Why we use stepped core in transformer?

 Usually, rectangular core is used for small rated transformer while the coil can be either square or rectangular. 

But for highly rated power transformer, we require circular coils only, so we have to use circular core(stepped core). Circular coil is preferred in power transformer due to its Superior mechanical characteristics. 

On circular coils, the mechanical force acts radially, so the stress is less and shape of coil do not change, however if we use rectangular coils, then the mechanical forces acts perpendicular to the coil results in creating high stress which would deform the coil shape. This is the reason why circular coils are preferred over other coils for power transformer.

 So the reason for circular core is that, if we use rectangular core, then the useful space is not utilized better resulting a air between the coil and core. This affects in increasing the length of mean-turn of the winding which further increase the over all cost of the transformer and and the reluctance of the flux. This is same, if we use square core. 

To avoid the overall losses and increased cost, the best option is to use stepped core, which utilizes the space better than the other type of core and the length of mean-turn of winding is lower compared to rectangular or square cores (shown below in fig.1). 

                               

 Thus for even bigger rated transformer, more steps of core should be engineered for better utilization of the space between the core and coil and for reducing the manufacturing cost (as in fig.2).

These are the different core and coil combination of a transformer. The direction indicates the acting mechanical forces on coil.

Hope  you have learnt something new and usefull.

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