A magnetic bearing is a type of bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact. For instance, they are able to levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear. Magnetic bearings support the highest speeds of any kind of bearing and have no maximum relative speed.
Active bearings have several advantages: they do not suffer from wear, have low friction, and can often accommodate irregularities in the mass distribution automatically, allowing rotors to spin around their center of mass with very low vibration.
Passive magnetic bearings use permanent magnets and, therefore, do not require any input power but are difficult to design due to the limitations described by Earnshaw's theorem. Techniques using diamagnetic materials are relatively undeveloped and strongly depend on material characteristics. As a result, most magnetic bearings are active magnetic bearings, using electromagnets which require continuous power input and an active control system to keep the load stable. In a combined design, permanent magnets are often used to carry the static load and the active magnetic bearing is used when the levitated object deviates from its optimum position. Magnetic bearings typically require a back-up bearing in the case of power or control system failure.
Magnetic bearings are used in several industrial applications such as electrical power generation, petroleum refinement, machine tool operation and natural gas handling. They are also used in the Zippe-type centrifuge,Charles, D., Spinning a Nuclear Comeback, Science, Vol. 315, (30 March 2007) for uranium enrichment and in turbomolecular pumps, where oil-lubricated bearings would be a source of contamination.
Design
An active magnetic bearing works on the principle of electromagnetic suspension based on the induction of eddy currents in a rotating conductor. When an electrically conducting material is moving in a magnetic field, a current will be generated in the material that counters the change in the magnetic field (known as Lenz's Law). This generates a current that will result in a magnetic field that is oriented opposite to the one from the magnet. The electrically conducting material is thus acting as a magnetic mirror.Basore P. A., "Passive Stabilization of Flywheel Magnetic Bearings," Master’s thesis, Massachusetts Institute of Technology (USA), 1980.Murakami C. and Satoh I., “Experiments of a Very Simple Radial-Passive Magnetic Bearing Based on Eddy Currents”, In Proceedings of the 7th International Symposium on Magnetic Bearings, March 2000.Bender D. and Post R. F., “Ambient Temperature Passive Magnetic Bearings for Flywheel Energy Storage Systems”, In Proceedings of the 7th International Symposium on Magnetic Bearings, March 2000.Moser R., Regamey Y. J., Sandtner J., and Bleuler H., “Passive Diamagnetic Levitation for Flywheels”, In Proceedings of the 8th International Symposium on Magnetic Bearings, 2002.Filatov A. V., McMullen P., Davey K., and Thompson R., “Flywheel Energy Storage System with Homopolar Electrodynamic Magnetic Bearing”, In Proceedings of the 10th International Symposium on Magnetic Bearings, 2006.Sandtner J. and Bleuler H., “Electrodynamic Passive Magnetic Bearings with Planar Halbach Arrays”, In Proceedings of the 9th International Symposium on Magnetic Bearings, August 2004.Sandtner J. and Bleuler H., “Passive Electrodynamic Magnetic Thrust Bearing Especially Designed for Constant Speed Applications ”, In Proceedings of the 10th International Symposium on Magnetic Bearings, August 2004.Amati N., De Lépine X., and Tonoli A., “Modeling of electrodynamic Bearings”, ASME Journal of Vibration and Acoustics, 130, 2008.Kluyskens V., Dehez B., “Dynamical electromechanical model for passive magnetic bearings”, IEEE Transactions on Magnetics, 43, pp 3287-3292, 2007.Kluyskens V., Dehez B., “Parameterized electromechanical model for magnetic bearings with induced currents”, Journal of System Design and Dynamics - Special Issue on the Eleventh International Symposium on Magnetic Bearings, 2009.
The hardware consists of an electromagnet assembly, a set of power amplifiers which supply current to the electromagnets, a controller, and gap sensors with associated electronics to provide the feedback required to control the position of the rotor within the gap. The power amplifier supplies equal bias current to two pairs of electromagnets on opposite sides of a rotor. This constant tug-of-war is mediated by the controller, which offsets the bias current by equal and opposite perturbations of current as the rotor deviates from its center position.
The gap sensors are usually inductive in nature and sense in a differential mode. The power amplifiers in a modern commercial application are solid state devices which operate in a pulse-width modulation configuration. The controller is usually a microprocessor or digital signal processor.
Two types of instabilities are typically present in magnetic bearings. Attractive magnets produce an unstable static force that decreases with increasing distance and increases at decreasing distances. This can cause the bearing to become unbalanced. Secondly, because magnetism is a conservative force, it provides little damping; oscillations may cause loss of successful suspension if any driving forces are present.
History The table below lists several early patents for active magnetic bearings. Earlier patents for magnetic suspensions can be found but are excluded here because they consist of assemblies of permanent magnets of problematic stability per Earnshaw's Theorem.
+ Early U.S. patents in active magnetic bearings Inventor(s) Year Patent number Title Beams, Holmes 1941 2,256,937 Suspension of Rotatable Bodies Beams 1954 2,691,306 Magnetically Supported Rotating Bodies Gilbert 1955 2,946,930 Magnetic suspension Beams 1962 3,041,482 Apparatus for Rotating Freely Suspended Bodies Beams 1965 3,196,694 Magnetic Suspension System Wolf 1967 3,316,032 Poly-Phase Magnetic Suspension Transformer Boden et al. 1968 DE1750602 Magnetische Lagerung (German patent) Lyman 1971 3,565,495 Magnetic Suspension Apparatus Habermann 1973 3,731,984 Magnetic Bearing Block Device for Supporting a Vertical Shaft Adapted for Rotating at High Speed Habermann, Loyen, Joli, Aubert 1974 3,787,100 Devices Including Rotating Members Supported by Magnetic Bearings Habermann, Brunet 1977 4,012,083 Magnetic Bearings Habermann, Brunet, LeClére 1978 4,114,960 Radial Displacement Detector Device for a Magnetic Bearings Croot, Estelle 1990 1,988,024,350 Further Improvements in Magnetic Bearings Meeks, Crawford R 1992 5,111,102 Magnetic Bearing Structure Croot, Estelle 1994 1,991,075,982 Non-linear Magnetic Bearing
Jesse Beams from the University of Virginia filed some of the earliest active magnetic bearing patentsBeams, J., Production and Use of High Centrifugal Fields, Science, Vol. 120, (1954)Beams, J., Magnetic Bearings, Paper 810A, Automotive Engineering Conference, Detroit, Michigan, USA, SAE (Jan. 1964) during World War II. The patents dealt with ultracentrifuges intended for the enrichment of isotopes of elements needed for the Manhattan Project. However, magnetic bearings did not mature until advances in solid-state electronics and modern computer-based control technology with the work of HabermannHabermann, H., Liard, G. Practical Magnetic Bearings , IEEE Spectrum, Vol. 16, No. 9, (September 1979) and Schweitzer.Schweitzer, G., Characteristics of a Magnetic Rotor Bearing for Active Vibration Control, Paper C239/76, First International Conference on Vibrations in Rotating Machinery, (1976) In 1987, Estelle Croot further improved active magnetic bearing technology,Estelle Croot, Australian Inventors Weekly, NSW Inventors Association, Vol. 3, (April 1987) but these designs were not manufactured due to expensive costs of production, which used a laser guidance system. Estelle Croot's research was the subject of three Australian patents and was funded by Nachi Fujikoshi, Nippon Seiko KK and Hitachi, and her calculations were used in other technologies that used rare-earth magnets but the active magnetic bearings were only developed to the prototype stage. Croot'sSawsan Ahmed Elhouri Ahmed, Nuha Abdallah Mohammed Babker & Mohamed Toum Fadel, "A Study on Classes of Magnetism," IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 6 Issue 4, 2348 – 7968, (2019). design also included an advance computerised control system, while the last design was a non-linear magnetic bearing.
Mary KasardaKasarda, M. An Overview of Active Magnetic Bearing Technology and Applications, The Shock and Vibration Digest, Vol.32, No. 2: A Publication of the Shock and Vibration Information Center, Naval Research Laboratory, (March 2000) reviews the history of active magnetic bearings in depth. She notes that the first commercial application of active magnetic bearings was in turbomachinery. The active magnetic bearing allowed the elimination of oil reservoirs on compressors for the NOVA Gas Transmission Ltd. (NGTL) gas pipelines in Alberta, Canada. This reduced the fire hazard allowing a substantial reduction in insurance costs. The success of these magnetic bearing installations led NGTL to pioneer the research and development of a digital magnetic bearing control system as a replacement for the analog control systems supplied by the American company Magnetic Bearings Inc. In 1992, NGTL's magnetic bearing research group formed the company Revolve Technologies Inc. for commercializing the digital magnetic bearing technology. The company was later purchased by SKF of Sweden. The French company S2M, founded in 1976, was the first to commercially market active magnetic bearings. Extensive research on magnetic bearings continues at the University of Virginia in the Rotating Machinery and Controls Industrial Research Program.
During the decade starting in 1996, the Dutch oil-and-gas company NAM installed twenty gas compressors, each driven by a 23-megawatt variable-speed-drive electric motor. Each unit was fully equipped with active magnetic bearings on both the motor and the compressor. These compressors are used in the Groningen gas field to extract the remaining gas from this large gas field and to increase the field capacity. The motor-compressor design was done by Siemens and the active magnetic bearings were delivered by Waukesha Bearings (owned by Dover Corporation). (Originally these bearings were designed by Glacier, this company was later taken over by Federal Mogul and is now part of Waukesha Bearings.) By using active magnetic bearings and a direct drive between motor and compressor (without having a gearbox in between) and by applying dry gas seals, a fully dry-dry (oil-free) system was achieved. Applying active magnetic bearings in both the driver and in the compressor (compared to the traditional configuration using gears and ball bearings) results in a relatively simple system with a very wide operating range and high efficiencies, particularly at partial load. As was done in the Groningen field, the full installation can additionally be placed outdoors without the need for a large compressor building.
Non-contacting permanent magnet bearings with electromotive stabilisation were applied for patent by R. G. Gilbert in 1955 (U. S. Patent 2,946,930) R. G. Gilbert, "Magnetic suspension" 1955 and K. Boden, D. Scheffer in 1968 (German Patent 1750602).K. Boden, D. Scheffer, "Magnetische Lagerung" 1968 These inventions provide the technological basis for a number of practical applications, some of which have reached the stage of industrial series production under licence from Forschungszentrum Jülich since about 1980.Johan K. Fremerey, "Permanentmagnetische Lager", November 2000 (in German)Johan K. Fremerey, "Permanent magnet bearings", March 2019