Transformer: Transformer is a device by which an alternating voltage may be decreased or increased. This is based on the principle of mutual-induction.
Construction: It consists of laminated core of soft iron, on which two coils of insulated copper wire are separately wound. These coils are kept insulated from each other and from the iron-core, but are coupled through mutual induction. The number of turns in these coils are different. Out of these coils one coil is called primary coil and other is called the secondary coil. The terminals of primary coils are connected to AC mains and the terminals of the secondary coil are connected to external circuit in which alternating current of desired voltage is required.
Step up Transformer: It 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’* 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 is and 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
Power Transmission Over Long Distances
The power (electrical energy) is transmitted to long distances by the use of transformers. The voltage output of the generator is stepped up by means of step up transformer, this steps down the current, so power loss I2R is significantly reduced. At the receiving station the voltage is stepped down to 220V for domestic supply
