High-speed compressor facility electromechanical design

Brown, Nathan K., Department of Engineering, University of Virginia
Barrett, Lloyd, Department of Mechanical and Aerospace Engineering, University of Virginia

The goal of this work was to develop the electromechanical design of a high-speed air compressor test rig. Research objectives of the test facility include the identification of radial and axial impeller loads at on and off-design operating conditions, and investigation into potential surge suppression control using the axial magnetic bearings to modulate the impeller tip clearance. This thesis describes the wide ranging mechanical and electromagnetic issues that were solved to produce a proper machine design.

High-speed induction motor housing design details are provided, including the housing and cooling system design, support bearing and lubrication selection, and electrical connections. Selection criteria for the flexible-disk pack coupling between the motor and test section are covered.

The majority of the design work focuses on the compressor spindle test section development. Analytical expressions are presented for radial AMB actuator static and dynamic force based on bearing geometry and power amplifier capacity. Radial load estimates are given, accounting for spindle imbalance, gravity, and aero side loads, the latter of which are derived from an empirical relationship. The maximum allowable radial AMB air gap is determined using the analytical force calculations combined with the expected loads. Thorough treatment of the Axial AMB actuator design method is presented, combining a mixture of analytical and numerical methods (FEMM) to achieve the desired load capacity and stress levels.

Subcritical operation of the machine is very desirable to prevent natural frequency problems within the compressor testing speed range. Critical speed maps and mode shapes are shown for the compressor test spindle, which indicate that the lowest frequency occurs well above the maximum operational speed of the machine.

Oil-impregnated bronze bushings, supported by viscoelastic o-rings comprise the auxiliary bearing system to catch the rotor in the event of a bearing failure or overload. The interference fits between the shaft and bearing components are carefully engineered to maintain contact pressure, yet not overstress at the worst-case conditions. Highstresses within the axial AMB rotor dictate an integral disk design to lower the maximum material stress.

Three AMB force measurement methods are discussed and their respective benefits compared: current-displacement, direct flux sensing, and coupled force models. Coupled force modeling is selected for the high-speed compressor radial AMB to provide low force measurement uncertainty with careful calibration, while not requiring additional hardware within the bearing housing. Concerns regarding dynamic eddy current effects drive the decision to implement the direct-flux sensing method for the axial AMB force measurement.

A numerical study is performed to investigate the feasibility of mounting Hall effect sensors within a modified radial AMB pole structure to provide accurate force measurement, while maintaining the actuator load capacity. In the end, it is determined that the required pole face modifications introduce appreciable uncertainties in the calibrated force. Enough so, the coupled force model alternative is a more attractive measurement option in terms of potential accuracy and reduced hardware requirements.

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MS (Master of Science)
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