Tuesday, July 12, 2011

Why is Cutting Height Mode (IHT Mode) so interesting?

In plasma cutting the cutting height is controlled by the arc voltage with the arc voltage being proportional to the distance between the cutting torch and the workpiece. The voltage varies by about 3 volts per millimetre so for example at a cutting height of 3 mm the arc voltage is 9 V. The operating voltage is, however, set at 110 volts. With an applied voltage of 101 volts the torch would touch the plate and with 119 volts the torch would be at a distance of 6 mm .

When you cut, for example, with settings of 100 Amps and 110 volts, the circuit resistance can be shown to be 1.1 ohms using Ohm's Law (R = V / I) . Imagine a cutting operation where one of the following is true; the workpiece is not lying flat on the table, the table is not properly earthed, a worn torch tip need to be changed or the work piece has been already repeatedly cut. All of these will affect the resistance in the plasma circuit. In my example just a change of 0.1 ohms will mean a change in voltage of 10 volts. Practical experience shows that the these variations in voltage can be up to 15 volts. This is where "cutting height mode" becomes of real interest because with this method it does not matter how big the difference is. Here we are setting the height and measuring the operating voltage. It always guarantees an exact cutting height and a consistently accurate cut.

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