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.
~~~~~~~~~~~~
Propeller head tech.
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
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.
~~~~~~~~~~~~
Propeller head tech.
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