Introduction to the Materials of Transformer Cores

Introduction to Transformer Core Materials

The transformer core is composed of silicon steel sheets. To reduce power loss (heat generation) within the core, it is built by stacking silicon steel sheets with a thickness of 0.23 to 0.5 mm. The silicon steel sheets used in transformers have a very high silicon content. Both sides of each sheet are coated with insulating varnish, which isolates the stacked sheets from one another. The thickness of this insulating varnish is only a few micrometers (μm).

Cold-rolled steel sheets are now widely used in power transformers. Compared to the hot-rolled steel sheets used in the past, they significantly reduce the weight and dimensions of the transformer for the same capacity (rating). This is because cold-rolled steel sheets exhibit very high magnetic permeability when magnetized along the rolling direction.

Silicon steel sheets belong to the category of soft magnetic materials. Magnetic materials possess anisotropy and various distinct characteristics. This means they have one or more directions with superior magnetic properties. When magnetized to saturation along these directions, the required energy is minimal; therefore, these directions are called "easy axes of magnetization." The energy required to saturate the material in other directions exceeds that of the easy axes. Consequently, when using various magnetic materials, it is crucial to pay attention to their specific magnetic orientation. Different magnetic materials demonstrate distinct advantages in their magnetic performance.

Silicon steel sheets are one of the key materials for manufacturing transformers, and their performance directly affects the transformer's overall performance and geometric dimensions. The properties of the silicon steel sheets must be clearly defined first, followed by determining the core limb diameter, cross-sectional area, and yoke cross-sectional area. The final specifications of the core are determined after the winding calculations are completed.

The silicon steel sheets used in cores include hot-rolled types (such as DR315-50, DR290-50) and cold-rolled types. The former has poorer performance and higher specific loss (loss per unit), while the latter offers better performance and lower specific loss, making it the preferred choice for low-loss transformers. Modern low-loss transformers utilize high-quality, grain-oriented cold-rolled silicon steel sheets.