Magnetic Bearings
Magnetic bearings have some distinct advantages. They do not generate wear and they do not need lubrication. These features make them attractive for vacuum applications. And their dynamic behaviour can be adjusted in a wide range, which allows active vibr
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MAGNETIC BEARINGS
G. Sebweltzer Institute for Meebanies, ETH Zurieb, Switzerland
ABSTRACT Magnetic bearings have some distinct advantages. They do not generate wear and they do not need 1ubri cation. These features make them at tractive for vacuum applications. And their dynamic behaviour can be adjusted in a wide range, which allows active vibration damping and control. This chapter presents the state of the art for the design of an electromagnetic bearing system. It introduces first the main elements and then discusses control and system aspects. Models for describing an elastic rotor and its active vibration control are included. The characteristics and the losses of such a suspension system are detailed. Several applications are demonstrated, indicated.
and future trends are
CONTENTS 1. 2. 3. 4.
5.
6. 7. 8.
Introduction Functional principle Design goals Elements of the magnetic bearing system 4.1 Model for the rotor 4.2 Sensor, controller, amplifier 4.3 Magnetic actuator System aspects 5.1 Control of rigid and elastic rotors 5.2 Characteristics of the magnetic bearing system 5.3 Losses Applications Conclusions References
N. F. Rieger (ed.), Rotordynamics 2 © Springer-Verlag Wien 1988
G. Schweitzer
544
1.
INTRODUCTION
Magnetic bearings can support a rotor in such a way that it levitates freely without any contact. Furthermore the dynamics of this suspension can be easily adjusted in a wide range for various applications. These two main properties already make the magnetic bearing a very attractive device for solving the classical bearing problem. On the other side the magnetic bearing is complex, expensive, usually not readily available from the shelf, and up to now only used for some advanced machinery. In the following the state of the art is presented so that future trends can be derived. Magnetic forces are generated either by permanent magnets, electrodynamically or electromagnetically. In the constant field of permanent magnets, however, a ferromagnetic body cannot hover in a stable way /BR 39/, and electrodynamic forces are usually too small or still too difficult to generate to be of actual technical interest. They are used where small forces are sufficient, for example in space applications for the support of flywheels or for a micro-g-platform in a near zero-g-environment. Or they are used where the high currents necessary for large forces are generated by means of cryogenics as in prototypes for an electrodynamically levitated high-speed vehicle. It is the electromagnetic force that is used most efficiently. For the l~vitation of guided vehicles a technology of its own has developed /GO 84/, that basically of course has some connection to magnetic bearings for rotors, too. Rotors have been supported magnetically at first for physical experiments. Spectacular 2.10E7 rpm have been reached while testing the strength of small steel balls under a centrifugal field of l.OE8 g /BY 46/. Since then the electromagnetic rotor bearing has been applied to solve a number of different technical problem
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