View Full Version : Arc length and heat?
07-01-2008, 07:59 AM
On a recent thread, some folks here said that if the arc length is too long, it increases the heat, and a shorter arc length decreases the heat.
That sort of makes sense to me, because the longer the arc is, the more resistance there is for the electrons to overcome to bridge the air gap...and resistance (in a stove heating element for example) creates heat.
On the other hand, I've read that as the arc length increases, the voltage increases, and the amperage decreases. Similarly, as the arc length decreases, the voltage decreases but the amperage increases.
That also makes sense to me, since longer arcs (like lightning strokes for example) require huge voltages whereas spark plug arcs are generally lower voltage than that.
That suggests to me that DEcreasing the arc length INcreases the heat, and INcreasing the arc length DEcreases the heat, because the heat is proportional to the amperage, and as the arc length decreases, the amperage goes up.
Can someone sort this out for me? I have a welding book on order (Lincoln's "Procedure Handbook of Arc Welding") but it hasn't arrived yet, and the information I'm finding on the web seems contradictory. One article I found suggested that as arc length decreases, the amperage and heat go up, but the penetration decreases, but that doesn't make sense to me. What gives? Help!
07-01-2008, 08:07 AM
Don't take this the wrong way, but you're WAAAAAYYYY overthinking this. For the amount of time you've been welding, there's nothing that will help you more right now than dropping your hood and burning some rod. Lots and lots of rods.
When you see how different things affect the weld and the arc, you'll learn how to compensate to make effective welds in any situation. THEN it's helpful to know the whys and wherefores and the theory that makes all of this work. Don't get me wrong, I'm an engineer and a techno-geek and I love to know how and why things work too, but at this point in your experience I think the theory is confusing you more than anything.
I will try to give you a short clear basic explanation.
There are two types of welders
CC = constant current used for stick (SMAW) and tig (GTAW).
these machines maintain control the amperage and arc length
controls the voltage or force of the arc.
CV = constant voltage used for solid (GMAW) or flux cored (FCAW)
wire welding, these machines maintain control of the voltage
and the wire feed speed is the amperage. Now this is where it
gets confusing with a constant voltage machine the arc length
stays constant but when you increase or decrease the contact
tip to work distance (refered to as wire stick out) the amperage
changes. With .035 solid wire a stick out change of 1/8" will result
in roughly a 50 amp change in current.
The explanations above should help you in your learning curve.
07-01-2008, 12:19 PM
No offense taken, Zrexxer. I agree I need to burn a lot more rod, but I also like to have a clue as to why one thing works and another thing doesn't. If it "makes sense" why something works or doesn't work, you can begin to understand the underlying principles, and then apply those principles to help solve problems in other situations, rather than just memorizing recipes. (Or at least that's the way I prefer to learn a new skill.) Otherwise, it just feels like poking around in the dark...it takes a lot longer to learn...and the frustration factor can become a stumbling block (or for me it can).
I understand where you're coming from, though, and appreciate the suggestions.
DrIQ, your explanation definitely gives me something to chew on!
07-01-2008, 08:58 PM
I wonder what kind of amperage there was on THIS (http://www.cnn.com/video/#/video/us/2008/07/01/sotvo.kare.man.lightning.kare)! :eek:
07-02-2008, 12:34 AM
Very low. Otherwise, he wouldn't give the interview.
07-02-2008, 07:23 AM
DrIQ - Now I know what the "CC" on my welding machine means...thanks!
I think I'm beginning to get the concept now.
If the current in amps remains the same regardless of arc length,
but the voltage goes up when you lengthen the arc length,
then the power transmitted, in watts, increases proportionally to the increase in voltage (V x A = W)
when you increase the arc length, right?
And that's why, when you lengthen the arc length, you get more heat, right?
Am I even close?
Thanks again for all the replies.
P.S. A friend of mine who's a weldor is lending me a book on welding this weekend, so maybe I'll slow down with all these dumb questions...
07-02-2008, 09:38 AM
Well - there's theory and there's reality. There is an excellent article on the Miller Web site that discusses long and short arc and how different machines react. They talk about a transformer machine versus an inverter, and a stick optimized versus a TIG optimized machine.
A CC machine does not actually hold the current constant - it "tries" to maintain the current constant.
A transformer based machine will show the current sloping down as the voltage increases - so a long arc will reduce the current input to the workpiece..
A machine optimized for TIG will hold the current much more constant as the voltage fluctuates with the arc length.
Miller explan it much better than I could.