ESAB Knowledge center.
What is arc voltage height control?
March 18, 2013
And why should I consider upgrading to a newer system?
When cutting with a CNC plasma shape cutting machine, a height control system is required to keep a constant distance between the tip of the plasma cutting torch and the metal plate being cut. As flat as a cutting table might look, the truth is that plates are never perfectly flat, and cutting tables never stay perfectly level. Without some way to maintain the gap between the torch and the plate, the torch would either get too high and loose the arc, or it would get too low and crash into the plate.
In order to maintain good cut edge quality and accurate part size, the height control system not only has to maintain a constant height, it also has to be the correct height. If the torch is too high while cutting there will be too much top bevel, and probably too much dross. If the torch is too close to the plate while cutting there will be a bottom bevel (under cutting), the torch may crash, and nozzle damage is likely. The height also affects the kerf width, which in tern affects the part size.
The most common way to maintain the correct cutting height is by using an “Arc Voltage Height Control” system. Arc voltage height control is an automatic system that maintains cutting height by maintaining a constant voltage through the arc. It works by measuring the voltage between the electrode in the torch and the plate that is being cut. The actual measurement is between two points in the circuit on each side of the arc, usually a point in the plumbing box where the torch lead is attached and a point in the plasma power supply where the work ground is attached.
The higher the torch is above the plate, the longer the arc is stretched, and therefore the voltage gets higher. As the torch cuts closer to the plate the arc gets shorter, which makes the voltage lower. An electronic feedback loop compares the actual voltage to a set-point, and then drives the motor on the torch lifter either up or down as required to maintain a constant voltage. The set-point comes from the operator either through a dial on a control box or through the cutting machine CNC.
It is important to set the correct voltage for each torch setup. Each setup will use a different nozzle, amperage, or speed, and thus requires a different voltage setting to maintain the optimum cutting height for that condition. There are too many possibilities for an operator to remember them all. Usually the optimum arc voltage setting is published in the plasma system’s cut data manual. In more modern plasma systems, the arc voltage setting is just one of many plasma setup parameters that are recorded in a database on the CNC control, so it can be set automatically.
When the arc voltage height control system doesn’t work properly, things can go very badly for either the cut parts or the torch itself. If the system is not calibrated properly it can also lead to poor cutting results. The accuracy of the plasma height control system is most critical when cutting small holes with plasma, using a system such as Hypertherm’s TrueHole, or ESAB’s Precision Hole Technology.
The latest technology in arc voltage height control will also automatically compensate for electrode wear. As an electrode is used, it wears a divot into the surface of the electrode, causing the portion of the arc that is inside the nozzle to get longer. In order to keep the same height between the tip of the nozzle and the plate, the actual arc voltage has to increase. With an older plasma system, the operator will have to manually increase the arc voltage setting as he notices the torch cutting closer to the plate. If he doesn’t, the torch will start to crash more frequently, and the operator will probably replace the electrode before it is completely worn out.
New systems, such as ESAB’s Smart Voltage Height Control, will automatically compensate for this wear over the life of the electrode. This eliminates the need for the operator to manually adjust the voltage setting, and allows the torch to keep cutting until the electrode is fully used. Not only does this make the system more automated, but it also helps keep the cut quality more consistent over the life of the electrode.