The main advantage of alternating currents over direct current is that, the alternating currents can be easily transferable from low voltage to high voltage or high voltage to low. Alternating voltages can be raised or lowered as per requirements in the different stages of electrical network as generation, transmission, distribution and utilization. This is possible with a static device called transformer. The transformer works on the principle of mutual induction. It transfer an electric energy from one circuit to other when there is no electrical connection between the tow circuits. Thus we can define transformer as below :
Key point : The transformer is a static piece of apparatus by means of which an electrical power is transformed from one alternating current circuit to another with the desired change in voltage and current, without any change in the frequency.
The use of transformers in transmission system is shown in the Fig 1.1.
|Fig. 1.1 Use of transformer in transmission system|
2. Principle of working
The principle of mutual induction states that when tow coils are inductively coupled and if current in one coil is changed uniformly then an e.m.f. gets induced in the other coil. This e.m.f can drive a current, when a closed path is provided to it. The transformer works on the same principle. In its elementary form, it consists of tow inductive coils which are electrically separated but linked through a common magnetic circuit. The two coils have high mutual inductance. The basic transformer is shown in the Fig 1.2.
One of the two coils is connected to source of alternating voltage. This coil in which electrical energy is fed with the help of source called primary winding (P). The other winding is connected to load. The electrical energy transformed to this winding is drawn out to the load.
|Fig.1.2 Basic transformer|
This winding is called secondary winding (S). The primary winding has N1 number of turns while the secondary winding has N2 number of turns. Symbolically the transformer is indicated as shown in the Fig 1.3.
When primary winding is excited by an alternating voltage, it circulates an alternating current. This current produces an alternating flux (Φ)which completes its path through common magnetic core as shown dotted in the Fig 1.2. Thus an alternating, flux links with the secondary winding. As the flux is alternating, according to Faraday's law of an electromagnetic induction, mutually induced e.m.f. gets developed in the secondary winding. If now load is connected to the secondary winding, this e.m.f. drives a current through it.
Thus through there is no electrical contact between the two windings, an electrical energy gets transferred from primary to the secondary.
Key point : The frequency of the mutual induced e.m.f. is same as that of the alternating source which is supplying energy to the primary winding.
3. Can D.C. Supply be used for Transformer?
The d.c. supply can not be used for the transformers.
The transformer works on the principle of mutual induction, for which current in one coil must change uniformly. If d.c. supply is given, the current will not change due to constant supply and transformer will not work.
Practically winding resistance is very small. For d.c., the inductive reactance XL is zero as d.c. has no frequency. So total impedance of winding is very low for d.c. Thus winding will draw very high current if d.c. supply is given to it. This may cause the burning of windings due to extra heat generated and may cause permanent damage to the transformer.
There can be saturation of the core due to which transformer draws very large current from the supply when connected to d.c.
Thus d.c. supply should not be connected to the transformers.