Electrical steel (lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is actually a special steel tailored to generate specific magnetic properties: small hysteresis area leading to low power loss per cycle, low core loss, and permeability.
Electrical steel is usually produced in cold-rolled strips under 2 mm thick. These strips are cut to shape to make laminations that are stacked together to make the laminated cores of transformers, and also the stator and rotor of electric motors. Laminations could be cut with their finished shape from a punch and die or, in smaller quantities, may be cut by a laser, or by Core cutting machine.
Silicon significantly raises the electrical resistivity in the steel, which decreases the induced eddy currents and narrows the hysteresis loop of the material, thus decreasing the core loss. However, the grain structure hardens and embrittles the metal, which adversely affects the workability from the material, particularly if rolling it. When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen must be kept low, as these elements indicate the actual existence of carbides, sulfides, oxides and nitrides. These compounds, even in particles no more than one micrometer in diameter, increase hysteresis losses while also decreasing magnetic permeability. The existence of carbon features a more detrimental effect than sulfur or oxygen. Carbon also causes magnetic aging whenever it slowly leaves the solid solution and precipitates as carbides, thus resulting in a rise in power loss as time passes. For these reasons, the carbon level is kept to .005% or lower. The carbon level can be reduced by annealing the steel in a decarburizing atmosphere, like hydrogen.
Electrical steel made without special processing to regulate crystal orientation, non-oriented steel, usually includes a silicon level of 2 to 3.5% and it has similar magnetic properties in every directions, i.e., it is isotropic. Cold-rolled non-grain-oriented steel is frequently abbreviated to CRNGO.
Grain-oriented electrical steel usually includes a silicon amount of 3% (Si:11Fe). It is processed in a manner that the optimal properties are developed in the rolling direction, as a result of tight control (proposed by Norman P. Goss) of the crystal orientation relative to the sheet. The magnetic flux density is increased by 30% inside the coil rolling direction, although its magnetic saturation is decreased by 5%. It can be employed for the cores of power and distribution transformers, cold-rolled grain-oriented steel is usually abbreviated to CRGO.
CRGO is usually supplied by the producing mills in coil form and must be cut into “laminations”, that happen to be then used to form a transformer core, that is an important part of any transformer. Grain-oriented steel is used in large power and distribution transformers and then in certain audio output transformers.
CRNGO is cheaper than core cutting machine. It is actually used when price is more important than efficiency and also for applications where direction of magnetic flux is not really constant, like in electric motors and generators with moving parts. It can be used when there is insufficient space to orient components to make use of the directional properties of grain-oriented electrical steel.
This material is actually a metallic glass prepared by pouring molten alloy steel onto a rotating cooled wheel, which cools the metal for a price around one megakelvin per second, so fast that crystals will not form. Amorphous steel is limited to foils around 50 µm thickness. It provides poorer mechanical properties and also as of 2010 it costs about twice as much as conventional steel, rendering it inexpensive exclusively for some distribution-type transformers.Transformers with amorphous steel cores may have core losses of just one-third those of conventional electrical steels.
Electrical steel is often coated to increase electrical resistance between laminations, reducing eddy currents, to provide potential to deal with corrosion or rust, as well as to serve as a lubricant during die cutting. There are many coatings, organic and inorganic, and also the coating used depends upon the application of the steel. The sort of coating selected is dependent upon the temperature treatment of the laminations, regardless of if the finished lamination will be immersed in oil, along with the working temperature in the finished apparatus. Very early practice was to insulate each lamination using a layer of paper or possibly a varnish coating, but this reduced the stacking factor from the core and limited the maximum temperature in the core.
The magnetic properties of electrical steel are influenced by heat treatment, as improving the average crystal size decreases the hysteresis loss. Hysteresis loss is dependent upon a regular 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.
Electrical steel can be delivered in a semi-processed state to ensure that, after punching the final shape, one final heat treatment can be applied to produce the normally required 150-micrometer grain size. Fully processed electrical steel is generally delivered by having an insulating coating, full heat treatment, and defined magnetic properties, for dexupky53 where punching does not significantly degrade the electrical steel properties. Excessive bending, incorrect heat treatment, or perhaps rough handling can adversely affect electrical steel’s magnetic properties and might also increase noise as a result of magnetostriction.
The magnetic properties of electrical steel are tested while using internationally standard Epstein frame method.
Electrical steel is far more costly than mild steel-in 1981 it absolutely was more than twice the fee by weight.
The dimensions of magnetic domains in crgo cutting machine could be reduced by scribing the surface of the sheet with a laser, or mechanically. This greatly cuts down on the hysteresis losses within the assembled core.