A practical transformer differs from the ideal transformer in many respects. The practical transformer has (i) iron losses (ii) winding resistances and (iii) magnetic leakage, giving rise to leakage reactances.

Iron losses

 Since the iron core is subjected to alternating flux, there occurs eddy current and hysteresis loss in it. These two losses together are known as iron losses or core losses. The iron losses depend upon the supply frequency, maximum flux density in the core, volume of the core etc. It may be noted that magnitude of iron losses is quite small in a practical transformer.

Winding Resistances

In practical transformer, each winding has some resistance. We can replace a practical transformer with an idealized transformer by including a lumped resistance equal to the winding resistance of series with each winding. R1 and R2, are the winding resistances of the primary and the secondary, respectively. The inclusion of the winding resistances dictates that:Practical transformer

  • The power input must be greater than the power output
  • The terminal voltage is not equal to the induced emf
  • The efficiency (the ratio of power output to power input) of a practical transformer is less than 100%.

Leakage Reactances

In an ideal transformer, alternating flux set up in the core and whole of this flux links with both primary and secondary windings. However, in an practical or actual transformer, both the windings produce some flux that links only with the winding that produces it.

Part of the flux, known as the leakage flux, does complete its path through air. Therefore, when both windings in a transformer carry currents, each creates its own leakage flux. The primary leakage flux set up by the primary does not link the secondary. Likewise, the secondary leakage flux restricts itself to the secondary and does not link the primary. The common flux that circulates in the core and links both windings is termed the mutual flux.Practical transformer 2The primary leakage flux is proportional to the primary current and secondary leakage flux proportional to the secondary current. The primary leakage flux produces self inductance L1  which in turn produces leakage reactance X1 ­. Similarly, secondary leakage flux produces leakage reactance X2