(i) Principle: It is based on the principle of mutual inductance and transforms the alternating low voltage to alternating high voltage and in this the number of turns in secondary coil is more than that in primary coil. (i. e.,NS > Np).
Working: When alternating current source is connected to the ends of primary coil, the current changes continuously in the primary coil; due to which the magnetic flux linked with the secondary coil changes continuously, therefore the alternating emf of same frequency is developed across the secondary.
Let Np be the number of turns in primary coil, NS the number of turns in secondary coil and f the magnetic flux linked with each turn. We assume that there is no leakage of flux so that the flux linked with each turn of primary coil and secondary coil is the same. According to Faraday’s laws the emf induced in the primary coil
If the resistance of primary coil is negligible, the emf (εp) induced in the primary coil, will be equal to the applied potential difference (Vp) across its ends. Similarly if the secondary circuit is open, then the potential difference VS across its ends will be equal to the emf (εS) induced in it; therefore
where r = NS/Np is called the transformation ratio. If ip and iS are the instantaneous currents in primary and secondary coils and there is no loss of energy; then For about 100% efficiency,
Power in primary = Power in secondary
(ii) Reasons for energy losses in a transformer:
(a) Joule Heating: Energy is lost in resistance of primary and secondary windings as heat (I2Rt).
(b) Flux Leakage: Energy is lost due to coupling of primary and secondary coils not being perfect, i.e., whole of magnetic flux generated in primary coil is not linked with the secondary coil.
(iii) When output voltage increases, the output current automatically decreases to keep the power same. Thus, there is no violation of conservation of energy in a step up transformer.
