Ferromagnetics: iron, nickel, cobalt, gadolinium and their different alloys — in magnetic field are magnetized. Стержень from ferromagnetic material, placed in magnetic field, for example in the field of the coil, streamline by current, finds strong magnetic properties. On the end of the rod which the magnetic flux leaves arises northern polyusn, and on its opposite end — the South Pole of S.
The magnetized rod removed from magnetic field somewhat keeps magnetic properties.
This its ostayotochny namagnicheniye. It is insignificant at the soft zheyolez, but is strongly expressed at hard-facing ferromagnetic alloys. The permanent magnetism at the magnetorigid materials applied to production of permanent magnets is rather big: cobalt steel, splayov magnico , etc.
The theory of ferromagnetics is very difficult, but for the main electrotechnical calculations it is not used. It is only possible to mention that the ferromagnetic body breaks up to obyolast of spontaneous magnetization inside — ferromagnetic domains, something similar to microscopic constant magyonita. These areas are rather big and contain the whole grupyopa of atoms. Under certain conditions domains on the surface of the ferromagnetic can be observed under the microscope. Magnetic forces between two next domains possess mutually demagnetizing action, i.e. aim to establish domeyona in opposite to the napravleyoniya ("antiparallelno"). In the reyozultata magnetic fields of domeyon are mutually compensated.
Under the influence of the external magnetic field the magnetic axis in domains turns and borders between them smeyoshchatsya. The last is characteristic difference of domains from voobrazhayeyomy elementary magnetics. Orientation of domains on the nayopravleniye of the field creates magnityony poles on the corresponding surfaces of the body. Forces of vneshyony magnetic field have to overcome counteraction of the internal forces in substance aiming to hold domains in peryovonachalny situation. Therefore than magnetic field, t is stronger. е the more its strength, the more is magnetized the ferromagnetic. At last, at considerable strength of the vneshyony field all domains are guided in the direction of the external field. This state is called as magnetic saturation of the ferromagnetic.
Change of situation and borders of domains is connected with smeshcheyoniy the atoms entering them and causes change of the shape of the nayomagnichivayemy body that the m and of N and t about with t р and to c and е й (Latin the words "stricio" — tension is called). In particular, iron and nickel rods are extended in the direction the namagnichivayoniya. This lengthening is not enough, about no more than one thousand, but has essential practical value. For example, it causes buzz of transformers.
For electrotechnical calculations important that the inyoduktion attitude towards strength, i.e. magnetic conductivity of ma, at feryoromagnetik is very big and changeable. Big magnetic conductivity of ferromagnetics is used to increase magnetic fields in electrical machines and apparayota. Inconstancy of magnetic conductivity significantly zayotrudnyat calculations. The dependence In from H at ferromagnetics is otnoyositelno difficult and cannot be expressed by any simple formula. For this reason it is necessary to use the graficheyosky image — the magnetization curve. The lower branch of such curve corresponds to the unsaturated condition of the ferromagnetic, the excess ("knee") of the curve — to the pereyokhod in the saturated state. The type of the magnetization curve opreyodelyatsya by properties of this magnetic medium.
Ferromagnetic cores of electrical machines and apparayot are calculated so that in working order devices magnetic induction corresponded to the certain part kriyovy core material magnetization.
In measuring apparatuses, devices of regulation and avtoyomatichesky protection it is necessary that magnetic induction изменялась in proportion to the magnetizing current. This condition buyodt is executed if during the operation of the device magnetic induction changes within the unsaturated part of the curve of the namagyonichivaniye.
For possibly best use of electrotechnical materials to receive the minimum of costs of materials of the power unit, it is reasonable to choose induction, primeryono corresponding to the magnetization curve knee.
If it is necessary that at fluctuations of the magnetizing current the magnetic flux excited by it changed a little (for example, in direct current generators), then magnetic induction has to correspond to the saturated branch of the magnetization curve.
The above-stated magnetization curve is the pervonachalyyony magnetization curve, i.e. the curve which can be poluyochit if to take completely degaussed sample of the mateyorial and to increase gradually, beginning from zero, its namagnichivayoyushchy current.
In operating conditions magnetic induction depends not only on magnetic field strength, but also on the previous magyonitny condition of the body. Process of magnetization okayozyvatsya partially irreversible is the phenomenon of the hysteresis (about Greek "delay"). Owing to the hysteresis at decrease in the magnetizing current magnetic induction decreases not on the curve of initial magnetization, and on the curve located above, than curve initial the namagnichivayoniya.
At the strength of the magyonitny field N equal to zero when the magnetizing current is switched off, in the ferromagnetic some ostatochyony induction of B0 will remain. That полностью to degauss the body, the neobyokhodim the field gradient of backward direction of Nanosecond called by coercive siyoly. To excite such degaussing field, необходимо to pass current of the return nayopravleniye on the magnetizing coil. If further to increase magnetic field strength of backward direction, then there will be the magnetic indukyotion of backward direction. Having increased current, it can be brought to the condition of saturation to which will correspond znacheyony to magnetic induction of Bm. If now to reduce namagniyochivayushchy current, then magnetic induction will decrease on the curve lying below abscissa axis, but same in the form as the curve of decrease of induction at positive weeding napravleyoniya. At reduction to zero magnetizing current in the ferromagnetic residual induction of backward direction — B0 will remain. If then again to include current in the pervonachalyyony direction and to gradually increase it to the value corresponding to saturation of the ferromagnetic, then the closed curve representing dependence of induction on field gradient at cyclic magnetic reversal will turn out. This curve is called the hysteresis curve. In conditions такого cyclic magnetic reversal anchors of machines of poyostoyanny current and cores of machines and alternating-current devices work.
The area of the hysteresis curve is proportional to energy expense for one magnetization cycle of unit of volume of the ferromagnetiyok. If magnetic induction In is measured in Wb / см2= in x sec./cm2, and magnetic field strength of N — in and/cm, then the area of the hysteresis curve will be measured in in x and x sec. / см3= by J/cm3.
For cyclically remagnetized cores to reduce losses in them from the hysteresis, it is desirable to apply ferromagnetic materials with very narrow hysteresis curve, for example electrotechnical sheet steel.
For permanent magnets material with big by the koeryotsitivny force and residual magnetization is necessary. At such m and of N and that е with t to and x materials the hysteresis curve has the bolyyoshy area, but permanent magnets are not exposed to cyclic magnetic reversal.Top