Magnetic Bearing

Magnetic bearing

GAP SENSOR ELECTRONICS

GAP

CONTROLLER

ROTOR

SENSORS

POWER AMPLIFIERS

A magnetic bearing Basic operation for a single axis 

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Design

An acti active ve magn magnet etic ic bear bearin ing g work workss on the the prin princi ci-ple ple of elec electromagneti tromagneticc suspen suspension sion   and consis consists ts of an A magnetic bearing  is a bearing a  bearing that  that supports a load uselectromagnet assembly, electromagnet  assembly, a set of power amplifiers which ing magnetic ing magnetic levitation. levitation. Magnetic bearings support movsuppl supply y current current to the electr electromag omagnets nets,, a controller controller,, and and gap ing parts without physical contact. For instance, they are sensors with associated electronics to provide the feedable to levitate a rotating shaft and permit relative motion back required to control the position of the rotor within with very low friction and no mechanical mechanical wear. Magnetic Magnetic the gap. The power power amplifie amplifierr supplies supplies equal bias curbearin bearings gs suppo support rt the high highes estt speed speedss of all kinds kinds of bearin bearing g rent to two pairs of electromagnets on opposite sides of and have no maximum relative speed. a rotor. This constant tug-of-war is mediated by the conPassive magnetic bearings use permanent magnets and, troller, which offsets the bias current by equal and oppotherefore, therefore, do not require any input power power but are difficult site perturbations of current as the rotor deviates deviates from its to design due to the limitations described by  Earnshaw’s center position. theorem.. Tech theorem Techniq niques ues using using   diamagnetic   materials materials are The gap sensors are usually inductive in nature and sense relatively undeveloped and strongly depend on material in a differe differentia ntiall mode. The power power amplifiers amplifiers in a modcharacteristics. characteristics. As a result, most magnetic bearings bearings are ern commercial application are solid state devices which active magnetic bearings, using electromagnets using  electromagnets which  which reoperate in a pulse a  pulse width modulation configuration. configuration. The quire quire conti continuo nuous us powe powerr input input and an acti active ve con contro troll syste system m controll controller er is usually usually a microprocessor or dig digital ital sig signal nal proto keep the load stable. In a combined combined design, permanent permanent cessor.. cessor magnets are often used to carry the static load and the active active magnetic bearing is used when the levitated levitated object object Active bearings have several advantages: they do not sufdeviates deviates from its optimum position. position. Magnetic bearings bearings fer fer from from wear, wear, have have low fric friction tion,, and can often often accommo accommo-typically require a back-up bearing in the case of power date irregularities in the mass distribution automatically, or control system failure. allowing rotors to spin around their center their  center of mass with mass  with very low vibration. Magnetic bearings are used in several industrial applications such as electrical power generation, petroleum re- Two types of instabilities are typically present in magfinemen fine ment, t, machine machine tool operatio operation n and natural gas han- netic bearings. Attractive Attractive magnets produce an unstable [1] dling. They are also used in the Zippe-type the  Zippe-type centrifuge, centrifuge, stati staticc force orce that that decre decrease asess with with incr increas easing ing distan distance ce and infor uranium for  uranium enrichment  enrichment   and in  turbomolecular pumps, pumps, creases at decreasing distances. This can cause the bearwhere oil-lubricated bearings would be a source of con- ing to become become unbalanc unbalanced. ed. Secondly Secondly,, because because magnetis magnetism m tamination. is a conservative a  conservative force, force, it provides provides little damping; oscilla1

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APPLIC APPLICA ATION TIONS  S 

tions may cause loss of successful successful suspension if any driving forces are present.

field field capacity capacity.. The motor-com motor-compre pressor ssor design design was done done by Siemens and the active magnetic bearings were delivered by  by   Waukesha Bearings   (owned by  Dover Corporation). Corporation). (Originally these bearings were designed by Glacier, this company was later taken over by Federal Mogul and is 2 Hi Hisstor tory now part of Waukes Waukesha ha Bearings.) By using active magnetic bearings and a direct drive between motor and comThe table below lists several early patents for active mag- pressor (without the gearbox in between) and applying netic bearings. Earlier patents for magnetic suspe suspensions nsions dry gas seals, a full dry-dry (oil free) system has been inApplying active magnetic magnetic bearings in the driver can be found but are excluded here because they consist stalled. Applying and in the compressor (compared to the traditional conof assemblies of permanent magnets of problematic stafigura figuratio tion n with with a gearb gearbox, ox, plain plain beari bearings ngs and a gastu gasturb rbin ineebility per Earnshaw’s per Earnshaw’s Theorem. Theorem. driver) results in a relatively simple system with a very Jessee Beam Jess Beamss from from the Uni Unive versi rsity ty of Virg Virginia inia filed filed some some of wide operating range and high efficiencies (particularly the earliest active magnetic bearing patents   [2][3] during at partial load). As done in the Groningen field, field, the full World War II. The patents dealt with ultracentrifuges with  ultracentrifuges in in- installation can additionally be placed outdoors without tended tended for the enric enrichm hment ent of isotop isotopes es of elem elemen ents ts neede needed d the need for a large compressor building. for for the  the   Manhattan Pro Project ject.. Howe Howeve ver, r, magne magnetic tic bearbear[8] ings did not mature until advances in solid-state electron- Meeks pioneered hybrid magnetic bearing designs (US which  permanent magnets provide magnets  provide ics and modern computer-based control technology with patent 5,111,102) in which permanent the work of Habermann[4] and Schweitzer.[5] In 1987, the bias field and active control coils are used for stabilEstelle Croot further improved active magnetic bearing ity and dynamic control. These designs using permanent technology, [6] but these designs were not manufactured magnets for bias fields are smaller and of lighter weight purely electr electromag omagneti neticc bearings. bearings. The electroni electronicc due to expensive costs of production, which used a laser than purely guidanc guidancee system. system. Estelle Estelle Croot’s research research was the subsub- control system is also smaller and requires less electrical ject ject of 3 Australian Australian patents patents and was funded funded by Nachi Nachi power because the bias field is provided by the permasuccessful applications applications of Fujikoshi, Nippon Seiko KK and Hitachi, and her cal- nent magnets. One of the most successful this technology was by Danfoss by  Danfoss, , a manufacturer of refrigculations were used in other technologies that used rare permanent magnet bias magearth magnets but the active magnetic bearings were only erator turbopumps. Using permanent netic bearings allows the compressors to operate at over developed to the prototype stage. 40,000 40,000 rpm instead of the traditiona traditionall 3,600 rpm. The Kasarda[7] reviews the history of active magnetic bear- Danfoss compressors are an order of magnitude smaller ings ings in depth depth.. She notes notes that that the first first comme commerc rcia iall appli applicaca- and lighter in weight than the lower speed compressors tion of active magnetic bearings was in  in   turbomachinery. turbomachinery. that they replace. replace. Thousands of compressors compressors using this The active magnetic bearing allowed the elimination of advanced magnetic bearing technology have been manuoil reservoirs on compressors for the NOVA Gas Trans- factured factured by Danfoss in Florida. mission mission Ltd. (NGTL) (NGTL) gas pipelines in pipelines  in Alberta, Canada. This reduced the fire hazard allowing a substantial re- As the development of the necessary components progressed, scienti scientific fic interes interestt in the field field also increased, increased, duction duction in insurance insurance costs. costs. The success success of these mag- gressed, peaking in the first International Symposium on Magnetic bearing installations led NGTL to pioneer the research search and devel developme opment nt of a digital digital magnetic magnetic bearing bearing netic Bearings held in 1988 in Zurich with the foundcontr control ol syste system m as a repla replace ceme ment nt for the analo analog g contr control ol syssys- ing of the International Society of Magnetic Bearings by Prof. Schwe Schweitze itzerr (ETHZ ( ETHZ), ), Prof. Allaire Allaire (Unive (Universi rsity ty of tems supplied by the American company Magnetic Magnetic Bear- Prof. Virginia), and a nd Prof. Okada (Ibaraki (Ib araki Universi Uni versity). ty). Since ings Inc. Inc. In 1992, NGTL’s magnetic magnetic bearing bearing research research then, the symposium has developed into a biennial congroup formed the company Revolve Technologies Inc. ference series with a permanent portal on magnetic magnetic bearfor for commerc commercial ializin izing g the digital digital magnetic magnetic bearing bearing technoltechnol- ference technology y where where all symposi symposium um contribu contribution tionss are ogy. The company was later later purchased by SKF by SKF of  of Swe- ing technolog made available. avail able. The web portal is supported by the interden. The French French company company S2M  S2M,, founded in 1976, was research and industrial community. community. Joining the the first to commercially market active magnetic bear- national research ings. Extensive research on magnetic bearings continues hall of fame and earning lifetime achievement awards in were Prof. Prof. Yoh Yohji Okada, Okada, Prof. Prof. Gerhard Gerhard Schwe Schweitze itzer, r, at the University the  University of Virginia in Virginia  in the Rotating Machinery 2012 were and Michael Swann of Waukesha Magnetic Bearings . and Controls Industrial Research Program . Starting from 1996 the Dutch oil and gas company NAM insta install lled ed over over a period period of 10 years years 20 large large E-moto E-motorr Appl Appliicati cation onss driven (with variable speed drive) gas compressors of 3 23 MW fully equipped with active magnetic bearings on both the E-motor E-motor and the compresso compressor. r. These These comprescompres- Magnetic bearing advantages include very low and presors are used in the Groningen gas field to extract the re- dictable friction, and the ability to run without lubricamaining gas from this large gas field and to increase the tion and in a vacuum. vacuum. Magnetic Magnetic bearing bearingss are increasincreas-

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ingly used in industrial machines such as compressors, turbines, pumps, motors and generators. Magnetic bearings are commonly used in watt-hour in  watt-hour meters by elec electri tricc utili utilitie tiess to meas measure ure home home powe powerr cons consump ump-tion. tion. They They are also used in high-preci high-precisio sion n instrume instruments nts and to support equipment in a vacuum, for example in flywheel energy storage   syste systems ms.. A flywhee flywheell in a vacvacuum has very very low wind resista resistance nce losses, losses, but con conven vention tional al bearings usually fail quickly in a vacuum due to poor lubricatio brication. n. Magnetic Magnetic bearings bearings are also used to support support maglev magle v trains trains in  in order to get low noise and smooth ride by eliminating physical physical contact surfaces. surfaces. Disadvantages Disadvantages include high cost, heavy weight and relatively large size. A new application of magnetic bearings is in artificial hearts. The use of magnetic suspension in ventricular assist sist devi device cess was was pione pioneer ered ed by Prof. Prof. Paul Paul Allair Allairee and Prof. Prof. Houston Wood at the University University of Virginia, Virginia, culminating culminating in the first magnetically magnetically suspended ventricular assist centrifugal pump (VA (VAD D) in 1999.

operate and works because the electrical currents generated by motion cause a restoring force.

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See also •

  Flywheel

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  Levitron

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Spin stabilized magnetic magnetic levitation

Ref Referen erence cess

[1] Charles, Charles, D., Spinning a Nuclear Comeback , Science, Vol. 315, (30 March 2007) [2] Beams, Beams, J. , Production and Use of High Centrifug Centrifugal al Fields , Science, Vol. 120, (1954)

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Futu Future re adva advanc nces es

[3] Beams, Beams, J. J. ,  Magnetic Bearings , Paper 810A, Automotive Engineering Conference, Detroit, Michigan, USA, SAE (Jan. 1964) [4] Habermann Habermann,H. ,H. , Liard, G.  Practical Magnetic Bearings  , IEEE Spectrum, Vol. 16, No. 9, (September 1979) [5] Schweitzer, Schweitzer, G. , Characteristics of a Magnetic Rotor Bearing for Active Vibration Control , Paper C239/76, First Internation ternational al Conf Conferen erence ce on Vibrati Vibrations ons in Rotating Rotating MachinMachinery, (1976) [6] Estelle Estelle Croot, Croot,  Australian Inventors Weekly, NSW Inventors Association, Vol. 3, (April 1987)

An axial homopolar electrodynamic bearing

With the use of an inducti induction-ba on-based sed levita levitation tion system system present in   maglev  technologies such as the   Inductrack system, magnetic bearings could replace complex control systems systems by using Halbach using  Halbach Arrays and Arrays  and simple closed loop coil coils. s. These These syste systems ms gain gain in simpl simplic icity ity,, but are less less adva advanntageous with regard to eddy to  eddy current los current  losses. ses. For rotating For rotating systems it systems  it is possible to use homopolar magnet designs instead instead of multipol multipolee Halbach Halbach structure structures, s, which which reduce reduce losses considerably. An example that has bypassed the Earnshaw’s theorem issues is the homopolar electrodynamic homopolar  electrodynamic bearing invented bearing  invented [9][10][11] by Dr Torbjörn Lembke. This is a novel type of electromagnetic bearing based on a passive magnetic technology. It does not require any control electronics to

[7] Kasar Kasarda, da, M.   An Overview Overview of Active Active Magnetic Magnetic Bearing Bearing Technology and Applications , The Shock and Vibration Dige Digest, st, Vol Vol.32 .32,, No. 2: A Publi Publicat catio ion n of the Shock Shock and Vibration Information Center, Naval Research Laboratory, (March 2000) [8] Meeks, C.R., “Magnetic “Magnetic Bearings - Optimum Design Design and Application”, Paper presented at the International Workshop on Rare Earth Cobalt Permanent Magnets, University of Dayton, Dayton, Ohio, October 14–17, 1974 [9]   “Design and Analysis of a Novel Low Loss Homopolar Electrodynamic Electrod ynamic Bearing.” Bearing.” Lembk  Lembke, e, Torbjörn. Torbjörn. PhD Thesis. Stockholm: Universitetsservice Universitetsservice US AB, 2005.  2005.  ISBN 91-7178-032-7 [10]  “3D-FEM Analysis of a Low Loss Homopolar Induction Bearing” Lembk Bearing”  Lembke, e, Torb Torbjörn. jörn. 9th Internatio International nal Symposium on Magnetic Bearings (ISMB9). Aug. 2004. [11]   Seminar Seminar at KTH – the Royal Institute Institute of Tec Technol hnology ogy Stockholm. Feb 24. 2010

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Furt Furthe herr read readin ing g •

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Schwe Schweitze itzer, r, G (2002). (2002). “Pro “P roc. c. 6th 6t h In Inte tern rnat at.. IFToMM Conf. on Rotor Dynamics”. Dynamics”. |chapter= ignored (help (help))   Chiba, A., Fukao, T., Ichikawa, O., Oshima, M., Takemoto, M., Dorrel, D. (2005).   Magnetic Bearings and Bearingless Drives . Newnes.  Schweitzer, G., Maslen, H. (2009).  Magnetic Bearings, ings, Theory, Theory, Design,a Design,and nd Applicati Application on to Rotating Rotating Machinery. Springer.   Jim Wilson (September 1999).   “Beating Demon Friction”.. Popular Mechanics. Friction”   Estelle Estelle Croot (1987–1995 (1987–1995). ). Improve Improved d Magnetic  IPAustr stral alia ia [Austra [Australi lian an Paten Patentt Office Office Bearings . IPAu database entries].  T. Lembke (2005).  PhD Thesis “Design and Analysis of a Novel Low Loss Homopolar Electrodynamic  Stockholm: Universi Universitetsservic tetsservicee US AB. Bearing” . Stockholm: ISBN 91-7178-032-7. 91-7178-032-7.

Exte Extern rnal al link linkss •

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Kinemati Kinem aticc Mod Model elss for De Desi sign gn Di Digi gital tal Lib Librar rary y (KMODDL) (KMODDL)  - Movies and photos of hundreds of working working mechani mechanicalcal-sys systems tems models models at Cornell Cornell UniUniversity. versity. Also includes includes an e-book an  e-book library of library  of classic texts on mechanical mechanical design and engineering. engineering.   MADYN2000, Rotordynami Rotordynamics cs Softw Software are   supports computercomputer-aid aided ed design design of Magnetic Magnetic Bearing Bearing controllers and provides multiple multiple analytic reports of design quality.

EXTE EXTERNA RNAL L LINKS  LINKS 

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