Core Cross Section of Transformer and Yoke Cross Section of Transformer

• For small core type transformers simple rectangular section limbs with rectangular coils are used. However for large capacity transformers circular coils are used instead of rectangular coils, as a circle gives reduced length of mean turn (due to minimum perimeter for a particular area) which reduces requirement of conductor material, which in turn reduces cost, weight of transformer, resistance of winding and I²R losses.

• If we use square core with circular coil it is found that lot of useful space is wasted. To solve this problem stepping of core is done.

• A cruciform core uses two stepped core, while as the transformer size increases more number of steps are used which reduces length of mean turn and consequently I²R losses. But stepped construction needs extra cost as different size laminations as required, also time and labour required for assembly increases.

• Hence proper balance need to be achieved between savings in material, increase in the labour cost etc. and optimum number of steps should be used, which will give benefits of less material, reduced losses and economical value. In shell type transformer usually simple rectangular core is used. 

Yoke Cross Section

• It is common practice to increase yoke cross section by 15 to 20 percent than that of the limb area of transformer which uses hot rolled steel. Since no winding is present on the yoke, its cross section is increased to reduce the no load current and iron losses (as area increases flux density decreases which reduces losses and no load current). 
• Further to simplify the core construction and to reduce the labour cost, yoke is not multi stepped like limb, it is either made rectangular or two stepped in cross section. However, this construction increases iron losses slightly due to cross fluxing between core and yoke.

It is to be noted that yoke area is taken equal to core area when CRGO steel is used for the construction of core. This is done because CRGO steels iron loss is low, hence no need to increase its area for reduction in iron loss.