Once a stable arc with proper thermal equilibrium has been established, it is essential to maintain it so that welds of consistent quality can be achieved. Normally, it is not difficult to re-ignite a stable arc, if it goes off momentarily. Whereas thousands of volts may be required to initiate an arc in TIG welding machine (gas tungsten arc welding), it may require only tens or at the most hundreds of volts to reignite it.
Maintenance of an arc when welding with an ac power source is rather a problem because the arc extinguishes every half cycle when the current is zero, that is, it will extinguish 100 times a second with a normal 50 hertz mains supply. For re-ignition the required voltage must be available at the time the current is zero. This is achieved in ac welding by keeping the current and voltage waves out of phase by using a power source with a low operating power factor of about 0.3.
For these conditions, almost the full OCV (open circuit voltage) is available to re-ignite the arc while the current is zero. The operating power factor of a power source can be improved while maintaining the ease of re-ignition only by using auxiliary means of maintaining or re-igniting the arc, for example, a high frequency high voltage spark gap oscillator can be used to supply a high voltage pulse at the appropriate instance. If such a technique is used to maintain the arc, then the power factor of normally adopted for gas tungsten arc welding using an ac welding power source. The situation can be further improved by using thoriated electrode with better electron emitting property.
Similarly, in arc welding machine (shielded metal arc welding) the electrode coatings with lower ionisation potential help in easy re-ignition of the welding arc.
In dc welding machine the arc maintenance is rather easy and it is only at the time of short-circuit between the electrode and the workpiece that the arc is extinguished. However, this problem is solved by providing suitable dynamic volt-ampere characteristics of the power source. Here again electrode coatings with low ionisation potential or with better emissivity can help in easy initiation and maintenance of the welding arc.
Role of Electrode Polarity
Arc welding machine can be carried out either by ac or dc. If ac is employed there is no question of electrode polarity as it changes every half cycle. However, if dc is used it is possible to make electrode either negative or positive.