Commonly used transformer cores are generally made of silicon steel sheets. Silicon steel is a steel alloy containing silicon (also known as "xī" in Chinese), with a silicon content ranging from 0.8% to 4.8%. Using silicon steel for transformer cores is because silicon steel itself is a ferromagnetic material with strong magnetic permeability. In energized coils, it can generate a larger magnetic induction intensity, thereby reducing the transformer's volume.
As we know, actual transformers operate under alternating current conditions. Power loss occurs not only in the resistance of the coils but also in the core under alternating current magnetization. The power loss in the core is commonly called "iron loss," which is caused by two factors: "hysteresis loss" and "eddy current loss."
Hysteresis loss is the iron loss generated in the core during the magnetization process due to magnetic hysteresis. The magnitude of this loss is proportional to the area enclosed by the material's hysteresis loop. Silicon steel has a narrow hysteresis loop, so using it for transformer cores results in lower hysteresis loss, significantly reducing heat generation.
Given these advantages of silicon steel, why isn't a solid block of silicon steel used for cores instead of processing it into sheets? This is because laminated cores can reduce another type of iron loss-"eddy current loss."
When a transformer operates, the alternating current in the coils generates an alternating magnetic flux. This changing flux induces currents in the core. These induced currents circulate in planes perpendicular to the magnetic flux direction, hence the term "eddy currents." Eddy current loss also causes the core to heat up. To minimize eddy current loss, transformer cores are constructed by stacking insulated silicon steel sheets. This creates narrow current paths with smaller cross-sectional areas, increasing the resistance along the eddy current paths. Additionally, the silicon in the steel increases the material's resistivity, further reducing eddy currents.
Transformer cores typically use 0.35mm thick cold-rolled silicon steel sheets cut into rectangular strips and stacked into "日" (ri) or "口" (kou) shapes. In theory, thinner sheets and narrower strips would better reduce eddy currents, lowering both losses and temperature rise while saving material. However, in practice, core manufacturing must balance these benefits against increased labor costs and reduced effective core cross-section. Therefore, optimal dimensions are chosen based on specific requirements.
Transformers operate on the principle of electromagnetic induction. Two windings-a primary and a secondary-are wound around a closed iron core. When an AC voltage is applied to the primary winding, alternating current flows, generating a magnetomotive force. This produces an alternating main flux in the core.
Regarding voltage transformation, Lenz's Law explains that the induced current's flux opposes the change in the original flux. When the primary flux increases, the secondary winding generates an opposing flux, resulting in a lower-grade alternating voltage. Thus, the core serves as the magnetic circuit of the transformer.
Silicon Steel Sheets
Electrician's silicon steel 薄板 (thin sheets) are commonly known as "xī gāng piàn" or silicon steel sheets. These contain 0.8%–4.8% silicon and are produced via hot or cold rolling. With thicknesses under 1mm, they form an independent category due to their specialized applications in electrical engineering. Silicon steel sheets exhibit excellent electromagnetic properties, making them indispensable in power, telecommunications, and instrumentation industries.
(1) Classification of Silicon Steel Sheets
A. Silicon Content:
Low-silicon sheets (≤2.8% Si): Used for motors due to their mechanical strength.
High-silicon sheets (2.8%–4.8% Si): Brittle but magnetically superior, used for transformer cores.
B. Manufacturing Process:
Hot-rolled: Being phased out in favor of cold-rolled varieties.
Cold-rolled:
Grain-oriented: For transformers and high-efficiency applications.
Grain-unoriented: For motors and general-purpose devices.
(2) Performance Indicators
A. Low Iron Loss: The primary quality metric, with lower loss indicating higher-grade material.
B. High Magnetic Induction: Reduces core size and material usage.
C. High Stacking Factor: Achieved through smooth, uniform sheet surfaces.
D. Good Stampability: Critical for small motor cores.
E. Surface Insulation & Weldability: Ensures reliable performance.
F. Magnetic Aging Resistance.
G. Annealed & Pickled: Standard post-processing for delivery.
(一) Hot-Rolled Electrical Silicon Steel Sheets (GB5212-85)
Made from low-carbon silicon-iron alloy, these sheets are hot-rolled to <1mm thickness. Divided into low-silicon (≤2.8%) and high-silicon (≤4.8%) grades.
(二) Cold-Rolled Electrical Silicon Steel Sheets (GB2521-88)
Produced via cold rolling with 0.8%–4.8% silicon content. Available in grain-oriented (for transformers) and grain-unoriented (for motors) types. Offer 10%–25% weight/volume reduction compared to hot-rolled sheets.
(三) Household Appliance Hot-Rolled Silicon Steel Sheets (GBH46002-90)
Denoted by "JDR" (Jia Dian Ri). Graded by iron loss (e.g., JDR540-50 for 5.40W/kg loss at 0.50mm thickness). Used in small motors for appliances like fans and washing machines.
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