Design and Simulation of an Active Load Balancing System for High-Speed, Magnetically Supported Rotors

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Title: Design and Simulation of an Active Load Balancing System for High-Speed, Magnetically Supported Rotors
Author: Robb, James Lawrence IV
Advisors: Dr. M.K. Ramasubramanian, Committee Member
Dr. Paul I. Ro, Committee Member
Dr. Kari Tammi, Committee Member
Dr. Gregory D. Buckner, Committee Chair
Abstract: Active magnetic bearings (AMBs) are being increasingly employed in the development of oil-free turbo machinery. One disadvantage of AMB systems, particularly AMB thrust bearings, is their limited dynamic load capacity relative to fluid film bearings. For centrifugal compressors, the most significant transient axial loads are associated with compressor surge, dictating that some of the AMB's load capacity be preserved to handle dynamic loads in this operating region. For other regions of operation, however, the AMB's load capacity may not be fully utilized, compromising compressor efficiency. One common solution to this problem involves the use of static balance pistons to keep thrust loads sufficiently small. Static balance pistons, however, employ seals that leak process gas flow and reduce machine performance. For these reasons, an active thrust load management system is sought. The active thrust balancing design proposed in this thesis seeks to improve the performance of AMB-supported turbo machines by maximizing load capacity and minimizing leakage across the machine's operating space. This design specifically targets high pressure ratio, single-overhung compressor systems that use magnetic thrust bearings. Detailed modeling and simulations are utilized to illustrate the limitations of magnetic thrust bearings and to discuss the pertinent design issues and benefits of regulating thrust loads. The modeling process addresses realistic dynamic effects such as amplifier saturation, magnetic flux saturation, and eddy currents. Simulation results are used to design an active thrust balancing system, and axial force and leakage flow characteristics of this active device are compared to a stationary design. The proposed active design is shown to offer average leakage reductions of 9.0% to 26.4% relative to static balancing devices. Finally, an observer-based controller is designed, and a gain-scheduling methodology is proposed to cover the compressor's full operating map.
Date: 2008-04-07
Degree: MS
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/1089


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