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IEC TR 62981:2017 – Studies and Comparisons of Magnetic Measurements on Grain-Oriented Electrical Steel – SST and Epstein Methods
💡 Key Insight: This technical report reveals that the relationship between Single Sheet Tester (SST) and Epstein frame measurements is not a simple constant conversion factor. The ratio PSST/PEpstein varies with magnetic polarization, material grade, and even individual manufacturer production characteristics, challenging long-held assumptions in the electrical steel industry.
1. Historical Background and the SST-Epstein Relationship
Grain-oriented electrical steel is the fundamental material used in transformer cores, large generators, and other electromagnetic devices where high magnetic permeability and low core loss are critical. For decades, the Epstein frame method (IEC 60404-2) has been the internationally accepted standard for measuring the magnetic properties of electrical steels. However, the Single Sheet Tester (SST) method (IEC 60404-3) offers significant advantages in terms of sample preparation simplicity, reduced material requirements, and faster measurement throughput.
A key question that has persisted in the industry is how to correlate measurements between these two fundamentally different test methods. The Epstein method uses a closed magnetic circuit with overlapping corner joints, while the SST uses a single sheet with a yoke-based magnetic circuit. These structural differences lead to systematic variations in measured values, particularly for specific total loss (Ps) and magnetic polarization (J).
⚠ Technical Challenge: Historically, manufacturers and users have attempted to apply simple correction factors (typically around 0.97-0.98 for PSST/PEpstein) to correlate SST results to Epstein values. This technical report demonstrates that such simplifications are inadequate and that the relationship is far more complex, depending on peak polarization, material characteristics, and even the specific design of the SST apparatus used.
2. International Comparison Study
2.1 Study Design and Participants
The core of this technical report is an extensive international comparison study involving laboratories from multiple countries. The study circulated six different grades of grain-oriented electrical steel samples, each measured by multiple laboratories using both SST and Epstein methods. The participants included steel manufacturers, transformer producers, and independent testing laboratories, ensuring a representative cross-section of the industry.
2.2 Key Findings
The results of the international comparison yielded several important findings that have significant implications for the electrical steel industry:
| Parameter | Finding | Practical Implication |
|---|---|---|
| PSST/PEpstein ratio | Varies from 0.92 to 1.08 depending on polarization level | Simple fixed conversion factors are unreliable |
| Repeatability | Both methods show good repeatability within the same laboratory | Each method is internally consistent |
| Reproducibility | SST shows better inter-laboratory reproducibility than Epstein | SST may be more suitable for standardization |
| Polarization dependence | Difference increases at higher polarization levels (J > 1.7 T) | Special attention needed for high-performance grades |
| Material grade influence | Conventional and high-permeability grades show different SST/Epstein relationships | Grade-specific correlation factors may be needed |
| Yoke design impact | Different SST yoke designs produce systematically different results | Standardization of SST design is critical |
2.3 The Four Chinese Laboratory Study
An important preliminary study conducted by four Chinese laboratories using six different SSTs with stacked yokes is included in the report. This study revealed that even SSTs of nominally identical design can produce measurable differences in results, highlighting the sensitivity of the measurement to yoke design, winding configuration, and calibration procedures.
✅ Engineering Insight: For transformer designers, the practical implication is that material specifications should clearly state which test method is used for acceptance testing. When SST results are used for material qualification, the correlation to Epstein values should be established through empirical testing for each specific material grade, rather than relying on generic conversion factors. This is particularly critical for premium grades operating at high flux densities (1.7-1.9 T).
3. Conclusions and Recommendations
3.1 Summary of Conclusions
The technical report concludes that while both SST and Epstein methods are valid for magnetic property measurement of grain-oriented electrical steel, they are not directly interchangeable. The relationship between them is influenced by:
- Peak magnetic polarization level during measurement
- Material grade and manufacturing process characteristics
- Specific SST design parameters, particularly yoke construction
- Sample preparation techniques and dimensions
- Calibration standards and reference materials used
3.2 Recommendations for Industry
Based on the findings, the report makes several recommendations:
- Method specification: Contracts and specifications should explicitly state which method (SST or Epstein) is used for determination of magnetic properties.
- Reference values: Independent SST reference values should be established for different grades of grain-oriented electrical steel to provide a common baseline for comparison.
- Yoke standardization: Further work is needed to standardize SST yoke design to reduce inter-laboratory variability.
- Continued investigation: The topic merits further study before formal standardization of conversion factors can be considered.
🚨 Critical Finding: The relative difference (PSST – PEpstein)/PEpstein can exceed 8% for certain material grades at high polarization levels. This magnitude of difference is significant for transformer loss evaluation and can impact commercial transactions, energy efficiency classification, and transformer performance guarantees. Engineers must account for these differences when designing to loss specifications derived from different measurement methods.
Frequently Asked Questions
Q1: Which test method is more accurate, SST or Epstein?
Both methods have comparable accuracy when properly calibrated and operated. Epstein has a longer historical track record and more extensive reference data, while SST offers better reproducibility between laboratories and requires smaller samples. The choice depends on the specific application requirements.
Q2: Can a universal conversion factor be established between SST and Epstein results?
Based on the extensive data presented in this technical report, a universal conversion factor is not feasible. The PSST/PEpstein ratio varies with multiple parameters including polarization level, material grade, and SST design. Any conversion must be empirically determined for each specific combination of material and test apparatus.
Q3: How should transformer manufacturers specify core loss requirements?
Transformer manufacturers should specify which test method will be used for acceptance testing and ensure that loss specifications are based on data from the same method. If SST data is used for design calculations, correlations to Epstein values should be verified through comparative testing on the specific material grades to be used.
Q4: What is the significance of the stacked yoke design in SST measurements?
The stacked yoke design is a critical factor affecting SST measurement accuracy. Differences in yoke material properties, lamination orientation, air gap lengths, and winding configurations all contribute to measurement variability. The report recommends ongoing efforts to standardize SST design to minimize these effects.
