By 803056
Date 11-19-2007 16:23
Edited 11-19-2007 16:37
You pretty well hit the nail on the head. The pinch effect, "controlled" by inductance, is primarily used when using the short circuiting mode of metal transfer.
I tell welders that inductance is like the shock absorbers on the front of the car. If the shocks are worn (little inductance) the car bounces uncontrollably (erratic unstable arc). If the shocks are too stiff, the ride is hard (too much inductance). In the case of the welding arc, the rate of change in the amperage relative to the arc voltage determines how the metal droplet detaches from the end of the electrode. If the rate of change is too rapid, the droplet detaches violently and produces excessive spatter. It the rate of change is too slow the metal droplet doesn't detach cleanly and the arc is unstable. I would have to do some serious digging through my old text books to go much beyond my brief description.
To the best of my recollection, inductance doesn't play a major roll in spray transfer.
Because welders are not taught electrical theory and don't understand the influence of inductance on a system, they had difficulty understanding how to set the inductance. Most manufacturers simply removed the variable inductor in favor of a fixed inductance that is best suited for welding carbon steels.
Slope is not quite the same in that slope is the relationship between the (static) voltage and (static) amperage of the power supply. Slope tells you the amount of change in amperage as you vary the arc voltage. Steep slope will produce a small change in amperage with a change in arc voltage. Flat slope results in a large change in amperage with a change in arc voltage. Slope doesn't indicate the rate of change. The primary difference between a CC and a CV power supply is the slope of the machine. The CC machine has a steep (drooper) slope and the CV has a flat (shallow) slope that results in an arc that maintains the proper arc length by varying the amperage (melt-off rate).
Most CC power supplies favored by pipewelders has a slope that is "shallow" to provide the ability to vary the amperage (thus penetration) by varying the arc length slightly. A steeper slope is favored by shop welders because slight fluctuations in arc length results in only a slight variations in amperage, i.e., it isn't as responsive and doesn't provide the "digging arc" many pipewelders favor.
Best regards - Al