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Tuesday, 26 March 2019

Transformer - Construction and Working Principle

The transformer is a static device which transfers the electrical energy from one circuit to another without change in its frequency. This transformation is done due to the Faraday's law of electromagnetic induction between the two circuits. Usually it has two windings, namely primary and secondary.

The primary winding receives the energy from a source where as the secondary winding delivers the induced voltage to a load. Both the primary and secondary winding coils are wound on a former or core materials. The use of core reduces the losses, such as hysteresis and eddy currents. The core may be the same or different for primary and secondary windings placed close to each other enough so that the magnetic lines of force from one coil will cut the turns of the other coil.
Diagram of a Transformer
The prime function of transformers is:

1. To transfer the power from one circuit to another acting as a coupling device.
2. The high or low output is obtained across the secondary winding.

It has several advantages, such as

1. No frictional losses.
2. Low maintenance cost because it does not requires much attention.

Transformer - Construction:

As explained in preceding section, commonly transformer consists of two windings, namely primary winding and secondary winding. These windings are wounded on a core material to reduce the losses such as eddy currents and hysteresis.

The core material used in transformer construction is of two types (Iron. Silicon.....). The core material is made of thin sheets of 0.35 mm to 0.5 mm thick. The sheets are available in different shapes, such as E, I, L, O, U and T which are laminated to minimize the eddy currents. The laminations are separated by an insulating materials, such as varnish, oxide layer etc.

The primary and secondary windings are wounded on the two limbs or on the central limb. According to the type of winding, there are two types of constructional methods [shown in Figure] in transformers. i.e.. shell type transformer and core type transformer. In core type construction most of the core is enclosed by the windings. However in the shell type transformer most of the windings are enclosed by the core. The core is made from laminations, usually the yoke is built up from a stack of laminations and the windings are formed around this. Once the windings have been formed extra laminations are added to form complete the core.

(a) Core Laminations
(b) Core Type Transformer
(c) Shell Type Transformer

Working Principle of Transformer:

The transformer works on the principle of "Electromagnetic Induction", Consider an ideal transformer (having no losses) consists of two stationary windings, namely primary and secondary.
The number of turns in primary is NP and NS is the number of turns in secondary. The primary windings are supplied with an ac voltage 'Vi' at a frequency 'f" hertz. It causes to flow an alternating current 'Ii' of same frequency of 'Vi'. As the current is alternating, it will produce an alternating flux (Φ) in the core which will be linked by both the primary and secondary windings. The varying flux causes to generate emf in two windings. The emf 'Ep' across the primary is the same magnitude of the same magnitude of source supply but opposite in phase.

This emf is also known back emf (or) 'counter emf' of the primary. The induced emf across secondary winding is known as 'mutually induced emf (Es). This emf is antiphase with supply voltage 'vi' and its magnitude is proportional to the rate of change of flux and the number of secondary turns. Further, this emf can be utilized to deliver power to any load connected across the secondary.

If a transformer has the number of turns in secondary is higher than primary winding. it is called step-up transformer and its emf is of high magnitude. Otherwise, if number of turns is less than primary, the transformer is said to be step-down transformer and the secondary emf is of smaller magnitude. The instantaneous values of applied voltage, number of turns, emfs are shown in the blog posts.


A transformer can be used for different purposes in various electronic and electrical circuits. The details are as follows:

(a) As a power transformer to provide desired ac voltage either single valued or multiple values.
(b) As matching device between amplifiers and speakers or-as inter stage coupling device between any two circuits to provide impedance matching for having maximum energy transfer.
(c) As a tuner in all communication devices for selection of proper signal in conjuction with capacitor (s).
(d) As an isolating transformer to reduce the change of electronic shock from one stage to another.

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