Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1998-11-16
2001-03-06
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S076000, C324S545000, C324S701000, C318S805000, C318S490000
Reexamination Certificate
active
06199023
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to fault detection in operating electric motors and, more particularly, to a method for removing spurious current signatures in the process of motor current signature analysis of such motors.
Motor current signature analysis (MCSA) is a technique for monitoring and diagnosing mechanical problems associated with rotating machines driven by electric motors as well as fault conditions which may exist in the motors themselves. MCSA is based upon the fact that variations in motor geometry, load torque or speed will cause corresponding changes in motor current. The interpretation of the changes in motor current can be used to draw conclusions as to the health of the motor or load. Numerous papers and patents have been published which discuss the MCSA process. For example, Kliman et al., “Noninvasive Detection of Broken Rotor Bars in Operating Induction Motors,”
IEEE Transactions on Energy Conversion,
Vol. 3, No. 4, December 1988, pages 873-879, discusses a non-invasive detection of broken rotor bars in operating induction motors. Another paper relating to the same subject is Kliman et al., “Methods of Motor Current Signature Analysis,”
Electric Machines and Power Systems,
No. 20, 1992, pages 463-474. The latter paper lists a number of other references on the same subject. All of these papers discuss how MCSA can be utilized to identify fault conditions in either the motor or in loads connected to the motor.
In general, some systems utilizing multiple motors do not have a stiff voltage source that is capable of maintaining a constant voltage irrespective of variations in any one of the motors. As a result, motor current signature analysis may identify a problem with a particular motor when in fact the problem arises from another motor in the same system. Consequently, a motor can be detected as having a broken bar or other defect, be taken off line, and then, on physical examination, be found to not have a defect. Signals representative of such false defect detections unpredictably appear and disappear and are sometimes referred to as “ghosts.”
SUMMARY OF THE INVENTION
One of the problems, recognized in the present invention, with motor current signature analysis is that spurious signatures are created in a motor from electromagnetic interference or from other loads which may be connected to the same voltage source as the motor of interest. These false indications of defects are often caused by the spurious signals generated in a typical weak voltage system such as may be found on ships, locomotives or other vehicles utilizing on-board generated power. Accordingly, it would be advantageous to provide a method which enables identification and removal of motor current anomalies which are caused by spurious or ghost signals on the power system and not by fault conditions associated with the motor.
In conventional motor current signature analysis, time functions or spectra of motor current are examined to determine the health or status of the motor or of the driven load since the driven load creates torque or speed variations on the motor shaft which are reflected as current variations in motor current. The motor current signals may be quite small, particularly in comparison with the amplitude of the fundamental frequency of current flowing in the motor. For example, the signals to be analyzed for signature analysis may be as much as 100 DB below the power component of motor current. In MCSA (motor current signature analysis), the transducer which is used for condition monitoring is the motor itself which is simultaneously serving as an energy converter for the load. While this poses data acquisition and processing problems, the situation is further complicated by the presence of other large power converters on the same or related buses driving their own loads and responding to the particular needs and peculiarities of their loads. If the bus were ideal and maintained a perfect sine wave at line frequency with constant amplitude and phase, spurious currents would not be an issue. However, even in a large base load power plant, the auxiliary power distribution system has considerable impedances due to transformers, switches, connections and the cabling itself. These problems will be especially severe in mobile power plants and distribution systems such as on naval vessels, air craft and locomotives where the generators are relatively small and closely matched to the anticipated load magnitude. Additionally, in such installations, the prime movers themselves may introduce variation in the voltage applied to the motors.
Previous attempts to resolve the problem of spurious or interfering voltages have approached the problem on a theory that the motor impedance or admittance ought to be independent of line conditions. This theory is based on the erroneous concept that the motor is a passive rather than an active device. Further, the motor is a highly non-linear active device. Even the traditional per phase steady state equivalent circuit will fail to account for bus interference promptly since the equivalent circuit is based on the assumption that voltage and current are proportional.
The present invention recognizes the active nature of a motor, that voltages are generated in the motor windings due to relative motion of the motor shaft or by variation of motor flux and that these variations may not be directly related to the applied voltage. In general, the invention is illustrated as a method for removing spurious signals in a process of motor current signature analysis of an electric motor by using an electronic model of the motor to identify motor current signature components that are spurious signals created by variations in the applied voltage. The method includes acquiring simultaneous measurements of voltage and current at the motor, applying the measured voltage to the motor model and determining an equivalent current produced in the motor model by the applied voltage, subtracting the equivalent current from the measured current to produce a corrected motor current and then processing the corrected motor current through conventional motor current signature analysis.
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Jeffery L. Kohler and Joseph Sottile, Incipient Failure Prediction For Electrical Machines, pp. 96-109.
Jonathan A. Siegler, Motor Current Analysis For The Diagnosis Of Fault Conditins In A Motor Driven Pump, pp. 230-241.
Ronald C. Tate, Current Analysis For the Condition Assessment of Shipboard Motor Driven Machinery, pp. 277-300, Proceedings of the 49th Meeting of the Society for Machinery Failure Prevention Technology, Apr. 18-20, 1995, Va.
Kliman, Noninvasive Detection of Broken Rotor Bars In Operating Induction Motors, pp. 873-879, IEEE, vol. 3, No. 4, Dec. 1988.
Chow, Motor Fault Detection and Diagnosis, Department of Electrical and Computer Enbgineering, North Carolina State University, Raleigh, NC.
G. B. Kliman, et al, Methods of Motor Current Signature Analysis, Electric Machines and Power Systems, 20:463-474, 1992.
GA Capolino, et al, Induction Machine Parameters Identification Comparison of Different Algorithms, Int. Conf. on Elect. Machines, Mass. Ins. of Technology, pp. 940-945, Aug. 13-15, 1990.
Agosti Ann M.
Breedlove Jill M.
Bui Bryan
Geneal Electric Company
Hoff Marc S.
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