TWI Industrial Member Report Summary 214/1983
By J C Needham and D F Percival
From an analysis of the short-circuiting MIG system, it is shown that for a basic power supply of e.m.f. E, total resistance R and overall inductance L, the average current I is given [by the equation:]
I = E - (M divided by M + I)VA, divided by R
where VA is the voltage of the arc itself and M the mean ratio of arc to short-circuit durations. This relation is independent of the inductance, the frequency of shorting or the amplitude of exponential current swings. The implications of this relation are discussed with respect to MIG welding at constant wire feed speed. It is suggested that the ratio M could serve as a suitable index to the operation of the system, as it is dependent only on the power supply voltage (less the total resistive drop) relative to the arc voltage. Monitoring the circuit resistance, as viewed from the power supply, also aids quality assurance in maintaining consistency of operation. Moreover the current excursion is largely independent of the circuit resistance, and the product of excursion with short circuit frequency provides a measure of the overall inductance while welding. Cyclograms of the welding arc show that the instantaneous voltage drop during short circuiting Vs is, for a 1mm diameter mild steel wire. given by 0.01i where i is the instantaneous current. Also the instantaneous arc voltage VA is given by 15 + 0.3i for argon 5%CO2 shielding gas.