Transformer Working Principle

A transformer works on the principle of electromagnetic induction between two or more coupled coils.

Consider a core type transformer as shown below. Here the secondary winding is left open circuited just for simplicity and an alternating voltage source V₁ is connected to the primary winding.

Due to the voltage source V₁  an alternating current I_e  starts flowing through N₁ turns. The current I_e  through primary winding produces the required mmf for flux production.

Primary mmf = N₁ I_e

But,       flux=mmf/reluctance

Therefore,   Φ =  N₁I_e / reluctance

Since, the magnetic flux path (mostly iron core) reluctance is almost constant and finite, therefore,

Φ ∝ I_e

Therefore, an alternating flux is produced.

This changing flux Φ links secondary winding and an emf e₂ is induced in it by law of electromagnetic induction given by

e₂ = -N₂(dΦ/dt)

As flux Φ also links primary winding, an emf e₁ is induced in it also by principle of self induction.

e₁ = -N₁(dΦ/dt)

Dividing the above equations :-

e₂ /e₁ = N₂/N₁

This equation is known as turns-ratio equation for a transformer.

The transformer working principle explained also explains why dc is not given to transformer ?

If the primary of a transformer is connected to dc supply, the primary will draw a steady current and hence produce constant flux. Consequently no back emf will be produced. The primary winding will draw excessive current due to low resistance of primary. The result is that the primary will overheat and burn out or fuse will blow.

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Related concepts :-
(1)Transformer construction and its types - shell and core type        transformer
(2)Open Circuit and Short Circuit test on Transformer