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Power Transmission


4 Active magnetic bearings have been available for decades, yet demand from design engineers for higher performance and reduced energy consumption is giving rise to heightened interest in these systems. Jon Severn reports on some of the latest developments and applications.


4 Les paliers magnétiques actifs sont présents sur le marché depuis des dizaines d’années, mais l’exigence des ingénieurs conception visant à atteindre des performances plus élevées et à réduire la consommation énergétique donne lieu à un intérêt accru pour ces systèmes. Jon Severn revient sur certains développements et applications qui ont vu le jour récemment.


4 Aktive Magnetlager sind seit Jahrzehnten verfügbar, allerdings ruft der Anspruch von Konstrukteuren hinsichtlich höherer


Performance und verringerten Energieverbrauchs verstärktes Interesse an diesen Systemen hervor. Jon Severn berichtet über einige der aktuellsten Entwicklungen und Anwendungen.


Active magnetic bearings prove attractive for demanding applications


F


or the vast majority of applications, engineers can choose the most appropriate rolling-element or plain bearings. Air bearings are suitable for only a limited range of applications,


but there is now increasing interest in magnetic bearings or, more precisely, active magnetic bearings. From early childhood, would-be engineers


understand that opposite poles of permanent magnets attract each other and like poles repel. By using electromagnets instead of permanent magnets, and by having a means to control the electromagnets, it is simple to conceive of a rotating shaft that is supported and guided in a non-contact manner by virtue of electromagnetic forces - and gravity, of course, depending on the orientation of the shaft. Clearly the main advantage of the magnetic


Fig. 1. Active magnetic bearings can achieve a surface speed of 250 m/s, which is around four times faster than conventional bearings.


bearing is that it is non-contact and, therefore, wear-free and virtually friction-free. This has benefits in terms of high reliability, reduced maintenance requirements, long and predictable life, and lower energy consumption - though the electromagnets will consume an amount of energy, typically in the order of tens of Watts, depending on the shaft size and radial loading. In some applications, such as turbomachinery, it is advantageous to eliminate the lubrication system that would otherwise be required for a conventional bearing arrangement. The lack of contact and lubrication also enables magnetic bearings to operate in hostile environments including those where temperature extremes (typically -256 to 220°C), vacuum, high pressure, steam or corrosive chemicals are encountered. Being non-contact components, magnetic bearings do not restrict the speed of rotation, though manufacturer SKF suggests an upper limit for the surface speed of 250 m/s or 4.5 million DN, where DN is the diameter of the rotor in mm multiplied by the speed in revolutions per minute (Fig. 1). To put this in context, conventional bearings with sophisticated lubrication systems would only be able to achieve speeds around one-quarter of this.


Bearing arrangement


A typical active magnetic bearing (AMB) system consists of a set of stationary electromagnets located around the ferromagnetic rotor (shaft),


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corresponding gap sensors, a control unit and power amplifiers for feeding power to opposite pairs of electromagnets. The controller uses data from the gap sensors


to adjust the power to the electromagnets in order that the shaft’s position is maintained to within microns, regardless of external influences such as radial loads. Whereas early controllers were analogue, the


current generation of controllers use digital signal processing and can cycle through the control loop thousands of times per second, enabling shaft speeds to be maintained in excess of 100 000 revolutions per minute.


Radial bearing assemblies


Most AMB systems have a pair of radial bearing assemblies, one at each end of the shaft, and a thrust bearing to maintain the shaft’s axial position. Because each radial bearing maintains the shaft’s position in two axes, and the thrust bearing controls the shaft’s position along its axis, the overall AMB


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