TRANSFORMER EFFICIENCY

by Sam Belkin, MSEE

The efficiency is defined as the ratio of the output welding power Pw to the input power Pin. The welding power Pw is a product of the welding current squared and the resistance of welded parts:

Pw = Iw² x Rw = 2000² x 10^-3 = 4000 VA.

Due to the negligible value of the inductance for absolute majority of resistance welding transformers we may express the welding power in either units VA or W.

The input power Pin is the product of primary voltage and current:

Pin = Vp x Ipmax = 120 x 90.15 = 10818 VA.

Efficiency of given machine:

h = Pw / Pin = 4000 / 10818 = 37 %.

We have already met this number when we checked the effective voltage applied to the primary circuit Vpe (see "Resistance in primary and secondary circuit"). These 37 % of efficiency are not very high and show the designer that he or she may make some improvements, especially in the primary circuit. But in practical welding it is not a very rare case when two thirds of the power are used to heat the welding machine, the shop, and the welder.

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