Design of a Magnetic Bearing for an Electrical Machine

Súčasný trend vývoja vysoko-otáčkových strojov otvára ďalšie oblasti bádania. Jednou z nich sú aj magnetické ložiská. Tradičný prístup uchytenia rotora sa stáva u týchto typov strojov prekážkou a limitujúcim parametrov vďaka vysokému treniu. Preto v aplikáciách s turbo pohonmi nachádzajú magnetické ložiská svoje uplatnenie. Rovnako aj vývoj elektroniky nastolil možnosti kvalitnejšieho a rýchlejšieho riadenia v zložitejších riadiacich štruktúrach. Najväčšou výhodou magnetických ložísk je ich chod bez mechanického trenia a takmer nulová údržba, avšak v porovnaní s tradičnými ložiskami zaberajú omnoho viac miesta a zvyšujú dĺžku hriadeľa. Za účelom eliminácie nárastu dĺžky hriadeľa sú čoraz častejšie konštruované systémy s integrovaným magnetickým zavesením rotora, čím sa dosiahne efektívne rozloženie jednotlivých častí v stroji. V tejto diplomovej práci je diskutovaný návrh magnetických ložísk pre motor k turbocirkulátoru 45000ot/min s výkonom 12kW. Dôraz pri vypracovaní tejto práce je kladený na praktický návrh magnetických ložísk a aj na voľbu parametrov. Analytický výpočet vyjadruje súvislosti medzi jednotlivými parametrami, avšak zanedbáva viacero parazitných vplyvov, takže samotný návrh je ďalej podrobený optimalizácii a numerickej analýze. K týmto analýzam je pridaná aj kontrola kritických otáčok. Výsledky prezentovanej práce budú následne využité na výrobu laboratórneho vzorku s porovnaním výsledkov.The current development in the field of electric machinery is focusing on high-speed electric machines. This opens also other fields related to high-speed machines. One of them are magnetic bearing systems. Tradition approach of using ball bearing brings a few problems in design dealing with friction at high speeds. Together with magnetic bearings, development is their control. Faster chips opened a new way of thinking of control and helped to evolve robust control loops. The biggest advantage of magnetic bearing is non-friction run and almost no maintenance. Compare to traditional ball bearing, a magnetic bearing system needs more space and in some applications could happen that the shaft will be twice as long. This problem can be solved designing complex system with motor and integrated magnetic bearing what leads to downsizing. In this master thesis, the design of magnetic bearing for 12kw, 45000rpm is discussed. It focuses on practical design and correlations between parameter selection. The analytical approach is used to sketch the design and optimization is done afterwards. Problem with an analytical design is that it doesn't cover all parasitic phenomenae and thus numerical modelling snd optimization are demanded. Also, the critical speed analyze is included in this thesis. The results of the work will be used for manufacturing prototype as an extension to the existing high-speed machine.

3 Dimensional Electromagnetic Analysis of an Axial Active Magnetic Bearing

In the rotating electrical machines, active magnetic bearing are basically performing the same role like mechanical bearings to support rotor. The function is based on the principle of magnetic levitation. The idea behind this involves creation of a magnetic field by supplying controlled currents in the bearing coil through amplifiers and complex power electronics. The accurate design of a magnetic bearing system incorporates many parameters before its implementation. The current work of the thesis encircles only the three dimensional (3D) modeling of axial active magnetic bearing (AMB). The static and dynamic models are analyzed for the bearing with a consideration of nonlinear material. In the study, the major emphasis is on the magnetic field, eddy current behavior and exerted magnetic forces in the magnetic bearing. The required input parameters for simulation are considered from the available two dimensional (2D) analysis for the same axial actuator. Elmer open source finite element tool is used in the entire work for making 3D simulations. Finally, the computed results are compared with the 2D case. As a part of the thesis work, a modified geometry is simulated to analyze eddy currents. The hypothesis in later task is the reduction of eddy current losses by providing a radial cut in the bearing ferromagnetic path. The radial cut brings asymmetry in the bearing and the three dimensional analysis provides the possibility to analyze the complete model. The results obtained in the above work provide a good understanding of 3D fields in axial AMB and the computed magnetic forces are in good agreement with the 2D results

Developments on Electrodynamic Levitation of Rotors

Magnetic bearings are systems capable of supporting rotors in absence of mechanical contact. Among many advantages with respect to ball and roller bearings are the possibilities of operating at extremely high rotational speeds and free of maintenance. Nevertheless, classical active magnetic bearings (AMB) are costly systems and may suffer from reliability problems. The most common types of passive magnetic bearings (PMB) based on the use of permanent magnet and reluctance forces are robust and relatively cheap but are affected by an intrinsic stability problem related to negative stiffness. The alternative of superconducting bearings has to deal with the difficulties for guaranteeing low temperatures for the superconducting materials to work; this represents a barrier for this technology. In the last decades an alternative for obtaining stable passive magnetic levitation has been searched, leading to the development of electrodynamic bearings. These systems, capable of realizing electrodynamic suspension for rotors using regular materials at room temperature, may be an alternative for the suspension of high rotational speed machines in the near future. The technological solutions proposed are still unable of devising a system capable of demonstrating the feasibility of this concept. Introduced in this context, this doctoral dissertation aims at developing models and design procedures to bring electrodynamic levitation of rotors closer to industrial applications. To this end, a large portion of the work is devoted to develop a unified model for representing the electromechanical interaction between rotor and stator generated by electrodynamic bearings of different types, namely homopolar and heteropolar configurations. The electromechanical model is developed taking advantage of the complex coordinate representation, typical in rotordynamics, in order to enable easy integration of the bearing's model with different rotordynamic models. An experimental validation of the model is carried out for homopolar configurations. The study of the dynamics of rotors on electrodynamic bearings is probably one of the most important aspects that must be dealt with before the bearings can reach the technological development needed to become industrially available. Bearing this in mind, the dynamics of a Jeffcott rotor and that of a four degree of freedom rotor are studied devoting special attention to the study of stability demonstrating the presence of unstable cylindrical and conical modes. The unbalance and frequency responses of the rotor on electrodynamic bearings are used to evidence the advantages and drawbacks between homopolar and heteropolar configurations. The studies are conduced using the state space formalism to obtain easy to manipulate system models. The modelling of the suspension evidences the strong coupling between the subsystems, showing that the influence of each subsystem on the rotordynamic stability is not obvious, thus complicating the design of the whole suspension. Considering an iterative design approach, the design of a test rig is presented. It is designed to test the validity of the models and the feasibility of radial electrodynamic suspension. A the mechanical layout of the test rig is developed to deal with the stability aspects introduced by the use of electrodynamic bearings

Stabilization of electrodynamic bearings with active magnetic dampers

Electrodynamic Bearings (EDBs) are a kind of passive magnetic bearings that exploits the interaction between the induced eddy currents in a conductor and a magnetic field to provide re-storing forces. They have been regarded as an appealing alternative to Active Magnetic Bearings (AMBs), having the ability to provide positive stiffness passively without introducing negative stiff-ness in any direction. Compared to AMBs, EDBs present advantages such as lower cost and higher reliability due to simplicity of configurations. One of the most interesting features of EDBs is the possibility to obtain stable levitation using standard conductive materials at room temperature, requiring no control systems, power electronics or sensors. Thus EDBs could be suitable solutions for highâspeed rotating machinery such as flywheels, small size compressors, centrifuges and vac-cum pumps. Despite these promising characteristics of EDBs, applications are still limited because of instability issues. The main problem is that the effect of the rotating damping force in EDBs causes unstable behavior of the rotor. In existing solutions, stabilization is achieved mainly by introducing non-rotating damping to the rotor with passive ways. Although stable levitation is possible, the effectiveness of the existing methods is still limited. A hybrid solution has been proposed in this thesis, where EDBs are combined with active magnetic dampers (AMDs). Using similar magnetic actuators as those used in classical active magnetic bearings (AMBs), nonârotating damping forces are applied on the rotor supported by EDBs to obtain stable operation. This system is designed to exploit the high reliability of EDBs, overcoming the stability problem by means of controllable AMDs. It results in increased global system reliability. In case of AMBs failure, the EDBs are able to guarantee a stable levitation down to a certain speed considered safe for touchâdown. During the operation speed range, the AMDs provide nonârotating damping to stabilize the rotor. This nonârotating damping can be easily tuned during rotor operation phase. At low speeds when the EDB forces are not sufficient to support the rotor, the active magnetic actuators work as AMBs to guarantee stable levitation of the rotor in a wide speed range. Besides, the EDBâAMD configuration also allows characterizeing in dynamic condition, which opens the possibility to establish damping strategy that can in perspective be implemented by totally passive means, such as eddy currents, elastomeric mounts. The combination of EDB and AMD forces are studied both analytically and experimentally. An analytical model of the system, as well as a test rig, has been built. Simulations and experi-mental tests validate the model and characterize the system. The effectiveness of the proposed solution is confirmed. The control strategy of AMDs and stabilizing alternatives of EDBs are dis-cussed consequently. The combination of EDB and AMD can be exploited to investigate easily dif-ferent damping strategies