JPH10335874A - Magnetic field canceller and feeder apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the effects of a magnetic field due to a current flowing in a current path by surrounding the current path with a tubular member and arranging magnetic circuits along the flowing direction of the current in the path. SOLUTION: This canceller 100 is equipped with a magnetic path member 20 for forming a flux passage and two magnetic sources 40 for generating a flux in the member 20. These sources 40 are arranged along the current path to form a tubular magnetic circuit surrounding a conductor 1 with the magnetic member 20 and generate a reverse magnetic field in the member 20 which is higher than that generated by an expected max. current flowing through the conductor 1, thereby lessening the effects of the magnetic field generated by the current flowing in the current path.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic field canceling device for reducing a magnetic field and a power supply equipment provided with the same, and more particularly, to a magnetic field suitable for reducing a magnetic field generated around a linear current. The present invention relates to a canceling device and a power supply facility provided with the canceling device.

[0002]

2. Description of the Related Art In order to reduce the influence of a magnetic field generated by a linear current flowing through a conductive wire, a parallel conductive wire 1 and a conductive wire 1, as shown in FIG. Flowing current is performed. The two conductors used for such a purpose are called a pair cable.

[0003] For relatively low-current alternating current, 2
Twisted pair of two conductors twisted together
air cable) may be used. By twisting the two conductors together, there is an effect that the influence from the outside approaches the two conductors evenly.

However, when the flowing current is large, for example, several tens of amperes to several hundreds of amperes, each of the conductors has a large cross-sectional area from the viewpoint of voltage drop due to resistance and heat generation. Conductors having such a cross-sectional area are difficult to twist. Therefore, in order to transmit a large current, two conductors are often used in parallel as a pair as shown in FIG.

On the other hand, as a method of reducing the influence of a magnetic field generated from an inductor such as a transformer, a material having a high magnetic permeability is used to surround a source of the magnetic field.

[0006]

However, in the case of power supply, currents of the same magnitude cannot be supplied to two conductors in some cases. For example, in a wiring method in which the ground is used as a common bus when supplying a direct current, the above-described paired wires cannot be adopted.

[0007] Even with alternating current flowing through a pair of wires, a phase difference may occur between the two conductors, resulting in a difference in the instantaneous value of the current.

On the other hand, it is difficult to reduce the influence of a magnetic field generated by a linear current flowing through the above-mentioned conductor by using a shielding member. The reason is considered as follows.

[0009] The magnetic flux of the magnetic field created by the linear current flowing through the conductor takes a closed path. Therefore, there are components that do not penetrate the shielding member. However, the method of using a material having a high magnetic permeability for the shielding member and surrounding the source of the magnetic field can only attenuate the magnetic flux through which the magnetic field lines pass through the shielding member. Therefore, components that do not penetrate the shielding member are radiated without being attenuated by the shielding member.

It is a first object of the present invention to provide a magnetic field canceling device capable of reducing the influence of a magnetic field due to a current flowing through a current path.

It is a second object of the present invention to provide a power supply facility capable of supplying power in a state in which the influence of a magnetic field due to a flowing current is reduced.

[0012]

According to a first aspect of the present invention, there is provided a magnetic field canceling apparatus for canceling a magnetic field generated by a current flowing through a current path. There is provided a magnetic field canceling device having a magnetic circuit surrounding the current path in an annular cross section.

According to a second aspect of the present invention, in a magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, a magnetic path member having an annular cross section; A magnetic source for supplying a magnetic flux, wherein the magnetic path member is formed in a cross-sectional shape having a through hole in which a current path can be arranged inside an annular cross-sectional space. Is provided.

According to a second aspect of the present invention, in a magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, the magnetic field canceling apparatus is formed in a cylindrical shape surrounding the current path, and the current flows through the current path. There is provided a magnetic field canceling device having a plurality of magnetic circuits arranged in a flowing direction.

According to a third aspect of the present invention, in a magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, a magnetic circuit having a path having an annular cross section is provided. There is provided a magnetic field canceling device having a magnetic path member having a cross-sectional shape having a gap through which a current path can pass in a part of the path.

According to a fourth aspect of the present invention, there is provided a magnetic field canceling device for canceling a magnetic field generated by a current flowing in a current path, comprising a magnetic circuit having a path having an annular cross section. There is provided a magnetic field canceling device comprising a nonmagnetic material over a path length longer than a length corresponding to an outer diameter of a current path.

In order to achieve the second object, according to a fifth aspect of the present invention, a conductor for energization and a magnetic field canceling device according to any one of the first to fourth aspects are provided. There is provided a power supply facility comprising:

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

In the following description, an imaginary unit (√ (−1))
Is represented by j. That is, j ² = −1 and j> 0. Also, an upward arrow (↑) is attached before a symbol to indicate that the symbol represents a vector quantity.

First, a first embodiment of the present invention will be described with reference to FIG.

In FIG. 1, a magnetic field canceling device 100 according to the present embodiment has a magnetic path member 20 for forming a magnetic flux path and a magnetic source 40 for generating a magnetic flux in the magnetic path member 20. It is composed.

The magnetic path member 20 is formed of a ferromagnetic material or a paramagnetic material. The magnetic path member 20 has a magnetic circuit in a shape surrounding the conductor 1 constituting the current path in an annular shape in cross section.
It is formed in a shape that is configured with 0.

The material forming the magnetic path member 20 preferably has a high magnetic permeability and a high saturation magnetic flux density.

Examples of the material having a high magnetic permeability include electromagnetic iron, permalloy, and ferrite.

The magnetic source 40 is for generating a magnetic flux in the magnetic path member 20 by the magnetomotive force. The magnetic source 40 is configured to generate a reverse magnetic field in the magnetic path member 20 with a magnitude greater than or equal to the magnetic field generated by the maximum current assumed for the current flowing through the conductor 1.

As the magnetic source 40, for example, a permanent magnet, an electromagnet or the like can be used.

Considering the direction along the current path (the longitudinal direction of the conductor 1), the magnetic source 40 forms a cylindrical magnetic circuit surrounding the conductor 1 together with the magnetic path member 20. Magnetic source 40
May be generated as long as it can generate a magnetic field that forms such a cylindrical magnetic circuit, and may be divided in a direction along a current path. For example, in FIG. 1, two magnetic sources 40 are arranged along a current path. Even if the magnetic sources 40 are arranged with a gap, a magnetic circuit continuous in the direction along the current path can be formed. For example, the magnetic flux generated by the magnetic source 40 and the magnetic resistance of the magnetic path member 20 By selecting the distribution, the magnetic flux can be distributed so that the cylindrical magnetic circuit is configured.

The number of the magnetic sources 40 is not limited to the above-mentioned two, but may be more. Of course, 1
One magnetic source 40 may form a cylindrical magnetic circuit over the entire length of the magnetic field canceling device 100.

The case where a plurality of cylindrical magnetic circuits are arranged in a direction along the current path will be described later. In this case, a plurality of magnetic path members 20 may be arranged with a gap, or a plurality of magnetic circuits arranged in the longitudinal direction of the magnetic path member 20 (along the current path) for one magnetic path member 20. May be.

Next, the operation of the magnetic field canceling device 100 configured as described above will be described with reference to FIG. FIG. 2 is a cross-sectional view taken along the arrow A in FIG.

First, consider a magnetic field ΔHr generated by a current ΔI flowing through the conductor 1. In FIG. 2, the current ΔI flows with a magnitude I in a direction from the front side of the paper to the back side of the paper. The magnetic field ΔHr created by this current ΔI is
A right-hand screw traveling in the direction of the current I occurs in the direction of rotation.
The magnitude Hr of the magnetic field ↑ Hr is proportional to the intensity I of the current ↑ I and inversely proportional to the distance r from the current ↑ I.

On the other hand, the magnetic path member 20 and the magnetic source 40
A magnetic circuit having a shape surrounding the conductor 1 in an annular cross section is formed.

The magnetic field ΔH generated in the magnetic circuit by the magnetic source 40 is the magnetic field ΔHr generated by the current ΔI.
Is generated in the opposite direction.

Here, if the magnitude Hr of the magnetic field generated by the current ΔI does not exceed the magnitude H of the magnetic field generated in the magnetic circuit, that is, if H ≧ Hr (7), the magnetic field No magnetic field appears outside the cancellation device 100.

Further, even if the magnitude Hr of the magnetic field generated by the current ΔI is larger than the magnitude H of the magnetic field generated in the magnetic circuit, the magnetic field canceling device 1
The magnitude Ho of the magnetic field ↑ Ho outside of 00 can be reduced to Ho = Hr-H (8).

On the other hand, even if the magnitude H of the magnetic field generated in the magnetic circuit is larger than the magnetic field ΔHr generated by the current ΔI, the magnetic field is not generated outside the magnetic field canceling device 100 according to the equation (7). Does not appear.

Therefore, when the current ΔI is a direct current, the magnitude of the magnetic field H generated in the magnetic circuit is equal to the magnitude Hr of the magnetic field generated by the current ΔI. The magnetic field outside the magnetic field canceling device 100 can be reduced without adjusting the magnetomotive force. Therefore, the magnetomotive force of the magnetic source 40 can be set to be larger than the expected magnitude Hr of the magnetic field. That is, it is easy to reduce the magnetic field even when the magnitude of the current ΔI is not known accurately.

When the current 上 記 I is a pulsating current whose magnitude I fluctuates with time, the maximum value Imax of the current ↑ I is larger than the magnitude Hrmax of the magnetic field generated by the current. By selecting the magnetic field magnitude H, the magnetic field can be reduced without changing the magnetic field magnitude H with time. That is, the magnetic source 40
Is sufficient if a steady magnetic field can be generated. Therefore, the magnetic field due to the pulsating current can be reduced by using a permanent magnet as the magnetic source 40.

Next, a second embodiment of the present invention will be described with reference to FIG. The magnetic field canceling device according to the present embodiment has the same basic configuration as the magnetic field canceling device according to the first embodiment, but differs in that the magnetic field generated by the magnetic source is an alternating magnetic field. The following description focuses on the differences.

In FIG. 3, an alternating current ΔI is flowing through the conductor 1 forming the current path. FIG. 3 shows that the alternating current ΔI
Is drawn from the front side of the paper to the back side of the paper.

First, a magnetic field ΔHr generated by an alternating current ΔI flowing through the conductor 1 will be considered. The magnetic field ↑ Hr is generated at each moment in a direction in which the right-hand screw traveling in the direction of the current ↑ I rotates. The magnitude Hr of the magnetic field ↑ Hr is proportional to the intensity I of the current ↑ I and inversely proportional to the distance r from the current ↑ I.
The direction and magnitude of the magnetic field are determined by the alternating current ΔI
It changes with the change of the direction and the size of.

In the present embodiment, the magnetic source 40 is
1, a coil 42, and an alternating current source 43.

The alternating current source 43 generates a magnetic field ΔH generated in the magnetic core 41 by a magnetic field ΔHr generated by the alternating current ΔI.
The alternating current ΔI _M is supplied to the coil 42 in a phase generated in the opposite direction. In addition, the magnitude of the alternating current ΔI _{M is determined by} the magnitude H of the magnetic field ΔH in each phase.
The coil 4 is set so that the magnitude of the magnetic field ΔHr is equal to or larger than the magnitude of the magnetic field ΔHr.
2 is supplied.

Here, as described above in the first embodiment,
At each instant, when the magnetic field ↑ Hr and the magnetic field ↑ H are opposite to each other and satisfy the expression (7), no magnetic field appears outside the magnetic field canceling device 100 at that moment.

Accordingly, the magnetomotive force (current ΔI _{M) of the} magnetic source 40 is set such that the current ΔI becomes equal to the magnitude Hr of the magnetic field ΔHr at a certain moment. ) Can be reduced without adjusting the magnetic field outside the magnetic field canceling device 100. For this purpose, for example, the magnetomotive force of the magnetic source 40 can be set to be larger than the expected magnetic field magnitude Hr. Therefore, even when the magnitude of the current ΔI is not accurately known, the magnetic field can be easily reduced.

[0046] Further, the time change of the magnitude I _M of the alternating current ↑ I _M, it is not necessary to follow the time variation of the magnitude of the alternating current ↑ I, the current satisfying the above expression (7) it may be the size I _M. Therefore, the magnetic field can be easily reduced even when the accurate time axis waveform of ΔI is not known. For example, even if the alternating current ΔI is a distorted time-axis waveform f including a harmonic component, the time-axis waveform g of the alternating current ΔI _M to be supplied by the alternating current source 43
Can be a simpler waveform than f that satisfies the above equation (7) at each phase. For example, a waveform such as a rectangular wave or a trapezoidal wave can be used.

Next, a third embodiment of the present invention will be described with reference to FIG.

Referring to FIG. 4, a magnetic field canceling device 200 according to the present embodiment includes a magnetic path member 20a and a magnetic source 40a for forming a first layer magnetic circuit, and a magnetic circuit member 20a and a magnetic source 40a for forming a second layer magnetic circuit. It has a magnetic path member 20b and a magnetic source 40b. The magnetic path member 20a forming the magnetic path of the magnetic circuit of the first layer is formed in a shape surrounding the conductor 1 forming the current path in an annular cross section. The magnetic path member 20b forming the magnetic path of the magnetic circuit of the second layer is formed in a shape surrounding the conductor 1 and the magnetic circuit of the first layer in an annular cross section.

In the magnetic circuit of the first layer and the magnetic circuit of the second layer, mutually opposite magnetic fields are generated by the magnetic sources 40a and 40b, respectively. For example, as shown in FIG. 4, the magnetic source 40a rotates clockwise to a magnetic field ΔHa generated in the magnetic circuit of the first layer.
Can turn the direction of the magnetic field ΔHb generated in the magnetic circuit of the second layer counterclockwise.

In FIG. 4, the current ↑ flowing through the conductor 1
Assuming that the positive direction of I is the direction from the front side to the back side of the paper, the magnetic field ΔHr generated when the current ΔI flows in the positive direction is the magnetic field generated in the magnetic circuit of the first layer. It can be reduced by ↑ Ha. The magnetic field ΔHr generated when the current ΔI flows in the negative direction
Can be reduced by the magnetic field ΔHb generated in the magnetic circuit of the second layer.

Accordingly, even when the alternating current ΔI flows through the conductor 1, the alternating magnetic field ΔH generated by the alternating current ΔI
The effect of r can be reduced outside the magnetic field canceling device 200. In particular, since the magnetic source 40a and the magnetic source 40b need only generate a steady magnetic field, they can be configured using permanent magnets.

Next, a fourth embodiment of the present invention will be described with reference to FIGS. The magnetic field canceling device according to the present embodiment has a plurality of canceling segments arranged along a current path.

In FIG. 5, the magnetic field canceling device 300 according to the present embodiment has a plurality of canceling segments 350 each forming a magnetic circuit that surrounds the current に I flowing through the conductor 1 constituting the current path in an annular cross section. It is composed. The plurality of cancel segments 350 are:
The conductors 1 are arranged along the conductors 1, and are arranged so as to surround the conductors 1.

Each of the above-mentioned cancel segments 350 corresponds to one of the first to third embodiments according to whether the current ΔI is a direct current or a pulsating current or an alternating current. Can be configured in the same manner as the magnetic field canceling device in the above.

Next, the operation of the magnetic field canceling device 300 will be described with reference to FIGS.

In FIG. 5, the length of the conductor 1 is 2 L, and the current flowing through the conductor 1 is ΔI. Further, it is assumed that the length of each of the cancel segments 350 along the conductor 1 is d, and that n cancel segments 350 are arranged.

In FIG. 5, the strength H of the magnetic field ΔH at a point L from both ends of the conductor 1 and a distance r from the center of the conductor diameter is represented by the following equation.

[0058]

(Equation 1)

However, L = nd.

Here, when deriving the equation (1), the length 2L
The magnitude H (see FIG. 6) of the magnetic field ΔH at a point at a distance r from the current ΔI flowing through the conductor 1 of FIG.

[0061]

(Equation 2)

The following expression was used.

In the equation (1), if r≫L, the magnitude H of the magnetic field is approximately

[0064]

(Equation 3)

Can be expressed as follows.

On the other hand, the magnitude of the magnetic field without the magnetic field canceling device 300 is

[0067]

(Equation 4)

Is obtained.

Comparing equations (3) and (4), the magnitude of the magnetic field when the magnetic field canceling device is applied (see FIG. 5) is the same as the magnitude of the magnetic field when the magnetic field canceling device is not used. Compared with this, the size is １ ／. This is an extremely valid result as a result of covering the cancel segment for half the length L of the conductor 2L through which the current flows.

Next, consider the case where r = L.

In equation (2), by substituting r = L, Tayl
or expand

[0072]

(Equation 5)

Is obtained. In equation (5), approximately

[0074]

(Equation 6)

Holds. That is, it is shown that the presence of the cancel segment in the finite length conductor is effective in reducing the near magnetic field.

In the first to fourth embodiments, a shielding member for covering the magnetic circuit can be provided. By surrounding the magnetic circuit from the outside with the shielding member, the influence of magnetic flux leaking from the magnetic circuit can be reduced. This can be explained as follows. The magnetic flux leaking from the magnetic circuit is such that a magnetic flux generated by a magnetic source leaks from a magnetic path, and such a magnetic flux appears radially to the outside. In order for these leaked magnetic fluxes to radiate outside, they must penetrate the shielding member. Paramagnetic or ferromagnetic shielding
It is effective against magnetic flux penetrating such a shielding member,
The effect of magnetic flux leaking to the outside can be reduced.

The material forming the shielding member is preferably a material having a high magnetic permeability. Materials with high magnetic permeability include:
Examples include electromagnetic iron, permalloy, and ferrite.

In the third embodiment, a magnetic circuit formed of a paramagnetic material or a ferromagnetic material can be provided between the first layer magnetic circuit and the second layer magnetic circuit. Thereby, the influence of the magnetic flux leaking between the magnetic circuit of the first layer and the magnetic circuit of the second layer can be reduced.

Incidentally, the magnetic flux leaking from each part of the magnetic circuit has an integral of 0 at the center. Therefore, by arranging the current path (for example, the conducting wire) at the center of the magnetic circuit, the interaction between the current flowing through the current path and the magnetic flux leaking from the magnetic circuit can be avoided.

In the above description, the case where the magnetic circuit is formed of a ferromagnetic or paramagnetic material over the entire length of the path of the magnetic circuit has been described, but the present invention is not limited to this. For example, a part of the path of the magnetic circuit can be formed of a non-magnetic material to such an extent that the influence of the leakage of the magnetic flux does not interfere. More specifically, for example, providing a gap, FIG.
As shown in FIG. 3, a part of the path is formed by a magnetic path member 20 made of a non-magnetic material.
V and the like. As a non-magnetic material,
For example, resin, aluminum, and the like can be given.

By providing the above gap, the gap can be passed through a conductor constituting a current path. For this reason, it is possible to attach and detach the magnetic field canceling device to and from the conductor without relatively moving the conductor in the penetration direction of the magnetic field canceling device and inserting the conductor. Therefore, attachment and detachment of the magnetic field canceling device can be easily performed.

Further, by providing the magnetic path member 20V made of the non-magnetic material at a part of the path, it is possible to attach and detach the magnetic path member 20V without affecting the magnetic force. For this reason, the arrangement of the conducting wire 1, inspection of the current path,
It becomes easy to perform operations such as attaching and detaching the magnetic field canceling device with the magnetic path member 20V removed.

[0083]

According to the present invention, the influence of the magnetic field due to the current flowing through the current path can be reduced. In particular, even when the current paths are single or asymmetrically arranged, the influence of the magnetic field due to the current flowing through each current path can be reduced.

The current flowing through the current path is a pulsating current,
Alternatively, even with an alternating current, the influence of the magnetic field created by the current can be reduced using a magnetic source that generates a steady magnetic field.

Further, by providing the above-mentioned magnetic field canceling device in the conducting wire for energization, power can be supplied in a state where the influence of the magnetic field generated by the energized current is reduced.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a magnetic field canceling device according to a first embodiment.

FIG. 2 is a cross-sectional view of the magnetic field canceling device according to the first embodiment, as viewed in the direction of arrow A in FIG.

FIG. 3 is an explanatory diagram showing a magnetic field canceling device having a magnetic source that generates an alternating magnetic field.

FIG. 4 is a sectional view showing a magnetic field canceling device in which a two-layer magnetic circuit is formed.

FIG. 5 is a perspective view showing a magnetic field canceling device having a plurality of canceling segments arranged along a magnetic path serving as a current path.

FIG. 6 is an explanatory diagram showing a relationship between a current flowing through a conductor and a magnetic field generated by the current.

FIG. 7 is a sectional view showing a magnetic field canceling device having a magnetic path member formed of a non-magnetic material in a part of a magnetic circuit.

FIG. 8 is a perspective view showing a conventional magnetic field canceling method using a pair of wires.

[Explanation of symbols]

1,2 ... conductor, 20, 20a, 20b ... magnetic path member, 20
V: magnetic path member of non-magnetic material, 40, 40a, 40b: magnetic source, 41: magnetic core, 42: coil, 43: alternating current source, 1
00, 200, 300 ... magnetic field canceling device, 350 ...
Cancel segment.

Claims (12)

[Claims] 1. A magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, comprising: a magnetic circuit surrounding the current path in an annular cross section. 2. The magnetic field canceling device according to claim 1, wherein the current flowing through the current path is a current whose polarity does not change, and the magnetic circuit has a current corresponding to a maximum value of the current flowing through the current path. A magnetic field canceling device comprising a magnetic source for generating a magnetic field having a magnitude equal to or greater than the magnitude of a magnetic field to be created. 3. The magnetic field canceling device according to claim 1, wherein the current flowing through the current path is an alternating current whose direction alternates between a first direction and a second direction. A first magnetic circuit and a second magnetic circuit surrounding the current path in two layers, wherein the first magnetic circuit has a current corresponding to a maximum value of the alternating current flowing in the first direction; A first magnetic source for generating a magnetic field having a magnitude equal to or greater than the magnitude of the magnetic field generated by the first magnetic source, wherein the second magnetic circuit has a current corresponding to a maximum value of the alternating current flowing in the second direction. A second magnetic source for generating a magnetic field having a magnitude equal to or greater than a magnetic field generated by the first magnetic source, wherein the first magnetic source and the second magnetic source generate magnetic fields in opposite directions to each other. Magnetic field canceling device. 4. The magnetic field canceling device according to claim 1, wherein the current flowing through the current path is an alternating current in which the direction in which the current flows alternately, and the magnetic circuit has a phase in which the direction of the alternating current alternates. A magnetic field canceling device comprising an alternating magnetic source for generating an alternating magnetic field whose direction is alternately changed. 5. A magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, comprising: a magnetic path member having an annular cross section; and a magnetic source for supplying a magnetic flux to the magnetic path member. The magnetic field canceling device, wherein the magnetic path member is formed in a cross-sectional shape having a through-hole in which a current path can be arranged inside an annular cross-sectional space. 6. The magnetic field canceling device according to claim 1, further comprising a ferromagnetic member formed in a shape covering the magnetic circuit. 7. The magnetic field canceling device according to claim 1, wherein the magnetic circuit is configured such that the current path can be arranged at a center of a cross section of the magnetic circuit. 8. A magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, wherein the magnetic field is formed in a cylindrical shape surrounding the current path, and a plurality of magnetic fields are arranged along the direction in which the current flows through the current path. A magnetic field canceling device having a circuit. 9. The magnetic field canceling device according to claim 8, wherein the magnetic circuits are arranged at predetermined intervals. 10. A magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, comprising a magnetic circuit having a path having an annular cross section, wherein the magnetic circuit has a gap through which the current path can pass. A magnetic field canceling device comprising a magnetic path member having a cross-sectional shape provided in a part of a path. 11. A magnetic field canceling device for canceling a magnetic field generated by a current flowing through a current path, comprising: a magnetic circuit having a path having an annular cross section, wherein the magnetic circuit has a length corresponding to an outer diameter of the current path. A magnetic field canceling device comprising a non-magnetic material over a longer path length. 12. A power supply facility comprising: a conductor for energization; and the magnetic field canceling device according to claim 1. Description: