Although in our calculations
input electrical power = output electrical power
we consider transformer in an ideal state means no loss of energy, but practically always there is some loss of energy during transformation from higher voltage to lower voltage and vice versa.
There are two possible reasons of energy loss.
1 - Primary and secondary coils are not perfect electrical conductors, always they have some electrical resistance in them. They heat up and lose some energy.
2 - When eddy current flow in the core, it becomes hot and there will be a loss of energy.
The second factor is really interested. We have studied that whenever there is change in the magnetic field (change of magnetic flux) accross the conductor, electric current induces accross it.
OK, when we say alternating magnetic field coming from primary coil through the core induces electric cuurent in the secondary coil (because magnetic field is changing accros teh secondary coil), then here is one quesion.
Does alternating magnetic field is not effecting the core, core is made up of iron, it is also conductor, why not electrical current is inducing accross it?
Answer is yes, electric current induces accross the core as well. That is called eddy current. As there is some electrical resistance in the core as well so heating effect produces and we lose some energy.
To reduce the heating effect of the eddy current in the core, the core is always laminated.
Note: The maximun efficiency of well designed transformer is 99%.
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