Working Principle of Single Phase Transformer

Principle of Working

AU: Dec.-13,17, May-13,14.

The principle of mutual induction states that when two coils are inductively coupled and if current in one coil is changed uniformly then an e.m.f. gets induced in the other coil. This e.m.f. can drive a current, when a closed path is provided to it. The transformer works on the same principle. In its elementary form, it consists of two inductive coils which are electrically separated but linked through a common magnetic circuit. The two coils have high mutual inductance. The basic transformer is shown in the Fig. 6.1.1.

• One of the two coils is connected to a source of alternating voltage. This coil in which electrical energy is fed with the help of source is called primary winding (P). The other winding is connected to load. The electrical energy transformed to this winding is drawn out to the load.

• This winding is called secondary winding (S). The primary winding has N₁ number of turns while the secondary winding has N₂ number of turns. Symbolically the transformer is indicated as shown in the Fig. 6.2.1.

• When primary winding is excited by an alternating voltage, it circulates an alternating current. This current produces an alternating flux (ϕ) which completes its path through common magnetic core as shown dotted in the Fig. 6.2.2. Thus an alternating, flux links with the secondary winding. As the flux is alternating, according to Faraday's law of an electromagnetic induction, mutually induced e.m.f. gets developed in the secondary winding. If now load is connected to the secondary winding, this e.m.f. drives a current through it.

• Thus though there is no electrical contact between the two windings, an electrical energy gets transferred from primary to the secondary. 

Key Point: The frequency of the mutually induced e.m.f. is same as that of the alternating source which is supplying energy to the primary winding.

1. Can D.C. Supply be used for Transformers?

• The d.c. supply cannot be used for the transformers.

•. The transformer works on the principle of mutual induction, for which current in one coil must change uniformly. If d.c. supply is given, the current will not change due to constant supply and transformer will not work.

• Practically winding resistance is very small. For d.c., the inductive reactance X_L is zero as d.c. has no frequency. So total impedance of winding is very low for d.c. Thus winding will draw very high current if d.c. supply is given to it. This may cause the burning of windings due to extra heat generated and may cause permanent damage to the transformer.

• There can be saturation of the core due to which transformer draws very large current from the supply when connected to d.c.

Thus d.c. supply should not be connected to the transformers.

Review Question

1. Explain the working principle of a single phase transformer.

AU: Dec.-13,17, May-13, 14, Marks 8