EI Transformer Core Lamination Thickness: How to Choose the Right mm Value

Content

1 EI Transformer Core Lamination Thickness: The Numbers You Need
2 Why Lamination Thickness Directly Controls Core Loss
3 Typical Lamination Thickness by Frequency, Size and Material Grade
4 How to Choose the Right Lamination Thickness
5 What to Verify When Sourcing Laminations or Finished EI Transformers
6 EI Core Lamination Thickness: Common Questions, Straight Answers

EI Transformer Core Lamination Thickness: The Numbers You Need

If you are specifying, repairing or redesigning an EI transformer, the first core question is usually: how thick is each lamination? For a mains-frequency transformer the practical answer is 0.35 mm or 0.50 mm. Most standard E-I lamination sizes, from about EI-28 up to EI-96, are stamped from 0.50 mm non-oriented silicon steel, while 0.35 mm is chosen when the design must reduce no-load loss, lower temperature rise, or cut audible noise. Textbook answers often quote 0.25–0.5 mm as a general range; commercial EI stampings, however, converge on 0.35 and 0.50 mm because the tooling and steel grades are standardized.

One point causes more specification errors than any other: lamination thickness is the thickness of a single steel sheet, not the width of the stacked core. A 40 mm core build, for example, contains 80 sheets of 0.50 mm steel, or about 114 sheets of 0.35 mm steel. Confusing these two values is the most common mistake in EI core ordering.

50/60 Hz standard transformers: 0.35–0.50 mm
400 Hz aerospace and industrial equipment: 0.10–0.20 mm
1–5 kHz inverter transformers: 0.05–0.10 mm
Above 10 kHz: ferrite or amorphous cores, not stamped laminations

Why Lamination Thickness Directly Controls Core Loss

Laminations exist to confine eddy currents. The alternating flux induces circulating currents inside the steel; a solid iron block would carry enormous eddy currents and overheat within minutes. Slicing the core into insulated sheets limits those currents to the volume of each thin sheet.

P_e = K_e × f² × B² × t²

In this eddy-current loss expression, t is the lamination thickness, f is the frequency, B is the peak flux density and K_e is a material constant. The t² term matters most: doubling the thickness increases the eddy-current component by roughly four times, while halving it cuts that component to one-quarter. Hysteresis loss, by contrast, depends on material and flux density, not on lamination thickness, so the total core-loss improvement from thinner steel is always smaller than the eddy-current reduction alone.

Relative eddy-current loss by lamination thickness (0.35 mm = 100)

0.20 mm

100

0.35 mm

204

0.50 mm

345

0.65 mm

Eddy-current loss scales with the square of lamination thickness; values are relative indices, not absolute watts.

In a typical 0.50 mm EI core at 50 Hz, the no-load loss splits roughly into 55% hysteresis, 40% eddy current and 5% stray loss:

Hysteresis 55%
Eddy current 40%
Stray 5%

Approximate loss split for a 0.50 mm M-45 grade core at 50 Hz and about 1.5 T; the eddy share falls when thinner steel is used.

Moving from 0.50 mm to 0.35 mm typically cuts the eddy-current share by half, which is why a 0.35 mm core runs measurably cooler and draws less no-load current. The penalty is higher material cost and slightly more sheets to stack for the same core area.

Typical Lamination Thickness by Frequency, Size and Material Grade

Frequency is the first filter when selecting thickness; material grade is the second. EI transformers normally use non-oriented silicon steel because the E-I shape has corners where grain orientation cannot be fully exploited. Common AISI designations are M-45, M-22 and M-19, and equivalent grades exist under IEC and most national standards.

Frequency, not core size, is the primary driver of lamination thickness.
Application	Typical thickness	Typical material
50–60 Hz mains transformers	0.35–0.50 mm	M-45, M-22, M-19 (non-oriented)
400 Hz aerospace and industrial equipment	0.10–0.20 mm	Thin non-oriented or grain-oriented strip
1–5 kHz inverter transformers	0.05–0.10 mm	Nickel-iron or thin silicon strip
Above 10 kHz	Ferrite core (not applicable)	Mn-Zn ferrite

AISI electrical-steel designations; IEC equivalents are commonly available.
Grade	Common thickness	Typical role
M-45	0.50 mm	General-purpose EI power transformers, cost-sensitive designs
M-22	0.50 mm	Industrial control and isolation transformers with moderate loss limits
M-19	0.35 mm	Higher-efficiency, medical and audio EI transformers
M-19 or better	0.27–0.30 mm	Premium low-loss designs where extra cost is acceptable

Relative total core loss index for both thicknesses; the gap widens with frequency because the eddy-current term scales with f².

How to Choose the Right Lamination Thickness

Choose with numbers, not habit. Most designers work through seven points before committing to a core specification:

Confirm the operating frequency and any derating requirement between 50 and 60 Hz.
Set the no-load loss ceiling from the customer specification, efficiency class or temperature-rise limit.
Estimate core loss at the planned flux density, usually 1.5–1.7 T for a 50 Hz EI core.
Compare 0.50 mm and 0.35 mm loss curves for the same silicon-steel grade.
Check stacking factor: about 0.95–0.97 for 0.50 mm and slightly lower for 0.35 mm because the insulating coating occupies a larger share of each sheet.
Confirm cost: thinner steel costs more per kilogram and needs more sheets to reach the same iron area.
Build a prototype and verify no-load current, temperature rise and audible noise before approval.

If the core window is already fixed and the transformer runs hot, 0.35 mm is the practical upgrade because it cuts the eddy-current share of loss. If the design is cost-driven and the temperature rise is acceptable, 0.50 mm remains the smarter default for 50/60 Hz work.

One hidden risk deserves attention: very thin sheets are harder to stamp cleanly, and burrs on the punched edges short-circuit adjacent laminations. Specify a burr-height limit in the purchase contract, otherwise the extra cost of thin steel may be lost to edge shorting.

What to Verify When Sourcing Laminations or Finished EI Transformers

The thickness on a datasheet is not always the thickness in the box. When qualifying a supplier, three measurements matter most: sheet thickness, burr height and stacking factor. Measure sheet thickness with a micrometer at three or more points and compare both the average and the spread with the ±0.03 mm tolerance typical of commercial electrical steel. Burr height at the punched edges should stay at or below 0.05 mm, ideally 0.03 mm for 0.35 mm steel. Stacking factor is the measured stack height divided by the product of the number of sheets and the nominal sheet thickness; expect at least 0.95 for varnished 0.50 mm laminations.

These acceptance checks catch most core specification errors before expensive rework.
Parameter	Typical acceptance	Why it matters
Sheet thickness	Nominal ± 0.03 mm	Eddy-current loss depends on t², so a tolerance shift changes the loss curve directly
Burr height	≤ 0.05 mm (0.03 mm ideal)	Burrs short adjacent laminations, raising no-load current and creating hot spots
Stacking factor	≥ 0.95 for 0.50 mm	A lower factor forces a taller stack and higher copper cost for the same iron area
Interlaminar coating	C-2, C-3 or annealed-grade coating	Coating integrity is what makes thin sheets behave like separate magnetic circuits
No-load loss at rated V/Hz	Within ±10% of the quoted value	Confirms that the delivered grade and thickness match the design

A manufacturer with an established EI transformer line normally supports this verification with production data. The Ningbo Chuangbiao team, introduced on the company About page, lists EI transformers as its specialty and states that every transformer passes a full electrical test before delivery. If your project needs a non-standard EI size or a particular silicon-steel grade, state the lamination thickness, loss limit and test conditions in a customization request so the factory measures exactly what you will measure.

EI Core Lamination Thickness: Common Questions, Straight Answers

Is 0.35 mm always better than 0.50 mm?

No. At 50/60 Hz, 0.35 mm reduces the eddy-current component of core loss by roughly half compared with 0.50 mm of the same grade, but the steel costs more and the stack needs more sheets. For small transformers with comfortable temperature margin, 0.50 mm is the economical, proven choice. Choose 0.35 mm when the specification sets a no-load loss limit, a low temperature-rise target or continuous-duty operation.

Will switching a hot transformer from 0.50 mm to 0.35 mm fix the overheating?

Probably, but it is not a drop-in swap. Thinner laminations lower the eddy-current part of core loss, so the core temperature falls and the no-load current drops. The number of sheets changes for the same core area, and the new no-load current, temperature rise and audible noise must be re-verified on a prototype before production.

What thickness should a 400 Hz EI transformer use?

About 0.10–0.20 mm. Since eddy-current loss scales with f², a core built with 0.50 mm steel would overheat quickly at 400 Hz. Moving to 0.20 mm cuts the eddy-current component to about 16% of its 0.50 mm value at the same flux density.

Can I measure lamination thickness without disassembling a finished transformer?

Only approximately. The core edge shows the sheet lines, but the outer sheet usually carries paint or varnish, so your reading includes the coating. For acceptance testing, use a spare lamination or an unfinished core, take readings at several points with a micrometer and average them instead of trusting a single measurement.

Bottom line: for most 50/60 Hz EI transformers, 0.50 mm is the cost-effective default and 0.35 mm is the lower-loss upgrade. Put the lamination thickness, steel grade, burr limit and no-load loss test into your inquiry, and review the company homepage for background on the product range before you commit to a supplier.