Electrical steel

Polycrystalline structure of electrical steel after coating has been removed.

Electrical steel (lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is a special steel tailored to produce specific magnetic properties: small hysteresis area resulting in low power loss per cycle, low core loss, and high permeability.

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 EDM.

Metallurgy

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 usually provoke brittleness during cold rolling). Manganese and aluminum can be added up to 0.5%.

Silicon significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and narrows the hysteresis loop of the material, thus lowering the core loss.[1] 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.[2]

Iron-silicon relay steel

Steel type Nominal composition[3] Alternate description
11.1% Si-FeSilicon Core Iron "A"[4]
1F1.1% Si-Fe free machiningSilicon Core Iron "A-FM"[5]
22.3% Si-FeSilicon Core Iron "B"[6]
2F2.3% Si-Fe free machiningSilicon Core Iron "B-FM"[6]
34.0% Si-Fe

Physical properties examples

  • Melting point: ~1,500 °C (example for ~3.1% silicon content)[7]
  • Density: 7,650 kg/m3 (example for 3% silicon content)
  • Resistivity: 47.2×10−8 (Ω·m) (example for 3% silicon content)

Grain orientation

Non-oriented electrical silicon steel (image made with magneto-optical sensor and polarizer microscope)

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.[8]

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.

Amorphous steel

This material is a metallic glass prepared by pouring molten alloy steel 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. It has poorer mechanical properties and as of 2010 it costs about twice as much as conventional steel, making it cost-effective only for some distribution-type transformers.[9] Transformers with amorphous steel cores can have core losses of one-third that of conventional electrical steels.

Lamination coatings

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.[10] 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.[11]

ASTM A976-03 classifies different types of coating for electrical steel.[12]

ClassificationDescription[13]For Rotors/StatorsAnti-stick treatment
C0Natural oxide formed during mill processingNoNo
C2Glass like filmNoNo
C3Organic enamel or varnish coatingNoNo
C3AAs C3 but thinnerYesNo
C4Coating generated by chemical and thermal processingNoNo
C4AAs C4 but thinner and more weldableYesNo
C4ASAnti-stick variant of C4YesYes
C5High-resistance similar to C4 plus inorganic fillerNoNo
C5AAs C5, but more weldableYesNo
C5ASAnti-stick variant of C5YesYes
C6Inorganic filled organic coating for insulation propertiesYesYes

Magnetic properties

The typical relative permeability (μr) of electrical steel is 4,000 times that of vacuum.

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.[11]

The magnetic properties of electrical steel are tested using the internationally standard Epstein frame method.[14]

Practical aspects

Electrical steel is much more costly than mild steel—in 1981 it was more than twice the cost by weight.[11]

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.[15]

See also

References

  1. ↑ K.H.J. Buschow et al, ed., Encyclopedia of Materials:Science and Technology, Elsevier, 2001, ISBN 0-08-043152-6 pp.4807-4808
  2. ↑ Y. Sidor, F. Kovac: Contribution to modeling of decarburization process in electrical steels
  3. ↑ "ASTM A867". ASTM A867. ASTM. Retrieved 1 December 2011.
  4. ↑ "Silicon Core Iron "A"". Retrieved 1 December 2011.
  5. ↑ "Silicon Core Iron "A-FM"". Retrieved 1 December 2011.
  6. 1 2 http://cartech.ides.com/datasheet.aspx?i=103&e=190&c=techart
  7. ↑ "Note on electromigration of grain boundaries in silicon iron" - Journal of Materials Science 10 (1975) - Letters
  8. ↑ Single Ended vs. Push Pull: The Deep, Dark Secrets of Output Transformers
  9. ↑ John Whincup, News Item Globe and Mail March 3rd, Federal Pioneer BAT, March 1983
  10. ↑ Beatty, Standard Handbook for Electrical Engineers 11th ed., pg. 4-111
  11. 1 2 3 Les Jump, Transformer Steel and Cores, Federal Pioneer BAT, March 1981
  12. ↑ "ASTM A976 - 03(2008) Standard Classification of Insulating Coatings by Composition, Relative Insulating Ability and Application". ASTM A976 - 03(2008). ASTM.
  13. ↑ "Classification of Insulating Coating for Electrical Steel" (PDF). Retrieved 27 March 2013.
  14. ↑ IEC 60404-2
  15. ↑ Richard de Lhorbe Steel No Lasers Here, Federal Pioneer BAT, June/July 1981

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