Plasma cutting with a MIG / Lift TIG (conversion question) ?

Not sure what you are getting at here. My "little" 55A plasma cutter consumes well over 9 kW at full tilt cutting thick material. A typical plasma cutting arc is DC, non-pulsed.
 
graham-xrf said:
Yes - I was reading that it could be typically 125V to 140V, though this might be a pulse average, or peaks in re-establishing the arc when regulating. What I was after was, with that 100V, what is the current, or average current, or instantaneous regulated current during a switcher ON period, anything!

This is because even 100V x 50A is 5kW, and I know the little plasma cutters sold do not boast this power, so this scenario is impossible - unless we are missing something!

An exact appreciation of what goes on in a plasma, and the ups and downs of voltage and current, and how this adds up to the energy being delivered, is not what we have.

I do get it that the question may involve a deeper understanding of plasma cutter operation and design, and that may not be a reasonable expectation here. That's OK. Folk here have provided me enough answers, especially that plasma cutting requires a different type of machine, and that a regular TIG cannot do it. I have seen an advertisement for a (cheapish) machine that can be switch selected to provide Plasma, or TIG, or stick weld, and this makes sense. If there are enough modern electronics inside, (ref @pontiac428 ) then one can design a special product that does this.
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I've read that some of these multi process welder plasma machines are the source of distrust for multi process welders. Both cheap and unreliable, because apparently the plasma function really does not get along with the welder side of the electronics and the gained a reputation for having a very short service life. Even with MP welders becoming popular ones including plasma cutters are limited to cheap imports. I don't think any of the big manufacturers offer one. There may be a good reason for that.
 
Aaron_W said:
I've read that some of these multi process welder plasma machines are the source of distrust for multi process welders. Both cheap and unreliable, because apparently the plasma function really does not get along with the welder side of the electronics and the gained a reputation for having a very short service life. Even with MP welders becoming popular ones including plasma cutters are limited to cheap imports. I don't think any of the big manufacturers offer one. There may be a good reason for that.
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My instincts follow yours. I accept that if the task is any form of "welding", the basic inverter controlled energy source can easily be arranged to suit different types of welding, MIG, TIG, etc. It is clearly possible to offer a welder that can do MIG and TIG and stick welding as a multi-option, with various software control configurations. Of course, these may be sold in any combination.

When it comes to plasma cutting, a multi-modes machine becomes quite awkward. Plasma really is better with a dedicated machine. It's not that it can't be done. The question is whether it should be done. This is, I think, better a case of designing something to do one thing well.

Starting a plasma
What is involved in plasma cutting is first the approximately 10kV radio frequency start spark, This is the same gadget as installed on switched start TIG, but this time is synchronized to a gas delivery pressure control. Depending on how sophisticated the plasma cutter is, so will the gas control timing be controlled. Traditionally, the shield gas in the outer stream is retained, and a pilot arc is started between the electrode and the nozzle using a switching designed for only this job. The start sequence also sends in pre-flow gases set at a much lower pressure so as not to blow out the arc. The gas has to be flowing before the arc starts, or the life of the torch parts becomes negligible.

The Pilot Arc
A little arc is first established betwwen parts inside the torch cup. As this precursor arc starts, the electronics increases the pilot arc current to about 18A to perhaps 22A, but it is a very brief pulse thing. The controlled current has to be in series with electronic ballast. The actual voltage across the arc ionized gas is low, and variable. The ionized pre-flow gas is pushed out of the nozzle, so hot plasma touches the work. At this point, the arc current can complete the external circuit back to the welder via the fat positive ground lead. As the external current increases past a minimum value, chosen to be around 7A to 10A in most designs, the pilot arc circuit cuts off the in-torch return route that was shunting the external path. The external arc current is still ramping up. Keep in mind that all this happens very quickly.

The Main Arc
At this stage, the gas pressure control ramps the gas pressure to the correct value for the torch settings. Various cups, nozzles and work thickness settings have the corresponding correct pressure. At the same time, the current control adjusts the arc current to the final cutting current value, typically 50 Amps or more, and keeps adjusting to have it stabilized there.

By now, the metal is melting and the central air gas column is cutting through. The air that is coming through had better be dry, and clean, or you may expect difficulties.

I expect there may be variations to this, depending on what tricks modern electronics can make happen. All the above I have found from searching and study, and none of it need necessarily be known to an experienced plasma cutter user, other than perhaps an appreciation that making it all happen just right might best be done with a machine dedicated to plasma cutting. Arguably, this path also helps with the costs. I trawled this stuff to help me choose, because on something like this, I want to only choose once. Even before I have made my first plasma cut, I don't want to start out working my way through various purchases, discovering from experience which I should not have gone for!
 
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matthewsx said:
That would explain why plasma cutters are so fiendishly expensive.
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There's a lot of detail in the torch. Basically, an arc is struck and the plasma is guided down to the work in a vortex of the working gas.

There are plenty of Chinesium units that can be purchased for around 400 $ (last I checked).
 
Aww, we should all know better by now... If an all-in-one machine craps out, you've got nothing to work with until you fix it or toss it (all in one dumpster). At least with discrete machines, you can still use the others when one is broken. And if you buy a good machine with local dealer support (they come in blue, usually) you can get the parts, schematics, and service you need to repair it long after it is out of production- decades, even.

I studied Chinese long enough to learn how to recognize the words, "no warranty express or implied" on a product box. It's on all of them, just look for the scribbles that look like chicken scratch. Caveat emptor.
 
pontiac428 said:
Aww, we should all know better by now... If an all-in-one machine craps out, you've got nothing to work with until you fix it or toss it (all in one dumpster). At least with discrete machines, you can still use the others when one is broken. And if you buy a good machine with local dealer support (they come in blue, usually) you can get the parts, schematics, and service you need to repair it long after it is out of production- decades, even.

I studied Chinese long enough to learn how to recognize the words, "no warranty express or implied" on a product box. It's on all of them, just look for the scribbles that look like chicken scratch. Caveat emptor.
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In general sentiment I tend to agree with this about multi-in one machines, but I will say the foot print is quite appealing. I have TIG, stick and MIG in a foot print only slightly larger than MIG alone (machine is marginally larger and there is a second tank for TIG). In my shop that is huge, errr it is not huge. ;) I would struggle finding space for 3 separate welders.

Blue and Red are each making several 3 in 1 machines. Both even offer some some 300A+ units intended for industrial use. Lincoln has an $11,000 815A multi-process machine so the idea of one welder to do it all is even gaining acceptance by professionals, it is not just a hobby machine thing.

My Miller 3 in 1 is basically two distinct welders (TIG / Stick and MIG) in the box sharing one power supply. Switching from one to the other is as simple as pulling the trigger, no need to swap leads or change settings.
 
Yeah, I understand the footprint thing. I always thought my way out of that was to upgrade to switching/digital welders... Some day.
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