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Answer Posted / praveen kandwal
A transformer consists of an metal core of laminated sheets about 0,35mm thick insulated from each other to reduce eddy current losses. The vertical parts are called the limbs and the horizontal parts are called the yokes. The two coils are wound on the limbs. One coil is called the primary coil and the other is called the secondary coil.
The metal core is made from a ferromagnetic metal selected because of its magnetic properties
An alternating voltage applied to the primary coil results in an alternating current which produces an alternating magnetic flux in the metal core. The current /flux level is just sufficient to induce an e.m.f that is practically equal and opposite to the applied voltage. The mean path of the alternating flux . Assuming all of the flux passes through the primary and the secondary winding,it induces the same e.m.f. in each turn of both coils. Hence if N1 and N2 are the number of turns in the primary and secondary coils respectively .
When the secondary winding is in open circuit its terminal voltage is the same as the induced e.m.f . The primary current is such that the induced e.m.f is equal and opposite to the applied alternating voltage.
When a load is connected to the secondary winding a current flows. This secondary current induces a magnetic flux which tends to reduce the current flow (Lenz's Law). The induced flux tends to reduce the magnetic flux and the resulting back back e.m.f.in the primary windigs This changes the difference between the applied voltage and the back e.m.f.in the primary windings from (say 0,05% to 1%). As a result the primary current increases, by say 20 times the no-load current. The demagnetising ampere turns of the secondary winding are thus neutralised by the increase in the primary ampere turns.. As the initial no-load primary ampere-turns are very small it can be stated that
Full load Ampere-Turns on (Primary) = Full load Ampere-Turns on (Secondary)
I1.N1 = I2.N2 ..therefore I1/I2= N2 /N1 = V2 /V1
In practice the above are all approximate
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