Design, modelling and experimental validation of a novel bearingless machine with fully passive magnetic suspension

Magnetic bearings constitute a convincing alternative to classical solutions such as ball and journal bearings by ensuring contactless guiding of the rotor, thereby eliminating the mechanical friction and wear as well as the necessity for lubrication. They are therefore suitable for applications requiring long lifetime, high purity environments, low losses and maintenance costs or rotating at high spin speeds. Nowadays, the vast majority of magnetic bearings rely on the control of the currents circulating in electromagnets so as to exert attractive forces on ferromagnetic materials fixed to the rotor. Nevertheless, their operation requires position sensors, controllers and power electronics that negatively affect the system reliability, compactness, complexity and costs. In contrast, passive magnetic bearings (PMBs) dispense with these elements since they solely lean on passive phenomena to ensure the suspension. Although bearings exclusively based on permanent magnets (PMs) is the most intuitive solution, their use is restricted by Earnshaw's theorem which states that the rotor can not be magnetically suspended only through static fields, thus preventing the realisation of a fully passive levitation. Electrodynamic bearings (EDBs) also belong to PMBs but evade from this theorem. Indeed, the electromagnetic restoring forces arise from the interaction between the magnetic field ensuing from permanent magnets and currents that are induced in bulk or short-circuited conductors due to their relative motion with respect to these PMs. Their implementations preferably satisfy the null-flux principle, meaning that the conductors link no flux when the rotor is centred. Consequently, there are no currents and therefore no losses generated in this specific position, thus improving the bearing energy efficiency. Numerous studies have been devoted to centring EDBs, designed to guide the rotor radial degrees of freedom, notably revealing that their radial motion is unstable in the absence of a significant amount of additional non-rotating damping. Contrariwise, despite their promising stability properties, electrodynamic thrust bearings (EDTBs), that support the rotor along its axis, have focused much less research efforts, the latter being mainly dedicated to one particular topology. Fully passive levitations based on its association with centring permanent magnet bearings have been successfully tested. Along with these experiments came up models allowing us to predict their rotor axial dynamic and quasi-static behaviours. However, their scope is limited to topologies comprising two phases. Moreover, even though these bearings are intended to be integrated into fully magnetic levitations, these models do not take into account the five independent degrees of freedom characterising the rotor suspension. In this context, this thesis introduces a generalisation of the topologies of electrodynamic thrust bearings described in the literature, encompassing those relying on axial flux and radial flux structures. Considering an arbitrary number of phases, an electromechanical model describing the rotor axial, radial and angular dynamics of this broad range of embodiments is developed. The resulting linear state-space representation depends on a reduced set of global parameters that can be identified through static finite element simulations or quasi-static experimental measurements. The model shows that EDTBs can exert, in addition to the axial restoring force and thanks to a specific armature winding connection, an angular electrodynamic torque counteracting the rotor tilt. Furthermore, four performance criteria related to the rotor axial levitation are derived on the basis of this dynamic model, thus enabling us to objectively compare the intrinsic qualities of these bearings. Detailed analyses of the axial stability are also carried out through analytical developments, stressing that EDTBs are inherently stable up to the speed corresponding to the winding electrical pole. This thesis further discloses that electrodynamic thrust bearings can combine both their suspension capabilities and the rotor drive through the same permanent magnet arrangements and within a single armature winding thanks to a smart connection of the power supply to the latter, yielding the first passively levitated self-bearing machine leaning on a combined winding. In this way, the benefits in terms of compactness, reliability, mechanical simplicity and cost of passive levitation are brought to the self-bearing technology. The electromechanical model related to EDTBs is extended so as to take into consideration the rotor spin dynamics, highlighting that the equations governing the magnetic suspension are exactly identical to those related to EDTBs, the bearing function being independent from the drive. A performance criterion dealing with the rotor drive is also introduced. Finally, the operation principle and the dynamic models concerning the rotor axial and tilt stabilisations achieved within electrodynamic thrust bearings as well as their combination to the drive within the underlying self-bearing machines are experimentally validated. The association of these machines with permanent magnet centring bearings is also investigated based on a reaction wheel demonstrator designed and sized following an optimal approach, proving that the proposed self-bearing machine can be the centrepiece of a magnetic levitation solely relying on passive phenomena and therefore constitutes a major advance in the field.(FSA - Sciences de l'ingénieur) -- UCL, 202