Take apart any motor, and you’ll find stacks of thin iron sheets piled up inside. Many people’s first reaction is:

“Why not just use a single piece of iron? Wouldn’t that be stronger and simpler?”

This is a very pertinent question. Intuitively, a solid piece of iron does indeed have many advantages:

· High mechanical strength: Will not loosen, will not delaminate.

· Simple processing: No cutting, stacking, or insulation required.

· Lower cost: Skip the complicated procedures.

However, the engineers still insisted on cutting the iron core into thin sheets measuring 0.10 to 0.50 mm thick and stacking them one by one—a process that is complex and more costly.

Why?

Because if you use a single solid piece of iron, the motor will have a fatal flaw— Eddy-current losses can cause a sharp drop in motor efficiency, ranging from 20% to 40%.

Even worse, the motor could burn out within just a few minutes after starting up.

Today, we’ll start with... Physical principles Let me explain this issue clearly.

01 Question: The Fatal Flaw in a Solid Block of Iron

To understand why you can't use a solid block of iron, you first need to grasp a physical phenomenon: Vortex.

1. What is eddy current?

Imagine a whirlpool in a river. When the flow of water encounters an obstacle, it forms a rotating vortex behind the obstacle.

The “eddy currents” in electromagnetism work on a similar principle.

When Alternating magnetic field When passing through a conductive material (such as iron), circular induced currents are generated within the material. These currents flow in loops around the material, much like whirlpools, which is why they’re called “eddy currents.” Vortex 。

This is an inevitable consequence of Faraday’s law of electromagnetic induction: a changing magnetic field → generates an electric field → drives the motion of electrons → and thus forms eddy currents.

What’s the problem with the 2-vortex?

The vortex itself isn't the problem. The problem lies in: Vortexes generate heat.

Just as an electric heating wire heats up when current flows through it, eddy currents flowing through iron also generate heat due to electrical resistance. This heat is entirely wasted—though it originates from electrical energy, it doesn’t perform any useful work; it simply dissipates as heat for no purpose.

This is it. Eddy current loss.

3 A Solid Block of Iron: A Paradise for Eddy Currents

Now let’s return to the issue of “a single piece of iron.”

In a solid piece of iron, eddy currents can form freely. A very large loop. Imagine that the vortex is like a giant “runway,” where electrons can charge straight through.

The longer the loop, the greater the current and the more heat generated.

Even worse, Eddy-current loss is proportional to the square of the material thickness. The formula is:

P is proportional to d² (P = power loss, d = thickness)

This means that if the core thickness is doubled, eddy-current losses will increase by a factor of four!

4. The complete calculation formula for eddy current loss

If you’d like to understand eddy-current losses more precisely, you can take a look at the complete calculation formula:

Pe = K × f² × Bm² × d² × V / ρ

Each term in the formula tells a story:

· Pe: Eddy current loss power (W) — the target we aim to reduce

· f: Magnetic field variation frequency (Hz)—the square of the frequency! The higher the frequency, the faster the loss increases.

· Bm: Maximum magnetic flux density (T)—the stronger the magnetic field, the more intense the eddy currents.

· d: Material thickness (m)—the square of the thickness! That’s why we need to cut it thin.

· V: Material volume (m³)—the larger the volume, the more eddy current paths there are.

· ρ: Material resistivity (Ω·m)—the higher the resistivity, the more eddy currents are suppressed.

· K: Coefficient (related to material shape and unit system)

What does this formula tell us?

The design logic of silicon steel sheets can be directly seen from the formula:

·The thickness d appears in the numerator and has a quadratic relationship with the loss. This means: halving the thickness reduces the loss to one-fourth of its original value. For example, cutting a solid 50-mm-thick iron block into 0.5-mm-thin sheets theoretically reduces the loss to one ten-thousandth of its original value.

·The resistivity ρ appears in the denominator, which means: the higher the resistivity, the lower the loss. That’s precisely why silicon is added—to increase the resistivity ρ and fundamentally suppress eddy currents.

·The frequency f also exhibits a quadratic relationship—this means that if the frequency doubles, the loss increases by a factor of four. That’s why high-frequency motors must use thinner silicon steel laminations.

From Formulas to Engineering Decisions

With this formula, engineers can make precise trade-offs:

·For 50Hz power-frequency motors: Since the frequency f is low, a thickness of 0.50mm can be used, resulting in low costs.

·For 400 Hz variable-frequency motors: Since f has increased by a factor of 8, f² has increased by a factor of 64. Therefore, it’s essential to use thinner laminations of either 0.35 mm or 0.2 mm.

·For high-speed motors (>1000 Hz): If f² exceeds 400 times, only an ultra-thin sheet of 0.10 mm can effectively control losses.

The physics behind the formula

This formula didn't come out of thin air—it originates from:

·Faraday's law of electromagnetic induction: A changing magnetic field induces an electromotive force.

·Ohm’s Law: Electromotive force drives current; the greater the resistance, the smaller the current.

· Joule's Law: Current passing through a resistor generates heat.

Starting from fundamental physics and through derivation, we arrive at this precise loss formula.

Actual data shows:

· In transformers, eddy-current losses can account for 10% to 30% of the total losses.

·In high-frequency motors, this ratio is even higher.

If a solid block of iron is used, the motor’s efficiency will drop sharply by 20% to 40%, and most of the electrical energy will be wasted as heat.

02

Solution: A Dual Breakthrough in Silicon Steel Sheets

Since a single solid piece of iron wouldn't work, the engineers came up with an ingenious solution: silicon steel laminations.

This solution addresses the eddy current problem from two directions simultaneously.

1. First trick: Slice thinly to physically block the vortex.

Imagine if you were to slice a large piece of iron into many thin sheets—would eddy currents still be able to flow as freely and unimpeded as before?

It's no longer possible.

Each thin silicon steel sheet is only 0.10–0.50 mm thick (slightly thicker than a sheet of paper); eddy currents can only occur in... Single sheet A small loop forms internally, making it impossible to bridge the gap between the pieces.

Even better, apply a layer of insulating varnish between each sheet to completely cut off the “inter-sheet pathways” for eddy currents.

As a result, the large eddy that could previously “plow straight through” the entire piece of iron is broken down into countless smaller eddies, each confined to a thin layer.

Remember that formula? P ∝ d²

When the thickness is reduced from (say) 50 mm to 0.5 mm, the loss decreases by a factor of (50/0.5)² = 10,000 times!

Of course, in reality, it’s not as simple as just slicing a solid 50mm block into thin sheets—but the principle is the same: The thinner the sheet, the shorter the eddy-current path, and the lower the loss.

2. Second technique: Add silicon to increase resistivity.

But simply slicing the material thin isn't enough. There's another crucial factor: the material's own resistivity.

Pure iron has a very low resistivity, making eddy currents easy to form. Could we perhaps increase iron’s resistivity to suppress these eddy currents?

Yes. The answer is to add silicon.

When silicon is added to iron, it significantly increases the material's resistivity. Silicon atoms enter the iron lattice, increasing the resistance to electron movement—much like placing countless speed bumps on a “vortex racetrack.”

The higher the resistivity, the harder it is for eddy currents to form, and the lower the losses will be.

The effect of silicon content on performance:

·Low-silicon steel (0.8%–2% Si): Moderate resistivity, low cost, suitable for general-purpose motors.

·Medium-silicon steel (2%-3.5% Si): Has higher resistivity and lower losses, and is the most widely used.

·High-silicon steel (>3.5% Si): Has very high resistivity and extremely low losses, but is difficult to process and costly.

The most commonly used in industry are silicon steel sheets with a silicon content of around 3%, which strike an optimal balance between performance and cost.

3 Thickness Selection: The Art of Engineering

You might ask: Since thinner wafers have lower loss, why not make them ultra-thin?

Because engineering is never about pursuing perfection—it’s about achieving balance.

The thinner the sheet, the lower the eddy-current loss—but:

·Higher manufacturing costs (cutting, coating, and lamination are all more complex)

· The weaker the mechanical strength (the thinner it is, the easier it is to deform)

· The more layers there are (the more space they occupy)

· The larger the proportion of the insulation layer (the lower the effective permeability),

Commonly used thicknesses in industry:

·0.50mm: Standard industrial motor, low cost

·0.35mm: High-efficiency motor with 20% lower losses

·0.20mm: High-frequency motors (such as variable-frequency motors) have lower losses.

·0.10 mm: Ultra-high-frequency applications, high cost

This is the wisdom of engineering: It’s not about achieving the thinnest possible design, but rather finding the optimal balance among performance, cost, and manufacturing processes.

03 Effect Comparison: Let the Data Speak for Themselves

So, just how effective are silicon steel sheets exactly? Let’s let the data speak for itself.

Comparison of the Impact of Silicon Steel Sheet Thickness on No-Load Losses

Note: The relative value of no-load loss is based on a rated magnetic flux density of 1.5 T at a power frequency of 50 Hz; the data are for reference only.

This table says it all: Silicon steel sheets reduce losses by 60% to 90%, converting wasted electrical energy into useful work.

2 Actual Cases: Magnetic Levitation High-Speed Motor

At high frequency 1000 Hz, 1.5 T Under rated operating magnetic flux density conditions, a 450-mm-diameter, 300-mm-high iron core operates continuously for 365 days a year (8,760 hours), with no-load losses consisting solely of iron losses in the core (without interference from mechanical losses or copper losses).

Note: Due to inconsistencies in material data among different suppliers in the market, this data is for reference only.

Compared to a solid pure iron core, 0.2mm non-oriented silicon steel... Approximately 600,000 yuan can be saved each year. This is just one maglev motor—yet worldwide, there are tens of thousands of such maglev motors. The energy savings and cost reductions they achieve are truly astonishing!

3 Why do different motors use different thicknesses?

You may have noticed that the thickness of silicon steel sheets varies depending on the application. The reason is:

Eddy-current loss is proportional to the square of the frequency: P ∝ f².

·Power-frequency motor (50Hz): Low frequency, small eddy currents—0.50mm is sufficient.

·Variable-frequency motors (200–400 Hz): High frequency leads to significant eddy currents, requiring a lamination thickness of 0.35 mm or even 0.20 mm.

·High-speed motors (>1000 Hz): With extremely high frequencies, they must use ultra-thin sheets measuring 0.10 mm.

The higher the frequency, the more urgent the demand for thin films.

04 Summary

Returning to the question at the beginning of the article: Why not use a single piece of iron?

Now the answer is clear:

Because a solid piece of iron generates significant eddy-current losses, causing the motor efficiency to plummet by 20% to 40% and turning most of the electrical energy into useless heat.

The design of silicon steel sheets represents engineers’ masterful application of physical laws:

·Slice into thin pieces → Physically block the eddy current path → Leveraging the relationship P ∝ d², reduce losses by a factor of thousands.

·Adding silicon → Increasing resistivity → Suppressing eddy current formation

·Choose the appropriate thickness—strike a balance among performance, cost, and process—not the thinnest, but the optimal.

This seemingly simple “heap of thin metal sheets” actually embodies profound physical principles and sophisticated craftsmanship.

From the solid iron cores of the 19th century, to the invention of silicon steel sheets in the early 20th century, and now to today’s ultra-thin, high-silicon steels, each advancement represents a deeper understanding and more sophisticated application of natural laws by humanity.

The layered, thin silicon steel laminations that make up the motor’s iron core, It’s not that engineers are “going overboard”—rather, they’re using the smartest possible approach to combat the inevitable eddy-current losses in the physical world.

The above article is sourced from a researcher of motor iron cores, author Zhang Lixun.