Transformer-Definition, working principle, Dot convention.

Definition:
The transformer is an electromagnetic energy conversion device, that transfers electrical energy from one circuit to another circuit without any change in frequency. 

During this process of energy transfer, the energy is being converted from electrical to magnetic and back again to electrical energy. The part of the transformer that receives electrical energy from supply mains is known as a primary winding and the part that delivers electrical energy to the load is known as the secondary winding.

Single phase transformer

The primary and secondary windings of the transformer are not electrically connected but are magnetically coupled. Thus, the electrical energy received by the primary winding is first converted into magnetic energy and then it is converted back into electrical energy in the secondary winding.

Working Principle of transformer:

The basic working principle of a transformer is "faraday's law of electromagnetic induction".     

According to faraday's law, whenever there is a change in flux linkages through the conductor, an emf is induced inside the conductor. The expression for the induced emf is given by 

N=number of turns

Now, the polarity of this induced emf is indicated by Lenz's law. Lenz's law states that the polarity of the induced emf will be such that it will oppose the cause producing it. This opposing nature is taken into consideration by putting a negative sign in the emf equation as shown below

Features of transformer:

1. It is a constant frequency device.
2. It is a constant power device i.e it transfers the approximately same amount of power from the primary to the secondary winding.
3. It is a constant flux device i.e the flux in the transformer remains constant irrespective of change in load.
4. It is a phase-shifting device that offers a 180° phase shift between primary to the secondary winding.

Dot convention:

In a transformer, the dot convention plays a very important role. For the parallel operation of the transformer, the dot polarity must be known. Hence, One must know how to place a dot in a transformer.

How to place a dot in a transformer?

While placing a dot, Lenz's law must be carefully taken into consideration. Let us place a dot for the transformer given in fig(a). The steps to be followed are:

Steps to be followed:

Let us consider a single-phase transformer as shown in fig(a)

fig(a)

1. let us assume a dot in the primary winding anywhere. for example, let the dot at primary winding is at point "A". At this point, the current(I) is entering the dot. This current set up flux ɸ₁ as shown in fig(a).
2. This flux links with the primary as well as secondary winding and according to faraday's law of electromagnetic induction, an emf is induced in both the windings. 
3. Now the polarity of the emf induced is given by Lenz's law. Lenz's law states that the polarity of the induced emf will be such that it opposes the cause producing it. Since the emf E₂ is induced due to flux ɸ₁. So, the emf E₂ should drive the current through load in such a direction that the flux due to _{I2 (i.e ɸ2)} opposes ɸ_1.
4. So to satisfy Lenz's law, the correct direction of current is leaving point C as shown in fig(b). Hence, point C will be marked as a dot.

fig(b)

In this way, the dot is placed in the transformer. One can also understand this by the source-load analogy. 

• On the primary side, the current enters the positive terminal, so the primary winding act as a load. On the secondary side, the current leaves the positive terminal, so the secondary winding act as a source. 

Note: In a transformer, if the current enters the dot at primary winding then it leaves the dot at the secondary winding.

This is some basic information about transformer and how to place dot in a transformer.