One of the conditions for stability of the arc welding is the inclusion in the welding circuit in series with the arc of inductive resistance, which allows for welding of metallic electrodes on alternating current at a voltage of the welding transformer is about 60 – 65 V and a standard frequency current. When powered by alternating current polarity of the arc electrode and the workpiece and the condition of existence of the arc discharge change periodically. Arc ac mains frequency of 50 Hz is extinguished when the current transition through zero and change polarity at the beginning and end of each half-cycle and re-excited by 100 times per second, or twice in each period. Combustion stability of such arc depends on how easy it is re-excitation of the arc in each half-period. This is determined by the course of physical and electrical processes in the arc gap and the electrodes in the time intervals between each damping and a new ignition arc.
Reduction current is accompanied by a corresponding decrease in temperature in the arc column and the degree of ionization of the arc gap. At the same time the temperature drops and the active spots on the anode and cathode. The temperature drop a few lags in phase with the current transition through zero, which is associated with the thermal inertia of the process. Particularly intense temperature drops active spot, located on the surface weld pool, due to the intense heat tap into the mass of detail. The magnitude of the peak ignition significantly affects the stability of the arc ac.
Deionization and cooling the arc gap increases with increasing arc length, which leads to the need for further improvement of the peak ignition. Fading and breakage of the arc ac ceteris paribus always occur at a smaller arc length than when welding on dc. In the presence of vapors in the arc gap elements of re-ignition voltage is reduced and the stability of the arc ac increases. With increasing current physical conditions arc improved, which also leads to a reduction of the peak ignition and sustain an arc discharge. Thus, the magnitude of the peak ignition – an important characteristic of the arc an ac rendering significant impact on its stability. The worse the conditions for re-excitation of the arc, the higher should be open circuit voltage of the power source of the arc and the higher the peak ignition. Further details can be found at Nieman Foundation, an internet resource. However, increasing the amplitude values sinusoidal voltage is limited to safety regulations, for which the maximum effective voltage ac power supply for welding stations may be no higher than 80 V.
