In an ideal transformer it is assumed that
windings have got no resistance and there is no leakage flux. But in actual practice it is not possible to have an ideal transformer.
Resistance. In an actual transformer the primary as well secondary winding possess some resistance due to which some voltage drop takes place in them.
Magnetic leakage. Magnetic flux cannot be confined into a desired path. The greater portion of the flux (i.e., the mutual flux) remains confined to the core and links both the windings but a small portion, called the leakage flux, completes its path through the air surrounding the coils. As shown in Fig. 22 each of the winding is associated with a leakage flux. Since the path of the leakage flux is
largely in air the leakage flux and the voltage induced by it vary linearly with the current. The primary leahage flux varies linearly with the primary current and the secondary leakage flux varies linearly with the secondary current. The effect of primary leakage may be simulated by assigning to the primary a leakage inductance (equal to primary leakage flux per ampere of primary current). The
reactance corresponding to this primary leakage inductance (i.e. 21Πf × primary leakage inductance) is known as primary leakage reactance X1. Similarly the effect of secondary leakage flux may be simulated by a secondary leakage inductance and the corresponding leakage reactance X2.
The terminal voltage V1 applied to the primary must have a component jI1 X1 to balance the primary leakage e.m.f. Similarly V2 i.e. the output voltage from secondary will be less than the induced e.m.f. E2 by a component jI2 X2 to account for the secondary leakage flux.
A transformer with winding resistance and magnetic leakage is equivalent to an idea transformer (having no resistance and leakage resistance) having resistive and inductive coil connected in series with each winding as shown in Fig. 23.