Main Functions of Transformer Cores

Main Functions of Transformer Cores:

Since transformers always operate under alternating current (AC), power losses occur not only in the resistance of the windings but also in the core due to magnetization by the alternating current. The transformer core consists of two parts: the core limbs (or core columns) and the yokes. The windings (coils) are mounted on the core limbs, while the yokes are used to complete the magnetic circuit. To reduce hysteresis and eddy current losses within the core, it is typically assembled by stacking silicon steel sheets according to specific rules. These sheets contain about 5% silicon, have a thickness of 0.35 to 0.50 mm, and are coated on both sides with insulating varnish 0.01 to 0.13 mm thick.

Hysteresis loss is the iron loss generated during the magnetization process due to the phenomenon of magnetic hysteresis. The magnitude of this loss is proportional to the area enclosed by the material's hysteresis loop. Since silicon steel has a narrow hysteresis loop, using it for the transformer core results in relatively small hysteresis loss, which significantly reduces the degree of heating. Additionally, the laminated (sheet) structure of the core helps minimize another type of iron loss—eddy current loss. When the transformer is in operation, the alternating current in the windings naturally produces an alternating magnetic flux. This changing magnetic flux induces a current within the core. This induced current circulates within planes perpendicular to the direction of the magnetic flux, hence the name "eddy current." Eddy current loss also causes the core to heat up. To reduce eddy current loss, the transformer core is stacked using mutually insulated silicon steel sheets. This forces the eddy currents to flow through narrow, elongated paths with smaller cross-sectional areas, thereby increasing the electrical resistance along the eddy current path. At the same time, the silicon content in the steel increases the material's resistivity, which also serves to reduce eddy currents.