Information about silicon steel - Knowledge

Silicon steel (also known as electrical steel or silicon electrical steel) is a specialized type of steel alloyed with silicon (Si). It is primarily used in the cores of transformers, electric motors, and generators due to its exceptional magnetic properties.

1.Silicon Iron-Based: Magnetic Properties

Iron–silicon alloys represent the most widely used and most cost effective core materials for devices used in electric power handling. Magnetic or power losses of electrical steels have to be optimized to improve the performance of electric power devices. Si improves the electric properties of the electrical steels by increasing the electrical resistance, decreasing the magnetic anisotropy and the magnetostriction. It also lowers the saturation magnetization and iron losses at high frequencies. Coarse grain size can effectively decrease the eddy current and hysteresis losses. Crystallographic texture is another important factor that needs to be controlled in electrical steels. Core losses of high silicon steel are affected by texture and grain size jointly in high frequency applications. Laminations and coatings are proved to be very useful in the reduction of power losses in electrical steels.

2.Si Steels

Silicon (Si) steels are alloys of Fe and Si having important electrical applications (motors, transformers, etc.). With low-C steels, nonoriented (NO) steels and grain-oriented (GO) Si steels are referred to as electrical steels. NO Si steels have applications in rotating machinery; GO Si steels are used in devices (e.g., transformers) where an EMD parallel to a roll direction is desired. GO Si steels are further subdivided into regular grain-oriented (RGO) and high-permeability grain-oriented (HGO) materials (Boll, 1994). The drive to develop Si steels was motivated by the large increase in the resistivity of Fe with Si additions.

3.A Core Materials

The use of silicon iron as a magnetic core material was introduced in the early 1900s. Silicon iron has a relatively high permeability, and the increased resistivity due to the silicon helped reduce the eddy losses. Until the late 1930s thin laminations of hot-rolled silicon iron, often referred to as silicon steel, were used almost exclusively in transformer cores. Since hot-rolled silicon iron has approximately the same magnetic properties in all directions in the plane of the laminations, a simple 90° butt overlap corner joint could be used for stacked cores.

In the late 1930s a new material, namely, grain-oriented silicon iron, was developed for transformers. This material had improved magnetic characteristics in the direction of rolling but poorer characteristics at right angles to this direction. Hot-rolled silicon iron is composed of randomly oriented crystals or grains. This characteristic results in equal properties in all directions. In grain-oriented silicon iron the additional cold-rolling and heat treatment processes used result in laminations, with a large percentage of the crystals oriented in the direction of rolling. A 45° mitered butt overlap joint is generally used in the corners of stacked cores with oriented material to reduce joint losses. The majority of transformers today uses grain-oriented material in lamination thicknesses of the order of 0.2 to 0.3 mm.

It is interesting to note that although the present magnetic materials used in cores have a crystalline structure, there is a new class of noncrystalline magnetic alloys in the research and development stage. These are the amorphous metals, whose amorphous state is produced by the very rapid quenching of the alloy melt.

Amorphous metal alloys have much lower loss levels than grain-oriented core materials. However, since these materials are produced, at present, in the form of a very thin ribbon, they are most easily applied to wound cores. At present a considerable number of distribution transformers have been manufactured using this material, and their performance in the field is being monitored.