| A single-structured hybrid gas-magnetic bearing(HGMB)is proposed and studied to meet the working requirements of high-speed low-temperature turbo expander which needs to be started and stopped frequently in engineering application.Different from the existing hybrid gas-magnetic bearing,the proposed bearing has single bearing structure,and integrates the bearing characteristics of gas bearings and active magnetic bearings(AMBs)with the hardware equipment of AMBs.In terms of bearing structure,the single-structured HGMB can save the processing and installation of conventional foil gas bearing or externally pressurized gas bearing,which can greatly simplify the bearing structure,reduce the processing difficulty and manufacturing cost.Compared with the traditional HGMB in side-by-side configuration,the proposed bearing can shorten the length of the rotating shaft and improve the critical speed of the rotor.In terms of bearing performance,the single-structured HGMB integrates the advantages of magnetic bearings and gas bearings.During the rotor start-up,shut-down and low-speed operation stage,the shaft is levitated by the AMBs,thus can effectively solve the friction and wear problems of self-acting gas bearings.After reaching the working speed,the rotating shaft is mainly carried by self-acting gas bearings to reduce or eliminate the power consumptions and heating of the AMBs,and reduce the electric heat leakage loss of the low-temperature turbo expander while saving energy.When the rotor system vibrates violently,the AMBs can increase the bearing damping,reduce the rotor vibration amplitude,improve the operation speed,and improve the safety and reliability of the HGMBs.In this study,the single-structured HGMB is proposed and the main research work is as follows.Firstly,in order to reduce the power consumption and heat generation of the AMBs for a less impact on the cryogenic turbo-expander system,the research is carried out from two aspects:control system and winding configuration.In terms of control strategies,considering the high nonlinearity and instability of the rotorbearing system caused by the zero-bias AMBs for reduced power consumptions and rotor heating,a mathematical model based on the AMB-rotor system is established,and the functional relationship between the control current I,the displacement X and the speed term Y is analyzed.The linear quantitative control algorithm is designed and verified on the horizontal and vertical axial AMB test-rig respectively.The results demonstrate that the appropriate control parameters can be figured out by mathematical modelling of the rotor-bearing system and this linear quantitative control algorithm shows good control performance and robustness in case of external disturbance.In addition,the power consumptions of the zero-bias axial AMBs during normal operation are dramatically reduced compared with the current-biased AMBs.In terms of winding configuration,this dissertation analyzes the comprehensive effects of two conventional winding configurations of the eightpole AMBs on power consumptions and dynamic characteristics of the AMB-rotor system.The results show that the AMB with orthogonal(Ortho)winding configuration has better rotation accuracy under static load and unbalanced mass,but the AMB with crossed(Cross)winding configuration has larger vertical load capacity,lower power loss under heavy load and reduced negative impact on the refrigeration process,which is more suitable for low-temperature devices such as the cryogenic turbo-expander.Secondly,according to the structural characteristics and operating conditions of the rigid self-acting gas bearings in single-structured HGMBs,the operating performance of the rigid self-acting gas bearing under the structure of the AMB is studied.The effects of structural and operating parameters of the rigid self-acting gas bearing on the static and dynamic characteristics of the gas bearing and the dynamic performances of rotor-bearing system are studied.The effects of the gas bearing parameters on the comprehensive performances of rotor system is obtained.The results show that the single operation parameter(operating speed,bearing load and gas viscosity)or single structural parameter(bearing diameter,bearing width and radius clearance)of the gas bearing has impacts on a plurality of bearing performances(critical speed,stability and vibration performance)of the rotorbearing system.And there is also a coupling between the structural parameters and operation parameters of the gas bearing.Therefore,in the design of a turbo expander supported by gas bearings,the coupling effects and comprehensive influences of various structural and operation parameters need to be fully considered to ensure the requirements of critical speed,stability and vibration performance.Thirdly,in consideration of various operation conditions of the turbo expander,such as startup and speed variation,the rotordynamic characteristics of the singlestructured HGMBs are studied.The journal orbit,supporting force,time domain and frequency domain displacement responses of the single-structured HGMB under different working speeds are analyzed.The transient response of the rotor during start-up is simulated.The results show that the proposed bearing can effectively suppress the instability and low-frequency vibration caused by gas film,which is conducive to the stable and safe operation of constant speed and startup.In addition,the proposed bearing has smaller vibration amplitudes because of enhanced bearing damping.The proposed bearing can save energy consumptions due to smaller magnetic force compared with pure AMB at steady-state operation.Finally,to avoid the power consumption of the AMB,it is further required that the single-structured HGMB can operate stably in aerodynamic mode,therefore the coupling problems in structure and operation of the proposed bearing needs to be solved.Because the nominal air gap of the AMB is usually more than 10 times larger than that of the gas bearing,it is necessary to solve the incompatibility of clearance dimensions between AMBs and gas bearings.In this chapter,a singlestructured HGMB-rotor system is designed for a specific turbo expander.To solve the structural coupling between the AMB and the gas bearing,the appropriate radius clearance is selected through dynamic analysis to meet the operation requirements of both the gas bearing and the AMB.On this basis,a rotor system supported by gas bearings is designed.Considering the mutual coupling between operation parameters and structural parameters of the gas bearing,as well as the comprehensive influences of operation parameters and structural parameters on the rotor dynamics performances.The designed rotor-bearing structure should meet the requirements of bearing capacity,critical speed,stability,and vibration performances.Then,the structures of the AMBs are designed,and the control strategy during the switching process from AMBs to gas bearings is put forward.The deformation and stress of the rotating shaft under high-speed rotation are analyzed to verify the feasibility of the proposed rotating shaft structure.A detailed processing scheme is formulated to lay a foundation for the processing and implementation of the single-structured HGMB. |
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