Why Silicon Steel Is Used in Electrical Machines?
Apr 30, 2024
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Silicon steel is a soft magnetic material that is used in electrical power transformers, motors and generators. It has a high silicon content of about 3.2 mass %, which increases the electrical resistivity of iron and, therefore, reduces eddy current losses. Grain-oriented silicon steel that is used for non-rotating applications, i.e. transformers, is characterised by a strong preferred crystallographic orientation. In iron the easiest directions of magnetisation are the <001> crystal directions. In grain-oriented silicon steel the Goss orientation, i.e. the {110}<001> orientation , is technologically realised to minimise magnetic losses in electrical transformers.
Texture and microstructure control in electrical steels
Grain-oriented silicon steel (Fe-Si steel; transformer steel; electrical steel) is a soft magnetic polycrystalline metallic alloy that is used as core material in electrical transformer cores and electrical motors.
In the power industry electrical voltage is almost always AC and has a rather low frequency, namely, 50-60 Hz. At these frequencies electrical eddy currents are generated in the transformer core. Alloying the Fe with Si has a large marked effect on the electric resistivity of the material, with an increase of a factor of 4 for 3wt%Si. Silicon also has the effect of reducing the magnetostriction (i.e. length change on magnetisation) and the magnetocrystalline anisotropy. In addition, the material is used in the form of laminations, typically 0.3 to 0.7mm thick. The addition of too much silicon makes the material extremely brittle and difficult to produce, giving a practical limitation of 4wt% to the amount of Si that can be added. Recently, a technique has been developed to produce laminations with >6wt% Si, by a SiCl4 chemical vapour deposition treatment to enrich the laminations with Si after forming the laminations. Typically most commercially available electrical steels will contain between 3 and 4 wt% Si.
Texture and microstructure control for electrical transformer applications
For transformer applications the flux lies predominantly in the length of the laminations and therefore it is desirable to enhance the permeability in this direction. This is achieved by various hot and cold rolling stages to produce textured sheets, known as grain-oriented silicon-steel, with the [001] direction in the length of the lamination. The <001> type crystal directions are the easy directions of magnetisation and hence the permeability is greater. Therefore, electrical steels are characterized by a pronounced Goss texture, i.e. a (110)<001> preferred crystal orientation.
Relevance of the Goss texture for electrical steels in case of transformer applications
Since the <001> crystallographic directions are the ones with the easiest directions of magnetisation and hence largest permeability electrical steels suited for transformer aplications are characterized by a pronounced Goss texture, i.e. a (110)<001> preferred crystal orientation relative to the main magnetic design direction of the transformer.
