Question

Q.32
Draw a schematic diagram of a step-up transformer. Explain the working principle. Deduce the
expression for the secondary to primary voltage in terms of the number of turns in the two
coils. In an ideal transformer, how is this ratio related to the currents in the two coils? How is
the transformer used in large scale transmission and distribution of electrical energy over long
distances?​

Answer 1

Answer:

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 N p 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 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. 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. Step up transformer is used at power house to transmit power at high voltage 11000V or 33000V. The current in wires at this voltage is quite small, so power loss I2R is negligible. At town, the step down transformer is used to supply power at 220V. This saves enormous electrical energy.