Analysis Guide For Variable Frequency Drive Operated Centrifugal Pumps
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The use of variable frequency drives (VFD) in pumping applications with variable-duty requirements provides the user with a variety of benefits, including potentially significant energy savings and improved reliability achieved by means of speed reduction and avoiding part-flow operation. Energy savings are primarily realized by running the equipment at high levels of efficiency and optimal operating speeds, matching the generated pump head to the exact system requirements without the use of energy consuming control valves. Running pumps at lower operating speeds and avoiding part-flow operation also positively influences component life and between maintenance intervals. The primary mechanical challenge of any VFD applications the wide continuous operating speed range. Excitation frequencies of fixed speed applications miss most natural frequencies of the structure, rotor, etc., and therefore potentially harmful resonance conditions often do not occur. This is no longer the case with VFD applications, where excitation frequencies become variable and the likelihood of encountering resonance conditions is greatly increased. Problems and failures in pumps and associated systems that are not caused by resonance are generally not VFD related and are therefore not discussed. The paper gives an overview of medium-voltage VFD technology as well as the main categories of resonance conditions of concern with regard to mechanical vibrations of pump/motor sets. The analytical and experimental identification of resonances related to lateral rotor, torsional rotor, structural, and acoustic dynamics are discussed in detail. The applicable set of analyses and, if necessary, the corresponding appropriate corrective measures, are designed to help ensure operation free of harmful resonance conditions and problems caused by excessive mechanical vibrations.
Kaiser, Thomas F.; Osman, Richard H.; Dickau, Ralph O. (2008). Analysis Guide For Variable Frequency Drive Operated Centrifugal Pumps. Texas A&M University. Turbomachinery Laboratories. Available electronically from