Magnetic bearing apparatus

Abstract

This magnetic bearing apparatus is provided with a stator core (15) that accommodates axial coils (10A, 10B) and radial coils (12A-12D). The stator core (15) is provided with: second axial stator cores (17A, 17B) positioned on both sides of a second disk (8) and between the second disk (8) and the axial coils (10A, 10B); a plurality of radial stator cores (18) around which the radial coils (12A-12D) are wound; and first axial stator cores (19A, 19B) positioned on both sides of a first disk (7).

Description

magnetic bearing device

[0001] The present invention relates to a magnetic bearing device capable of supporting a rotor in a non-contact manner.

[0002] Magnetic bearings are bearings that can support the rotor without contact using electromagnetic force, eliminating friction loss and enabling the rotor to rotate at ultra-high speeds. Magnetic bearings also have the advantage of not requiring lubrication, which means that the system environment is not polluted, and they have a long life and can be operated in vacuum, extremely low temperature, and high temperature environments.

[0003] FIG. 14 is a schematic diagram showing the structure of a conventional magnetic bearing. As shown in FIG. 14, the stator 500 of the magnetic bearing is composed of an axial stator section 501 and a radial stator section 502. The axial stator section 501 has two axial coils 512 wound around a rotor shaft 510. The radial stator section 502 has a four-slot structure and four radial coils 513 for generating radial forces (only two radial coils 513 are shown in FIG. 14). The radial stator section 502 forms a magnetic circuit in a plane perpendicular to the axial direction of a rotor 520. The rotor 520 has a disk 525. This magnetic bearing supports the rotor 520 by generating a magnetic attraction force between the disk 525 of the rotor 520 and a stator core 526.

[0004] P. Imoberdorf, C. Zwyssig, SD Round and JW Kolar, "Combined Radial-Axial Magnetic Bearing for a 1 kW, 500,000 rpm Permanent Magnet Machine," APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition, Anaheim, CA, USA, 2007, pp. 1434-1440, doi: 10.1109 / APEX.2007.357705.

[0005] However, the above-mentioned magnetic bearing has a problem in that the opposing area between the disk 525 and the stator core 526 (the area surrounded by the dotted line) is small, and the supporting force (also called supporting force density) of the rotor 520 per unit volume is small.

[0006] Therefore, the present invention provides an improved magnetic bearing device that can increase the bearing force (bearing force density).

[0007] In one aspect, there is provided a magnetic bearing device comprising: a rotor shaft; a first disk protruding radially outward from an outer circumferential surface of the rotor shaft; a second disk protruding radially outward from an outer circumferential surface of the first disk; an axial coil arranged on both sides of the second disk; a plurality of radial coils arranged radially outside the axial coil; and a stator core accommodating the axial coil and the plurality of radial coils, wherein the stator core comprises a pair of first axial stator cores located on both sides of the first disk, a plurality of pairs of second axial stator cores located on both sides of the second disk and between the second disk and the axial coil, and a plurality of radial stator cores each wound with the plurality of radial coils.

[0008] In one aspect, the axial coil surrounds the periphery of the first disk. In one aspect, the thickness of the first disk is greater than the thickness of the second disk. In one aspect, the plurality of radial coils surround the periphery of the axial coil. In one aspect, the magnetic bearing device further includes a pair of side disks protruding radially outward from the outer circumferential surface of the first disk, the second disk is located between the pair of side disks, and the stator core further includes a pair of side stator cores arranged outside the pair of side disks in the axial direction of the rotor shaft.

[0009] In one aspect, the magnetic bearing device further includes a current controller that controls currents supplied to the axial coil and the plurality of radial coils. In one aspect, the current controller is configured to supply current to one of the axial coils located on both sides of the second disk while supplying current to the plurality of radial coils. In one aspect, the current controller is configured to supply currents of the same magnitude and the same direction to the plurality of radial coils. In one aspect, the current controller is configured to supply currents to two radial coils located on opposite sides of the plurality of radial coils while supplying current to the axial coil. In one aspect, the current controller is configured to supply currents to the two radial coils so that the two radial coils generate magnetic flux in the same direction within the second disk. In one aspect, the current controller is configured to supply currents in opposite directions to the axial coil.

[0010] The magnetic bearing device includes a first disk and a second disk facing the stator core. Therefore, compared to conventional magnetic bearings, the facing area between the stator core and the rotor is increased, resulting in a high bearing force (high bearing force density).

[0011] 1. A sectional perspective view showing an embodiment of a magnetic bearing device. 2. A longitudinal sectional view of the magnetic bearing device shown in FIG. 1. 3. A sectional view taken along line A-A in FIG. 2. 4. A perspective view showing another embodiment of a magnetic bearing device. 5. A longitudinal sectional view of the magnetic bearing device shown in FIG. 4. 6. A graph showing axial forces generated by the magnetic bearing device described with reference to FIGS. 1 to 3, the magnetic bearing device described with reference to FIGS. 4 and 5, and a conventional magnetic bearing device. 7. A graph showing an example of radial forces. 8. A diagram explaining an embodiment of a current-flow method for generating an axial magnetic attractive force. 9. A diagram explaining an embodiment of a current-flow method for generating an axial magnetic attractive force. 10. A graph showing the analysis results of axial forces in the embodiment described with reference to FIGS. 8 and 9. 11. A diagram explaining an embodiment of a current-flow method for generating a radial magnetic attractive force. 12. A diagram explaining an embodiment of a current-flow method for generating a radial magnetic attractive force. 13. A graph showing the analysis results of radial forces in the embodiment described with reference to FIGS. 11 and 12. 14. A schematic diagram showing an example of a conventional magnetic bearing.

[0012] Embodiments of the present invention will now be described with reference to the drawings. Fig. 1 is a cross-sectional perspective view showing one embodiment of a magnetic bearing device, Fig. 2 is a longitudinal cross-sectional view of the magnetic bearing device shown in Fig. 1, and Fig. 3 is a cross-sectional view taken along line A-A in Fig. 2. The magnetic bearing device comprises a rotor 1, a magnetic bearing 2 that supports the rotor 1 in a non-contact manner by magnetic attraction, and a current controller 3 that controls the operation of the magnetic bearing 2.

[0013] The rotor 1 is a rotatable structure. Applications for the rotor 1 rotatably supported by the magnetic bearing 2 are not particularly limited. For example, the rotor 1 may be part or all of a rotor included in a rotary machine such as a liquid pump or a vacuum pump. In particular, the magnetic bearing device can support the rotor 1 without mechanically contacting the rotor 1, thereby preventing wear and frictional heat from occurring in the rotor 1. Therefore, the magnetic bearing device is suitable for applications in which it supports rotors that rotate at high speeds, such as the turbine of a turbomolecular pump or the impeller rotor of a liquid hydrogen pump that transports liquid hydrogen, which has a low specific gravity.

[0014] The rotor 1 has a rotor shaft 5, a first disk 7 protruding radially outward from the outer peripheral surface of the rotor shaft 5, and a second disk 8 protruding radially outward from the outer peripheral surface of the first disk 7. The rotor shaft 5, the first disk 7, and the second disk 8 are concentric. The rotor shaft 5, the first disk 7, and the second disk 8 are an integral structure and rotate together. The first disk 7 has a larger diameter than the rotor shaft 5, and the second disk 8 has a larger diameter than the first disk 7. The second disk 8 is located at the center of the first disk 7 in the axial direction of the rotor 1. The thickness (axial width) of the first disk 7 is larger than the thickness (axial width) of the second disk 8. The rotor 1 of this embodiment is a dual-disk rotor having the first disk 7 and the second disk 8.

[0015] The magnetic bearing 2 includes a pair of axial coils 10A, 10B arranged on both sides of the second disk 8, a plurality of radial coils 12A, 12B, 12C, and 12D arranged radially outside the axial coils 10A, 10B, and a stator core 15 that houses the axial coils 10A, 10B and the plurality of radial coils 12A, 12B, 12C, and 12D. The pair of axial coils 10A, 10B are arranged to surround the first disk 7. More specifically, these axial coils 10A, 10B are arranged radially outside the first disk 7 and on both sides of the second disk 8 in the axial direction of the rotor 1. The central axes of the axial coils 10A, 10B coincide with the axis of the rotor 1.

[0016] The multiple radial coils 12A to 12D surround the pair of axial coils 10A and 10B and are arranged along the outer circumferential surfaces of the axial coils 10A and 10B. As shown in FIG. 3 , in this embodiment, four radial coils 12A, 12B, 12C, and 12D are arranged at equal intervals along the circumferential direction of the rotor 1. In one embodiment, five or more radial coils may be provided. The central axis of each of the radial coils 12A, 12B, 12C, and 12D is perpendicular to the axis of the rotor 1. Therefore, each of the radial coils 12A, 12B, 12C, and 12D forms a magnetic circuit in a plane perpendicular to the axial direction of the rotor 1.

[0017] The stator core 15 is made of a magnetic material such as silicon steel and covers the axial coils 10A and 10B, the plurality of radial coils 12A to 12D, the first disk 7, and the second disk 8. As shown in Fig. 2, the stator core 15 has a pair of first axial stator cores 19A and 19B, a plurality of pairs of second axial stator cores 17A and 17B, and a plurality of radial stator cores 18.

[0018] The pair of first axial stator cores 19A, 19B are located on both sides of the first disk 7 in the axial direction of the rotor 1. More specifically, the first axial stator cores 19A, 19B have first opposing surfaces 19c, 19d that face both side surfaces 7a, 7b of the first disk 7. The first axial stator cores 19A, 19B extend radially inward from an outer wall portion 21 of the stator core 15.

[0019] Multiple pairs of second axial stator cores 17A, 17B are provided corresponding to the multiple radial coils 12A to 12D. In this embodiment, four radial coils 12A to 12D are provided, and therefore four pairs of second axial stator cores 17A, 17B are provided. FIG. 2 shows only two pairs of second axial stator cores 17A, 17B. Each pair of second axial stator cores 17A, 17B is located on both sides of the second disk 8 and between a pair of axial coils 10A, 10B. More specifically, the second axial stator core 17A is located between the second disk 8 and the axial coil 10A, and the second axial stator core 17B is located between the second disk 8 and the axial coil 10B. The second axial stator cores 17A, 17B have second opposing surfaces 17c, 17d that face both side surfaces 8a, 8b of the second disk 8.

[0020] The plurality of radial stator cores 18 are provided corresponding to the plurality of radial coils 12A to 12D. In this embodiment, four radial coils 12A to 12D are provided, and therefore four radial stator cores 18 are provided. Only two radial stator cores 18 are illustrated in Fig. 2. The plurality of radial stator cores 18 are located radially outward of the second disk 8 and the plurality of pairs of second axial stator cores 17A, 17B.

[0021] 3, a plurality of radial stator cores 18 surround the second disk 8. Each radial stator core 18 has an inner peripheral surface 18a that faces the outer peripheral surface 8c of the second disk 8. A plurality of radial coils 12A to 12D are wound around the plurality of radial stator cores 18, respectively. In this embodiment, four radial coils 12A to 12D are wound around four radial stator cores 18, respectively.

[0022] As shown in Figure 2, the multiple pairs of second axial stator cores 17A, 17B are respectively connected to multiple radial stator cores 18, and the multiple radial stator cores 18 extend radially inward from the outer wall portion 21 of the stator core 15.

[0023] 1 to 3, the thickness of the first disk 7 is greater than the thickness of the second disk 8, and the second disk 8 is located at the center of the outer circumferential surface of the first disk 7. With this configuration, it is possible to reduce magnetic flux leakage from the radial stator core 18 to the first axial stator cores 19A, 19B.

[0024] 1 to 3 has first opposing surfaces 19c, 19d and second opposing surfaces 17c, 17d, which increases the opposing area between the rotor 1 and the stator core 15. Specifically, the magnetic bearing 2 shown in Figures 1 to 3 has an opposing area that is three times or more the opposing area of ​​the conventional magnetic bearing shown in Figure 14. Therefore, the magnetic bearing device according to the embodiment described with reference to Figures 1 to 3 can achieve a high supporting force (supporting force density).

[0025] The magnetic bearing device includes a plurality of displacement sensors (not shown) for measuring the axial displacement and radial displacement of the rotor 1. The measured values ​​of the axial displacement and radial displacement of the rotor 1 are sent to a current controller 3. The current controller 3 controls the currents to be supplied to the axial coils 10A, 10B and the plurality of radial coils 12A to 12D based on the measured values ​​of the axial displacement and radial displacement of the rotor 1. The rotor 1 is supported in a non-contact manner by the magnetic attractive forces generated by the axial coils 10A, 10B and the plurality of radial coils 12A to 12D.

[0026] The current controller 3 includes a storage device 3a storing a program for controlling the position of the rotor 1, a calculation device 3b that executes calculations according to instructions included in the program, and a power supply 3c that supplies current to the pair of axial coils 10A, 10B and the plurality of radial coils 12A-12D. The current controller 3 includes at least one computer. The storage device 3a includes a main storage device such as a random access memory (RAM) and an auxiliary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). Examples of the calculation device 3b include a central processing unit (CPU) and a graphics processing unit (GPU). However, the specific configuration of the current controller 3 is not limited to these examples, as long as the current controller 3 can control the current to be supplied to the pair of axial coils 10A, 10B and the plurality of radial coils 12A-12D based on measured values ​​of the axial displacement and radial displacement of the rotor 1.

[0027] Fig. 4 is a perspective view showing another embodiment of a magnetic bearing device, and Fig. 5 is a longitudinal sectional view of the magnetic bearing device shown in Fig. 4. The configuration and operation of this embodiment not specifically described are the same as those of the embodiment described with reference to Figs. 1 to 3, so duplicated descriptions will be omitted. The sectional view shown in Fig. 3 also applies to this embodiment shown in Figs. 4 and 5.

[0028] The magnetic bearing device further includes a pair of side disks 24A and 24B in addition to the first disk 7 and second disk 8. The rotor shaft 5, first disk 7, second disk 8, and pair of side disks 24A and 24B are concentric. The rotor shaft 5, first disk 7, second disk 8, and pair of side disks 24A and 24B are integrally configured and rotate as a single unit.

[0029] The side disks 24A, 24B protrude radially outward from the outer peripheral surface of the first disk 7. The second disk 8 is located between the pair of side disks 24A, 24B. In this embodiment, the side disks 24A, 24B are located on both ends of the first disk 7. The rotor 1 of this embodiment is a multi-disk rotor having the first disk 7, the second disk 8, and the side disks 24A, 24B.

[0030] In one embodiment, each of the side disks 24A, 24B has a larger diameter than the second disk 8. Furthermore, in one embodiment, the thickness of each of the side disks 24A, 24B is smaller than the thickness of the second disk 8. The pair of axial coils 10A, 10B and the plurality of radial coils 12A to 12D are located between the pair of side disks 24A, 24B. That is, the pair of side disks 24A, 24B are arranged outside the pair of axial coils 10A, 10B and the plurality of radial coils 12A to 12D in the axial direction of the rotor 1.

[0031] The stator core 15 includes a pair of side stator cores 26A, 26B disposed outside the pair of side disks 24A, 24B in the axial direction of the rotor shaft 5. The side stator cores 26A, 26B extend radially inward from the outer wall portion 21 of the stator core 15, and the first axial stator cores 19A, 19B extend radially inward from the side stator cores 26A, 26B. The side stator cores 26A, 26B have third opposing surfaces 26c, 26d that face outer surfaces 24c, 24d of the side disks 24A, 24B.

[0032] The embodiment described with reference to Figures 4 and 5 has first opposing surfaces 19c, 19d, second opposing surfaces 17c, 17d, and third opposing surfaces 26c, 26d, which further increases the opposing area between the rotor 1 and the stator core 15. Specifically, the magnetic bearing 2 shown in Figures 4 and 5 has an opposing area that is about seven times the opposing area of ​​the conventional magnetic bearing shown in Figure 14. Therefore, the magnetic bearing device according to the embodiment described with reference to Figures 4 and 5 can achieve a higher supporting force (supporting force density).

[0033] FIG. 6 is a graph showing examples of the axial force generated by the magnetic bearing device described with reference to FIGS. 1 to 3 (hereinafter referred to as Model A), the axial force generated by the magnetic bearing device described with reference to FIGS. 4 and 5 (hereinafter referred to as Model B), and the axial force generated by the conventional magnetic bearing described with reference to FIG. 14 (hereinafter referred to as Model C).

[0034] Models A, B, and C were designed so that their outer diameters were the same. Models A, B, and C generated axial forces by passing current through only one of the two axial coils. The current value was a low current of 0 to 0.1 A. As can be seen from Figure 6, Models A and B can generate large axial forces with a relatively small current compared to Model C. In addition, Model B can generate a larger axial force than Model A.

[0035] FIG. 7 is a graph showing an example of the radial force generated by model A, the radial force generated by model B, and the radial force generated by model C.

[0036] Models A, B, and C were designed so that they had the same outer diameter. Models A, B, and C generated radial forces by passing current through only one of the four axial coils. The current value was a high current of 0 to 1 A. As can be seen from Figure 7, Models A and B can generate a larger radial force than Model C. In addition, Model B can generate a larger radial force than Model A.

[0037] Next, an embodiment of a novel method for supplying current to the axial coils 10A, 10B and the radial coils 12A to 12D will be described. The following description applies to each embodiment described with reference to Figures 1 to 5. The supply of current to the axial coils 10A, 10B and the radial coils 12A to 12D is controlled by a current controller 3.

[0038] 8 and 9, an embodiment of a current supply method for generating a magnetic attraction force in the axial direction will be described. The current controller 3 is configured to supply current to one of the axial coils 10A and 10B while supplying current to all of the radial coils 12A to 12D. In the example shown in FIG. 8, current flows only through the right-side axial coil 10A, and no current flows through the left-side axial coil 10B.

[0039] The current controller 3 is configured to supply current of the same magnitude and the same direction to all of the radial coils 12A to 12D. In the example shown in Figure 9, the radial coils 12A to 12D generate radially outward forces as indicated by the white arrows. These radially outward forces have the same magnitude and therefore cancel each other out.

[0040] When current is supplied to one of the axial coils 10A while current is supplied to the radial coils 12A to 12D, as shown in FIG. 8 , magnetization and demagnetization occur at the first opposing surfaces 19c and 19d of the first axial stator cores 19A and 19B and the side surfaces 7a and 7b of the first disk 7 (see the dotted circles). In the example shown in FIG. 8 , magnetic flux increases between the first opposing surface 19c of the right-side first axial stator core 19A and the right side surface 7a of the first disk 7, while magnetic flux decreases between the first opposing surface 19d of the left-side first axial stator core 19B and the left side surface 7b of the first disk 7. Therefore, a difference in magnetic flux density occurs between the left and right sides of the rotor 1. As a result, in the example shown in FIG. 8 , a magnetic attraction force toward the right is generated, as indicated by the white arrow, and the rotor 1 is displaced to the right.

[0041] Although not shown, when generating a magnetic attraction force toward the left, the current controller 3 similarly supplies current to all of the plurality of radial coils 12A to 12D, while supplying current only to the left-side axial coil 10B. Fig. 8 shows the magnetic bearing device described with reference to Figs. 1 to 3, but the magnetic bearing devices described with reference to Figs. 4 and 5 also operate in the same way.

[0042] 10 is a graph showing the analysis results of the axial force in the embodiment described with reference to FIGS. 8 and 9. Current was passed through only one axial coil 10A, varying from 0 to 0.2 A, and current was passed through the four radial coils 12A to 12D in four patterns: 0 A, 0.1 A, 0.2 A, and 0.3 A. As can be seen from FIG. 10, the axial force increased as the current passed through the four radial coils 12A to 12D increased. This is because the increase in current passed through the four radial coils 12A to 12D created a larger difference in magnetic flux density between the left and right sides of the rotor 1.

[0043] Next, one embodiment of a current supply method for generating a radial magnetic attraction force will be described with reference to Figures 11 and 12. The current controller 3 is configured to supply current to both axial coils 10A and 10B while supplying current to two radial coils located on opposite sides of the plurality of radial coils 12A to 12D. As shown in Figure 11, the current controller 3 supplies currents in opposite directions to the axial coils 10A and 10B.

[0044] In the example shown in FIG. 12 , the current controller 3 supplies current to two radial coils 12A and 12C located opposite each other, and does not supply current to the other two radial coils 12B and 12D. The two radial coils 12A and 12C to which current is supplied generate magnetic flux in the same direction within the second disk 8. In the example shown in FIG. 11 , the two radial coils 12A and 12C generate upward magnetic flux within the second disk 8. In addition to energizing one radial coil 12A, energizing the opposite radial coil 12C so that it generates magnetic flux in the same direction strengthens the radial magnetic flux. At the same time, energizing the two axial coils 10A and 10B so that they form symmetrical magnetic circuits with respect to the second disk 8.

[0045] When current is supplied to both axial coils 10A, 10B while current is supplied to two radial coils 12A, 12C located on opposite sides of each other, magnetization and demagnetization occur at the opposing surfaces of the two radial stator cores 18 and the second disk 8 (see the dotted circles), as shown in Fig. 11 . In the example shown in Fig. 11 , magnetic flux increases between the upper radial stator core 18 and the second disk 8, while magnetic flux decreases between the lower radial stator core 18 and the second disk 8. Therefore, a difference in magnetic flux density occurs between the top and bottom of the rotor 1. As a result, in the example shown in Fig. 11 , an upward magnetic attraction force is generated, and the rotor 1 is displaced upward.

[0046] Although not shown, when generating a magnetic attraction force in a downward, rightward or leftward direction, the current controller 3 similarly supplies current to the two radial coils located opposite each other while supplying current to both axial coils 10A, 10B. Although Fig. 11 shows the magnetic bearing device described with reference to Figs. 1 to 3, the magnetic bearing devices described with reference to Figs. 4 and 5 operate in the same manner.

[0047] Fig. 13 is a graph showing the analysis results of the radial force in the embodiment described with reference to Figs. 11 and 12. Current was passed through the two radial coils 12A and 12C while varying from 0 to 1 A, and current was passed through the two axial coils 10A and 10B in three patterns: 0 A, 0.2 A, and 0.4 A. As can be seen from Fig. 13, the radial force increased as the current passed through the two axial coils 10A and 10B increased. This is because the increase in current passed through the two axial coils 10A and 10B caused a greater difference in magnetic flux density between the top and bottom of the rotor 1.

[0048] The magnetic bearing devices of the above-described embodiments are three-degree-of-freedom magnetic bearing devices. The current application methods described with reference to Figures 8 to 13 are suitable for three-degree-of-freedom magnetic bearing devices, and are capable of improving the bearing force density.

[0049] The above-described embodiments have been described for the purpose of enabling a person of ordinary skill in the art to practice the present invention. Various modifications of the above-described embodiments would be obvious to a person skilled in the art, and the technical concept of the present invention may be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be interpreted in the broadest scope in accordance with the technical concept defined by the claims.

[0050] The present invention can be used in a magnetic bearing device that can support a rotor in a non-contact manner.

[0051] REFERENCE SIGNS LIST 1 rotor 2 magnetic bearing 3 current controller 3a storage device 3b arithmetic device 3c power supply 5 rotor shaft 7 first disk 7a, 7b side surface of first disk 8 second disk 8a, 8b side surface of second disk 10A, 10B axial coil 12A, 12B, 12C, 12D radial coil 15 stator core 17A, 17B second axial stator core 17c, 17d second opposing surface 18 radial stator core 18a inner peripheral surface of radial stator core 19A, 19B first axial stator core 19c, 19d first opposing surface 21 outer wall portion 24A, 24B side disk 24c, 24d outer surface of side disk 26A, 26B side stator core 26c, 26d third opposing surface

Claims

1. A magnetic bearing device comprising: a rotor shaft; a first disk protruding radially outward from the outer circumferential surface of the rotor shaft; a second disk protruding radially outward from the outer circumferential surface of the first disk; an axial coil arranged on both sides of the second disk; a plurality of radial coils arranged radially outward from the axial coil; and a stator core accommodating the axial coil and the plurality of radial coils, the stator core comprising: a pair of first axial stator cores located on both sides of the first disk, a plurality of pairs of second axial stator cores located on both sides of the second disk and between the second disk and the axial coil, and a plurality of radial stator cores each wound with the plurality of radial coils.

2. A magnetic bearing device as claimed in claim 1, wherein said axial coil surrounds the periphery of said first disk.

3. A magnetic bearing device as claimed in claim 1, wherein the thickness of said first disk is greater than the thickness of said second disk.

4. A magnetic bearing device according to claim 1, wherein said plurality of radial coils surrounds said axial coil.

5. A magnetic bearing device as described in claim 1, further comprising a pair of side disks protruding radially outward from the outer circumferential surface of the first disk, the second disk being positioned between the pair of side disks, and the stator core further comprising a pair of side stator cores arranged outside the pair of side disks in the axial direction of the rotor shaft.

6. A magnetic bearing device according to claim 1, further comprising a current controller for controlling currents supplied to said axial coil and said plurality of radial coils.

7. A magnetic bearing device as claimed in claim 6, wherein said current controller is configured to supply current to one of said axial coils disposed on either side of said second disk while supplying current to said plurality of radial coils.

8. A magnetic bearing device according to claim 7, wherein said current controller is configured to supply currents of the same magnitude and direction to said plurality of radial coils.

9. The magnetic bearing device according to claim 6, wherein the current controller is configured to supply current to two radial coils located opposite each other among the plurality of radial coils while supplying current to the axial coil.

10. A magnetic bearing device according to claim 9, wherein said current controller is configured to supply current to said two radial coils such that said two radial coils generate magnetic flux in the second disk in the same direction.

11. A magnetic bearing device as claimed in claim 9, wherein said current controller is configured to supply currents in opposite directions to said axial coils.

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