Tendinte La Table Fesi

8. Tendinţe în creşterea performanţelor tabelelor electrotehnice În perioada de la 1940 până în 1960, pierderile în fier au scăzut mult prin dezvoltarea de materiale magnetice cu cristale orientate având orientare controlată. Aceastea au permis creşterea conţinutului de siliciu la 3,25%, fără o scădere semnificativă a inducţiei de saturaţie. Ulterior, pierderile în fier au continuat să scadă prin:  reducerea conţinutului de impurităţi,  îmbunătăţirea izolaţiei,  reducerea grosimii,  alte îmbunătăţiri. Pierderile în fier în materiale magnetice s-au diminuat în continuare prin integrarea într-o mai mare măsură a cristalelor de orientatate (anii 1970) şi aplicarea (anii 1980) în practică a tehnologiei de rafinare a domeniilor magnetice fapt ce duce la scădea pierderilor prin histeresis. Odata cu alierea cu siliciu in scopul reducerii pierderilor, rezulta si o coborire a inductiei de saturatie a materialelor deci cele doua performante de baza sunt intr-o contradictie de termeni: reducind pierderile se reduce si capacitatea de magnetizare. Silicon is the primary alloying element in electrical steels. It is added because it increases the volume resistivity of the steel and thereby reduces the eddy current component of core loss. Silicon is more effective in this respect than any other element which may be conveniently added. Magnetic characteristics are of first importance and are dependent on processing as well as on chemical composition. Form metallic materials, the conductivity will influence the facility to generate eddy currents on the material's surface.

Texture GOSS (GO) The GO sheet metallurgy is based on a highly oriented grain texture: the Goss texture. Rolling induced shearing causes, in the body-centred-cubic structure, glide along the atomic planes of highest density. One thus obtains a A10) [001] texture called "cube on edge", characterized by an easy magnetization axis in the sheet plane and very close to the rolling direction. A secondary recrystallization anneal leads to grain size increase through the anomalous selective growth of some of them. The classical GO sheets, 0.23 to 0.30 mm thick, are manufactured by continuous casting and a series of hot rolling and annealing steps. The final step is coating with a phosphatizing solution. The high permeability sheets ("Hi-B"), 0.23 to 0.30 mm thick, are obtained by a single but very strong cold rolling process around 250°C, with a reduction rate larger than 80%. After secondary recrystallization, the grain size can reach 30 mm. Finally the sheet is coated with a phosphatizing colloid containing silicon. Beyond its protective and insulating function, we will see further on that this coating plays an important role in the magnetic properties [11].

[Alex Hubert et Rudolf Schäfer "Magnetic Domains" Springer 1998] Domain structure optimization The quest for the Goss texture aims at an ideal domain structure presenting elongated 1800 domains parallel to the rolling direction. The history of loss reduction in GO sheets follows closely that of texture optimization. The reduction of dynamic and hysteresis losses requires the elimination of hindrances of any nature which slow down the wall motion on the one hand, and the refinement of the domain width on the other hand. When impurities and cold-rolling stresses have been thoroughly eliminated, wall pinning mainly comes from magnetostatic effects. Too large a discontinuity of the magnetization vector component perpendicular to an interface causes the onset of harmful secondary structures: closure domains at the grain boundaries, spike domains at the surface of the material (fig. 16.9).

Metallurgists therefore worked at improving the key parameter, grain orientation. However experiment shows that it is good to retain a slight misorientation of the easy magnetization direction with respect to the sheet plane. As long as the misorientation angle does not exceed about 2°, the decrease in magnetostatic energy occurs through a width reduction of the 180° domains without creating spike domains. The corresponding increase of the total wall area also favours high quasi-static permeability. Furthermore, too large grains are undesirable: from a given grain size (around 0.5 mm), the decrease in hysteresis loss is smaller than the increase of the dynamic losses resulting from the domain widening. The protective coating, only 2 or 3 ixm thick, exerts on the sheet a longitudinal stress of several MPa. On account of magnetoelastic coupling, the easy magnetization axis tends to be parallel to the rolling direction. The result is a reduction of the secondary (closure domain) structure, and a refinement of the 180° domain main structure. Finally, an especially efficient method of domain refinement consists in producing on the sheet surface a set of lines perpendicular to the lamination direction, and regularly spaced by a few mm to 30 mm (fig. 16.10) [7]. This treatment can be made using several processes based on two different principles. Mechanical scratching, spark- machining, continuous or pulsed laser irradiation, and plasma beams give rise to a system of tensile stresses parallel to the lamination direction. These treatments do not withstand a possible final anneal designed to relax machining stress.

Tratarea prin iradiere LASER

. THIN IRON-SILICON SHEETS

When the working frequency increases, too thick a sheet is less and less efficiently used because only two thin sheets contribute to induction on account of the skin effect. A rigorous calculation of the latter is only possible in a material with linear response and constant permeability |J., which obviously is not the case. However one obtains useful orders of magnitude by using as average permeability the slope of a hysteresis loop branch. With a relative permeability of 20,000, typical of a GO sheet, the skin thickness 8 = ^jp/n\if reaches 0.35 mm at 50 Hz. and falls below 0.1 mm at 1 kHz. This is the reason why the general trend towards an increase in operating frequency requires thinner products. One speaks of thin sheets below 0.2 mm. As for classical thickness sheets, they are marketed under GO and NO forms. As an example, a 0.1 mm thick GO sheet dissipates 15 W.kg at 400 hertz under 1.5 tesla, whereas a NO sheet of the same thickness dissipates at the same frequency 14 W.kg under 1 tesla.

Deşi din punct de vedere fenomenologic pierderile de magnetizare au cele trei componente în practică este util ca acestea să se poată calcula printr-o funcţie care să aibă un singur termen. Pentru proiectarea de transformatoare, maşini electrice rotative, bobine care funcţionează într-un domeniu de frecvenţe şi de valori ale inducţiei este avantajoas ca pierderile să poată fi calculate cu o relaţie de forma [9]: α

 f  B pov = p1 ⋅      f1   B1 

β

Unde: p1 – pierderile la o anumită valoare a inducţiei şi a frecvenţei. For others magnetization frequencies and others magnetization induction we must analytical know the losses dependence between this parameters.