The welding transformer

Thumbnail of a small electric arc welding transformer

This photograph shows the construction of the transformer used in a small (150 ampere), inexpensive electric arc welding unit. You see that it is essentially a shell type of core. The windings are on separate bobbins, with the primary on the right and the secondary on the left. Between them however, is an important modification to the magnetic design.

Observe the separate bar protruding from the back of the transformer. This bar, comprising a set of stacked laminations, is linked by a lead screw to the current adjustment control on the front panel. By turning the current control wheel the position of the bar may be altered.

The flux distribution in a
welding transformer without leakage shunt With the bar out in the position shown, the core behaves in much the same way as an ordinary mains transformer. That is, the primary and secondary share the same magnetic flux path. This strong linkage results in the highest current being available to the welding electrode.

The flux distribution in a
welding transformer with leakage shunt When the control wheel is rotated so that the bar is withdrawn back into the core then the flux generated by the primary winding need no longer travel through the secondary in order to form a closed loop. It instead has the option of following the shorter path through the bar; and this is what most of it will decide to do.

There are three effects as a result of this change in the flux path due to this 'magnetic shunt':
Last modified: 2006 August 6th.