Mechanical Vibration Problems With Variable Frequency Drives

Variable frequency drives are now frequently used to power large horsepower pipeline pumps to eliminate power costs wasted by throttling, to reduce inrush current on motor start, and to provide greater operating flexibility. This variable speed operation, however, can cause vibration problems in the pump and motor bearings, and in the couplings that are not normally experienced with fixed speed pumps. Enbridge Pipe Line has installed a number of variable frequency drives on units ranging from 2500 to 5000 hp in size and discovered unexpected torsional and lateral resonance vibration in the equipment at certain operating speeds. This presentation discusses the tests that were conducted to determine the source of the vibration, the theoretical analysis of the vibration modes and equipment responses, and the solutions that were implemented to reduce or eliminate the vibration. Recommendations are given for design principles that could be incorporated into the initial pump station design in order to avoid vibration problems with variable speed operation.