In Transformer if a current carrying conductor is placed near by the second conductor, then the changing magnetic field in current carrying conductor induces a voltage in the second coil. Based on this principle, it is illustrated that the transformer should consists of two coils placed near by as shown in below figure.
The coil which is connected to the power supply is called primary winding and the coil which is at load side is called secondary coil. the power is transferred from primary to secondary. In some cases the power supply can also be connected to secondary winding also.
An AC voltage is connected to a primary conductor, shown as the left hand solid bar in the above figure. In response to the voltage, an AC current flows, setting up a time-varying magnetic field surrounding the primary conductor. A secondary coil, shown as the right-hand solid bar, is located in proximity to the primary conductor so that the magnetic flux surrounding the primary conductor links the secondary circuit. According to the Faraday’s law of induction, there will be an induced voltage E in the secondary coil which is placed in that time varying magnetic field.
The construction configuration of transformer shown above is not adequate for an efficient transmission of power from primary to secondary. Because the total flux produced in primary conductor cannot be linked to the secondary conductor due to the leakage of the flux. There will not be a prop[er way to transmit the flux from primary to secondary and flux will scatter as its own way which results in magnetic losses. In order to improve the efficiency, the magnetic flux needs to channeled in such a way that the maximum flux produced in primary are linked to secondary. This is accomplished by using magnetic core material having the low reluctance path to the magnetic flux (iron core).
The construction of transformer with magnetic core material is shown in the figure. With this arrangement certainly the maximum flux produced in primary is linked to secondary through the iron channel. Thus the efficiency of the transformer can be increased and as well as the cost of the transformer also increases. But it is worth doing it.
Presently in all the transformers that are used commercially, are equipped with sheet steel metal core to produce a constant path to magnetic flux with less air gap. The important property of the core is that it should have high permeability constant ( μ).
The permeability constant is defined as the ratio of magnetic flux density(B) to the field intensity (H).
μ = B/H (N/A2)