CRGO lamination magnetostriction: why it matters for transformer noise

1. Magnetostriction in CRGO: the short version engineers actually use

Magnetostriction in grain-oriented electrical steel is one of the main drivers of transformer core vibration and acoustic noise. That isn’t new; it’s been shown across lab studies, full-core tests, and recent reviews on transformer vibration.

A few working points, just to align language:

• Frequency: With sinusoidal excitation, magnetostriction is dominated by a 2f component (100 Hz at 50 Hz systems, 120 Hz at 60 Hz), with higher harmonics extending well into the audible band.
• Amplitude: For typical CRGO, magnetostrictive strain is on the order of a few ×10⁻⁶ in normal operating induction ranges (around 1.7–1.9 T). Low-noise steels aim to push that closer to ~1.5×10⁻⁶ or lower at defined test points.
• Waveform shape: DC bias, harmonics in flux, and stress state distort that clean butterfly curve, skewing the spectrum and often increasing audible components.

None of that makes noise by itself. The lamination stack does. The gaps, the clamping, the coating, the way your CRGO is cut and assembled. That’s where magnetostriction turns into a vibrating mechanical system.

2. Why purchasing should care about magnetostriction

From a buying perspective, “magnetostriction” can feel like one more obscure line on a steel datasheet. Yet it ties directly into cost items you actually see:

• Oversized tank stiffeners and damping
• Extra acoustic insulation around the transformer
• Rework if the factory noise test fails
• Site complaints, penalties, even relocation in worst cases

Suppliers of GO steels now explicitly position “low magnetostriction” and “noise reduction” as value points, not just academic properties.

So the question for purchasing isn’t “What is magnetostriction?” It’s: “How much magnetostriction can I afford, given my noise target and cost structure?”

That’s where a simple mental map of CRGO options helps.

3. CRGO lamination magnetostriction tiers (practical view)

Not all CRGO is equal. And not all “low noise” claims mean the same thing in a stack. Based on published low-noise patents, CRGO datasheets, and measured core tests, you can think in ranges like this:

Note: Numbers below are indicative, not guaranteed values. Always refer to the specific mill’s datasheet and test method.

*Rough engineering-level descriptions; different mills specify at different test points (B, f, λ definition).

Material choice alone won’t guarantee your target dB(A). But it sets the floor. If you start with a noisy steel, the best stack in the world can only do so much.

4. Where lamination stacks turn magnetostriction into noise

Magnetostrictive strain is microscopic. Noise in the test bay is not. The bridge between them is mechanical:

1. Lamination thickness and grade
   • Thinner CRGO and high-permeability grades reduce losses and can help with noise by limiting flux peaks.
   • But if the laminations are poorly stacked, extra interfaces mean more places to vibrate.
2. Stress in the stack
   • Cutting, punching, bending, and clamping all modify local stress.
   • CRGO datasheets warn that extremely low magnetostriction values are hard to keep in a real core because of these fabrication stresses.
   • A bit of controlled tensile film tension on the sheet surface can reduce magnetostriction and change its spectrum, as low-noise steel patents emphasize.
3. Coating and surface treatment
   • Stress-relief coatings and tension coatings simplify the domain pattern and can halve magnetostriction at normal inductions according to certain CRGO datasheets.
   • That’s a big lever, but it assumes the coating survives core manufacturing without damage.
4. Stacking pattern: step-lap vs. butt
   • Step-lap cores spread flux and reduce local saturation. Multiple studies and field experience show step-lap stacking can reduce core noise on the order of 3–6 dB when applied correctly.
   • But the benefit depends on tight tolerances in cutting and assembly. Misaligned steps reintroduce local stress and air gaps.
5. Bonding and clamping strategy
   • Flexible bonding between laminations can damp magnetostrictive vibrations significantly, as demonstrated in classic work on bonded silicon-iron stacks.
   • Overly rigid clamping at a few points can create local hot-spots in stress and noise; distributed clamping with controlled pressure tends to be quieter.

Summary: CRGO grade sets the potential. The lamination stack decides how much of that potential you actually get.

5. Design knobs that actually move the noise level

For engineers, it helps to sort all the options into a few controllable “knobs”:

5.1 Flux density & core cross-section

You already juggle core loss and material cost when setting design induction. Magnetostriction adds another axis:

• Higher B → larger magnetostrictive strain and usually stronger 2f component.
• Lower B → less strain, often smoother waveform, but a larger core and more steel.

Noise-critical designs sometimes deliberately use:

• Slightly lower design induction
• Premium low-magnetostriction steel

Taken together, that combination can meet both loss and noise limits without heroic tank damping.

5.2 Control of harmonic content and DC bias

Power quality at the primary doesn’t stay perfect. DC bias and harmonic distortion in B:

• Distort magnetostriction waveforms
• Increase higher-frequency components
• Push the core to vibrate in ways the basic 2f model doesn’t capture

From a design and grid-integration standpoint, this means:

• Being conservative with induction on systems where DC bias is expected (HVDC injection, large renewable systems near the transformer).
• Specifying acceptable harmonic conditions and DC bias assumptions in technical documents, not leaving them implicit.

5.3 Choice of CRGO and coating technology

When you evaluate CRGO offers, magnetostriction-related details to note:

• Whether the grade is standard, high-permeability, or “low-noise” with explicit λ limits.
• Presence and type of domain refinement (laser scribing, etching grooves, etc.).
• Coating type: is it marketed as “tension coating”, “stress-relief coating”, or similar, and does the datasheet link it to magnetostriction reduction?

These are not marketing trivia. They tell you how the steel will behave once cut, stacked, clamped.

5.4 Lamination geometry and stack design

For a lamination supplier or transformer OEM, key choices include:

• EI vs. step-lap vs. wound core geometries
• Sheet thickness (0.30, 0.27, 0.23 mm and thinner)
• Stacking factor targets and allowable burr height
• Whether to bond, varnish, or leave laminations free with traditional clamping

Each combination shifts how magnetostriction is transmitted to the tank and to the surrounding air. A comprehensive 2024 review shows that models which include lamination-level magnetostriction and stacking details match measured vibration and noise much better than simplified ones.

6. How to read CRGO offers through a noise lens

Let’s move to the practical buying side.

When you receive offers for CRGO lamination stacks or cores, a “noise-aware” reading looks at more than just core loss and price.

6.1 Check for real magnetostriction information

Questions to ask suppliers:

1. Is λ specified?
   • At what B and frequency?
   • Is it a guaranteed maximum, or just a typical value?
2. Is there any mention of low-noise or acoustic testing?
   • Some mills and core makers show noise tests for standard designs at specified B and load.
3. Do they reference specific coating or domain refinement aimed at noise?
   • Look for mentions of tension films, domain-refined grades, or noise-focused product families.

If the offer simply says “equivalent to Grade X” with no λ, you’re flying blind on a property that directly affects dB(A).

6.2 Clarify stack design details in the RFQ

It helps to describe, explicitly, the lamination stack expectations:

• Required stacking pattern (e.g. 7-step lap, defined overlap lengths)
• Maximum burr and lamination edge quality
• Allowed stress-relief annealing steps, if any
• Whether bonding/varnish is allowed, preferred, or forbidden

Technical papers on bonded laminations show that the bonding method can change measured vibration by a large margin. If you don’t mention it, suppliers will optimize for cost and throughput, not necessarily for noise.

6.3 Align test conditions

Noise disputes often come from mismatched expectations:

• Factory tests at one induction vs. site operation at another
• Different measurement distances or background levels
• Use of different standards for sound pressure or sound power

Since magnetostriction is very sensitive to B and waveform, you want:

• Agreed test induction(s) and tap settings
• Defined load condition for noise testing (no-load or specified load)
• Stated measuring distance and environment

You can’t change physics later with paperwork.

7. Quick checklist before you sign off a lamination supplier

For purchasing and engineering teams working together, here’s a compact checklist:

1. Steel grade & λ data
   • Does the offer include magnetostriction information, not just loss?
   • Is a low-magnetostriction or “low-noise” grade available as an option?
2. Coating & domain refinement
   • Is the coating described and tied to noise or magnetostriction performance?
   • Any constraints on heat treatment that might degrade those properties?
3. Stacking approach
   • Is step-lap or another low-noise pattern defined?
   • Tolerances on burrs, gap, alignment spelled out?
4. Bonding/clamping concept
   • Are laminations bonded, banded, or just clamped?
   • Has that configuration been used in low-noise projects before?
5. Test and documentation
   • Will the supplier share lab data or past project references for noise-critical transformers using similar CRGO and stack designs?

If most boxes are blank or answered vaguely, you know the risk is being transferred to you.

8. FAQ: CRGO lamination magnetostriction & transformer noise

Key takeaway

For a lamination stack supplier or transformer OEM, CRGO lamination magnetostriction is not just a materials property; it’s a contract outcome.

If you choose the right steel, specify the lamination stack properly, and align tests with reality, transformer noise becomes predictable and manageable instead of a late-stage surprise.