Transformer

Basic Working of Transformer
E.M.F Equation of transformer
Voltage Transformation ratio (K)


Introduction

A transformer is a static piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or increase in current .

It has an input side (primary) and an output side (secondary). Electrical energy applied to the primary is converted to a magnetic field which in turn, induces a current in the secondary which carries energy to the load connected to the secondary. The energy applied to the primary must be in the form of a changing voltage which creates a constantly changing current in the primary, since only a changing magnetic field will produce a current in the secondary.










In brief, a transformer is a device that

(a) transfers electric power from one circiut to another.

(b) it does so without a change of frequency.

(c) it accomplishes this by electromagnetic induction and

(d) where the two circuit are in mutual inductive influence of each other.

The Basic Working of Transformer

In its most basic form a transformer consists of :

(1)A primary coil or winding.
(2)A secondary coil or winding.
(3)A core that supports the coils or windings.


It consist of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. If one coil (primary) is connected to source of alternating voltage , an alternating flux is set up in the laminated core, most of which is linked with the other coil in which it produces mutually-induced e.m.f. (according to Faraday's Laws of Electromagnetic Induction) . If the second coil (secondary) circuit is closed, a current flows in it and so electric energy is transferred (entirely magnetically) from the first coil to the second coil.
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E.M.F Equation of transformer

Let               N1 = No. of turns in primary
                    N2 = No. of turns in secondary
                    Øm = Maximum flux in core in webers = Bm x A
                    f = Frequency of a.c. input in Hz.









The flux increases from it's zero value to maximum value Øm in one quarter of the cycle i.e. in 1/4 f second.

Therefore, r.m.s value of e.m.f./turn = 4.44 Øm volts

Now, r.m.s value of induced e.m.f in the whole primary winding
                          = ( induced e.m.f. / turn ) x No. of primary winding

                               E1 = 4.44 f N1Øm ------------------- (i)

Similarly, r.m.s. value of e.m.f. induced in secondary is,

                               E2 = 4.44 f N2Øm ------------------- (ii)

Voltage Transformation ratio (K)

From the above equations (i) and (ii), we get





(i) If K > 1 , then the transformer is called step-up transformer.
(ii) If K < 1 , then the transformer is called step-down transformer.
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