الجمعة، 3 يونيو 2011

Construction of Single Phase Transrformer

       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 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. 1(a) shows the schematic representation of the core type transformer while the Fig 1(b) shows the view of actual construction of the core type transformer.
Fig.  1  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 tow 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 joint. 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 laimnations are required to be removed.
       The Fig. 2(a) shows the schematic representation while the Fig. 2(b) shows the outaway view of the construction of the shell type transformer.
Fig 2  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 Fug. 3.
Fig. 3  Berry type transformer
       The transformers are generally kept in tightly fitted sheet metal tanks. The tanks are constructed of specified high quality steel plate cut, formed and welded into the rigid structures. All the joints are painted with a solution of light blue chalk which turns dark in the presence of oil, disclosing even the minutes leaks. The tanks are filled with the special insulating oil. The entire transformer assembly is immersed in the oil. Oil serves two functions : i) Keeps the coil cool by circulation and   ii) Provides the transformers an additional insulation.
       The oil should be absolutely free from alkalies, sulphur and specially from moisture. Presence of very small moisture lowers the dielectric strength of oil, affecting its performance badly. Hence the tanks are sealed air tight to avoid the contact of oil with atmospheric air and moisture. In large transformers, the chambers called breather are provided. The breathers prevent the atmospheric moisture to pass on to the oil. The breathers contain the silica gel crystal which immediately absorb the atmospheric moisture. Due to long and continuous use, the sludge is formed in the oil which can contaminate the oil. Hence to keep such sludge separate from the oil in main tank, an air tight metal drum is provided, which is placed on the top of tank. This is called conservator.     

4. Comparison of Core and Shell Type Transformers 

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