Making electrodynamic centering self-bearing machines dynamically stable using homopolar current

Details

Supervisors

Dehez, Bruno ; Van Verdeghem, Joachim

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en génie de l'énergie , à finalité spécialisée

Abstract

enSelf-bearing machines combine the functions of rotor drive and rotor guidance within the machine. This type of machine offers a high degree of compactness by integrating these two functions. This makes them particularly attractive for applications involving high rotational speeds or requiring high power density. In addition to these advantages, there are benefits of magnetic levitation which eliminates the need for lubrication, drastically reduces the friction and, consequently, wear. Initially, the rotor guidance within the machine was based on the principle of active magnetic bearings. This approach requires specific components to regulate the current flowing through the windings to generate the appropriate guiding forces. This has led to an interest in passive stabilisation techniques. The integration of passive levitation systems in self-bearing machines has been made possible by the principle of electrodynamic bearings. In centering configurations, these bearings can generate both the drive torque and a passive radial restoring force within a single multi-function winding thanks to an appropriate connection of the coils to the power supply. However, theoretical and experimental investigations of electrodynamic centering self-bearing machines have shown that they suffer from intrinsic dynamic instability. This instability occurs over the entire speed range. To date, only the addition of external damping systems have stabilised the rotor. Due to these limitations, research in this field has focused more on self-bearing machine with electrodynamic thrust which generates passive axial restoring force. In thrust configuration, the axial dynamic instabilities occur only at high rotational speeds. Recently, it has been demonstrated that these instabilities can be eliminated through intrinsic damping by considering nonlinear electrodynamic effect. Indeed, this damping strategy is based on an additional force component arising from the second-order dependency of the flux linkages with the rotor eccentricity. On the basis of these new findings regarding the second-order consideration in electrodynamic thrust self-bearing machines, it was decided to investigate their impact on electrodynamic centering self-bearing machines. This master thesis demonstrates that new force components arise when the second-order dependency of fluxes is taken into account. This effect has not yet been observed because it requires the presence of a homopolar current which is often prevented by the standard connection of electrical machines. First, the electromechanical model of the electrodynamic centering self-bearing machine is established considering nonlinearity of flux linkages. Then, the purely passive behaviour is described to highlight analytically the origin of the instability of these kind of machines. The stability analysis shows that the instability is only exacerbated by the induced homopolar current. To solve this instability, a pseudo-passive method based on the addition of a negative homopolar resistance is proposed. It is demonstrated that this method is not feasible due to dynamic effects. To counteract these dynamic effects, an active control method of the homopolar current via the power supply is proposed to regulate additional forces acting on the rotor. Based, on this control law, two intrinsically damping strategies are developed. These damping strategies allow the rotor to stabilise either in a predefined radius orbit or at the centred position. These methods solve the instability of electrodynamic centering self-bearing machines. Finally, an experimental validation is described to prove the feasibility of the model using a prototype developed during previous work. In this phase, the new force components are validated and the feasibility using them to counteract the force causing the instability is demonstrated.