In a welding arc, where is the heat generated?

Factual Questions
1

[ul]
[li]In the arc?[/li][li]At the positively-charged electrode?[/li][li]At the negatively-charged electrode?[/li][/ul]

I know in an electrical circuit, energy is dissipated at the load, which I assume is the arc. But the arc doesn’t seem very high resistance, e.g. an arc welder may work at 30V, 200A, meaning a resistance of 0.15Ω. Won’t energy then be dumped in other high-resistance parts of the circuit? Also, it seems only the end of the arc at the workpiece will provide any useful heat.

I thought heat might also be generated by electrons crashing into the positive electrode, each providing 30eV. Does this mean the workpiece has to be positive? How does AC welding work then?

2

30 V, 200 A means 6 kW - seems like plenty of effect to me. There is certainly heat generated in the arc - that’s why it glows!

3

when you have an arc, the electrodes get hot at the area the arc touches (depends on the electrode dimensions).

4

If you hold the rod too tight against what you are werding it will stick to the metal you are welding and the entire rod will heat up. When you are arcing the point of highest resistance is where it is attempting to make contact at the target point. This is where it would get hottest.

5

Your calculations are wrong.
An arc welder might require 220v at 30A from the mains power supply, but there is a mondo transformer inside the box which steps the voltage way down, and boosts the current.

My MIG welder outputs 170A on high - this implies that a .1Ω arc will dissipate 170^2 x .1 = 2,890W all in a fraction of a cubic inch, which is why it gets so hot.

6

I just grabbed 30V 200A from http://weldingweb.com/showthread.php?t=93171 but I’ve found that they are usually around this value. The voltage seems very low, I know.

7

No, the voltage seems right.
Welders work on high current. You can weld with a 12v car battery and carbon electrodes.
The high current requires huge cables to manage the voltage drop, so that most of the power ends up in the arc.

8

All I can recall is that the arc is a high temperature plasma, and there must be a resistive load (or something equivalent) somewhere else in the welder. You don’t want it in the electrodes, they would melt.

9

Well you want it in the workpiece “electrode” (“electrode” here referring to the part that connects the arc), don’t you?

Actually, isn’t there a type of welding where the electrode provides the filler material? Stick welding?

10

Yes but I think the heat comes from the plasma. You only want the end of the electrode to melt, not the whole thing.

11

:nitpick:
You do want the whole thing to melt.
Just not, all at once. :smiley:

D&R

12

I think most of the heat comes from where the central zone of the arc hits the positive electrode. In a carbon arc lamp, there is a pool of molten carbon in a pit in the face of the positive electrode at around 5 kK that radiates the most strongly. There are smaller shares of heat generated in the arc and where it hits the negative electrode. In an AC arc, the two electrodes keep trading their status as the positive electrode, and there are two equally heated places where they are hit by the arc.