Torsional vibration problems in rotating machinery can be difficult to recognize. Unlike radial vibration which can be easily measured with readily available sensors, torsional vibration is more difficult to measure because it involves the twisting of shafts while the machine is rotating. It can typically only be measured with special devices such as strain gauges or torsional lasers. As a result, torsional problems typically go unnoticed until something fails.
Variable frequency drives (VFDs) are commonly used to vary the speed of various types of rotating machinery to efficiently control the capacity. Typical machinery applications include pumps, fans, blowers and compressors. However, VFDs can induce dynamic torques which can excite torsional natural frequencies, leading to undetected failures.
It is not likely further testing will help with the understanding of the failure mechanism. While it may be possible to make VFD configuration changes, the VFD manufacturer has not been helpful in the failure investigation. So, this might end up being a trial and error process. It is also possible to change to a different VFD manufacturer or model. However, changing to a torsionally resilient type coupling is a simple change and has a high chance of solving the problem. Based on this, the decision was made to change to a torsionally resilient type coupling. A couple of different coupling types are being evaluated. It will be necessary to do a torsional rotordynamic analysis as part of the retrofit.
VFDs can also cause torsional vibration problems. And, these can be difficult to analyze. In some cases, field testing is needed. Even though the testing led to inconclusive results, it did show that high dynamic torque was not the problem. This provides the confidence that installing a more tolerant coupling will solve the problem.
Patrick J. Smith