Power transformers work depending upon the principle of mutual induction between two or more magnetically coupled windings. The proper mutual induction requires an efficient linkage of magnetic flux between the windings. Therefore there must be a low reluctance magnetic path for flowing the maximum flux through the windings. The magnetic core serves the purpose. It is made of the material of high permeability. Therefore it allows the maximum magnetic flux to flow through it. Generally, the transformer core is just like a rectangular frame on which the windings are wound.
Transformer Core Material
The material must be of high electrical resistance and of low hysteresis loss. Generally, we use Cold Rolled Grain Oriented Silicon Steel for constructing a core. The grain orientation reduces the hysteresis loss of the power transformer.
Transformer Core Losses
Although steel as a transformer core material reduces the reluctant of the magnetic path and efficiently facilitates the flux to flow through it. But it also introduces some specific losses inside the core. These losses are eddy current loss and hysteresis loss. We call these two losses collectively as transformer core losses. Obviously, core losses affect the efficiency of a power transformer.
Transformer Core Laminations
The core is not generally made of the solid transformer core material that is solid CRGO Silicon Steel. Instead in a number of properly sized, shaped and laminated sheets of CRGO Silicon Steel are assembled together to form the transformer core. This arrangement increases the overall resistance of the core. Therefore it reduces the overall Eddy current loss of the transformer.
Transformer Core Types
The main purpose of a transformer core is to provide a closed magnetic path to the magnetic flux. Without disturbing the closeness of the magnetic path we can design two types of core. These are the core type and shell type. In the core type of design, the windings surround the core. Besides it, the cell type core surrounds the windings.
Both of the transformer core types have their own advantages. The core type of design is more easily repairable on-site. The cell type of design is more robust but it is difficult to repair on site. Less flux leakage takes place in the shell-type core.
Construction of Transformer Core
For small size transformer, we normally use the simple rectangular cross-section of the core limbs. A rectangular limb holds rectangular winding on it. But for a bigger power transformer, it is not practically economical to use rectangular winding. So here we circular winding. Now if we use a circular winding on a rectangular limb, there will be unused spaces in between the edges of the limb and inner periphery of the winding as shown in the figure below.
Stepped Transformer Core
Although, theoretically there will be no problem of using a circular winding on a rectangular limb. But for the better utilization of the space available inside the transformer, we need to have circular cross-sectional limbs of the core. It optimizes the size of a bigger power transformer. Obviously, for a perfectly circular cross-section of limbs, each of the laminations will have a unique different size. Because of that the manufacturing process and cost become quite laborious and high respectively. In these cases, we use a stepped core as shown in the figure below.
An increasing number of steps in the cross-section of the core limb increases the manufacturing cost and labor. Therefore optimum steps are used in the core. For a large power transformer, we use a double-stepped core. Then for a further larger power transformer, we use a triple stepped core. For a very large power transformer, we generally use four or more stepped core.