Electrical steel (lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is a special steel tailored to create specific magnetic properties: small hysteresis area causing low power loss per cycle, low core loss, and permeability.
Electrical steel is normally manufactured in cold-rolled strips below 2 mm thick. These strips are cut to contour around make laminations that happen to be stacked together to form the laminated cores of transformers, and the stator and rotor of electric motors. Laminations may be cut for their finished shape by way of a punch and die or, in smaller quantities, can be cut by a laser, or by cut to length machine.
Silicon significantly improves the electrical resistivity in the steel, which decreases the induced eddy currents and narrows the hysteresis loop from the material, thus lowering the core loss. However, the grain structure hardens and embrittles the metal, which adversely affects the workability of the material, specially when rolling it. When alloying, the concentration amounts of carbon, sulfur, oxygen and nitrogen needs to be kept low, as these elements indicate the inclusion of carbides, sulfides, oxides and nitrides. These compounds, even during particles as small as one micrometer in diameter, increase hysteresis losses whilst decreasing magnetic permeability. The presence of carbon carries 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 causing an increase in power loss after a while. For these reasons, the carbon level is kept to .005% or lower. The carbon level may be reduced by annealing the steel within a decarburizing atmosphere, for example hydrogen.
Electrical steel made without special processing to regulate crystal orientation, non-oriented steel, usually carries a silicon level of 2 to 3.5% and possesses similar magnetic properties in all directions, i.e., it is actually isotropic. Cold-rolled non-grain-oriented steel is frequently abbreviated to CRNGO.
Grain-oriented electrical steel usually features a silicon degree of 3% (Si:11Fe). It is processed in a manner that this optimal properties are developed in the rolling direction, because of a tight control (proposed by Norman P. Goss) from the crystal orientation relative to the sheet. The magnetic flux density is increased by 30% within the coil rolling direction, although its magnetic saturation is decreased by 5%. It is employed for the cores of power and distribution transformers, cold-rolled grain-oriented steel is frequently abbreviated to CRGO.
CRGO is often provided by the producing mills in coil form and must be cut into “laminations”, that happen to be then used to form a transformer core, which happens to be an important part of any transformer. Grain-oriented steel is utilized in large power and distribution transformers and then in certain audio output transformers.
CRNGO is more affordable than cut to length. It is used when cost is more important than efficiency and for applications the location where the direction of magnetic flux will not be constant, like electric motors and generators with moving parts. It can be used if you have insufficient space to orient components to take advantage of the directional properties of grain-oriented electrical steel.
This product is really 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 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, making it cost-effective only for some distribution-type transformers.Transformers with amorphous steel cores could have core losses of merely one-third that of conventional electrical steels.
Electrical steel is normally coated to enhance electrical resistance between laminations, reducing eddy currents, to supply resistance to corrosion or rust, and to serve as a lubricant during die cutting. There are various coatings, organic and inorganic, and the coating used is dependent upon the use of the steel. The particular coating selected is determined by the heat treatments for the laminations, if the finished lamination is going to be immersed in oil, and also the working temperature in the finished apparatus. Very early practice would be to insulate each lamination using a layer of paper or even a varnish coating, but this reduced the stacking factor of your core and limited the most temperature from the core.
The magnetic properties of electrical steel are determined by heat treatment, as boosting the average crystal size decreases the hysteresis loss. Hysteresis loss depends upon a typical test and, for common grades of electrical steel, may cover anything from a couple of to 10 watts per kilogram (1 to 5 watts per pound) at 60 Hz and 1.5 tesla magnetic field strength.
Electrical steel could be delivered inside 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 often delivered with the 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, as well as rough handling can adversely affect electrical steel’s magnetic properties and could also increase noise because of magnetostriction.
The magnetic properties of electrical steel are tested utilizing the internationally standard Epstein frame method.
Electrical steel is a lot more costly than mild steel-in 1981 it was actually over twice the cost by weight.
How big magnetic domains in Transformer core cutting machine might be reduced by scribing the top of the sheet with a laser, or mechanically. This greatly cuts down on the hysteresis losses inside the assembled core.