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Construction of a Transformer – Core Type and Shell Type
The transformers are designed such that it approaches the characteristics of an ideal transformer. For this, following design considerations are incorporated
- The core is made of magnetic materials (like silicon steel) which have high permeability and low hysteresis loss. Also, the core is made laminated to reduce the eddy current loss. Therefore, by doing these the iron losses or core losses can be reduced considerably and hence the no-load current.
- Instead of placing primary winding on one limb and the secondary winding on the other limb, in practice, the one half of each winding is wound on one limb and the other half of each winding on the second limb. This ensures tight magnetic coupling between the two windings. As a result, the leakage flux is greatly reduced in the transformer.
- The resistances of the transformer windings R1 and R2 are minimised to reduce the I2R losses and hence the temperature rise of the transformer.
By incorporating these constructional features, the efficiency of the transformer is increased considerably.
A transformer consists of primary and secondary windings put on a magnetic core. The magnetic core is used to provide a definite path to the magnetic flux. The core of the transformer is made from thin sheets, called laminations, of silicon steel. The laminations are insulated from one another by the layer of heat resistant enamel insulation coating and are clamped together.
Depending upon the arrangement of the primary and secondary windings on the core, the transformers are of two types
- Core Type Construction
- Shell Type Construction
Core Type Transformer Construction
In core type construction of the transformer, the magnetic core consists of two vertical lags called limbs and two horizontal sections called yokes. In order to reduce the leakage flux to its minimum value, half of each winding is placed on each lag of the core (see the figure).
The low voltage (lv) winding is placed next to the core and the high voltage (hv) winding is placed around the low voltage winding. This reduces the requirement of insulating material. Hence, the primary and secondary windings are arranged as concentric coils, thus known as concentric winding or cylindrical winding.
The core type construction of transformer is easier to dismantle for maintenance. The natural cooling is good in the core type transformer. Therefore, core type transformers are suitable for high voltage and small output applications.
Shell Type Transformer Construction
In the shell type construction of the transformer, the magnetic core consists of three vertical lags and two horizontal sections. Both the primary and secondary windings are wound on the central limb and the two outer limbs provide the low reluctance flux path (see the figure).
Therefore, the shell type construction involves the use of a double magnetic circuit. The low voltage (lv) winding is placed next to the core (on the central limb) and around the low voltage winding the high voltage (hv) winding is placed. This arrangement reduces the requirement of insulating material.
The shell type construction of the transformer provides better support against the electromagnetic forces between the current carrying conductors, which are very high under short circuit conditions.
In shell type transformers, a shorter magnetic path is available, hence it requires a small magnetising current. The natural cooling is poor in shell type transformer, because the coils are placed on the central limb. The shell type transformers are mainly used in low voltage and high output applications.
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