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The Experimental Rotordynamic Stability Evaluation Method Using Magnetic Excitation System for an Integrally Geared Compressor


 In Turbomachinery the rotordynamic design is highly important to keep lower shaft vibration in the high speed rotating machinery for personal safety & machine protection. To resolve same, the authors have developed a direct rotor excitation technique using the magnetic exciters designed in-house. The development of compact magnetic exciters enables to be incorporated into the casing and can be an effective method of exciting rotor vibration modes. Four magnetic exciters are installed facing the back side of an integrally geared compressor impeller in the circumferential direction. Test procedure implemented a rotational excitation method to excite forward mode and backward mode independently based on phase controlled magnet currents. In this case rotor has totally ten shaft vibration sensors which detect the rotor vibration modes excited by magnetic excitation technique. The directional frequency response functions (dFRFs) are separate results of forward mode component and backward mode component included in FRF. It achieves more accurate modal analysis to obtain natural frequency and log decrement. The dFRFs with rotational excitation denotes enough amplitude response and thus the magnetic excitation method proves to be more effective. This study successfully carry’s out the measurement of natural frequencies and log decrements of operating rotor. The comparison test results of tilting pad journal bearing (TPJB) with and without the integral squeeze film damper (ISFD), that show the ISFD reduces natural frequency and considerably increases log decrements. These test results demonstrated the effectiveness of the developed rotor excitation method for integrally geared compressor pinion rotors.