How Does Electrical Steel Work?

Iron alloy optimized for magnetic properties

Electrical steel (E-steel, lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is speciality steel used in the cores of electromagnetic devices such as motors, generators, and transformers because it reduces power loss. It is an iron alloy with silicon as the main additive element (instead of carbon).

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Metallurgy

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Electrical steel is an iron alloy which may have from zero to 6.5% silicon (Si:5Fe). Commercial alloys usually have silicon content up to 3.2% (higher concentrations result in brittleness during cold rolling). Manganese and aluminum can be added up to 0.5%.[1]

Silicon increases the electrical resistivity of iron by a factor of about 5; this change decreases the induced eddy currents and narrows the hysteresis loop of the material, thus lowering the core loss by about three times compared to conventional steel.[1][2] However, the grain structure hardens and embrittles the metal; this change adversely affects the workability of the material, especially when rolling. When alloying, contamination must be kept low, as carbides, sulfides, oxides and nitrides, even in particles as small as one micrometer in diameter, increase hysteresis losses while also decreasing magnetic permeability. The presence of carbon has a more detrimental effect than sulfur or oxygen. Carbon also causes magnetic aging when it slowly leaves the solid solution and precipitates as carbides, thus resulting in an increase in power loss over time. For these reasons, the carbon level is kept to 0.005% or lower. The carbon level can be reduced by annealing the alloy in a decarburizing atmosphere, such as hydrogen.[1][3]

Iron-silicon relay steel

[edit] Steel type Nominal composition[4] Alternate description 1 1.1% Si-Fe Silicon Core Iron "A"[5] 1F 1.1% Si-Fe free machining Silicon Core Iron "A-FM"[6] 2 2.3% Si-Fe Silicon Core Iron "B"[7] 2F 2.3% Si-Fe free machining Silicon Core Iron "B-FM"[7] 3 4.0% Si-Fe Silicon Core Iron "C"[8]

Physical properties examples

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• Melting point: ~1,500 °C (example for ~3.1% silicon content)[9]
• Density: 7,650 kg/m3 (example for 3% silicon content)
• Resistivity (3% silicon content): 4.72×10−7 Ω·m (for comparison, pure iron resistivity: 9.61×10−8 Ω·m)

Grain orientation

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Electrical steel made without special processing to control crystal orientation, non-oriented steel, usually has a silicon level of 2 to 3.5% and has similar magnetic properties in all directions, i.e., it is isotropic. Cold-rolled non-grain-oriented steel is often abbreviated to CRNGO.

Grain-oriented electrical steel usually has a silicon level of 3% (Si:11Fe). It is processed in such a way that the optimal properties are developed in the rolling direction, due to a tight control (proposed by Norman P. Goss) of the crystal orientation relative to the sheet. The magnetic flux density is increased by 30% in the coil rolling direction, although its magnetic saturation is decreased by 5%. It is used for the cores of power and distribution transformers, cold-rolled grain-oriented steel is often abbreviated to CRGO.

CRGO is usually supplied by the producing mills in coil form and has to be cut into "laminations", which are then used to form a transformer core, which is an integral part of any transformer. Grain-oriented steel is used in large power and distribution transformers and in certain audio output transformers.[10]

CRNGO is less expensive than CRGO. It is used when cost is more important than efficiency and for applications where the direction of magnetic flux is not constant, as in electric motors and generators with moving parts. It can be used when there is insufficient space to orient components to take advantage of the directional properties of grain-oriented electrical steel.

• Magnetic domains and domain walls in oriented silicon steel (image made with CMOS-MagView)
• Magnetic domains and domain walls in oriented silicon steel (image made with CMOS-MagView)
• Magnetic domains and domain walls in non-oriented silicon steel (image made with CMOS-MagView)

Amorphous steel

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This material is a amorphous metal, or metallic glass, prepared by pouring molten alloy onto a rotating cooled wheel, which cools the metal at a rate of about one megakelvin per second, so fast that crystals do not form. Amorphous steel is limited to foils of about 50 μm thickness. The mechanical properties of amorphous steel make stamping laminations for electric motors difficult. Since amorphous ribbon can be cast to any specific width under roughly 13 inches and can be sheared with relative ease, it is a suitable material for wound electrical transformer cores. In , the price of amorphous steel outside the US was approximately $.95/pound compared to HiB grain-oriented steel which costs approximately $.86/pound. Transformers with amorphous steel cores can have core losses of one-third that of conventional electrical steels.

Lamination

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Electrical steel is usually manufactured in cold-rolled strips less than 2 mm thick. These strips are cut to shape to make laminations which are stacked together to form the laminated cores of transformers, and the stator and rotor of electric motors. Laminations may be cut to their finished shape by a punch and die or, in smaller quantities, may be cut by a laser, or by wire electrical discharge machining.

Electrical steel is usually coated to increase electrical resistance between laminations, reducing eddy currents, to provide resistance to corrosion or rust, and to act as a lubricant during die cutting. There are various coatings, organic and inorganic, and the coating used depends on the application of the steel.[11] The type of coating selected depends on the heat treatment of the laminations, whether the finished lamination will be immersed in oil, and the working temperature of the finished apparatus. Very early practice was to insulate each lamination with a layer of paper or a varnish coating, but this reduced the stacking factor of the core and limited the maximum temperature of the core.[12]

ASTM A976-03 classifies different types of coating for electrical steel.[13]

Classification Description[14] For Rotors/Stators Anti-stick treatment C0 Natural oxide formed during mill processing No No C2 Glass like film No No C3 Organic enamel or varnish coating No No C3A As C3 but thinner Yes No C4 Coating generated by chemical and thermal processing No No C4A As C4 but thinner and more weldable Yes No C4AS Anti-stick variant of C4 Yes Yes C5 High-resistance similar to C4 plus inorganic filler Yes No C5A As C5, but more weldable Yes No C5AS Anti-stick variant of C5 Yes Yes C6 Inorganic filled organic coating for insulation properties Yes Yes

Magnetic properties

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The typical relative permeability (μr) of electrical steel is 4,000-38,000 times that of vacuum, compared to 1.003- for stainless steel.[15][16][17]

The magnetic properties of electrical steel are dependent on heat treatment, as increasing the average crystal size decreases the hysteresis loss. Hysteresis loss is determined by a standard Epstein tester and, for common grades of electrical steel, may range from about 2 to 10 watts per kilogram (1 to 5 watts per pound) at 60 Hz and 1.5 tesla magnetic field strength.

Electrical steel can be delivered in a semi-processed state so that, after punching the final shape, a final heat treatment can be applied to form the normally required 150-micrometer grain size. Fully processed electrical steel is usually delivered with an insulating coating, full heat treatment, and defined magnetic properties, for applications where punching does not significantly degrade the electrical steel properties. Excessive bending, incorrect heat treatment, or even rough handling can adversely affect electrical steel's magnetic properties and may also increase noise due to magnetostriction.[12]

The magnetic properties of electrical steel are tested using the internationally standard Epstein frame method.[18]

The size of magnetic domains in sheet electrical steel can be reduced by scribing the surface of the sheet with a laser, or mechanically. This greatly reduces the hysteresis losses in the assembled core.[19]

Applications

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Non-grain-oriented electrical steel (NGOES) is mainly used in rotating equipment, for example, electric motors, generators and over frequency and high-frequency converters. Grain-oriented electrical steel (GOES), on the other hand, is used in static equipment such as transformers.[20]

See also

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• Ferrosilicon, starter material for silicon steel
• Mumetal
• Permalloy
• Supermalloy

References

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When thoughts turn to the modernization and decarbonization of our transportation infrastructure, one imagines it to be dominated by exotic materials. EV motors and wind turbine generators need magnets made with rare earth metals (which turn out to be not all that rare), batteries for cars and grid storage need lithium and cobalt, and of course an abundance of extremely pure silicon is needed to provide the computational power that makes everything work. Throw in healthy pinches of graphene, carbon fiber composites and ceramics, and minerals like molybdenum, and the recipe starts looking pretty exotic.

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As necessary as they are, all these exotic materials are worthless without a foundation of more familiar materials, ones that humans have been extracting and exploiting for eons. Mine all the neodymium you want, but without materials like copper for motor and generator windings, your EV is going nowhere and wind turbines are just big lawn ornaments. But just as important is iron, specifically as the alloy steel, which not only forms the structural elements of nearly everything mechanical but also appears in the stators and rotors of motors and generators, as well as the cores of the giant transformers that the electrical grid is built from.

Not just any steel will do for electrical use, though; special formulations, collectively known as electrical steel, are needed to build these electromagnetic devices. Electrical steel is simple in concept but complex in detail, and has become absolutely vital to the functioning of modern society. So it pays to take a look at what electrical steel is and how it works, and why we’re going nowhere without it.

Iron vs. Steel

The idea for a feature about electrical steel came from a story bemoaning delays plaguing renewable energy projects in the United States, mainly due to supply chain issues with the transformers needed to upgrade and expand the electrical grid. Building wind and solar farms is one thing; connecting them to the existing grid is another, one that often requires building completely new substations and refurbishing existing ones to gather the output of geographically dispersed generators and boost it to an appropriate voltage for long-haul transmission. Substations need transformers, often lots of them, and transformers are large, complicated devices that more often than not are custom-built. Lead times on large power transformers now routinely exceed 150 weeks!

The root cause of the three-year wait for large power transformers comes down to raw material supply chain problems, particularly with electrical steel. The electrical steel market is global both on the supply and demand side, so disruptions in one part of the world can ripple through the entire market. The electrical steel market’s current disruptions can be blamed on a host of factors: pandemic-era shutdowns of mines and factories, international sanctions, tariffs and trade disputes, off-shoring of manufacturing, and probably about a dozen other things. What it all means, though, is too little of this specialized material to go around.

So what is electrical steel? In some ways, the name is a misnomer; while electrical steel alloys are formulated specifically to change their electrical characteristics, these changes result in different magnetic properties, which is the key to understanding what they are and why they’re important. Electrical steel, which is used in the cores of nearly every device that uses magnetism, is probably better called “magnetic steel.” The material does have a few other monikers that better reflect this, such as “relay steel” and “transformer steel,” but the name “silicon steel” is perhaps most chemically descriptive, for reasons that will soon become obvious.

All steels are simply alloys composed primarily of iron and carbon, and electrical steel is no different. Pure iron is quite soft and ductile; the addition of carbon in just the right amounts serves as a hardening agent that gives the alloy its increased tensile strength and other desirable properties. Being primarily composed of a metallic element, steel is a good conductor of electricity. That sounds like it would be a beneficial property, and indeed it can be, as in the case of automotive electrical systems, which often use the steel body and chassis as a low-impedance return path.

Hysteresis Control

However, in electromagnetic assemblies like motors, generators, and transformers, carbon steel’s conductivity ends up causing problems that need to be solved. This has to do with the ferromagnetic properties of the iron in the steel, such as magnetic permeability and magnetic coercivity. Magnetic permeability measures the degree to which an external magnetic field, such as from a coil of wire carrying an electric current, induces a magnetic field in a material. Permeability is what makes steel stick to a permanent magnet — the magnet induces a temporary magnetic field in the high-permeability steel, causing the two to stick together. Coercivity, on the other hand, measures the degree to which a ferromagnetic material can resist becoming demagnetized by an externally applied field.

Electromagnetic components like transformers exhibit hysteresis, which just refers to how the history of a system can affect its current state. In an electromagnet, for instance, the core stays magnetic for a while after the current stops flowing in the coil. Likewise, in a transformer, the magnetic field created in the core by the alternating current passing through the coil doesn’t instantly collapse when the current reverses polarity. Rather, it lags somewhat, creating the characteristic shape seen on a hysteresis loop diagram, which maps the magnetic force applied by the coil against the magnetic field density in the iron core.

The details of why the magnetic hysteresis loop diagram looks the way it does aren’t really important to understanding electrical steel except to say that the narrower the loop, the more efficient the transformer (or motor or generator). This is because the induced magnetic field in the core spends less time opposing the magnetic field in the coils. But this leads to a paradox: pure iron has a very slender hysteresis loop, while alloying iron with carbon widens the loop. It seems like steel is therefore a poor choice for transformer cores than pure iron. What gives?

As with everything in engineering — and life in general — there are tradeoffs. While pure iron may be the best choice in terms of minimizing hysteresis losses, iron is a soft, ductile metal that would be difficult to form into mechanically stable transformer cores. The problem would be even worse in motors and generators, where rotors and stators need to stand up to the torque produced or applied. Steel is the material of choice for these applications, but the trick is to alloy it in a way that makes it mechanically strong while minimizing electrical losses.

Eddy Current Losses

But wait — hysteresis losses aren’t the only losses electrical steel needs to deal with. There are also losses thanks to the familiar eddy currents, which are electrical currents induced in metals by magnetic lines of force passing through them. Eddy currents both physically oppose the torque of motors and generators, and dissipate electrical energy through heating. Since the magnitude of eddy currents is proportional to the area of the conductor — the iron core, in this case — it pays to reduce the size of the core. Or, as is more typically the case, to build cores from stacks of thin laminations, each electrically isolated from each other but which act as a monolithic component magnetically.

Also, eddy currents are inversely proportional to the resistivity of the core material. Put all these together and what you need is a material with the magnetic permeability and coercivity of pure iron, the structural properties of carbon steel, and electrical properties that land somewhere in between. That’s electrical steel.

The magic ingredient that gives electrical steel its desirable properties is silicon. Like all metals, iron is electrically conductive thanks to unpaired electrons in its outer orbital shells. Silicon, on the other hand, is a semiconductor with a higher resistivity (measured in ohm-meters, or Ω·m). When silicon is alloyed with iron and carbon at a concentration of between 3% and 6% by weight of finished metal, it increases the resistivity of the resulting steel. This occurs thanks to a combination of refining the grain structure (smaller grains mean higher resistivity) and forming a solid solution, where silicon dissolves into the iron-carbon matrix and reduces the number of free electrons available carry charge. The lower resistivity of silicon steel narrows the hysteresis loop and helps reduce the losses due to eddy currents compared to carbon steel, while maintaining a lot of the magnetic properties of pure iron and delivering the structural properties necessary for the application.

Going With The Grain

Because almost all electrical steel is used to make laminated cores, rotors, and stators, it’s usually manufactured as cold rolled coil stock less than 2 mm thick but sometimes as thin as 0.2 mm. Electrical steels are categorized by their grain structure and orientation. Non-oriented grain steels are cheaper to manufacture and have similar magnetic properties in all directions. This isotropism makes non-oriented grain steel more suitable for use in applications like motors and generators where the magnetic flux is constantly changing.

For applications where the magnetic flux doesn’t change much with time, like the large power transformers that are in such short supply these days, grain-oriented electrical steel is desirable. The magnetic lines of force in transformers mainly line up with the long axis of the laminations, so core material needs to have greater magnetic permeability in that direction. Grain-oriented steels suit this application better because the magnetic moments in the metal line up in the direction of rolling, giving strongly anisotropic magnetic properties. The first person to make grain-oriented silicon steel was a fellow named Norman Goss, who in invented a method that alternates cold-rolling and heat treatment of silicon steel to produce a steel with grains lined up in the direction of rolling. The process results in steel with a distinctive appearance known as “Goss texture.”

Since most electrical steel is destined to be laminated, coil stock is also often coated with various non-conductive materials at the factory. Coatings can be as simple as varnish or enamel coating, which are often used for coil stock destined for rotors and stators, to glass and even ceramic coatings.

Only about 1% of the 2 billion metric tons of steel produced in was electrical steel. It’s an impressive amount, to be sure, but we’re going to have to find a way to keep up with demand for non-oriented electrical steel for hybrid and EV traction motors, as well as the grain-oriented steel needed to build all the new grid components and charging stations they’ll need. Here’s hoping manufacturers find a way to keep the magic going.