Non-Linear Behaviors in the Motion of a Magnetically Supported Rotor on the Catcher Bearing During Levitation Loss: An Experimental Description

Abstract

Recent developments improving load capacity foretell the practical implementation of Active Magnetic Bearings (AMB) on industrial level, pushed by the advantages of reduced wear and higher speeds that they make available. However, the possibility of an eventual power failure forces the use of back-up (catcher) bearings, which usually are of the ball bearing type. The back-up bearings present a clearance between the shaft and the inner race, such that there is not contact during normal operation. On a power failure or an emergency stop, the rotor is only supported by the catcher bearings. Thus, the rotor motion within the clearance results on a succession of impacts, contact and non-contact intervals producing a non-linear behavior of the system. The complexity of this non-linear behavior prevents the use of traditional methods for the design of the catcher bearings, calling for the need of extensive experimentation and previous experience in their dimensioning process. Here, the response of a rigid rotor, supported by a pivot on the drive side and a magnetic bearing on the other side, is measured during the emergency stop from several operating speeds. Non-linear analysis tools, such as Poincare´ Maps and Bifurcation Diagrams, are employed to demonstrate the non-linear characteristics of the motion, which in some conditions is shown to become chaotic with a vibration limit cycle. The rotor motion on the catcher bearings (with the magnetic bearing deactivated) is measured at constant running speeds. The limit cycles and chaotic attractors are described, showing the relation of the non-linear effects to the rotational speed.