Working Principle of a Transformer

Updated: Mar 11

A transformer is a static piece of equipment and it does not have any moving part in it. The transformer has primary and secondary winding. Both primary and secondary winding is mounted separately on the CRGO steel core. The winding connected to AC supply is called the primary winding, and the winding connected to load is called secondary winding. The primary and secondary of a transformer can be connected to either AC supply side or load side. The transformer transfers the power from one side to other side without changing the supply frequency.

The transformer functions on the principle of electromagnetic induction and, therefore transformer functions with AC supply only. The electromagnetic induction principle invented by Michael Faraday is known as Farady's Law of Electromagnetic Induction or Farad's Law. According to Farady's Law, EMF is induced in a conductor when it is placed in a changing magnetic field. If the conductor circuit is closed circuit, current starts flowing in the circuit. The magnitude of the EMF induced in a conductor depends on number of turns in a coil and rate of change of magnetic field.


As per Lenz's Law, the direction of EMF induced in the primary is in opposite direction of the applied voltage.


When primary winding of a transformer is energized with AC supply, the magnetic flux is produced. The magnetic flux produced in the primary travels through the magnetic core, and the flux gets linked to secondary winding by mutual induction. The flux also gets linked to primary winding by self induction.It is desired to have maximum linking of flux to primary and secondary winding of the transformer to have better regulation and efficiency. The flux linking mechanism of a transformer is as shown below.

The magnitude of the EMF induced in the primary and secondary winding depends on the number of turns and the rate of change of flux linking to primary and secondary winding. The magnitude of induced EMF in the primary winding is;





Where, f = frequency, N1= Number of turns of primary


The minus sign shows that induced emf in the primary and secondary winding oppose the applied voltage.


Similarly, voltage induced in the secondary winding is given as.





The induced emf limits the primary current because it opposes the applied voltage. In absence of emf, the transformer draws enormous current and winding is apt to fail. The emf is induced only when if AC supply is fed to primary or secondary of the transformer. The transformer does not function if DC supply is fed to primary of the transformer.


DC supply should never be fed to transformer, Why?


If DC supply is fed to transformer, no back emf is induced in the primary because the rate of change of current in the case of DC supply is zero. In absence of emf, the current through the transformer primary is given by;









From above, it is lucid that the current is limited by the primary winding resistance when DC is fed to primary of the transformer. The primary winding resistance is very low and hence the transformer primary draw very huge current and winding insulation gets failed. Therefore, DC supply should never be fed to transformer.


Turn Ratio of a Transformer


The ratio of primary turns to secondary turns is known as turn ratio of a transformer.






If turn ratio a>1, the transformer is called step down transformer. If turn ratio a<1, the transformer is called step up transformer.


Voltage Transformation Ratio of a Transformer


The ratio of secondary voltage to primary voltage is known as voltage transformer ratio of transformer.






If K>1, the transformer is called step up transformer, and if K<1, the transformer is called step down transformer.


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