In this topic, you study Practical Transformer.
The ideal transformer having no core losses, winding resistances and magnetic leakage. We have to take into account the effects of these factors while analysing the behaviour or the performance of the transformers used in actual practice.
Effect of Core Losses
Since the iron core of the transformer is subjected to an alternating flux, core losses (i.e. hysteresis and eddy current losses) are always present to some extent even though the core is laminated to reduce eddy currents and hysteresis is minimised by the use of high
Fig. 3.13 : Phasor diagram for the practical transformer on no load
grade silicon steel (refer to the Section 3.14). It follows, therefore, that even under no-load condition (i.e. with the secondary winding open circuited), the source must supply enough power to the primary winding to overcome the core losses. Therefore, in the case of transformers which are used in actual practice, on no load i.e. with no current in the
Effect of Resistance and Leakage Reactance of Windings
In practical transformer, each winding of the transformer possesses certain finite resistance which apart from causing a power loss, produces a voltage drop in it on current flow. Further, as an electric current, magnetic flux cannot be completely confined into a desired path. Therefore, all the magnetizing flux produced by the primary winding of a transformer does not link the secondary, but a part of it completes its magnetic circuit by passing through the air rather than around through the core as shown in Fig. 3.14. Such a flux is called the primary leakage flux.
Fig. 3.14 : Primary and secondary leakage fluxes
This leakage flux does not contribute to the transfer of energy from the primary to the secondary and thus serves no useful purpose, since it fails to link the secondary winding to the primary winding. Secondary winding also produces opposing (in accordance with Lenz’s law) flux due to the induced current flowing through it on load. Major portion of this flux links with the primary winding through the core. However, certain amount of this flux links only with the secondary winding through air as shown in Fig. 3.14. This is termed as the secondary leakage flux. These primary and secondary leakage fluxes produce in their respective windings emfs of self-induction which are proportional to the current and the frequency. They are, therefore, equivalent to an inductance placed in series with each wining, the reactance of which is called the leakage reactance of the winding. Similar to resistance, both primary and secondary leakage reactances cause a voltage drop in the respective windings. The combined effect of the voltage drop due to resistance and leakage reactance of the windings of a transformer is ultimately to change its secondary terminal voltage on load from its no-load value for a given applied voltage across the primary as discussed in the next few articles. In practice, the leakage reactance is minimized by placing the windings on both the limbs of the core (half on each) and also by subdividing and interleaving the diffeRnt sections of the windings.