The present invention relates to a magnetic bearing device comprising a stator and a rotor supported in the stator for rotation around a rotation axis.
Active magnetic bearings are often used to support moving parts without physical contact. If the moving part is a rotor supported for rotation around a rotation axis, it is known to control both radial and axial displacements of the rotor by active magnetic bearings.
In a commonly employed design, an active radial magnetic bearing comprises a magnetically soft stator core having structures that extend radially inwards, commonly called “teeth”. Each stator tooth forms an air gap with the rotor, which also is magnetically soft. Stator windings are provided on the stator teeth or between adjacent stator teeth. A current in a stator winding causes a magnetic flux through the stator core, the associated stator teeth and the rotor, which gives rise to an attractive reluctance force between the stator and the rotor. The region between adjacent teeth is commonly called a “slot”, and therefore this design type is often called a “slotted” design.
This traditional design is not optimally suited for rotors that have small diameter (e.g., a diameter below a few centimeters). With small-diameter rotors, the length of the air gap between the stator teeth and the rotor becomes comparatively large in relation to the rotor diameter, leading to a comparatively high magnetic reluctance. As a consequence, comparatively large windings are required for generating sufficient bearing forces. This adds to both cost and size.
It is therefore desirable to provide a magnetic bearing device for controlling at least radial displacements of a rotor that can be constructed in a compact and cost-efficient manner, making it particularly well-suited for small rotors.
The publication C. Zwyssig et al., “High-Speed Magnetically Levitated Reaction Wheel Demonstrator”, Proceedings of the 2014 International Power Electronics Conference—ECCE Asia (IPEC 2014), Hiroshima, Japan, pp. 1707-1714, May 2014, discloses a dual hetero-/homopolar slotless self-bearing motor. In the heteropolar part, the rotor comprises diametrically magnetized permanent magnets. Motor windings and bearing windings are placed in an air gap between the rotor and a slotless stator core. In the homopolar part, the rotor comprises two axially-magnetized permanent magnets with opposite polarities. The corresponding stator portion is ironless. It comprises two axial bearing windings, each axial bearing winding being wound azimuthally around the rotor axis and being placed in a location where it sees mainly a radial magnetic field, and a plurality of radial bearing windings, the radial bearing windings being axially arranged between the axial bearing windings. The turns of the radial bearing windings extend essentially on a cylindrical surface.
The same design is also disclosed in M. Kaufmann et al., “High-Speed Magnetically Levitated Reaction Wheels for Small Satellites”, Proceedings of the 23rd International Symposium on Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM 2016), Anacapri, Italy, pp. 28-33, June 2016, and in A. Tüysüz et al., “A 300′000-r/min Magnetically Levitated Reaction Wheel Demonstrator”, IEEE Transactions on Industrial Electronics, Vol. 66, No. 8, pp. 6404-6407, August 2019.
This design requires considerable space, and the bearing forces that can be generated in the homopolar part are rather limited.
The publication T. Baumgartner, J. W. Kolar, “Multivariable State Feedback Control of a 500 000-r/min Self-Bearing Permanent-Magnet Motor”, IEEE/ASME Transactions on Mechatronics, Vol. 20, No. 3, pp. 1149-1159, June 2015 discloses a self-bearing motor. The rotor comprises three permanent magnets: an axially magnetized permanent magnet arranged between two diametrically magnetized permanent magnets. An axial bearing winding surrounds the axially magnetized permanent magnet, and axial bearing forces are generated by interaction with this magnet. Motor windings and bearing windings are placed in an air gap between the diametrically magnetized permanent magnets and a slotless magnetic stator core. Radial bearing forces are caused by interaction with the diametrically magnetized permanent magnets. Also in this design, the turns of the radial bearing windings extend essentially on a cylindrical surface.
This design is relatively complex.
The publication D. Steinert et al., “Slotless Bearingless Disk Drive for High-Speed and High-Purity Applications”, IEEE Transactions on Industrial Electronics, Vol. 61, No. 11, pp. 5974-5986, November 2014 discloses a bearingless motor with toroidally wound windings on a ring-shaped, slotless magnetic stator core. The rotor consists of a heteropolar, diametrically magnetized, ring-shaped magnet. Stability of the rotor against axial and tilt displacements is given by passive reluctance forces, and no axial bearing windings are provided. A bearingless motor with a heteropolar ring-shaped rotor, defining one or two pole pairs, is also disclosed in D. Steinert et al., “Evaluation of One- and Two-Pole-Pair Slotless Bearingless Motors with Toroidal Windings”, IEEE Transactions on Industry Applications, Vol. 52, No. 1, pp. 172-180, January/February 2016.
Applicability of this design is limited to extremely short, ring-shaped rotors.
It is an object of the present invention to provide a magnetic bearing device for controlling at least radial displacements of a rotor that can be constructed in a compact and cost-efficient manner, making it particularly well-suited for small rotors.
This object is achieved by a magnetic bearing device having the features of claim 1. Further embodiments of the invention are laid down in the dependent claims.
Accordingly, a magnetic bearing device is provided, comprising:
By providing the at least one radial bearing winding in a toroidal configuration on a closed magnetic core in combination with a rotor comprising an axially magnetized permanent magnet, a very compact design can be achieved, while large radial bearing forces can be obtained.
The core preferably comprises or consists of a magnetically soft material. The closed magnetic core will be generally ring-shaped. Advantageously, the closed magnetic core does not have any teeth that extend inward from a closed ring portion of the core toward the rotor, and accordingly it does not have any voids or “slots” between the teeth. Such a magnetic core can accordingly be considered “slotless”.
In particular, the closed magnetic core can have the shape of a toroid. A toroid is a surface of revolution with a hole in the middle, like a doughnut. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its edges, then a rectangle-section toroid is produced. A closed magnetic core in the shape of a toroid is called a toroidal magnetic core. While it is advantageous if the closed magnetic core has exactly toroidal shape, the closed magnetic core can deviate from an exact toroidal shape and can, for instance, have the shape of a toroidal polyhedron.
In the context of the present disclosure, a winding is to be understood to have “toroidal configuration” if the winding extends around the ring formed by the closed magnetic core, each of its turns passing through the central hole of the ring and returning at the outside of the ring. A winding of toroidal configuration does not need to and generally will not extend along the entire circumference of the closed magnetic core with respect to its axis of revolution. In fact, the winding may extend along only a small fraction of said circumference. In the following, the terms “winding having toroidal configuration” and “toroidal winding” will be used as synonyms.
Preferably at least two radial bearing windings are provided on a common closed magnetic core, each of these windings having a toroidal configuration and being arranged to interact with a permanent magnetic field generated by the at least one axially magnetized permanent magnet. It is then possible to supply currents to the at least two radial bearing windings in such a manner that the magnetic fluxes generated by each of said currents are oriented along different circumferential directions inside the closed magnetic core. This is advantageous for maximizing the obtainable radial forces. In particular, it is advantageous to provide at least one pair of radial bearing windings arranged on diametrically opposite sides of the rotor.
In some embodiments, the magnetic bearing device comprises first and second magnetic guide rings, each of the first and second magnetic guide rings surrounding the rotor, the first magnetic guide ring being arranged on a first axial side of the at least one radial bearing winding, and the second magnetic guide ring being arranged on a second axial side of the at least one radial bearing winding opposite to the first axial side. The first magnetic guide ring can be configured to guide the permanent magnetic field between a first axial end portion of the at least one permanent magnet and a first axial end portion of the closed magnetic core, and the second magnetic guide ring can be configured to guide the permanent magnetic field between a second axial end portion of the at least one permanent magnet and a second axial end portion of the closed magnetic core. Like the closed magnetic core, the magnetic guide rings advantageously comprise or consist of a magnetically soft material. Each magnetic guide ring preferably has toroidal shape, in particular, the shape of a flat circular disk with a central hole. Each magnetic guide ring preferably has an outer diameter that is at least as large as the outer diameter of the closed magnetic core. For constructive reasons, it can be preferable that the outer diameter of the magnetic guide ring is at least as large as the maximum outer diameter of the radial bearing windings. The first and second guide rings are preferably part of the stator.
In some embodiments, the rotor comprises at least one magnetic guide disk arranged axially adjacent to the at least one permanent magnet, the at least one magnetic guide disk being configured to concentrate the permanent magnetic field in a radial gap region between the rotor and the closed magnetic core. Each magnetic guide disk advantageously comprises or consists of a magnetically soft material. The purpose of the at least one magnetic guide disk is the following: By having a toroidal configuration, the at least one radial bearing winding comprises a plurality of axial conductor portions (i.e., conductor portions that extend essentially parallel to the rotation axis or, equivalently, to the central axis of the closed magnetic core, or that extend at least in a direction having a large axial component) arranged in said radial gap region. The at least one magnetic guide disk will then act to concentrate the permanent magnetic field in a region where these inner axial conductor portions are disposed, thus helping to maximize the obtainable radial forces.
In some advantageous embodiments, the rotor comprises first and second permanent magnets that are magnetized along the rotation axis with opposite polarities. In this manner, the permanent magnetic field will have a particularly high radial component around the axial location where the first and second permanent magnets meet, enabling the generation of particularly strong radial bearing forces.
In such embodiments, it is advantageous if the rotor comprises a magnetic guide disk that is arranged axially between the first and second permanent magnets, preferably immediately adjacent to both permanent magnets. The magnetic guide disk will then act to further concentrate the permanent magnetic field in the radial gap between the rotor and the closed magnetic core in a region where the radial component of the permanent magnetic field is already particularly high, thus further aiding in the generation of strong radial bearing forces. Again, the magnetic guide disk preferably comprises or consists of a magnetically soft material.
In some embodiments, the stator comprises first and second closed magnetic cores that surround the rotor at different axial locations along the rotation axis, at least one first radial bearing winding that is arranged in a toroidal configuration around the first closed magnetic core, and at least one second radial bearing winding that is arranged in a toroidal configuration around the second closed magnetic core. Both the first and second radial bearing windings are arranged to interact with the permanent magnetic field generated by the at least one axially magnetized permanent magnet to obtain a radial bearing force when current is supplied to the first and second radial bearing windings. In this case, additional magnetic guide rings at the axial ends of the radial bearing windings can readily be dispensed with, since the closed magnetic cores will generally already guide the permanent magnetic field to a sufficient extent.
This design is particularly advantageous if the at least one axially magnetized permanent magnet has only one single polarity along the rotation axis, i.e., if its polarity does not change along the rotation axis. In particular, the rotor can comprise only a single, axially magnetized permanent magnet.
If the at least one permanent magnet has only a single polarity along the rotation axis, it is particularly advantageous if the rotor comprises first and second magnetic guide disks arranged at axially opposite ends of the at least one permanent magnet. The first magnetic guide disk can then be configured to concentrate the permanent magnetic field in a radial gap region between the rotor and the first closed magnetic core, and the second magnetic guide disk can be configured to concentrate the permanent magnetic field in a radial gap region between the rotor and the second closed magnetic core. Again, these guide disks preferably comprise or consist of a magnetically soft material.
In some embodiments, the magnetic bearing device further comprises at least one axial bearing winding wound azimuthally around the rotor, the at least one axial bearing winding being arranged to interact with the permanent magnetic field generated by the at least one axially magnetized permanent magnet to obtain an axial bearing force when current is supplied to the at least one axial bearing winding. In some embodiments, the magnetic bearing device comprises two axial bearing windings arranged on axially opposite sides of the at least one radial bearing winding. In other embodiments, the magnetic bearing device comprises at least one axial bearing winding that is arranged in a radial gap between the rotor and the closed magnetic core, in particular, between the rotor and the at least one radial bearing winding. It is of course also possible to combine these two possibilities.
In some embodiments, the magnetic bearing device comprises at least one radial displacement sensor for detecting radial displacements of the rotor, and a controller configured to receive signals from the at least one radial displacement sensor and to generate radial control signals for the at least one radial bearing winding so as to manipulate a radial position of the rotor. In particular, the controller can be configured to stabilize the rotor in a desired radial positon.
The magnetic bearing device can further comprise at least one axial displacement sensor for detecting axial displacements of the rotor, and the controller can be further configured to receive signals from the at least one axial displacement sensor and to generate axial control signals for at least one axial bearing winding so as to manipulate an axial position of the rotor, in particular, to stabilize the rotor in a desired axial position.
Generally the magnetic bearing device will comprise more than one radial bearing winding, in particular, two, three, four, six or more radial bearing windings distributed over the circumference of the at least one closed magnetic core. It is then advantageous to connect the radial bearing windings as follows: If the magnetic bearing device comprises at least one pair of radial bearing windings arranged on diametrically opposite sides of the rotor in a toroidal configuration on a common closed magnetic core, it is advantageous to connect the radial bearing windings of the pair in an anti-series or anti-parallel configuration. If the magnetic bearing device comprises at least three radial bearing windings distributed along the circumference of a common closed magnetic core in a toroidal configuration, it is advantageous to connect the radial bearing windings in a star configuration or in a delta configuration. Both possibilities can be combined if at least six radial bearing windings are present, e.g., if three pairs of two diametrically opposed radial bearing windings are present. In this case, it is advantageous to connect the windings of each pair in an anti-series or antiparallel configuration and to connect the pairs in a star or delta configuration.
Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,
In
A rotor 10 comprises a hollow tube 11 made of a non-magnetic material like austenitic steel, titanium, aluminum or fiber-reinforced plastics. In order to minimize losses by eddy currents, it may be advantageous to manufacture hollow tube 11 from a material having high electrical resistivity. Inside the hollow tube 11, a first permanent magnet 21 and a second permanent magnet 22 are held. Both permanent magnets 21, 22 are axially magnetized. They have opposite polarities. Axially between and immediately adjacent to the permanent magnets 21, 22, a magnetic guide disk 23 is held. The magnetic guide disk 23 is made of a magnetically soft material, i.e., of a ferro- or ferrimagnetic material having high magnetic permeability, low coercivity (e.g., below 100 A/m) and low hysteresis. Suitable magnetically soft materials are well known in the art. Examples include supermalloy, permalloy, electrical steel and so-called soft ferrites.
A stator 30 surrounds the rotor 10. The stator 30 comprises a ring-shaped closed magnetic core 31 made of a magnetically soft material, the magnetic core 31 surrounding the rotor 10. Six radial bearing windings 32 in toroidal configuration are provided on the magnetic core 31. The radial bearing windings 32 are illustrated in
First and second axial bearing windings 41, 42 are provided on both axial sides of the radial bearing windings 32. Each axial bearing winding 41, 42 extends azimuthally around the rotor 10 and is arranged coaxially with the ring-shaped magnetic core 31. Also the axial bearing windings 41, 42 are shown in a highly simplified manner only. In particular, it is to be understood that each of the axial bearing windings 41, 42 can have an arbitrary number of turns.
Axially adjacent to each axial bearing winding 41, 42, on the axial side that faces away from the radial bearing windings 32, a magnetic guide ring 51, 52 made of a magnetically soft material is arranged. Each magnetic guide ring 51, 52 extends around the rotor 10, having the shape of a flat disk with a central hole through which the rotor 10 passes. A cylindrical magnetic guide sleeve 55 made of a magnetically soft material surrounds the radial bearing windings 32, the axial bearing windings 41, 42, and the magnetic guide rings 51, 52, touching the magnetic guide rings 51, 52 at their respective outer circumference. The magnetic guide rings 51, 52 and the magnetic guide sleeve 55 together form a housing 50 of the magnetic bearing device.
A plurality of radial displacement sensors 61 is disposed at regular intervals around the circumference of the rotor in a region outside the housing 50. The hollow tube 11 of the rotor 10 is closed at one end with an end plate 12. An axial displacement sensor 62 is arranged on the rotation axis R, facing the target plate 12. The radial and axial displacement sensors 61, 62 can be, for instance, eddy current sensors as they are well known in the art. Electrically conducting targets for the displacement sensors can be provided on the outer circumference of hollow tube 11 and on end plate 12.
A controller 70 receives displacement signals from the radial and axial displacement sensors 61, 62 and derives control signals for the radial and axial bearing windings 32, 41, 42, based on the displacement signals. The controller 70 further comprises power amplifiers for driving the radial and axial bearing windings 32, 41, 42.
The magnetic bearing device is operated to maintain rotor 10 in a desired position without contact to the stator. In the present example, the desired position is the position shown in
The magnetic field distribution inside the magnetic bearing device will now be explained with reference to
The oppositely magnetized permanent magnets 21, 22 generate a homopolar permanent magnetic field, i.e., a magnetic field that is rotationally symmetric around the rotation axis R assuming ideal isotropic conditions in the permanent magnets 21, 22 and a fully rotationally symmetric rotor 10 and stator 30. The magnetic field gives rise to a magnetic flux. The magnetic flux is concentrated and deflected from the axial into the radial direction by magnetic guide disk 23. The magnetic flux exits magnetic guide disk 23 essentially along the radial direction and crosses the radial gap between the rotor 10 and the ring-shaped magnetic core 31. In this region, i.e., on the inside of ring-shaped magnetic core 31, the flux density B has a large radial component. The flux then enters the magnetic core 31 and is deflected inside the magnetic core 31 essentially into opposite axial directions. Near the axial ends of magnetic core 31, the magnetic flux exits magnetic core 31 and returns into the permanent magnets 21, 22, passing through the magnetic guide rings 51, 52. Together with the cylindrical guide sleeve 55, they essentially confine the magnetic field to the inside of housing 50.
Radial bearing forces are generated as follows. As apparent from
Axial bearing forces primarily result from the interaction of currents in the axial bearing windings 41, 42 with the radial component of the magnetic flux density at the locations of these windings. A net axial bearing force can be readily generated by supplying currents of identical magnitude and identical directions to axial bearing windings 41, 42.
Numerical simulations have shown that the following relative proportions between the various parts of the magnetic bearing device are advantageous:
These dimensional considerations are not only applicable to the first embodiment illustrated in
A second embodiment of a magnetic bearing device is illustrated in
A third embodiment of a magnetic bearing device is illustrated in
A fourth embodiment of a magnetic bearing device is illustrated in
A fifth embodiment of a magnetic bearing device is illustrated in
Radial forces are generated in a similar manner as described above for the first embodiment. For instance, radial bearing windings 32A, 32B arranged on the same ring-shaped magnetic core 31A, 31B, but on diametrically opposite radial sides of the rotor 10 are operated with currents that have the same magnitude, but opposite directions, as in the first embodiment. Likewise, radial bearing windings 32A, 32B arranged on the same radial side of the rotor, but on different ring-shaped magnetic cores 31A, 31B are operated with currents that have the same magnitude, but opposite directions.
It should be noted that the return current in each turn of the radial bearing windings 32A, 32B, which flows on the outside of the ring-shaped cores 31A, 31B, gives rise to an oppositely oriented force, which however acts between the cores 31A and 31B, thus does not result in a force on the rotor.
Also axial forces are generated in a similar manner as described above for the first embodiment, except that currents are supplied to the axial bearing windings 43A, 43B with equal magnitude, but opposite directions.
In a variant, the axial bearing windings can also be provided axially adjacent to the radial bearing windings, as in the first embodiment.
Possible arrangements of toroidal bearing windings on a ring-shaped magnetic core and possible connections between the toroidal bearing windings will now be explained with reference to
As explained above, in order to create radial bearing forces along the X direction, bearing windings L1 and L3 should be supplied with currents of equal magnitude, but opposite directions: i1+i3=0. Likewise, in order to create radial bearing forces along the Y direction, bearing windings L2 and L4 should be supplied with currents of equal magnitude, but opposite directions: i2+i4=0.
These requirements can be readily fulfilled by connecting the toroidal bearing windings on diametrically opposite sides of the rotor either in anti-series or anti-parallel configuration. Such a configuration at the same time ensures that the integral of the flux density inside the ring-shaped magnetic core along the circumferential direction is always zero, i.e., there is no net flux along the ring-shaped magnetic core. Such a net flux would be undesirable because it would contribute to magnetic saturation of the ring-shaped core without contributing to the radial bearing forces.
A connection in anti-series configuration is illustrated in
The concept of connecting pairs of toroidal bearing windings on diametrically opposite sides of the rotor in anti-series or anti-parallel configuration can be readily generalized to situations with different numbers of toroidal bearing windings.
In order to create radial bearing forces along the X direction, currents of equal magnitude, but opposite directions can be supplied to bearing windings L1 and L4. However, also currents of equal magnitude and equal direction supplied to bearing windings L2 and L6 and currents of the same magnitude, but the opposite direction supplied to bearing windings L3 and L5 will give rise to a net bearing force along the X direction. In order to create radial bearing forces along the Y direction, currents of equal magnitude and equal direction are supplied to bearing windings L2 and L3, while currents of equal magnitude and the opposite direction are supplied to bearing windings L5 and L6. In the same spirit, radial bearing forces can be created along any arbitrary lateral direction in the XY plane
A convenient way of connecting bearing windings L1 to L6 is illustrated in
As illustrated in
From the above description it will be readily apparent to a person skilled in the art that many modifications are possible without departing from the scope of the present invention. In particular, it is possible to employ other numbers of radial bearing windings than in the above examples. Instead of mounting the permanent magnet(s) and the magnetic guide disk(s) in a hollow tube, it is also possible to mount these elements on a common central rod. The magnetic guide sleeve can be left away in any of the present examples. The radial and axial displacement sensors can be configured differently than described and can be located in different positions than shown. Many other modifications are possible.
Number | Date | Country | Kind |
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19195639.0 | Sep 2019 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2020/073591 | 8/24/2020 | WO | 00 |