The development of a flexible rotor active magnetic bearing system
Ranft, Eugén Otto
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The School of Electrical and Electronic Engineering at the North-West University is in the process of developing an Active Magnetic Bearing (AMB) research laboratory. The aim is to establish a knowledge base on AMBs in support of industries that make use of this environmentally friendly technology. AMB technology is seen as one of the technology drivers for the Pebble Bed Modular Reactor (PBMR) currently in development in South Africa and is predicted to become largely conventional in this application. In the process of developing an AMB laboratory some basic models are constructed to establish infrastructure for research investigations. The aim of this project is to develop a flexible rotor double radial AMB system. The system comprises a laminated heteropolar magnetic actuator, eddy-current position sensors, switch-mode power amplifiers and a digital controller. Emphasis is placed on stable suspension of a flexible rotor through the first three critical frequencies. This project also caters for future work on high speed losses in AM6 systems. A design process comprising aspects of modelling and analysis is developed, implemented and verified for a flexible rotor AMB system. The design commences with a system specification followed by an iterative process comprising electromagnetic design, detailed system modelling and rotordynamic analysis, and is concluded with design implementation and verification. The system design includes two interchangeable rotors; a flexible rotor for rotordynamic analyses and a rigid rotor for high speed loss analyses. The flexible rotor system is specified to experience the first three critical frequencies up to an operating speed of 10,000 rpm. The rigid rotor maximum operating speed is specified as 30,000 rpm. Rotor stability at critical frequencies places specific constraints on the equivalent stiffness and damping parameters of the AMB. An iterative design process is then initiated by an analytical electromagnetic design of the radial AMBs conducted in MathCAD® The magnetic actuator utilizes a 0.6 mm air gap and has a maximum load capacity of 500 N. A force slew rate specification of 5x10~N /s is obtained from the system's equivalent stiffness (500 N/mm) and damping (2.5 N.s/mm) parameters resulting in a 3 kVA power amplifier requirement. These parameters are used in the detailed MATLAB® modelling of the system. Stiffness and damping parameters as well as system dynamic response are verified and used to design a flexible rotor. The magnetic bearing locations, displacement sensor locations and rotordynamic response are verified using finite element methods. The design of the rotor stands central to the iterative design process since it impacts on the forces experienced by the AMBs as well as the critical frequencies of the AMB system. The most important outcome of the iterative design process is a dimensioned electromagnetic configuration and two rotor designs. The flexible rotor spans 500 mm and weighs 7.72 kg whereas the rigid rotor has the same length and weighs 12.5 kg. A centre mass on the flexible rotor lowers the first three critical frequencies to below the maximum operating speed. A 3 kVA (300 V, 10 A) switch-mode, current controlled power amplifier (PA) is developed in-house as part of the outcome of the study. The topology used is a two-quadrant controlled H-bridge, switched at 100 kHz and controlled in current-mode. The design is thoroughly verified through a process of prototyping and includes aspects of electromagnetic compatibility and protection in terms of over-current and temperature. The PA exhibits a 6 kHz bandwidth and linear characteristics and plays a critical role in the AMB system performance. The AMB controller is realised with a dSPACE® real-time development tool (DS1104), located inside a personal computer (PC). The rotational speed is monitored with an optical speed sensor while the shaft is propelled via an air turbine unit. Once constructed the actual AMB stiffness and damping parameters as well as its dynamic response are obtained. Discrepancies between the analytically predicted, simulated and experimentally obtained results are addressed and clarified. The sensitivity of the system to parameter changes is obtained as a measure of marginal stability. The rotordynamic response is characterised by measuring the rotor displacement at pre-defined locations as the rotor traverses the critical frequencies. These results show good correlation with the predicted rotordynamics. This study emphasises the importance of extensive modelling and analyses in the design of AMB systems to guarantee the required performance of the end product in terms of its dynamic performance and stability. The most important outcome of this project is a working high speed AMB model complete with integrated control. The system is versatile and allows for a variety of investigations including advanced control investigations and high speed magnetic bearing loss analyses. This project uniquely contributes to the research currently underway in the field of AMBs in the School of Electrical and Electronic Engineering.
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