PDA

View Full Version : Some thoughts about Arc Voltage

MotoFab
10-04-2006, 10:04 PM
I have been doing a little thinking about different electrical power supplies and their use in various welding processes.

All welding processes that use an arc, seek to control both the amperage going through that arc, and the arc voltage across it.

But that last part about controlling the voltage across the arc is a little tricky, as it isn't really possible.

The arc is a plasma, and the voltage of any particular plasma cannot be controlled by varying the potential across it.

I thought I'd offer this information to those interested in technical matters.

Notes: In this little treatise, the terms arc and plasma are used interchangeably. Some of the information is presented in absolute terms because from a practical viewpoint it is. There is caveat or exception to the absolutes, but they are reasonably outside the scope of this brief, and of welding in the most common forms discussed in this and similar forums.

~~~~~~~~~~~~

The welding arc is a plasma, the so-called 4th state of matter, and is an interesting physical phenomenon.

The arc has a voltage, and that voltage is based on arc length and whatever gaseous or particulate elements are in the arc.

The arc voltage can only be changed by changing the arc length, or changing the gaseous elements in the arc.

The arc voltage cannot be controlled by any power supply. It is a physical phenomenon.

~~~~~~~~~~~~

A few interesting features of plasma are:

1) At the moment the plasma forms, it becomes a 'short circuit' of a sort. Some science considers it a negative resistance.

2) For any set of conditions, there is an arc voltage. And no arc can be maintained below that based-on-conditions voltage.

3) In addition, no arc voltage can be increased by any power source. All energy directed into the plasma gets converted to heat.

Understand that last one? A whole power station can be poured into the arc, and the arc voltage, which is given by arc length and the elements in the arc, will not change.

~~~~~~~~~~~~

But in practice, the arc length does increase. It does so by liquefying and vaporizing the conductors on either side of the arc. Some of the metal actually becomes part of the plasma. That is why plasma is considered the 4th state of matter. Solid, fluid, gaseous, plasma.

This liquefying or gasification of the metal increases the arc length until everything conducting the arc either melts or vaporizes. At some point the distance between the conductors becomes too great to maintain an arc with the available voltage, and the arc extinguishes. I'm an expert at doing this during stick welding.

~~~~~~~~~~~~

How is controlled welding for any length of time possible without melting everything?

Ahaa, the MIG and Stick process have a trick. The arc length (and voltage) is maintained by continuing to replace the liquefied electrode.

And TIG has a trick too. Enough heat is removed from the electrode that the electrode can survive a little while.

Plasma Arc Welding is trickier still. The rushing concentric gas stream around the electrode 'pushes' the plasma out to just the tip of the electrode. Because the inner 'high speed' gas stream pushes the arc out to the tip of the electrode, the arc touches only a small surface of the electrode. The small contact area of the arc limits the amount of heat that can be conducted into the electrode. That is why the electrode tip melts less, and lasts so long. The same tungsten used in a TIG electrode lasts so long in PAW because much less heat from the arc can conduct into the electrode.

~~~~~~~~~~~~

What does this all mean to welding?

For welding, and given the physics, the output voltage can be adjusted only by arc length and shield gas.

And since adjusting the arc length and shield gas is the only way to adjust arc voltage, controlling the arc length in any particular welding process is elementary.

I realize that to the more experienced welders here that this is more or less well known.

~~~~~~~~~~~~

And so - With any given welding process, and with a given shield gas, the arc voltage can only be adjusted with arc length. The welding processes differ mostly in how they go about controlling the arc length.

1) With TIG and PAW, the arc voltage can only be adjusted with arc length. And torch placement sets the arc voltage.

2) With MIG, also the arc voltage can only be adjusted with arc length. And arc length can be controlled either solely with wire speed, or by wire speed in combination with amp output.

Practically this means that either:

a) The wire speed is fixed, and the amperage is varied. And the arc length is adjusted by melting the wire faster or slower.

b) Or . . The amp output is fixed, and the wire speed is varied. And the arc length is adjusted by feeding the wire faster or slower.

How does the wire drive, or amp control, know how fast the wire electrode is melting? In a way, it doesn't care. The control method sort of doesn't think about wire melt rate. The control pretty much focuses on the arc voltage, and controls the motor speed, or amp output, to maintain that voltage. Arc voltage = arc length.

Sure, the melt rate will be what it will be. And there are fancy-featured wire drives that measure drive roller rate, and provided with the filler wire type and diameter, give a panel readout of pounds per hour.

3) With STICK too, the arc voltage can only be adjusted with arc length. Electrode placement sets the arc voltage, and electrode is fed into the arc at a rate that matches the melting rate.

Well, it is by many of you guys. I'm not there yet. I mean, I know about the electronics and all, but have almost no skill or experience with the physical aspects of say making a good stick bead.

~~~~~~~~~~~~

But what about the voltage settings on the machine? Doesn't that control the arc voltage? If there are readers still interested in the topic, I'll try and answer that.

- Jim

Dave Normandeau
11-03-2006, 01:12 AM
Wow, I quite enjoyed that. If you perhaps could would you be able to explain the "arc force" knob on a welding machine,,, since you made the rest of it so clear. I have been welding for over 5 years and i am just begining to get a feel for how the arc force affects my weld but i still do not fully understand it. From what i have been told it affects the 'slope' of the amperage and voltage but i really don't understand how and why yet.

MotoFab
11-03-2006, 06:24 AM
You have me there Dave. I may be able to understand what is meant by their use of the term "arc force" if I read the sell sheet. It may mean Volts, Amps or some 'dual' or 'combination' function. But that's only a guess. What brand and model machine do you have?

I've seen that the 'slope' or 'ramp up/down' of the amperage is called different things on different machines.

- Jim

Dave Normandeau
11-04-2006, 02:20 AM
Well I have seen it called different things and i do believe they do the same thing. We have a miller 400 amp unit that has the knob and on that one it is called "arc control". I just know that when i turn it up "stick welding" it tends to dig in harder and it spatters more called a 'crisp arc' i think. And u turn it down and it is less spatter and seems to look a little nicer seems to weld a little easier and smoother but doesnt penetrate as well. I have been told that when you mig weld it affects the inductance "i think" and that apparantly makes the arc diameter larger or smaller. I do believe they are entirely different things when mig as compared to while stick welding.. I will have to dig up my manual tomorrow if i remember and perhaps that will help to clarify it for you.

MotoFab
11-04-2006, 04:10 PM
From your description it sounds like it could be AC balance.

Maybe ask about that in the TIG forum? From the people who use that feature every day.

- Jim

smithboy
11-04-2006, 06:09 PM
Found this for ya. Inductance and arc force are pretty much interchangable.

http://www.esabna.com/EUWeb/MIG_handbook/592mig3_6.htm

Low inductance setting:- Gives higher short circuiting frequency and relatively cold welding
High inductance setting:- Gives lower short circuiting frequency and longer arcing periods between short circuiting giving hotter welding

Dave Normandeau
11-04-2006, 11:45 PM
From your description it sounds like it could be AC balance.

Maybe ask about that in the TIG forum? From the people who use that feature every day.

- Jim

I don't think so (well at least with what i am doing) because i am just doing dc welding???

MotoFab
11-05-2006, 02:17 AM
:waving: hi fafa

enlpck
11-07-2006, 05:19 PM
On at least some machines, the 'arc force" varies the open circuit voltage, and with it the I-V curve. Higher OCV and steeper curve will give a more biting arc-- as you shove in (with stick) the current goes up rapidly, digging in. Lower OCV and flatter curve dig less.

pulser
11-08-2006, 02:53 PM
Nice explaination of arc voltage Motofab.

I was scratching my head one day, cause I had three identical automated plasma systems, and one of them was running about 20 volts at the power supply, and the other two were running the normal 14 volts or whatever. As the thread explains, turns out the high voltage was caused by a corroded power cable that had added a bunch of resistance in the circuit. So measuring voltage at the power supply in this case is a good diagnostic on your circuit resistance. But, in a practical sense, all three welds were identical because the arc voltage was unchanged and the power supply was still supplying the correct current.

Now if you're running an automatic voltage control (AVC), it's best to measure voltage as close to the arc as possible, so that changes in circuit resistance don't change your arc length.

MotoFab
11-08-2006, 08:00 PM
Nice explaination of arc voltage Motofab.

. . . measuring voltage at the power supply in this case is a good diagnostic on your circuit resistance. But, in a practical sense, all three welds were identical because the arc voltage was unchanged and the power supply was still supplying the correct current.

Now if you're running an automatic voltage control (AVC), it's best to measure voltage as close to the arc as possible, so that changes in circuit resistance don't change your arc length.

Grazie, and yes I see what you mean. The AVC unit responds with . . . well, it responds like any controller would I guess. By changing what its control outputs can control, based on its inputs and control program.

And no question Pulser, measuring the voltage at the torch is more valid as the actual arc voltage.

But a problem, with the cables for example, can be 'interpreted' from a voltage measured at the supply or at the torch.

The controller can 'interpret' a rise in circuit resistance, from the cable resistance say, either by measuring the voltage at the supply, or at the torch end.

Whether or how it makes that interpretation is based on the control program.

You know? Based on which end of the cable the measurement is taken, the 'nominal resistance' of the cables is added or subtracted from the volts, amps, resistance equation.

And heck, if we're measuring at the torch, then let's live large and also measure at the supply. This eliminates the 'interpretation', but maybe more important, it allows for continued operation should one or the other of the 'voltage sense' leads fail. The ability to the machine to operate with a failure, and to shut down the machine for repairs at will, can save a lotto money.

- Jim