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Research Papers: Gas Turbines: Structures and Dynamics

On the Use of Actively Controlled Auxiliary Bearings in Magnetic Bearing Systems

[+] Author and Article Information
Iain S. Cade, M. Necip Sahinkaya, Clifford R. Burrows, Patrick S. Keogh

Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK

J. Eng. Gas Turbines Power 131(2), 022507 (Dec 24, 2008) (10 pages) doi:10.1115/1.2982159 History: Received April 08, 2008; Revised July 24, 2008; Published December 24, 2008

Auxiliary bearings are used to prevent rotor/stator contact in active magnetic bearing systems. They are sacrificial components providing a physical limit on the rotor displacement. During rotor/auxiliary bearing contact significant forces normal to the contact zone may occur. Furthermore, rotor slip and rub can lead to localized frictional heating. Linear control strategies may also become ineffective or induce instability due to changes in rotordynamic characteristics during contact periods. This work considers the concept of using actively controlled auxiliary bearings in magnetic bearing systems. Auxiliary bearing controller design is focused on attenuating bearing vibration resulting from contact and reducing the contact forces. Controller optimization is based on the H norm with appropriate weighting functions applied to the error and control signals. The controller is assessed using a simulated rotor/magnetic bearing system. Comparison of the performance of an actively controlled auxiliary bearing is made with that of a resiliently mounted auxiliary bearing. Rotor drop tests, repeated contact tests, and sudden rotor unbalance resulting in trapped contact modes are considered.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Rotor/auxiliary bearing contact forces

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Figure 2

Rotor/magnetic bearing/active auxiliary bearing system

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Figure 3

(a) Static auxiliary bearing clearance circle and (b) dynamic auxiliary bearing contact region. ((c) and (d)) The active auxiliary bearing position with and without contact, respectively. Values normalized with respect to the rotor/stator clearance gap.

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Figure 4

Augmented rotor/auxiliary bearing system block diagram including weighting transfer functions

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Figure 5

(1) PID controlled auxiliary bearing singular value response, (2) σ¯[Wyb(iω)], (3) σ¯[Wub(iω)], and (4) active auxiliary bearing system closed loop response

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Figure 6

(a) Rotor displacements normalized with respect to rotor/stator clearance, csi, for a passive auxiliary bearing. (b) Rotor deviation beyond clearance circle (xpi,ypi). (c) Rotor/bearing contact forces. Rotor operating speed Ω=1000 rad/s.

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Figure 7

(a) Rotor displacements normalized with respect to rotor/stator clearance, csi, for an active auxiliary bearing. (b) Active auxiliary bearing displacement (xbi,ybi)/(csi−cbi). (c) Rotor/bearing contact forces. Rotor operating speed Ω=1000 rad/s.

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Figure 8

(a) Rotor displacements normalized with respect to rotor/stator clearance, csi, for a passive auxiliary bearing. (b) Rotor deviation beyond clearance circle (xpi,ypi). ((c) and (d)) Rotor/bearing contact forces. Rotor operating speed Ω=500 rad/s.

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Figure 9

(a) Rotor displacements normalized with respect to rotor/stator clearance, csi, for an active auxiliary bearing. (b) Active auxiliary bearing displacement (xbi,ybi)/(csi−cbi). ((c) and (d)) Rotor/bearing contact forces. Rotor operating speed Ω=500 rad/s.

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Figure 10

(a) Rotor displacements normalized with respect to rotor/stator clearance, csi, for a passive auxiliary bearing. (b) Rotor deviation beyond clearance circle (xpi,ypi). ((c) and (d)) Rotor/bearing contact forces. Rotor operating speed Ω=500 rad/s.

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Figure 11

(a) Rotor displacements normalized with respect to rotor/stator clearance, csi, for an active auxiliary bearing. (b) Active auxiliary bearing displacement (xbi,ybi)/(csi−cbi). ((c) and (d)) Rotor/bearing contact forces. Rotor operating speed Ω=500 rad/s.

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