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IEC TR 61807: Magnetic Properties of Electrical Steel Sheet – Measurement and Evaluation Guide
Key Insight
IEC TR 61807 provides the authoritative measurement methodology for evaluating the magnetic properties of electrical steel sheet and strip, establishing the foundation for core loss specifications used in transformer, motor, and generator manufacturing worldwide.
IEC TR 61807 provides the authoritative measurement methodology for evaluating the magnetic properties of electrical steel sheet and strip, establishing the foundation for core loss specifications used in transformer, motor, and generator manufacturing worldwide.
1. Scope and Measurement Principles
IEC TR 61807, published in 1999, addresses the measurement of magnetic properties of electrical steel sheet and strip intended for use in power, distribution, and small transformers as well as rotating electrical machines. The standard covers both grain-oriented (GO) silicon steel (used primarily in transformers) and non-oriented (NO) grades (used primarily in motors and generators). It specifies measurement methods for specific total loss, magnetic polarization, relative permeability, and apparent power at industrial frequencies (50 Hz and 60 Hz) and, for certain grades, at higher frequencies up to 400 Hz.
Two principal measurement methods are defined: the Epstein frame method (IEC 60404-2) and the single sheet tester (SST) method (IEC 60404-3). IEC TR 61807 provides guidance on selecting between these methods based on material grade, sheet dimensions, and required accuracy. The Epstein method is the traditional reference, using a standardized 25 cm square specimen with primary and secondary windings distributed along four arms. The SST method offers faster measurement on a single rectangular sheet and is increasingly used for quality control in steel production lines.
Engineering Insight: The Epstein frame remains the arbitration method for commercial transactions between steel producers and transformer manufacturers. Discrepancies between Epstein and SST measurements for the same material can reach 3–5% for core loss; IEC TR 61807 recommends establishing a correlation curve between the two methods before using SST for acceptance testing. Always specify which method governs in procurement contracts.
2. Key Measurement Parameters and Specifications
2.1 Specific Total Loss
The most critical parameter for electrical steel is the specific total loss (Ps), expressed in watts per kilogram (W/kg) at a specified magnetic polarization (typically 1.5 T or 1.7 T for GO steels at 50 Hz). IEC TR 61807 mandates that losses be measured under sinusoidal flux conditions with a form factor of the induced secondary voltage within 1.11 ± 1%. Deviation from sinusoidal conditions introduces measurement errors that can exceed 5% for modern high-permeability grades due to their sharp saturation knee.
| Material Grade | Thickness (mm) | Core Loss @ 1.5 T, 50 Hz (W/kg) | Core Loss @ 1.7 T, 50 Hz (W/kg) | Typical Application |
|---|---|---|---|---|
| M085-23P (Hi-B GO) | 0.23 | 0.62 | 0.95 | Large power transformers |
| M095-27P (GO) | 0.27 | 0.75 | 1.15 | Distribution transformers |
| M130-30S (GO) | 0.30 | 1.05 | 1.55 | Medium transformers |
| M250-35N (NO) | 0.35 | 2.50 | — | Small motors, generators |
| M400-65N (NO) | 0.65 | 4.00 | — | Large motors, alternators |
2.2 Magnetic Polarization and Permeability
For grain-oriented steels, IEC TR 61807 specifies measurement of magnetic polarization (J) at magnetic field strengths of 800 A/m (J800) and 2500 A/m (J2500). These values correlate with the material’s ability to maintain low core loss at high operating flux densities — a critical parameter for transformer design where the core is operated near the knee of the B-H curve. Modern Hi-B (high-permeability) grades achieve J800 values of 1.89–1.93 T, compared to 1.80–1.85 T for conventional GO grades.
Design Warning: The apparent power (S) measurement specified in IEC TR 61807 is often overlooked but is essential for transformer design. Apparent power reflects the magnetizing current requirement, which determines the no-load current and reactive power consumption of the transformer. For Hi-B grades, apparent power at 1.7 T can be 10–15% higher than conventional GO grades of the same loss class, meaning lower loss does not automatically translate to lower magnetizing current.
3. Specimen Preparation and Measurement Conditions
Specimen preparation significantly affects measurement results. IEC TR 61807 provides detailed requirements for:
- Cutting orientation: For GO steels, Epstein strips must be cut with the rolling direction (RD) and transverse direction (TD) specimens in specific ratios. The standard specifies both “whole strip” (all strips in RD) and “half-and-half” (equal RD and TD) configurations for different purposes.
- Stress relief annealing: After cutting, specimens for guaranteed values must undergo stress relief annealing at 780–820 °C in a non-oxidizing atmosphere to eliminate cutting-edge strain that can increase core loss by 5–15%.
- Magnetizing and measuring windings: The Epstein frame uses a standardized turns ratio of 700 primary and 700 secondary turns. The SST yoke must provide a magnetic path with cross-section at least 10 times that of the test specimen to avoid systematic errors.
- Temperature control: Measurements must be performed at 23 °C ± 5 °C, as core loss varies approximately 0.3–0.5% per °C for silicon steel.
4. Frequently Asked Questions
Q1: What is the difference between specific total loss and magnetic polarization measurement?
Specific total loss (W/kg) measures the energy dissipated as heat in the material under alternating magnetization — this determines transformer efficiency and operating temperature. Magnetic polarization (T) measures the material’s achievable flux density at a given magnetizing force — this determines the core cross-section needed for a given voltage rating. Both are essential but serve different design purposes.
Q2: Why does IEC TR 61807 reference IEC 60404-2 and IEC 60404-3?
IEC 60404-2 contains the detailed test method for the Epstein frame, while IEC 60404-3 covers the single sheet tester. IEC TR 61807 is a technical report that provides application guidance, interpretation of results, and engineering recommendations — it does not duplicate the test methods but rather explains how to apply them correctly for electrical steel evaluation.
Q3: Can IEC TR 61807 be used for amorphous metal (metallic glass) cores?
The measurement principles apply, but the test methods require modification. Amorphous metals have much higher electrical resistivity (130 μΩ·cm vs. 45 μΩ·cm for silicon steel) and are only 0.020–0.025 mm thick, requiring special Epstein frames with higher turns density and lower flux densities (typically 1.3–1.4 T) to avoid saturation. The standard acknowledges this but does not provide detailed procedures — refer to ASTM A927 for standardized amorphous metal testing.
Q4: How should I interpret deviations between measured and guaranteed core loss values?
IEC TR 61807 recommends that acceptance testing account for measurement uncertainty, which for well-maintained Epstein frames with calibrated wattmeters is approximately ±2–3% at 95% confidence. Most procurement contracts specify a tolerance of +5% (manufacturer’s advantage) above the guaranteed value, meaning measured loss can exceed the guaranteed value by up to 5% without penalty. Systematic deviations beyond 5% typically indicate material non-conformity or improper specimen preparation.
