Why Transformers Use Laminated Silicon Steel

Why Silicon Steel Sheets Are Used in Transformer Cores – Reducing Eddy Current Loss

Why reduce the other type of iron loss—eddy current loss?
When a transformer operates, alternating current flows through its windings, producing a correspondingly alternating magnetic flux. This changing flux induces currents within the iron core. These induced currents circulate in planes perpendicular to the direction of the magnetic flux, forming closed loops—hence they are called eddy currents. Eddy current losses also cause the core to heat up.

Why Are Transformer Cores Made of Silicon Steel Sheets?

Silicon steel—a steel alloy containing silicon (also known as "silicon" or "Si") with a silicon content between 0.8% and 4.8%—is commonly used for transformer cores. The reason lies in silicon steel’s strong magnetic permeability. As a highly efficient magnetic material, it can produce a high magnetic flux density when energized, allowing transformers to be made more compact.

As we know, real-world transformers operate under alternating current (AC) conditions. Power losses occur not only due to resistance in the windings but also within the iron core due to cyclic magnetization. This core-related power loss is known as "iron loss", which consists of two components:

• Hysteresis loss

• Eddy current loss

Hysteresis loss arises from the magnetic hysteresis phenomenon during the magnetization process of the core. The magnitude of this loss is proportional to the area enclosed by the material’s hysteresis loop. Silicon steel has a narrow hysteresis loop, resulting in lower hysteresis loss and significantly reduced heating.

Given these advantages, why isn’t a solid block of silicon steel used for the core? Why is it instead processed into thin sheets?

The answer is to reduce the second component of iron loss—eddy current loss.

As previously mentioned, the alternating magnetic flux induces eddy currents in the core. To minimize these currents, transformer cores are constructed from thin silicon steel sheets that are insulated from each other and stacked together. This design confines eddy currents to narrow, elongated paths with smaller cross-sectional areas, thereby increasing the electrical resistance along their flow paths. Additionally, the addition of silicon in the alloy increases the electrical resistivity of the material itself, further suppressing eddy current formation.

Typically, transformer cores use cold-rolled silicon steel sheets about 0.35 mm thick. Based on the required core dimensions, these sheets are cut into long strips and then stacked in “日” (double-window) or single-window configurations.

In theory, the thinner the sheet and the narrower the strips, the smaller the eddy current loss—resulting in lower temperature rise and reduced material usage. However, in actual manufacturing, designers do not optimize solely based on minimizing eddy currents. Using extremely thin or narrow strips would greatly increase production time and labor while reducing the effective cross-sectional area of the core. Therefore, when fabricating silicon steel cores, engineers must carefully balance technical performance, manufacturing efficiency, and cost to select the optimal dimensions.