There are two main types of transformers which can be categorized by the shape of the magnetic core. These are core type transformers, and shell-type transformers (and less commonly, air core transformers). We shall discuss here the core type transformer.
The core of single phase core type transformer is a single window core. This means there are two limbs as shown in the figure below.
Both LV and HV windings are wrapped on both limbs. Normally both windings are divided into parts and one part of both windings are wrapped on one limb and other parts or halves are wrapped on the other limb of the core.
The LV winding is first wrapped on the core limb surface with sufficient insulation between the core body and winding. Then the LV winding, HV winding is wrapped with sufficient insulation between LV winding and HV winding. In this way, both limbs are covered with LV then HV windings.
This arrangement makes the transformer economical. Since the potential difference between grounded core and HV winding is maximum so insulation cost between them is also maximum. To avoid this insulation cost the LV winding is placed nearer to the core.
In smaller size core type transformer the core of square or rectangular cross-section is used. Because it is easier to construct laminated square or rectangular cross-sectional core. It is not also hard to wrap winding on that square or rectangular cross-sectional core limbs. This is a cost-effective process of manufacturing smaller transformers.
But in case of large transformers, it is not easy to wrap the winding conductor in square or rectangular forms. Also to optimise the use of copper conductor, the round cylindrical shaped windings are the best choice. But round cylindrical winding on a square cross-sectional core limb causes a significant amount of unused space in between winding and core. This is not a good approach for large transformers as the size of the transformer matters. To reduce these unused spaces, stepped cross-sectional core is utilised where laminations of different dimensions are carefully staged to form nearly circular cross-section. The cross section may be one-stepped, two-stepped, and multi-stepped depending on the size, design and economical optimization of the transformer.
The main disadvantage of core type transformer is the leakage flux. The leakage flux in this arrangement is more than that of shell type transformer. It affects the performance and efficiency of the transformer but still this arrangement is best approach for large sized transformers because of easier access to the windings during maintenance. If any permanent damage occurs in the inner winding, it can be rectified by removing outer winding only which is not so smooth in the case of shell type transformer.
For three phase transformer, there are three limbs in the core. Each limb carriers LV and HV windings of one phase of the three phase system. The construction of core of a three phase transformer is shown below.