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 ϕ 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 i
p 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
i.e. step up transformer increases the voltage.
