The transformer is very simple in construction and consists of magnetic circuit linking with two windings known as primary and secondary windings.
Besides magnetic circuit and windings it consists of a suitable container for the assembled core and windings, such as a tank, a suitable medium for insulating the core and windings from its container such as transformer oil, suitable bushings (either of porcelain, oil-filled or condenser type) for insulating and bringing out terminals of the windings from the container, temperature gauge for measurement of temperature of hot oil or hottest spot temperature, and oil gauge to indicate the oil level inside the tank.
Some transformers are also provided with conservator tank in order to slowdown deterioration of oil and keep the main tank full of oil, emergency vent to relieve the pressure inside the tank in case the? pressure inside the transformer rises to a danger point and gas operated relay (Buchholz relay) in order to give alarm to indicate the presence of gas in case of some minor fault and take out the transformer out of circuit in case of serious fault.
Magnetic circuit consists of an iron core. Since core is magnetic link between the two systems in a transformer and it itself contains a lot a energy, it is not by any means the passive component it would first appear.
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Ideally the steel, of which the transformer core is made, is simple in chemical composition, the hot rolled steel has 4-5% silicon and the cold rolled has 3%. Now-a-days cold rolled steel is used almost exclusively inspite of its high price, because flux density 30% higher can be used. This is because of high permeability and lower losses at given flux density.
Since flux is an alternating one, in order to reduce eddy current losses the core is laminated. The thickness of laminations or stampings varies from 0.35 mm to 0.5 mm. The laminations are insulated from each other by coating them with a thin coat of varnish. The iron core is rectangular in cross-section for small size transformers and circular for large size ones to reduce the quantity of copper required.
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The electric circuit consists essentially of two windings, each being split up into two equal number of coils and the coils of two windings are arranged on each limb of iron core to reduce the magnetic leakage. In order to minimise the amount of high voltage insulation, low voltage coils are placed adjacent to the iron.