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Commonly used transformer cores are generally made of silicon steel sheets. Silicon steel is a silicon-containing steel with a silicon content of 0.8 to 4.8%. The iron core of the transformer is made of silicon steel because silicon steel itself is a magnetic substance with strong magnetic permeability. In the energized coil, it can produce a larger magnetic induction intensity, which can reduce the volume of the transformer. The induced current generated in the iron core circulates in a plane perpendicular to the direction of the magnetic flux, so it is called an eddy current.
The role of transformer core
The actual transformer always works in an alternating state, and the power loss is not only in the resistance of the coil, but also in the iron core under the magnetization of the alternating current. The power loss in the iron core is usually called "iron loss". The iron loss is caused by two reasons, one is "hysteresis loss" and the other is "eddy current loss".
Hysteresis loss is the iron loss caused by the presence of hysteresis in the magnetization process of the iron core. The size of this loss is proportional to the size of the area enclosed by the material's hysteresis loop. The hysteresis loop of silicon steel is narrow, and the iron core of the transformer with it has a small hysteresis loss, which can greatly reduce the degree of heat generation.
Since silicon steel has the above advantages, why not use a whole piece of silicon steel as an iron core, but also process it into a sheet?
This is because the sheet iron core can reduce another type of iron loss-"eddy current loss". When the transformer is working, there is alternating current in the coil, and the magnetic flux it produces is of course alternating. This changing magnetic flux induces 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, so it is called an eddy current. Eddy current loss also heats the core. In order to reduce the eddy current loss, the core of the transformer is laminated with silicon steel sheets that are insulated from each other, so that the eddy current flows in a narrow and long loop through a smaller cross-section to increase the resistance of the eddy current circuit; at the same time, the silicon in the silicon steel makes The increase in the resistivity of the material also plays a role in reducing the eddy current.
As the iron core of the transformer, a 0.35mm thick cold-rolled silicon steel sheet is generally selected. According to the required iron core size, it is cut into long slices, and then overlapped into a "day" shape or a "kou" shape. In principle, to reduce the eddy current, the thinner the thickness of the silicon steel sheet, the narrower the spliced sheet and the better the effect. This not only reduces eddy current loss and temperature rise, but also saves silicon steel sheet materials. But when actually making silicon steel sheet iron core. It is not only from the above-mentioned favorable factors, because the production of the iron core in that way will greatly increase the man-hours and reduce the effective cross-section of the iron core. Therefore, when using silicon steel sheets to make transformer cores, it is necessary to start from the specific situation, weigh the pros and cons, and choose the best size.
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