1. Core Loss, Defined
Core loss — also called iron loss — is the electrical energy converted to heat inside a magnetic core every time the magnetic field reverses. It is the reason a transformer hums warm and a motor runs warm even when doing no mechanical work. For a buyer, it is the single most direct quality number on an electrical steel certificate, and it is what the grade is priced on.
1.1 Hysteresis loss
Every magnetization cycle traces a hysteresis loop. The area inside the loop is energy — the work required to move the magnetic domains back and forth — and it is lost as heat on every cycle. Hysteresis loss is proportional to frequency and to the loop area, which is small when the steel is clean, the grain size is controlled, and the domain walls move easily. Impurities, inclusions and internal stress all enlarge the loop and raise the loss.
1.2 Eddy current loss
An alternating flux induces circulating currents inside the steel, exactly like currents in a transformer winding — and like any current in a resistor, they heat the material. Eddy loss grows with the square of frequency and with the square of lamination thickness, which is why electrical steel is delivered thin (0.35, 0.50 mm and less) and why going thinner cuts loss so effectively. Raising silicon content raises electrical resistivity, which shrinks the eddy currents — the second big lever.
2. How Core Loss Is Measured
2.1 The Epstein frame
The reference method builds a square magnetic circuit from overlapping strips cut from the sheet — typically with half the strips along the rolling direction and half across it — wound with magnetizing and sensing windings. The frame measures the power loss of the stack under sinusoidal flux at a controlled induction and frequency. Epstein results are the numbers that appear on certificates and in standards, and they are the numbers behind the grade name.
2.2 The single sheet tester
Production control uses the single sheet tester (SST), which magnetizes one full-width sheet between yoke poles. It is faster and needs far less material than an Epstein frame, which is why mills use it for coil-to-coil testing. The two methods agree closely when calibrated to the same conditions, but they are not identical — an SST reading and an Epstein reading on the same material can differ by a small margin, so compare like for like.
2.3 Reading the units
Core loss is reported as W/kg at a stated induction and frequency. The standard reference in the Chinese naming system is 1.5 T at 50 Hz — written P1.5/50. Some grades are also quoted at 1.0 T (P1.0/50) or at 400 Hz for high-frequency applications. The induction and frequency are not optional decoration: a 4.70 W/kg figure at 1.5 T/50 Hz is meaningless without its conditions, and you cannot compare two grades unless the conditions match.
3. Reading a Grade Name: 50WW470
The GB naming convention for non-oriented electrical steel packs three facts into one string:
- 50 — nominal thickness, 0.50 mm (35WW300 is 0.35 mm).
- WW — 无取向, non-oriented (the grain-oriented family uses different designators).
- 470 — guaranteed core loss, 4.70 W/kg, at the reference condition (1.5 T, 50 Hz).
Lower loss numbers mean better magnetic quality and higher price. A 50WW270 costs more than a 50WW470 for a reason: it saves real watts in the machine it goes into.
4. Grade Comparison Table
Typical values for common non-oriented grades in the 0.35 and 0.50 mm classes — the original data point of this guide. Guaranteed limits and typical magnetic induction are per the governing standard; confirm the exact certified values on the mill certificate.
| Grade | Thickness (mm) | Core loss P1.5/50, typ. guarantee (W/kg) | Magnetic induction B50 (T, typ.) | Typical use |
|---|---|---|---|---|
| 35WW300 | 0.35 | ≤ 3.0 | ~1.66 | High-efficiency motors, small transformers |
| 50WW270 | 0.50 | ≤ 2.7 | ~1.65 | Premium / IE4-class motors |
| 50WW400 | 0.50 | ≤ 4.0 | ~1.69 | General industrial motors |
| 50WW470 | 0.50 | ≤ 4.7 | ~1.70 | Standard motors, generators |
| 50WW600 | 0.50 | ≤ 6.0 | ~1.71 | Cost-driven motors, small appliances |
| 50WW800 | 0.50 | ≤ 8.0 | ~1.72 | Small motors, ballasts, relays |
Typical orientation values only. B50 figures are indicative; certified values come from the mill certificate against the governing standard (e.g. GB/T 2521).
5. What Drives Core Loss
5.1 Silicon content
Silicon is the electrical steel workhorse: it raises resistivity (shrinking eddy currents), raises permeability, and lowers magnetostriction. The trade is formability — more silicon makes the steel harder and more brittle to punch. Non-oriented grades sit in a practical silicon range that balances magnetic quality against stampability, which is why the motor shop and the mill negotiate the grade together.
5.2 Thickness
Eddy loss scales with the square of thickness, so halving the gauge roughly quarters the eddy component at a given frequency. That is why premium motor grades drop from 0.50 mm to 0.35 mm and below. The catch: thinner laminations cost more to roll and handle, and the gain matters most at higher frequencies.
5.3 Grain orientation
Grain-oriented (GO) steel is processed so the easy magnetization direction aligns with rolling — loss is very low along that axis and poor across it. Non-oriented (NO) steel trades that peak performance for isotropic behavior: nearly equal properties in every direction. Motors need NO because the flux rotates; transformers need GO because the flux is one-directional. The two are not interchangeable despite both being "electrical steel".
5.4 Annealing and stress
Magnetic properties are structure-sensitive: cold work, punching and assembly stress all degrade them. The mill controls final annealing to set the grain size and remove rolling stress; the motor shop controls punching quality and decides whether to stress-relief anneal the laminations. Every step between coil and stator is a chance to add loss back.
6. How to Reduce Core Loss — Mill and Shop
Loss reduction is a shared job. At the mill: higher silicon, thinner gauge, cleaner steel chemistry, controlled grain size, and a final anneal tuned to the grade. At the shop:
- Stress relief annealing after punching — the classic fix for edge damage; it recovers the magnetic structure around cut edges and typically pays for itself in efficiency on higher-grade motors.
- Sharp, well-maintained tooling — a dull die work-hardens a wider band of edge, adding loss and burr.
- Careful stacking and clamping — high clamp pressure and interlaminar shorts from burr create circulating currents between sheets.
- Clean, burr-free edges — burrs bridge adjacent laminations and create eddy paths that should not exist. The burr story is covered in our edge burr guide.
7. Core Loss and Motor Efficiency
Core loss is a direct efficiency tax. In a typical induction motor, core loss sits alongside copper loss and mechanical loss in the loss budget, and the motor efficiency class (IE2/IE3/IE4) is won or lost on reducing these. A useful engineering rule of thumb: cutting core loss by roughly 0.5 W/kg in the lamination steel can improve motor efficiency by on the order of 0.3% — small-sounding, but decisive at utility scale where a fraction of a percent is the difference between efficiency classes. The exact relationship depends on machine design, but the direction is always the same: lower-loss steel is the cheapest efficiency upgrade that is bought once and never maintained.
8. Non-Oriented vs Grain-Oriented
| Dimension | Non-oriented (NO) | Grain-oriented (GO) |
|---|---|---|
| Magnetic behavior | Isotropic — similar in all directions | Anisotropic — very low loss along rolling |
| Typical loss at 1.5 T/50 Hz | ~2.7 – 8.0 W/kg by grade | Lower along the rolling direction |
| Typical thickness | 0.35 / 0.50 mm common | Thinner — 0.23 – 0.35 mm common |
| Primary use | Motors, generators, rotating machines | Transformers, cores with directional flux |
| Processing | Punched into stator / rotor laminations | Cut and stacked; punching less common |
HS-FINEB's electrical steel range follows this split — the non-oriented 50WW and B50A families for motors, and the thinner high-permeability families for transformer work. The grade matrix on our products page lists the full family set.
9. What to Check When Buying
- The test conditions behind the W/kg figure — induction and frequency must match your application.
- Thickness and coating — the insulation coating matters for stack factor and eddy behavior; confirm the coating type.
- Burr level after slitting — slitting damage raises edge loss; the burr limits in our edge burr guide apply here too.
- Whether the grade tolerates your punching route — higher-silicon grades are harder and need better tooling; confirm before tooling the die.
- The certificate — core loss, induction and thickness should all be on the mill certificate, not estimated.
10. Frequently Asked Questions
Short answers here; the full schema FAQ follows below.
- Is lower W/kg always better? For efficiency, yes — for cost and stampability, no. The grade is a trade-off, and the right choice depends on the machine and its duty.
- Can I anneal electrical steel at my shop? Stress relief annealing is a standard motor-shop step, but the cycle must suit the grade and coating — wrong temperature ruins the insulation coating.
- Do the numbers change with frequency? Yes, sharply. Eddy loss rises with f², so a grade quoted at 50 Hz is different in behavior at 400 Hz or 1 kHz applications.