The reason why transformers use silicon steel sheets for their cores.

To reduce another type of iron loss—"eddy current loss." When a transformer is operating, there is alternating current in the coils, and the magnetic flux it generates is, of course, also alternating. This changing magnetic flux induces a current in the iron core. The induced current in the iron core circulates in a plane perpendicular to the direction of the magnetic flux, hence the name eddy current. Eddy current loss also causes the iron core to heat up.

Why are silicon steel sheets used for transformer cores? Its advantages:

Common transformer cores are generally made of silicon steel sheets. Silicon steel is a type of steel alloyed with silicon (also called silicide), with a silicon content of 0.8-4.8%. Silicon steel is used for transformer cores because it is a highly permeable magnetic material. In an energized coil, it can produce a larger magnetic induction intensity, thus allowing the transformer's size to be reduced.

We know that actual transformers always operate under alternating current conditions. Power loss occurs not only in the resistance of the coils but also in the iron core under magnetization by alternating current. The power loss in the iron core is usually called "iron loss," which is caused by two factors: "hysteresis loss" and "eddy current loss."

Hysteresis loss is the iron loss caused by the hysteresis phenomenon during the magnetization process of the iron core. The magnitude of this loss is directly proportional to the area enclosed by the hysteresis loop of the material. Silicon steel has a narrow hysteresis loop, so using it for transformer cores results in less hysteresis loss, significantly reducing heat generation.

Since silicon steel has the above advantages, why not use a solid block of silicon steel for the core, instead of processing it into sheets?

This is because the laminated core can reduce another type of iron loss—"eddy current loss." When a transformer is operating, there is alternating current in the coils, and the magnetic flux it generates is, of course, also alternating. This changing magnetic flux induces a current in the iron core. The induced current in the iron core circulates in a plane perpendicular to the direction of the magnetic flux, hence the name eddy current. Eddy current loss also causes the iron core to heat up. To reduce eddy current losses, the transformer core is made of stacked silicon steel sheets insulated from each other. This forces the eddy currents to flow in narrow, elongated paths through smaller cross-sections, increasing the resistance in the eddy current path.  Simultaneously, the silicon in the steel increases the material's resistivity, further reducing eddy currents.

Transformer cores are typically made from 0.35mm thick cold-rolled silicon steel sheets. These sheets are cut into rectangular strips according to the required core dimensions and then stacked in an "E" or "I" shape. Theoretically, to minimize eddy currents, thinner silicon steel sheets and narrower strips would be more effective. This would not only reduce eddy current losses and temperature rise but also save on silicon steel material. However, in practice, when manufacturing silicon steel cores, one cannot solely consider these advantages, as doing so would significantly increase manufacturing time and reduce the effective cross-sectional area of the core. Therefore, when making transformer cores from silicon steel sheets, the specific circumstances must be considered, and a balance of advantages and disadvantages must be weighed to select the appropriate dimensions.