Definition of transformer

Transformer is an ac machine that

  • transfer the electrical energy from one electric circuit to another(energy transfer usually takes place with a change of voltage)
  • does so without a change of frequency
  • based on principle of electromagnetic induction and
  • has electric circuits that are linked by a common magnetic circuit.
  • static machine and has high efficiency(in range of 96-99%).

Why transformer is never operated on dc?

=If a rated dc voltage is applied to the primary of a transformer,the flux produced in the transformer core will no vary but remain constant in magnitude and there will be no emf induced in the secondary winding expect at the moment of switching on.Thus transformer is not capable of lowering or raising the dc voltage.If dc supply is connected to the transformer primary or secondary,it may burn out since there is no self-induced emf and heavy current will flow through it.

Working principle of transformer

The working principle of transformer is based on the principle that energy may be efficiently transferred by induction from one set of coils to another by means of a varying magnetic flux provided that the both sets of coils are on a common magnetic circuit.

Theory of transformer

when the primary winding is connected to an ac supply mains,a current flows through it.Primary winding is linked with an iron core.Hence current flowing through primary winding produces an alternating flux Φ in the core.since the flux is alternating and links with the secondary winding. Thus the emf is induced in the secondary winding.The frequency of induced emf in secondary winding is the same as that of the flux or that  of the supply voltage.The induced emf in the secondary winding enables it to deliver current to an external load connected across it.Thus without any change in the frequency energy is transferred from primary winding to the secondary winding by the means of electromagnetic induction.

When the transformer raises the voltage i.e when the output voltage is higher than its input voltage ,it is called the step-up transformer.when the transformer lowers the voltage ,it is called the step-down transformer.

Transformer parts and construction

Transformer consists of a soft iron or silicon steel core and two windings placed on it.The windings are insulated from both the core and each other.The core is built up of thin soft iron or silicon steel lamination to provide a path of low reluctance to the magnetic flux.

Why does a transformer has iron core?

=The use of the iron or steel for transformer core ensures a high permeability of the magnetic circuit.Because of high permeability the magnitude of exciting current necessary to create the required flux in the core is small.The presence of steel core also causes 100% of magnetic flux created by primary to be linked with secondary.

primary winding:The winding connected to the supply mains is primary.

secondary winding:The winding connected to the load is secondary.

High voltage (hv) winding:The winding connected to the high-voltage.

Low-voltage(lv) winding:The winding connected to the lower voltage.


[For design of transformer]


Theory and performances of electric machine by JB Gupta



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