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That is why the iron body, which has a magnetic permeability hundreds and thousands of times greater than jio, absorbs lines of force. Magnetic protection is based on this phenomenon.
That is why an iron body, which has a magnetic permeability hundreds and thousands of times greater than q0, absorbs lines of force. Magnetic protection is based on this phenomenon.
It should be noted that the lower the power consumption of an electrodynamic device, the weaker its own magnetic fields and the stronger the influence of external fields. Such devices require better magnetic protection, have a more complex design and are more expensive. Electrodynamic devices have a relatively low quality factor and do not withstand mechanical influences - shock, shaking and vibration.
It should be noted that the lower the power consumption of an electrodynamic device, the weaker its own magnetic fields and the stronger the influence of external fields. Such devices require better means - magnetic protection, have a more complex design and are more expensive.
The magnetic history of the tape is important for the subsequent accumulation of information. One of them is heating the sample to a temperature above the Curie point, followed by cooling in magnetic protection. The resulting natural demagnetized state is called the absolute zero state.
In the case of a magnetic field, thin iron walls do not protect the internal space: magnetic fields pass through the iron, and some magnetic field appears inside the vessel. Only with sufficiently thick iron walls can the weakening of the field inside the cavity become so strong that magnetic protection becomes practical, although even in this case the field inside is not completely destroyed. And in this case, the weakening of the field is not the result of its break on the surface of the iron; The magnetic field lines do not break off at all, but still remain closed, passing through the iron. By graphically depicting the distribution of magnetic field lines in the thickness of the iron and in the cavity, we obtain a picture (Fig. 283), which shows that the weakening of the field inside the cavity is the result of a change in the direction of the field lines, and not their break.
In the case of a magnetic field, thin iron walls do not protect the internal space: magnetic fields pass through the iron, and some magnetic field appears inside the vessel. Only with sufficiently thick iron walls can the weakening of the field inside the cavity become so strong that magnetic protection becomes of practical importance, although even in this case the field inside is not completely destroyed. And in this case, the weakening of the field is not the result of its break on the surface of the iron; The magnetic field lines do not break off at all, but still remain closed, passing through the iron. By graphically depicting the distribution of magnetic field lines in the thickness of the iron and in the cavity, we obtain a picture (Fig. 283), which shows that the weakening of the field inside the cavity is the result of a change in the direction of the field lines, and not their break.
Usually several options are calculated and the optimal one is selected. The presented methodology for calculating an electrodynamic wattmeter applies only to devices with a moving part installed on cores and is incomplete (for example, the issue of magnetic protection and etc.
In Fig. 237 shows an example of the location of induction lines in the case of a body with a high magnetic permeability μ and having a cavity. A rare arrangement of induction lines inside a cavity indicates a weak magnetic field inside the cavity. In practice, massive iron cases are used for magnetic protection.
To do this, the tunnel contact was placed in a hollow waveguide immersed in a cryostat. To avoid any kind of interference, the system was surrounded by magnetic protection.
Currently, astronauts often find themselves in areas of increased radiation. To protect against it, you need a magnetic field that bends the trajectory of charged particles and diverts radiation. For this purpose, the spacecraft must have an installation that creates magnetic protection using superconducting solenoids.
The influence of the magnetic properties of matter on the distribution of the magnetic field. If you make a ferromagnetic body in the form of a ring, then magnetic field lines will practically not penetrate into its internal cavity (Fig. 102) and the ring will serve as a magnetic shield protecting the internal cavity from the influence of the magnetic field. Magnetic protection of electrical measuring instruments and other electrical devices from the harmful effects of external magnetic fields is based on this property of ferromagnetic materials.
The picture that we observe when creating magnetic protection superficially resembles the creation of electrostatic protection using a conductive shell. In the case of electrostatic protection, the metal walls can be as thin as desired. It is enough, for example, to silver the surface of a glass vessel placed in an electric field so that there is no electric field inside the vessel, which breaks off at the metal surface. In the case of a magnetic field, thin iron walls do not protect the internal space: magnetic fields pass through the iron, and some magnetic field appears inside the vessel. Only with sufficiently thick iron walls can the weakening of the field inside the cavity become so strong that magnetic protection becomes practical, although even in this case the field inside is not completely destroyed.
This is where the trick ends. Now we need physics: how to get a protective layer of balls. Physics is simple, it is taught in the seventh grade: you need to use magnets. Where the pipe bends, place a magnet outside. It is interesting to note that shot blasting machines for hardening parts were widely used at least a quarter of a century before the appearance of copyright certificate No. 2N1 207 for magnetic protection.
MAGNETIC SHIELDING(magnetic protection) - protection of an object from magnetic influences. fields (constant and variable). Modern Research in a number of fields of science (geology, paleontology, biomagnetism) and technology (space research, nuclear energy, materials science) is often associated with measurements of very weak magnetic fields. fields ~10 -14 -10 -9 T in a wide frequency range. External magnetic fields (for example, the Earth's field T with T noise, magnetic noise from electrical networks and urban transport) create strong interference with the operation of highly sensitive devices. magnetometric equipment. Reducing the influence of magnetic fields strongly determines the possibility of conducting magnetic fields. measurements (see, for example, Magnetic fields of biological objects).Among the methods of M. e. the most common are the following.
The shielding effect of a hollow cylinder made of a ferromagnetic substance with ( 1
- external cylinder surface, 2
-internal surface). Residual magnetic 
field inside the cylinder
Ferromagnetic screen- sheet, cylinder, sphere (or shell of any other shape) made of material with high magnetic permeability m low residual induction In r and small coercive force N s. The principle of operation of such a screen can be illustrated using the example of a hollow cylinder placed in a homogeneous magnetic field. field (fig.). External induction lines mag. fields B when passing from the medium to the screen material, the external fields become noticeably denser, and in the cavity of the cylinder the density of the induction lines decreases, i.e., the field inside the cylinder turns out to be weakened. Field weakening is described by f-loy
Where D- cylinder diameter, d- thickness of its wall, - mag. permeability of the wall material. To calculate the effectiveness of M. e. volumes decom. configurations often use file
where is the radius of the equivalent sphere (almost the average value of the screen dimensions in three mutually perpendicular directions, since the shape of the screen has little effect on the efficiency of the magnetoelectric system).
From formulas (1) and (2) it follows that the use of materials with high magnetic field. permeability [such as permalloy (36-85% Ni, rest Fe and alloying additives) or mu-metal (72-76% Ni, 5% Cu, 2% Cr, 1% Mn, rest Fe)] significantly improves the quality of screens (at iron). Seemingly obvious way to improve shielding due to the thickening of the wall, it is not optimal. Multilayer screens with gaps between layers work more efficiently, for which the coefficients are shielding is equal to the product of coefficient. for dept. layers. It is multilayer screens (external layers of magnetic materials that are saturated at high values IN, internal - from permalloy or mu-metal) form the basis of the designs of magnetically protected rooms for biomagnetic, paleomagnetic, etc. research. It should be noted that the use of protective materials such as permalloy is associated with a number of difficulties, in particular with the fact that their magnesium. properties under deformation and that means. heat deteriorate, they practically do not allow welding, which means. bends and other mechanical loads In modern mag. Ferromagnets are widely used in screens. metal glasses(metglasses), close in magnetic. properties to permalloy, but not so sensitive to mechanical influences. The fabric, woven from metglass strips, allows the production of soft magnets. screens of arbitrary shape, and multilayer shielding with this material is much simpler and cheaper.
Screens made of material with high electrical conductivity(Cu, A1, etc.) serve to protect against alternating magnetic fields. fields. When changing external mag. fields in the walls of the screen arise inductively. currents that cover the shielded volume. Magn. the field of these currents is directed opposite to the external one. indignation and partially compensates for it. For frequencies above 1 Hz coefficient. shielding TO increases in proportion to frequency:
Where - magnetic constant, - electrical conductivity of the wall material, L- screen size, - wall thickness, f- circular frequency.
Magn. screens made of Cu and A1 are less effective than ferromagnetic ones, especially in the case of low-frequency electromagnetic. fields, but ease of manufacture and low cost often make them more preferable for use.
Superconducting screens. The action of this type of screens is based on Meissner effect- complete displacement of magnet. fields from a superconductor. With any change in external mag. flow in superconductors, currents arise, which, in accordance with Lenz's rule compensate for these changes. Unlike ordinary conductors, inductive superconductors. the currents do not fade and therefore compensate for the change in flux during the entire period of existence of the external current. fields. The fact that superconducting screens can operate at very low temps and fields not exceeding critical. values (see Critical magnetic field), leads to significant difficulties in the design of large magnetically protected “warm” volumes. However, the discovery oxide high temperature superconductors(OBC), made by J. Bednorz and K. Müller (J. G. Bednorz, K. A. Miiller, 1986), creates new opportunities in the use of superconducting magnets. screens. Apparently, after overcoming the technological difficulties in the manufacture of SBCs, superconducting screens will be used from materials that become superconductors at the boiling point of nitrogen (and in the future, possibly at room temperatures).
It should be noted that inside the volume magnetically protected by the superconductor, the residual field that existed in it at the moment of the transition of the screen material to the superconducting state is preserved. To reduce this residual field it is necessary to take a special measures. For example, transfer the screen to a superconducting state at a low magnetic field compared to the earth's. field in the protected volume or use the “inflating screens” method, in which the folded shell of the screen is transferred to a superconducting state and then expanded. Such measures make it possible, for now, to reduce residual fields to a value of T in small volumes limited by superconducting screens.
Active interference protection carried out using compensating coils that create a magnetic field. a field equal in magnitude and opposite in direction to the interference field. When added algebraically, these fields cancel each other out. Naib. Helmholtz coils are known, which are two identical coaxial circular coils with current, separated by a distance equal to the radius of the coils. Fairly homogeneous mag. the field is created in the center between them. To compensate for three spaces. components require a minimum of three pairs of coils. There are many options for such systems, and their choice is determined by specific requirements.
An active protection system is typically used to suppress low-frequency interference (in the frequency range 0-50 Hz). One of its purposes is post compensation. mag. Earth's fields, which require highly stable and powerful current sources; the second is compensation for magnetic variations. fields, for which weaker current sources controlled by magnetic sensors can be used. fields, e.g. magnetometers high sensitivity - squids or fluxgates To a large extent, the completeness of compensation is determined by these sensors.
There is an important difference between active magnetic protection. screens. Magn. screens eliminate noise throughout the entire volume limited by the screen, while active protection eliminates interference only in a local area.
All magnetic suppression systems interference need anti-vibration. protection. Vibration of screens and magnetic sensors. The field itself can become a source of additions. interference
Lit.: Rose-Ince A., Roderick E., Introduction to Physics, trans. from English, M., 1972; Stamberger G. A., Devices for creating weak constant magnetic fields, Novosibirsk, 1972; Vvedensky V.L., Ozhogin V.I., Ultrasensitive magnetometry and biomagnetism, M., 1986; Bednorz J. G., Muller K. A., Possible high Tc superconductivity in the Ba-La-Cr-O system, "Z. Phys.", 1986, Bd 64, S. 189. S. P. Naurzakov.
Shielding of magnetic fields can be done by two methods:
Shielding using ferromagnetic materials.
Shielding using eddy currents.
The first method is usually used when shielding constant MFs and low frequency fields. The second method provides significant efficiency in shielding high-frequency MPs. Due to the surface effect, the density of eddy currents and the intensity of the alternating magnetic field decreases exponentially as one goes deeper into the metal:
A measure of the field and current reduction, which is called the equivalent penetration depth.
The smaller the penetration depth, the greater the current flows in the surface layers of the screen, the greater the reverse MF created by it, which displaces the external field of the interference source from the space occupied by the screen. If the screen is made of a non-magnetic material, then the shielding effect will depend only on the conductivity of the material and the frequency of the shielding field. If the screen is made of ferromagnetic material, then, other things being equal, a large e will be induced in it by the external field. d.s. due to the greater concentration of magnetic field lines. With the same specific conductivity of the material, the eddy currents will increase, which will lead to a smaller penetration depth and a better shielding effect.
When choosing the thickness and material of the screen, one should not proceed from the electrical properties of the material, but be guided by considerations of mechanical strength, weight, rigidity, resistance to corrosion, ease of joining individual parts and making transition contacts between them with low resistance, ease of soldering, welding, etc.
From the data in the table it is clear that for frequencies above 10 MHz, copper and, especially, silver films with a thickness of about 0.1 mm provide a significant shielding effect. Therefore, at frequencies above 10 MHz, it is quite acceptable to use screens made of foil getinax or fiberglass. At high frequencies, steel provides a greater shielding effect than non-magnetic metals. However, it is worth considering that such screens can introduce significant losses into the shielded circuits due to high resistivity and the phenomenon of hysteresis. Therefore, such screens are applicable only in cases where insertion losses can be ignored. Also, for greater shielding efficiency, the screen must have less magnetic resistance than air, then the magnetic field lines tend to pass along the walls of the screen and penetrate less into the space outside the screen. Such a screen is equally suitable for protection against the influence of a magnetic field and for protecting the external space from the influence of a magnetic field created by a source inside the screen.
There are many grades of steel and permalloy with different magnetic permeability values, so the penetration depth must be calculated for each material. The calculation is made using the approximate equation:
1) Protection from external magnetic field
The magnetic field lines of the external magnetic field (the induction lines of the magnetic field of interference) will pass mainly through the thickness of the walls of the screen, which has low magnetic resistance compared to the resistance of the space inside the screen. As a result, the external magnetic field of interference will not affect the operating mode of the electrical circuit.
2) Shielding your own magnetic field
Such shielding is used if the task is to protect external electrical circuits from the effects of the magnetic field created by the coil current. Inductance L, i.e. when it is necessary to practically localize the interference created by inductance L, then this problem is solved using a magnetic screen, as shown schematically in the figure. Here, almost all of the field lines of the inductor coil will be closed through the thickness of the screen walls, without going beyond their limits due to the fact that the magnetic resistance of the screen is much less than the resistance of the surrounding space.
3) Dual screen
In a double magnetic screen, one can imagine that part of the magnetic lines of force that extend beyond the thickness of the walls of one screen will be closed through the thickness of the walls of the second screen. In the same way, one can imagine the action of a double magnetic screen when localizing magnetic interference created by an element of an electrical circuit located inside the first (inner) screen: the bulk of the magnetic field lines (magnetic scattering lines) will close through the walls of the outer screen. Of course, in double screens the wall thicknesses and the distance between them must be rationally chosen.
The overall shielding coefficient reaches its greatest magnitude in cases where the thickness of the walls and the gap between the screens increases in proportion to the distance from the center of the screen, and the value of the gap is the geometric average of the wall thicknesses of the adjacent screens. In this case, the shielding coefficient is:
L = 20lg (H/Ne)
The production of double screens in accordance with this recommendation is practically difficult for technological reasons. It is much more expedient to choose a distance between the shells adjacent to the air gap of the screens that is greater than the thickness of the first screen, approximately equal to the distance between the stack of the first screen and the edge of the shielded circuit element (for example, an inductor coil). The choice of one or another thickness of the walls of the magnetic shield cannot be made unambiguous. The rational wall thickness is determined. screen material, interference frequency and specified shielding coefficient. It is useful to consider the following.
1. As the frequency of interference increases (the frequency of the alternating magnetic field of interference), the magnetic permeability of materials decreases and causes a decrease in the shielding properties of these materials, since as the magnetic permeability decreases, the resistance to magnetic flux provided by the screen increases. As a rule, the decrease in magnetic permeability with increasing frequency is most intense for those magnetic materials that have the highest initial magnetic permeability. For example, sheet electrical steel with a low initial magnetic permeability changes little in the value of jx with increasing frequency, and permalloy, which has large initial values of magnetic permeability, is very sensitive to an increase in the frequency of the magnetic field; its magnetic permeability drops sharply with frequency.
2. In magnetic materials exposed to high-frequency magnetic field interference, the surface effect is noticeably manifested, i.e., the displacement of magnetic flux to the surface of the screen walls, causing an increase in the magnetic resistance of the screen. Under such conditions it seems almost useless to increase the thickness of the screen walls beyond those occupied by the magnetic flux at a given frequency. This conclusion is incorrect, because an increase in wall thickness leads to a decrease in the magnetic resistance of the screen even in the presence of a surface effect. In this case, the change in magnetic permeability should be taken into account at the same time. Since the phenomenon of the surface effect in magnetic materials usually begins to affect itself more noticeably than the decrease in magnetic permeability in the low-frequency region, the influence of both factors on the choice of screen wall thickness will be different at different frequency ranges of magnetic interference. As a rule, the decrease in shielding properties with increasing interference frequency is more pronounced in screens made of materials with high initial magnetic permeability. The above-mentioned features of magnetic materials provide the basis for recommendations on the selection of materials and wall thickness of magnetic screens. These recommendations can be summarized as follows:
A) screens made of ordinary electrical (transformer) steel, which have a low initial magnetic permeability, can be used if necessary to ensure low shielding coefficients (Ke 10); such screens provide an almost constant shielding coefficient over a fairly wide frequency band, up to several tens of kilohertz; the thickness of such screens depends on the frequency of the interference, and the lower the frequency, the greater the thickness of the screen required; for example, with a magnetic interference field frequency of 50-100 Hz, the thickness of the screen walls should be approximately 2 mm; if an increase in the shielding coefficient or a larger screen thickness is required, then it is advisable to use several shielding layers (double or triple screens) of smaller thickness;
B) It is advisable to use screens made of magnetic materials with high initial permeability (for example, permalloy) if it is necessary to ensure a large shielding coefficient (Ke > 10) in a relatively narrow frequency band, and it is not advisable to choose the thickness of each magnetic screen shell more than 0.3-0.4 mm; the shielding effect of such screens begins to decrease noticeably at frequencies above several hundred or thousand hertz, depending on the initial permeability of these materials.
Everything said above about magnetic shields is true for weak magnetic interference fields. If the screen is located close to powerful sources of interference and magnetic fluxes with high magnetic induction arise in it, then, as is known, it is necessary to take into account the change in magnetic dynamic permeability depending on the induction; It is also necessary to take into account losses in the thickness of the screen. In practice, such strong sources of magnetic interference fields, in which one would have to take into account their effect on screens, are not encountered, with the exception of some special cases that do not provide for amateur radio practice and normal operating conditions for widely used radio devices.
Test
1. When using magnetic shielding, the screen must:
1) Have less magnetic resistance than air
2) have magnetic resistance equal to air
3) have greater magnetic resistance than air
2. When shielding magnetic field Grounding the shield:
1) Does not affect shielding effectiveness
2) Increases the efficiency of magnetic shielding
3) Reduces the effectiveness of magnetic shielding
3. At low frequencies (<100кГц) эффективность магнитного экранирования зависит от:
a) Screen thickness, b) Magnetic permeability of the material, c) Distance between the screen and other magnetic circuits.
1) Only a and b are correct
2) Only b and c are true
3) Only a and c are true
4) All options are correct
4. Magnetic shielding at low frequencies uses:
1) Copper
2) Aluminum
3) Permalloy.
5. Magnetic shielding at high frequencies uses:
1) Iron
2) Permalloy
3) Copper
6. At high frequencies (>100 kHz), the effectiveness of magnetic shielding does not depend on:
1) Screen thickness
2) Magnetic permeability of the material
3) Distances between the screen and other magnetic circuits.
Used literature:
2. Semenenko, V. A. Information security / V. A. Semenenko - Moscow, 2008.
3. Yarochkin, V. I. Information security / V. I. Yarochkin - Moscow, 2000.
4. Demirchan, K. S. Theoretical foundations of electrical engineering, volume III / K. S. Demirchan S.-P, 2003.
Protective measures against the effects of MF mainly include shielding and “time” protection. Screens must be closed and made of soft magnetic materials. In some cases, it is sufficient to remove the worker from the zone of influence of the MF, since with the removal of the source of PMF and PeMF, their values quickly decrease.
As personal protective equipment against the action of magnetic fields, you can use various remote controls, wooden pliers and other remote-based manipulators. In some cases, various blocking devices can be used to prevent personnel from being in magnetic fields with induction levels higher than recommended values.
The main protective measure is precautionary:
It is necessary to avoid prolonged stay (regularly for several hours a day) in places with high levels of industrial frequency magnetic fields;
The bed for night rest should be kept as far as possible from sources of prolonged exposure; the distance to distribution cabinets and power cables should be 2.5 - 3 meters;
If there are any unknown cables, distribution cabinets, transformer substations in or adjacent to the room, removal should be as much as possible; optimally, measure the level of electromagnetic radiation before living in such a room;
When installing electrically heated floors, choose systems with a reduced magnetic field level.
Structure of protection measures against magnetic fields
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Name of protection measures |
Collective defense |
Personal protection |
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Organizational protection measures |
Treatment and preventive measures |
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The use of visual warnings about the presence of MP |
Conducting a medical examination upon hiring |
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Posting posters and notices listing basic precautions |
Periodic medical examinations and medical observations of personnel |
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Conducting lectures on occupational safety when working with MF sources and preventing overexposure from their exposure |
Objective information about the level of intensity in the workplace and a clear understanding of their possible impact on the health of workers |
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Reducing the impact of related production factors |
Conducting instructions on safety rules when working in conditions of exposure to MP |
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Time protection measures |
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Development of an optimal work and rest regime for the team with the organization of working hours with the minimum possible time contact with the MP |
Being in contact with the MP only for production needs with clear regulation of the time and space of actions performed |
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Protection measures through rational placement of objects |
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Placement of magnetic materials and magnetic devices at a sufficient distance (1.5-2 m) from each other and from workplaces |
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Prevention of the creation of additional sources of MF (“soft magnetic” materials) by removing them from the MF coverage area of powerful installations | ||
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Engineering and technical protection measures |
Storage and transportation of magnetic products in “yokes”, devices or devices that completely or partially close the magnetic field |
Use of tools, manipulators for individual use with a remote operating principle |
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Application of closed screens made of soft magnetic materials |
The use of blocking devices that make it possible to turn off equipment generating MFs if various parts of the body enter the induction zone of strong MFs |
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Bibliography:
Dovbysh V. N., Maslov M. Yu., Sdobaev Yu. M. Electromagnetic safety of elements of energy systems. 2009.
Kudryashov Yu. B., Perov Yu. F. Rubin A. B. Radiation biophysics: radio frequency and microwave electromagnetic radiation. Textbook for universities. - M.: FIZMATLIT, 2008.
Website http://ru.wikipedia.org
SanPiN 2.1.8/2.2.4.2490-09. Electromagnetic fields in industrial conditions Intro. 2009–05–15. M.: Publishing house of standards, 2009.
SanPiN 2.2.2.542–96 "Hygienic requirements for video display terminals, personal electronic computers and organization of work"
Apollonsky, S. M. Electromagnetic safety of technical equipment and humans. Ministry of Education and Science of Russia. Federations, State education institution of higher education prof. Education "Northern-West State Extramural Technical University". St. Petersburg: Publishing House of North-West Technical University, 2011