A transformer consists of three major parts namely a primary winding, a secondary winding and a magnetic core. The primary winding is one that used to input the supply and secondary winding is one that used to take output. The magnetic core is used to confine the magnetic flux to a definite path.

We design a transformer in such a way that it approaches the characteristics of an ideal transformer. In practice, we incorporate the following design features for transformer construction −

• The core of the transformer is made up of high grade silicon steel which has high permeability and low hysteresis loss.

• The core is laminated to minimize the eddy current loss.

• It is a usual and more efficient practice to wind one-half of the primary and secondary windings on one limb instead of placing primary on one limb and secondary on the other. This ensures tight magnetic coupling between the two windings and hence reduces the leakage flux considerably.

• The winding resistances R₁ and R₂ are reduced as much as possible so that they cause lowest I²R loss and temperature rise and ensure higher efficiency.

Transformer Construction

A transformer can be constructed in the following two ways −

• Core Type Transformer Construction

• Shell Type Transformer Construction

Core Type Construction of Transformer

In the core type construction of the transformer, the magnetic core has two vertical lags (called limbs) and two horizontal sections (called yokes). The half of the primary winding and the half of the secondary winding are placed around each limb as shown in Figure-1.

This arrangement of windings minimizes the leakage flux. In practice, the low-voltage winding (it could be primary or secondary) is placed next to the core and the high-voltage winding is placed around the low-voltage winding. This considerably reduces the requirement of insulating material.

The main advantage of the core-type construction of transformers is that it is easier to dismantle for repair and maintenance. The core-type construction is most suitable for high-voltage and high-power transformers because in the core type construction, the nature cooling is more efficient.

Shell Type Construction of Transformer

In the shell-type construction of transformers, both primary and secondary windings are wound on the central limb, while the two outer limbs complete the low reluctance flux paths as shown in Figure-2.

In this case, each winding is sub-divided into sections, and the low-voltage (lv) winding sections and high-voltage (hv) winding sections are alternatively put in the form of a sandwich. Therefore, this type of winding is also called as sandwich winding or disc winding.

The shell-type construction of transformers provides better mechanical support against electromagnetic forces between the current-carrying windings. Also, this transformer construction provides a shorter path for magnetic flux and hence requires small magnetizing current. The shell-type construction is more suitable for low voltage transformers because of poor nature cooling due to the embedding of the windings.