacitors

I have a cracked capacitor on my old Lincoln SP-200 mig welder. The plan is to replace all seven caps but I’m not sure if the 75vdc and the +50% higher tolerance will work.


The old capacitor specs are

MEPCO/ELECTRA
11,000uF +/- 20%
50vdc 60 surge
85C max ambient

If possible, I would like to use these for $23.

Genteq

Specifications

• Capacitance: 11,000uF
  
• Voltage: 75VDC
  
• Tolerance: -10+50%
• Temp: 85°C
  
• Lead Type: 2 High Post Screw Terminals (Screws/washers Included)
  
• Diameter: 2"
  
• Height: 2 1/8"
  
• 2019 Date Of Manufacture

Thanks,
Andy

The higher voltage rating should not be a problem, I am not qualified to make the judgement call on the tolerance rating.

If they fit , they will work just fine...Phil

Ripple current rating matters more. Put a thermocouple on the cap and monitor its temperature. Heat drives the water out and the capacitor fails. Sometimes explosively.

Thanks for all the help. Good thing I didn’t order the caps because there’s more going on. It’s amazing how far people will go to screw you over. Here’s the story, the seller wasn’t there when I tried the welder out. It worked fine on the lower settings but wasn’t working on the high voltage. While standing next to the welder, I called him.

He said he’ll check it out in the am but everything worked fine when he left that afternoon. He sends me a video of him changing it to the high voltage and it was welding fine. I specifically asked him if he was welding on high voltage....he said yes. I show up the next day to try it out and he has it strapped to a skid. I took him at his word and bought it. I wire it up a couple days later and guess what? It’s blowing crap out of the capacitor bank and doesn’t work on high voltage. Now the old guy won’t return my calls.

I took the capacitor bank out yesterday and one is blown wide open. I started looking around and one side of the mag-amp coil is completely melted down. I need to weld 1/4” in one pass so I’ll probably end up scrapping the machine.

Andy

I have not had the opportunity to try it yet, but I hear a can of spray foam in a tail pipe does wonders, let us know how it works

Andy,

Sorry to hear your bad deal. What you show in your picture is the achilles heal of that model I came to find out also. Not sure what happens first to cause this, but remember reading it had to do with diodes going bad first(?). Somewhere online is an article of a guy who rolls his own part, but he was a EE by trade.

I have the same welder in the back room with the same problem. Bought it new in the 80's and was a fine machine, paying it's way each and every day. Mine started to make an irritating humming sound one day when welding and went up in smoke in less than half a minute.

Let me know if you happen to find any new parts source, they are a great machine when they work!

Gus

So... capacitors, diodes, and welders are a chicken-and-egg type situation.

Diodes rectify, capacitors absorb and store... but the inductors (the transformer) are doing a job trading voltage for current...

And the output of those inductors goes to a big diode bridge, then to an ARC.

ARCs occur for one of two reasons: First, is that there's sufficient voltage across a gaseous gap to excite the gas in the gap, and carry lots of current... and SECOND, is that there's a high enough level of high-frequency AC to cause the same circumstance.

Diodes and capacitors are really good at what they do, but the circumstances they see in low-voltage DC are entirely different from the circumstances they get from high-frequency AC.

A DC welder generates HUGE amounts of high-frequency (aka 'radio frequency') energy... and that means there's some really nasty high-voltage/high frequency components rolling across the electrode cabling.

RF currents into a general-purpose capacitor them to expire after a very short life. Diodes, likewise, don't like the high RF spikes that occur, and they'll short or blow open.

When I work on a welder, I make it a habit to go to ratings that are much higher (in voltage, better ESR characteristics, etc), and do the same for diodes. I add belt-to-suspenders by adding additional (small value/high voltage/ capacitors across the diodes of the bridge, and at other obvious places in a unit) to help suppress the magnitude of RF noise, so that the diodes and capacitors have a chance at better lifespan. The additional cost that the factory saw, was probably higher, and deemed unnecessary, but in MY shop, the doubling of cost of a $0.52c capacitor is entirely insignificant compared to the peace-of-mind it offers.

If you replace the bad parts with equivalent parts (in a machine that is NOT entirely slagged, as that one sounds as if it is, now ) you can expect it to work as well as it did originally. That may be OK, or poorly, or very well, depending on how good the design is.

They may be (in their unknown quantity and unknown ripple current rating) perfectly fine. Or not.

I have NO idea what went bad in the machine you have. I do not know if the capacitors you saw in it were in any way correct parts. They may not have been, and if so, are a lousy thing to base replacements on. (not that it is an issue now)

However, This machine seems to be 200A, 60% duty, single phase. Open circuit voltage is 40V max.

The 60% duty may be high enough for the capacitors to be an issue for ripple current. They are low-mass parts, and may heat significantly in the 6 minutes on out of 10 minutes elapsed time that is implied. It depends on the cooling, and on what the parts were originally supposed to be.

We do not even have any pictures. Of the setup, or of the damage.

However.....

The RF generated is not generally an issue with the large electrolytics, since they are not very good at those frequencies, and do not pass large ripple currents at those frequencies. And the inductor will likely cut them down considerably, even though IT is likely not very good at high frequencies either. I would not obsess about that.

Adding parallel smaller capacitors is likely not even a good idea. It seems as though it would be great, but they tend to resonate with the stray inductances around them and cause more trouble than they are worth. It is better to use a smaller capacitor in the first place, but in a welder that is not practical.

So let's do some educated guessing.... Even though you think the thing is trash. (pics?)

The 200A at 26V is equivalent to a load impedance of about 0.13 ohms.

You suggested part from Digikey (Cornell-Dubilier) has a rating of 24A approximately. The multiplier for a reasonable temperature is 1.5, so that is raised to 36A. Then 6 of them would be rated at the full 200A as ripple current, and they likely would not see that. The duty cycle may or may not be good for them, if they heat more, the allowable ripple current is less.

Time constant of 6 parts is about a half cycle, which is sorta OK in general, since the capacitors are not expected to hold up the voltage perfectly.

It could work.

JST, post #5 shows the melted down coil. I can take more pictures of the capacitor bank and inside the welder if needed.

Thanks everyone for all the info. I know it’s the wrong direction to go but I’m pissed off enough about this welder to try and fix it. The plan is to make the new coil first and then order the seven new capacitors.

Andy

Well that could work, if you do it carefully. Is that the output coil? It does not look quite like it, but I have no good impression of scale in the pic.

It would be good to try to get a handle on what caused the mess to begin with. Otherwise the same thing may happen again.

Seven parts will have a bit of "cushion" so that is better than 6.

What you see in the picture is the Magnetic Amplifier coil. According to the schematic I saw, the SP-200 brings in single phase to the primary of the power transformer, one leg goes to a pair of diodes, the other passes through two phased legs of a magnetic amplifier (figure-8) where swamping current apparently charges the center coil of the three, saturating the two outer sides where the large, flat mag-amp coils pass.

The part that I find peculiar, is that one leg of the mag-amp looks like it was physically welded ON... like something metal fell in there, and shorted it to secondary's return, and I can't tell for certain from the resolution of the snapshot, but it looks like it was bouncing around in a couple spots.

IF this mag-amp failure occurred as a result of a bad connection, the hot-spot would be from that location, and percolate gradually out. If the mag-amp fractured, it would do same, and open up from that point until failure. Without seeing closer, it's hard for me to figure how this happened, but it isn't pretty, and fixing that winding will be an interesting challenge.

Because of how the mag-amp is damaged, the success of trying to recover the rest of this machine relies on finding some replacement for that mag-amp winding... either that, or bypassing that side, and figuring that he'll only have partial range control...

But I can understand why OP would be miffed about the deal. I wouldn't jump to a conclusion that someone sold it with knowledge and intent, because I've seen plenty of things like this happen without such. Things happen over time, things in shipment. I've gotten so that my S.O.P. for anything that comes to me, gets opened up for an internal inspection prior to a smoke test... I guess I prefer to have the covers off, so that the smoke has an unimpeded exit path... and while that's said in humor, there's inherent truth in that I've seen smoke, and with the cover off, I saw it's source and path quick enough to knock the power off and move in for a really close look, to get a quick resolution.

With regard to JST's note on capacitors, it has been my experience in welders, radio transmitters, and railway traction inverters that capacitors are most heavily stressed and rapidly degraded by voltages in excess of their working ratings, and while a DC component, or a low-frequency AC component would pass, there are many opportunities for very high frequencies to develop, or find their way into circuit areas where such are neither intended, or expected by the designers. Anything that intendionally develops an arc, either AC or DC, is subject to creating a cornucopia of wideband high-frequency noise, and it's like mice- all the protective circuitry in the world will NOT stop it from finding ways into places we really don't want it... and the weakest points will advertise themselves in many ways. I've grown old enough to expect the unexpected, which I guess means I probably just expect everything, and assume basically nothing. I DO know, that when my designs incorporate belt-and-suspenders for things I didn't expect, I had no failures, and I've also come to the conclusion that if a part failed, up-rating the part's limits was always the proper course of action.

But as for this SP-200, I've never made, or attempted to repair a mag-amp. Men built it, so it can be done, but I haven't done that YET... I hope a solution can be found for it.

Mag amp, eh? did not see that on the schematic I saw, but I am not certain it was the right one.

EDIT: Found a better schematic, from Lincoln, which should be the right one. The mag amps are indeed shown in the position you indicate. None of the control circuitry is shown, it is more of a wiring diagram.

Your point is taken, although transmitters are a really bad case.... However, it is worth noting that welders, with the arc, are related to Tesla coils.

You CAN get voltages from induction via resonances and coupled coils. However, the welder designers normally have been aware of this, and take case of it in the design.

The things that kill capacitors, especially electrolytic, are excessive voltage, and heat. Heat comes internally mostly from current, so too much current is bad. Once capacitors start to fail, they can finish quickly.

If HF and resonances kill them, it will be from one of the two main causes of voltage and heat,with the heat likely being from excess current. (OK, the dielectric material dissipates some heat as well, the amount depends on many tings we do not know about the circuit)

When considering the things that can happen, you just cannot calculate for many of the resonances unless you know more than you typically do about the circuit. That;'s what planning, and prototypes, are for.

When repairing, it is not always good to second guess, because the cure can be worse than the disease... I have seen an effect made worse, many times, by well-meaning techs who add parts or change them for better parts. You pays yer money and you takes your chance.

For the mag-amp, Bill (9100) has lots of experience with them, while I have very little past knowing how they work, and probably calculating one or two several decades ago. You might see what he says about that coil.

Thanks for all the advise. Once I started taking the Mag-amp coil apart and seeing how it was all put together, it became clear fixing it was out of wheelhouse. I didn’t feel like going through the hassle of selling a broken welder to someone else so I had my son push it to the road. It was only out there for 20min before a scrapper grabbed it.