JP2014043115A - Coil unit used in non contact power supply system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coil unit which reduces the possibility of cracks and chips of a magnetic core and is used in a non-contact power supply system.SOLUTION: A coil unit is used in a non contact power supply system which supplies electric power from a power supply device 100 to a power receiving device 200 in a non contact manner and comprises: a power transmission coil 112 which generates magnetic flux; and a magnetic core part 108 which collects the magnetic flux generated by the power transmission coil 112. The magnetic core part 108 comprises: a first magnetic core member 108c which is disposed in a direction along the magnetic flux generated by the coil; and a second magnetic core member 108d which is disposed adjacent to the first magnetic core member 108c and has a length shorter than the first magnetic core member 108c.

Description

  The present invention relates to a coil unit used in a non-contact power feeding system that transmits power in a non-contact manner from a built-in primary coil to a secondary coil mounted in an electric propulsion vehicle such as an electric vehicle or a hybrid vehicle. .

  FIG. 8 is a schematic diagram showing a configuration of a conventional non-contact power feeding device 6. In FIG. 8, the non-contact power feeding device (primary side) F connected to the power panel of the power source 9 on the external ground side supplies power to the power receiving device (secondary side) G mounted on the electric propulsion vehicle. , They are arranged to face each other through an air gap that is a void space without physical connection. In this arrangement state, when a magnetic flux is formed by the primary coil 7 provided in the power feeding device F, an induced electromotive force is generated in the secondary coil 8 provided in the power receiving device G. Electric power is transmitted to the coil 8 in a non-contact manner.

  The power receiving device G is connected to, for example, the in-vehicle battery 10, and the in-vehicle battery 10 is charged with the electric power transmitted as described above. The on-vehicle motor 11 is driven by the electric power stored in the battery 10. Note that, during the non-contact power supply process, for example, the wireless communication device 12 exchanges necessary information between the power supply device F and the power reception device G.

  FIG. 9 is a schematic diagram illustrating the internal structure of the power feeding device F and the power receiving device G. In particular, FIG. 9A is a schematic diagram illustrating an internal structure when the power feeding device F is viewed from above and the power receiving device G is viewed from below. FIG. 9B is a schematic diagram illustrating an internal structure when the power feeding device F and the power receiving device G are viewed from the side.

  In FIG. 9, the power feeding device F is an example of a coil unit and includes a primary coil 7, a primary magnetic core 13 having a flat plate shape, a back plate 15, a cover 16, and the like. The power receiving device G is another example of a coil unit. Briefly speaking, the power receiving device G has a symmetrical structure with the power feeding device F, and has a flat secondary coil 8, a secondary magnetic core 14, and a back plate. 15, a cover 16 and the like, and the surfaces of the primary coil 7 and the primary magnetic core 13 and the surfaces of the secondary coil 8 and the secondary magnetic core 14 are respectively molded resin 17 mixed with a foam material 18. Covered and fixed.

  That is, both the power feeding device F and the power receiving device G are filled with the mold resin 17 between the back plate 15 and the cover 16, and the primary coil 7, the secondary coil 8, the primary magnetic core 13, and the secondary magnetic core core inside. 14 surfaces are coated and fixed. The mold resin 17 is made of, for example, silicon resin. By hardening the interior in this way, the primary and secondary coils 7 and 8 are positioned and fixed, and the mechanical strength is ensured and the heat dissipation function is also exhibited. That is, the primary and secondary coils 7 and 8 generate heat due to Joule heat through an exciting current, but are radiated and cooled by heat conduction of the mold resin 17 (see, for example, Patent Document 1).

  The following Patent Documents 2 and 3 also disclose a flat primary magnetic core and a secondary magnetic core.

JP 2008-87733 A JP 2010-93180 A JP 2010-119187 A

  However, if the primary magnetic core or the secondary magnetic core is flat, each magnetic core may be broken or chipped due to vibration, external impact or load.

  In particular, considering the case where the power receiving device G is mounted on an electric propulsion vehicle, the secondary magnetic core may protrude from the power receiving device G when a large external impact is applied to the vehicle. This is especially true when a crushable body is used.

  SUMMARY OF THE INVENTION The main object of the present invention is to provide a coil unit that can be used in a non-contact power feeding system that can reduce the risk of cracking and chipping of a magnetic core. It is.

  In order to achieve the above object, the present invention provides a coil unit used in a non-contact power feeding system that supplies power from a power feeding device to a power receiving device in a non-contact manner, the coil generating a magnetic flux, and generated by the coil. A magnetic core that collects the magnetic flux, and the magnetic core is disposed adjacent to the first magnetic core member and the first magnetic core member that is disposed in a direction along the magnetic flux generated by the coil. And a second magnetic core member whose length is shorter than that of the first magnetic core member.

  According to the present invention, the magnetic core unit is provided with two types of magnetic core members, ie, a first magnetic core member and a second magnetic core member. Therefore, when viewed as a single member, it is possible to make it smaller compared to a flat magnetic core. As a result, the entire magnetic core portion is more susceptible to vibrations, external impacts and weights than conventional configurations. It becomes possible to raise tolerance with respect to. As a result, it is possible to reduce the breakage or chipping of the magnetic core member.

The schematic diagram which shows the example of installation of the non-contact electric power feeding system provided with the non-contact electric power feeder which concerns on this invention 1 is an external perspective view of the non-contact power feeding device of FIG. The perspective view which shows the internal structure of the non-contact electric power feeder of FIG. Longitudinal sectional view of the non-contact power feeding device along line IV-IV in FIG. FIG. 2 is an external perspective view in which a part of the non-contact power feeding device of FIG. Schematic diagram showing a top view of the magnetic core 108 shown in FIG. Schematic diagram showing the magnetic core members 108a and 108b shown in FIG. Schematic diagram showing the configuration of a conventional non-contact power feeding device Schematic diagram showing the internal structure of the non-contact power feeding device of FIG. 6 and the power receiving device arranged to face this power feeding device

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

FIG. 1 is a schematic diagram illustrating an installation example of a non-contact power supply system S including a non-contact power supply apparatus 100 according to the present invention. In FIG. 1, the non-contact power feeding system S includes a non-contact power feeding device (hereinafter simply referred to as “power feeding device”) 100 arranged at a predetermined place on the ground and a power receiving device 200 installed on the moving body side. ing. This non-contact power feeding system S is typically used for charging the electric propulsion vehicle 400, for example. In this case, the power receiving device 200 is installed in an electric propulsion vehicle 400 as a moving body, and the power feeding device 100 is typically installed on the ground.
However, the present invention is not limited to this, and the power supply apparatus 100 may be buried in a parking space, for example, or configured to be movable.

2 is an external perspective view of the power feeding apparatus 100 shown in FIG. 1, and FIG. 3 shows the internal structure of the power feeding apparatus 100 shown in FIG. 2, particularly when the cover 114 shown in FIG. 2 is removed. It is a perspective view. 4 is a longitudinal sectional view of the power feeding apparatus 100 taken along line IV-IV in FIG.

  As shown in FIGS. 2 to 4, the power feeding device 100 is an example of a coil unit, and is a heat conducting member 104, a coil base 106, a magnetic core 108, and a mica plate 110 that are sequentially placed and fixed on the bottom plate 102. , A power transmission coil 112, and an electronic component group 116 mounted and fixed on the bottom plate 102 at a position spaced apart from these components, a heat conducting member 104, a coil base 106, a magnetic core 108, a mica plate 110, a power transmission The coil 112 and the electronic component group 116 are covered with a cover 114.

  The bottom plate 102 has a substantially rectangular shape, and its lower surface serves as an installation surface.

  A partition wall 118 is formed on the cover 114, and the partition wall 118 has an annular (tubular) shape with an outer diameter of φ and a height of h. The partition wall 118 is integrally formed with the cover 114 so as to protrude downward from the cover 114, and is dimensioned so that the lower end of the partition wall 118 can reach the bottom plate 102, and may be applied to the cover 114 from above. The strength against the load (for example, human weight) is secured. The heat conducting member 104, the coil base 106, the magnetic core 108, the mica plate 110, the power transmission coil 112, and the electronic component group 116 are disposed outside the partition wall 118.

  The heat conducting member 104 has a substantially annular (cylindrical) shape with an inner diameter slightly larger than the outer diameter φ of the partition wall 118 and a predetermined height h1. If a conductive material is used as the material of the heat conducting member 104, it is magnetically coupled by the magnetic field formed by the power transmission coil 112, and an induced current flows to cause unnecessary heat generation, leading to problems such as a decrease in power supply efficiency. A material other than a metal or a conductive material is used as the material of the heat conducting member 104. For example, a resin containing a ceramic or a filler that improves the heat conductivity is used.

  Further, the thermal conductivity of the heat conducting member 104 is preferably higher than the thermal conductivity of air in a predetermined temperature range (for example, 150 ° C. or lower), and is set to 1 W / mK or higher, for example. However, the heat conductivity of the heat conducting member 104 is preferably as high as possible.

  The heat conduction member 104 is fixed to the bottom plate 102 with, for example, bolts or the like in a state where the partition wall 118 is inserted into a through hole formed in the central portion in the radial direction. A coil base 106 is placed, and a magnetic core 108 for concentrating the magnetic flux generated by the power transmission coil 112 is placed on the annular coil base 106.

  The coil base 106 and the magnetic core 108 substantially have an inner diameter slightly larger than the outer diameter φ of the partition wall 118, have an annular shape with a thickness t1, and are formed at the center in the radial direction. The partition wall 118 is inserted through the through hole and is disposed on the heat conducting member 104.

  In addition, an annular mica plate 110 as an electrical insulating plate is placed on the magnetic core 108, and the mica plate 110 has an inner diameter substantially larger than the outer diameter φ of the partition wall 118, A partition wall 118 is inserted through a through hole formed in the radial center.

  The power transmission coil 112 is wound so that its inner diameter is substantially larger than the outer diameter φ of the partition wall 118, and has an annular shape with a thickness t2 (including the thickness of the mica plate 110). . A partition wall 118 is inserted through a hole in the central portion in the radial direction of the power transmission coil 112. In addition, although the power transmission coil 112 is comprised with the copper wire etc., in order to simplify illustration, it is drawn in disk shape, for example in FIG.

  The electronic component group 116 is necessary for the operation of the power feeding apparatus 100 (operation for performing non-contact power feeding to the power receiving apparatus 200), such as a capacitor.

  2 and 3, the bottom plate 102 is designed such that its long side is along the traveling direction of the electric propulsion vehicle 400 (indicated by an arrow A). Note that the direction indicated by the arrow A is also the front-rear direction of the power supply apparatus 100.

  Further, the heat conducting member 104, the magnetic core 108, the power transmission coil 112, and the like are arranged on the front side or the rear side of the power feeding device 100 or the bottom plate 102 when viewed from the electronic component group 116. As a result, a space is created behind or in front of the power feeding device 100 or the bottom plate 102, and the electronic component group 116 is disposed in this space while being isolated from the power transmission coil 112 and the like.

  As shown in FIG. 4, the heat conducting member 104, the coil base 106, the magnetic core 108, the mica plate 110, and the power transmission coil 112, which are sequentially laminated on the bottom plate 102, are held by a resin coil holder 120. Projecting portions (not shown) are provided at a plurality of locations on the outer peripheral surface of the coil holder 120, and are attached to the bottom plate 102 with bolts or the like through the projecting portions.

  The heat conductive member 104, the coil base 106, the magnetic core 108, and the mica plate 110 may be fixed to each other with an adhesive or the like.

  Furthermore, the heat conducting member 104, the coil base 106, the magnetic core 108, the mica plate 110, and the power transmission coil 112 are surrounded by a cylindrical shield member 122 provided on the radially outer side. The shield member 122 surrounds the power transmission coil 112 with the bottom plate 102 and the shield member 122.

  The cover 114 is made of resin concrete or FRP (Fiber Reinforced Plastics), and includes a first upper wall 114a, a second upper wall 114b, a first upper wall 114a and / or a second upper wall 114b. And a plurality of side surfaces 114c extending obliquely toward the bottom, and are attached to the bottom plate 102 by, for example, bolts or the like. Therefore, the heat conducting member 104, the coil base 106, the magnetic core 108, the mica plate 110, and the power transmission coil are constituted by the first upper wall 114a, the second upper wall 114b, the plurality of side surfaces 114c, and the bottom plate 102 of the cover 114. 112, a space capable of accommodating the electronic component group 116 is formed.

  As shown in FIGS. 4 and 5, the cover 114 is formed with a partition wall 124 extending from a connection portion between the first upper wall 114 a and the second upper wall 114 b toward the bottom plate 102. Has a plate-like shape having a predetermined thickness t3 and a height (maximum value) of h2, and has a heat insulating property. In addition, the partition wall 124 is provided between the power transmission coil 112 and the electronic component group 116 so as to cross the bottom plate 102 to the left and right (in the direction of the arrow A). The cover 114 is integrally formed.

In particular, as shown in FIGS. 4 and 5, the cover 114 is designed so that the first upper wall 114 a covers the power transmission coil 112 and the second upper wall 114 b covers the electronic component group 116. The partition wall 124 extending downward from the connection portion between the first upper wall 114 a and the second upper wall 114 b is dimensioned so that the lower end thereof reaches the bottom plate 102. As a result, the space in the cover 114 (the space surrounded by the bottom plate 102 and the cover 114) is a first housing space S1 that houses the power transmission coil 112 and the like, and a second housing space that houses the electronic component group 116. Partitioned by a partition wall 124 into S2. As a result, Joule heat generated from the power transmission coil 112 is blocked by the partition wall 124 having heat insulation performance, and is prevented from being transmitted to the electronic component group 116.

  3 and 5, one end of the power transmission coil 112 is connected to the lead wire 130, the other end of the power transmission coil 112 is connected to another lead wire 132, and the lead wire 130 is connected to the cover 114. The lead wire 132 is connected to the electronic component group 116 and penetrated through one of the side walls 114c and led out to the outside. .

  Therefore, the lead wires 130 and 132 need to penetrate the partition wall 124. However, when the partition wall 124 is integrally formed with the cover 114, wiring of the lead wires 130 and 132 in the cover 114 is extremely difficult.

  Therefore, in the present embodiment, a substantially rectangular notch is provided in a part of the partition wall 124, and a heat insulating member 126 having approximately the same shape as the notch and having two through holes 126 a is fitted into the notch of the partition wall 124. The heat insulating member 126 is attached to the bottom plate 102 after the lead wires 130 and 132 are inserted into the two through holes 126a. In addition, bushes 134 are respectively attached to the two through holes 126a to improve the heat insulating performance of the heat insulating member 126.

  This configuration is advantageous in terms of workability because all the components except the cover 114 can be attached to the bottom plate 102 and the lead wires 130 and 132 are wired, and then the cover 114 can be attached to the bottom plate 102.

  Note that it is preferable to apply an adhesive or the like around the heat insulating member 126 so as to be in close contact with the partition wall 124.

  In addition, the partition wall 124 may have a double structure with an air layer interposed therebetween, and the double structure partition wall is further excellent in terms of heat insulation performance.

  Now, referring further to FIG. 4, the distance from the upper surface of the bottom plate 102 to the upper end of the power transmission coil 112 is D1 (≈h1 + t1 + t2), and the distance from the upper surface of the bottom plate 102 to the lower surface of the first upper wall 114a of the cover 114 Is D2, the distance D2 is set larger than the distance D1 (D2> D1). That is, a gap having a predetermined height (D2-D1) is formed between the power transmission coil 112 and the cover 114. The height of the gap is preferably set equal, and this gap is an air layer.

  Further, since the total height of the heat conducting member 104, the coil base 106, the magnetic core 108, the mica plate 110, and the power transmission coil 112 is different from the height of the electronic component group 116, the first upper wall 114a from the bottom plate 102 is used. And the distance from the bottom plate 102 to the second upper wall 114b are set to be different from each other. In the example of FIG. 4, the height of the first upper wall 114a is set higher than the height of the second upper wall 114b.

The contactless power supply device 100 having the above configuration may be buried in a parking space shallowly. In addition, the non-contact power feeding apparatus 100 itself may be configured to be movable. In any case, the power transmission coil 112 and the electronic component group 116 are arranged in line along the traveling direction of the electric propulsion vehicle 400. At this time, the first upper wall 114a of the cover 114
Since the height of the second upper wall 114b is different from the height of the second upper wall 114b, it is possible to instantaneously identify the front and rear (direction) of the power supply apparatus 100, and the power supply apparatus 100 can be easily attached. Can do.

  After installation, the power receiving device 200 (see FIG. 1) mounted on the electric propulsion vehicle 400 and the power feeding device 100 are positioned so as to face each other through the air gap. In such a positioned state, power is transmitted from the power feeding device 100 to the power receiving device 200 in a contactless manner.

  Note that the power supply apparatus 100 is installed so that the bottom plate 102 faces downward, whether it is embedded or movable. Therefore, in the case of the movable power supply apparatus 100, there is a possibility that a person may touch the cover 114. Further, even in the case of embedding, it is not possible to embed it so deeply, and in some cases, it is assumed that the cover 114 is exposed.

  Therefore, in the present embodiment, an air layer that exhibits a heat insulating effect is provided between the power transmission coil 112 and the cover 114, and the cover 114 that can be touched by the person, particularly the top plate portion ( The Joule heat from the power transmission coil 112 is transmitted to the first upper wall 114a as much as possible. That is, it is possible to provide the power supply apparatus 100 that can reduce overheating of the cover 114.

  A heat insulating member having a lower thermal conductivity than air can be interposed in the air layer between the power transmission coil 112 and the cover 114. Instead of the air layer, a heat insulating member having a lower thermal conductivity than air is used. By providing, transmission of Joule heat from the power transmission coil 112 to the cover 114 can be further reduced.

  Further, as described above, the heat conducting member 104 has a thermal conductivity larger than that of air. If there is an air layer in the part of the heat conducting member 104, it will move upward by convection when the air is heated, so there will be a high temperature part above the air layer, and heat transfer cannot be expected downward, By disposing a solid having a higher thermal conductivity than air, heat transfer is merely performed by heat conduction and there is no direction of heat transfer, so heat can be efficiently guided downward. Thus, Joule heat generated from the power transmission coil 112 is transmitted to the bottom plate 102 via the heat conducting member 104 and is dissipated. More specifically, for example, when the power feeding device 100 is installed on an installation surface such as the ground, heat is transmitted from the bottom plate 102 to the installation surface and dissipated. When an air layer (gap) is interposed between the bottom plate 102 and the installation surface, heat is dissipated from the bottom plate 102 to the air layer. In other words, it is possible to further reduce the transmission of Joule heat toward the top plate (first upper wall) 114a.

  Further, since the distance between the upper end of the power transmission coil 112 and the lower surface of the first upper wall 114a is set to be equal, Joule heat can be evenly transmitted to the first upper wall 114a. It is possible to prevent a local temperature rise of the upper wall 114a.

  Further, as described above, the Joule heat generated from the power transmission coil 112 can be prevented from being transmitted to the electronic component group 116 by being blocked by the partition wall 124 having heat insulating performance. Thus, according to this Embodiment, the non-contact electric power feeder which can reduce the overheating of an electronic component can be provided.

  In FIG. 1 to FIG. 5, the magnetic core 108 is illustrated on a flat plate for the sake of simplicity, but actually, the magnetic core 108 is configured as illustrated in FIG. 6. 6 is a schematic diagram showing a top view of the magnetic core core portion 108 shown in FIG.

  In FIG. 6, the magnetic core 108 is a hatched portion, and is composed of a plurality of magnetic core members 108a (shown by right-down hatching) and 108b (shown by left-down hatching). For convenience, in FIG. 6, reference numeral 108a is assigned to one magnetic core member, and reference numeral 108b is assigned to two magnetic core members.

  Each of the magnetic core members 108a and 108b is made of a high magnetic permeability material typified by ferrite. A plurality of the magnetic core members 108a are arranged along the magnetic flux generated from the power transmission coil 112 on the inner peripheral side of the power transmission coil 112 (that is, near the center C of the power transmission coil 112). More specifically, when the power transmission coil 112 has a substantially concentric shape, the magnetic core member 108 a is radially arranged on the coil base 106 with the center C of the power transmission coil 112 as a reference.

  A plurality of magnetic core members 108b are arranged on the outer peripheral side of the power transmission coil 112 (that is, not near the center C of the power transmission coil 112) along the magnetic flux generated by the power transmission coil 112 in the same manner as the magnetic core member 108a. Yes. More specifically, some of the plurality of magnetic core members 108b are continuously arranged with the magnetic core member 108a in the direction along the magnetic flux generated by the power transmission coil 112. That is, when the combination of the magnetic core members 108a and 108b is the first magnetic core member 108c and the power transmission coil 112 has a substantially concentric shape, they are arranged radially with respect to the center C of the power transmission coil 112. .

  In the present embodiment, the magnetic core members 108a and 108b constituting the first magnetic core member 108c are rectangular parallelepipeds having substantially the same cross-sectional shape perpendicular to the magnetic flux from the power transmission coil 112, as shown in FIG. Has a shape.

  In such a configuration, the magnetic flux generated in the power transmission coil 112 enters and exits from the inner peripheral side end surface of the magnetic core member 108b and the outer peripheral side end surface of the magnetic core member 108a, and passes through the magnetic core members 108a and 108b. At this time, since the magnetic flux is concentrated on the inner peripheral side of the power transmission coil 112, the magnetic flux is also collected from the side surface to the magnetic core materials 108a and 108b having a low magnetic resistance. In the innermost circumference of the core core material 108a where the magnetic flux is most concentrated, a cross-sectional area that is not magnetically saturated is secured. If the innermost cross-sectional area of the magnetic core material 108a, 108b is minimized and the cross-sectional area is equal to or greater in the other portions, the magnetic flux density is reduced and magnetic saturation does not occur.

  When the magnetic core members 108a and 108b are formed of substantially the same rectangular parallelepiped as described above and a plurality of such combinations 108c are arranged radially, two magnetic core members adjacent in the arc direction are used. Since a relatively large gap is generated between the two layers 108b, the magnetic flux passing through the gap increases and spreads three-dimensionally. The member 108d is disposed on the coil base 106 (see FIG. 6). In other words, the second magnetic core member 108d having a shorter length than the first magnetic core member 108c is disposed next to the first magnetic core member 108c. The arranged second magnetic core member 108d can collect the magnetic flux, and can prevent problems such as an increase in loss in the metal part near by the spread of the magnetic flux and a decrease in the self-inductance of the coil.

As described above, in this embodiment, the magnetic core 108 is disposed under the power transmission coil 112. The magnetic core 108 is not a single flat plate, but includes a relatively small first magnetic core 108c and second magnetic core 108d. Accordingly, when viewed from the first magnetic core member 108c and the second magnetic core member 108d alone, they are resistant to vibrations, external impacts, and load as compared with the flat magnetic cores. It becomes possible to reduce that 108a, 108b is cracked or chipped. In particular, since the magnetic core members 108a and 108b are arranged so as to spread in a plane parallel to the installation surface, the load is frequently applied in the vertical direction. The magnetic core member 108d has a short length in the direction perpendicular to the weight, and thus the resistance is increased.

  In the above embodiment, the case where the magnetic core 108 is provided in the power supply apparatus 100 has been described. However, as described in the related art, the power receiving device 200 has a symmetrical structure with the power feeding device 100, so that the magnetic core 108 can be incorporated into the power receiving device 200.

  Moreover, in the above embodiment, the 1st magnetic core member 108c was comprised by the combination of 2 members, the magnetic core member 108a and the magnetic core member 108b. However, the present invention is not limited to this, and the first magnetic core member 108c may be composed of one member as long as the first magnetic core member 108c is longer than the second magnetic core member 108d.

  The non-contact power feeding device according to the present invention can reduce overheating of electronic components, and is thus suitable for charging electric propulsion vehicles, for example.

100 contactless power supply device, 102 bottom plate, 104 heat conduction member,
106 coil base, 108 magnetic core part, 108a, 108b magnetic core member, 118c first magnetic core member, 118d second magnetic core member,
110 mica board, 112 power transmission coil, 114 cover,
114a first upper wall, 114b second upper wall, 114c side wall,
116 electronic component group, 118 partition wall, 120 coil holder,
122 shield member, 124 partition wall, 126 heat insulation member, 126a through hole,
130,132 lead wire, 134 bush, 200 power receiving device,
400 Electric propulsion vehicle, S Non-contact power supply system.

Claims (1)

A coil unit used in a non-contact power supply system that supplies power from a power supply device to a power reception device in a contactless manner,
A coil for generating magnetic flux;
A magnetic core for collecting magnetic flux generated in the coil, and
The magnetic core portion is
A first magnetic core member disposed in a direction along the magnetic flux generated by the coil;
A coil unit including a second magnetic core member disposed next to the first magnetic core member and having a shorter length than the first magnetic core member.