EP1129459A1

EP1129459A1 - Magnetic core that is suitable for use in a current transformer, method for the production of a magnetic core and current transformer with a magnetic core

Info

Publication number: EP1129459A1
Application number: EP99963240A
Authority: EP
Inventors: Detlef Otte; Jörg PETZOLD
Original assignee: Vacuumschmelze GmbH and Co KG
Current assignee: Vacuumschmelze GmbH and Co KG
Priority date: 1998-11-13
Filing date: 1999-11-15
Publication date: 2001-09-05
Anticipated expiration: 2019-11-15
Also published as: WO2000030131A1; US6580347B1; EP1129459B1; JP2002530853A; KR20010080442A; KR100606514B1; DE59909661D1

Abstract

The magnetic core (M) consists of a coiled amorphous ferromagnetic alloy strip (B) Its saturation-permeability is greater than 20 000 and lower than 300 000. The saturation-magnetostriction of the magnetic core (M) is less than 0.5 ppm. The core (M) is substantially free from mechanical stress and has an anisotropic axis (A) along which the magnetization of the magnetic core (M) can be oriented in a particularly easy manner and which is perpendicular to a plane in which the center line of the strip (B) runs. The composition of the alloy essentially corresponds to the formula Coa(Fe1-xMnx)bNicXdSieBfCg whereby X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge, P, a g is indicated in atom % and a, b, c, d, e, f, g and x meet the following conditions: 40 </= a </= 82; 3 </= b </= 10; 0 </= c </= 30; 0 </= d </= 5; 0 </= e </= 20; 7 </= f </= 26; 0 </= g </= 3; with 15 </= d + e + f + g </= 33 and 0 </= x </= 1.

Description

description

Magnetic core suitable for use in a current transformer, method for producing a magnetic core and current transformer with a magnetic core.

The invention relates to a magnetic core which is suitable for use in a current transformer, a method for producing such a magnetic core and a current transformer with such a magnetic core.

Energy meters are used to record the energy consumption of electrical devices and systems in industry and households. The oldest principle used is that of the Ferraris meter. The Ferraris counter is based on the

Energy metering via the rotation of a disk connected to a mechanical counter, which is driven by the fields of corresponding field coils that are proportional to the current or voltage. For expanding the functional options of energy meters such as For multi-tariff operation or remote reading, electronic energy meters are used, in which the current and voltage are recorded via inductive current and voltage converters.

A special application in which particularly high accuracy is required is the detection of energy flows in the area of electricity supply companies. On the one hand, the amounts of energy generated by the respective power plants and fed into the high-voltage grids must be precisely determined, on the other hand, the changing proportions of consumption or delivery in traffic between the energy supply companies are of great importance for billing. The energy meters used for this are

Multifunction built-in devices, their input signals for current and voltage from the respective high and Medium voltage systems can be tapped via cascades of current and voltage transformers and their output signals are used for digital and graphic registration or display as well as for control purposes in the control rooms. The first converters on the network side are used for electrically isolated transformation of the high current and. Voltage values, for example 1 to 100 kA and 10 to 500 kV, to values that can be handled in control cabinets, the second transform them in the actual energy meter to the signal levels required by the measuring electronics in the range of less than 10 to 100 mV.

FIG. 1 shows an equivalent circuit diagram of such a current transformer and the areas of the technical data that can occur in various applications. A current transformer 1 is shown here. The primary winding 2, which carries the current Ip _r i _m and a secondary winding 3, which carries the measuring current I _sec , is located on a magnetic core 4, which is composed of an amorphous soft magnetic tape. The secondary current I _sec automatically adjusts itself so that the primary and secondary ampere turns are ideally of the same size and oppositely directed. The course of the magnetic fields in such a current transformer is shown in FIG. 2, losses in the magnetic core not being taken into account. The current in the secondary winding 3 then adjusts itself according to the law of induction in such a way that it tries to prevent the cause of its formation, namely the change in the magnetic flux in the magnetic core 4 over time.

In the ideal current transformer, the secondary current multiplied by the ratio of the number of turns is negatively equal to the primary current, which is illustrated by equation (1):

| ^al = -Iprim * (N i _pr _m / N _sec) (1) This ideal case is never achieved because of the losses in the load resistor 5, in the copper resistor 6 of the secondary winding and in the magnetic core 4.

In the real current transformer, the secondary current therefore has an amplitude error and a phase error compared to the above idealization, which is described by equation (2):

^• rreal _ xideal amplitude error: F (I) = ^sec ^ ec; Phase error: φ = φ (Islc ^] ) - Φ (-Iprim) ( ² )

Isec

The output signals of such a current transformer are digitized, multiplied, integrated and stored. The result is an electrical quantity that is available for the stated purposes.

The electronic energy meters used for energy metering in these applications work "indirectly", so that only purely bipolar, zero-symmetrical alternating currents have to be measured in the meter itself. For this purpose, current transformers are used, which are constructed with magnetic cores made of highly permeable materials and to achieve small measurement errors via a small phase error φ with a large number, i.e. typically 2500 and more, secondary windings must be equipped.

Current transformers are known for imaging purely bipolar currents, the magnetic cores of which consist of highly permeable crystalline alloys, in particular nickel-iron alloys, which contain approximately 80% by weight of nickel and are known under the name "Permalloy". These have a fundamentally very low phase error φ. However, they have the disadvantage that this phase error φ varies greatly with the current Ip _r i _m to be measured, which is synonymous with the modulation of the converter core. For precise current measurement with changing loads with these converters therefore a complex linearization in the energy meter is required.

Furthermore, current transformers are known which work on the basis of ironless air coils. This principle is known as the so-called Rogowski principle. Here, the influence of the modulation on the phase error is eliminated. However, since the requirements for the immunity to interference of such current transformers must be very high in order to enable a verifiable energy metering, these constructions are equipped with complex shielding against external fields, which means a high expenditure on materials and installation and is therefore cost-intensive.

Solutions are also known in which one with a

Air-sheared (sheared) ferrite shell core is used as the magnetic core. These current transformers have a very good linearity, however, due to the relatively low permeability of the ferrites, a very high number of turns in connection with a very large-volume magnetic core is required in order to achieve a low phase angle with the current transformer. These current transformers based on ferrite shell cores also have a high sensitivity to external external fields, so that shielding measures must also be taken there.

The invention is based on the object of specifying a magnetic core which, when used in a current transformer, permits a higher measuring accuracy of a current to be measured compared to the prior art. Furthermore, a method for producing such a magnetic core and a current transformer with such a magnetic core are to be specified.

The task is solved by a magnetic core which is used for

Is suitable for use in a current transformer and which is characterized in that it consists of a wound tape an amorphous ferromagnetic alloy, has a saturation permeability that is greater than 20,000 and less than 300,000, has a saturation magnetostriction, the amount of which is less than 0.5 ppm and is essentially free of mechanical stresses. The magnetic core has a magnetic anisotropy axis, along which the magnetization of the magnetic core aligns particularly easily and which is perpendicular to a plane with which a center line of the strip runs, ie which runs perpendicular to the direction of the wound strip. The alloy has a composition consisting essentially of the formula

Co _a (F _eι _ _x Mn _x ) _b Nι _c X _d SieB _f C _g

where X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge, P, a to gm atomic% are given and where a, b, c, d, e, f, g and x meet the following conditions:

40 <a <82; 3 <b <10; 0 <c <30; 0 <d <5; 0 <e <20; 7 <f <2 6; 0 <g <3; with 15 <d + e + f + g <33 and 0 <x <1.

The permeability relates to an applied field strength, which lies m of the plane, m the center line of the band, and the induction caused thereby.

It has been shown that the dependence of the permeability on the magnetization is very small in such a magnetic core. The hysteresis loop of the magnetic core is therefore very narrow and linear.

Since the permeability is very high at over 20,000 and is also essentially independent of the bias, the absolute phase error and the absolute

Amplitude error of a current transformer with such a magnetic core is very small. The absolute amplitude error can be less than 1%. The absolute phase error can be less than 0.1 °.

In addition to the magnetic core, the current transformer has at least one primary winding and one secondary winding, to which a load resistor is connected in parallel and which terminates the secondary circuit with low resistance.

It has also been shown that the hysteresis loop of the magnetic core has a high linearity. So a permeability ratio μ ^ 5 / μ4 <1.1 and a

Permeability ratio μιo / ^u O, 5 <1/25, where μo 5, V-4, mo and μ] _5 are the permeabilities at a field amplitude H of 0.5, 4, 10 and 15 mA / cm.

The small saturation magnetostriction and the alignment of the anisotropy axis have a particularly advantageous effect on the high linearity of the hysteresis loop.

Due to the good linearity, the phase and amplitude errors are essentially independent of the current to be measured.

Since the absolute phase error, the absolute amplitude error and the dependency of the errors on the current to be measured are very small, the current transformer can carry out a very exact current detection.

The invention is based on the finding that a magnetic core with the described properties can be produced with the alloy of the described composition by means of a suitable heat treatment. Many parameters are matched to each other so that the magnetic core has the properties described. The following describes a heat treatment, which is a method for producing a magnetic core and also solves the task:

After production and winding of the tape to the magnetic core, the magnetic core is brought to a target temperature

(Relaxation temperature) heated between 380 ° C and 500 ° C. The magnetic core is cooled from the target temperature to room temperature, a magnetic field H> 100 A / cm, better> 1000 A / cm being switched on at the latest from the Curie temperature of the alloy, which is parallel to the anisotropy axis of the magnetic core to be generated. The Curie temperature T _c is the temperature at which spontaneous magnetization of the alloy begins. Depending on the alloy composition, which determines the position of the Curie temperature and the permeability level to be achieved, cooling takes place at rates between 0.1 and 10 K / min. The temperature-time profile can be stationary, non-linear, steady or discontinuous. The cooling time can be between 0.25 and 60 hours.

The target temperature is chosen so that it is below the crystallization temperature of the alloy. The target temperature is preferably at least 100 ° C. below the crystallization temperature of the alloy.

Furthermore, the target temperature is chosen so that a very small saturation agnetostriction is achieved with the alloys described. The target temperature required for this depends on the ratio of Fe, Mn to Co. The larger this ratio, the smaller the target temperature is chosen in order to obtain the smallest possible saturation magnetostriction.

The heating simultaneously compensates for mechanical stresses and a small saturation magnetostriction. A particularly high linearity of the hysteresis loop can be achieved if the ratio of the mechanical elastic tension tensor of the magnetic core multiplied by the saturation magnetostriction to the uniaxial anisotropy is less than 0.5.

It has been shown that the duration described for cooling leads to the result that an anisotropy which is sufficiently high for good loyalty of the hysteresis loop is achieved with a high saturation permeability. The described elimination of magnetostriction and tension makes it possible to produce highly linear hysteresis loops with very high permeabilities despite very small values of the uniaxial anisotropy. The longer the cooling in the magnetic field takes, the smaller it is

Saturation permeability and the higher the anisotropy. This is because atomic regions of the alloy which have magnetic dipole moments gradually align themselves more and more spatially below the Curie temperature in the magnetic field, so that a preferred direction for magnetization is formed, that is to say the anisotropy axis is formed. The more pronounced this orientation in the magnetic field, the greater the uniaxial anisotropy, but the lower the permeability.

The alignment processes taking place in the magnetic field depend on the temperature in two ways. The higher the temperature, the more mobile the atomic and the easier it is to align. The lower the temperature, the greater the driving force of the magnetic field on the magnetic dipole moments of the atomic areas, that is, the stronger the aligning force that acts on the atomic areas. These factors have been optimally coordinated with one another by the cooling time described, so that an anisotropy which is sufficiently high for good linearity is achieved with a high permeability. The magnetic field is chosen such that the

Saturation magnetization of the magnetic core in its axial direction is safely achieved.

In order to achieve high permeabilities, the composition of the alloy is selected such that the Curie temperature, taking into account other parameters to be optimized, e.g. a high saturation induction, is as small as possible The Curie temperature is, for example, between 190 ° C.

270 ° C. This is advantageous for technical and economic reasons because, for reasons of linearity, it is not possible to cool field-free below the Curie temperature. A lowering of the Curie temperature is first achieved in that the metalloid content, i.e. the proportion of Si and B is increased, the saturation induction also decreasing at the same time. On the other hand, if Mn additions are added within the ranges discussed, the Curie temperature can be lowered while maintaining the saturation induction.

At the same time, increasing the metalloid content taking into account other parameters to be optimized, such as the saturation magnetostriction, an increase in the crystallization temperature is achieved. This is advantageous because a high crystallization temperature enables a better aging behavior of the magnetic core and a high target temperature and thus a better compensation of the mechanical tension.

Furthermore, when choosing the composition of the alloy, it was taken into account that the saturation induction of the magnetic core is as large as possible. This is advantageous since, in the case of large saturation induction, the linearity range is expanded and thus a higher current can be measured reliably before saturation is reached and the linearity of the current mapping is thereby destroyed. The saturation induction is over the greater the ratio of Co, Fe, Mn to the rest of the alloy. At the same time, the crystallization temperature decreases.

Due to the high permeability, the current transformer can have a particularly small volume with precise current detection.

With regard to the required properties, particularly good current transformers can be achieved by using amorphous, ferromagnetic alloys which have a magnetostriction value | λ _s | Have <0.1 ppm, the alloy having a composition which essentially consists of the formula

Co _a (F _eι ., _Χ Mn _x ) _b Ni _c X _d Si _e B _f C _g

where X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge and P, a to g are given in atomic% and where a, b, c, d, e, f, g and x meet the following conditions:

63 <a <73; 3 <b <10; 0 <c <5; 0 <d <3; 12 <e <19; 7 <f <20; 0 <g <3 with 20 <d + e + f + g <30 and x <0, 5.

A further improvement can be achieved with current transformers which contain amorphous, ferromagnetic alloys of the type mentioned above, in which a, b, c meet the following condition:

68 <a + b + c <75.

The above-mentioned alloy systems are characterized by very linear, extremely narrow hysteresis loops, with a permeability μ4> 120,000 at a field amplitude H of 4 mA / cm being easily adjustable using the described method. The alloy systems according to the invention are almost free of magnetostriction. The magnetostriction is preferably set by a heat treatment, so that linear hysteresis loops with an induction range that can be used widely due to the high saturation reduction of B _s = 0.5 to 0.7 T and a very good frequency response with regard to permeability and comparatively low magnetic reversal losses can be produced.

Such high linear current transformers are achieved with the particularly highlighted alloy compositions, since a zero crossing of the saturation magnetostriction can be set with an adapted heat treatment. In addition, it can be exploited that in the usual heat treatment for property assessment

Temperature dependency of the permeability is in or very close to the zero crossing.

Due to the high degree of saturation, very high currents can be measured before saturation is reached, thereby disturbing the amperity of the current mapping. By fine-tuning the ratio of silicon to boron and the ratio of Co, Fe, Mn to the rest of the alloy, a particularly high saturation reduction can be achieved. SJ ch increases the saturation reduction by increasing the proportion of the ferromagnetic elements Co and Fe, but also by Mn compared to the total metalloid content. In addition, due to its 4 valence electrons, Si lowers the magnetic moment more than B with only 3 valence electrons. In this way, the saturation reduction with a constant total metalloid content can be further increased by a favorable fine tuning of B to Si. The magnetostriction, which becomes more negative as the metalloid content decreases, must then be adjusted again via the Fe content to such an extent that the zero crossing can finally be reached by the target temperature. By fine-tuning the iron content to the manganese content, when selecting a suitable target temperature, a saturation magnetostriction can be achieved, the amount of which is less than 0.1 or even 0.05 ppm. Due to the small saturation magnetostriction, the storanisotropy which competes with uniaxial anisotropy is particularly small. In this way, a good linearity of the hysteresis loop can be achieved even with small uniaxial anisotropies, which are a prerequisite for high permeability.

The magnetic core preferably has no air gap. A current transformer with a magnetic core without an air gap has a particularly high immunity to external magnetic fields without additional shielding measures. The magnetic core is, for example, a closed, air-gap-free ring core, oval core or rectangular core. Assigns the band as in the case of the toroid

Rotational symmetry axis, the anisotropy axis is parallel to the rotational symmetry axis.

With regard to the eddy current losses and thus the course of the permeability, a thickness d <26 μm has proven to be a favorable range for the band thickness of the band. On the other hand, in order to achieve a linear hysteresis loop that is as linear as possible, a band thickness d> 15 μm has been found. In the case of the alloys according to the invention, the surface-related proportion of the storanisotropies can be surprisingly greatly reduced as a result.

Particularly small coercive fields and thus a particularly good linearity of the hysteresis loop is achieved if the tape is provided with an electrically insulating layer on at least one surface. On the one hand, this results in a better relaxation of the core, and on the other hand, particularly low eddy current losses can be achieved through the electrically insulating layer. For example, the tape is provided with the electrically insulating layer on at least one of its two surfaces before winding. Depending on the requirements for the quality of the insulating layer, a dip, continuous, spray or electrolysis process is used for this.

Alternatively, the wound magnetic core is subjected to immersion insulation before being heated to the target temperature, so that the tape is provided with the electrically insulating layer. An immersion process under negative pressure has proven to be particularly advantageous.

When selecting the insulating medium, care must be taken that on the one hand it adheres well to the surface of the tape, and on the other hand does not cause a surface reaction which can damage the magnetic properties. In the alloys in question here, oxides, acrylates, phosphates, silicates and chromates of the elements calcium, magnesium, aluminum, titanium, zirconium, hafnium, silicon have proven to be effective and contractual insulators. Magnesium is particularly effective here, which is applied to the strip surface as a liquid magnesium-containing precursor and during a special process that does not influence the alloy

Heat treatment m converts a dense layer containing magnesium, whose thickness D can be between 25 nm and 3 μm. The actual insulator layer made of magnesium oxide is then formed at the temperatures of the magnetic field heat treatment described above.

The secondary winding of the current transformer can have a number of windings which is less than or equal to 2200. The primary winding of the current transformer can have a number of turns equal to three. The current transformer can be designed for a primary current that is less than or equal to 20A. Heating to the target temperature takes place as quickly as possible. For example, heating to the target temperature occurs at a rate between 1 to 15 K / mm.

For example, the magnetic core is held at the target temperature for between 0.25 and 4 hours in order to achieve the best possible balance of the mechanical stresses. The higher the target temperature, the shorter this time.

The cooling between the relaxation temperature and the Curie temperature also takes place as quickly as possible, e.g. with rates of 0.5 - 10 K / mm. The cooling rate regulates the proportion of the free volume and thus the atomic

Alignment capability, which is available at lower temperatures for setting the anisotropy. After the Curie temperature has been reached, the applied field, which is perpendicular to the direction of the strip, is cooled with 0.1 - 5 K / mm. This cooling rate is chosen so that a uniaxial anisotropy of the desired size is produced by the atomic reorientation under the driving force of the magnetic field. Since this uniaxial anisotropy is reciprocal to permeability, high permeability can be set with high cooling rates.

However, if a somewhat higher magnetic field-induced uniaxial anisotropy is to be set to linearize the hysteresis loop or to increase the anisotropic depth, a stationary temperature plateau can be inserted below the Curie temperature. The temperature should be chosen so low that the magnetic moments are as high as possible, but on the other hand so high that the kinetics of the alignment processes are still fast enough. Depending on the effect, the length of the

Temperature plateaus with an applied magnetic field are between 0.1 and 24 h. To produce the magnetic core, for example, an amorphous band is first produced from a melt using the rapid solidification technology known per se, which is described, for example, in DE 37 31 781 Cl. The amorphous alloy strip is then wound tension-free to the magnetic core. To reduce the storanisotropies, the procedure is preferably such that the strip has a low surface roughness.

The heat treatment is carried out in such a way that the value of the saturation magnetostriction λ _s changes during the heat treatment by an amount m positive direction depending on the alloy composition, until it changes in the range λ _s | <0.5 ppm, preferably | λ _s | <0.05 ppm. This value can also be achieved if the amount of λ _s in the "as quenched" state of the strip, that is to say directly after the casting process, is clearly above this value.

Depending on the alloy used, the magnetic core can be flushed with a reducing or at least passive protective gas, so that no oxidations or other reactions can occur on the surface of the strip, apart from the self-passivating and at the same time electrically insulating, extremely thin metalloid oxide layers that are permissible in certain cases .

The magnetic core treated in this way is finally solidified, e.g. provided by drinking, coating, encasing with suitable plastic materials and / or encapsulation and in each case with at least the secondary winding of the current transformer.

Exemplary embodiments of the invention are explained in more detail below with reference to the figures.

Figure 3 shows schematically the course of a heat treatment of a magnetic core. FIG. 4 shows the dependencies of the permeabilities of the magnetic core and the permeabilities of permalloy cores on an induction amplitude, which is caused by an exciting

Magnetic field is generated.

FIG. 5 shows the dependence of the amplitude error and the phase error on the current to be measured (primary current).

Figure 6 shows schematically the magnetic core, which consists of a tape with an insulating layer, and its anisotropy axis.

Figure 6 is not to scale and shows only a few turns for the sake of clarity.

With a magnetic core M weighing only 3.3 g made of an amorphous ferromagnetic alloy with the composition C057 7Fe3,3Mθ_ 5Si] _g 5JA0 5, a current transformer with a primary number of turns N] _ = 3 and a secondary number of turns N2 = 2000 could be produced, which has a load resistance of 100 ohms in the secondary circuit was terminated with low resistance.

For this purpose, the magnetic core M, which consisted of a tape B coated with an approximately 250 nm thick insulating layer S made of magnesium oxide, was subjected to the heat treatment shown in FIG. First, the magnetic core M was heated at a rate of approximately 420 K / h to a target temperature of approximately 458 ° C. within one hour and held there for approximately 1.5 hours. This was followed by cooling at a rate of about 120 K / h to about 220 ° C. in about two hours and at a rate of about 60 K / h to about room temperature in about three hours. The cooling at the rate of 60 K / h took place in a transverse magnetic field which was parallel to an axis of rotational symmetry of the magnetic core M. An anisotropy axis A parallel to the magnetic field was formed along which the magnetization of the magnetic core M aligns itself particularly easily (see FIG. 6).

In this example, the magnetostriction was reduced by the heat treatment from λ _s = - 13.5 * 10 ^{~ 8} to the very small value of - 1.2 * 10 ^{~ 8} . At the same time, the mechanical stresses previously existing in the wound magnetic core M were almost completely eliminated and the condition | σ | «0 was fulfilled, where σ is the mechanical elastic stress tensor. This was the prerequisite for high

Permeabilities were created and μ (50 Hz) = 177,000 was actually achieved. A favorable combination with high permeability and very good linearity (i.e. | λs | «0 and | σ |« 0) was achieved.

The hysteresis loop was so linear that the modulation dependence of the permeability shown in FIG. 4 is approximately constant. Comparable properties were also measured at target temperatures of T _σ = 449 ° C. For comparison, FIG. 4 shows the modulation dependency of the permeability of conventional permalloy alloys.

The curves of phase errors φ and amplitude errors F measured after winding on the described current transformer are shown in FIG. 5. The comparison to conventional permalloy alloys shows the advantages of current transformers made of magnetostriction-free, highly permeable amorphous cores.

The current transformer had an average phase error φ of 0.19 ° and an linearity of the phase angle Δφ over a current range of 0.1 to 2 A of less than 0.02 °. The permeability of this amorphous heat-treated ferromagnetic alloy is 192000 at a field amplitude H of 4 mA / cm. With the magnetic core M used is a ring band core measuring 19 x 15 x 5 mm with an iron cross section of Ap _e = 0.081 cm ² .

Similar good current transformers could be made with magnetic cores from the following alloys:

^Co 67, 62 ^Fe 3.7 ^Mo l, 5 ^si 16.5 ^B 10, 68 ^Co 68.2 ^Fe 3.9 ^Mo l, 5 ^si 16.3 ^B 10, l Co ₆ 7.65 ^Fe 3.4 ^Mn l, θSil6.75 ^M ° 0.2 ^B ll. 0 Cθ68.3 ^Fe 3.4 ^Mn l, 0 ^si 16.5 ^Mo 0.5 ^B 10.3 ^Co 68.2 ^Fe 4, l ^Ni l, 4 ^si 14.7 ^c 0.2 ^B ll, 4.

In contrast to these examples, using one of the alloys already described (the composition Cθ67 7Fe3, gMθ] _ 5Si] _g _/ 5 ^B 10, 5) made them significantly worse

Magnetic properties achieved if the heat treatment was carried out in a different way. In a first modification, the target temperature was increased to 510 ° C. with the intention of even better relaxation. However, the highly non-linear hysteresis loop that subsequently occurred had an initial permeability of only 9,400 due to strong interference anisotropies due to the onset of crystallization.

If, on the other hand, the relaxation was carried out at T _σ = 400 ° C, the linearity of the hysteresis loop also deteriorated, in which case the initial permeability was 97,000.

After a rapid cooling in the transverse field at 2.5 K / min instead of 1 K / min (see FIG. 3), the loop also rounded because of the now extremely small uniaxial anisotropy K _u . The initial permeability was therefore only 127,000.

After a slow cooling in the cross-field at 0.5 K / min, the loop kept its pronounced linearity. The bigger one However, uniaxial anisotropy energy also resulted in a reduced permeability of only 139,000.

Claims

claims

1. magnetic core which is suitable for use in a current transformer, characterized in that it consists of a wound band (B) made of an amorphous, ferromagnetic alloy,

it has a saturation permeability that is greater than 20,000 and less than 300,000,

it has a saturation magnetostriction, the amount of which is less than 0.5 ppm,

- it is essentially free of mechanical tension,

it has an anisotropy axis (A) along which the magnetization of the magnetic core (M) aligns itself particularly easily and which is perpendicular to a plane in which a center line of the band (B) runs,

- The alloy has a composition consisting essentially of the formula

Co _a (F _eι _ _x Mn _x ) _b Ni _c X _d Si _e B _f C _g

where X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge, P, a to g are given in atomic% and where a, b, c, d, e, f, g and x meet the following conditions:

40 <a <82; 3 <b <10; 0 <c <30; 0 <d <5; 0 <e <20; 7 <f <26; 0 <g <3; with 15 <d + e + f + g <33 and 0 <x <1.

2. Magnetic core according to claim 1, d a d u r c h g e k e n n z e i c h n e t that a, b, c, d, e, f, g and x meet the following conditions:

63 <a <73, 3 <b <10; 0 <c <5; 0 <d <3; 12 <e <19; 8 <f <20; 0 <g <3; with 20 <d + e + f + g <30 and x <0.5.

3. magnetic core according to claim 2,

d a d u r c h g e k e n n z e i c h n e t that a, b and c meet the following conditions:

68 <a + b + c <75.

4. Magnetic core according to claim 3, d a d u r c h g e k e n n z e i c h n e t that the amount of saturation magnetostriction is less than 0.1 ppm.

5. Magnetic core according to one of claims 1 to 4, characterized in that the magnetic core (M) has a saturation magnetization B _s of 0.5 to 0.7 T.

6. Magnetic core according to one of claims 1 to 5, d a d u r c h g e k e n n z e i c h n e t that the band (B) has a thickness d of 15 microns <d <26 microns.

7. Magnetic core according to one of claims 1 to 6, so that the tape (B) is provided at least on one surface with an electrically insulating layer (S).

8. Magnetic core according to claim 7, that a layer of magnesium oxide is provided as the electrically insulating layer (S).

9. Magnetic core according to claim 8, so that the electrically insulating layer (S) has a thickness D of 25 nm <D <lμm.

10. magnetic core according to one of claims 1 to 9, characterized in that it is designed as a closed, air-gap-free ring core, oval core or rectangular core.

11. Magnetic core according to one of claims 1 to 11, α a d u r c h g e k e n n z e i c h n e t that the ratio of its mechanical elastic tension tensor multiplied by the

Saturation magnetic striction to its uniaxial anisotropy is less than 0.5.

12. A current transformer for alternating current with a magnetic core according to one of claims 1 to 11, wherein the current transformer in addition to the magnetic core (M) as a transformer core made up of at least one primary winding and at least one secondary winding, to which a load resistor is connected in parallel and which terminates the secondary circuit in a median manner, consists.

13. Current transformer according to claim 12, characterized in that the secondary winding has a Wmdungszahl N _sec <2200, wherein the primary winding has a Wmdungszahl Np _rιm = 3 and the current transformer is designed for a primary current I _prιm <20 A.

14. A method for producing a magnetic core according to one of claims 1 to 11,

in which, after production and winding of the tape (B) to form the magnetic core (M), the magnetic core (M) is heated to a target temperature between 380 ° C and 500 ° C,

- at which the magnetic core (M) is cooled from the target temperature to room temperature, whereby at the latest from the Curie temperature of the alloy, a magnetic field of H> 100 A / cm is switched on, which is parallel to the anisotropy axis (A) of the magnetic core (M) ,

- at which the rate of cooling from the Curie temperature to room temperature is between 0.1 and 5 K / mm.

15. The method according to claim 14,

- in which the heating to the target temperature takes place at a rate between 1 to 15 K / min, - in which the magnetic core (M) is kept at the target temperature for between 0.25 and 4 hours.

16. The method according to claim 14 or 15,

- At which the cooling is interrupted at an intermediate temperature which is below the Curie temperature, at which the magnetic core (M) is held between 0.1 and 24 hours.

17. The method according to any one of claims 14 to 16, - in which the cooling takes place at rates between 0.1 and 15 K / min.

18. The method according to claim 17,

- in which cooling to the Curie temperature takes place at a rate between 0.5 and 10 K / min.

19. The method according to any one of claims 14 to 18,

- In which the tape (B) is provided with an electrically insulating layer (S) on at least one of its two surfaces before winding.

20. The method according to any one of claims 14 to 18,

- In which the magnetic core (M) is subjected to immersion insulation before heating to the target temperature, so that the strip (B) is provided with an electrically insulating layer (S).