A transformer is a device used to change the relative amplitudes of voltages and current in an AC circuit. A transformer consists of primary and secondary windings whose magnetic fluxes are linked by a ferromagnetic core. The principle of working of a transformer is based on the fact that two coils that are wound on the same iron core have the property of mutual inductance, because a change in flux and hence in e.m.f in one coil produces, via the iron ore, corresponding change in the other coil.
If we take this arrangement above, and apply alternating voltage to one coil, it will induce an alternating e.m.f in the other coil; this is called the transformer effect. The coil or winding to which the supply is connected is called the primary and the winding from which the induced voltage is taken is called the secondary.
Using Faraday’s law of induction and neglecting losses, the voltage per turn of wire is the same for both the primary and secondary windings, because the windings experience the same alternating magnetic flux. Therefore, the primary and secondary voltages (Vp and Vs) are related by:
Where, Np is the number of turns in the primary winding, Ns is the number of turns in the secondary winding and Φ is the magnetic flux linked between the two coils. Thus the secondary voltage is related to the primary voltage by:
If Ns > NP, it is called a step-up transformer because the voltage increases. If Ns < NP it is called a step-down transformer because the voltage decreases. If Ns = NP, it is called an isolation transformer and the output voltage is the same as the input voltage. All transformers electrically isolate the output circuit from the input circuit. If we neglect losses in the transformer due to winding resistance and magnetic effects, the power in the primary and secondary circuits is equal and is given by:
Thus, a step-up transformer results in lower current in the secondary and a step-down transformer results in higher current. An isolation transformer has equal alternating currents in both the primary and secondary. Please note that, any DC component of voltage or current in a transformer primary will not appear in the secondary. Only alternating currents are transformed.
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