Why are iron cores added to transformers?

The iron core is one of the most critical components of a transformer, and it is an essential element that enables the transformer to operate as intended. It plays a vital role in the fundamental operation of the transformer’s current and voltage regulation.

The iron core also constitutes a high-value part of the transformer, making it prone to theft in many cases. If the iron core is removed, the entire transformer will fail to function properly. So why is an iron core necessary for transformers?

Transformer iron cores are mostly fabricated from silicon steel sheets. Transformers operate under alternating current, and power loss occurs not only in the resistance of the coils but also within the iron core when magnetized by alternating current. Such power loss inside the core is collectively referred to as "core loss", which stems from two factors: hysteresis loss and eddy current loss.

Hysteresis loss is a type of core loss generated by the hysteresis effect during the magnetization of the iron core. The magnitude of this loss is proportional to the area enclosed by the material’s hysteresis loop. Silicon steel features a narrow hysteresis loop; using it for transformer cores significantly reduces hysteresis loss and lowers heat generation. Laminated core sheets mitigate the second form of core loss: eddy current loss.

When a transformer is energized, alternating current flows through its coils and generates alternating magnetic flux. This varying flux induces circulating electric currents within the iron core. These induced currents loop along planes perpendicular to the magnetic flux, hence the name eddy currents, which also heat up the core and cause power loss.

To minimize eddy current loss, transformer cores are assembled from stacked silicon steel sheets insulated from one another. This design forces eddy currents to travel through narrow, elongated conductive paths with smaller cross-sections, raising the resistance of the eddy current loops. Additionally, silicon contained within the steel increases the material’s electrical resistivity, further suppressing eddy currents.

As an indispensable component of transformers, the iron core delivers powerful control over voltage and current. Transformers offer numerous performance advantages, and the integration of an iron core greatly stabilizes current and voltage output while comprehensively boosting the overall operational performance of the device.