Main Functions of Transformer Cores

Primary Functions of Transformer Cores:

Practical transformers always operate under alternating current (AC) conditions. Power losses occur not only in the resistance of the coils but also within the core as it is magnetized by the alternating current. A transformer core consists of two main parts: the core legs and the yokes. The coils (windings) are mounted on the core legs, while the yokes serve to close the entire magnetic circuit. To reduce hysteresis and eddy current losses within the core, it is typically constructed by stacking silicon steel laminations according to specific rules. These laminations contain approximately 5% silicon, have a thickness of 0.35 to 0.50 mm, and are coated on both sides with an insulating varnish layer 0.01 to 0.13 mm thick.

Hysteresis loss is a type of iron loss generated during the magnetization process due to the hysteresis phenomenon. The magnitude of this loss is directly proportional to the area enclosed by the material's hysteresis loop. Because silicon steel features a narrow hysteresis loop, using it for transformer cores results in lower hysteresis losses, which significantly reduces the core's heating.

Laminated cores also help minimize another type of iron loss: eddy current loss. During transformer operation, the alternating current in the coils generates an alternating magnetic flux. This changing flux induces currents within the core. These induced currents circulate in planes perpendicular to the direction of the magnetic flux, and are therefore known as eddy currents. Eddy current losses similarly cause the core to heat up. To reduce eddy current losses, the transformer core is built from mutually insulated silicon steel laminations. This forces the eddy currents to flow through narrow, elongated loops with smaller cross-sectional areas, thereby increasing the electrical resistance along the eddy current paths. Additionally, the silicon in the steel increases the electrical resistivity of the material, which further helps to reduce eddy currents.