Controlled Escape from Trapped Contact Modes in Magnetic Bearing Systems

  • Fawaz Saket

Student thesis: Doctoral ThesisPhD


Rotors supported by active magnetic bearings under contact-free levitation have many advantages, such as allowing near frictionless rotation and high rotational speeds. They also provide the designer the capability to achieve increased machine power density. However, magnetic bearings possess limited load capacity and operate under active control. Under certain operational conditions, the load capacity may be exceeded or a transient fault may occur. Touchdown bearings or bushes are required in such systems, from the operational and design points of view, to prevent contact between the rotor and stator laminations causing damage to the system.Rotor/touchdown bearing contact can occur in fault conditions and under external disturbances, even if the magnetic bearings are fully functional. If the rotor makes contact with the touchdown bearings, the ensuing rotor dynamics may result in transient or sustained contact dynamics. These dynamics involve transmitting a range of stresses under distortional strains. Such stresses occur under different contact modes, can be of very short or long duration, and can affect the life span of touchdown bearings. Touchdown bearings thus characterise significant safety and reliability aspects of active magnetic bearing systems. Maximisation of the performance and operational life of magnetic bearing systems dictates the need for minimisation of rotor/touchdown bearing contact. Magnetic bearing forces may have the capability to restore contact-free rotor levitation, though this will require appropriate control strategies to be devised. An understanding of the contact dynamics is required, together with the relationship between these and magnetic bearing control forces.In this thesis, rotor/touchdown bearing contact conditions are identified and investigated dynamically. An active magnetic bearing system with a long flexible rotor is considered. A nonlinear system model is employed, and a speed range covering three of the rotor’s critical frequencies is taken into account. Different types of transient and steady-state contact modes are identified, including non-persistent and persistent trapped contact modes of varying contact force levels and time durations. Design methodology is presented for a force measurement system capable of providing rotor/touchdown bearing contact force related data, based on experimental strain measurement. The system is implemented and a calibration method of assessing magnetic bearing forces based on rotor/touchdown bearing contact is demonstrated. The frequency dependent behaviour of the active magnetic bearing system is considered using evaluated force and phase measurements. Force measurements covering one of the rotor’s critical speeds are experimentally validated using an open-loop control strategy, aimed at attenuating rotor vibration in contact cases. Rotor recovery from a persistent rub contact mode through employing synchronous active magnetic bearing forces is demonstrated and discussed. The range of magnetic bearing control forces capable of contact elimination is also explored for different running speeds. The new control method presented provides insight into potential control methods capable of achieving rotor/touchdown bearing contact recovery utilizing experimental force data in contact conditions. This will contribute towards improving safety and reliability aspects of active magnetic bearing systems, which undergo operational conditions leading to rotor/touchdown bearing contact.
Date of Award1 Nov 2016
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorPatrick Keogh (Supervisor)

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