Composite-type electromagnet and radial magnetic bearing

[0002] 1. Field of the Invention

[0003] The present invention relates to a composite-type electromagnet structured by a combination of an electromagnet having a control coil wound on an electromagnet core and a radial permanent magnet, and to a radial magnetic bearing provided with this composite-type electromagnet.

[0004] 2. Description of the Related Art

[0005] Radial magnetic bearings, which are used for rotatably supporting a rotary shaft of a rotary apparatus such as a motor without making contact therewith, are structured by position sensors, controlling electromagnets and biasing electromagnets. However, there are proposals for employing an electromagnet for both controlling and biasing in order to simplify the structure. The position sensor is used to detect displacement of the rotary shaft in the radial direction and provides a detection signal to a magnetic bearing control device. The biasing electromagnet provides a biasing magnetic flux of a constant magnitude to the rotary shaft through a gap between the rotary shaft and the radial magnetic bearing. The controlling electromagnet serves to apply, in accordance with an output signal from the magnetic bearing control device, a controlling magnetic flux to the rotary shaft via the gap and return the displaced rotary shaft to a predetermined position and hold it in that position.

[0006] Because the biasing magnetic field is constant, if it is supplied by a permanent magnet, the biasing electromagnet is unnecessary. If this is done, the lack of need of a biasing current to be flown to an excitation coil of the biasing electromagnet greatly reduces power consumption and heat generation of the radial magnetic bearing. Furthermore, because the number of cores and coils for the biasing electromagnet are reduced, the apparatus size and cost can be reduced. Because these various merits are available, proposal and development are being made for the radial magnetic bearings adopting a composite-type electromagnet combined with a biasing permanent magnet and a controlling electromagnet.

[0007] The radial magnetic bearing conventionally disclosed has, as shown in FIG. 10, a plurality of biasing-magnetic-field permanent magnets 54a,54b,54c,54darranged equidistant at inner positions in a widthwise center of an annular yoke 57. These permanent magnets are clamped between an inner surface of the annular yoke 57 and back surfaces of inner magnetic poles 52a,52b,52c,52d.Each inner magnetic pole has an excitation coil 53a,53b,53c,53dprovided on a periphery of a leg thereof. Further, a pair of disc-formed outer magnetic poles 55, 56 defining a plurality of magnetic poles are provided on respective sides of the annular yoke 57 so that the inner magnetic poles and the outer magnetic poles can form a magnetic circuit. Thus, the radial magnetic bearing conventionally disclosed is characterized in that an attracting force of the biasing permanent magnet can be adjusted by varying the thickness of the backside of the inner magnetic pole and a gap between a backside side position and the outer magnetic pole.

[0008] Meanwhile, another radial magnetic bearing is a radial magnetic bearing low in current consumption structured as shown FIG. 11. That is, a housing 67 has electromagnet cores 62a,62b,62c,62din a squared-U form circumferentially arranged around a rotary shaft 69. These electromagnet cores are wound around with excitation coils 63a,63b,63c,63d,and biasing permanent magnets 64a,64b,64c,64darranged in one-part section thereof. Also, the rotary shaft 69 has a rotor core 66 as a circumferential laminate with an I-type electromagnetic steel plate disposed facing the electromagnet cores 62a,62b,62c,62d.Based on a position sensor signal for detecting a radial position of the rotary shaft 69, a magnetic bearing control device not shown supplies a control current to the excitation coils 63a,63b,63c,63d,thereby controlling a radial position of the rotary shaft 69.

[0009] Furthermore, another radial magnetic bearing is a magnetic bearing having, as shown in FIG. 12, a stator arranged around a rotary shaft 79 to apply magnetism to the rotary shaft 79 from the stator to hold the rotary shaft 79 constant on the axis, thus being simple in structure and usable for a large-sized bearing. That is, the stator is structured by electromagnet cores 72a,72b,72c,72dextending along an axial direction and arranged circumferentially, control coils 73a,73b,73c,73dwound around the electromagnet cores, a pair of disc-formed stator yokes 75, 76 respectively fixed on the ends of the electromagnetic cores, a biasing permanent magnet 74 that is a cylindrical permanent magnet connecting between inner sides of the control coils of the stator yokes and magnetized in the axial direction.

[0010] In the meanwhile, although not limited to those as the above for use in radial magnetic bearings, in a composite-type electromagnet combined with a permanent magnet and an electromagnet, a magnetic member forming an electromagnet core and a permanent magnet has a bonded junction, e.g. by an epoxy adhesive or joined by mechanically clamping. In the junction between the permanent magnet and the electromagnetic core thus bonded, there always exists a gap though different in degree due to assembling. Though being dependent upon electromagnet size, forming accuracy, etc., the gap is nearly several hundreds μm, which in some cases is an air gap and in other cases a presence of a non-magnetic material such as adhesive. In both cases, it is a factor for preventing magnetic flux flow. This results in problems such as failing to obtain a desired magnetic force or imbalance in magnetic force or further occurrence of magnetic flux leak to the outside thus causing adverse effects upon external apparatuses.

[0011] As concerned with putting into practical use a composite-type electromagnet or a radial magnetic bearing having this composite-type electromagnet, there is a problem with prevention of magnetic flux flow in the junction of a magnetic-material electromagnet core and a permanent magnet, besides a problem with adjustment of a biasing magnetic field.

[0012] The problem that the invention is to solve is, in a composite-type electromagnet or a radial magnetic bearing having this composite-type electromagnet comprising an electromagnet having a controlling coil wound around an electromagnet core of a magnetic material and a biasing permanent magnet having a junction between the electromagnet core and the permanent magnet, to minimize the leak of magnetic flux occurring at the junction not to reduce the amount of magnetic flux available and prevent bad effects upon an external member, apparatus or the like due to leak magnetic flux.

[0013] In order to solve the above problem, a composite-type electromagnet comprises an electromagnet having a controlling coil wound around an electromagnet core of a magnetic material and a biasing permanent magnet having a junction between the electromagnet core and the permanent magnet, wherein a filling material made of a material having a high permeability and a high saturation magnetic flux density is disposed in the junction.

[0014] FIG. 1 is a cross sectional view of a radial magnetic bearing of one embodiment of the present invention;

[0015] FIG. 2 is a longitudinal sectional view of the radial magnetic bearing of one embodiment of the present invention;

[0016] FIG. 3 is a partial perspective view of an electromagnet core, a controlling coil and a biasing permanent magnet;

[0017] FIG. 4 is a partial sectional view showing a junction of the electromagnetic core and the biasing permanent magnet with a gap exaggerated;

[0018] FIG. 5 is a controlling block diagram of the radial magnetic bearing of FIG. 1;

[0019] FIG. 6 is a figure for explaining the operation of the radial magnetic bearing of one embodiment of the invention;

[0020] FIG. 7 is a figure for explaining the operation of the radial magnetic bearing of one embodiment of the invention;

[0021] FIG. 8 is a figure for explaining the operation of the radial magnetic bearing of one embodiment of the invention;

[0022] FIG. 9 is a figure for explaining the operation of the radial magnetic bearing of one embodiment of the invention;

[0023] FIG. 10 is a figure showing a first example of a conventional radial magnetic bearing, wherein FIG. 10A is a cross sectional view, and FIG. 10B is a longitudinal sectional view.

[0024] FIG. 11 is a figure showing a second example of a conventional radial magnetic bearing wherein FIG. 11A is across sectional view, and FIG. 11B is a longitudinal sectional view; and

[0025] FIG. 12 is a figure showing a third example of a conventional radial magnetic bearing, wherein FIG. 12A is a cross sectional view, and FIG. 12B is a longitudinal sectional view.

[0026] A radial magnetic bearing of one embodiment of the present invention is structured by composite type electromagnets structured by four electromagnet cores 2a,2b,2c,2d,four biasing permanent magnets 4a,4b,4c,4dand eight control coils 5a,5b,5c,5d,5e,5f,5g,5h,radial position sensors 6x,6y,a cylindrical case 7 of a non-magnetic material accommodating the composite type electromagnet part and the position sensors, and a rotary shaft 9, as shown by a cross sectional view in FIG. 1 and a partial longitudinal sectional view in FIG. 2 and a control block diagram in FIG. 5. The rotary shaft 9 of a non-magnetic material, constituting a part of the rotor, has a magnetic material target 8 formed on a surface thereof.

[0027] The electromagnet core 2a-2dhaving two protrusion poles comprises laminated core pieces fabricated through blanking a material such as a silicon steel plate into a predetermined shape, as shown by a partial perspective view in FIG. 3. Meanwhile, the biasing permanent magnet 4a-4dis a permanent magnet generally in a rectangular-parallelepiped form, similarly as shown by a perspective view in FIG. 3. The four electromagnet cores 2a-2deach having two poles and the four biasing permanent magnets 4a-4dare alternately arranged to form one annular body, and accommodated and fixed within the non-magnetic cylindrical case 7.

[0028] The electromagnet cores 2a-2dand the biasing permanent magnets 4a-4dhave junctions Q at two points for each or totally 8 points. The junction Q is disposed therein with a filling material M having high permeability and saturation magnetic flux density, as shown exaggeratedly with the gap in FIG. 4. In the case of mechanically joining the junction Q, the junction Q is filled with a viscous material such as grease prepared based on a material high in permeability and saturation magnetic flux density or disposed with a magnetic fluid. This conspicuously decreases magnetic resistance of the junction Q down to nearly that of the magnetic-member electromagnet core. The junction Q may be joined with an adhesive prepared based on a material high in permeability and saturation magnetic flux density. In such a case, mechanical joining is unnecessary or auxiliary. Here, the grease or adhesive prepared based on a material having high permeability and saturation magnetic flux density is to be obtained by mixing a powder, for example, of pure iron, silicon steel or ferrite in silicone grease or an epoxy or acrylic adhesive and thoroughly stirring them.

[0029] In the composite-type electromagnet above, the electromagnet core 2ais formed at opposite ends with protruding poles 3hand 3a,the electromagnet core 2bat opposite ends with protruding poles 3band 3c,the electromagnet core 2cat opposite ends with protruding poles 3dand 3e,and the electromagnet core 2dat opposite ends with protruding poles 3fand 3g.The control coil 5a,5b,5c,5d,5e,5f,5g,and 5hare respectively wound around the protruding poles 3a,3b,3c,3d,3e,3f,3g,and 3h.As shown in FIG. 5, a control current is supplied from a magnetic bearing control device 10 to the control coils 5a-5h.

[0030] That is, the control coils 5aand 5band the oppositely-arranged control coils 5eand 5fare connected in series and supplied with a control current from a Y-axis current amplifier section 12yof the magnetic bearing control device 10. The control coils 5cand 5dand the oppositely-arranged control coils 5gand 5hare connected in series and supplied with a control current from an X-axis current amplifier section 12xof the magnetic bearing control device 10.

[0031] With such arrangement and connection, a horse-hoof form of +Y-axis composite-type electromagnet section 1ais composed of the biasing electromagnet 4aand the protruding pole 3aand control coil 5awound thereon as well as the protruding pole 3band the control coil 5bwound thereon. A horse-hoof form of −Y-axis composite-type electromagnet section 1cis composed of the biasing electromagnet 4cand the protruding pole 3eand control coil 5ewound thereon as well as the protruding pole 3fand the control coil 5fwound thereon. A horse-hoof form of +X-axis composite-type electromagnet section 1ais composed of the biasing electromagnet 4band the protruding pole 3cand control coil 5cwound thereon as well as the protruding pole 3dand the control coil 5dwound thereon. Further, a horse-hoof form of −X-axis composite-type electromagnet section 1dis composed of the biasing electromagnet 4dand the protruding pole 3gand control coil 5gwound thereon as well as the protruding pole 3hand the control coil 5hwound thereon.

[0032] In each composite-type electromagnet section, a biasing magnetic field is generated by the biasing permanent magnet 4a-4d,as shown by a dotted-line magnetic flux line in FIG. 1. On the same axis, as shown in FIG. 5 to FIG. 9, a control magnetic field through one composite-type electromagnet section is generated in the direction of intensifying the biasing magnetic field and through the other composite-type electromagnet section arranged opposite is in the direction of weakening the biasing magnetic field. That is, when the control coils 5aand 5bin the +Y-axis composite-type electromagnet section 1agenerate a control magnetic field in the same direction as the biasing magnetic field, the oppositely-arranged control coils 5eand 5fin the −Y-axis composite-type electromagnet section 1cgenerate a control magnetic field in the reverse direction to the biasing magnetic field. Similarly, when the control coils 5cand 5din the +X-axis composite-type electromagnet section 1bgenerate a control magnetic field in the same direction as the biasing magnetic field, the oppositely-arranged control coils 5gand 5hin the −X-axis composite-type electromagnet section generate a control magnetic field in the reverse direction to the biasing magnetic field.

[0033] Accordingly, when the rotary shaft 9 displaces from a predetermined position, the magnetic bearing control device 10 calculates a deviation between a position signal and target value given from the radial position sensors 6xand 6yand performs feedback control to reduce the deviation to zero, thereby returning the rotary shaft 9 to the predetermined position. That is, the magnetic bearing control device 10 has a control section 11 to perform predetermined operation, such as PID, on the deviation between the position signal and target value given from the radial position sensors 6xand 6yand generates a control signal to control the current amplifier section 12xand 12y,thereby varying a control current as an output thereof. Thereupon, the magnetic fields generated by the eight control coils 5a-5hvary. This varies the electromagnetic force by the +Y-axis composite-type electromagnet section 1a,−Y-axis composite-type electromagnet section 1b,+X-axis composite-type electromagnet 1cand −X-axis composite-type electromagnet 1d,thus returning the rotary shaft 9 to the predetermined position.

[0034] Hereunder, the operation of the radial magnetic bearing according to the invention will be explained in further detail with reference to FIG. 6 to FIG. 9. As discussed above, closed circuits are respectively formed in the direction of arrows through the +Y-axis composite-type electromagnet section 1a,−Y-axis composite-type electromagnet section 1b,+X-axis composite-type electromagnet 1c,and −X-axis composite-type electromagnet section 1dby the biasing permanent magnets thereof. Consequently, the rotary shaft 9 undergoes a magnetic attracting force due to biasing magnetic flux in equal amounts in four directions of +X axis, −X axis, +Y axis and −Y axis, being rotatably supported in a predetermined radial position.

[0035] It is herein assumed that disturbance occurs and the rotor displaces to move the rotary shaft 9 toward −Y. The Y-axis position sensor 6ydetects a displacement of the rotary shaft 9 and inputs a detection signal to the control section 11 of the magnetic bearing control device 10. The control section 11 compares the detected signal from the radial position sensor 6ywith a Y-axis direction target value signal to calculate a deviation signal, performs predetermined operation processing such as PID on the deviation signal to output a control signal, and supplies a control current through the Y-axis current amplifier section 12yto the control coils 5aand 5bof the +Y-axis composite-type electromagnet section 1aand the control coils 5eand 5fof the −Y-axis composite-type electromagnet section 1c.Thereupon, as shown in FIG. 6, a closed-circuit magnetic path with a control magnetic flux Φ1 is formed including the protruding poles 3aand 3band the target 8. Further, a closed-circuit magnetic path with a control magnetic flux Φ3 is formed including the protruding poles 3eand 3fand the target 8. The control magnetic flux Φ1 is in the same direction as the direction of a biasing magnetic flux Φ. Consequently, the two magnetic fluxes are added together to thereby intensify the magnetic attracting force by the +Y-axis composite-type electromagnet section 1a.On the other hand, because the control magnetic flux Φ3 is in a direction reverse to a direction of the biasing magnetic flux Φ, the two magnetic fluxes are subtracted to thereby reduce the magnetic attracting force by the −Y-axis composite-type electromagnet section 1c.Due to this, the rotary shaft 9 immediately is returned to the predetermined position in the Y-axis direction and held in that position.

[0036] In the case that the rotary shaft 9 shifts toward +Y, the Y-axis current amplifier section 12ysupplies a control current to the control coils 5aand 5bof the +Y-axis composite-type electromagnet section 1aas well as the control coils 5eand 5fof the −Y-axis composite-type electromagnet section 1c,in the direction reverse to the control current supplied upon shifting the rotary shaft 9 toward −Y. Thereupon, as shown in FIG. 7, a closed-circuit magnetic path with a control magnetic flux Φ1 is formed including the protruding poles 3aand 3band the target 8. Further, a closed-circuit magnetic path with a control magnetic flux Φ3 is formed including the protruding poles 3eand 3fand the target 8. The control magnetic flux Φ1 is in the direction reverse to the direction of a biasing magnetic flux φ. Consequently, the two magnetic fluxes are subtracted to thereby reduce the magnetic attracting force by the +Y-axis composite-type electromagnet section 1a.On the other hand, because the control magnetic flux Φ3 is in the same direction as the direction of the biasing magnetic flux φ, the two magnetic fluxes are added together to thereby intensify the magnetic attracting force by the −Y-axis composite-type electromagnet section 1c.Due to this, the rotary shaft 9 is immediately returned to the predetermined position in the Y-axis direction and held in that position.

[0037] Assuming that the rotary shaft 9 shifts toward −X, the X-axis position sensor 6xdetects a displacement of the rotary shaft 9 and inputs the detected signal to the control section 11 of the magnetic bearing control device 10. The control section 11 compares the detected signal from the radial position sensor 6xwith a Z-axis direction target value signal to calculate a deviation signal, performs predetermined operation processing such as PID on the deviation signal to output a control signal, and supplies a control current through the X-axis current amplifier section 12xto the control coils 5cand 5dof the +X-axis composite-type electromagnet section 1band the control coils 5gand 5hof the −X-axis composite-type electromagnet section 1d.Thereupon, as shown in FIG. 8, a closed-circuit magnetic path with a control magnetic flux Φ2 is formed including the protruding poles 3cand 3dand the target 8. Further, a closed-circuit magnetic path with a control magnetic flux Φ4 is formed including the protruding poles 3gand 3hand the target 8. The control magnetic flux Φ2 is in the same direction as the direction of a biasing magnetic flux Φ. Consequently, the two magnetic fluxes are added together to thereby intensify the magnetic attracting force by the +X-axis composite-type electromagnet section 1b.On the other hand, because the control magnetic flux Φ4 is in the direction reverse to the direction of the biasing magnetic flux Φ, the two magnetic fluxes are subtracted to thereby reduce the magnetic attracting force by the −X-axis composite-type electromagnet section 1d.Due to this, the rotary shaft 9 is immediately returned to the predetermined position in the X-axis direction and held in that position.

[0038] In the case that the rotary shaft 9 shifts toward +X, the X-axis current amplifier section 12xsupplies a control current to the control coils 5cand 5dof the +X-axis composite-type electromagnet section 1bas well as the control coils 5gand 5hof the −X-axis composite-type electromagnet section 1d,in the direction reverse to the control current supplied upon shifting the rotary shaft toward −X. Thereupon, as shown in FIG. 9, a closed-circuit magnetic path with a control magnetic flux Φ2 is formed including the protruding poles 3cand 3dand the target 8. Further, a closed-circuit magnetic path with a control magnetic flux Φ4 is formed including the protruding poles 3gand 3hand the target 8. The control magnetic flux Φ2 is in the direction reverse to the direction of a biasing magnetic flux φ. Consequently, the two magnetic fluxes are subtracted to thereby reduce the magnetic attracting force by the +X-axis composite-type electromagnet section 1b.On the other hand, because the control magnetic flux Φ4 is in the same direction as the direction of the biasing magnetic flux φ, the two magnetic fluxes are added together to thereby intensify the magnetic attracting force by the −X-axis composite-type electromagnet section 1d.Due to this, the rotary shaft 9 is immediately returned to the predetermined position in the Y-axis direction and held in that position.

[0039] In the above, the invention was explained on its structure and operation by exemplifying the composite-type electromagnet for a radial magnetic bearing of FIG. 1. The invention is applicable not only, of course, to a composite-type electromagnet for a conventional radial magnetic bearing having a permanent magnet to provide a biasing magnetic field but also to the other structure of a composite-type electromagnet comprising an electromagnet having a control coil wound around a magnetic-material electromagnet core and a biasing permanent magnet having a junction to the electromagnet core. Also, the form of electromagnet cores 2a-2dand biasing permanent magnets 4a-4dconstituting the composite-type electromagnet applied to the radial magnetic shown in FIG. 1 bearing is not limited to those shown in FIG. 1 to FIG. 3, but may be in other form provided that an annular body is formed arranged, at an equal interval, with two protruding poles on each of four horse-hoof composite-type electromagnets, i.e. totally eight protruding poles on which control coils are wound.

[0040] The present invention is characterized by, in a composite-type electromagnet comprising an electromagnet having a controlling coil wound around an electromagnet core of a magnetic material and a biasing permanent magnet having a junction between the electromagnet core and the permanent magnet, disposing in the junction a filling material made of a material having a high permeability and a high saturation magnetic flux density. Accordingly, it can be realized to minimize the leak of magnetic flux occurring at the junction, not to reduce the amount of magnetic flux available, and to prevent adverse effects upon an external member, apparatus or the like due to leak magnetic flux.

[0041] Also, the radial magnetic bearing according to the present invention has a composite-type electromagnet to provide a controlling magnetic field by a controlling electromagnet and a biasing magnetic field by a biasing permanent magnet to have a filling material having high permeability and a high saturation magnetic flux density disposed in a junction between the biasing permanent magnet and an electromagnet core of the controlling electromagnet. Accordingly, there is no possibility that magnetic flux leak outside through the junction. Consequently, the biasing magnetic flux and the controlling magnetic flux are all effectively utilized, enhancing bearing rigidity. Moreover, further structural simplification and size and weight reduction were achieved as compared to the conventional radial magnetic bearing having a biasing electromagnet.