Electrical steel (lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) can be a special steel tailored to make specific magnetic properties: small hysteresis area causing low power loss per cycle, low core loss, and high permeability.
Electrical steel is often made in cold-rolled strips less than 2 mm thick. These strips are cut to contour around make laminations which are stacked together to make the laminated cores of transformers, and also the stator and rotor of electric motors. Laminations could be cut to their finished shape by a punch and die or, in smaller quantities, could be cut by a laser, or by Core cutting machine.
Silicon significantly raises the electrical resistivity from the steel, which decreases the induced eddy currents and narrows the hysteresis loop of the material, thus reducing the core loss. However, the grain structure hardens and embrittles the metal, which adversely affects the workability in the material, specially when rolling it. When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen has to be kept low, because these elements indicate the actual existence of carbides, sulfides, oxides and nitrides. These compounds, in particles no more than one micrometer in diameter, increase hysteresis losses whilst decreasing magnetic permeability. The existence of carbon features a more detrimental effect than sulfur or oxygen. Carbon also causes magnetic aging if it slowly leaves the solid solution and precipitates as carbides, thus leading to an increase in power loss after a while. For these reasons, the carbon level is kept to .005% or lower. The carbon level might be reduced by annealing the steel in a decarburizing atmosphere, such as hydrogen.
Electrical steel made without special processing to control crystal orientation, non-oriented steel, usually carries a silicon amount of 2 to 3.5% and contains similar magnetic properties in all of the directions, i.e., it can be isotropic. Cold-rolled non-grain-oriented steel is usually abbreviated to CRNGO.
Grain-oriented electrical steel usually has a silicon degree of 3% (Si:11Fe). It is actually processed in such a way that the optimal properties are developed in the rolling direction, as a result of tight control (proposed by Norman P. Goss) from the crystal orientation in accordance with the sheet. The magnetic flux density is increased by 30% within 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 often abbreviated to CRGO.
CRGO is often supplied by the producing mills in coil form and needs to be cut into “laminations”, which can be then used to make a transformer core, that is an important part of any transformer. Grain-oriented steel is commonly used in large power and distribution transformers as well as in certain audio output transformers.
CRNGO is less costly than core cutting machine. It can be used when price is more valuable than efficiency as well as for applications where the direction of magnetic flux is not really constant, as with electric motors and generators with moving parts. It can be used if you find insufficient space to orient components to benefit from the directional properties of grain-oriented electrical steel.
This material can be a metallic glass prepared by pouring molten alloy steel onto a rotating cooled wheel, which cools the metal for a price of approximately one megakelvin per second, so quickly that crystals do not form. Amorphous steel is limited to foils around 50 µm thickness. It has poorer mechanical properties and also as of 2010 it costs about double the amount as conventional steel, so that it is cost-effective only for some distribution-type transformers.Transformers with amorphous steel cores might have core losses of merely one-third that relating to conventional electrical steels.
Electrical steel is usually coated to improve electrical resistance between laminations, reducing eddy currents, to deliver effectiveness against corrosion or rust, and also to act as a lubricant during die cutting. There are many coatings, organic and inorganic, along with the coating used depends upon the application of the steel. The type of coating selected is determined by the heat therapy for the laminations, regardless of if the finished lamination will probably be immersed in oil, as well as the working temperature from the finished apparatus. Very early practice ended up being to insulate each lamination using a layer of paper or even a varnish coating, but this reduced the stacking factor from the core and limited the utmost temperature in the core.
The magnetic properties of electrical steel are dependent on heat treatment, as enhancing the average crystal size decreases the hysteresis loss. Hysteresis loss depends on a typical 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 might be delivered in a semi-processed state to ensure, after punching the ultimate shape, a final heat treatment can be applied to produce the normally required 150-micrometer grain size. Fully processed electrical steel is often delivered by having an insulating coating, full heat treatment, and defined magnetic properties, for dexupky53 where punching will 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.
The magnetic properties of electrical steel are tested using the internationally standard Epstein frame method.
Electrical steel is more costly than mild steel-in 1981 it was actually more than twice the cost by weight.
The dimensions of magnetic domains in Transformer core cutting machine may be reduced by scribing the top of the sheet by using a laser, or mechanically. This greatly reduces the hysteresis losses in the assembled core.