TRG-42
02-14-2007, 07:26 PM
Since there has been recent discussion regarding arc force / dig / etc on engine drives here is the theory in a long and potentially boring nutshell
The term Arc Force actually means different things depending on the particular machine
In generally Arc Force ( traditional ) / Dig ( Miller Trade Name )when stick welding is the following
- as voltage decreases, current spikes ( think of it as a tail on the volt amp curve ) . Please see diagram of Miller VA curve below.
- the key point is as the electrodes arc voltage approaches zero ( starts to short ) the current increases ( to a varying degree determined by Dig / arc force setting - the 0% to 100% )
http://i30.photobucket.com/albums/c347/TRG42/MillerDig.png
Why would you want Arc Force / Dig ?
If you have trouble with, for example, E7018 sticking you can turn up arc force. As you short out the electrode because you got too close the current shoots up, burning back. You can take an electrode and push it straight through a 1/2" plate with arc force turned all the way up !
If you are pipe welding, and the gap is tighter, by pushing in ( shortening arc length, therefore arc voltage ) the current goes up ( see bottom of VA curve shown above ) to ensure adequate penetration
As arc force goes up the arc generally gets harsher, especially if you start to shorten arc length
What Machines have this
- almost all stick machines now, and Miller engine drives
Key Point - all these machines have a TRADITIONAL textbook CC curve ( dropper ) Again, refer to VA curve above
Conclusion - Arc Force ( traditional ) / Dig are pretty rudimentary soloutions to increase current depending on arc length
Lincolns Arc Control ( their version of arc force ) on Chopper Machines
This is kinda the same as Arc Force ( traditional ) and Dig except the VA curve ( when you set it in CC downhill pipe mode ) has slope
http://i30.photobucket.com/albums/c347/TRG42/ArcControl.png
Again, if you turn up the Arc Control, essentially the software will cause the VA curve to provide more amps as voltage approaches zero ( electrode starts to get closer )
Advantage over Traditional Arc Force /Dig
Although traditional arc force will cause the current to vary , it only starts to vary as electrode starts to drop well below 20 arc volts ( once again, refer to miller VA curve above ). Slope only starts at the "tail end"
As a result, the current does not vary that much unless you get real close. Most people actually want this, because this is true constant current
However, if you are a pipeline welder, they prefer the more slopy VA curve, which is what ALL Lincoln generators will do and all Lincoln Chopper machines on "Downhill Pipe CC setting " ( Lincoln choppers on standard CC behave like all Miller VA curve
This is why guys who want a generator arc but dont want a generator use Lincoln Ranger 305s / Vantages because you can emulate ( not perfectly as you will see ) the VA characteristic of a true generator
Key Point - Chopper machines on Downhill Pipe ( even they are on constant current ) have not so constant current because of the sloping VA curve ( see VA curve of chopper above )
This is far more refined way to vary amps as voltage changes. It will vary it continously, not only radically as voltage approaches zero ( electrode starts to short ). in additon traditional CC curves with arc force can only INCREASE current as electrode starts to short
A sloping VA curve indicates that current can also decrease as arc length increases
Example, you can swing a 125 amp arc from 140 to 100 if you wanted to.
Conclusion - Lincolns arc control on chopper machines not only incorporates traditional arc force, but also can play with the VA curve yielding ( depending if you are on CC or Downhill Pipe setting ) far more current control ( yeah i know its suppose to be constant current ) than you can ever hope to get with a traditional CC machine
King of Variable Arc - The generator !
The mainstay of the pipewelding industry is the generator machine. how does the arcs VA curve vary
- VA curve has slope . As in "Downhill Pipe " on chopper machines, sloping VA curves yields far greater control on the arc ( IF the operator knows what he is doing )
- VA cuve is variable . This is the main reason that generator machines rule when it comes to pleasing stick welding connisors
Please see VA curve of a Classic 300D below showing the numerous VA curves the machine is capable of
As you can see below ( its not as confusing as it looks ! ) the VA curve is unlike a true CC curve because it has slope . Again, this means that as voltage changes current also changes
ALL other CC machines have basically one volt amp curve. If you don't like it too bad because thats the curve. On a generator machine, as you can see, there are numerous VA curves
More importantly you can , hit a desired current on two or three taps
Key Point - this is NOT like a range switch on other engine drives ( it does chang the range ) but you can hit a given current on numerous ranges ( way more overlapp )
The purpose of this is you can hit a given current with a more sloped or less sloped tap .
Advantage is you can have a 160 amp arc , for example, that has lots of variation as arc length changes, or a 160 amp arc with less variation as arc length changes
http://i30.photobucket.com/albums/c347/TRG42/VACurveClassic.jpg
Here is a more clear picture of the choice of two VA curves for a given current ( load point - where machine has been set for - is where the two graphs intersect ). Depending on which VA curve, the current will vary differently as V changes
http://i30.photobucket.com/albums/c347/TRG42/GeneratorControl.png
Also notice that not only the slope is different, but the current as voltage approaches zero ( starts to short ) is noticably higher on the bottom curve. This means there would be more current as electrode starts to short. This is very similar to more arc force
Last, you can play with the engine RPM to vary the OCV ( open circuit voltage )
This is different from variable slope on VA curve because the slope does not change but rather it moves up or down ( varys OCV )
Variation is about 100 to 150 rpm btw
http://i30.photobucket.com/albums/c347/TRG42/CustomArc1.jpg
http://i30.photobucket.com/albums/c347/TRG42/CustomArc2.jpg
Conclusion - Generator Machines
Not only do you have a better VA curve for Pipe Welding, you can choose from different VA slopes
You can also vary the OCV to change the arc characteristic
You can also roll the rack ( I think I am starting to get too long winded so i quite while Im ahead )
Are these arc force ( in the traditional sense ) , NO . Do they vary the arc characteristics ( far more variation and control compared to conventional arc force ) YES .
This way there is way more control over the arc with a generator machine than you can hope to get with even a chopper machine, or a traditional rectified alternator
