TORSIONAL (ROTATIONAL) VIBRATION: EXCITATION OF SMALL ROTATING MACHINES

Abstract

The investigation of torsional vibration of rotating machines and rotational vibration of structural systems has been hindered by the lack of suitable transducers and torsional exciters. The advent of the laser torsional vibrometer has provided an accurate non-contact measurement technique for rotating machines. There is a requirement for further advancement in torsional excitation methods, particularly for low cost, low torque applications. In this paper, experimental results are presented which demonstrate that a commercially available a.c. servo-drive may be used as a combined drive motor and torsional exciter for small rotating test rigs. Two optimum servo-amplifier settings were identified and the maximum sinusoidal dynamic torque capacity running unloaded (below 1400 r.p.m.) was measured to be 1–10 Nm (r.m.s.), at frequencies up to at least 2 kHz. Random excitation was also possible at lower frequencies. The servo-drive will find applications in torsional vibration testing of small rotating machines and potentially for structural testing. The exciter has been characterized (i.e., the torsional stiffness and damping were measured) over the full speed range (2000 r.p.m.), so that the performance when attached to a particular test system may be predicted. Because previous researchers have demonstrated that inconsistent results may be obtained when experimentally measuring torsional damping levels, independent frequency and time domain techniques were used to determine the torsional stiffness and damping properties of the servo-drive, over a range of operating conditions of speed, load and torsional excitation level. A high degree of consistency was obtained between the results provided by using the different techniques under appropriate test conditions. The servo-drive was found to be a non-linear SDOF torsional system, with stiffness and damping levels dependent on motor speed, torsional excitation level and load inertia.