The Active Magnetic Bearing Enables Optimum Control of Machine Vibrations

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Abstract
The active magnetic bearing is based on the use of forces created by a magnetic field to levitate the rotor without mechanical contact between the stationary and moving parts. A ferromagnetic ring fixed on the rotor “floats” in the magnetic field generated by the electromagnets, which are mounted as two sets of opposing pairs. The current is transmitted to the electromagnetic coils through amplifiers. The four electromagnets control the rotor’s position in response to the signals transmitted from the sensors. The rotor is maintained in equilibrium under the control of the electromagnetic forces. Its position is determined by means of sensors which continuously monitor any displacements between rotor and stator through an electronic control system. As in every control system, damping of the loop is provided by means of a phase advance command from one or more differenciating circuits of the position error signal. The vibrations of the rotor and stator of a machine are generated by different forces: - centrifugal forces due to the misalignment between the geometrical axis and the inertial axis of the rotor (unbalance), - reaction forces due to aerodynamical forces on the rotor and stator blades. The active magnetic bearing allows the decrease and in many cases the fully cancelling of effects of these forces i.e. the vibrations of the machine. The inertial forces can be cancelled by shifting the axis of rotation of the rotor from the geometrical axis to the inertial axis (this system is usually called automatic balancing). The reaction forces due to aerodynamical effects can be cancelled by the creation by the magnetic bearings of forces in opposition. The vibrations are measured on the stator by accelerometers, and the signals drive magnetic bearings which generate forces having the same amplitude but in phase opposition. The improvement in vibrations amplitude usually ranges from 20 Db to 40 Db over a large band of frequencies.

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