When current flows through a conductor, a magnetic field forms around the conductor. If that conductor is bent into a loop, the magnetic field becomes more concentrated. If the conductor is formed into a coil, the magnetic field becomes stronger. Adding an iron core inside the coil increases flux density because iron has high magnetic permeability.

Permeability is a material’s ability to concentrate magnetic flux. Iron has high permeability and, therefore, low reluctance. Air has low permeability and, therefore, high reluctance. Reluctance is the magnetic equivalent of electrical resistance. It opposes the production of magnetic flux.

In a transformer, the core is designed to provide a low-reluctance path for magnetic flux. The winding produces magnetomotive force, commonly described in ampere-turns. This magnetomotive force drives magnetic flux through the core.

When the core or winding condition changes, the magnetic circuit changes. If the reluctance changes, the excitation current may also change. That is the foundation of excitation current testing.

Instead of directly measuring the magnetic flux inside an energized transformer, which is impractical, technicians measure the current required to produce that flux. That current becomes a diagnostic indicator.

Tech-Tip Takeaway: Excitation current testing works because transformer windings and cores behave as magnetic circuits. A change in reluctance, flux path, or winding condition can manifest as a change in the measured current.