Design and Evaluation of Hybrid Magnetic Bearings for Turbo Compressors

Abstract

Recently, high capacity and high efficiency turbo machines such as turbo blowers and turbo compressors have been being developed. To keep up with this trend, magnetic bearings are increasingly being applied to turbo machines instead of air foil bearings. In this study, a hybrid magnetic bearing composed of a permanent magnet and an electromagnet was applied to 300-HP (horsepower) turbo compressors with rated speeds of 50,000 rpm. The length of the shaft should be reduced as short as possible so that the compressor rotor can rotate stably while maintaining a small vibration at a rotation speed of 50,000 rpm. In this study, the additional shaft length for the axial gap sensor is eliminated by applying a new layout in which the axial gap sensor is placed on the inner surface of the thrust magnetic bearing. No-load tests and full-load tests in the manufactured turbo compressors were performed to evaluate the performance of the designed magnetic bearings. The biggest obstacle to this development is the heat problems caused by compressor impellers and high-speed motors. Thanks to the cooling system using water and air, the problem of thermal expansion of the rotor in the axial direction can be avoided. Thrust forces were estimated using the control current for thrust magnetic bearings. The experiment is also performed to evaluate the vibration of each turbo compressor and the results are presented. Based on the evaluated vibration, it has been confirmed that the hybrid magnetic bearings and the new axial gap sensor layout can support the turbo compressors stably.