Because a sheet-like iron core can reduce another type of iron loss—eddy current loss. When a transformer is working, there is an alternating current in the coil, and the magnetic flux it generates is also alternating. This changing magnetic flux induces a current in the iron core. The induced current generated 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 the core of transformers? Their advantages:
Transformer cores are typically made of silicon steel sheets. Silicon steel is a type of steel containing silicon (silicon is also called silicide), with a silicon content ranging from 0.8% to 4.8%. Silicon steel is used for transformer cores because it is a highly permeable magnetic material. In a current-carrying coil, it can generate a large magnetic induction intensity, thus allowing for a reduction in the size of the transformer.
We know that actual transformers always operate under alternating current, and power losses occur not only in the resistance of the coils but also in the iron core under the magnetization of 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 that occurs in the iron core during magnetization due to the hysteresis phenomenon. 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, resulting in lower hysteresis loss in transformer cores and significantly reducing heat generation.
Given the advantages of silicon steel mentioned above, why not use a single piece of silicon steel for the core instead of processing it into sheets?
This is because the sheet-like iron core can reduce another type of iron loss—eddy current loss. When a transformer is working, there is an alternating current in the coil, and the magnetic flux it generates is also alternating. This changing magnetic flux induces a current in the iron core. The induced current generated 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 loss, the transformer core is made of stacked silicon steel sheets that are insulated from each other, so that the eddy currents pass through a smaller cross-section in a narrow loop, thereby increasing the resistance in the eddy current path; at the same time, the silicon in the silicon steel increases the resistivity of the material, which also helps to reduce eddy currents.
Transformer cores typically use 0.35mm thick cold-rolled silicon steel sheets. These are cut into long strips according to the required core dimensions and then overlapped into a "日" (sun) or "口" (square) shape. In theory, to reduce eddy currents, thinner silicon steel sheets and narrower overlapping strips are better. This not only reduces eddy current losses and temperature rise but also saves on silicon steel material. However, in practice, the manufacturing of silicon steel cores is not solely based on these advantages. Such a method would significantly increase manufacturing time and reduce the effective cross-section of the core. Therefore, when using silicon steel sheets to manufacture transformer cores, the specific circumstances must be considered, weighing the pros and cons to select the optimal dimensions.
