ARC CHARACTERISTICS
In order to understand the different output characteristics that are required by the welding power source it is necessary to understand the load which is fed by the welding power source. In arc welding processes the load is nothing but the arc itself. In order to select the right source, it is essential to know the load characteristic, namely arc characteristics.A general arc characteristic is shown in Fig.(a) Since the negative slope region (dotted line) of the arc occurs at very low arc currents, it is usually neglected and a simplified arc characteristic is assumed as shown in Fig.(b).
It is also known that as the arc length changes the characteristic (arc voltage for a given current) changes. Longer arcs require higher arc voltages Fig.(a) For the same arc length if we increase the arc current then the arc voltage increases (as though the arc is a pure resistive load obeying ohm's law) Fig.(b).Therefore the arc voltage is function of two variables namely arc current and length for given gas.
Fig. shown how the operating pointis achieved by the intersection of the source and arc characteristics. In Fig.(a) the source has high open circuit voltage (OCV). The characteristic is drooping. Hence, there is a lot of difference between OCV and actual arc voltage. Where as the drooping curve meets the X-axis at a point called short circuit current (lSC). In Fig .(b), the power source has low open circuit voltage (OCV) and the source characteristic is such that the output voltage changes slightly with load current. Therefore, the arc voltage is almost equal to the source OCV. We can fix arc voltage with flat characteristic power source by controlling the OCV.
For changes in arc length the arc characteristic will change. But, this will have different effects with flatand drooping characteristic power sources. As shown in Fig. (a) with a drooping characteristic power source and changing arc lengths we can see that the change in arc voltage is considerable. Where as with a flat characteristic power source, the change in current is considerable, but the change in voltage is less, as shown in Fig .(b). With the above things considered, let us see now what type of output characteristics are required for each of the arc welding processes.