Construction of Transformer

TRANSFORMER CONSTRUCTION

All transformer has the following essential elements.

(i) Primary winding: The winding which is connected to ac supply is called primary winding.

(ii) Secondary winding: The winding which is connected to load is called secondary winding.

(iii) Transformer core: The two windings of the transformer is magnetically coupled through the core. The core is constructed of transformer sheet steel laminations to provide continuous magnetic path with a minimum of air gap included.

The core is made up of silicon steel to reduce the hysteresis loss. The laminated construction of the core minimise the eddy current loss. The laminations are insulated from each other by light coat of core plate varnish (or) by an oxide layer on the surface. The thickness of the lamination varies from 0.35 mm for a frequency of 50 Hz to 0.5 mm for a frequency of 25 Hz.

Other necessary parts are

• A suitable container for the assembled core and windings.

• A suitable medium for insulating the core and its windings from each other and from the container.

• Suitable bushings for insulating and bringing the terminals of the windings out of the case.

Types of Transformer

The two basic types of transformer construction are:

1. The core type

2. The shell type.

Another recent development is spiral‒core or wound core type.

1. Core Type Transformer

The completed magnetic circuit of the core type transformer is in the shape of the hollow rectangle, exactly as shown Fig. 3.2.

Here the windings surround the considerable part of the transformer core as shown in Fig. 3.2.

There are two limbs in a core type transformer. The two windings are placed in each limb. In core type transformer there is only one path for the magnetic flux.

In the core type transformer, the individual laminations are cut in long 'L' shape. In order to minimize the air gap in the joints between the strips, the layers are stacked alternately as shown in Fig. 3.3.

2. Shell Type Transformer

In shell type transformer, the steel core surrounds a major part of the windings as shown in Fig. 3.4.

There are three limbs in the shell type transformer. The primary and secondary windings are placed in the central limb as shown in Fig. 3.4.

The windings occupy the entire space in both windows of the core. The entire flux passes through the central limb, but divides between the outside limb. Here the individual laminations are cut in the form of long strips of E and I. After the coils are put in place, the core is butted together and held in place by two clamps one at top and other at bottom.