Design and control for hybrid magnetic thrust bearing for turbo refrigerant compressor

21,000RPM, 145RT class turbo refrigerant compressor which is supported by magnetic bearings are under development. Oil-free bearing has merits that the continuous operation is possible because the lubrication system is not required and the refrigerant is not contaminated by the oil. This paper presents the design procedure of thrust hybrid magnetic bearing (HMB) and its controller for turbo refrigerant compressor. Thrust HMB consists of permanent magnet and electromagnet and it is designed to support 2,000N thrust force by compressor at 21,000rpm operating speed. After building the thrust bearing test rig and control system with PID controller, plant model of hybrid thrust bearing was established by measuring FRF(frequency response function) from the test rig. Based on FRF, PID gain was optimized and the test rotor was successfully levitated with 4.7μm standard deviation from the reference axial position. The additional DOB(disturbance observer) was designed to attenuate the low frequency disturbance and the measured error sensitivity transfer function shows that DOB works well as it has designed.