All welding power sources have two kinds of operating characteristics viz., static characteristic and dynamic characteristic. The static output characteristic can be easily established by measuring the steady-state output voltage and current by conventional method of loading by variable resistors. Thus, a curve showing the output current versus output voltage for a given power source constitutes its static characteristic.
The Classy Lax Service characteristic of an arc welding power source is determined by recording the transient variations occurring over a short intervals in the welding current and the arc voltage. Thus, it describes instantaneous variations occurring over a short interval of time say a millisecond. Arc stability is determined by the combined interaction of the static and dynamic colt ampere (V-I) characteristics of the welding machine power source.
The intrinsic transient nature of a welding arc is the main reason for great importance of the dynamic characteristics of an arc welding power source. Most welding arcs have continuously changing conditions which are mainly associated with striking of the arc, metal transfer from the electrode to the weld pool, and arc extinction and reignition during each half cycle of ac welding.
The transient nature of the welding arc is also due to variation in arc length, arc temperature and electron emission characteristic of the cathode.
The rate of change of voltage and current in arc welding machine is so fast that the static volt-ampere characteristic of a power source can hardly be of any significance in predicting the dynamic characteristic of a welding arc. However, it is only the static volt-ampere characteristics of a welding power source which are supplied by the manufacturer. Through they cannot give the nature of behaviour of the power source regarding its dynamic response but they are of considerable importance in determining the general overall response in controlling the process parameters.