Abstract:
Provided is a cable enabling to reduce leakage flux and to restrict an increase of high-frequency resistance. A magnetic shield is provided to enable to reduce leakage of magnetic flux to an outside, and two first conductive wires and two second conductive wires having different phases from each other are adjacent to each other and arranged annularly to enable to disperse the magnetic flux, to restrict a proximity effect, and to restrict an increase of high-frequency resistance.

Description:
TECHNICAL FIELD 
     The present invention relates to a cable for high-frequency alternating-current power transmission. 
     BACKGROUND ART 
     Conventionally, as a cable for high-frequency alternating-current power transmission, one including a magnetic shield is proposed (e.g., refer to Patent Literature 1). In the cable described in Patent Literature 1, an outside of a pair of electric wires having different phases from each other is covered with a magnetic shield to reduce leakage flux, which is particularly problematic at the time of transmission of high-frequency alternating-current power. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 10-116519 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the cable described in Patent Literature 1, providing a magnetic shield  104  causes magnetic flux B to concentrate between an electric wire  102  and an electric wire  103  as illustrated in  FIG. 4A , which causes a problem in which a cross-sectional area of a region V easily carrying current is smaller than in a configuration of providing no magnetic shield illustrated in  FIG. 4B  due to a proximity effect, and in which high-frequency resistance increases. 
     An object of the present invention is to provide a cable enabling to reduce leakage flux and to restrict an increase of high-frequency resistance. 
     Solution to Problem 
     A cable according to the present invention is a cable provided with a magnetic shield and having a pair of electric wires transmitting alternating-current power and includes a first electric wire as a first side of the pair of electric wires having a plurality of first conductive wires and a second electric wire as a second side of the pair of electric wires having a plurality of second conductive wires. The first conductive wires and the second conductive wires are arranged alternately and disposed annularly in a circumferential direction of the cable. 
     According to the present invention described above, by providing the magnetic shield, leakage flux is reduced. Also, by alternately arranging the first conductive wires and the second conductive wires having different phases from each other, each of the conductive wires is adjacent to two conductive wires having the other phase. Accordingly, concentration of magnetic flux is prevented further, and a proximity effect is restricted further than in a configuration in which each of the conductive wires is adjacent to one conductive wire having the other phase. 
     At this time, in the cable according to the present invention, each of the number of the first conductive wires and the number of the second conductive wires is preferably two. 
     According to this configuration, since a cross-sectional area of a space surrounded by the conductive wires disposed annularly is minimum, an occupancy ratio of the electric wires in a cross-sectional area of the entire cable is higher, and an outside diameter of the cable can be shortened while securing the cross-sectional area of the electric wires. Also, a configuration in which each of the conductive wires is adjacent to two conductive wires having the other phase can be simplified most. 
     Also, in the cable according to the present invention, an end portion of the first electric wire is preferably provided with a first terminal having a first terminal surface parallel to an opposing direction of the first conductive wires and an axial direction of the cable, an end portion of the second electric wire is preferably provided with a second terminal having a second terminal surface parallel to the first terminal surface, the two first conductive wires preferably extend in the axial direction and are collectively connected to the first terminal, and the two second conductive wires are preferably bent in a plane parallel to the second terminal surface, extend in the axial direction to avoid interference with the first terminal, and are collectively connected to the second terminal. 
     According to this configuration, since the first terminal surface and the second terminal surface are parallel to each other, a connecting structure to an outside can be simplified. Also, since the first conductive wires extend linearly in the axial direction and are connected to the first terminal, the two first conductive wires can have equal length dimensions. Also, since the second conductive wires are bent in the plane parallel to the second terminal surface and extend in the axial direction, the two second conductive wires can have equal length dimensions. Circulating current is prevented from being generated respectively in the first electric wire and the second electric wire. At this time, a difference of the length dimensions between each first conductive wire and each second conductive wire having different phases from each other does not contribute to circulating current. 
     Furthermore, in the cable according to the present invention, a circumference of each of the first conductive wires and the second conductive wires is preferably provided with an insulating cover. 
     According to this configuration, since the insulating cover secures a distance between each first conductive wire and each second conductive wire, insulation between the conductive wires can be secured, and concentration of magnetic flux can further be restricted. 
     Advantageous Effects of Invention 
     With the above cable according to the present invention, a magnetic shield is provided to enable to reduce leakage flux, and magnetic flux is dispersed, and a proximity effect is restricted to enable to prevent an increase of high-frequency resistance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view illustrating a cable according to an embodiment of the present invention, and  FIG. 1B  is a cross-sectional view illustrating magnetic flux thereof. 
         FIG. 2A  is a side view illustrating a terminal structure of the cable, and  FIG. 2B  is a cross-sectional view. 
         FIG. 3  is a graph illustrating frequency dependence of high-frequency resistance of the cable and conventional cables. 
         FIGS. 4A and 4B  are cross-sectional views illustrating the conventional cables. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinbelow, embodiments of the present invention will be described based on the drawings. 
       FIGS. 1A and 1B  are cross-sectional views illustrating a cable  1  according to an embodiment of the present invention.  FIGS. 2A and 2B  illustrate a terminal structure of the cable  1 .  FIG. 2A  is a side view while  FIG. 2B  is a II-II cross-sectional view.  FIG. 3  is a graph illustrating frequency dependence of high-frequency resistance of the cable  1  and conventional cables.  FIGS. 4A and 4B  are cross-sectional views illustrating the conventional cables. Although current and a direction of magnetic flux at a certain instant are illustrated in each of  FIGS. 1A, 1B, 4A, and 4B , the direction and magnitude shall change every second since current is alternating in the present embodiment. 
     In  FIGS. 1A to 2B , the cable  1  is a cable connecting an instrument such as a power supply device and a matching unit to a coil for wireless power feeding to transmit high-frequency alternating-current power and is configured to include a first electric wire  2  as a electric wire connecting one side of the instrument to one side of the coil, a second electric wire  3  as a electric wire connecting the other side of the instrument to the other side of the coil, an insulating cover  4  covering a first conductive wire  21  and a second conductive wire  31  respectively constituting the first electric wire  2  and the second electric wire  3 , an inner sheath  5  bundling the first electric wire  2  and the second electric wire  3 , a magnetic shield  6  provided outside the inner sheath  5 , and an outer sheath  7  provided outside the magnetic shield  6 . 
     Here, in the present embodiment, a right-left direction in  FIGS. 1A and 1B  is referred to as an X direction, an up-down direction in  FIGS. 1A and 1B  is referred to as a Y direction, and a right-left direction in  FIG. 2A  (an axial direction of the cable) is referred to as a Z direction. Also, a left side in  FIG. 2A  (a side connected to the instrument) is referred to as a Z-direction instrument side while a right side (a side connected to the coil) is referred to as a Z-direction coil side. 
     The first electric wire  2  is configured to include the two first conductive wires  21  and a first terminal  22  to which the two first conductive wires  21  are connected at an end portion on the Z-direction instrument side. Each of the first conductive wires  21  is a litz wire for reduction of high-frequency resistance, for example. 
     The second electric wire  3  is configured to include the two second conductive wires  31  and a second terminal  32  to which the second conductive wires  31  are connected at an end portion on the Z-direction instrument side. Each of the second conductive wires  31  is a litz wire for reduction of high-frequency resistance, for example. 
     As for the insulating cover  4 , a thickness in a radial direction is set to enable to withstand voltage between wires, and the covers covering the adjacent conductive wires abut on each other. 
     The inner sheath  5  is made of resin, forms the cable to have a circular cross-section to keep positional relationship among the first conductive wires  21  and the second conductive wires  31  covered with the insulating covers  4 , and is provided to secure a predetermined separation dimension between each of the conductive wires  21  and  31  and the magnetic shield  6 . 
     The magnetic shield  6  is made of a material with high magnetic permeability and covers a circumference of the inner sheath  5 . When current flows in the first conductive wires  21  and the second conductive wires  31  to cause a magnetic field to be generated, magnetic flux B preferentially passes through the magnetic shield  6  to prevent the magnetic flux B from leaking outside. 
     The outer sheath  7  is made of resin and covers an outside of the magnetic shield  6  to protect the cable  1  from external mechanical impact and the like. 
     Next, the positional relationship among the first conductive wires  21  and the second conductive wires  31  and flow of the magnetic flux B will be described. 
     As illustrated in  FIGS. 1A and 1B , the two first conductive wires  21  are arranged in the Y direction while the two second conductive wires  31  are arranged in the X direction. That is, the first conductive wires  21  and the second conductive wires  31  are arranged alternately and disposed annularly. In such arrangement, at a moment at which positive current flows toward a front side of the drawing sheet in the Z direction in the first conductive wires  21  and at which positive current flows toward a rear side of the drawing sheet in the Z direction in the second conductive wires  31 , the magnetic flux B flowing inside the cable  1  is as illustrated in  FIG. 1B . That is, the magnetic flux B passes inside the magnetic shield  6  on an outer side in a radial direction of each of the conductive wires  21  and  31  and concentrates between the adjacent conductive wires. 
     Next, configurations of the first electric wire  2  and the second electric wire  3  at the end portions on the Z-direction instrument side will be described. 
     As illustrated in  FIGS. 2A and 2B , the first conductive wires  21  extend toward the Z-direction instrument side, get closer to each other, and are connected to a swage portion  22   a  of the first terminal  22 . At this time, the first conductive wires  21  have a mutual opposing direction thereof directed in the Y direction and have approximately equal length dimensions. On the other hand, the second conductive wires  31  are bent in a YZ plane (bent to an upper side in the Y direction in FIG.  2 A), extend toward the Z-direction instrument side, get closer to each other, and are connected to a swage portion  32   a  of the second terminal  32 . At this time, the second conductive wires  31  have a mutual opposing direction thereof directed in the X direction and have approximately equal length dimensions. Furthermore, a first terminal surface  22   b  and a second terminal surface  32   b  as connection parts to outsides in the first terminal  22  and the second terminal  32  are arranged in an approximately equal plane approximately parallel to the YZ plane and at approximately equal positions in the Z direction. 
     The present embodiment exerts the following effects. 
     That is, since the magnetic flux B flowing inside the cable  1  when current flows in the first electric wire  2  and the second electric wire  3  concentrates on both the adjacent sides of each of the conductive wires  21  and  31 , a cross-sectional area of a region V easily carrying current is larger, and high-frequency resistance decreases further than in a comparative example illustrated in  FIG. 4A , in which the single first electric wire  102  and the single second electric wire  103  are provided. At this time, frequency dependence characteristics of high-frequency resistance of a cable  201  according to a conventional example provided with no magnetic shield  6  illustrated in  FIG. 4B , a cable  101  according to the comparative example, and the cable  1  according to the present embodiment are ones represented by a dashed-dotted line, a dashed line, and a solid line in  FIG. 3 , respectively. The high-frequency resistance in the present embodiment is higher than that in the conventional example but is lower than that in the comparative example at frequency of approximately 30 kHz or higher. In particular, at 20 to 200 kHz, which is used for wireless power feeding, the high-frequency resistance in the present embodiment is much better than that in the comparative example. Also, the magnetic flux B preferentially passes through the magnetic shield  6  to prevent the magnetic flux B from leaking outside. 
     Also, respectively providing the two first conductive wires  21  and the two second conductive wires  31  enables a cross-sectional area of a space surrounded by the conductive wires disposed annularly to be minimum. Also, a configuration in which each of the conductive wires  21  and  31  is adjacent to two conductive wires having the other phase can be simplified most. 
     Furthermore, arranging the first terminal surface  22   b  and the second terminal surface  32   b  in the approximately equal plane approximately parallel to the YZ plane and at the approximately equal positions in the Z direction simplifies a connecting structure to the instrument. At this time, since the two first conductive wires  21  as well as the two second conductive wires  31  have the approximately equal length dimensions, circulating current is prevented from flowing respectively in the first electric wire  2  and the second electric wire  3 . 
     It is to be noted that the present invention is not limited to the above embodiment, includes other configurations and the like that can achieve the object of the present invention, and includes the following modifications. 
     For example, although the first electric wire  2  and the second electric wire  3  respectively have the two first conductive wires  21  and the two second conductive wires  31  in the above embodiment, the first electric wire  2  and the second electric wire  3  may respectively have the three or more ones so that the numbers thereof may be equal. In the case in which the numbers increase, the cross-sectional area of the aforementioned space is larger, and this space can be provided with a coaxial cable for signal transmission and reception, for example. 
     Also, although the first terminal  22  and the second terminal  32  are provided on the Z-direction instrument side in the above embodiment, these components can be omitted. The conductive wires may be connected to an instrument provided with as many connection parts as the number of the conductive wires, and the conductive wires having the same phases may be electrically connected inside the instrument. Alternatively, the first terminal  22  and the second terminal  32  may be provided at both end portions in the Z direction. According to this configuration, not only the coil and the instrument but also two instruments can be connected to each other, and high-frequency alternating-current power can be transmitted. 
     Also, although the circumference of each of the conductive wires  21  and  31  is covered with the insulating cover  4  in the above embodiment, an inside of the inner sheath  5  may entirely be filled with an insulator, for example. Any configuration in which the conductive wires are kept insulated is available. 
     Also, although the resin-made outer sheath  7  is provided in the above embodiment, the outer sheath  7  may be made of metal. According to this configuration, the cable can be protected reliably. Moreover, in a case in which the outer sheath  7  is made of metal with high magnetic permeability, the outer sheath  7  can function as a magnetic shield, and the magnetic shield  6  can be omitted for cost reduction. 
     Although the best configuration, method, and the like for carrying out the present invention are disclosed in the above description, the present invention is not limited thereto. That is, although the present invention is mainly illustrated and described based on a specific embodiment, those skilled in the art can modify the aforementioned embodiment in various forms in terms of the shapes, materials, quantities, and other detailed configurations without departing from the technical spirit and objective scope of the present invention. 
     Accordingly, since the above description disclosed by limiting the shapes, materials, and the like is illustrative only to facilitate understanding of the present invention and does not limit the present invention, description using names of members from which part of limitations of these shapes, materials, and the like or all of the limitations have been removed shall be included in the present invention. 
     REFERENCE SIGNS LIST 
     
         
           1  Cable 
           2  First electric wire 
           3  Second electric wire 
           4  Insulating cover 
           6  Magnetic shield 
           21  First conductive wire 
           22  First terminal 
           22   b  First terminal surface 
           31  Second conductive wire 
           32  Second terminal 
           32   b  Second terminal surface