THE PREDICTION AND EXPERIMENTAL MEASUREMENT OF TORSIONAL VIBRATIONS IN VARIABLE FREQUENCY DRIVE CONTROLLED INDUCTION MOTOR MACHINERY

Abstract

The performance of detailed, mechanical system analyses of rotating machinery is an important step in avoiding potential failure due to excitations on the system. Excitations such as compressors, pump, turbines and motors and can generate both steady-state and transient vibrations leading to fatigue failure. With the growing use of motors as a means to drive machinery, the popularity of Variable Frequency Drives (VFDs) to control motor speed has also increased. These speed controllers present many useful benefits to the industry, however, an increasing number of system failures caused by VFDs have been recorded in the literature. These failures are due to undesirable torque harmonics caused by the interaction of both the switching frequency and carrier frequency of the VFD. When close to a natural frequency of the mechanical system, these unwanted torque frequencies can cause large vibration amplitudes leading to fatigue failure. In this thesis, an in-house “VFD Software” which numerically analyzes the coupled electrical and mechanical side of a rotordynamic system to output the system response and predicted fatigue life is further developed. The accuracy of the fatigue life prediction is improved by allowing for a strain life approach to be performed over the more conservative and less accurate stress life method. This method incorporates the material ductility and strain into the life prediction and treats the life degrading factors of surface finish, size, stress concentration, and notch sensitivity as functions of life instead of constants. Furthermore, the transient analysis of the software is also improved by allowing the users to manually input a drive torque as a series of torque frequencies, amplitudes, and phase angles in the case that the electrical system harmonics are previously known. To further study the presence and accurate prediction of torque harmonics and vibrations in rotating machinery, a VFD test rig has also been designed and fabricated. The test rig incorporates the use of a VFD controlled, 2-pole induction motor to drive a shaft supported by two ball bearings and a tilting pad bearing. A torque transducer, torsional vibration geared sleeve, and 4 Bently probes are used to measure the dynamic torques and torsional vibrations along with other system responses. Initial experimental results show the presence of a torque harmonic spectrum which interacts with the software predicted natural frequency of the system. This spectrum can be correlated to integer harmonics of the running speed and values predicted by the VFD Software. The rig also proves to be able to measure small vibration amplitudes that are predicted by the software from known torque frequencies and amplitudes.