High speed flexible rotor active magnetic bearing control
Marais, Charl Henri
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The School of Electrical, Electronic and Computer Engineering at the North-West University is in the process of establishing a knowledge base on Active Magnetic Bearings (AMBs). In support of this initiative this project is aimed at characterising an in-house developed double radial heteropolar AMB system. Before characterising the AMB system the acoustic noise problem of the system had to be addressed and reduced to an acceptable level. To reduce the acoustic noise of the system a noise analysis was done to determine the source of the noise. The analysis revealed radiated noise from the electromagnets and power amplifiers (PA) and conducted noise on the signals to and from the controller. The conducted noise is reduced by using anti-aliasing (AAF) and anti-imaging filters (AIF) before and after the controller. The effect of the radiated noise is reduced by synchronising the sampling of the sensor signals with the switching of the PAS. The characterisation of the AMB system starts with a Mass-Spring-Damper (MSD) simulation which is a linear representation of the AMB system. This simulation is used to understand the basic principles of a second order system and to compare its response to the nonlinear AMB simulation. The following step in characterising the AMB system is to determine the effect of filters on the nonlinear AMB simulation and to determine the simulation characteristics. These characteristics are compared to the MSD simulation and the actual AMB system. The characteristics compared between the MSD and AMB simulations are the static, second order and dynamic stiffness. The actual AMB system was characterised before and after the AAF and AIF were implemented. This provided the opportunity to determine the effects of the filters on the actual system and not just from simulations. The characteristics measured on the actual AMB system include the static stiffness, dynamic stiffness, rotor dynamics and system sensitivity. The stiffness characteristics of the actual AMB system showed good correlation with the linear and nonlinear simulations. The measured results showed a decrease in static stiffness and an increase in system sensitivity because of the AAF, AIF and controller pole. It also showed that the effects of the filters can be reduced by moving the controller pole to a higher frequency. The characterisation of the double radial heteropolar AMB system provides a fundamental understanding of the AMB performance aiding the AMB design process.
- Engineering