Transformer

A transformer is a electrical device that transfers electrical energy from one electrical circuit to another cuircuits. A varying current in any one coil of the transformer produces a varying Magnetic flux, which in turn induces a varying Electromotive force across any other coils wound around the same core.

1.1 Transformer construction –

TRANSFORMER CONSTRUCTION
There are two general types of transformers, the core type and the shell type. These two types differ from each other by the manner in which the windings are wound around the magnetic core.
The magnetic core is a stack of thin silicon-steel laminations about 0.35 mm thick for 50Hz transformers. In order to reduce the eddy current losses, these laminations are insulated from one another by thin layers of varnish. For reducing the core losses, nearly all transformers have their magnetic core made from cold-rolled grain-oriented sheet steel
(C.R.G.O.). This material, when magnetized in the rolling direction, has low core loss and high permeability.

In the core-type, the windings surround a considerable part of steel core as shown in Fig. 1.1 (a). In the shell-type, the steel core surrounds a major part of the windings as shown in Fig. 1.1 (b). For a given output and voltage rating, core-type transtormer requires less iron but more conductor material as compared to a shell-type transformer. The vertical portions of the core are usually called limbs or legs and the top and bottom portions are called the yoke.This means that for single-phase transformers, core-type has two-legged core whereas shell-type has three-legged core.

In iron-core transformers, most of the flux is confined to high permeability core. There is,however, some flux that leaks through the core legs and non-magnetic material surrounding the core. This flux, called leakage flux, links one winding and not the other. A reduction in this leakage flux is desirable as it improves the transformer performance considerably. Consequently, an effort is always made to reduce it. In the core-type, transformer this is achieved by placing half of the low voltage (L.V.) winding over one leg and other half over the second leg or limb. For the high voltage winding also, half of the winding is over one leg and the other half over the second leg, Fig. 1.1 (a). L.V. winding is placed adjacent to the steel core and H.V. winding outside, in order to minimise the amount of insulation required.

In the shell type transformer, the L.V. and H.V. windings are wound over the central limb and are interleaved or sandwiched as shown in Fig. 1.1 (b). Note that the bottom and top L.V. coils are of half the size of other L.V. coils. Shell-type transtormers are preferred for low- voltage, low-power levels, Whereas core-type construction is used for high-voltage, high-power transformers.

In core-type transformer, the flux has a single path around the legs or yokes, Fig. 1.1 (a). In the shell-type transformer, the flux in the central limb divides equally and returns through the outer two legs as shown in Fig. 1.1 (b).

There are two types of windings employed for transformers. The concentric coils are used for core type transformers as shown in fig.1.1(a) and interleaved (or sandwiched) coils for shell type transformers as shown in figure 1.1(B).

one type of laminations for the core and shell type of transformers is illustrated in figure 1.2( a )and (b)respectively. The steal core is assembled in such a manner that the butt joints in adjacent layers are staggered as illustrated in fig. 1.2(c)The staggering of the butt joints avoids continuous air gap and therefore the reluctance of the magnetic circuit is not increased. At the same time,a continuous air gap would reduce the mechanical strength of the core, and therefore, the staggering of the butt joints is essential.

During the transformer construction, first the primary and secondary windings are wound, then the laminations are pushed through the coil openings, layer by layer and the steel core is prepared. The laminations are then tightened by means of clamps and bolts.

Low power transformers are air-cooled whereas large power transformers are immersed in oil for better cooling. In oil-cooled transformers, the oil serves as a coolant and also as an insulating medium.

For power frequency range of 25 to 400 Hz, tranformers are constructed with 0.35 mm thick sllicon-steel lamination. For audio-frequency range of 20 to 20,000 Hz, iron core with suitable refinements is uaed.For high frequencies employed in communication circuits, core is made up of powdered ferromagnetic alloy. In special cases, the magnetic circuit of a transformer may be made of non-magnetic material and in such a case, the transformer is referred to as an air-core transformer. The air-core transformer is primarily used in radio devices and in certain types of measuring and testing instruments. Cores made of soft ferrites are also used for pulse transformers as well as for high frequency electronic transformers.