Core type and Shell type transformers....

In practical transformer design the manufacturer has the choice between two different basic constructions: 1.Core type Construction. 2.Shell type Construction. The one or the other of these Constructions has no influence on operational characteristics or the service reliability of the transformer, but there are essentially differences in the manufacturing process. Each manufacturer chooses the concept that he finds most convenient from a manufacturing point of view and tends to use this concept for the whole production volume. The basic difference between these two transformers are: 1.The core type has two limbs & shell type has three limbs. 2.Core type has longer mean length of iron core & shorter mean length of coil turn. Shell type has shorter mean length of iron core & longer mean length of coil turn. 3.In core type transformers the LV(low voltage) coil is wound next to the core & HV(high voltage) coil is wound on the LV coil after the insulation layer. In Shell type transformers the LV & HV windings are sandwiched between each other. Transformers used in practice are of extremely large variety depending upon the end use. In addition to the transformers used in power systems, in power transmission and distribution, a large number of special transformers are in use in applications like electronic supplies, rectification, furnaces, traction etc. Here the focus is on power transformers only. The principle of operation of these transformers also is the same but the user requirements differ. Power transformers of smaller sizes could be air cooled while the larger ones are oil cooled. These machines are highly material intensive equipments and are designed to match the applications for best operating conditions. Hence they are ‘tailor made’ to a job. This brings in a very large variety in their constructional features. Here more common constructional aspects alone are discussed. These can be broadly divided into 1. Core construction 2. Winding arrangements 3. Cooling aspects 3.1 Core construction Transformer core for the power frequency application is made of highly permeable material. The high value of permeability helps to give a low reluctance for the path of the flux and the flux lines mostly confine themselves to the iron. Relative permeability μr well over 1000 are achieved by the present day materials. Silicon steel in the form of thin laminations is used for the core material. Over the years progressively better magnetic prop- erties are obtained by going in for Hot rolled non-oriented to Hot rolled grain oriented steel.