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.