The dəmir əsas is the main part of the magnetic circuit in the transformer. It is usually composed of hot-rolled or cold-rolled silicon steel sheets with high silicon content, thickness of 0.35 or 0.5 mm, and coated with insulating paint. The dəmir əsas is divided into two parts: an iron core column and an iron yoke, and the iron core column is sleeved with windings; the iron yoke is used to close the magnetic circuit.

There are two basic forms of core structure: core type and shell type.

Shell-type transformer, the iron core is made of high-quality, non-aging, cold-rolled, grain-oriented, high-permeability silicon steel sheets. There are two parallel magnetic circuits, and the iron core is arranged horizontally, so that the iron core can surround the coil. Shell transformers have a core with regular width silicon steel perforations stacked on the bottom tank flange to form lap joints, the upper tank is lowered to the wire core and coil assembly, which can then be welded to the bottom tank to achieve the fixation of the iron core. The iron core is also fastened to the external equipment due to the tension of the side plate of the fuel tank, which reduces noise and eliminates the possibility of loosening during operation; at the same time, the narrow iron core greatly improves the cooling effect. Different from the traditional core-type transformer, the shell-type transformer has an outer iron-type structure, and the iron core does not need to be equipped with oil passages to improve the cooling effect of the iron core.