Construction of a Transformer

The basic parts of a transformer consist of two coils having mutual inductance and a core material. Apart from these there are other parts such as the outer container, insulating and cooling medium, bushings to obtain the terminals of the transformers out. During the construction of transformer first the windings are wound and then the core laminations are pushed through openings of coil and the core is prepared. Then the laminations are tightened with clamps and bolts.

Core of a transformer

The core is generally made of silicon steel laminations insulated from each other by a light coating of varnish. The most popular material for core is cold-rolled grain oriented sheet steel (C.R.G.O.). It reduces the core loss and has high permeability when magnetized in rolling direction. The laminations are used to reduce the eddy currents. For a 50 Hz transformer the thickness of laminations is around 0.35 mm while for a 25 Hz transformer it is 0.5 mm. The laminations are cut in L, T and I shapes and are assembled to form a core. The vertical parts of the core are usually called limbs or legs and the horizontal parts are called yoke. The core is assembled in such a manner that adjacent layers are staggered. It avoids continuous air gap, provide low reluctance and provide good mechanical strength.

For high frequency transformers which are employed in communication circuits the core is made of powdered ferromagnetic alloy.

Types of Core construction:

Generally there are two types of construction of core based on the manner in which the winding and the core are placed. They are

Core type
Shell type

Core type Transformer:

Core type transformer

In core type, the windings surround a considerable part of the core and require more conductor material but less iron material. For a single phase transformer core type has two legs. To reduce leakage flux half of the both low voltage (LV) and high voltage (HV) windings together are placed on one leg and the remaining halves are placed on the other leg. To minimise the insulation requirement LV windings which are easy to insulate are placed near the core and the HV winging is placed outside.

Shell type Transformer:

shell type transformer construction

In shell type transformer, the core surrounds the major part of the windings and require more core material and less conductor material. For a single phase shell type transformer there are three limbs. Both the LV and HV windings are wound around the central limb and are sandwiched over one another.

Air-core transformers:

This type of transformer is used in radio devices and certain measuring instruments. The core in these transformers is made on non-magnetic materials.

Cruciform of Core:

Two stepped core

For small transformers the limbs of the core are rectangular but it is not preferred for large transformers. In large sized transformers, circular cylindrical coils are used; for such transformers the cruciform core is used with multiple steps. It gives better space factor. To form the steps different sizes of the lamination are required. To form a single step core, it require to different sized laminations. Generally three stepped core is the most common.

Transformer tank:

The assembly of the power transformer is placed in a suitable container or casing and it can be called as a transformer tank.

Cooling:

Low power transformers are generally air cooled whereas large power transformers are immersed in oil for cooling. The circulating oil in the transformer apart from cooling the transformer provides better insulation. In some transformers the sides of the transformer are corrugated with radiators mounted on the sides which provide a path for better cooling.

Also Read:

Transformer
Different Core materials used in transformer