The power loss in the iron core includes "hysteresis loss" and "eddy current loss". Hysteresis loss is the energy consumed by the repeated magnetized domains in the alternating magnetic field. The hysteresis loss per unit volume is proportional to the area of the hysteresis loop and the frequency f of the alternating magnetic field (V is the volume of the magnetic conductor). Therefore, the hysteresis loss of the iron core can be reduced by selecting magnetic materials with narrow hysteresis loops, and the hysteresis loss of materials such as electrical steel used in transformers and motors is low.

Eddy current loss The alternating magnetic flux generates induced electromotive force in the  iron core and causes a circulation called eddy current, which circulates in the plane perpendicular to the magnetic flux, and the power loss generated by eddy current; Therefore, the eddy current loss of the iron core can be reduced by increasing the resistance of the iron core . The electrical steel sheet insulated by the core is stacked to make the eddy current increase the resistance on the eddy current path through a small section in the narrow and long circuit. At the same time, the silicon in the silicon steel increases the resistivity of the material and also plays a role in reducing the eddy current.

The biggest reason for choosing a square iron core is the convenience of winding the coil, the use of this shape of the iron core can be wound alone in the machine, and then set on the iron core, suitable for mass production in the actual project, of course, this shape of the iron core also has shortcomings, that is, there is an air gap between the lamination resulting in magnetic leakage.

In fact, there is also a circular iron core, this iron core is made of electrical steel strip directly rolled iron core, no air gap, small magnetic leakage, magnetic properties are better than square; However, due to its coil winding trouble, not easy to mechanized mass production, fewer applications, usually used for precision current transformers (different from power transformers, transformers are usually used for measurement).