1、外文资料 Magnetic Bearings Come Of Age Magnetic-bearings, which support shafts with magnetic levitation rather than mechanical contact, have been in industrial use for decades. Magnetic bearings offer a host of advantages to users, including high-speed capabilities and the ability to operate lubrication
2、-free and in vacuum environments. They generate no friction, experience minimal wear, and operate contamination free with extremely low vibration. And the bearings can precisely control shaft position, measure external forces acting on the shaft, and even monitor a machines operating condition. Rece
3、nt technological developments, especially in digital processing and control, have made magnetic bearings a more-robust and cost-effective design solution than ever. Todays bearings are suitable for a wide range of applications, from semiconductorfabrication equipment to microturbines,from refrigerat
4、ion compressors to vacuum pumps. MAGNETIC-BEARING BASICS: Magnetic-bearing systems electromagnetically suspend shafts by applying electric current to a bearings ferromagnetic materials. The systems have three main elements: bearing actuators, position sensors, and controller and control algorithms.
5、Typical units consist of two magnetic radial bearings and one magnetic thrust bearing. They control the shaft along five axes: two axes for each radial bearing and a fifth axis along the shaft. Magnetic bearings have stationary and rotating components the stator and rotor, respectively. The radial m
6、agnetic bearing stator resembles an electricmotor stator. The radial stator is formed by a buildup of laminations, each of which is shaped with poles. The laminations stack together, and coils of wire are wound around each pole. Controlled electric currents passing through the coils produce an attra
7、ctive force on the ferromagnetic rotor and levitate it within an air gap. The gap usually measures about 0.5 mm but, in some applications, can be as large as 2 mm.The rotor fits over the shaft, which is in the air gap but need not be centered. This is useful in applications where it is valuable to c
8、ompensate for wear, or if the shaft oscillates such as in machine-tool grinding processes where the wheel wears over time. A magnetic thrust bearing provides axial control. The thrust-bearing rotor is a solid steel disk attached to the shaft and positioned at a preset distance from the stator on one
9、 or both sides. During operation, electromagnetic forces produced by the stator act on the rotor and control axial movement. Magnetic-bearing arrangements also include touchdown or auxiliary bearings. Their main function is to support the shaft when the machine is idle and protect machine components
10、 in case of power outage or failure. The touchdown bearings inner ring is smaller than the magnetic-bearing air gap to prevent potential damage if the shaft delevitates. CONTROL SYSTEMS: The control system regulates bearing current and, thus, the force of the bearings. During operation, radial and a
11、xial position sensors feed data on shaft location and movement to the controller. It compares actual and desired shaft position, calculates the force required to maintain the shaft in the preset position and, if necessary, commands the amplifier to adjust the electric current to raise or lower the l
12、evel of magnetic flux. The main parts of the control system are the digital signal-processing (DSP) electronics, a power supply, and amplifiers. Generally, the larger the machine, the larger the amplifiers. Controller size also depends on the dynamic load capacity required, which is typically greate
13、r in heavy machines. The shaft can be controlled through Single Input/Single Output (SISO) or Multiple Input/ Multiple Output (MIMO) algorithms for high-speed and more-demanding applications. The controller typically measures and processes position signals at 10-kHz frequency, enabling precise contr
14、ol of machinery rotating at speeds of 100,000 rpm and higher. A significant benefit of magnetic-bearing technology is that the controller functions as a built-in condition-monitoring system, providing extensive real-time information and making other monitoring devices unnecessary. Software, such as
15、MBScope from SKF, provides detailed diagnostic information about machine health and helps schedule preventive maintenance more effectively. The software includes configuration tools for tuning input parameters and checking clearances prior to start-up. Its viewing tools include real-time monitoring
16、of positions, currents, and forces； an alarm log that captures all system variables before and after an unusual event； and short or long-term trending. This lets users view information in various formats for bearing tuning and machine diagnostics. Adaptive vibration control (AVC) is another importan
17、t tool. AVC computes the forces necessary to cancel out vibration in two ways. One is to let the shaft rotate around its geometric center and tightly control shaft displacement, eliminating runout caused by imbalance. This is useful in high-precision applications, such as machine tools. The other wa
18、y is to rotate the shaft around its center of mass to reduce vibrations transferred to the housing or casing (to 0.01 m m). This is a valuable feature in turbomolecular pumps and other semiconductor-manufacturing equipment. AVC can increase machine reliability and the time between service intervals.
19、 Its adaptive feature minimizes vibrations even with rotor fouling over time and, by canceling out process disturbances, can extend equipments operating range. DESIGN CONSIDERATIONS: The ultimate goal of magnetic-bearing design is reliable, noncontacting rotation over the machines entire speed range
20、. It is also essential to meet OEM and end-user cost targets without compromising performance. Reducing the size of digital-control systems means more cost-efficient solutions, and compact magnetic-bearing designs can lead to smaller, more-robust machines. When developing magnetic-bearing systems, m
21、ain factors to consider are the speeds, loads, and operating environment. The mechanical strength of the shaft typically limits speed. Surface speeds of 3.5 X 10 6 DN (diameter in mm 3 rpm) are possible. Static capacity the maximum force magnetic bearings generate to lift the shaft is a function of
22、variables such as amplifier current, surface area of the magnetic poles, number of coil windings, and air-gap dimensions. A good rule of thumb is 75 lb of force/sq in. of bearing. Dynamic capacity the rate at which magnetic bearings change the applied force is determined by a single variable, amplif
23、ier voltage. Consider, for example, a 150-N magnetic bearing connected to a 2-A/40-V control system. Switching to a larger, 200-N bearing with more coil turns, a larger magnetic pole area, and so on, will increase static capacity. If the controller remains the same, however, there will be no effect
24、on dynamic capacity the ability to handle shaft imbalances and other dynamic forces during operation. Conversely, retaining the 150-N magnetic bearing but switching to a 3-A/50-V control system will increase the units dynamic capacity but have no effect on static capacity. DIVERSE APPLICATIONS： The
25、unique design and wide-ranging capabilities of magnetic bearings offer solutions in a host of diverse applications. One example is semiconductorfabrication, particularly front-end operationsinvolving the production of silicone wafers. Magnetic bearings can improve yields in these operations, which a
26、re highly sensitive to contamination and vibration. For instance, magnetic bearings permit edge rotation of 300-mm wafers, allowing convenient access to both wafer sides. Because magnetic bearings have an air gap, they are ideal for certain biological and pharmaceutical applications. Blood cells or other liquids can pass through the air gap without damage. Refrigeration compressors are another important application. Magnetic bearings can run at the high speeds required by new-generation refrigerants and, unlike conventional oil-lubricated bearings, they pose minimal risk of contamination.
