Variable inductance coil device

Abstract: The variable inductance coil device having an outer magnetic member, a bobbin member, a coil member and an inner magnetic member. A female thread (thread portion) is provided in the inner periphery of the tube of the bobbin member, and a male thread (thread portion) which meets with the female thread of the tube is provided in the outer periphery of the inner magnetic member. The inductance varies accurately by rotating and moving the inner magnetic member. Since the outer magnetic member is formed in the closed shape, the leakage flux can be lowered.

Description: Background of the Invention Field of the Invention This invention relates to a variable inductance coil device such as a transformer or a choke coil. Description of the Prior Art For a magnetic core which is used in a transformer or a choke coil, an E-E type (Japanese Patent Publication No. 50372/1980), an E-I type (Japanese Patent Publication No. 24363/1981) and a drum type have been conventionally well-known in the art. In the E-E type magnetic core, a pair of E-shaped cores made of magnetic material such as ferrite is positioned so that each leg of the cores is opposed each other, wherein a gap is provided between each end of the center legs in order to prevent magnetic saturation. The E-I type magnetic core combines an E-shaped core and an I-shaped core, wherein there is a gap provided on the end of the center leg of the E-shaped core. The drum type core literally uses the drum-shaped core. However, a method for winding wire around the above-mentioned magnetic core having the gap has frequently caused inductance errors which are induced by dimensional errors in the magnetic core, dimensional errors caused during manufacturing of the gaps, and errors in magnetic permeability of the core. For example, if a choke coil has an effective permeability of around 100, the errors of the inductance is ±21 % in the E-E type and ±16 % in the E-I type. In case of the drum-type magnetic core, the inductance error is relatively small for ±6 %. However, as illustrated in a diagram of Figure 7 showing distribution of leakage flux (unit in the diagram is expressed in gauss), the leakage flux near the drum core turns out to be very large, about 20 gauss. In the US-Document, US 3 500 274 a variable inductance coil device is disclosed comprising an outer magnetic member, a coil member, an inner magnetic member with a thread portion. In the disclosed embodiment of the before mentioned document the inner magnetic member extends outside of the outer magnetic member and also the inner magnetic member is not completely surrounded by the outer magnetic member. As a consequence of that, it is not possible to adjust the gap between the inner and outer magnetic member very precisely, so that the inductance can be variate in little ranges. Finally in the British document GB 1 518 938 a variable electrical choke is disclosed showing another constitution as it is described in the preamble of claim 1 which means that the inner magnetic member of the embodiment shown in the British document is not surrounded by the outer magnetic member. Furthermore the embodiment provides non-magnetic inserts between the inner magnetic member and the outer magnetic member so that their relative prositions are fixed. Therefore the variability of the inductance is very limited. Summary of the Invention An objective of the present invention is to provide a variable inductance coil device having a highly accurate inductance. In order to accomplish the above-described objective, the present invention is as claimed in claim 1. In the variable inductance coil device designed as above, the inductance can be accurately varied because the thread portion is provided therein and thus the relative movement of the inner magnetic member can be performed precisely. The relative movement can be easily adjusted by engaging a tool in the stopper so as to rotate the inner magnetic member. Furthermore, the outer magnetic member itself is formed in a closed shape, so the leakage flux can be decreased. Therefore, it is possible to provide a high precision variable inductance coil device of small inductance errors and small leakage flux. Brief Description of the Drawing Figure 1 is a perspective view showing one preferred embodiment of the variable inductance coil device of the present invention. Figure 2 is an exploded perspective view of the preferred embodiment. Figure 3 is a perspective view of a main part of a bobbin member of the preferred embodiment. Figure 4 is a diagram showing a variation of the inductance when either one of members in the embodiment is moved. Figure 5 is a plan view showing the distance between a gap and the inner magnetic member in the preferred embodiment. Figure 6 is a diagram showing a distribution of the leakage flux. Figure 7 is a diagram showing a distribution of the leakage flux of the conventional drum-type coil device. Figure 8 is a perspective view showing one preferred embodiment of the outer magnetic member having a half-moon shaped groove for restricting the horizontal position of the bobbin member, a hole for inserting a tool in order to rotate the inner magnetic member, and a gap provided in a magnetic path. Figure 9A is a perspective view showing one preferred embodiment of a hexagon-shaped outer magnetic member. Figure 9B is a perspective view showing one preferred embodiment of a tube-shaped outer magnetic member Figure 10A is a perspective view showing one preferred embodiment of the inner magnetic member wherein the stopper for the rotating tool is formed in a concaved square-shape. Figure 10B is a perspective view showing one preferred embodiment of the inner magnetic member wherein the stopper is formed in a projected hexagon-shape. Figure 10C is a perspective view showing one preferred embodiment of the inner magnetic member wherein the stopper is formed in a projected square-shape. Description of the Preferred Embodiments The preferred embodiments of the present invention are described in detail in reference to Figures 1-10C. A variable inductance coil device 1 in Figure 1 includes an outer magnetic member 2, a bobbin member 3, a coil member 4 and an inner magnetic member 5. The outer magnetic member 2 comprises a magnetic material such as ferrite made from manganese, iron or zinc. The outer magnetic member 2 is formed in a square shape, that is a closed shape, comprising four side plates 20a-20d having a thickness T of 2 millimeters. As shown in Figure 2, the outer magnetic member 2 includes: V-shaped cutouts 21a and 21b which are provided in both of upper and bottom sides of the side plate 20a, a half-moon shaped cutout 22 which is provided in the upper side of the corresponding side plate 20c, and gap grooves 23a and 23b having a depth D of 0.5 millimeter which are provided in inner walls of both side plates 20a and 20c. The cutouts 21a and 21b are engaged in a projection 31b of the bobbin member 3 so as to restrict the horizontal position of the bobbin member 3. Since the cutouts 21a and 21b are provided in both of the upper and bottom sides on the side plate 20a, it is applicable to other bobbin members having other shapes. The half-moon shaped cutout 22 is provided for inserting a tool into the inner magnetic member 5. The gap grooves 23a and 23b are provided for forming gaps between the outside of the coil member 4 and the outer magnetic member 2 so that fringing flux caused around the coil member 4 (wire) is decreased and eddy current loss in the coil member 4 (wire) is also lowered. As shown in Figure 2, the bobbin member 3 formed integrally by an injection molding is made of a resin and comprises: a tube 30, a L-shaped part 31 which is connected to the end of the tube 30, and a base 32 which is connected to the L-shaped part 31. In an inner periphery of the tube 30, female thread 30a is formed, and the coil member 4 is adapted to be wound around an outer periphery of the tube 30. A space S between the end of the L-shaped part 31 and the base 32 is about 2-2.2 millimeters so as to restrain the position of the outer magnetic member 2 in an axial direction. As shown in Figure 3, in a horizontal part 31a of the L-shaped part 31, there is the projection 31b which engages in the cutout 21b of the outer magnetic member 2 so that the movement of the outer magnetic member 2 in the horizontal direction can be restrained thereby. The inner magnetic member 5 comprises a magnetic material such as ferrite which is baked metallic oxide made from manganese, iron or zinc and formed in a bar shape. As shown in Figure 2, a male thread 5a which mates with the female thread 30a of the tube 30 is formed in an outer periphery of the inner magnetic member 5, and a hexagon-shaped concave portion 5b is formed as a stopper on an end surface of the inner magnetic member 5. The hexagon-shaped concave portion 5b is provided to insert a hexagon-shaped wrench therethrough in order to rotate the inner magnetic member 5. In the following, a method for assembling the preferred embodiments is described. First, the coil member 4 is wound on the outer periphery of the tube 30 of the bobbin member 3. Then, as shown in Figure 2, the male thread 5a of the inner magnetic member 5 is screwed into the female thread 30a of the tube 30 of the bobbin member 3 so that the inner magnetic member 5 can be inserted inside the tube 30. Next, the outer magnetic member 2 is positioned at the outside of the tube 30 to form the device as shown in Figure 1. In a further step, a hexagon wrench bar is inserted into the hexagon concave portion 5b of the inner magnetic member 5 so that the inductance is adjusted to desirable values by rotating the inner magnetic member 5. The effect of the preferred embodiment is described in reference to Figures 4 and 5. Figure 4 is a diagram showing the fluctuation of the inductance when either one of the outer magnetic member 2, the coil member 4 or the inner magnetic member 5 is moved relatively with other members. The vertical axis shows the inductance (µH). The lower horizontal axis shows the distance L (mm) between the gap groove 23a in the side plate 20a and the inner magnetic member 5, and the upper horizontal axis shows the distance (mm) between the gap groove 23a and the coil member 4 as shown in Figure 5. In the Figure 4, a curve a shows the test result when only the outer magnetic member 2 is moved, a curve b shows when only the inner magnetic member 5 is moved, and a straight line c shows when only the coil member 3 is moved. In accordance with Figure 4, the coil device in the preferred embodiment can obtain a wide variable range of the inductance for 29.2 % as shown in the curve b . Even if only the outer magnetic member 2 is moved, the wide variable range of the inductance can be obtained for 38.4 % as shown in the curve a . Similarly, when only the coil member 3 is moved, the wide variable range can be also obtained for 38.0 % as shown in the straight line c . In addition, the inductance can be easily and accurately adjusted by rotating the inner magnetic member 5, and it is possible to provide a precise coil device having small errors in the inductance. Figures 6 and 7 show the distribution of the leakage flux for the variable inductance coil device of the present invention and the conventional drum type coil device respectively. The unit of the numbers in the drawings is expressed in gauss (1 gauss = 10 -4 T). The measurement of the leakage flux for both devices has been performed with equal drive current value, number of windings of the coil, and coil inductance value. In this preferred embodiment, the outer magnetic member 2 is formed in the closed shape; thus, the leakage flux produced around the outer magnetic member 2 is about 3 gauss as shown in Figure 6. This is one-sixth of the leakage flux of the conventional drum-type coil device in Figure 7; the present invention has realized a lower leakage flux. In addition, the fringing flux interlinked on the coil member 4 is lowered by the gap grooves 23a and 23b provided in the outer magnetic member 2, so that the eddy current loss on the coil member 4 is also lowered. Furthermore, the present invention can have various arrangements within the scope of the invention other than the preferred embodiment described in the foregoing. Although the present invention is described in the preferred embodiment that the inner magnetic member 5 is moved, other mechanism is also possible. For example, both of the outer magnetic member 2 and the coil member 4 can be moved, or either one of the members can be moved as well. For the outer magnetic member 2, as shown in Figure 8, a V-shaped cutout 21a' can be formed only in the upper side of the side plate 20a instead of the cutouts 21a and 21b in both sides. The shape of the cutout can be half-moon as long as it can restrain the horizontal position of the outer magnetic member 2 when it is engaged with the projection part 31b. When a gap 24 is provided on the magnetic path, a highly accurate inductance can be obtained even though the leakage flux cannot be lowered. In addition, a hole 22' as shown in Figure 8 can be acceptable instead of the half-moon shaped cutout 22 in Figure 2 if the tool can be inserted therethrough and the inner magnetic member 5 can be rotated thereby. Furthermore, the shape of the outer magnetic member 2 can be either a hexagon-shaped tube 2' or a tube 2" as shown in Figures 9A-9B. For the inner magnetic member 5, the shape of the concave portion 5b can be either one of a square concave portion 5b', a hexagon projection 5c, or a square projection 5c' as shown in Figures 10A-10C as long as the inner magnetic member 5 can be rotated by the tool.

Claims: A variable inductance coil device comprising: an outer magnetic member (2); a coil member (4) positioned inside said outer magnetic member (2); an inner magnetic member (5) positioned inside said coil member (4), wherein a thread portion enables said inner magnetic member (5) to move relatively with respect to the other members (2), characterised in that said outer magnetic member (2) comprises a magnetic material to form a substantially closed loop ; said inner magnetic member (5) forms a gap at each of its two end surfaces with respect to said outer magnetic member (2) and said inner magnetic member (5) is movable with respect to said outer magnetic member (2) to adjust said gaps at each end of said inner magnetic member (5) at the same time. A variable inductance coil device as defined in claim 1, wherein said inner magnetic member which can move relatively includes an engagement portion so as to rotate said inner magnetic member.

Inventors: ITO SHINICHIRO, KINOSHITA YUKIHARU, SAITOH YUTAKA, ITO, SHINICHIRO, KINOSHITA, YUKIHARU, SAITOH, YUTAKA
Applicants: TDK CORP, TDK CORPORATION
Application year: 1993
Main classification: H01F21
IPC codes: H01F21, H01F17