Explain the underlying principle and working of a step-up Transformer.
Principle: It is a device which converts high voltage AC into low voltage AC and vice-versa. It is based upon the principle of mutual induction. When alternating current is passed through a coil, an induced emf is set up in the neighboring coil.
Working: When an alternating current is passed through the primary, the magnetic flux through the iron core changes, which does two things. It produces emf in the primary and an induced emf is also set up in the secondary. If we assume that the resistance of primary is negligible, then the back emf will be equal to the voltage applied to the primary.
∴ V1 = -N1 dΦ/dt
and V2 = -N2 dΦ/dt
Where, N1 and N2 are number of turns in the primary and the secondary respectively and V1 and V2 are their voltages respectively.
In a step-up transformer: N2 > N1, therefore, V2 > V1
Such a device cannot be used to step up a dc because a dc voltage cannot produce a changing flux, which is very essential to the production of induced emf in the secondary.