Construction of Transformer

CONSTRUCTION OF TRANSFORMER

The simple elements of a transformer consists of two coils having mutual inductance and a laminated steel core. The two coils are insulated from each other and the steel core other necessary parts are core and windings. In all types of transformers, the core is constructed of transformer sheet steel laminations assembled to provide a continuous magnetic path with a minimum of air-gap included.

The steel used is of high silicon content, sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the laminations being insulated from each other by a light coat of core-plate varnish (or) by an oxide layer on the surface. The thickness of laminations varies from 0.35 mm to 0.5 mm. The core laminations are joined as shown in Figure 3.42 (b).

Constructionally, the transformers are of two general types, distinguished from each other by in which the primary and secondary coils are placed around the laminated core. The two types are known as

(i) Core type

(ii) Shell type

In both core and shell type transformers, the individual laminations are cut in the form of long strips of 'L', 'E' and 'I' as shown in Figure 2.30.

The assembly of the complete core for the two types of transformers as shown on Figure 3.43.

Core-Type Transformer

The coils used are form-wound and are of the cylindrical type. The general form of these coils may be circular (or) oval (or) rectangular. In small size core-type transformers, a simple rectangular core is used with cylindrical coils which are either circular (or) rectangular in form.

But for large size core type transformers, round (or) circular cylindrical coils are used which are so wound as to fit over a cruciform core section as shown in Figure 2.32.

The circular cylindrical coils are used in most of core-type transformer because of their mechanical strength. Such cylindrical coils are wound in helical layers with the different layers insulated from each other by paper, cloth, mica board (or) cooling ducts.

Shell-Type Transformers

In these case also, the coils are form-wound but are multi-layer disc type usually wound in the form of pancakes. The different layers of such multi-layer disc are insulated from each other by paper. The complete winding consists of stacked disc with insulation space between the coils-the spaces forming horizontal cooling and insulating ducts. A shell type transformer may have a simple rectangular form as shown in Figure 2.33.

A very commonly used shell type transformer is the one known as Berry Transformer so called after the name of its designer and is cylindrical in form. The transformer core consists of laminations arranged in groups which radiate out from the centre as shown in Figure 3.47.

It may be pointed out that cores and coils of transformers must be provided with rigid mechanical bracing in order to prevent movement and possible insulation damage.

Good bracing reduces vibration and the objectionable noise-a humming sound- during operation.

The spiral-core transformer employs the newest development in core construction. The core is assembled of a continuous strip (or) ribbon of transformer steel wound in the form of a circular (or) elliptical cylinder. Such construction allows the core flux to follow the grain of the iron.

Transformers are generally housed in lightly-fitted sheet-metal, tank filled with special insulating oil. This oil has been highly developed and its function is two-fold.