The use of the motor as a transducer to monitor pump conditions
1995
Motor current and power analysis methods have been developed to assist in the condition monitoring of a variety of motor-driven devices. The successful implementation of motor current signature analysis (MCSA) as a diagnostic for valves led to its application to other devices and to refinements in the methodologies used. A variety of pump applications, ranging from 5 to over 1200 horsepower have been analyzed, including low and high specific speed and suction specific speed pumps. For some of the pumps, the full range of flow conditions from shutoff to runout has been studied. Motor current and power analysis have been found to provide information that is complementary to that available from conventional diagnostics, such as vibration and pressure pulsation analysis. Inherent signal filtering associated with rotor to stator magnetic field coupling does limit the high frequency response capability of the motor as a transducer; as a result certain phenomena, such as vane pass energy, is not readily apparent in the motor electrical signals. On the other hand, the motor-monitored parameters have often been found to be much more sensitive than vibration transducers in detecting the effects of unsteady flow conditions resulting from both system and pump specific sources such as suction cavitation. By combining motor equivalent circuit models with pump performance characteristics, shaft power and torque fluctuation estimates have been assessed. The usefulness of motor data in assessing some common sources of pump problems, such as mechanical and hydraulic imbalance, misalignment, and unstable flow conditions is shown. The results of testing several motor-driven pumps, including comparisons with vibration and pressure pulsation analysis are discussed. The development of a single figure of merit for pump load stability (as a function of pump flow rate and type) is presented.
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