What does BH curve represent for silicon steel sheet?
A BH curve for silicon steel sheet plots flux density B against field strength H, and therefore it predicts how much magnetising current a core will draw. Moreover, the enclosed loop area equals heat generated per cycle. Jiangsu Zhongtong Weiye Special Steel Co., Ltd. supplies coil whose curves you can inspect before ordering. Below is a guided reading of that graph, point by point.
Reading the Curve: Six Points, One Story
Point 1 — the origin. Both axes start at zero, because the sample begins demagnetised. Any offset here suggests residual magnetisation from prior testing.
Point 2 — the initial slope. B climbs fast while H rises slowly, since domain walls move freely. Consequently, this slope defines initial permeability, and steeper is better.
Point 3 — the knee. Around 1.5 T the slope bends, because domain rotation replaces wall motion. Beyond this knee, each extra ampere-turns buys less flux.
Point 4 — saturation. Near 2.0 T the curve flattens almost horizontally. Therefore, extra current yields almost no gain, and windings simply heat.
Point 5 — the return path. On the way down, B lags H, so the descending trace sits above the ascending one. That gap creates the loop.
Point 6 — the enclosed area. Multiply loop area by frequency and you obtain hysteresis loss per kilogram, whereas eddy currents add a separate term.
Each point answers a design question, and together they form a complete magnetic fingerprint.
Permeability Is a Slope, Not a Constant
Permeability equals the local slope, and that value changes continuously along the curve. Near the origin it peaks, then collapses toward saturation. Hence a single permeability number cannot describe a whole core.
Engineers therefore quote two figures. Amplitude permeability suits power devices, because it averages over a full cycle. Incremental permeability suits small-signal work, since it tracks a minor loop. Both come from the same measured data, yet they answer different questions.
Our note on flux density measurement explains how B50 and B8 are read. Separately, the saturation magnetisation analysis covers the upper limit.
Loop Area Equals Heat
A narrow loop means low hysteresis loss, and grain-oriented grades achieve exactly that. Consequently, transformer cores built from oriented sheet run cooler at the same flux.
Non-oriented grades show a wider loop, yet they tolerate rotating flux far better. Therefore, motor designers accept the wider loop as a fair trade.
In practice, the loop narrows when grain size grows and impurities fall. It widens again when stamping stress accumulates, so annealing after punching restores the original shape.
Temperature and Frequency Move the Curve
Heat lowers saturation flux density, because thermal agitation disrupts domain alignment. Moreover, hysteresis loss usually rises with temperature in electrical steel.
Frequency adds eddy current loss, and that term grows with the square of frequency. Subsequently, the dynamic loop becomes fatter and more distorted than the quasistatic one.
Hence request curves measured at your real operating point, not at room temperature and 50 Hz alone. Otherwise, the design margin you calculated may vanish on site.
What to Ask the Mill For
- B-H data at the specified frequency, not just loss figures.
- Saturation induction, quoted as B50 or B8.
- Coercive force, which reveals loop width directly.
- Test method, either Epstein frame per IEC 60404 or a single-sheet tester.
- Sample orientation, because oriented sheet is directional.
Without those five items, two quotations are not comparable. Furthermore, a certificate that lists loss alone hides the loop entirely.
How Grade Choice Follows the Curve
Static cores want a steep slope plus a narrow loop, so grain-oriented grades win. M4 Grain Oriented Electrical Steel delivers that profile for 0.27–0.30 mm cores.
Rotating machines need uniform behaviour in every direction, although the loop widens slightly. M19 Non Oriented Electrical Steel covers mainstream stators, while M36 Non Oriented Electrical Steel serves cost-sensitive frames.
Frequently Asked Questions
Effectively yes, since the loop is the curve traced in both directions.
Higher hysteresis loss, therefore more heat per cycle.
Typically 1.9–2.05 T, although alloying and texture shift it slightly.
Grain size, purity, and coating all alter the shape, so compare like-for-like tests.
Only hysteresis loss. Eddy current loss needs thickness and resistivity data.
What We Do Not Do
- We do not roll, anneal, or coat steel.
- We do not run magnetic testing in-house.
- We do not guarantee performance for your finished design.
Consequently, we pass mill curves through unchanged and document their test conditions.
Specify Your Order
- State the operating flux and frequency.
- Name the grade, or give the loss limit.
- Give gauge and slit width, then confirm tonnage and port.
We reply with stock, price, and available B-H data within one working day.
- Email: vera.liang@ztwysteel.com
- Location: Xishan District, Wuxi, Jiangsu, China
- Supply range: CRGO M4/M5 at 0.23–0.35 mm, CRNGO M19–M47 at 0.35–0.65 mm
- Slitting tolerance: ±0.05 mm
- Documentation: MTC EN 10204 3.1, core loss and B-H data per ASTM A343 / IEC 60404-2
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