Transformer - Introduction & Basic

What is an electrical transformer? 

An electrical transformer is a static device which consists of two stationary electrical circuits connected to each other by a common magnetic circuit for the purpose of transferring electrical energy between them.

This transfer of electrical energy takes place without the change in frequency.

Transformer works on the principle of Electromagnetic Induction. Electromagnetic Induction is one of the basic law of Electrical Engineering. You can read about Electromagnetic Induction.

CONSTRUCTION & WORKING PRINCIPLE  OF TRANSFORMER

A simple transformer consists of two coils wounded on a core as shown in the figure.

There is no electrical connection between these two coils.

When an alternating voltage source V is applied to the coil 1, an alternating current I1 flows in it. This alternating current produces an alternating magnetic flux in the core. This alternating flux links with the turns  T1 of the coil 1 and induces an emf E1 by self-induction.

Thus, all the flux produced by the coil 1 also links with the turns T2 of the coil 2 and thereby inducing an emf E2 by mutual induction.

If load is connected to the coil 2, then an alternating current I2 will flow through it and hence electrical energy will delivered to the load without change in frequency.

Coil 1 which receives the electrical energy from the source of a.c supply is called primary winding.

Coil 2 which is connected to the load and delivers the energy to the load is called secondary winding.

Note that the transformer can only work with the alternating source. It cannot be used for transferring electrical energy in DC form because transformer works on the principle of Electromagnetic Induction and it requires that current should be alternating in nature. So, transformer cannot work on DC.

EMF EQUATION OF THE TRANSFORMER

As we have studied that an emf is induced in both the coils or windings. So the magnitude of the emf can be calculated from the emf equation of the transformer which is ;

E = 4.44* fi * f * T

Where , E = Magnitude of emf induced.
             fi = Maximum flux in Weber.
             f  = Frequency in Hertz.
             T = Number of turns in the coil.
With the help of the emf equation, emf induced in the primary as well as secondary winding can be calculated by providing the suitable number of turns.
It shows that winding having higher number number of turns will have higher voltage and hence it is called high voltage  (hv) winding.
Winding having lower number number of turns will also have lower voltage is called low voltage (lv) winding.

TRANSFORMATION RATIO

The ratio of emf induced in the primary winding to the emf induced in the secondary winding is called transformation ratio. It is denoted by 'k'.

k = E1/E2 = T1/T2

Transformation ratio can also be defined as the ratio of number of turns of primary winding and number of turns of secondary winding. It is also called turn ratio.

STEP-UP & STEP-DOWN TRANSFORMER

Transformer in which the secondary voltage is greater than the primary voltage is called step-up transformer.
Step-up transformer has turn ratio smaller than 1.
Transformer in which the secondary voltage is smaller than the primary voltage is called step-down transformer.
Step-down transformer has turn ratio greater than 1.
Transformer having secondary voltage equal to the primary voltage is called isolation transformer or one-to-one transformer.
Such transformer has turn ratio equal to 1.