Electrical steel

Electrical steel, also called lamination steel, silicon electrical steel, silicon steel or transformer steel, is specialty steel tailored to produce certain magnetic properties, such as a small hysteresis area (small energy dissipation per cycle, or low core loss) and high permeability.

The material is usually manufactured in the form of cold-rolled strips less than 2 mm thick. These strips are called laminations when stacked together to form a core. Once assembled, they form the laminated cores of transformers or the stator and rotor parts 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 erosion.

Metallurgy

Electrical steel is an iron alloy which may have from zero to 6.5% silicon (Si:5Fe). Silicon significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and thus reduces the core loss. Manganese and aluminum can be added up to 0.5%.

Increasing the amount of silicon inhibits eddy currents and narrows the hysteresis loop of the material, thus lowering the core losses. However, the grain structure hardens and embrittles the metal, which adversely affects the workability of the material, especially when rolling it. When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen must be kept low, as these elements indicate the presence of carbides, sulfides, oxides and nitrides. These compounds, 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 steel in a decarburizing atmosphere, such as hydrogen.

Physical properties examples

Melting point ~1500 °C. (example for ~3.1% silicon content) ^[1]

Density - 7650 kg/m^3 (example for 3% silicon content)

Resistivity - 47.2E-8 (Ω m) (example for 3% silicon content)

Grain orientation

There are two main types of electrical steel: grain-oriented and non-oriented.

Grain-oriented electrical steel usually has a silicon level of 3% (Si:11Fe). It is processed in such a way that the optimum 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. Due to the special orientation, 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 high-efficiency transformers, electric motor and generators.

Non-oriented electrical steel usually has a silicon level of 2 to 3.5% and has similar magnetic properties in all directions, which makes it isotropic. It is less expensive and is used in applications where the direction of magnetic flux is changing, such as electric motors and generators. It is also used when efficiency is less important or when there is insufficient space to correctly orient components to take advantage of the anisotropic properties of grain-oriented electrical steel.

Lamination coatings

Electrical steel is usually coated to increase electrical resistance between laminations, to provide resistance to corrosion or rust, and to act as a lubricant during the cutting. There are various coatings, organic and inorganic, and the coating used depends on the application of the steel.^[2] 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. Former 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. ^[3].

Magnetic properties

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 test 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. Semi-processed electrical steels are delivered in a state that, after punching the final shape, a final heat treatment develops the desired 150-micrometer grain size. The fully processed steels are usually delivered with insulating coating, full heat treatment, and defined magnetic properties, for applications where the punching operation does not significantly degrade the material properties. Excessive bending, incorrect heat treatment, or even rough handling of core steel can adversely effect its magnetic properties and may also increase noise due to magnetostriction ^[3]

Amorphous steel

For certain transformers, cores made of amorphous steel are used. This material is a metallic glass prepared by pouring molten alloy steel on a rotating cooled wheel, which cools the metal so quickly (a rate of about one million degrees per second) that crystals do not form. The resulting amorphous metal transformers may have losses due to the core material only one-third that of conventional steels. However, its high cost (about twice that of conventional steel) and lower mechanical properties make use of amorphous steel economical only for certain distribution-type transformers. ^[4]

Practical concerns

Core steel is much more costly than the mild steel used for apparatus tanks, generator frames, etc. - in 1981 it was more than twice the cost per unit weight. ^[5]

The size of magnetic domains in the sheet 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. ^[6]

See also

• Ferrosilicon, starter material for silicon steel

References

1. ^ "Note on electromigration of grain boundaries in silicon iron" - JOURNAL OF MATERIALS SCIENCE 10 (1975) - LETTERS
2. ^ Beatty, Standard Handbook for Electrical Engineers 11th ed., pg. 4-111
3. ^ ^{a b} Les Jump
4. ^ John Whincup, News Item Globe and Mail March 3rd, Federal Pioneer BAT, March 1983
5. ^ Les Jump, Transformer Steel and Cores, Federal Pioneer BAT, March 1981
6. ^ Richard de Lhorbe Steel No Lasers Here, Federal Pioneer BAT, June/July 1981

External links

• Electrical Machinery Optimization
• Allegheny Ludlum
• Encyclopedia Britannica
• Seco/Warwick
• Transformer Scrap, Transformer Core, Cold Rolled Steel
• EMERF, the Electric Motor Education and Research Foundation
• Summary of Silicon Steels