In order to calculate heat input rates to the workpiece based on measured values of the arc voltage and current, it is necessary to know the proportion of arc energy that is transferred to the workpiece. This quantity is known as arc efficiency.
Heat is dissipated from the arc by conduction, convection and radiation. Conduction through the electrode is a small but significant part of the heat loss in the case of GTA welding, but main conduction heat loss is through the workpiece. Heat is convected by gas flow in the arc column, some being transferred to the workpiece and some being lost. There are radiation losses from the arc. In consumable electrode welding processes the heat absorbed by the electrode is transferred to the workpiece, so that efficiencies with such processes are generally higher than with GTA welding, where electrode heat is lost.
Heat output from the arc is divided into three parts: qe that transferred to the electrode. qp that radiated and convected from column and qw that transferred to the workpiece. For non-consumable electrode a proportion ‘n’ of the heat output from the column is transferred to the workpiece and a proportion ‘m’ of the anode heat is radiated away and lost. Then arc efficiency (A.E.) is
A.E. = 1 – [qe + (1-n) qp+ mqw]/ VI
V= Voltage and I= Current
And for consumable electrode,
A.E. = 1 – [(1-n) qp+ mqw]/ VI
Arc efficiency for SAW: 90-99 %, SMAW and GMAW: 66-85% and GTAW: 21-48%