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IEC/TR 62581 โ Magnetostriction Measurement Methods for Electrical Steel
Electrical steel is the backbone of transformers, motors, and generators. Its magnetostriction behaviour directly influences audible noise, vibration, and long-term mechanical fatigue. IEC/TR 62581 provides standardized methods to characterize this critical property using both single sheet testers (SST) and Epstein frames under applied mechanical stress.
💡 Why it matters: Magnetostriction-induced noise is often the dominant noise source in power transformers at no-load. Accurate measurement allows manufacturers to select steel grades that minimize acoustic impact in residential and urban installations.
1 ⚙️ Scope and Purpose of IEC/TR 62581
IEC/TR 62581, published in 2010 as a Technical Report, describes methods for measuring the magnetostriction characteristics of electrical steel sheets by means of single sheet testers and Epstein test specimens. Unlike a normative standard, a Technical Report provides guidance and state-of-the-art methodologies that may evolve as measurement technology advances.
The document addresses a critical gap: prior to this report, there was no internationally agreed procedure for measuring magnetostriction under mechanical stress — a condition that occurs in every real-world transformer core due to clamping forces and thermal expansion. The report covers:
- Measurement of magnetostriction under applied compressive and tensile stress
- Single sheet tester (SST) configurations with stress-applying devices
- Epstein frame adaptations for magnetostriction characterization
- Optical sensor techniques for strain measurement
- Data acquisition and processing procedures
2 📐 Test Methods and Measurement Setup
2.1 Single Sheet Tester (SST) Method
The SST method is the primary approach described in IEC/TR 62581. A test specimen — typically 30 mm wide and 300–500 mm long — is placed between two yokes that provide a closed magnetic path. An optical sensor (usually a laser-based or capacitive device) measures the elongation of the specimen with sub-micrometre resolution.
Key components of the SST setup include:
| Component | Function | Typical Specification |
|---|---|---|
| Magnetizing yokes | Provide closed magnetic circuit | Laminated silicon steel, low-loss grade |
| Primary winding | Excites the specimen to target flux density | Turns count per manufacturer protocol |
| Secondary (B-coil) | Measures magnetic flux density | Few turns of thin wire on specimen |
| Optical sensor | Measures magnetostrictive strain | Resolution ≤ 0.1 µm/m |
| Stressing device | Applies compressive/tensile stress | 0–20 MPa range typical |
| Data acquisition system | Captures strain waveform synchronously | ≥ 16-bit ADC, ≥ 10 kHz sampling |
⚠️ Important: Air flux compensation is essential in the SST method. Without it, the measured strain includes a component from the air gap between yoke and specimen, leading to significant overestimation of magnetostriction values.
2.2 Epstein Frame Method
The Epstein frame — standardized in IEC 60404-2 for core loss measurement — is adapted in IEC/TR 62581 for magnetostriction characterization. The classic 25 cm Epstein frame uses strips cut at 0° and 90° to the rolling direction, arranged in a square. Magnetostriction is measured by bonding strain gauges directly to the inner surfaces of the strips or by using external optical sensors.
The Epstein method offers the advantage of well-established magnetic circuit geometry but is less suited for applying external stress compared to the SST method. The report acknowledges this limitation and recommends the SST approach for stress-dependent measurements.
2.3 Stress Application and Measurement Protocol
A defining feature of IEC/TR 62581 is the ability to measure magnetostriction as a function of applied mechanical stress. In real transformer cores, the steel is subjected to compressive stresses from clamping bolts (typically 0.5–5 MPa) and from thermal expansion constraints. The standard test protocol involves:
- Specimen preparation: cut strips with controlled orientation to rolling direction
- Demagnetization of the specimen before each measurement
- Application of controlled stress via the stressing device
- Magnetization at specified flux densities (typically 1.0 T to 1.7 T) and frequencies (50 Hz or 60 Hz)
- Simultaneous recording of strain waveform and magnetic waveform
- Calculation of peak-to-peak strain (λpp) and harmonic content
3 📊 Engineering Design Insights and Applications
Understanding magnetostriction data from IEC/TR 62581 testing is vital for several engineering domains:
3.1 Transformer Noise Prediction
The magnetostrictive strain of electrical steel at 50/60 Hz is typically in the range of 1–10 µm/m (peak-to-peak). Since the fundamental frequency of magnetostriction is twice the magnetizing frequency (100 Hz for 50 Hz systems), transformer noise is dominated by this double-frequency component. Using IEC/TR 62581 data, designers can predict the no-load noise level of a transformer with greater accuracy than using catalogue loss values alone.
3.2 Material Selection and Core Design
| Steel Grade | Typical λpp at 1.5 T (µm/m) | Core Loss W/kg at 1.5 T/50 Hz | Application |
|---|---|---|---|
| Conventional grain-oriented (CGO) | 3.0 – 5.0 | 1.00 – 1.20 | Distribution transformers |
| High-permeability grain-oriented (HiB) | 2.0 – 4.0 | 0.85 – 1.00 | Power transformers |
| Domain-refined HiB (DR) | 1.5 – 3.0 | 0.75 – 0.90 | High-efficiency transformers |
| Non-oriented (NO) steel | 1.0 – 3.0 | 2.5 – 5.0 | Rotating machines |
✅ Design tip: For noise-sensitive installations (hospitals, residential areas), selecting domain-refined HiB steel can reduce magnetostriction-induced noise by 3–6 dB compared to CGO grades, potentially eliminating the need for additional acoustic enclosures.
3.3 Impact of Stress on Performance
Research has shown that even moderate compressive stress (2 MPa) can increase magnetostriction by 50–100% for grain-oriented steels. This means that careless clamping during core assembly can negate the benefits of premium steel grades. The IEC/TR 62581 measurement methodology enables manufacturers to:
- Qualify steel suppliers with stress-dependent magnetostriction data
- Optimize clamping pressure in core assembly
- Validate finite-element models of transformer noise
- Support contractual noise guarantees with quantitative data
🚨 Critical consideration: Never assume catalogue magnetostriction values apply to your installed condition. Clamping stress, stacking factor, and thermal cycling all alter the effective magnetostriction. Always request stress-dependent test data per IEC/TR 62581.
Frequently Asked Questions
Q1: What is the difference between a Technical Report (TR) and an International Standard (IS)?
A Technical Report provides informative guidance and state-of-the-art methodologies, whereas an International Standard contains normative requirements that must be followed for compliance. IEC/TR 62581 is a TR because magnetostriction measurement under stress is still an evolving field with no single consensus method.
Q2: Can IEC/TR 62581 methods be used for non-oriented electrical steel?
Yes. While the report primarily focuses on grain-oriented steel (used in transformers), the SST and Epstein methods are equally applicable to non-oriented grades used in motors and generators. The key difference is that non-oriented steel exhibits nearly isotropic magnetostriction, so specimen orientation is less critical.
Q3: Why is magnetostriction measured at twice the supply frequency?
Magnetostriction is a function of the absolute value of flux density — the steel elongates regardless of whether B is positive or negative. This produces two strain cycles per magnetizing cycle, resulting in a fundamental frequency of 2×fsupply. For 50 Hz systems, the dominant magnetostriction frequency is 100 Hz.
Q4: How does temperature affect magnetostriction measurements?
Temperature influences both the magnetic domain structure and the elastic properties of the steel. At elevated temperatures (above 80°C), magnetostriction generally decreases slightly. However, thermal gradients can introduce measurement artefacts due to differential expansion. IEC/TR 62581 recommends stabilizing specimen temperature before measurement.
