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

The rotordynamic design is quite important to keep lower shaft vibration in the high speed rotating machinery. In particular, the integrally geared compressors have overhang impeller and heavy bearing load due to gear mesh, therefore it may become severe rotordynamic stability condition. The compressor venders usually make efforts to separate the rotor natural frequencies from rotating speed and getting higher log decrement for lower vibration amplitude. However, these considerations of rotordynamic design rely on the predictions. Once realize the measurement technique of the rotor natural frequency and log decrement, the compressor venders can demonstrate mechanical reliability against rotor vibration. Such a measurement has to perform under actual operating condition, because the bearing coefficients significantly depend on the bearing load and rotational speed. Generally, it is difficult to evaluate the rotor log decrement from steady operating vibration measurement, thus some researchers attempt shaking test to excite the rotor vibration modes. Since the excited vibration responses shaking from the outside of the casing are relatively small, further effective excitation method is desired for the rotor inside the casing. The API standard exemplifies the rotor excitation method using the magnetic bearings. Research in the past has established a rotordynamic measurement test sequence applying the magnetic bearings for the inline centrifugal compressors. However, most integrally geared compressors have no extra space to install the magnetic bearings on the rotor, because there are a lot of components on the rotor, i.e., shaft seals, bearings, impellers and a gear mesh. Hence, it is difficult to apply the magnetic bearings to the integrally geared compressors for rotordynamic stability evaluation.

In this paper, the authors have developed a direct rotor excitation technique using the magnetic exciters designed in-house. The development of compact magnetic exciters enables to incorporate into the casing and can be an effective method of exciting rotor vibration modes. Four magnetic exciters are installed facing the back side of impeller in the circumferential direction. The magnets dispose outside the impeller to generate a bending moment that excites the rotor vibration mode though the generated forces acting in the axial direction of the rotor. The rotor has two rotating directions of vibration mode, one is along the rotational direction and the other is in the opposite direction. Test procedure implement a rotational excitation method to excite each directional mode independently based on phase controlled magnet currents. The excitation in the rotating direction is called forward rotational excitation, and the excitation in the opposite direction is called backward rotational excitation, respectively. This rotor has totally four proximity probes, two each at 1st stage and 2nd stage respectively which is commonly utilized near the journal bearings in X-Y direction to monitor shaft vibration. These probes will detect the shaft vibrations generated 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 excitation denotes enough amplitude response and thus the magnetic excitation method proves to be effective. In this study successfully carry out the measurement of natural frequencies and log decrements of operating rotor. In a tilting pad journal bearing (TPJB) case, the predicted natural frequencies are in good agreement with tested results. And test results quantitatively prove that the actual rotor log decrement is greater than 0.1 with enough rotor damping. The natural frequency of the rotor with an integral squeeze film damper (ISFD) bearing is much lower than the value with TPJB (without ISFD). The calculated first forward mode log decrement well corresponds with the measured value. However, the measured second forward mode does not reach the predicted damping performance. These test results demonstrate the effectiveness of the developed rotor excitation method for integrally geared compressor pinion rotors.