When MIG welding in the dip transfer mode, the welding electrode touches the weld pool, causing a short circuit. During the short circuit, the arc voltage drops to nearly zero. This change in the arc voltage results in a corresponding change in the welding curcuit current, the precise magnitude of which depends on the slope of the output characteristic (the resistance of the welding circuit), Fig. 1.
If the constant voltage power supply responded instantly, very high current would immediately begin to flow through the welding circuit. The rapid rise in current to a high value would melt the short-circuited electrode free with explosive force, dispelling the weld metal and causing considerable spatter.
Inductance is the property in an electrical circuit that slows down the rate of current rise, Fig. 2. The current travelling through an inductance coil creates a magnetic field. This magnetic field creates a current in the welding circuit that is in opposition to the welding current. Increasing the inductance will also increase the arc time and decrease the frequency of short-circuiting.
For each electrode feed rate, there is an optimum value of inductance. Too little inductance results in excessive spatter. If too much inductance is used, the current will not rise fast enough and the molten tip of the electrode is not heated sufficiently causing the electrode to stub into the base metal. Modern electronic power sources automatically set the inductance to give a smooth arc and metal transfer.