Design und Sensorless Control of a Magnetic Bearing System

In the present project a novel design of a radial magnetic bearing system will be investigated and optimized. Compared to the state of the art, this concept includes a overall consideration of efficiency and economy including the magnetic bearing, sensorless control and power electronics. Therefore the radial magnetic bearing will be constructed with permanent magnets for bias flux generation and consists of only three concentrated coils for force generation in two directions. Additionally the homopolar structure reduces losses during high rotation speed. For design of the magnetic bearing Finite element simulation will be used. The examined electromagnetic properties like inductance behaviours and flux distribution will be used in further dynamic simulations in Matlab/Simulink to get a model of the magnetic bearing. The well known INFORM-method from the field of sensorless control of PM-drives will be used for sensorless control. Because of avoiding external position sensors a sensorless method reduces the hardware costs dramatically. The influence of parameters of the sensorless method like bandwidth, accuracy, nonlinearities and eddy currents on the closed loop behaviour have to be investigated and optimized. The proposed three phase structure of the magnetic bearing allows using a standard three phase voltage inverter, which is state of the art for many years in industry. Different methods for current measurement are known and will be compared within this project. Compared to the standard inverters with eight power switches the simple three phase inverter structure reduces costs and losses during operation. Based on the improvement in each part of the magnetic bearing system, the whole system can be optimized in efficiency and economy. This new concept has lower costs in manufacturing and operation and will increase the field of possible applications of magnetic bearings.

The goal of this research project was the investigation and development of a sensorless method for determination of the rotor displacement of an active magnetic bearing. Thus, usually used external position sensors for stabilization of the principally unstable behavior of active magnetic bearings is omitted and replaced by mathematical methods. The sensorless rotor position determination is based on the INFORM method, which was developed in the 1990s to detect the angular rotor position of a synchronous machine without any external angular sensor. With basic investigations to proof the applicability of this method to magnetic bearings and further by an iterative optimization of the bearing topology, the voltage source inverter and the detailed implementation of the INFORM method in the control software, all project goals were achieved successfully.Low hardware cost: The developed prototypes can be manufactured with known methods and process of classical electric machines. Because of the three phase magnetic bearing concept conventional three phase voltage source inverters with accordant power electronic components can be used. The sensorless method does not require high computational power and can be implemented in a small digital signal processor. Only small adaptations of the inverter hardware are required.Low operational cost: The sensorless method is able to operate at bearings equipped with permanent magnets providing the bias flux in the homopolar bearing. By this low losses at high electromagnetic forces are achieved.Sensorless control: The functionality of the sensorless control was verified in a final prototype bearing. This prototype setup consists of a reluctance synchronous machine equipped with two radial active magnetic bearings. Each provides a two degree of freedom stabilization. The axial direction is stabilized passively by the prototype arrangement. Finally the stability of the sensorless control approach is as well verified during rotation.