GB2593539A - Magnetic damper, method of damping and turbomlecular pump - Google Patents

Magnetic damper, method of damping and turbomlecular pump Download PDF

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Publication number
GB2593539A
GB2593539A GB2004522.5A GB202004522A GB2593539A GB 2593539 A GB2593539 A GB 2593539A GB 202004522 A GB202004522 A GB 202004522A GB 2593539 A GB2593539 A GB 2593539A
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GB
United Kingdom
Prior art keywords
rotor
magnetic
stator
directing
magnetic flux
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
GB2004522.5A
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GB202004522D0 (en
Inventor
Dudley Brewster Barrie
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Edwards Ltd
Original Assignee
Edwards Ltd
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Publication date
Application filed by Edwards Ltd filed Critical Edwards Ltd
Priority to GB2004522.5A priority Critical patent/GB2593539A/en
Publication of GB202004522D0 publication Critical patent/GB202004522D0/en
Publication of GB2593539A publication Critical patent/GB2593539A/en
Pending legal-status Critical Current

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
    • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
    • F04D19/00—Axial-flow pumps
    • F04D19/02—Multi-stage pumps
    • F04D19/04—Multi-stage pumps specially adapted to the production of a high vacuum, e.g. molecular pumps
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
    • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
    • F04D19/00—Axial-flow pumps
    • F04D19/02—Multi-stage pumps
    • F04D19/04—Multi-stage pumps specially adapted to the production of a high vacuum, e.g. molecular pumps
    • F04D19/048—Multi-stage pumps specially adapted to the production of a high vacuum, e.g. molecular pumps comprising magnetic bearings
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
    • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
    • F04D29/00—Details, component parts, or accessories
    • F04D29/66—Combating cavitation, whirls, noise, vibration or the like; Balancing
    • F04D29/661—Combating cavitation, whirls, noise, vibration or the like; Balancing especially adapted for elastic fluid pumps
    • F04D29/668—Combating cavitation, whirls, noise, vibration or the like; Balancing especially adapted for elastic fluid pumps damping or preventing mechanical vibrations
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16F—SPRINGS; SHOCK-ABSORBERS; MEANS FOR DAMPING VIBRATION
    • F16F15/00—Suppression of vibrations in systems; Means or arrangements for avoiding or reducing out-of-balance forces, e.g. due to motion
    • F16F15/02—Suppression of vibrations of non-rotating, e.g. reciprocating systems; Suppression of vibrations of rotating systems by use of members not moving with the rotating systems
    • F16F15/03—Suppression of vibrations of non-rotating, e.g. reciprocating systems; Suppression of vibrations of rotating systems by use of members not moving with the rotating systems using magnetic or electromagnetic means
    • F16F15/035—Suppression of vibrations of non-rotating, e.g. reciprocating systems; Suppression of vibrations of rotating systems by use of members not moving with the rotating systems using magnetic or electromagnetic means by use of eddy or induced-current damping
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
    • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
    • F04D19/00—Axial-flow pumps
    • F04D19/02—Multi-stage pumps
    • F04D19/04—Multi-stage pumps specially adapted to the production of a high vacuum, e.g. molecular pumps
    • F04D19/042—Turbomolecular vacuum pumps
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05D—INDEXING SCHEME FOR ASPECTS RELATING TO NON-POSITIVE-DISPLACEMENT MACHINES OR ENGINES, GAS-TURBINES OR JET-PROPULSION PLANTS
    • F05D2260/00—Function
    • F05D2260/90—Braking
    • F05D2260/903—Braking using electrical or magnetic forces
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
    • F16C32/00—Bearings not otherwise provided for
    • F16C32/04—Bearings not otherwise provided for using magnetic or electric supporting means
    • F16C32/0406—Magnetic bearings
    • F16C32/0408—Passive magnetic bearings
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
    • F16C32/00—Bearings not otherwise provided for
    • F16C32/04—Bearings not otherwise provided for using magnetic or electric supporting means
    • F16C32/0406—Magnetic bearings
    • F16C32/0408—Passive magnetic bearings
    • F16C32/041—Passive magnetic bearings with permanent magnets on one part attracting the other part
    • F16C32/0412—Passive magnetic bearings with permanent magnets on one part attracting the other part for radial load mainly
    • F16C32/0414—Passive magnetic bearings with permanent magnets on one part attracting the other part for radial load mainly with facing axial projections
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16F—SPRINGS; SHOCK-ABSORBERS; MEANS FOR DAMPING VIBRATION
    • F16F2222/00—Special physical effects, e.g. nature of damping effects
    • F16F2222/06—Magnetic or electromagnetic

Landscapes

  • Engineering & Computer Science (AREA)
  • General Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Acoustics & Sound (AREA)
  • Aviation & Aerospace Engineering (AREA)
  • Magnetic Bearings And Hydrostatic Bearings (AREA)
  • Non-Positive Displacement Air Blowers (AREA)
  • Structures Of Non-Positive Displacement Pumps (AREA)

Abstract

Magnetic damper 5 for dampening radial movement of a rotor shaft 9, the damper having a magnetic flux-conductor 16 mounted to the rotor 9, and a magnet 30 and an electrical conductor ring 50 mounted to the stator. The stator may also have magnetic flux-conductors 40 for directing the magnetic field from the magnet to the conductor rings 50, which may be copper. As the rotor 9 moves radially, eddy current is generated on the conductor rings 50, dissipating the kinetic energy, dampening the variation. The magnet 30 may be an electromagnet. The magnetic flux conductors 16, 40 may be formed of a soft magnetic material with higher magnetic permeability and a lower electrical conductance than the conductor rings 50, having a relative magnetic permeability above 100. The amount of damping may be controlled by adjusting the electrical current in the electromagnet 30. Other aspects include a turbomolecular pump having said damper, and a method of damping.

Description

MAGNETIC DAMPER, METHOD OF DAMPING AND TURBOMLECULAR
PUMP
FIELD OF THE INVENTION
The field of the invention relates to a method of magnetic damping and in particular, to an eddy current magnetic damper for damping radial movement of a rotor of a pump mounted to rotate within a stator and to a turbomolecular pump comprising such a damper.
BACKGROUND
In the field of pumps, and in particular high-speed pumps mounted on passive magnetic bearings, effective radial damping of the rotor is an important requirement.
Eddy current damping is a generally a preferred damping mechanism for such pumps. However, for eddy current damping to provide a stabilising effect at frequencies below the rotational frequency of the pump the damping energy should be dissipated in the stator and not the rotor. Eddy current dampers have therefore generally been configured by attaching rotating magnets to the rotor and a copper plate for the conduction of the eddy currents and the corresponding dissipation of the energy to the stator.
However, a drawback of this is that rotating magnets are subject to centrifugal stress particularly in high speed machines and therefore need to be manufactured to a high precision. They also represent a parasitic mass which may be undesirable for rotordynamic reasons.
It would be desirable to provide an improved eddy current damper for damping radial movement of the rotor of a pump. -2 -
SUMMARY
A first aspect provides an eddy current magnetic damper for damping radial movement of a rotor of a pump mounted to rotate within a stator, said magnetic damper comprising: a magnet mounted on said stator; said rotor; and comprising at least one magnetic flux-directing rotor structure configured to extend towards a portion of said stator; an electrical conductor fixed to said stator and arranged between said portion of said stator and said at least one magnetic flux-directing rotor structure, such that flux flowing from said magnet through said at least one magnetic flux-directing rotor structure passes through said electrical conductor and radial movement of said magnetic damper relative to said stator generates eddy currents within said electrical conductor.
The inventor of the present invention recognised the disadvantages of mounting a magnet on a rotor and realised that a similar damping effect could be provided by using a stationary magnet on the stator and directing the flux from the magnet through the rotor using a magnetic flux-directing structure. Arranging the stationary magnet and flux-directing structure such that magnetic flux from the magnet is directed to flow from the stator through the flux-directing structure on the rotor, means that radial movement of the rotor will change the flux running between the rotor and stator. Providing an electrical conductor within the flux path between the rotor and stator but attached to the stator, means that eddy currents will be generated within the conductor in response to this radial rotor movement. The eddy currents generated in this way provide the desired damping effect to the radial movement that generated them.
In this way the rotor is provided with a magnetic flux-directing structure rather than a magnet, which structure is generally cheaper, easier to manufacture, more robust and often lighter than the magnet would be. The magnet on the stator can be manufactured to lower tolerances than would be required were the magnet to be mounted on the rotor and with an appropriately arranged flux-directing structure substantial damping can be achieved. -3 -
In some embodiments, the magnetic damper further comprises a magnetic flux-directing stator structure configured to channel flux from said magnet towards said at least one magnetic flux-directing rotor structure, said electrical conductor being arranged between said magnetic flux-directing stator structure and said magnetic flux-directing rotor structure.
Although in some embodiments, the magnet on the stator may be arranged adjacent to the flux-directing portion of the rotor such that the electrical conductor may be mounted on or at least adjacent to the magnet and no further flux-structure is required on the stator, in other embodiments the magnet may be more remote from the rotor. Allowing the magnet to be provided at a more remote location allows for a greater flexibility in the design and in particular in the form of the magnetic damper. It may also make manufacture simpler and can still provide effective damping provided that the magnetic flux can be effectively channelled from the magnet to the magnetic flux-directing portion of the rotor. This can be done using a magnetic flux-directing stator structure arranged to direct flux from the magnet towards the magnetic flux-directing rotor structure. A magnetic flux-directing structure is one with a relatively high magnetic permeability compared to the materials around it, so that magnetic flux will preferentially travel through the structure. In an embodiment with a magnetic flux-directing stator structure the electrical conductor will be located in the gap between the stator and rotor flux-directing structures, such that the magnetic flux will flow through the electrical conductor from the stator to the rotor.
In some embodiments, at least a portion of said magnetic flux-directing rotor structure and said magnetic flux-directing stator structure are configured to be separated by an axial gap, said electrical conductor being located within said axial gap.
In order to provide effective radial damping, it may be preferable to have the electrical conductor in an axial gap between the rotor and stator flux-directing structures, as radial movement will generally provide increased flux variations -4 -and therefore effective eddy current generation in this configuration. However, in other embodiments, where for example, there may be considerable axial movement of the rotor then it may be preferable to have the electrical conductor within a radial gap (see figure 8). With this arrangement the size of the gap will change with radial movement and it is this movement that generates changes in magnetic reluctance and eddy currents are generated in response to this.
It should be noted that in this context the axial direction is the direction of the axis of rotation of the rotor, while the radial direction is perpendicular to and passing io through the axis of rotation.
In some embodiments, said rotor comprised a rotor damping structure (10) extending radially from a shaft of said rotor, said magnetic flux-directing rotor structure extending from said rotor damping structure towards said portion of said stator; wherein said rotor damping structure is configured such that a magnetic reluctance of said rotor damping structure and a gap between said rotor damping structure and said stator varies with radial position.
In order to provide increased eddy currents, the magnetic flux should be concentrated within a portion of the rotor damping structure and should change significantly between the structure and any gap, such that radial movement of this portion of the structure provides significant changes in the magnetic flux. Thus, the rotor may comprise a rotor damping structure configured such that reluctance changes radially and the magnetic flux is concentrated in a portion of the rotor comprising the rotor magnetic flux-directing structure and the gap adjacent to this where magnetic reluctance is lower. The electrical conductor may be located within the gap at this point and the magnetic flux is thus, concentrated here and radial movement changes the magnetic flux flowing through the conductor generating eddy currents. Other parts have a higher reluctance and thus, the magnetic flux travelling through those parts is reduced. -5 -
In some embodiments, said rotor damping structure is configured such that a thickness of said rotor damping structure varies abruptly with radial position such that a gap between said rotor damping structure and said stator varies with radial position.
Although the change in reluctance with radial position may be provided by forming the flux-directing rotor structure of different materials with different magnetic permeabilities, in some embodiments, it is provided by forming the damping structure with different thicknesses along its radial length. In some embodiments, the thickness varies in a step wise manner, such that there may be one or more thicker portions that are provided by the magnetic flux rotor directing structures and which form a portion that extends close to the electrical conductor and provides a reduced gap and reduced magnetic reluctance between the rotor and stator, thereby concentrating the flux in this area.
In some embodiments, said rotor damping structure comprises a plurality of magnetic flux-directing rotor structures extending towards said stator arranged at different radial position on said rotor damping structure.
In some embodiments, the rotor damping structure has an indented cross section, that is it has a plurality of portions having different thicknesses along its radial length. These thicknesses vary abruptly in a stepwise manner. These radial variations in thickness are provided by provided by a number of magnetic flux-directing rotor structures extending axially out from the rotor damping structure towards the conducting plate on the stator. In effect, the rotor configured in this way provides a plurality of salient pole pieces that are configured to extend close to said electrical conductor.
In some embodiments, where said electrical conductor is mounted on the magnet, the rotor may comprise two rotor damping structures each comprising at least one magnetic flux-directing rotor structure and each extending on either -6 -side of said magnet such that each of said magnetic flux-directing rotor structures extend close to respective poles of said magnet.
In some embodiments, said magnetic flux-directing stator structure is configured to extend on either axial side of said at least a portion of said magnetic flux-directing rotor structure, said magnetic damper comprising two electrical conductors located between said magnetic flux stator directing structure and each axial side of said magnetic flux-directing rotor structure.
The magnetic flux forms a loop from one pole of the magnet back to the other, thus, a magnetic flux-directing structure that directs the flux from one pole through the rotor portion and back to the other pole, may be an effective structure for channelling the flux through the rotor damping portion and this structure allows the provision of two conducting plates and thus, increased eddy current generation.
In some embodiments, said magnetic flux-directing structure is formed of a soft magnetic material with a higher magnetic permeability and a lower electrical conductance than said electrical conductor.
In some embodiments, the magnetic flux-directing structure is formed of a material with a relative permeability of more than 100 and the electrical conductor is formed of a material with a resistivity of less than 7 X 10-8 Om.
The flux-directing structures should be formed of a soft magnetic material operable to channel flux and yet not become permanently magnetised to any great degree. Different types of iron are effective materials for this although any material with a relative magnetic permeability above 100 is effective as one of the flux-directing structures. The electrical conductor must allow eddy currents to flow and thus have a high conductivity -copper is a good material for this. -7 -
In some embodiments, said magnetic flux-directing rotor and stator structures are each formed of a material with a relative magnetic permeability above 100, preferably above 140.
Although the two flux-directing structures may be formed of different materials, in some embodiments they are formed of the same material.
In some embodiments said magnet is substantially ring shaped.
The magnet should extend around the rotor and in particular, around the flux-directing rotor structure and thus it may conveniently be a ring magnet, although other arrangements where one or more magnets substantially surround the rotor structure are effective.
In some embodiments the electrical conductor comprises a disc-shaped plate.
Where for example, there are stator magnetic flux-directing portions that extend around either axial side of the rotor, then the electrical conductor may conveniently take the form of a disc shaped plate mounted on the stator and in particular, on the magnetic flux-directing portion of the stator. In other embodiments, the electrical conductor may take the form of a coating or plating on the stator or on the magnet on the stator.
In some embodiments, said electrical conductor is formed of a material with an electrical resistivity below 7 X 10-8 Om.
The electrical conductor should preferably have a low resistivity and may in some embodiments, comprise copper.
In some embodiments, said magnet comprises a permanent magnet while in others said magnet comprises an electromagnet. -8 -
A permanent magnet will work effectively and may be lower cost than an electromagnet, however, an electromagnet may offer potential for further control of the system.
In some embodiments, said magnet comprises an electromagnet, said magnetic damper further comprising control circuitry configure to control an amount of damping by controlling electrical current in said electromagnet.
A second aspect provides a turbomolecular pump comprising: a rotor rotatably io mounted within a stator on at least one passive magnetic bearing, said turbomolecular pump further comprising a magnetic damper according to a first aspect.
Although this magnetic damper is effective for damping any rotor in a rotating machine, it is particularly applicable to vacuum pumps and particularly to turbomolecular pumps. These pumps rotate at high speeds and often have rotors mounted on passive magnetic bearings, which bearings are not effectively damped. Thus, a magnetic damping system may be particular effective for such a pump.
A third aspect provides a method of radially damping a rotor of a turbomolecular pump, said method comprising: mounting a magnet on said stator; mounting a magnetic flux-directing rotor structure on said rotor such that it extends towards said a portion of said stator; fixing an electrical conductor to said stator such that said electrical conductor is located between said portion of said stator and said magnetic flux-directing rotor structure, such that flux from said magnet flowing through said magnetic flux-directing rotor structure passes through said electrical conductor and radial movement of said magnetic flux-directing rotor structure relative to said stator generates eddy currents within said electrical conductor.
Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be -9 -combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.
Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 shows a magnetic eddy current damper according to a first embodiment; Figure 2 schematically shows the magnetic flux lines running through the magnetic eddy current damper of Figure 1; Figure 3 shows the magnetic field variation with radial distance through the upper and lower copper plates of the magnetic eddy current damper of Figure 1; Figure 4 shows a further embodiment of a magnetic eddy current damper; Figure 5 shows a magnetic eddy current damper with an internal stator; Figure 6 shows a further embodiment of a magnetic eddy current damper; Figure 7 shows an embodiment of a magnetic eddy current damper comprising an electromagnet; Figure 8 shows an embodiment, where the gap between rotor damper and conducting plate is an axial gap; Figure 9 shows a turbomolecular pump according to an embodiment; and Figure 10 schematically shows a flow diagram illustrating steps in a method of providing magnetic damping to a rotor according to an embodiment.
DESCRIPTION OF THE EMBODIMENTS
Before discussing the embodiments in any more detail, first an overview will be provided.
Effective radial damping is critical to the performance of a turbomolecular pump that has a rotor mounted on one or more passive magnetic bearings. For many -10 -reasons eddy current damping is preferred to mechanical damping such as that using tuned mass dampers with elastomer springs. However, for eddy current damping to be effective at damping fluctuations with a frequency below the frequency of rotation of the rotor, the eddy current damping must dissipate energy in the stator rather than in the rotor. Thus, a conventional eddy current damper for a pump has involved mounting magnet rings to the rotor and a copper plate to the stator. However, rotating magnets are subject to centrifugal stress, must be manufacture to high precision and also represent a significant parasitic mass.
Embodiments address this by providing an eddy current damper where the magnet and conductive plate are mounted on the stator and a flux-directing rotor structure or salient pole is provided on the rotor. This extends towards the conductive plate, such that the flux from the magnet on the stator flows through the flux-directing structure and through the conductive plate. Thus, any radial movement of the rotor causes variations in this magnetic flux flowing through the flux-directing structure and conductive plate and generates eddy currents within the plate which act to damp the radial movement.
Thus, embodiments provide an eddy current damper that is particularly suitable for a single active-axis magnetic bearing turbomolecular pump. Construction of the damper simple and is able to be done with low cost materials and iron rotor parts.
Figure 1 schematically shows a magnetic eddy current damper 5 according to an embodiment. Magnetic eddy current damper 5 comprises a rotor damping structure 10 extending from the rotor shaft towards a magnet 30 attached to the stator. Magnet 30 comprises magnetic flux-directing formations 40 extending on either side of the magnet towards the rotor damping structure 10. These magnetic flux-directing portions 40 concentrate the magnetic flux from the magnet 30 and direct it towards the rotor damping structure 10. Rotor damping structure 10 is formed of a disc comprising a shoulder portion 11 which attaches to a shaft of the rotor, a narrower protruding portion 12 and a flux-directing portion 16 which extends towards the flux-directing portions 40 of the stator and which owing to its high permeability and lower reluctance, tends to concentrate the flux from magnet 30 within this portion of the rotor protrusion. In effect portion 16 forms a salient pole for the rotor.
In this embodiment there are two copper plates which take the form of discs 50 which are attached to the stator and are located in the air gap between the stator and the salient pole 16 portion of the rotor damping structure 10. Copper is used in this embodiment owing to its high electrical conductivity, although other conductive materials may also be used.
Although copper has a high electrical conductivity it has a low magnetic permeability that is similar to the permeability of air. However owing to the proximity of the salient pole portion 16 of the rotor, magnetic flux is concentrated from the flux-directing structure 40 through the copper plate 50 towards salient pole 16 such that any radial movement of the rotor will bend the flux lines flowing through the copper plate 50 into salient pole 16 and generate eddy currents which will act to damp such movement.
Figure 2 shows schematically the magnetic flux lines with the structure of Figure 1. This figure illustrates how the magnetic flux from magnet 30 is concentrated within the areas of higher magnetic permeability. It should be noted that the flux-directing structures within the stator or rotor are preferably formed of some soft magnetic material such as iron which material is not permanently magnetised but does have a high permeability to a magnetic field, such that magnetic flux lines will preferentially flow through this material and can thus, be directed through the conductive plate 50.
Figure 3 shows the changes in flux density with radius in both the upper and lower copper discs 50 of figures 1 and 2. As can be seen there is a large change in magnetic field within the copper disc close to the edges of the salient pole 16.
-12 -It is this high gradient in the magnetic flux that is particularly effective at generating eddy currents with the radial movement of this portion. Thus, an arrangement where a protruding portion from the rotor provides low reluctance for some of the radial distance and high reluctance for others provides an effective shape for generating eddy currents. In this embodiment, this step change in reluctance is provided by the form of the salient pole 16, that extends far closer to the conductive plate 50 than other portions of the rotor damping structure 10. In other embodiments, the step change may be provided by forming the damping structure 10 of materials of a different permeability.
Figure 4 shows an alternative embodiment, where there are multiple salient poles 16 on the rotor damping structure 10. In this embodiment the axis of rotation 8 and the rotor shaft 9 are shown with the rotor damping structure 10 extending radially from the rotor shaft 9. In this embodiment, magnet 30 is again a ring type magnet extending around the rotor shaft 9 and comprising magnetic flux-directing portions 40 for directing the magnetic flux from the ring magnet 30 towards either side of the rotor damping structure 10. In this embodiment, the rotor damping structure 10 has two salient poles 16 which each concentrate magnetic flux to different radial portions of the copper discs 50. Thus, radial movement of the rotor shaft 9 and correspondingly of the portion 10 and the salient poles 16 will generate eddy currents at multiple points within the copper plates50 and act to dampen the radial movement.
Figure 5 shows an alternative embodiment where the rotor is the outer portion of the pump and the stator is within the rotating rotor. Thus, in this embodiment the ring magnet 30 is attached to the static shaft of the stator. Rotor 60 extends around the shaft of the stator and comprises two rotor damping structures 10 extending inwardly therefrom towards ring magnet 30. One of the rotor damping structures 10 is close to one pole of the magnet and the other to the other pole.
In this embodiment there are again two flux-directing structures or salient poles 16 extending from each rotor damping structure 10 towards the ring magnet. In this embodiment, there is no requirement for magnetic flux-directing structures on -13 -the stator as the salient poles extend towards conductive plate 50 mounted on the magnet 30 itself. Thus, in this embodiment the longer magnetic flux path is through the rotor rather than the stator.
Copper plates 50 are mounted on the poles of the magnet and thus, flux from the magnet is directed through the copper plates 50 towards the salient poles 16 of the rotor. Radial movement of the rotor will be damped by eddy currents generated within the copper plates 50. Although the plates 50 are referred to as copper plates and copper is a preferred material for these plates, they can be formed of other materials such as a copper alloy or aluminium.
Figure 6 shows a further embodiment where again the flux-directing structure for the stator is not required and the longer flux path extends through the rotor. In this embodiment, the rotor is again within the stator and has a central rotating shaft 9 and the rotor damping structure is formed of two discs 10 which extend on either side of the magnet 30 such that magnetic flux from each pole of magnet 30 is directed to the two discs 10 through two conductive plates 50. The electrically conductive plates 50 are again mounted directly on either pole of the magnet 30.
Figure 7 shows a further example embodiment analogous to that of Figure 4, but with the permanent magnet 30 of figure 4 being replaced by an electromagnet 31 comprising coils configured to carry electrical current and generate a controllable magnetic field, control of the current flowing through the coils varying the strength of the magnetic field and thus, the amount of damping.
Figure 8 schematically shows an alternative arrangement where the conducting plates 50 are arranged in a radial gap between the stator and the rotor. This allows for greater axial movement of the rotor without clashing of the damper components. Radial movement will cause the gaps 52 to change in size as can be seen from the Figure. This change in gap size results in a change in magnetic reluctance and eddy currents being generated and damping of the radial movement. In this embodiment some axial damping is also provided as axial -14 -movement causes the salient poles 16 to move relative to the copper plates 50 generating eddy currents at multiple points within the copper plates 50 which eddy currents provide damping of the axial movement.
Figure 9 schematically shows a turbomolecular pump according to an embodiment. Turbomolecular pump 70 comprises a rotor shaft 9 which extends into pumping chamber 68,which pumping chamber houses rotor blades 82 and stator blades 84. The rotor is mounted on radial bearings 74 and axial bearings 76, which include an active axial magnetic bearing 76and two passive radial magnetic bearings 74. There is a motor 72 for driving the rotor and there may be control circuitry (not shown) associated with the motor and active bearing for controlling both the motor and the active bearing. There is also two magnetic eddy current dampers 90 according to an embodiment, mounted on the rotor shaft to provide radial damping of the rotor.
Figure 10 schematically shows steps in a method for providing a rotor with a magnetic eddy current damping means. The steps of the method may be performed in any order and involve mounting a magnet on a static part or stator of the pump. Mounting a rotor damping structure such that it extends radially from the shaft of the rotor, the rotor damping structure comprising at least one magnetic flux-directing rotor structure which extends towards a portion of the stator. Mounting a conductive plate on the portion of the stator between the magnetic flux-directing rotor structure and the stator such that flux from the magnet flowing through the magnetic flux-directing rotor structure passes through the electrical conductive plate and radial movement of the magnetic flux-directing rotor structure relative to the stator generates eddy currents within the electrical conductive plate and in this way dissipates energy and provides the desired radial damping effect. This method may be a method performed in the manufacture of a turbopump, or it may be a method performed in the retrofitting of a magnetic damper to an existing turbopump.
-15 -Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.
-16 -
REFERENCE SIGNS
eddy current dumper 8 axis of rotation 9 rotor shaft 10 rotor damping structure 11 shoulder for attachment to rotor shaft 12 extension connecting shoulder to salient poles 16 rotor flux-directing structure or salient poles ring magnet in 31 electromagnet stator flux-directing structure conductive plate 52 gap rotor 68 pumping chamber turbomolecular pump 72 motor 74 radial bearings 76 axial bearings 82 rotor blades 84 stator blades magnetic eddy current damper

Claims (15)

  1. -17 -CLAIMS1. An eddy current magnetic damper for damping radial movement of a rotor of a pump mounted to rotate within a stator, said magnetic damper comprising: a magnet mounted on said stator; said rotor comprising at least one magnetic flux-directing rotor structure configured to extend towards a portion of said stator; an electrical conductor fixed to said stator and arranged between said portion of said stator and said at least one magnetic flux-directing rotor structure io such that flux flowing from said magnet through said at least one magnetic flux-directing rotor structure passes through said electrical conductor and radial movement of said magnetic damper relative to said stator generates eddy currents within said electrical conductor.
  2. 2. A magnetic damper according to claim 1, further comprising a magnetic flux-directing stator structure configured to channel flux from said magnet towards said at least one magnetic flux-directing rotor structure, said electrical conductor being arranged between said magnetic flux-directing stator structure and said at least one magnetic flux-directing rotor structure.
  3. 3. A magnetic damper according to claim 1, wherein at least a portion of said at least one magnetic flux-directing rotor structure and said magnetic flux-directing stator structure are configured to be separated by an axial gap, said electrical conductor being located within said axial gap.
  4. 4. A magnetic damper according to any preceding claim, comprising a rotor damping structure extending radially from a shaft of said rotor, said magnetic flux-directing rotor structure extending from said rotor damping structure towards said portion of said stator; wherein said rotor damping structure is configured such that a magnetic reluctance of said rotor damping structure and a gap between said rotor damping structure and said stator varies with radial position.
  5. -18 - 5. A magnetic damper according to claim 4, wherein said rotor damping structure is configured such that a thickness of said rotor damping structure varies abruptly with radial position such that a gap between said rotor damping structure and said stator varies with radial position.
  6. 6. A magnetic damper according to claim 5, wherein said rotor damping structure comprise a plurality of magnetic flux-directing rotor structures extending towards said stator arranged at different radial position on said rotor damping io structure.
  7. 7. A magnetic damper according to any preceding claim, wherein said magnetic flux-directing stator structure is configured to extend on either axial side of said at least one magnetic flux-directing rotor structure, said magnetic damper comprising two electrical conductors fixed to said stator and located between said magnetic flux stator directing structure and each axial side of said at least one magnetic flux-directing rotor structure.
  8. 8. A magnetic damper according to any preceding claim, wherein said magnetic flux-directing structure is formed of a soft magnetic material with a higher magnetic permeability and a lower electrical conductance than said electrical conductor.
  9. 9. A magnetic damper according to any preceding claim, wherein said magnetic flux-directing rotor and stator structures are each formed of a material with a relative magnetic permeability above 100.
  10. 10. A magnetic damper according to any preceding claim, wherein said magnet is substantially ring shaped.
  11. -19 - 11. A magnetic damper according to any preceding claim, wherein said electrical conductor is formed of a material with an electrical resistivity below 7 X10-8 Om.
  12. 12. A magnetic damper according to any preceding claim, wherein said magnet comprises an electromagnet, said magnetic damper further comprising control circuitry configure to control an amount of damping by controlling electrical current in said electromagnet.io
  13. 13. A magnetic damper according to any preceding claim, wherein said said pump comprises a turbomolecular pump.
  14. 14. A turbomolecular pump comprising: a rotor rotatably mounted within a stator on at least one passive magnetic bearing, said turbomolecular pump further comprising a magnetic damper according to any preceding claim.
  15. 15. A method of providing radial damping to a rotor of a turbomolecular pump, said method comprising: mounting a magnet on said stator; mounting a magnetic flux-directing rotor structure on said rotor such that it extends towards a portion of said stator; fixing an electrical conductor to said stator such that said electrical conductor is arranged between said portion of said stator and said magnetic flux-directing rotor structure, such that flux from said magnet flowing through said magnetic flux-directing rotor structure passes through said electrical conductor and radial movement of said magnetic flux-directing rotor structure relative to said stator generates eddy currents within said electrical conductor.
GB2004522.5A 2020-03-27 2020-03-27 Magnetic damper, method of damping and turbomlecular pump Pending GB2593539A (en)

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GB2004522.5A GB2593539A (en) 2020-03-27 2020-03-27 Magnetic damper, method of damping and turbomlecular pump

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GB2593539A true GB2593539A (en) 2021-09-29

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2621342A (en) * 2022-08-09 2024-02-14 Leybold Gmbh Eddy current damper and vacuum pump

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1471843A (en) * 1974-02-08 1977-04-27 Hitachi Ltd Arrangement of rotar stator and eddy current damper
JPS541764A (en) * 1977-06-03 1979-01-08 Hitachi Ltd Eddy current system vibration controller
JPS59217030A (en) * 1983-05-24 1984-12-07 Toshiba Corp Suppressing device for vibration of rotary body

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1471843A (en) * 1974-02-08 1977-04-27 Hitachi Ltd Arrangement of rotar stator and eddy current damper
JPS541764A (en) * 1977-06-03 1979-01-08 Hitachi Ltd Eddy current system vibration controller
JPS59217030A (en) * 1983-05-24 1984-12-07 Toshiba Corp Suppressing device for vibration of rotary body

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2621342A (en) * 2022-08-09 2024-02-14 Leybold Gmbh Eddy current damper and vacuum pump

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