Turbo-Molecular Pump Levitated By Unbiased Active Magnetic Bearing

A turbo molecular pump (TMP) is an axial flow compressor used for production of high and ultrahigh vacuum conditions. Ultrahigh vacuum is required for several industrial applications such as semiconductor technology or materials research. Usually the rotor of a TMP is balanced contactless by using magnetic bearings. They are used to achieve reliable high speed operation without any wear and lubrication. Conventional pumps are equipped with actively controled biased magnetic bearings. This paper gives information and results of an research project which aim was the development of a “low cost” drive system for turbo-molecular pumps. The significant feature of the development is the application of active magnetic bearing without using magnetic bias. Advantages and disadvantages of magnetic bearing without premagnetization are considered. As a consequence of the lack of magnetic bias the unbiased magnetic bearing is much more nonlinear than the biased magnetic bearing. Due to nonlinearity a nonlinear fuzzy controller is employed in order to determine the actuating currents. The paper shows that by using active magnetic bearing without magnetic bias a stable position control is feasible. Advantageously the lack of bias leads to a significant reduction of power losses. Furthermore the simplified power electronics decreases the costs due to the reduction of electronic components. For determination of axial and radial shaft displacement contactless and low-cost inductive sensors were developed. They operate according to transformer principle. While keeping the construction of TMP as compact as possible a significant effort has to be done in order to obtain an efficient electromagnetic screening effect. Further on the use of brushless DC motor (500 W, 36.000 r.p.m.) in place of squirrel-cage motor decreases the power loss of the complete system even more. The computer-aided control of a motor occurs sensorless. The lack of compulsory angle resolver and speed sensor leads to a reduction of electronic components and production costs.