Transformer

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For a given power requirement, one has the choice of the relative values of I_rms and E_rms. That is, for the product to be a constant, we can choose a relatively large current I and a relatively small potential difference v or just the reverse. In an electric power distribution system, it is desirable - both for reasons of safety and the efficient design of equipment; to have relatively low voltage at both the generating end and receiving end. But for transmission of electrical energy from the generating plant to the consumer, we want the lowest practical current so as to minimize the I²R energy dissipation in transmission line. This mismatch between the requirements for transmission and consumption calls for a device which raises or lowers the potential difference in a circuit, keeping the product I_rmsE_rms essentially constant. This device is a transformer whose operations are based on Faraday's law of induction.

An electrical device is used to change the AC voltage. A transformer which increases the AC voltage is called a 'step up transformer' and a transformer which decreases the AC voltage is called a 'step down transformer'.

Principle

A transformer is based on the principle of mutual induction.

Construction

It consists of a soft iron core made of laminated sheets well insulated from each other. The coils P₁ P₂ and S₁ S₂ are wounded on the same core.

The coil P₁ P₂ is a primary coil connected to AC source and S₁ S₂ is a secondary coil connected across a load resistance R.

Working

As the current in the primary varies, the flux linked with P₁ P₂ and hence S₁ S_2, changes_.

If N_p is the number of turns in P₁ P_2, and E_p is the alternating EMF fed to P₁ P₂ at instant t under ideal conditions;

Self induced EMF in P₁ P₂ at instant t = EMF fed to P₁ P₂ at this instant.

Assuming there is no flux leakage, the rate of change of flux through each turn of S₁ S₂ is df/dt,

Since

For Step Up Transformers

E_s > E_p

i.e., K >1 N_s > N_p

For a Step Down Transformers

E_s < E_p

i.e., K <1 n_s < N_p

If we assume there is no loss of power,

Out put power = Input power

E_sI_s = E_pI_p

Energy Losses in a Transformer

(i) Copper loss is the energy lost due to heating of copper coils of transformers.

(ii) Iron loss due formation of induced current in the iron line resulting in lot of heat.

(iii) Leakage of magnetic flux. All flux linked with primary may not be linked with secondary.

(iv) Magnetostriction i.e., humming noise of a transformer.