JP2016151393A - Heat exchanger and magnetic heat pump device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat exchanger that can increase the speed of a magnetic heat pump device.SOLUTION: A heat exchanger 10 used in a magnetic heat pump device 1 comprises: at least one foil material 12 consisting of a magnetocaloric effect material; and a container 13 including inner wall surfaces 134 and 135 that support the foil material 12. The foil material 12 comprises: a first bent part 123 bent in a U-shape; and first and second planar parts 121 and 122 mutually connected via the first bent part 123. The inner wall surfaces 134 and 135 comprises: a first groove 134a into which an end part 121b of the first planar part 121 and an end part 122b of the second planar part 122 are inserted; and a second groove 135a opposed to the first groove 134a and into which the first bent part 123 is inserted.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a heat exchanger used in a magnetic heat pump apparatus using a magnetocaloric effect, and a magnetic heat pump apparatus including the heat exchanger.

  A technique of manufacturing a heat exchanger having a microchannel by laminating plate-like material pieces made of a magnetocaloric effect material and having grooves (see, for example, Patent Document 1).

JP 2014-44003 A

  In the above technique, the groove is formed by cutting, cutting, polishing, or the like on the material piece. Therefore, the material piece needs a certain thickness (0.4 mm). Therefore, the heat transfer time from the inside to the surface of the magnetocaloric effect material is long, and the time required for heat exchange of the magnetocaloric effect material becomes long, and the magnetic heat pump device cannot be sufficiently speeded up.

  On the other hand, the time required for heat exchange of the magnetocaloric effect material can be shortened by using the foil-like magnetocaloric effect material. However, if the magnetocaloric effect material is simply made into a thin foil shape, it becomes difficult to form a narrow groove in the support member for inserting the foil material, and the foil-like magnetocaloric effect material can be arranged at a narrow pitch. There is a problem that it becomes difficult.

  The problem to be solved by the present invention is to provide a heat exchanger capable of increasing the speed of a magnetic heat pump device, and a magnetic heat pump device including the heat exchanger.

  [1] A heat exchanger according to the present invention includes a magnetocaloric effect material having a magnetocaloric effect, and is a heat exchanger used in a magnetic heat pump device, and is at least one foil composed of the magnetocaloric effect material And a support member that supports the foil material, and the foil material is connected to each other via a first bent portion bent in a U-shape and the first bent portion. First and second planar portions, wherein the support member is inserted with an end portion of the first planar portion and an end portion of the second planar portion. A heat exchanger having a groove and a second groove provided to correspond to the first groove and into which the first bent portion is inserted.

  [2] In the above invention, the first bent portion may be provided over the entire area of the foil material in the direction along the bending center line of the first bent portion.

  [3] In the above invention, the foil material further extends from at least one of an end portion of the first planar portion and an end portion of the second planar portion, and is bent into a U shape. A second bent portion may be provided, and the second bent portion may be interposed between an end portion of the first planar portion and an end portion of the second planar portion.

  [4] In the above invention, the heat exchanger includes a plurality of the foil members, and the support members are inserted with end portions of the first and second planar portions of the foil members, respectively. A plurality of the first grooves and a plurality of the second grooves that are provided so as to correspond to the first grooves, respectively, and into which the first bent portions of the foil material are respectively inserted. You may do it.

  [5] In the above invention, the heat exchanger includes a container having the support member, and the container has a first opening through which fluid flows into or out of the container, and the fluid A second opening that flows out of or into the container, a direction from the first opening toward the second opening, a fold line of the first bent portion, May be substantially parallel.

  [6] A magnetic heat pump device according to the present invention includes the heat exchanger, a magnetic field changing unit that applies a magnetic field to the magnetocaloric effect material and changes the magnitude of the magnetic field, and the container through a flow path. And a fluid supply for supplying the fluid from the container to the first or second external heat exchanger in conjunction with the operation of the magnetic field changing means. And a magnetic heat pump device.

  According to the present invention, the end portions of the first and second planar portions of the foil material are inserted into the first groove, and the first bent portion is inserted into the second groove. That is, by inserting both ends of the folded foil material into the first and second grooves, the foil material is supported by the support member. For this reason, since the total thickness of the foil material is increased and the formation of the first and second grooves of the support member is facilitated, the speed of the magnetic heat pump device can be increased.

  Further, in the present invention, the first bent portion of the foil material is bent in a U shape so that the first bent portion is attached in a direction in which the first planar portion and the second planar portion are separated from each other. Therefore, the foil material can be stably fixed to the support member.

FIG. 1 is a diagram showing an overall configuration of a magnetic heat pump device according to an embodiment of the present invention, and shows a state where a piston is in a first position. FIG. 2 is a diagram illustrating the overall configuration of the magnetic heat pump device according to the embodiment of the present invention, and is a diagram illustrating a state where the piston is in the second position. FIG. 3 is a cross-sectional view of the MCM heat exchanger in the embodiment of the present invention, which is a cross-sectional view cut along the refrigerant flow direction. 4 is a cross-sectional view taken along line IV-IV in FIG. FIG. 5 is a perspective view showing a foil material in the embodiment of the present invention. FIG. 6 is a cross-sectional view taken along line VI-VI in FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  1 and 2 are diagrams showing the overall configuration of the magnetic heat pump device in the present embodiment, FIGS. 3 and 4 are diagrams showing an MCM heat exchanger in the present embodiment, and FIGS. 5 and 6 are foil materials in the present embodiment. FIG.

  The magnetic heat pump device 1 in the present embodiment is a heat pump device using a magnetocaloric effect, and as shown in FIGS. 1 and 2, first and second MCM heat exchangers 10 and 20, The piston 30, the permanent magnet 40, the low temperature side heat exchanger 50, the high temperature side heat exchanger 60, the pump 70, the pipes 81 to 84, and the switching valve 90 are provided.

  The first and second MCM heat exchangers 10 and 20 in the present embodiment correspond to an example of a heat exchanger in the present invention, and the piston 30 and the permanent magnet 40 in the present embodiment serve as an example of a magnetic changing unit in the present invention. The low temperature side heat exchanger 50 and the high temperature side heat exchanger 60 correspond to an example of the first and second heat exchangers in the present invention, and the pipes 81 to 84 in the present embodiment are flow paths in the present invention. It corresponds to an example, and the pump 70 and the switching valve 90 in the present embodiment correspond to an example of the fluid supply means in the present invention.

  As shown in FIGS. 3 and 4, the first MCM heat exchanger 10 includes a plurality of foil materials 12 and a container 13 that accommodates the stacking foil materials 12. In addition, since the 1st MCM heat exchanger 10 and the 2nd MCM heat exchanger 20 have the same structure, the structure of the 1st MCM heat exchanger 10 is demonstrated in detail below, and 2nd The configuration of the heat exchanger 20 will be omitted.

  Each foil material 12 is composed of a magnetocaloric effect material (MCM) having a magnetocaloric effect. When a magnetic field is applied to the foil material 12 made of this MCM, the magnetic entropy is reduced by aligning the electron spin, and the foil material 12 generates heat and the temperature rises. On the other hand, when the magnetic field is removed from the foil material 12, the electron spin becomes messy and the magnetic entropy increases, and the foil material 12 absorbs heat and the temperature decreases.

  The foil material 12 is formed by bending a single MCM thin plate. This MCM thin plate before bending has a thickness of about 0.1 mm and a rectangular shape. Note that the thickness of the MCM thin plate is not particularly limited to the above, and it is sufficient that the MCM thin plate has a thickness of about 0.05 mm to 0.1 mm.

  The foil material 12 formed by bending the MCM thin plate has, as shown in FIGS. 5 and 6, a first and second planar portions 121 and 122, and a first bent portion 123. ing. The first planar portion 121 is a flat portion spreading in a planar shape, and is continuously connected to the first bent portion 123 at one end 121a thereof. Similarly to the first planar portion 121, the second planar portion 122 is a flat portion spreading in a planar shape, and is continuously connected to the first bent portion 123 at one end portion 122a. Has been. That is, the first planar portion 121 and the second planar portion 122 are continuously connected to each other via the first bent portion 123.

The first bent portion 123 is formed by bending the approximate center of the MCM thin plate into a U shape by about 180 °. The first bent portion 123 is provided over the entire area of the foil material 12 in the direction along the bending center line FL. As the first bent portion 123 is bent, the first planar portion 121 and the second planar portion 122 are opposed to each other, and the first planar portion 121 and the second planar shape are opposed to each other. A first gap 125 is formed between the portion 122. The bending radius r ₁ of the first bent portion 123 is preferably, for example, about half of the thickness of the MCM thin plate (that is, r ₁ = 0.05 mm).

Furthermore, the foil material 12 in the present embodiment has a second bent portion 124. The second bent portion 124 is formed by bending a portion further extending from the other end 122b of the second planar portion 122 into a U shape by 180 ° or more, and the second bent portion An end portion of 124 is in contact with the vicinity of the other end portion 122 b of the second planar portion 122. The second bent portion 124 is interposed between the other end 121 b of the first planar portion 121 and the other end 122 b of the second planar portion 122. The bending radius r ₂ of the second bent portion 124 is not particularly limited, but is preferably equal to or less than the bending radius r _{1 of} the first bent portion 123 in order not to make the first gap 125 too large ( r ₂ ≦ r ₁ ).

  In place of the end 122 b of the second planar portion 122, the second bent portion 124 may be provided at the other end 121 b of the first planar portion 121. Alternatively, the second bent portion 124 may be provided at both the end portion 121 b of the first planar portion 121 and the end portion 122 b of the second planar portion 122.

  The MCM constituting the foil material 12 is not particularly limited as long as it is a magnetic material. For example, the MCM has a Curie temperature (Curie point) in a normal temperature range of about 10 ° C. to 30 ° C., and exhibits a high magnetocaloric effect in the normal temperature range. It is preferable that it is a magnetic body. Specific examples of such MCMs include gadolinium (Gd), gadolinium alloys, lanthanum-iron-silicon (La-Fe-Si) compounds, and the like.

  Moreover, as a concrete manufacturing method of the MCM thin plate which comprises the foil material 12, the following methods can be illustrated. That is, first, bulk gadolinium (Gd) is crushed and then melted to form a plate having a thickness of about 1 mm. Next, the plate material is rolled a plurality of times until the thickness of the plate material becomes about 0.1 mm, and then heated at a temperature of 700 ° C. to 800 ° C. for about 1 hour. Thereby, the MCM thin plate which has the ductility which can be bend | folded can be formed.

  The plurality of foil members 12 described above are accommodated in the container 13 of the first MCM heat exchanger 10 as shown in FIGS. 3 and 4. The container 13 has first and second inner wall surfaces 134 and 135 that support the plurality of foil materials 12, and the plurality of foil materials 12 are supported by the first and second inner wall surfaces 134 and 135. In this state, it is accommodated in the internal space 131 of the container 13. The first and second inner wall surfaces 134 and 135 in the present embodiment correspond to an example of a support member in the present invention.

  Both the first inner wall surface 134 and the second inner wall surface 135 are in the refrigerant flow direction CL in the container 13 (that is, the direction from the first opening 132 of the container 13 toward the second opening 133). The planes are substantially parallel and are provided to face each other.

  A plurality of first grooves 134 a are formed in the first inner wall surface 134. Each of the first grooves 134a is a linear groove having a width of about 0.3 mm, and extends substantially parallel to the flow direction CL. The plurality of first grooves 134a are arranged at substantially equal intervals at intervals of about 0.3 mm.

  A plurality of second grooves 135 a are also formed in the second inner wall surface 135. Each of the second grooves 135a is a linear groove having a width of about 0.3 mm and extends substantially parallel to the flow direction CL. The plurality of second grooves 135a are arranged at substantially equal intervals with an interval of about 0.3 mm, and are provided so as to face the first grooves 134a.

  Each foil material 12 has a container 13 such that the bending center line FL (see FIGS. 5 and 6) of the first bent portion 123 is substantially parallel to the refrigerant flow direction CL in the container 13. Is housed in the internal space 131. The other end portions 121 b and 122 b of the planar portions 121 and 122 of the foil material 12 are inserted into the first grooves 134 a of the first inner wall surface 134, and the first bending of the foil material 12 is performed. The portion 123 is inserted into the second groove 135 a of the second inner wall surface 135. Thereby, each foil material 12 is supported by the first and second inner wall surfaces 134 and 135.

  At this time, in the present embodiment, the foil material 12 is bent in a U shape at the first bent portion 123, and the first planar portion 121 and the second planar portion are formed by the first bent portion 123. 122 are biased in directions away from each other. For this reason, the other end portions 121 b and 122 b of the planar portions 121 and 122 are stably fixed to the first groove 134 a of the first inner wall surface 134.

  Further, as described above, the grooves 134a and 135a of the inner wall surfaces 134 and 135 are formed at substantially equal intervals. For this reason, the second gap 126 is formed between the foil materials 12 adjacent to each other in a state where the plurality of foil materials 12 are supported by the inner wall surfaces 134 and 135.

  Two openings 132 and 133 are formed in the internal space 131 of the container 13. As shown in FIG. 1, the first opening 132 communicates with the low temperature side heat exchanger 50 via the first low temperature side pipe 81. On the other hand, the second opening 133 communicates with the high temperature side heat exchanger 60 via the first high temperature side pipe 83.

  Similarly, the container 23 of the second MCM heat exchanger 20 has an internal space 231 in which the foil material 22 is accommodated and the first and second openings 232 and 233 are formed, as shown in FIG. doing. The first opening 232 communicates with the low-temperature side heat exchanger 50 via the second low-temperature side pipe 82, whereas the second opening 233 and the second high-temperature side pipe 84 via The high temperature side heat exchanger 60 is communicated.

  The foil material 22 of the second MCM heat exchanger 20 has the same configuration as the foil material 12 of the first MCM heat exchanger 10. Further, the container 23 of the second MCM heat exchanger 20 has a plurality of grooves formed at substantially equal intervals, like the first MCM heat exchanger 20, and supports the foil material 22. A pair of inner wall surfaces are provided.

  For example, when the air conditioner using the magnetic heat pump device 1 in the present embodiment functions as cooling, the room is cooled by exchanging heat between the low temperature side heat exchanger 50 and the indoor air, The heat is radiated to the outside by performing heat exchange between the high temperature side heat exchanger 60 and the outside. On the other hand, when the air conditioner functions as heating, the room is warmed by exchanging heat between the high temperature side heat exchanger 60 and the indoor air, and the low temperature side heat exchanger 50 and the outdoor side. Heat is absorbed from outside by exchanging heat with other air.

  As described above, a circulation path including the four heat exchangers 10, 20, 50, 60 is formed by the two low temperature side pipes 81, 82 and the two high temperature side pipes 83, 84. The refrigerant is pumped into the circulation path. Specific examples of the refrigerant include liquids such as water, antifreeze, ethanol solution, or a mixture thereof. The refrigerant in the present embodiment corresponds to an example of a fluid in the present invention.

  The two MCM heat exchangers 10 and 20 are accommodated inside the piston 30. The piston 30 can reciprocate between the pair of permanent magnets 40 by an actuator (not shown). Specifically, the piston 30 can reciprocate between a “first position” as shown in FIG. 1 and a “second position” as shown in FIG. .

  Here, the “first position” refers to a piston in which the first MCM heat exchanger 10 is not interposed between the permanent magnets 40 and the second MCM heat exchanger 20 is interposed between the permanent magnets 40. 30 positions. On the other hand, the “second position” is a piston in which the first MCM heat exchanger 10 is interposed between the permanent magnets 40 and the second MCM heat exchanger 20 is not interposed between the permanent magnets 40. 30 positions.

  The permanent magnet 40 may be reciprocated instead of the first and second MCM heat exchangers 10 and 20. Alternatively, an electromagnet having a coil may be used in place of the permanent magnet 40. In this case, a mechanism for moving the MCM heat exchangers 10, 20 or the magnet becomes unnecessary.

  The switching valve 90 is provided in the first high temperature side pipe 83 and the second high temperature side pipe 84. The switching valve 90 switches the refrigerant supply destination by the pump 70 to the first MCM heat exchanger 10 or the second MCM heat exchanger 20 in conjunction with the operation of the piston 30 described above, The connection destination of the side heat exchanger 60 can be switched to the second MCM heat exchanger 20 or the first MCM heat exchanger 10.

  Next, operation | movement of the magnetic heat pump apparatus 1 in this embodiment is demonstrated, referring FIG.1 and FIG.2.

  First, when the piston 30 is moved to the “first position” shown in FIG. 1, the foil material 12 of the first MCM heat exchanger 10 is demagnetized to lower the temperature, while the second MCM heat exchanger. The 20 foil members 22 are magnetized and the temperature rises.

  At the same time, the switching valve 90 causes the pump 70 → the first high temperature side pipe 83 → the first MCM heat exchanger 10 → the first low temperature side pipe 81 → the low temperature side heat exchanger 50 → the second low temperature side pipe. A first path consisting of 82 → second MCM heat exchanger 20 → second high temperature side pipe 84 → high temperature side heat exchanger 60 → pump 70 is formed.

  For this reason, the refrigerant is cooled by the foil material 12 of the first MCM heat exchanger 10 whose temperature has decreased due to demagnetization, the refrigerant is supplied to the low temperature side heat exchanger 50, and the low temperature side heat exchanger 50 is cooled. Is done. At this time, the refrigerant passes through the first and second gaps 125 and 126 formed by the foil material 12 in the first MCM heat exchanger 10 and comes into contact with the foil material 12, so that the refrigerant is Cooled by the foil material 12.

  On the other hand, the refrigerant is heated by the foil material 22 of the second MCM heat exchanger 20 that has been magnetized and the temperature has risen, and the refrigerant is supplied to the high temperature side heat exchanger 60, and the high temperature side heat exchanger 60 is Heated. At this time, the refrigerant passes through the first and second gaps formed by the foil material 22 in the second MCM heat exchanger 20 and comes into contact with the foil material 22, so that the refrigerant is the foil material 22. Heated by.

  Next, when the piston 30 is moved to the “second position” shown in FIG. 2, the foil material 12 of the first MCM heat exchanger 10 is magnetized to increase the temperature, while the second MCM heat exchange is performed. The foil material 22 of the container 20 is demagnetized and the temperature is lowered.

  At the same time, the switching valve 90 causes the pump 70 → second high temperature side pipe 84 → second MCM heat exchanger 20 → second low temperature side pipe 82 → low temperature side heat exchanger 50 → first low temperature side pipe. A second path consisting of 81 → first MCM heat exchanger 10 → first high temperature side pipe 83 → high temperature side heat exchanger 60 → pump 70 is formed.

  For this reason, the refrigerant is cooled by the foil material 22 of the second MCM heat exchanger 20 whose temperature has decreased due to demagnetization, the refrigerant is supplied to the low temperature side heat exchanger 50, and the low temperature side heat exchanger 50 is cooled. Is done. At this time, the refrigerant passes through the first and second gaps formed by the foil material 22 in the second MCM heat exchanger 20 and comes into contact with the foil material 22, so that the refrigerant is the foil material 22. Cooled by.

  On the other hand, the refrigerant is heated by the foil material 12 of the first MCM heat exchanger 10 that has been magnetized and the temperature has risen, and the refrigerant is supplied to the high temperature side heat exchanger 60, and the high temperature side heat exchanger 60 is Heated. At this time, the refrigerant passes through the first and second gaps 125 and 126 formed by the foil material 12 in the first MCM heat exchanger 10 and comes into contact with the foil material 12, so that the refrigerant is Heated by the foil material 12.

  Then, the reciprocating movement between the “first position” and the “second position” of the piston 30 described above is repeated, and the foil members 21 in the first and second MCM heat exchangers 10 and 20 are repetitively moved. By repeating the application / removal of the magnetic field to / from 22, the cooling of the low temperature side heat exchanger 50 and the heating of the high temperature side heat exchanger 60 are continued.

  As described above, in this embodiment, the foil material 12 is supported by the container 13 by inserting both ends of the folded foil material 12 into the grooves 134 a and 135 a of the inner wall surfaces 134 and 135 of the container 13. For this reason, the total thickness of the foil material 12 is increased (in this embodiment, the total thickness of the foil material 12 is 3.0 mm (= 0.1 mm [thickness of the first planar portion 121] +0.05 mm [first Bend radius of the bent portion 123] × 2 + 0.1 mm [thickness of the second planar portion 122])), and the first and second grooves 134a and 135a can be easily formed. The speed can be increased.

  In the present embodiment, the first bent portion 123 of the foil material 12 is bent into a U shape, and the first bent portion 123 and the second bent portion 122 are formed by the first bent portion 123. Are biased away from each other. For this reason, the other end portions 121b and 122b of the planar portions 121 and 122 can be stably fixed to the first groove 134a of the first inner wall surface 134.

  Moreover, in this embodiment, since the 1st bending part 123 is provided over the whole region of the foil material 12 in the direction along the bending centerline FL, the intensity | strength of the foil material 12 can be improved.

  In the present embodiment, the second bent portion 124 is interposed between the other end 121 b of the first planar portion 121 and the other end 122 b of the second planar portion 122. Further, it is possible to prevent the entrance / exit of the first gap 125 from being blocked during the circulation of the refrigerant.

  The embodiment described above is described for facilitating the understanding of the present invention, and is not described for limiting the present invention. Therefore, each element disclosed in the above embodiment is intended to include all design changes and equivalents belonging to the technical scope of the present invention.

  The configuration of the magnetic heat pump device described above is an example, and for example, the heat exchanger according to the present invention may be applied to other magnetic heat pump devices of an AMR (Active Magnetic Refrigeration) system.

  Moreover, although the above-mentioned embodiment demonstrated the example which applied the magnetic heat pump apparatus to the air conditioning apparatus for household use or a motor vehicle, it is not limited to this in particular. For example, by selecting an MCM having an appropriate Curie temperature according to the application, the magnetic heat pump device according to the present invention can be applied to an application in a cryogenic temperature region such as a refrigerator or an application in a certain high temperature region. May be.

DESCRIPTION OF SYMBOLS 1 ... Magnetic heat pump apparatus 10 ... 1st MCM heat exchanger 12 ... Foil material 121 ... 1st planar part
121a, 121b ... end 122 ... second planar portion
122a, 122b ... end 123 ... first bent portion 124 ... second bent portion 125 ... first gap 126 ... second gap 13 ... container 131 ... inner space 132 ... first opening 133 ... second Opening 134 ... first inner wall surface 134a ... first groove 135 ... second inner wall surface 135a ... second groove 20 ... second MCM heat exchanger 22 ... foil material 23 ... container 231 ... internal space 232 ... first 1 opening 233 ... 2nd opening 30 ... piston 40 ... permanent magnet 50 ... low temperature side heat exchanger 60 ... high temperature side heat exchanger 70 ... pump 81-82 ... 1st-2nd low temperature side piping 83-84 ... 3rd-4th low temperature side piping 90 ... Switching valve

Claims (6)

A heat exchanger comprising a magnetocaloric effect material having a magnetocaloric effect and used in a magnetic heat pump device,
At least one foil material composed of the magnetocaloric effect material;
A support member for supporting the foil material,
The foil material is
A first bent portion bent into a U-shape;
And first and second planar portions connected to each other via the first bent portion,
The support member is
A first groove into which an end of the first planar portion and an end of the second planar portion are inserted;
And a second groove provided to correspond to the first groove and into which the first bent portion is inserted. The heat exchanger according to claim 1,
The first bent portion is a heat exchanger provided over the entire area of the foil material in a direction along the bending center line of the first bent portion. The heat exchanger according to claim 1 or 2,
The foil material has a second bent portion that extends further from at least one of an end portion of the first planar portion and an end portion of the second planar portion and is bent into a U shape. And
The second bent portion is a heat exchanger interposed between an end portion of the first planar portion and an end portion of the second planar portion. It is a heat exchanger as described in any one of Claims 1-3,
The heat exchanger includes a plurality of the foil materials,
The support member is
A plurality of the first grooves into which ends of the first and second planar portions of the foil material are respectively inserted;
A heat exchanger having a plurality of second grooves provided to correspond to the first grooves, respectively, into which the first bent portions of the foil material are respectively inserted. It is a heat exchanger as described in any one of Claims 1-4,
The heat exchanger includes a container having the support member,
The container is
A first opening through which fluid flows into or out of the container;
A second opening through which the fluid flows out of or into the container;
A heat exchanger in which a direction from the first opening toward the second opening and a bending center line of the first bent portion are substantially parallel. The heat exchanger according to any one of claims 1 to 5,
A magnetic field changing means for applying a magnetic field to the magnetocaloric material and changing the magnitude of the magnetic field;
First and second external heat exchangers respectively connected to the vessel via a flow path;
And a fluid supply means for supplying the fluid from the container to the first or second external heat exchanger in conjunction with the operation of the magnetic field changing means.

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