A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact, for example, they can levitate a rotating shaft and permit relative motion without friction or wear. They are in service in such industrial applications as electric power generation, petroleum refining, machine tool operation and natural gas pipelines. They are also used in the Zippe-type centrifuge ^[1] used for uranium enrichment. Magnetic bearings support the highest speeds of any kind of bearing, they have no known maximum relative speed. Image File history File links Magneticbearings. ... A bearing is a device to permit constrained relative motion between two parts, typically rotation or linear movement. ... This article is about magnetic levitation. ... The Zippe-type centrifuge is a device designed to collect Uranium-235. ...

Description

It is difficult to build a magnetic bearing using permanent magnets due to the limitations imposed by Earnshaw's theorem, and techniques using diamagnetic materials are relatively undeveloped. As a result, most magnetic bearings require continuous power input and an active control system to hold the load stable. Because of this complexity, the magnetic bearings also typically require some kind of back-up bearing in case of power or control system failure. Earnshaws theorem states that a collection of point charges cannot be maintained in a stable stationary equilibrium configuration solely by the electrostatic interaction of the charges. ... Diamagnetism is a very weak form of magnetism that is only exhibited in the presence of an external magnetic field. ...

Two sorts of instabilities are very typically present with magnetic bearings. Firstly attractive magnets give an unstable static force, decreasing with greater distance, and increasing at close distances. Secondly since magnetism is a conservative force, in and of itself it gives little if any damping, and oscillations will cause loss of successful suspension if any driving forces are present, which they very typically are. A conservative force is a force which is path-independent. ...

The use of an induction-based levitation system present in cutting-edge MAGLEV technologies, magnetic bearings could do away with complex control systems by using Halbach arrays and simple closed-loop coils. Look up Maglev in Wiktionary, the free dictionary. ... A Halbach array, showing the orientation of each pieces magnetic field. ...

Basic Operation

Basic Operation for a Single Axis

An active magnetic bearing (AMB) 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. These elements are shown in the diagram. The power amplifiers supply 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 but opposite perturbations of current as the rotor deviates by a small amount from its center position. Image File history File links Amb2. ... Image File history File links Amb2. ... An electromagnet is a type of magnet in which the magnetic field is produced by the flow of an electric current. ... Basic Principles A controller is the brain component of a system that monitors certain input variables and adjusts other output variables to achieve the desired operation. ...

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 (PWM) configuration. The controller is usually a microprocessor or DSP. Pulse-width modulation of a signal or power source involves the modulation of its duty cycle to either convey information over a communications channel or control the amount of power sent to a load. ... A microprocessor incorporates most or all of the functions of a central processing unit (CPU) on a single integrated circuit (IC). ... The abbreviation DSP can refer to: // Digital signal processing, the study of signals in a digital representation and their processing methods Digital signal processor, a specialized microprocessor designed specifically for digital signal processing Dylan Server Pages, a web template engine using the Dylan language Delivery Service Partner, a type of...

Applications

Magnetic bearing advantages include very low and predictable friction, ability to run without lubrication and in a vacuum. Magnetic bearings are increasingly used in industrial machines such as compressors, turbines, pumps, motors and generators. Magnetic bearings are commonly used in watt-hour meters by electric utilities to measure home power consumption. Magnetic bearings are also used in high-precision instruments and to support equipment in a vacuum, for example in flywheel energy storage systems. A flywheel in a vacuum has very low windage losses, but conventional bearings usually fail quickly in a vacuum due to poor lubrication. Magnetic bearings are also used to support maglev trains in order to get low noise and smooth ride by eliminating physical contact surfaces. Disadvantages include high cost, and relatively large size. Typical US domestic electricity meter An electric meter or energy meter is a device that measures the amount of electrical energy supplied to a residence or business. ... NASA G2 flywheel Flywheel Energy Storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. ... Transrapid Shanghai Maglev Train stopping at terminus Longyang Road station Transrapid Shanghai Maglev Train Inside the Shanghai Transrapid maglev Inside the Shanghai Transrapid maglev VIP section Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains) using electromagnetic force. ...

A very interesting new application of magnetic bearings is their use in artificial hearts. The use of magnetic suspension in ventricular assist devices was pioneered by Prof. Paul Allaire and Prof. Houston Wood at the University of Virginia culminating in the first magnetically suspended ventricular assist centrifugal pump (VAD) in 1999^{[citation needed]}. A Ventricular assist device, or VAD, is mechanical device that is used to partially or completely replace the function of a failing heart. ...

History

The evolution of active magnetic bearings may be traced through the patents issued in this field. 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. A patent is a set of exclusive rights granted by a government to an inventor or applicant for a limited amount of time (normally maximum 20 years from the filing date, depending on extension). ... Earnshaws theorem states that a collection of point charges cannot be maintained in a stable stationary equilibrium configuration solely by the electrostatic interaction of the charges. ...

Early active magnetic bearing patents were assigned to Jesse Beams^[2][3] at the University of Virginia during World War II and are concerned with ultracentrifuges for purification of the isotopes of various elements for the manufacture of the first nuclear bombs, but the technology did not mature until the advances of solid-state electronics and modern computer-based control technology with the work of Habermann^[4] and Schweitzer^[5]. Extensive modern work in magnetic bearings has continued at the University of Virginia in the Rotating Machinery and Controls Industrial Research Program. The first international symposium for active magnetic bearing technology was held in 1988 with the founding of the International Society of Magnetic Bearings by Prof. Schweitzer, Prof. Allaire (University of Virginia), and Prof. Okada (Ibaraki University). Since then there have been nine succeeding symposia. Kasarda^[6] reviews the history of AMB in depth. She notes that the first commercial application of AMB’s was with turbomachinery. The AMB 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. (MBI). In 1992, NGTL's magnetic bearing research group formed the company Revolve Technologies Inc. to commercialize the digital magnetic bearing technology. This firm was later purchased by SKF of Sweden. The French company S2M, founded in 1976, was the first to commercially market AMB’s. Extensive research on magnetic bearings continues at the University of Virginia in the Rotating Machinery and Controls Industrial Research Program [[1]]. Jesse Beams was an American physicist (1898-1977) at the University of Virginia. ... The University of Virginia (also called U.Va. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... S2M, formerly Société de Mécanique Magnétique, is a French company based in Saint-Marcel, Eure, Normandie that produces magnetic bearings and high speed motors. ...

References

1. ^ Charles, D., Spinning a Nuclear Comeback, Science, Vol.315, (30March 2007)
2. ^ Beams, J. , Production and Use of High Centrifugal Fields, Science, Vol. 120, (1954)
3. ^ Beams, J. , Magnetic Bearings, Paper 810A, Automotive Engineering Conference, Detroit, Michigan, USA, SAE (Jan. 1964)
4. ^ Habermann,H. , Liard, G. Practical Magnetic Bearings , IEEE Spectrum, Vol. 16, No. 9, (September 1979)
5. ^ Schweitzer, G. , Characteristics of a Magnetic Rotor Bearing for Active Vibration Control, Paper C239/76, First International Conference on Vibrations in Rotating Machinery, (1976)
6. ^ Kasarda, 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)

• Schweitzer, G (2002). "Active Magnetic Bearings – Chances and Limitations" (PDF).
• Chiba, A., Fukao, T., Ichikawa, O., Oshima, M., Takemoto, M., Dorrel, D. (2005). Magnetic Bearings and Bearingless Drives. Newnes. 
• Maslen, E. H. (1999). Course notes on Magnetic Bearings (PDF). 
• Jim Wilson (1999-Sep). Beating Demon Friction. Popular Mechanics.

PDF is an abbreviation with several meanings: Portable Document Format Post-doctoral fellowship Probability density function There also is an electronic design automation company named PDF Solutions. ... PDF is an abbreviation with several meanings: Portable Document Format Post-doctoral fellowship Probability density function There also is an electronic design automation company named PDF Solutions. ...

See also

• Maglev