Switchgear

Abstract

To reduce induction heating caused by an eddy current by strengthening the effect of magnetic driving by strengthening a magnetic field of a component orthogonal to an arc.SOLUTION: A switch gear (10) comprises: a stationary side electrode (11) which is provided in a space which is filled with an insulation gas; a movable side electrode (12) which is in contact with and separated from the stationary side electrode in the space; a field relaxation shield (30) which is provided so as to cover the stationary side electrode; and permanent magnets (41 and 42) which are provided in the stationary side electrode and the movable side electrode. The switchgear (10) magnetically drives an arc (AR) which is generated between the stationary side electrode and the movable side electrode by magnetic fields generated by the permanent magnets. The field relaxation shield includes: a shield main body (33) which is provided at a position enclosing a contact portion of the stationary side electrode with the movable side electrode; and a nonmagnetic layer (34) which is formed at an outer surface side of the shield main body. The shield min body is formed by compression-molding magnetic substance particles on which insulation coating is performed.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a switchgear, and more particularly to a switchgear used for power distribution equipment. [Background technology]

[0002] Patent Document 1 discloses a switchgear having a second electrode that moves in contact with and away from a first electrode, which is in a closed state when the electrodes come into contact with each other and in an open state when the electrodes move away from each other. In the switchgear, an arc is generated between the electrodes as soon as the electrodes move away from the closed state. Here, a configuration is disclosed in which a permanent magnet is installed inside each electrode, and a Lorentz force is generated by the magnetic field of the permanent magnet, which magnetically drives the arc and improves the arc-extinguishing performance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2022 / 230095 Summary of the Invention [Problem to be solved by the invention]

[0004] In the configuration in which the permanent magnets are installed, a magnetic field is generated in a direction perpendicular to the arc, but if the strength is insufficient, the Lorentz force in the direction driving the arc is weak, making it difficult to obtain a sufficient magnetic driving effect. In addition, in the configuration around each electrode, eddy currents are generated due to changes in current, and it is necessary to reduce induction heating caused by these eddy currents.

[0005] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a switching device that can strengthen the magnetic field component perpendicular to the arc, thereby enhancing the effect of magnetic driving and reducing induction heating due to eddy currents. [Means for solving the problem]

[0006] One embodiment of the switching device of the present invention comprises a fixed electrode provided in a space filled with an insulating gas, a movable electrode that operates in the space to contact and separate from the fixed electrode, an electric field mitigation shield provided to cover the fixed electrode, and a permanent magnet provided on at least one of the fixed electrode and the movable electrode, and is a switching device that magnetically drives an arc generated between the fixed electrode and the movable electrode by a magnetic field generated by the permanent magnet, wherein the electric field mitigation shield comprises a shield main body provided at a position surrounding the contact portion of the fixed electrode with the movable electrode, and a non-magnetic layer formed on the outer surface of the shield main body, and the shield main body is formed by compression molding of insulated coated magnetic particles. Effect of the Invention

[0007] According to the present invention, the shield body of the electric field mitigation shield is formed by compression molding of magnetic particles, so that the magnetic field generated by the permanent magnet is directed from the permanent magnet to the shield body, and the component of the magnetic field perpendicular to the arc can be strengthened. This can further increase the effect of magnetically driving the arc by the Lorentz force, and improve the arc extinguishing performance. In addition, since the magnetic particles forming the shield body are insulated, induction heating due to eddy currents can be reduced. [Brief description of the drawings]

[0008] [Figure 1] 2 is a schematic partial cross-sectional view of a switching device according to an embodiment in an open state; FIG. [Diagram 2] 2 is a schematic partial cross-sectional view of a switching device according to an embodiment in a closed state; FIG. [Diagram 3] 11 is an explanatory diagram showing a state in which an arc is generated between a fixed electrode and a movable electrode; FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] An opening and closing device according to an embodiment of the present invention will be described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiment, and can be modified as appropriate within the scope of the present invention. In the following drawings, some components may be omitted for convenience of explanation. Here, in this specification and claims, "front" and "rear" are used as references to the directions indicated by the arrows in each drawing.

[0010] FIG. 1 is a schematic partial cross-sectional view of a switchgear according to an embodiment in an open state. FIG. 2 is a schematic partial cross-sectional view of a switchgear according to an embodiment in a closed state. As shown in FIGS. 1 and 2, a switchgear 10 includes a fixed electrode 11 and a movable electrode 12 arranged side by side in the front-rear direction. In this embodiment, the fixed electrode 11 is arranged at the front, and the movable electrode 12 is arranged at the rear. The switchgear 10 includes a housing (not shown) that forms a sealed internal space, and the fixed electrode 11 and the movable electrode 12 are arranged in the internal space. When the switchgear 10 is a gas-insulated switchgear, the internal space is filled with an insulating gas such as sulfur hexafluoride (SF6) or dry air, but the internal space may be kept vacuum instead of the insulating gas.

[0011] The switchgear 10 further includes a fixed-side conductive holding part 13 and a movable-side conductive holding part 14 that are energized through a conductor (not shown) that constitutes a circuit. The fixed-side conductive holding part 13 and the movable-side conductive holding part 14 are provided in a roughly cylindrical shape with an axial direction facing the front-rear direction. The fixed-side conductive holding part 13 holds the fixed-side electrode 11 and electrically connects the circuit to the fixed-side electrode 11. The movable-side conductive holding part 14 holds the movable-side electrode 12 and electrically connects the circuit to the movable-side electrode 12. In this embodiment, the electrodes 11, 12 and the conductive holding parts 13, 14 constitute a disconnector that opens and closes the circuit.

[0012] The movable electrode 12 is formed in a round shaft shape extending in the front-rear direction. The movable electrode 12 is moved in the front-rear direction by an operation mechanism (not shown). The operation mechanism transmits a driving force from a driving source such as an appropriate rotating mechanism or a cylinder to the movable electrode 12 via a lever or a link mechanism.

[0013] The movable electrode 12 is located at the rear and separated from the fixed electrode 11 in the open circuit state shown in Fig. 1, and is located at the front and contacts the fixed electrode 11 at its tip end (front end) in the closed circuit state shown in Fig. 2. Thus, the movable electrode 12 is provided so as to be capable of opening and closing, moving toward and away from the fixed electrode 11, and the direction of the opening and closing movement of the movable electrode 12 is parallel to the front-rear direction.

[0014] The fixed side electrode 11 has a fixed side main contact 16 and a fixed side arc contact 17 that can be brought into contact with and separated from the movable side electrode 12, and the fixed side arc contact 17 is arranged to be displaceable in the forward / backward direction, which is the same as the operating direction of the movable side electrode 12.

[0015] The fixed main contact 16, not shown in detail, is made of an annular conductor having an opening diameter slightly smaller than the outer diameter of the movable electrode 12. The fixed main contact 16 is provided with multiple grooves at predetermined intervals in the circumferential direction or multiple strips arranged in the circumferential direction to provide flexibility or elasticity in the radial direction. The fixed main contact 16 also has a ring-shaped or spiral tension spring 16a on its outer periphery, which exerts a force in the direction of reducing the opening diameter. Therefore, in the closed state shown in FIG. 2, the fixed main contact 16 comes into contact with the outer periphery of the tip side of the movable electrode 12 in the forward position with an appropriate contact pressure, allowing current to flow.

[0016] The fixed side main contact 16 is supported in contact with an annular protrusion 13a formed at the rear end of the fixed side conductive holding part 13, allowing electrical current to flow between the fixed side main contact 16 and the fixed side conductive holding part 13.

[0017] The fixed side arc contact 17 includes a contact body 18 in the shape of a round shaft having an outer diameter smaller than the opening diameter of the fixed side main contact 16, and a disk-shaped base 19 connected to the rear end of the contact body 18 and having a larger diameter than the contact body 18. In the closed state shown in Fig. 2, the contact body 18 of the fixed side arc contact 17 has a tip (rear end) in contact with the tip of the movable side electrode 12 to allow current to flow.

[0018] The fixed-side arc contact 17 is disposed in a movable space 13b formed in a round hole shape at the axial center position of the fixed-side conductive holding part 13 so as to be displaceable in the front-rear direction. An annular wall part 13c protruding inward is formed in the middle part of the movable space 13b in the front-rear direction. In the fixed-side arc contact 17, a contact body 18 is inserted into the annular wall part 13c, and a base part 19 abuts against the front surface of the annular wall part 13c, thereby restricting rearward displacement of the fixed-side arc contact 17.

[0019] Annular sliding parts 21, 22 are embedded in the inner circumference of the annular wall part 13c and the outer circumference of the base part 19. The sliding parts 21, 22 are made of wear rings or the like that serve as low-friction sliding members. The inner circumference of the sliding part 21 provided on the annular wall part 13c slides against the outer circumference of the contact body 18 that displaces in the front-rear direction. The inner circumference of the sliding part 22 provided on the base part 19 slides against the inner circumference of the fixed-side conductive holding part 13 (the surface on which the movable space 13b is formed) when displacing in the front-rear direction. The sliding of the sliding parts 21, 22 guides the displacement of the fixed-side arc contact 17 in the front-rear direction.

[0020] In the fixed-side arc contact 17, a contact band 24 is provided on the outer periphery of the base 19. Electricity is passed between the inner circumferential surface of the fixed-side conductive holding part 13 and the base 19 of the fixed-side arc contact 17 via the contact band 24, and the electric current is maintained regardless of the forward and backward displacement of the fixed-side arc contact 17.

[0021] A spring member 25 is accommodated in the movable space 13b of the fixed-side conductive holding part 13, in front of the fixed-side arc contact 17. In this embodiment, the spring member 25 is configured by a compression coil spring, and its front end side is fixed by a pressing member 26 that closes the front part of the movable space 13b. The spring member 25 is disposed between the pressing member 26 and the base 19 of the fixed-side arc contact 17. In this state, the spring member 25 is compressed from its natural length and stored in energy, applying an elastic force in a direction that displaces the fixed-side arc contact 17 backward.

[0022] 1, when the rear surface of the base 19 is pressed against the front surface of the annular wall portion 13c by the elastic force of the spring member 25, the rearward displacement of the fixed-side arc contact 17 is restricted and the fixed-side arc contact 17 is positioned in the front-rear direction. In this state, the rear end surface of the fixed-side arc contact 17 (contact body 18) is set to be located rearward of the rear surface of the fixed-side main contact 16 and forward of the front end of the cover member 31 described later.

[0023] The switching device 10 further includes an electric field mitigation shield 30 attached to the rear end side of the fixed side conductive holding portion 13 so as to cover the fixed side electrode 11 from behind, and a cover member 31 provided in an opening 30a of the electric field mitigation shield 30.

[0024] The opening 30a of the electric field mitigation shield 30 is formed so as to open in the front-rear direction at the rear of the electric field mitigation shield 30, and is formed to have an opening diameter larger than the outer diameter of the movable electrode 12. Therefore, the tip (front end) side of the movable electrode 12 can be inserted into and removed from the opening 30a of the electric field mitigation shield 30 in the front-rear direction.

[0025] The cover member 31 is provided along the edge of the opening 30a in the electric field reduction shield 30. More specifically, the cover member 31 is formed in an annular shape corresponding to the opening 30a, and a circumferential groove that receives the edge of the opening 30a is formed on the outer circumferential surface. Thus, the cover member 31 is U-shaped in cross section and covers the edge of the opening 30a. Even inside the annular cover member 31, the tip (front end) side of the movable-side electrode 12 can be inserted and removed in the front-rear direction.

[0026] The electric field mitigation shield 30 includes a shield body 33 formed in a dome shape bulging backward, and a nonmagnetic layer 34 formed so as to be laminated on the outer surface side of the shield body 33. The shield body 33 forms roughly the entire shape of the electric field mitigation shield 30, and is provided at a position surrounding the inner circumferential surface of the fixed-side main contact 16, which is the contact portion with the movable-side electrode 12 of the fixed-side electrode 11, and the rear end of the fixed-side arc contact 17.

[0027] The shield body 33 and cover member 31 of the electric field mitigation shield 30 are formed by compressing and molding raw material powder made of insulating coated magnetic particles. An example of the raw material powder is soft magnetic powder coated with an insulating film. The compression molding is performed by filling the raw material powder into a molding die and compressing it with a predetermined press pressure.

[0028] The components of the raw material powder are not particularly limited as long as they exhibit soft magnetism, and examples thereof include pure Fe, Fe-Si alloy, permalloy, etc. The raw material powder is preferably produced by, for example, a water atomization method, and its particle size is preferably several tens to several hundreds of μm (for example, 100 μm). In addition, the raw material powder is preferably a flat powder obtained by performing flattening processing on granular powder (for example, a method of mechanically flattening by a dry method).

[0029] Furthermore, in order to remove distortion caused during pressure molding, the shield body 33 and cover member 31 obtained by pressure molding may be appropriately heat-treated. When heat-treating, it is preferable to form an insulating coating of a highly heat-resistant material on the raw material powder. Examples of such highly heat-resistant materials include phosphate compounds and magnesium oxide (MgO), which are already well known.

[0030] Furthermore, the raw material powder may contain a binder. The type of binder is not particularly limited, but examples thereof include polyimide, epoxy resin, phenol resin, silicone resin, etc. The content of the binder is preferably 1.0 mass% or less of the entire raw material powder, and more preferably 0.5 mass% or less. If the content of the binder is high, the non-magnetic portion inside the compact increases, which may result in insufficient magnetic properties.

[0031] The non-magnetic layer 34 may be made of a metal such as copper or aluminum, or a non-metal, and a material having a significantly lower magnetic permeability than the shield body 33 is used.

[0032] The electric field reduction shield 30 is attached at its front end to the fixed-side conductive holding part 13 via a connecting member 35. The connecting member 35 is composed of a fastening member such as a screw member or a bolt, and penetrates the electric field reduction shield 30 from the outside to the inside, with its tip side screwed into the fixed-side conductive holding part 13 and its head in contact with the non-magnetic layer 34. In addition, the connecting member 35 is made of a conductive material, and the fixed-side conductive holding part 13 and the non-magnetic layer 34 are kept at the same potential via the connecting member 35.

[0033] Here, permanent magnets 41, 42 are provided on both the fixed electrode 11 and the movable electrode 12. The permanent magnet 41 provided on the fixed electrode 11 is embedded in the rear end (tip) side of the fixed arc contact 17 and is arranged so as not to be exposed when viewed from the movable electrode 12 side. The permanent magnet 42 provided on the movable electrode 12 is embedded in the front end (tip) side of the movable electrode 12 and is arranged so as not to be exposed when viewed from the fixed electrode 11 side.

[0034] In the closed circuit state shown in Fig. 2, the permanent magnets 41, 42 are disposed close to each other in the front-rear direction. Here, the permanent magnets 41, 42 are disposed so that the same poles face each other, for example, so that the rear part of the permanent magnet 41 provided on the fixed electrode 11 and the front part of the permanent magnet 42 provided on the movable electrode 12 are both N poles (see Fig. 3).

[0035] The movable side electrode 12 is supported by the movable side conductive holder 14 via two sliding parts 44. The sliding parts 44 are configured similarly to the sliding parts 21 described above, and guide the displacement of the movable side electrode 12 in the front-rear direction.

[0036] The movable-side conductive holding part 14 is provided with a contact 46 that comes into contact with the movable-side electrode 12 when the movable-side conductive holding part 14 is inserted therethrough. The contact 46 is configured similarly to the fixed-side main contact 16 described above, and electrical current can be conducted between the movable-side conductive holding part 14 and the movable-side electrode 12 via the contact 46.

[0037] An electric field mitigation shield 48 is attached to the front end side of the movable-side conductive holding part 14. The electric field mitigation shield 48 is provided so as to cover the contacts 46 from behind, and is formed in the same shape as the above-mentioned electric field mitigation shield 30. Furthermore, the electric field mitigation shield 48 may be made of the same material as the above-mentioned electric field mitigation shield 30, or may be made of a different material, as long as it can mitigate electric field concentration around the attachment position.

[0038] 2, the movable electrode 12 moves forward and is inserted into the fixed main contact 16. Also, the front end of the movable electrode 12 comes into contact with the rear end of the fixed arc contact 17, pushing the fixed arc contact 17 forward, so that the fixed arc contact 17 is in the advanced position and the spring member 25 is compressed. In the closed state, the elastic force of the spring member 25 presses the rear end of the contact body 18 of the fixed arc contact 17 against the front end of the movable electrode 12, and the fixed main contact 16 comes into contact with the outer circumferential surface of the front end of the movable electrode 12 with an appropriate contact pressure.

[0039] In the closed state, a current path is formed that flows via the movable side electrode 12 and the fixed side main contact 16 to the fixed side conductive holding part 13, and a current path is formed that flows via the movable side electrode 12, the fixed side arc contact 17, and the contact band 24 to the fixed side conductive holding part 13.

[0040] 1 from the closed state, the movable electrode 12 is moved backward from the position shown in Fig. 2. In the initial stage of this movement, the fixed arc contact 17 is also displaced forward following the movement of the movable electrode 12 due to the elastic force of the spring member 25, and the electrical connection between the movable electrode 12 and the fixed arc contact 17 is maintained. On the other hand, when the movable electrode 12 moves backward and comes out of the inside of the fixed main contact 16, the fixed main contact 16 is separated from the movable electrode 12. Therefore, in the fixed electrode 11, the current path via the fixed main contact 16 is interrupted, and a current flows through the current path via the fixed arc contact 17.

[0041] When the rear end of the fixed arc contact 17 (contact body 18) is displaced rearward from the rear end of the fixed main contact 16, the base 19 of the fixed arc contact 17 abuts against the annular wall 13c, and the rearward displacement of the fixed arc contact 17 is stopped. In this state, the movable electrode 12 continues to move rearward, and the movable electrode 12 is separated from the fixed arc contact 17 inside the electric field mitigation shield 30. When the movable electrode 12 is separated from the fixed arc contact 17, an arc AR (see FIG. 3) is generated between them. Therefore, the position where the arc AR is generated in the fixed electrode 11 becomes the fixed arc contact 17, and it is possible to prevent the arc AR from being generated in the fixed main contact 16. The extension direction of the arc AR is irregular but generally along the front-rear direction.

[0042] Fig. 3 is an explanatory diagram showing a state in which an arc is generated between the fixed electrode and the movable electrode. In this embodiment, when the movable electrode 12 and the fixed arc contact 17 are separated from each other and an arc AR is generated, a magnetic field is generated between them by the permanent magnets 41, 42. The magnetic field lines indicating the direction of the magnetic field are indicated by the arrows in Fig. 3.

[0043] The shield body 33 and the cover member 31 of the electric field mitigation shield 30 arranged near the permanent magnets 41 and 42 are formed using magnetic particles. Therefore, the magnetic field generated by the permanent magnets 41 and 42 is attracted to the shield body 33 and the cover member 31, and immediately after extending in the front-rear direction from the surface where the N pole is formed, a component perpendicular to the front-rear direction (a component parallel to the up-down direction in FIG. 3) is strongly generated by curving. This makes it possible to strengthen the Lorentz force in the direction that drives the arc AR generated by the magnetic field of the permanent magnets 41 and 42, and to magnetically drive the arc AR well in the circumferential direction. Therefore, the heat of the arc AR is transported to the surroundings, the conductivity of the arc AR is lost, and the time until the arc AR disappears can be shortened. As a result, it is possible to improve the arc extinguishing performance, prevent the arc AR from sticking, and reduce the amount of wear of the movable side electrode 12 and the fixed side arc contact 17.

[0044] Furthermore, since the electric field mitigation shield 30 of this embodiment has a non-magnetic layer 34, which has the same potential as the fixed-side conductive holding part 13, even if the shield body 33 is made of a magnetic material, it is possible to mitigate electric field concentration around the electrodes 11, 12. Furthermore, since the magnetic particles forming the shield body 33 are insulated, it is possible to suppress the generation of eddy currents in the shield body 33, and it is possible to reduce induction heating due to eddy currents. This is because the eddy currents are confined inside the insulated magnetic particles, making the current path smaller than in normal magnetic materials.

[0045] Moreover, since the cover member 31 is provided at the opening 30a of the electric field mitigation shield 30, it is possible to prevent the arc AR from being diverted to the electric field mitigation shield 30, and to protect the periphery of the opening 30a. Furthermore, it is easy to adjust the cover member 31, which is a magnetic body, to be disposed close to the permanent magnets 41, 42. This makes it possible to further strengthen the component of the magnetic field of the permanent magnets 41, 42 that is perpendicular to the front-rear direction, and to further enhance the effect of magnetically driving the arc AR.

[0046] Furthermore, since an arc AR occurs at the fixed arc contact 17 at the fixed electrode 11, the fixed main contact 16 can be protected and its lifespan can be extended. When the movable electrode 12 moves away from the fixed arc contact 17, they are located inside the electric field mitigation shield 30. This makes it easier to apply a magnetic force that attracts the magnetic fields of the permanent magnets 41, 42 to the shield body 33 and the cover member 31 at the timing when the arc AR starts to occur.

[0047] In addition, since the permanent magnets 41, 42 are provided on both the fixed electrode 11 (fixed arc contact 17) and the movable electrode 12 and are arranged so that the same poles face each other, a strong component perpendicular to the arc AR can be generated in the magnetic field of the permanent magnets 41, 42.

[0048] Furthermore, since the permanent magnets 41, 42 are embedded in the fixed arc contact 17 and the movable electrode 12 and are arranged without being exposed, the permanent magnets 41, 42 can be protected from being directly exposed to the arc AR.

[0049] The present invention is not limited to the above-mentioned embodiment, and can be modified in various ways. In the above-mentioned embodiment, the size, shape, orientation, etc. shown in the attached drawings are not limited to these, and can be modified as appropriate within the scope of the effects of the present invention. In addition, the present invention can be modified as appropriate without departing from the scope of the purpose of the present invention.

[0050] In the above embodiment, the permanent magnets 41, 42 are provided on both the fixed-side arc contact 17 (fixed-side electrode 11) and the movable-side electrode 12, but as long as the direction of the magnetic field can be set in the same manner as described above, one of the permanent magnets 41, 42 may be omitted. In other words, it is sufficient that at least one of the fixed-side arc contact 17 (fixed-side electrode 11) and the movable-side electrode 12 is provided with the permanent magnets 41, 42.

[0051] Furthermore, the cover member 31 may be omitted as long as commutation of the arc AR from the fixed-side arc contact 17 (fixed-side electrode 11) and the movable-side electrode 12 to the electric field mitigation shield 30 can be prevented.

[0052] In addition, the above-mentioned configuration has been described as being applied to the disconnector of the switchgear 10, but it is not limited to being applied to a disconnector, and can also be applied to a grounding switch, a disconnector with a grounding switch, etc. that open and close the electric circuit of a power system. [Explanation of symbols]

[0053] 10: Switchgear 11: Fixed side electrode 12: Movable side electrode 16: Fixed side main contact 17: Fixed arc contact 30: Electric field mitigation shield 30a: opening 31: Cover member 33: Shield body 34: Non-magnetic layer 41: Permanent magnet 42: Permanent magnet AR: Arc

Claims

1. a fixed electrode provided in a space filled with an insulating gas; a movable electrode that moves within the space to come into contact with and separate from the fixed electrode; an electric field mitigation shield provided to cover the fixed electrode; a permanent magnet provided on at least one of the fixed electrode and the movable electrode, and an arc generated between the fixed electrode and the movable electrode is magnetically driven by a magnetic field generated by the permanent magnet, The electric field mitigation shield includes a shield body provided at a position surrounding a contact portion of the fixed electrode with the movable electrode; a non-magnetic layer formed on an outer surface side of the shield body, The switchgear, wherein the shield body is formed by compression molding of insulatingly coated magnetic particles.

2. an opening through which the movable electrode is inserted and removed is formed in the electric field mitigation shield; The opening and closing device according to claim 1 , further comprising a cover member provided along an edge of the opening and formed by compression molding of insulatingly coated magnetic particles.

3. The fixed electrode has a fixed main contact that is movable and can be brought into contact with the movable electrode. a fixed arc contact that is provided displaceable in a direction of movement of the movable electrode and that generates the arc when the fixed arc contact is separated from the movable electrode, 3. The switchgear according to claim 1, wherein the movable electrode is separated from the fixed arc contact inside the electric field mitigation shield.

4. 3. The switchgear according to claim 1, wherein the permanent magnets are provided on both the fixed electrode and the movable electrode.

5. the permanent magnet provided on the fixed electrode is arranged so as not to be exposed when viewed from the movable electrode side, 3. The opening and closing device according to claim 1, wherein the permanent magnet provided on the movable electrode is arranged so as not to be exposed when viewed from the fixed electrode side.

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