What is Transformer? – Definition and Meaning

Definition of Transformer:

Transformer is a device which transfers electrical energy from one circuit to another through the medium of magnetic field and without change in the frequency. The electrical circuit which receives electrical power from the source is called the primary winding and another circuit which delivers the electric energy to load is called secondary winding.

There is many other definition of transformer. It can also be defined as a device which either step-up or step-down voltage with corresponding decrease and increase in current respectively.

Transformer is an electromagnetic energy conversion device, since the energy received by the primary is first converted into magnetic energy and then it is reconverted into useful electric energy in the secondary winding.


In the above figure, the winding connected to supply main is primary winding and the winding connected to load is secondary winding. Primary winding takes alternating current from the supply main to create alternating magnetic field in the core of transformer. This magnetic field links with the secondary winding and as per the faraday’s law, an emf is induced. When the secondary is connected to load, a current starts flowing through the load due to the induced emf in the secondary winding. This method of transfer of energy is known as Transformer Action.

Thus electric energy is transferred from primary to secondary winding without having actual electrical interconnection between the two windings. This is due to their magnetic coupling. This coupling magnetic field allows transfer of electric power in either direction i.e. from high voltage circuit to low voltage circuit or from low voltage circuit to high voltage circuit.

If the secondary winding has more number of turns than the primary winding, then the secondary voltage is higher than the primary voltage and the transformer is called the Step-up transformer.

If the primary winding has more number of turns than the secondary winding, then the secondary voltage is lower than the primary voltage and hence this transformer is called the Step-down transformer.

It must be noted that, a step-up transformer can also be used as step-down transformer. In this case, the secondary of step-up transformer becomes the primary of step-down transformer. Thus, a transformer can only be termed as step-up or step-down transformer when it is put into service. Therefore it is always a good idea to refer a transformer winding as high voltage winding and low voltage winding instead of primary and secondary winding.

Why Transformer is called Static Device?

In transformer the transfer of electric energy from one circuit to another circuit take place without the use of any moving part. This is the reason; it is called a Static Device. Since there is no any moving part in transformer, therefore it has the highest possible efficiency out of all electrical machines. The efficiency of transformer is of the order of 99%.

Requirement of Transformer:

Transformer plays a vital role in the entire power system. Be it power generation, transmission or distribution, transformer is required. In fact, AC system is so popular just because of transformer. Let us consider this aspect and try to realize the importance and necessity of transformer.

Electric power is generated by the alternator at 11 kV or 16.5 kV or 21.5 kV. This power is then required to be transmitted at the load center. To transmit the power, it is stepped up to higher voltage level to minimize the transmission losses. Let the power is generated at 16.5 kV and transmitted through 220 kV Grid.  For this purpose, 16.5 kV / 220 kV Step-up Transformer is used. Again, at the load center, this voltage is stepped down to 33 kV using 220 / 33 kV Step-down transformer. Then 33 kV / 11 kV and finally 11 kV / 415 V step-down transformer is used for electric power distribution to households.

In addition, it is also used widely in electronic circuits as isolation transformer. Therefore, transformers are built in many different sizes. For electronic and control purpose, the weight of transformer may be of the order of few gram whereas for high voltage power circuit, it may weight around hundreds of tonnes.

Function of Transformer:

The important tasks performed by transformer are as below:

  • To increase or decrease voltage level from one circuit to another circuit. For increasing voltage level, a step-up transformer is used while for decreasing voltage level, a step-down transformer is used.
  • A transformer is useful for matching source and load impedance to achieve maximum power transfer.
  • Transformer is also used to isolate one circuit to another circuit. These transformers are called isolation transformer. In isolation transformer, the number of turns in primary and secondary winding is same. Therefore, it does not alter the voltage level in the two electric circuits but electrically isolate them.

Rating of Transformer:

Transformer manufacturer fixes a nameplate on the transformer, on which rated output, frequency, voltage etc. are mentioned. A typical nameplate rating of single phase transformer is as follows:

Rated Output: 20 kVA

Rated Voltage: 3300 / 220 V

Frequency: 50 Hz

Here, rated voltage 3300 / 220 V means that, this transformer can either step-up voltage from 220 V to 3300 V or step-down voltage from 3300 V to 220 V at constant frequency of 50 Hz.

It should also be noted that rated output is given in terms of apparent power (kVA) not in kW (Active Power). You are requested to read “Why Transformer Rated in kVA and not in kW?

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