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

[graham-xrf]

I ask this from a start point of sheer ignorance, and because I figure that the difference seems hardly more than screwing on a plasma cutting torch and changing the gas to shop air, or maybe adapting a plasma cutting end bits onto a recycled old TIG torch. So far as I can tell, a settable 200A machine would not know the difference on the business end, and could be used.

Folk here are very practical, but I know the devil is in the detail. So I ask, is any of this notion just plain stupid? What might be involved, if it can or should be done at all?

I am guessing that if there is a hangup, it most likely will be to do with starting the arc?

 

[General Zod]

Not possible. Welders cannot produce the arc voltage needed to sustain a plasma cutting arc. That simple.

 

[matthewsx]

That would explain why plasma cutters are so fiendishly expensive.

 

[graham-xrf]

Not possible. Welders cannot produce the arc voltage needed to sustain a plasma cutting arc. That simple.

Thanks much Z, that's good to know.

For one who starts out not knowing anything, just looking at everything searched, and seeing the principle of operation, nowhere I found was this explicitly stated.

Clearly the variety of common settable plasma currents were usually less than the 200A available from my machine.
When one looks at (say) the cheap, lightweight, and it has to be said, damn tiny things like the $150 HERO CUT from Amazon, there seemed every reason for me to think that such a conversion would hardly be more than using a plasma torch on a basically TIG machine, and going for shop air! There is even a video out there on how to adapt an old TIG torch to use Plasma cut consumables.

A TIG arc is already a plasma state, so if it is the VOLTAGE that makes the difference, then naturally one would ask, exactly how high does it need to get? If the voltage has to be higher (to suit plasma cutting), then, for a given power, the current has to be lower. For a whole bunch of those admittedly crap-looking Chinesium kits, they do not look as if they are "higher power", and clearly 50A to 80A is enough to cut some sheet metal.

I do note that at the prices some import plasma cutters can now be had, it seems hardly worth messing with an ill-judged "conversion". The cheapest of the "better rated" plasma cutters on Amazon seem to cost around $270.

 

[Janderso]

A TIG arc is already a plasma state, so if it is the VOLTAGE that makes the difference, then naturally one would ask, exactly how high does it need to get? If the voltage has to be higher (to suit plasma cutting), then, for a given power, the current has to be lower

This made my head hurt.

 

[pontiac428]

Arc start is like 400v, and the plasma arc is around 60v, plus plasma cutting heads focus the gas and arc into a very tight cone. Plasma cutters weren't really a thing in the days of analog electrics, it took those great digital innovations of the 70's and 80's and switching power supplies to bring the technology to the shop. Must be something to that. I own a robust but somewhat simple plasma machine from the 90's, and that thing is stuffed full of circuit boards to provide the control to sustain a stable cutting arc. My transformer TIG would be too "dumb" to work as a plasma power supply for sure.

 

[graham-xrf]

This made my head hurt.

This is easy!
Power(Watts) = Voltage x Current.

You know from the start how much power the kit can deliver (e.g.2kW) , and the input voltage (e.g. 220 VAC)
The current it takes from the socket will be 2000/220 = 9.09Amps
Expect that because it is not 100% efficient, it may take about 10Amps or so.

Now move to the other end. Take out (say) 60Amps.

The most volts the machine could ever muster to deliver that continuously would be 2000/60 = 33Volts.
We know this is not what really happens. The plasma machine switching regulator winds up 200V to 400V (no current yet) and starts the arc.
In milliseconds, that high voltage collapses to a much lower voltage sustained across the arc.
The machine then controls the current in the arc, millisecond by millisecond.

Any other numbers (voltage x current) giving an answer exceeding the power of the machine gets you something for nothing - impossible!
If there is one thing we know about energy done with electrons, it is that the amounts are exact, and unforgiving. There is so escaping a simple multiplication!

 

[graham-xrf]

Arc start is like 400v, and the plasma arc is around 60v, plus plasma cutting heads focus the gas and arc into a very tight cone. Plasma cutters weren't really a thing in the days of analog electrics, it took those great digital innovations of the 70's and 80's and switching power supplies to bring the technology to the shop. Must be something to that. I own a robust but somewhat simple plasma machine from the 90's, and that thing is stuffed full of circuit boards to provide the control to sustain a stable cutting arc. My transformer TIG would be too "dumb" to work as a plasma power supply for sure.

Thanks John - my searches have started to yield the detail in plasma technology.
The "resistance" in a plasma ionized gas is indeed unstable, wobbling about near zero, flaring up and subsiding as the molten metal under it moves. It takes very high speed switching regulated current control to keep the arc going.

A good example of plasma from crude AC volts is in a fluorescent lamp tube. When the gas in the tube ionizes, the tube becomes a near short circuit which, left to itself, would pop all the house fuses instantly. The only thing that limits the current is the ballast choke in series, basically lots of turns of fine wire on an iron magnet core. The choke provides inductance, which acts to limit the current in AC circuits by it's impedance. The actual resistance in the wire is low, so the choke does not get very hot. Sadly, what chokes can do is frequency-conscious, so they do not work for DC currents.

 

[General Zod]

A high quality plasma cutter will sustain 100V to 140V at the arc.

 

[graham-xrf]

A high quality plasma cutter will sustain 100V to 140V at the arc.

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.