Construction of Single Phase Transformers

Construction of Single Phase Transformers

AU: Dec.-08, May-13

The various constructions used for the single phase transformers are,

1. Core type 2. Shell type and 3. Berry type

1. Core Type Transformer

• It has a single magnetic circuit. The core is rectangular having two limbs. The winding encircles the core. The coils used are of cylindrical type. As mentioned earlier, the coils are wound in helical layers with different layers insulated from each other by paper or mica. Both the coils are placed on both the limbs. The low voltage coil is placed inside near the core while high voltage coil surrounds the low voltage coil. Core is made up of large number of thin laminations.

• As the windings are uniformly distributed over the two limbs, the natural cooling is more effective. The coils can be easily removed by removing the laminations of the top yoke, for maintenance.

• The Fig. 6.4.1 (a) shows the schematic representation of the core type transformer while the Fig. 6.4.1 (b) shows the view of actual construction of the core type transformer.

2. Shell Type Transformer

• It has a double magnetic circuit. The core has three limbs. Both the windings are placed on the central limb. The core encircles most part of the windings. The coils used are generally multilayer disc type or sandwich coils. As mentioned earlier, each high voltage coil is in between two low voltage coils and low voltage coils are nearest to top and bottom of the yokes.

• The core is laminated. While arranging the laminations of the core, the care is taken that all the joints at alternate layers are staggered. This is done to avoid narrow air gap at the joint, right through the cross-section of the core. Such joints are called over lapped or imbricated joints. Generally for very high voltage transformers, the shell type construction is preferred. As the windings are surrounded by the core, the natural cooling does not exist. For removing any winding for maintenance, large number of laminations required to be removed.

The Fig. 6.4.2 (a) shows the schematic representation while the Fig. 6.4.2 (b) shows the outaway view of the construction of the shell type transformer.

3. Berry Type Transformer

• This has distributed magnetic circuit. The number of independent magnetic circuits are more than 2. Its core construction is like spokes of a wheel. Otherwise it is symmetrical to that of shell type.

• Diagramatically it can be shown as in the Fig. 6.4.3.

4. Comparison of Core and Shell Type Transformers

The comparison of core type and shell type transformers is given in the Table 6.4.1.

Review Questions

1. Explain the constructional details of core and shell type transformers. AU: Dec.-08, May-13, Marks 8

2. State the functions of oil used in transformer.

3. Discuss the difference between core type and shell type of construction.