Rotordynamic Design and Control of Three-Stage Centrifugal Compressor with Magnetic Bearings

Demand for turbo chillers is increasing for cooling facilities such as large buildings and data centers, and the application of oil-free technology, known for its eco-friendly high efficiency, is expanding to large scale projects. In this study, a large capacity magnetic bearings and rotor dynamics design are described for the development of an oil-free turbo chiller with 2400-3000 USRT performance. First, the rotor and magnetic bearings supporting the three-stage impeller of the large capacity compressor were designed and developed. A constant speed motor rotating at 50 Hz was applied, and the shaft was designed to be long and heavy because of the high load aerodynamic design at low rotation speed. Therefore, the focus was placed on designing sufficient separation margin for the first bending mode. Second, it was important to design the stability of the magnetic bearings with notch filters and PID position control. It is necessary to verify the stability of the bearing support over a wide load range, as efficiency must be maintained not only under maximum load but also during load fluctuations. The system identification of magnetic bearings was used to optimize the controller, and the control stabilization was satisfied through the operation data. Based on these studies, the operation of large capacity turbo chillers was stabilized and the product was successfully commercialized. The chiller introduced in this study was installed in the Middle East and proved its performance by applying a large capacity magnetic bearing for the first time in the oil-free turbo chiller industry.