GB2453673A

GB2453673A - Magnetic core method for production thereof and earth leakage circuit breaker

Info

Publication number: GB2453673A
Application number: GB0819475A
Authority: GB
Inventors: Markus Brunner; Joerg Petzold
Original assignee: Vacuumschmelze GmbH and Co KG
Current assignee: Vacuumschmelze GmbH and Co KG
Priority date: 2006-04-25
Filing date: 2007-04-25
Publication date: 2009-04-15
Anticipated expiration: 2027-04-25
Also published as: WO2007122592A3; GB0819475D0; GB2453673B; DE102006019613A1; DE102006019613B4; GB201017171D0; WO2007122592A2

Abstract

The invention relates to a magnetic core made from a wound soft magnetic strip, for low frequency applications designed to have an increased relative permeability r by means of reduced mechanical tensions. The above is achieved by means of coating at least one side of the soft magnetic strip with a static friction reducing material which is partly burnt off during the thermal treatment of the magnetic core. The residue thereof remains such that in cross-section the magnetic core has an alternating layer of soft magnetic material and a layer of residue of a static friction reducing material.

Description

Magnet core, process for producing a magnet core and residual current dce The invention relates to a magnet core for low frequency applications which is wound from a soft magnetic strip. It also relates to a process for producing such a magnet core, and a residual current device with a magnet core.

Magnet cores which are formed from a spiral-wound metal strip, so-called toroidal strip cores', are used, for example, in current transformers, output transformers, current-compensated noise suppression chokes, residual current limiters, storage chokes, single-circuit chokes, transductor chokes and sum or differential current transformers for Fl-safety switches. As such, they are used in the low frequency range at typical operating frequencies of 50 to 60 Hz and at least below the kilohertz range. Unlike magnet cores used in higi; frequency applications, no particular attention need be paid to insulating the individual strip layers against one another to avoid eddy currents, since at low frequencies the voltage between the individual strip layers is typically less than 100 pV. At this voltage the intrinsic oxide layer on the strip surfaces is generally adequate for insulation.

However, the demands placed on magnet cores for low frequency applications in terms of magnetic properties are high: fault current transformers for a.c.-sensitive residual current devices, for example, must provide a secondary voltage which is at least sufficient to trigger the magnet system of the release relay responsible for cut-off. Since the objective is to achieve the mc-t compact current cransrmer ign possible, as well as high induction at the typical operating frequency of 50 Hz, the material used for the magnet core must above all have the highest possible relative permeability Pr. The geometry of the magnet core itself, together with the material properties combined with the technological processing of the material, by means of heat treatment, for example, have a significant effect on relative permeability.

In order to reach sufficiently high relative permeability values it was hitherto necessary to achieve the lowest possible saturation magnetostriction constant A of < 2ppm or even < 0.3pprn. A further important prerequisite was to achieve the most geometrically perfect strips possible with the minimum number of structural defects. However, such a small saturation magnetostriction constant X can only be achieved easily with a few alloys and, in addition, it is almost impossible to achieve an exact alloy composition without impurities in industrial-scale production.

It would however be possible to achieve high relative permeability with numerous other alloy compositions if the magnet core were free of mechanical stresses.

Mechanical stresses may be introduced into the magnet core as it is wound from one or more strips and/or in the case of subsequent heat treatment. The relationship between the freedom from stress of the magnet core and high relative permeability is mentioned in JP 63-

115313, for example.

US 2005/0221126 Al discloses a method for introducing stresses into strip cores in a specific manner in order to improve the magnetic properties of strip cores for high frequency applications.

The object of the invention is therefore to specify a magnet core wound from a soft magnetic strip for low frequency applications with a relative permeability p increased by the reduction of mechanical stresses.

A further object of the invention is to specify a process for producing such a magnet core.

This object is achieved in the invention by means of a magnet core for low frequency applications consisting of a spiral-wound, soft magnetic strip, said soft magnetic strip having on at least one side a layer made of the ignition residue of a static-friction-reducing material such that layers of the soft magnetic material and layers of the ignition residue of a static- friction-reducing material alternate through the cross-section of the magnet core.

The invention is based on the knowledge that mechanical stresses introduced during the winding of the magnet core or its subsequent heat treatment are caused by the fact that the shape of the soft magnetic strip is not ideal. Such variations from an ideal shape are twofold: Firstly, the surface of the soft magnetic strip is not ideally smooth, but possesses a certain surface roughness. The sur�ce roughnL within t.e micro-and nano-scale range and is subject to certain S fluctuations during the industrial-scale manufacture of the strips by casting using *the rapid solidification process. It varies both between individual casting batches and within a single batch due to casting wheel wear during production. Ideally, surface roughness Ra as defined in DIN 4762/ISO 4287/1, measured as the arithmetical mean of the two strip sides (Ra1+Ra2)/strip thickness, assumes values of approximately 1%.

Typically, however, the values achieved are between 2% and 6%, and in some cases as high as approximately 15%.

Secondly, rather than achieving ideal strip geometry with exactly plane-parallel surfaces industrial-scale manufacture produces variations from this geometry in the form of bulges and/or wedge or bone shapes.

The consequences of these variations from ideal strip shape are local or whole-layer tensions in the magnet core created during the winding process by local or surface fluctuations in the thickness and surface roughness of the individual strip layers. At the same time, geometrical surface defects lead to the interlocking of the strip layers, thereby preventing the relief of mechanical stresses in the magnet core during heat treatment. The almost ubiquitous combination of surface roughness and structural defects over large surface areas leads to particularly high surface pressures at the points where the strip layers touch, and thus to particularly high, local static friction. Due to the fundamental relationship hoc between relative permeability Pr, saturation magnetostriction constant A and mechanical stress o, if the saturation magnetostriction constant is not sufficiently small such distortions lead to inadequate and widely spread relative permeability values.

One fundamental concept of the invention is to compensate for micro-and nano-scale variations from ideal geometry leading to the interlocking of the strip layers and thus to distortions in the magnet core by the introduction of a static-friction-reducing material between the individual strip layers. This may be carried out either before or after the magnet core is wound, but should take place prior to heat treatment.

Following heat treatment at least the ignition residue of the static-friction-reducing material remains in the magnet core, whilst at least as long as the duration of the heat treatment is sufficient any loss on ignition is consumed.

The magnet core disclosed in the invention is subjected to heat treatment at temperatures typically below 600°C. Adsorptively or chemically bound water and, where, applicable other solvents are lost at these temperatures, although typically not all the organic component of the static-friction-reducing material is consumed. For this reason, the term ignition residue' is used here and below to refer to that part of the static-friction-reducing material which remains between th strlp Lyers following heat treatment. It is not therefore necessarily only mineral substances.

The magnet core disclosed in the invention has a relative permeability Pr of 400,000 �= Pr �= 800,000. Ifl the past such high relative permeability values were achieved by the use of rnagnetostriction-free materials.

This considerably reduced the number of suitable alloys. The magnet core disclosed in the invention offers the advantage that it is possible to relieve mechanical stresses and thus achieve a high relative permeability value with other non-magnetostriction-free alloys by the use of a static-friction-reducing material which exercises a lubricating effect on the individual strip layers during heat treatment. Such alloys can thus also be used in the low frequency applications descried above.

The particles of the static-friction-reducing material advantageously have a particle diameter d of less than 500 nm. Here the particle diameter d refers to the maximum diameter of a particle since the static-friction-reducing particles are not necessarily spherical, but can also be cylindrical or flake-shaped, for example. Particles with diameters over 500 nm tend towards the formation of agglomerates and can form irregularly shaped, sintered bodies which render the lubricating effect of the static-friction-reducing material required to achieve a high relative permeability ineffective.

In an advantageous embodiment the particles of the static-friction-reducing material have particle diartIeteb d below 60 nm, in paricu1ar peferab; particle diameters of 20 nm or below. This is because, as experiments have shown, a reduction in static-friction-reducing effect can be observed from particle sizes above 20 nm. If possible, therefore, particles with diameters greater than 20 nm should not be used where particularly high relative magnet core permeability is required.

The finished magnet core, i.e. the magnet core following completion of the heat treatment, typically has a nano-crystalline soft magnetic strip. However, both amorphous and crystalline strips are also conceivable depending on the intended use of the magnet core.

A number of different alloy compositions can conceivably be used for the magnet core disclosed in the invention. Since it is not necessary to eliminate the saturation magnetostriction constant, common iron-based alloys can be used, and even residual impurities which cannot generally be avoided completely can be tolerated without any undesirable effect on magnetic properties.

In one embodiment of the invention the alloy composition of the soft magnetic strip is essentially as follows: FeaCobCucSidBeMf, M being at least one of the elements V, Nb, Ta, Ti, Mo, W, Zr and Hf; a, b, c, d, e and f being specified in percent per atom; 0 �= b �= 20; 0.5 �= C �= 2; 6.5 �= d �= 18; 5 �= e �= 14; 1 �= f �= 6; d + e > 16; a + b + c + d + e + f = 100 and it being ssib1e t. pThc.oalt wholly or partially by nickel.

S

The iron cross-sectional area AFe of the magnet core is defined by the formula: Ap = D -D, where Da is the external diameter of the magnet core, D1 is its internal diameter, h is the width of the strip and r the fill factor 0% �= r �= 100%, ri > 40% being typical of magnet cores.

Spaces between the strip layers which necessarily result from the non-ideal strip geometry reduce the iron cross-sectional area Ap of the magnet core. To a certain extent the fill factor represents a measure of the degree of variation in the strip from an ideal, plane-parallel sheet.

In one embodiment after heat treatment the magnet core has a fill factor ri of more than 80%, whilst in alternative embodiments the fill factor is 70% �= r �= 80% or 65% �= r �= 70%.

The effective surface roughness RT of the strip is advantageously 1% �= RT �= 12%, and more advantageously 1% �= RT �= 6% or even 1% �= RT �= 4%. The saturation magnetostriction constant X of the magnet core is advantageously less than 6 ppm and the ratio of remanence induction to saturation induction BR/Es is greater than 40%.

In one embodiment of the invention the magnet core has a remanence induction to saturation induction ratio BR/Es of 1% �= BR/ES �= 30% auf, in alternative embodiments it is 30% �= Bp/B5 �= 80% or 80% �= BR/Es �= 97%.

Materials suitable for use as static-friction-reducing materials are those which can be applied easily, by spraying, rolling or deposition from a solution or gas phase or by solid particle deposition from a suspension, for example, and which reduce the static friction between the individual strip layers and in this manner generate a lubricating effect as the magnetic core is wound and during the relief of mechanical stresses during the subsequent heat treatment. However, there are also other requirements which a suitable static-friction-reducing material must meet. For example, where possible the material should not form film-like, tension-creating layers or generate tension-creating corrosion layers as is the case, for example, with aluminium silicate, lithium silicate and magnesium oxide coatings applied from an aqueous solution. In addition, the static-friction-reducing material should neither trigger undesirable surface reactions or diffusion processes within the strip, generate tension-creating shrinking effects.

Suitable static-friction-reducing materials include the following: nanodispersed Si02 which is commercially available under the name of Aerosil' or HDK' and has a similar thermal coefficient of expansion to the strip; nanodispersed Si02 with magnesium methylate; pigment soot; carbon nanotubes or C60 fullerene. In such cases. s pSsIt1e, where applicb1e, to avoid corrosive surface attack by the use of water-free solvents such as alcohols, ketones and ether alcohols.

As experiments have shown, it is also possible to achieve an improvement in the magnetic properties of a magnet core with a coating of alkylates of metals or transition metals, although these layers do not have any static-friction-reducing effect. Their effect is due to the loosening of the magnet core when fully or partially consumed during heat treatment. They are therefore suitable for use as additives to the static-friction-reducing material, for example. Zirconium propylate, aluminium butylate or a magnesium methoxide solution, for example, are suitable additives, as is boron nitride.

One process disclosed in the invention for producing a magnet core for low frequency applications comprises at least the following steps: an amorphous, soft magnetic strip, which may be manufactured using the rapid solidification process, for example, is provided and coated with a static-friction-reducing material. The strip is then wound into a magnet core, typically a toroidal strip core, and the magnet core is subject to heat treatment in order to impart the desired magnetic properties, the static-friction-reducing material exercising a lubricating effect on the strip and enabling the individual strip layers to slide over one another more easily. Alternatively, it is possible to carry out the coating process after the strip has been wound into a magnet core, coating the magnet core using the economical process of dip coating, for example.

Both versions of the process permit the production of a magnet core which is largely free of mechanical stresses and thus can have a high relative permeability value. While in the first version the stresses occurring during winding using traditional processes are avoided from the outset, in the second these stresses generally continue to occur as before.

In the second version of the process, however, the static-friction-reducing material considerably facilitates the relief of mechanical stresses from the magnet core during heat treatment and therefore this version of the process also provides a magnet core which is largely free of mechanical stresses.

The process disclosed in the invention therefore provides at least two mechanisms for stress relief which can be used individually or in combination. These are, firstly, the loosening of the core due to the at least partial consumption of the static-friction-reducing material during heat treatment which gives the core sufficient freedom for the relief of stresses, and, secondly, a lubricating effect between the strip layers which is produced by the rolling or sliding properties of spherical, cylindrical or flake-shaped particles or entire layers. Here, at the temperatures occurring during heat treatment which do not exceed 600°C, the lubricating effect in particular plays a significant role, while loosening due to consumption is relatively weak. This is due to the fact that at these relatively low temperatures it is primarily water or solvents which escape from the static-friction-reducing naterial, while the orynir crrncrent i ii. gerly completely consumed.

S

The process disclosed in the invention can be used to achieve a hysteresis curve either in the form of an F-loop or in the form of a Z- and R-loop. However, since it is primarily the R-loop which achieves the highest permeability values, the magnet core must be of particularly stress-free design in order to avoid damaging anisotropisms. In order to achieve an R-loop, the heat treatment is advantageously carried out field-free, i.e. in the absence of a magnetic field strong enough to have an adverse effect on the magnetic properties of the magnet core, and at a temperature T of 505°C �= T �= 600°C.

The static-friction-reducing material can be applied by deposition from a gas phase or a solution, by solid particle deposition from a suspension or using the sol-gel process, for example.

It is advantageous to achieve a relative static-friction-reducing material surface density p of 10 mg/rn2 �= p �= 600 mg/rn2, or better still of 20 mg/rn2 �= p �= 300 mg/rn2 or even of 50 mg/rn2 �= p �= 150 mg/rn2. This ensures, on one hand, that a sufficient static-friction-reducing effect is achieved and, on the other, that the layer of static-friction-reducing material does not become too thick. For if the layer is too thick, the fill factor of the magnet core drops too far during heat treatment when a significant part of the static-friction-reducing material is consumed and the magnet core becomes too loose' and thus too sensitive for subsequent handling.

For this reason, layer thicknesses d of the static-friction-reducing material prior to heat treatment of d < 5 p m, preferably of d < 1 p m or even d < 0.5 p m or d < 0.2 p m are advantageous.

The process disclosed in the invention has the advantage that it permits the production of a magnet core with particularly favourable magnetic properties, in particular with a high relative permeability value, without great technical difficulty. The process eliminates dependence on special, often costly alloy compositions and renders the complete removal of impurities in the alloy unnecessary. At the same time, it renders superfluous complex selection procedures designed to achieve the most perfect strip surfaces possible. Thus the process disclosed in the invention can be used to reduce the production costs for toroidal strip cores by simple means. In addition, since compensation is provided for structural defects in the strip, it is also possible to avoid the undesirable excessive spread of relative permeability values.

The magnet core disclosed in the invention is particularly suitable for use in a residual current device since, due to its high relative permeability, it provides an adequately high secondary voltage to trigger the magnet system of the release relay responsible for cut-off.

Embodiments of the invention are explained in greater detail below with reference to the attached drawings.

Fig. 1 shows a schematic cross-section through a S magnetic toroidal strip core.

Fig. 2a shows a schematic section of a plurality of strip layers in the toroidal strip cores described in the first step of the process disclosed in the invention.

Fig. 2b shows the same section after a subsequent process step.

Fig. 2c shows the same section after a further process step.

Fig. 3 shows various embodiments of the process disclosed in the invention in tabular form.

Identical parts are designated by the same reference numerals in all figures.

Fig. 1 shows a cross-section through a magnet core (1), or more exactly through a magnetic toroidal strip core, consisting of numerous strip layers 3. The magnet core 1 is wound from a soft magnetic, typically amorphous strip 2 onto a cylinder 4, which is later removed. The strip 2 does not present ideal geometry with smooth, exactly plane-parallel surfaces 5. In fact, its thickness varies locally causing surface roughness, and in addition the strip 2 may present geometric defects such as bulges and/or wedge-or bone-shaped cross-sectional areas. In Fig. 1 the surface roughness of the iij 2 s i1iust:td by hooks 6.

Due to the lack of ideal geometry presented by the strip 2 as caused by the hooks 6 and other irregularities, when the magnet core 1 is wound the static friction between the individual strip layers 3 which is significantly higher than would be presented by an ideal strip leads to mechanical stresses in the magnet core 1. These mechanical stresses could be relieved during the heat treatment of the magnet core 1 following winding. However this is completely or at least partially prevented by the sustained interlocking of the strip layers 3.

Fig. 2 shows steps of a process as disclosed in the invention for producing a magnet core 1. In the version of the process illustrated, first the magnet core 1 is wound, then its strip layers 3 are coated with a static-friction-reducing material by dip coating, for example. However, it is equally possible to coat the strip prior to winding, either by dipping or by a continuous process, for example. The magnet core 1 is then subjected to heat treatment during which the static-friction-reducing material is partially or extensively consumed. Here the freedom from stresses of the magnet cores 1 is achieved both by the lubricating effect of the static-friction-reducing material and by a loosening of the magnet cores due to the partial consumption of the static-friction-reducing material, since the magnet core 1 is able to expand during heat treatment.

Fig. 2a shows a section from the wound magnet core with three strip layers 3 of the soft magnetic strip 2. Due to h:onsiderable enlargement ar(1 2.fl jfltrCStZ simplification, the strip layers 3 are represented as being flat rather than curved. The surface 5 of the strip 2 is not perfectly flat, but has micro-and nano-scale geometrical defects in the manner of surface roughness which is indicated by hooks 6. Superimposed on these short-wave' oscillations are long-wave' structural defects extending along and across the strip. As a result, spaces 7 form between the strip layers 3 as the magnet cores is wound. These spaces are undesirable because they reduce the iron cross-sectional area of the magnet core and, in addition, lead to mechanical tensions.

Fig. 2b represents the section from the magnet core after the strip 2 has been coated with a static- friction-reducing material 8. The static-friction-reducing material 8 sheathes the strip 2 and thus forms intermediate layers 10 between the strip layers 3, resulting in a layer sequence comprising alternating layers of soft magnetic strips 2 and static-friction-reducing material 8. The new surface 9 of the strip 2 is significantly smoother than the original, and as such significantly closer to an ideal surface. If the layer is composed of spherical or cylindrical particles or of graphitic layers, it already has a marked lubricating effect which facilitates the sliding of the strip layers and thus relieves stresses in the core.

After coating, the magnet core is heat-treated in a retort or continuous furnace. Fig. 2c indicates that during this heat treatment the static-friction-reducing material is partially or extensively consumed. What remains is essentially t1-. init�cz &.1uc:: of the static-friction-reducing material which now forms the intermediate layers 10 between the strip layers 3. The material lost due to consumption leads to a loosening of the magnet core. If the ignition residue also has the aforementioned lubricating effect, these two fundamental mechanisms both contribute to the relieving of stresses in the core.

Fig. 3 is a table representing various embodiments of the invention. In addition to nano-dispersed Si02, carbon nanotubes, fullerene and pigment soon are also used as static-friction-reducing materials for coating.

Layers of MgO, zirconium propylate and aluminium buthylate are also examined. Ensembles of 50-100 core are coated and heat treated. The magnetic properties, in particular relative permeability Pr at a frequency of 50 Hz, are then measured. The range of relative permeability is specified as 2s/< Pr >, where s is standard deviation.

As a comparative example, ensembles of uncoated reference cores were produced using the traditional process. The table shows that static-friction-reducing layers of nano-dispersed S102 in particular result in high levels of relative permeability. Layers of MgO, zirconium propylate or aluminium buthylate cause slight tensions or are largely passive. These layers can nevertheless lead to improved magnetic properties of strip cores since the extensive consumption of the layer during heat treatment will permit the loosening of the core and thus the relief of mechanical stresses.

Nano-dispersed Si02 with small additions of MgO as an additive also permits the loosening of the core due to consumption during heat treatment, although the percentage of MgO must be kept low enough to prevent the coating from presenting any tension-creating effect. Thus the positive effects of Si02 and MgO coating can be combined.

It has been shown that the particle size of nano-dispersed Si02 plays a not un-negligible role. Tests using finely dispersed Si02 with a particle size of 500 nm or 1000 nm show a clear deterioration in magnetic properties, in particular a reduction in permeability to values around 200,000. A slight, though not yet significant deterioration in magnetic properties can also be measured at a particle size of 60 nm and above.

Furthermore, the concentration of the Si02 in the dispersion also influences the magnetic properties of the core. In concentrations of more than approximately 8%, as opposed to concentrations of 1.5% to 5%, it is possible to measure slight deteriorations which increase further at higher concentrations.

Carbon nanotubes, fullerene and pigment soot do not form tension-creating layers. They exercise a lubricating effect which can be exploited as the core is being wound and also as it relaxes during heat treatment.

List of reference numerals I Mayi-c-Dr 2 Strip 3 Strip layer 4 Cylinder Surface 6 Hook 7 Space 8 Static-friction-reducing material 9 Coated surface Intermediate layer 11 Ignition residue

Claims

Claims J Magnet cre fDr low frequency app' !c.ain consisting of a spiral-wound, soft magnetic strip, said soft magnetic strip having on at least one side a layer comprising the ignition residue of a static-friction-reducing material such that layers of soft magnetic material and ignition residue of a static-friction-reducing material alternate through the cross-section of the magnet core.
2. Magnet core in accordance with claim 1, the magnet core having a relative permeability Pr where 400,000 �= p-�= 800,000.
3. Magnet core in accordance with claim 1 or 2, the particles of the static-friction-reducing material having a particle diameter d where d < 500 nm.
4. Magnet core in accordance with claim 1 or 2, the particles of the static-friction-reducing material having a particle diameter d where d < 60 nm.
5. Magnet core in accordance with claim 1 or 2, the particles of the static-friction-reducing material having a particle diameter d where d �= 20 nm.
6. Magnet core in accordance with one of claims 1 to the soft magnetic strip being nano-crystalline.
7. Magnet core in accordance with one of claims 1 to the soft magnetic strip being crystalline.
8. Magnet core in accordance with one of claims 1 to 5' the soft magnetic strip being amorphous.
9. Magnet core in accordance with one of claims 1 to the soft magnetic strip essentially comprising the alloy composition FeaCobCucS dBeMf, M being at least one of the elements V, Nb, Ta, Ti, Mo, W, Zr and Hf; a, b, c, d, e and f being specified in percent per atom; 0 �= b �= 20; 0.5 �= C �= 2; 6.5 �= d�= 18; 5 �= e �= 14; 1 �= f �= 6; d + e > 16 and a + b + c + d + e + f = 100; and cobalt being able to be wholly or partially replaced by nickel.
10. Magnet core in accordance with one of claims 1 to the magnet core having a fill factor of > 80% after heat treatment.
11. Magnet core in accordance with one of claims 1 to the fill factor r being 70% �= r �= 80%.
12. Magnet core in accordance with one of claims 1 to the fill factor ri being 65% �= �= 70%.
13. Magnet core n ccr.ne:t cn of claims 1 zc' the magnet core having an effective surface roughness RT where 1% �= RT �= 12%.
14. Magnet core in accordance with one of claims 1 to the effective surface roughness RT being 1% �= RT �= 6%.
15. Magnet core in accordance with one of claims 1 to the effective surface roughness RT being 1% �= RT �=

AC *10
16. Magnet core in accordance with one of claims 1 to the magnet core having a saturation magnetostriction constant A where ? < 6 ppm.
17. Magnet core in accordance with one of claims 1 to the magnet core having a remanence induction to saturation induction ratio BR/Bs of BR/Bs > 40%.
18. Magnet core in accordance with one of claims 1 to the magnet core having a remanence induction to saturation induction ratio BR/Bg of 1% �= BR/Bs �= 30%.
19. Magnet core in accordance with one of claims 1 to the magnet core having a remanence induction to saturation induction ratio BR/BS of 30% �= BR/Bs �= , r -
20. Magnet core in accordance with one of claims 1 to the magnet core having a remanence induction to saturation induction ratio BR/Bs of 80% �= BR/Bs �= 97%.
21. Magnet core in accordance with one of claims 1 to the static-friction-reducing material provided being nano-dispersed S102.
22. Magnet core in accordance with one of claims 1 to the static-friction-reducing material provided being nano-dispersed Si02 with magnesium methylate.
23. Magnet core in accordance with one of claims 1 to the static-friction-reducing material provided being pigment soot.
24. Magnet core in accordance with one of claims 1 to the static-friction-reducing material provided being carbon nanotubes.
25. Magnet core in accordance with one of claims 1 to the static-friction-reducing material provided being C60 fullerene.
26. Magnet core in accordance with one of claims 1 to 25, an alkylate of a metal or transition metal being provided as an additive to the static-friction-reducing material.
27. Magnet core in accordance with one of claims 1 to boron nitride being provided as an additive to the static-friction-reducing material.
28. Process for producing a magnet core for low frequency applications comprising the following steps: -provision of an amorphous, soft magnetic strip; -coating of the strip with a static-friction- reducing material; -coiling of the strip into a magnet core; -subjection of the magnet core to heat treatment, the static-friction-reducing material exercising a lubricating effect on the strip.
29. Process for producing a magnet core for low frequency applications comprising the following steps: -provision of an amorphous, soft magnetic strip; -coiling of the strip into a magnet core; -coating of the strip layers forming the magnet core with a static-friction-reducing material; -subjection of the magnet core to heat treatment, the static-friction-reducing material exercising a lubricating effect ii
30. Process in accordance with claim 28 or 29,

the heat treatment being carried out field-free in

the absence of a magnetic field.
31. Process in accordance with one of claims 28 to 30, the heat treatment being carried out at a temperature T where 505°C �= T �= 600°C.
32. Process in accordance with one of claims 28 to 31, nano-dispersed Si02 being used as the static-friction-reducing material.
33. Process in accordance with one of claims 28 to 31, nano-dispersed Si02 with magnesium methylate being used as the static-friction-reducing material.
34. Process in accordance with one of claims 28 to 31, pigment soot being used as the static-friction-reducing material.
35. Process in accordance with one of claims 28 to 31, carbon nanotubes being used as the static-friction-reducing material.
36. Process in accordance with one of claims 28 to 31, C60 fullerene being used as the static-friction-reducing material.
37. Process in accordance with one of claims 28 to 36, an alkylate of a metal or transition metal being used as an additive to the static-friction-reducing material.
38. Process in accordance with one of claims 28 to 36, boron nitride being used as an additive to the static-friction-reducing material.
39. Process in accordance with one of claims 28 to 38, the surface coverage density p of the static-friction-reducing material being 10 mg/rn2 �= p �= 600 mg/rn2.
40. Process in accordance with one of claims 28 to 38, the surface coverage density p of the static-friction-reducing material being 20 mg/rn2 �= p �= 300 mg/rn2.
41. Process in accordance with one of claims 28 to 38, the surface coverage density p of the static-friction-reducing material being 50 mg/rn2 �= p �= 150 mg/rn2.
42. Process in accordance with one of claims 28 to 41, the layer thickness d of the static-friction-reducing material prior to heat treatment being d < pm.
43. Process in accordance with one of claims 28 to 41, the layer thickness ci of the static-friction-reducing material prior to heat treatment being d 1 pm.
44. Process in accordance with one of claims 28 to 41, the layer thickness d of the static-friction-reducing material prior to heat treatment being d < 0.5 pm.
45. Process in accordance with one of claims 28 to 41, the layer thickness d of the static-friction-reducing material prior to heat treatment being d < 0.2 pm.
46. Process in accordance with one of claims 28 to 45, the static-friction-reducing material being applied by deposition from the gas phase or from a solution.
47. Process in accordance with one of claims 28 to 45, the static-friction-reducing material being applied using the sol-gel process.
48. Process in accordance with one of claims 28 to 45, the static-friction-reducing material being applied by solid particle deposition from a suspension.
49. Residual current device with a magnet core in accordance with one of claims 1 to 27.