ACTIVE MAGNETIC BEARING：主动磁轴承.docVIP

INTRODUCTION Active magnetic bearings (AMB) have been designed to overcome the deficiencies of conventional journal or ball bearings. They have the ability to work in vacuum with no lubrication and no contamination, or to run at high speed. Today, magnetic bearings have been introduced into the industrial world as a very valuable machine element with quite a number of novel features, and with a vast range of diverse applications. Let us discuss key features such as load, size, stiffness, temperature, precision, speed, losses and dynamics. PRINCIPLE 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. 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 due to control current as the rotor deviates by a small amount from its center position. Schematic Diagram of AMB LOAD The term load touches upon basic properties of magnetic bearings. The load capacity depends on the arrangement and geometry of the electromagnets, the magnetic properties of the material, and of the control laws. The load carrying capacity of an AMB depends on the magnetomotive force i.e., the product of the maximum current imax and winding number n. The load that can be carried depends on possible heat dissipation. Therefore, one limitation for a high static load is the adequate dissipation of the heat generated by the coil current due to the Ohm resistance of the windings. This is called soft limitation on load. When the current imax reaches a value where the flux generated will cause saturation then the carrying force has reached its maximal value. Any overload beyond that will cause the rotor to break away from its centre position and touch down on its retainer bearings. This is hard limitation It has been found that the maximum specific carrying force of 32 N/cm2 (or 0.32 MPa) can be generated which is considerably lower than that for oil lubricated bearings, which is about four times as high. Using expensive cobalt-alloys with a saturation flux density of over 2 Tesla, from which a specific carrying force of up to 60 N/cm2 will result Geometry of a radial bearing d Inner diameter (bearing diameter) da Outer diameter dr Rotor diameter c Leg width di Shaft diameter l Bearing length h Winding head height b Bearing width (magnetically active part) An Slot cross section (winding space) p Pole shoe width s0 Nominal air gapSTIFFNESS The stiffness of a bearing is the ratio of the supported load with respect to the resulting displacement of that load High stiffness can be obtained by using PID controller The PID controller brings the position the shaft to the same position after the load and thus the rotor shows a behavior that cannot be obtained with classical bearings SPEED It can be discussed under 3 categories Rotational speedv Circumferentialv speed Supercritical speedv Rotational speed In todays industrial applications rotational speed range of about 3kHz to 5kHz has reached. Problems arise from eddy current and hysteresis losses in the magnetic material, air losses, and the related requirements for power generation and adequate heat dissipation if the rotor runs in vacuum.Circumferential Speed The circumferential speed is a measure for the centrifugal load and leads to specific requirements on design and material. The centrifugal load leads to tangential and radial stresses in the rotor. The tangential stress is the most critical one. Highest stress values occur at the inner boundaries of a rotor disc Rotor speeds of up to 340 m/s in the bearing area can be reached with iron sheets from amorphous metal (metallic glass), having good magnetic and mechanical properties.Supercritical Speed A rotor may well have to pass one or more critical bending speeds in order to reach its operational rotation speed. In classical rotor dynamics this task is difficult to achieve. In AMB technology it is the controller that has to be designed carefully to enable a stable and well-damped rotor behaviour. SIZE In principle, there appears to be no upper limit for the bearing size, it can be adapted to any load. Small bearings are of special interest to micro-techniques. Potential applications are video heads, medical instruments, hard disk drives, and optical scanners. The challenge lies in simplifying the design and in the manufacturing process. HIGH TEMPERATURE AMB In order to utilize the full advantages of active magnetic bearings, operation in gas turbine and aircraft engines requires that the magnetic bearing should work properly at high temperatures Challenges in designing such bearings consist in material evaluation, manufacturing process and high temperature displacement sensor development. Operating temperatures of up to 550 C have been reached, at rotor speeds of 30,000 rpm. Such a performance cannot be reached by any other kind of bearing. Experimental tests were quite successful, but the long-term exposure to high temperature needs further research, as the actual results are not yet convincing Test Rig For High Temperature AMB LOSSES With contact-free rotors there is no friction in the magnetic bearings. The operation of active magnetic bearings causes much less losses than operating conventional ball or journal bearings Losses can be grouped into losses in Stator partv Rotorv part Stator Loss Stator losses are mainly from copper losses in the windings of the stator and from losses in the amplifiers. The copper losses are a heat source, and, if no sufficient cooling is provided, can limit the control current and hence the maximal achievable carrying forceRotor Loss These losses comprise iron losses caused by hysteresis and eddy currents, and air drag losses. The losses heat up the rotor, cause a breaking torque on the rotor, and have to be compensated by the drive power of the motor In general, the eddy current losses are the largest ones. The iron losses depend on the rotor speed, the material used for the bearing bushes The iron losses in the rotor can limit operations, as in particular in vacuum applications it can be difficult to dissipate the generated heat. The hysteresis losses arise if the B-H-curve travels along a hysteresis loop The eddy-current losses arise when the flux density within the iron core changes. A compact core acts like a short circuit winding and generates large eddy currents. The eddy-current losses can be reduced by dividing the iron core in insulated sheets or in particles (sinter cores). The smaller these divisions, the smaller the eddy-current losses. a) Iron Core b) SheetPRECISION Precision in rotating machinery means most often how precise can the position of the rotor axis be guaranteed Active magnetic bearings levitate an object, with feedback control of measured displacement sensor signal. The performance of AMB systems is therefore directly affected by the quality of a sensor signal MATERIALS Common AMB materials areSilicon alloysCobalt alloys such as HipercoADVANTAGES OF AMB Lubrication free Reliability Operation in vacuum Reduced energy consumption Condition monitoring High speed capability No oil contamination Precision DISADVANTAGES Requires auxiliary bearing Load carrying capacity is limited Requires continuous power supply APPLICATIONS Refineries and petrochemical plants Offshore and under sea gas extraction Cryogenic pumps, compressor and expanders Industrial refrigeration and air conditioning Turbo blowers, Turbo booster pumps, Turbo molecular pumps etcCONCLUSIONS The maximal load depends on design The specific load depends on the available ferromagnetic material and its saturation properties, and is therefore limited to 32 to 60 N/cm2 Circumferential speeds, causing centrifugal loads, are limited by the strength of material. Values of about 250 to 300 m/s have been realized with actual design Supercritical speed means that one or more critical speeds can be passed by the elastic rotor. It appears to be difficult to pass more than two or three High temperature bearings have been realized, running in experiments at an operating temperature of 550 C. The losses of magnetic bearings at operating speed are much smaller than that of classical bearings. Eddy current losses will limit the rotation frequency of massive rotors, the air drag will be crucial at high circumferential speeds A high precision of the position of the rotor axis (in the range of mm) requires high resolution sensors and adequate signal processing to separate disturbance signals from the desired onesReference: /Thread-magnetic-bearing-4413?pid=43324#ixzz1Icq7O122IntroductionA 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 used for uranium enrichment. Magnetic bearings support the highest speeds of any kind of bearing; they have no known maximum relative speed.HistoryThe 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 Earnshaws Theorem.Early active magnetic bearing patents were assigned to Jesse Beams 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 and Schweitzer. 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 reviews the history of AMB in depth. She notes that the first commercial application of AMBs 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, NGTLs 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 AMBs. Extensive research on magnetic bearings continues at the University of Virginia in the Rotating Machinery and Controls Industrial Research Program.DescriptionIt is difficult to build a magnetic bearing using permanent magnets due to the limitations imposed by Earnshaws 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.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.With the use of an induction-based levitation system present in cutting-edge MAGLEV technologies such as the Inductrack system, magnetic bearings could do away with complex control systems by using Halbach arrays and simple closed loop coils.Basic OperationBasic Operation for a Single AxisAn 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.T