MM200 capacitors, revisited

[knothead]

I have a MM200 that I want to install new capacitors in. Previous posts on the subject indicate that I'm going to see an improvement in the weld quality and a higher voltage. Higher voltage will be a problem for me. I'm using the lowest setting already. If voltage goes up I have no room for adjustment. Any thoughts? Should I go with a lower rated capacitor?

[duaneb55]

You lost me.

Good capacitors (whether new or used) are meant to maintain a constant arc voltage while welding to compensate for operator variables such as slight changes in stick out and travel speed.

So if the existing capacitor(s) in a machine have exceeded their life expectancy resulting in inconsistent or low arc voltage then yes, new caps should solve that issue and restore performance. However, actual output voltage is determined by a units main transformer and not the caps.

What exactly are you trying to accomplish? Specifically, what voltage do you believe you're needing?

[knothead]

If I'm using an old machine with old capacitors, does that mean that the machine's output voltage is likely to be lower than if it had new capacitors?

I'm welding 304ss, .035 thickness, without a back purge. I'm running silicon bronze wire, .023 diameter.

I'm experiencing what I would describe as an unstable arc. It welds fine for the first 5-10 seconds, then it gets erratic. If I pause, and then restart, it's corrected, but only briefly.

I've looked for grounding issues and I'm presently clean connections inside the machine as suggested on another thread.

[knothead]

And so my concern is that if I replace the capacitors the voltage will go up and I'll start having sugaring problems on the backside.

[Bluewelders]

Just replace the capacitors with the proper size.
Messing with the capacitor size will just cause problems.
You may have a capacitor going short that is heating and causing the instability.
If a capacitor goes short it can blow the diodes.

[tadawson]

If I'm using an old machine with old capacitors, does that mean that the machine's output voltage is likely to be lower than if it had new capacitors?

I'm welding 304ss, .035 thickness, without a back purge. I'm running silicon bronze wire, .023 diameter.

I'm experiencing what I would describe as an unstable arc. It welds fine for the first 5-10 seconds, then it gets erratic. If I pause, and then restart, it's corrected, but only briefly.

I've looked for grounding issues and I'm presently clean connections inside the machine as suggested on another thread.

No - as already noted, the output voltage of the unit is not determined by the capacitors. The caps have a value, typically in micro farads, of how much charge they can hold. They also have a voltage rating, of the maximum they can *tolerate* . . . Think of it as one of your welding tanks . . . They have an internal volume, and pressure rating . . . exceed the pressure, and they fail . . . just like the voltage rating on a cap . . . but if you have the same amount of gas in a 40cf 2400 psi rated cylinder vs a 40cf 3000 psi rated cylinder, the pressure will be the same . . . just like a cap . . .

- Tim

[duaneb55]

As Bluewelders stated, you may have a bad capacitor (or more) that is preventing the cap bank from maintaining a steady weld arc voltage. As they have a finite life, replacing them with new may correct your problem.

As for any sugaring, that would simply be a result of no back purging IMO.


What's your machine's SN?

[knothead]

Jf928813
and
Jh213789

I'm successfully welding now without sugaring and without a back purge. My concern is that if the voltage is currently lower than the rated output, and by replacing the capacitors the voltage goes up to it's rated output, then I might have a sugaring problem. I ordered one set of capacitors. Waiting for UPS.

[tadawson]

If the old caps had been pulling enough power to sag the voltage, they would have been on fire or blown apart by now . . .

[walker]

Since you are having problems, and are going to have the machine opened up, take the time to ubolt all the cable connections (one at a time) wire brush and reassemble. Pay special attention to the back side of the din conecctors, I have seen several machines that have had melt downs there.Clean your voltage selector contacts as well. Also, the thing that made the biggest difference in performance on my machine was to pull the contactor points and clean them. After that and new capacitors it welds beautifully.

[Bluewelders]

If the old caps had been pulling enough power to sag the voltage, they would have been on fire or blown apart by now . . .

You are probably right,but the caps stealing even 1 volt would make a large difference while welding.
The difference between 40amps and 30amps is only about 1/2 a volt.

[duaneb55]

If the old caps had been pulling enough power to sag the voltage, they would have been on fire or blown apart by now . . .

Not necessarily - based on my experience.

As capacitors have a finite life, just one or more "tired" caps will result in a banks' inability to maintain design arc voltage. Have seen the same with a blown cap or an open cap in a bank.

[N2 Welding]

Do capacitors life depend on age from manufacture date or hours of use. What are the biggest factors in cap degradation?

[Bluewelders]

The biggest factor seems to be the temperature that they see.
High temps of the environment and ripple will all cause heating.
Letting them set for years will cause heating when the plate oxide coating reforms.
The configuration of the capacitors will also have an effect.
Capacitors have a wattage dissipation value and using more small capacitors for the same capacitance, will usually run cooler.

[N2 Welding]

Thank you

So heat and sitting around for long periods unused are problematic.

[knothead]

https://farm6.staticflickr.com/5466/...b5722ab9_b.jpg

[knothead]

This is what one of my new capacitors looks like. I hesitate to put it in. Anyone here know enough about capacitors to know whether I should use it or send it back?

[knothead]

I think I just answered my own question. The new capacitors read zero ohms, except the dented one which read resistance, and it's constantly changing.

[knothead]

I put the 7 good capacitors in and took out for a ride. It fixed my erratic arc problem. I'll ask miller4less to replace the bad capacitor and I'm going to order a set for my other mm200.

[pin2hot]

I think I just answered my own question. The new capacitors read zero ohms, except the dented one which read resistance, and it's constantly changing.

All should behave like that. An ohmmeter applies a small DC voltage and measures the current and from that determines the resistance. When first applying the ohmmeter leads, the capacitor should read a low resistance and then climb as it charges. Switch the ohmmeter leads and see what happens. It should be a zero (or negative) resistance. The capacitor will discharge thru the ohmmeter and then start charging at the opposite polarity, and again the resistance reading will climb as it charges.

I would probably return the dented one.

[pin2hot]

I guess I didn't understand by what you meant by, "it's constantly changing."

To add to what I wrote yesterday, you should only connect an ohmmeter to a discharged capacitor. I was considering the ones that you had taken out of the box.

Another way to test a capacitor is to connect the ohmmeter and let the capacitor charge. The ohms should be infinite or close when charged to the meters output voltage. Remove the leads and switch the meter to DC volts and retouch the leads to the capacitor. You should see the voltage that the cap is charged to. The voltage will decrease as the cap discharges thru the meter.

An open or shorted cap won't have a charge.

And remember a capacitor may have a dangerous to lethal charge, particularly from something like a microwave oven.

[knothead]

By constantly changing I meant that the reading was floating up and down. I'll check them all again tomorrow.

[Denis G]

When capacitors get old the Equivalent Series Resistance (ESR) goes up and those capacitors get hotter (it makes them age even faster and more likely to fail). With some multi-meters you can measure both capacitance and ESR and check them against datasheets to see if they are really out of spec.
https://en.wikipedia.org/wiki/Equiva...ies_resistance