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The transformer is a static device which converts the magnetic energy into electrical energy. It consists of two or more than two stationary circuits interlinked by a common magnetic circuit; the energy transfer takes place through this circuit without having any change in frequency from one circuit to another.

The transformer consists of two windings. The winding which is connected with the supply a.c. voltage is called the primary winding, and the winding which is connected to load and delivers the energy to load is called secondary winding.**E.M.F. Equation of a Transformer**

Let the flux at any point is given by

φ = φ_{m} sin ω t

The instantaneous e.m.f. induced in a coil of T turns linked by this flux is given by Faraday's law as

The above equation can also be written as

Where E_{m} = Tωφ_{m} = maximum value of e.

For sine wave, the r.m.s. value is given by

This is called as e.m.f. equation of transformer.

Where,

φ_{m} is the maximum flux in webers (Wb)

f is the frequency in hertz (Hz)

E is the voltage in volts

T is number of turns in winding

The primary r.m.s. voltage is

E_{1} = 4.44φ_{m} fT_{1}

The secondary r.m.s. voltage is

E_{2} = 4.44φ_{m} fT_{2}**Voltage Ratio and Turns Ratio**

The ratio E/T is called voltage per turn.

As we know

Form equation 1 and 2

The ratio is called truns ratio.**Step - up transformer:** These are the transformer in which the output voltage is higher than the input voltage.**Step - down transformer:** These are the transformer in which the output voltage is less than the input voltage.

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