Mutual Flux

The total flux in the core of the transformer is common to both the primary and secondary windings. It is also the means by which energy is transferred from the primary winding to the secondary winding. Since this flux links both windings, it is called mutual flux. The inductance which produces this flux is also common to both windings and is called mutual inductance.

The following figure shows the flux produced by the currents in the primary and secondary windings of a transformer when source current is flowing in the primary winding.

FIGURE

When a load resistance is connected to the secondary winding, the voltage induced into the secondary winding causes current to flow in the secondary winding. This current produces a flux field about the secondary (shown as broken lines) which is in opposition to the flux field about the primary (Lenz's law). Thus, the flux about the secondary cancels some of the flux about the primary. With less flux surrounding the primary, the Back-EMF is reduced and more current is drawn from the source. The additional current in the primary generates more lines of flux, nearly re-establishing the original number of total flux lines.