The magnetic performance of silicon steel sheets, including core loss, saturation magnetic flux density, and permeability, is closely related to temperature. This relationship is crucial for the application of silicon steel sheets.
Definition of Silicon Steel Sheets
Silicon steel sheets are a low‑carbon silicon‑iron soft magnetic alloy, typically containing 0.5%–4.5% silicon. They feature high resistivity, high permeability, and low core loss, and are primarily used to manufacture the cores of transformers, electric motors, generators, and other electrical equipment, thereby improving efficiency and performance. Silicon steel sheets are divided into grain‑oriented (GO) and non‑grain‑oriented (NGO) types.
Key Magnetic Performance Indicators of Silicon Steel Sheets
The magnetic performance indicators of silicon steel sheets are critical parameters for evaluating their performance, mainly including:
| Magnetic flux density (B): Represents the magnetic flux per unit volume, reflecting the material’s magnetization capability. Higher B reduces excitation current and increases power density.
| Core loss: Energy dissipated as heat when the sheet is magnetized in an alternating magnetic field. Lower core loss indicates better material performance and higher equipment efficiency.
| Thickness: Directly influences eddy‑current loss, which is proportional to the square of the sheet thickness. Thin sheets are preferred for high‑frequency applications, while thicker sheets can be used at low frequencies.
| Stacking factor: Ratio of the net volume of effective magnetic material to the total volume (including insulation and gaps) in a core. A higher stacking factor reduces magnetic reluctance, improves permeability, and enhances efficiency.
| Relative permeability (μr): Describes how easily the material can be magnetized. Most silicon steel sheets exhibit nonlinear behavior and require a magnetization curve to describe saturation characteristics.
Silicon Steel Sheets and Temperature
Core Loss (Pt)
Total core loss (Pt) of silicon steel sheets consists of hysteresis loss (Ph) and eddy‑current loss (Pe).
| Hysteresis loss (Ph) decreases significantly with rising temperature, becoming the dominant factor in the temperature dependence of core loss. The reduction is due to decreased magnetocrystalline anisotropy constants and stress relaxation, which facilitate domain‑wall movement and lower the energy required for magnetization (i.e., a smaller hysteresis loop area).
| Eddy‑current loss (Pe) remains roughly constant or changes only slightly. While the resistivity (ρ) of silicon steel increases with temperature (positive temperature coefficient), which tends to reduce Pe, the simultaneous decrease of saturation flux density (Bs) raises the required dH/dt (or dB/dt) for a given flux change, potentially increasing Pe. These opposing effects largely cancel each other out.
Overall trend of Pt
Above room temperature (typically >0 °C), the pronounced drop in Ph leads to a net decrease in Pt with temperature, which is beneficial for devices operating at elevated temperatures—the actual core loss at operating temperature is often lower than the nominal value measured at room temperature.
Below room temperature (typically <0 °C), Ph increases, causing Pt to rise as temperature falls. This must be considered for equipment used in cold environments.
The minimum Pt usually occurs between 50 °C and 100 °C (depending on grade and composition). Beyond this range, although Ph continues to fall, the reduction of Bs and possible increase of Pe or abnormal losses cause Pt to level off or slightly rise. Within the common operating window (80 °C–150 °C), Pt generally remains below its room‑temperature value.
Saturation Magnetic Flux Density (Bs)
Bs decreases with temperature. Higher thermal agitation disrupts the ordered alignment of magnetic moments, lowering the maximum flux density achievable at saturation.
Approximate rate: about a 0.2% reduction per 1°C increase (exact values vary with grade).
Curie temperature (Tc): Around 730°C–750°C for silicon steel; above this, the material loses ferromagnetism and becomes paramagnetic, causing Bs to drop to zero. This marks the absolute upper temperature limit for silicon‑steel applications.
Permeability (μ)
The relationship is more complex, influenced by temperature, magnetic field strength, and steel grade.
Initial permeability (μi) and maximum permeability (μm):
At low to moderate temperatures, stress relaxation and reduced anisotropy increase domain‑wall mobility, often raising permeability.
At higher temperatures, the sharp decline of Bs requires larger H to achieve the same B, leading to a decrease in permeability. Near the Curie point, the loss of ferromagnetism causes a rapid drop.
For typical GO and NGO silicon steel sheets within the normal operating range (room temperature to ~150 °C), permeability usually shows a slight rise at lower temperatures followed by a gradual decline at higher temperatures. The exact magnitude depends on the specific grade and the measurement point on the B‑H curve.
Engineering Significance of the Temperature‑Magnetic Performance Relationship
| Core‑loss reduction (above room temperature): The most important impact. Motors and transformers operating at their rated load experience lower actual core loss at operating temperatures (e.g., 80°C–150°C) than the nominal loss measured at 20°C–25°C, improving overall efficiency. Design calculations must incorporate temperature coefficients.
| Decrease of saturation flux density: Limits the maximum output capability at high temperatures. Designers must ensure that, at the highest expected operating temperature, the core flux density of silicon steel sheets stays below the temperature‑adjusted Bs to avoid excessive saturation, which would cause a surge in excitation current and higher losses.
| Permeability variation: Affects excitation (no‑load) current. Within the usual temperature range, the effect is less pronounced than core loss and Bs, but it still needs consideration for precise modeling and high‑performance designs.
| Low‑temperature effects: In cold environments (e.g., wind turbines, electric vehicles in winter), increased core loss reduces efficiency, and excitation characteristics may change, requiring design accommodations.
| Temperature‑rise management: Although core loss may drop with temperature, copper loss and other losses increase due to higher winding resistance. The final steady‑state temperature rise results from the balance of all losses and cooling conditions, so thermal design must account for the temperature‑dependent material properties.
| Measurement standards: To enable consistent comparison, magnetic parameters—especially core loss—are typically specified at a standard temperature (e.g., 20°C, 23°C, or 25°C).
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