Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2000-06-06
2001-05-22
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S064000, C702S060000, C324S701000
Reexamination Certificate
active
06236947
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally pertains to a device and method for evaluating and reporting motor condition and performance and more particularly, to an improved method and apparatus for evaluating motor performance and assessing motor condition while the motor is operating.
BACKGROUND OF THE INVENTION
Motors, particularly electrical motors, play a key role in industry. Such motors are used to drive fans, pumps, compressors, valves, and many other machines. It is potentially very costly to allow a significant problem to go on unnoticed in either the motor or the motor driven machine. It is also costly and very time consuming to attempt to repair a nonexistent problem. With present methods of analyzing motor performance, these costly situations often occur. Thus there is a need for an improved diagnostic method and apparatus for use with motors and motor driven machines.
Prior art motor monitors do not report exact shaft speed, comprehensive motor condition, or efficiency of a continuously running motor without the use of intrusive sensors. Such prior art monitors also do not provide automatic probe misplacement detection and correction, nor do they statistically analyze data on a per load basis, to provide a meaningful view.
Prior art motor monitors also have serious deficiencies in measuring the exact speed of an electric motor. Generally, the motor shaft speed is measured in one of several ways, all of which are very inconvenient to a user. One method of measuring shaft speed is to use a tachometer attached to a shaft of the motor. This requires a mechanical connection which, in turn, mandates shutting the motor down. Alternatively, a reflective tape or a notch can be placed on the shaft to provide a once-per-revolution phase reference and speed indicator. However, in addition to requiring a motor shut down, the speed accuracy is limited because the timing sample occurs only once per shaft revolution. Another method of determining shaft speed is to calibrate the motor driver to measure shaft speed based on load and torque. This is an indirect measurement, requires software specialized to the particular motor and drive, and is only suitable for use on a very small number of motors.
Prior art motor monitors for detecting winding faults require the use of coils intrusively placed within the motor, or alternately, monitoring the neutral return line for radio frequency evidence of arcing. These monitors require extra hardware beyond the normally used three current and three voltage probes. Additionally, in most motors the neutral wire is unavailable. Accordingly, to implement comprehensive detection, all three electrical leads have to be monitored with high frequency probes in addition to the usual power frequency probes. This involves multiple additional sensors and channels, high frequency sensors, and extra signal conditioning.
Prior art motor monitors for detecting broken rotor bars generally search for sidebands of the first harmonic of the synchronous frequency for detecting broken rotor bars. This method of detection is not totally reliable because torque variations in the load as well as the use of certain external equipment give rise to interfering sidebands. For example, the use of belt drives can produce such interfering tones. Also, air gap asymmetries between the rotor and stator can give rise to artifactual tones in that region of the spectrum.
Another shortcoming of prior art motor monitors is the need to take the motor off-line in order to ascertain resistive and inductive unbalance. This is a great inconvenience when dealing with continuous-duty motors. In addition, the off-line measurements may not be truly representative of the on-line resistive or inductive balance, since operational stresses may significantly change the state of balance. Further, prior art motor monitors for measuring efficiency require a dynamometer to measure the load.
The present invention provides a motor monitor which measures a motor's efficiency without the requirement of a dynamometer. In addition, the present invention performs two unique functions which improve the efficiency measurement: (a) correcting for unbalance in the power source voltage; and (b) compensating for line loss. The present invention provides a probe configuration for obtaining motor data and a processor for analyzing the motor data. A memory is included for storing motor data (e.g., in a database), which allows for comparisons to be made between the motor's present condition and performance and motor historical data to determine the presence of trends, and data on similar “sister” motors to ascertain normality with respect to a population of similar motors. The database accumulates knowledge about the motor, which may be sorted by load. The information provided by the present invention is useful for changing operating load, scheduling maintenance, planning for replacement, or even shutting down, as is appropriate. Furthermore, the system compares multiple motors under similar operating conditions so that comparisons can be made, which are useful, for instance, in making purchasing decisions.
The present invention determines shaft speed, condition, and performance of a three phase induction motor, and more specifically measures the exact shaft speed, detects stator winding shorts, detects broken rotor bars, and measures output and efficiency using only current and voltage measurements, in conjunction with a knowledge of approximate shaft speed, motor resistance, and historical electrical data from the same motor under an uncoupled condition. In addition, the present invention provides features of great convenience to a user which prior art systems do not offer, such as automatic probe check and software correction for misplaced probes.
The present invention can be used to provide either on-line continuous monitoring of condition and performance, or as a periodic inspection tool. The present invention also provides detailed analysis capability for intensive diagnostic work and a database for determining trends and relative performance (compared to similar motors). The present invention avoids the limitations and problems with the prior art by providing new, meaningful representations of the data, comparing results with historical data and sister motors and by providing on-line monitoring or periodic inspection functions, as well as automatic probe misplacement detection and correction. Algorithms are implemented to precisely determine motor speed, winding faults, and broken rotor bars from nonintrusive current measurement, and to determine efficiency without a dynamometer, by using a reasonable generic estimate of minor motor losses, in conjunction with prior uncoupled motor data.
SUMMARY OF THE INVENTION
Briefly stated, the present invention comprises a first method of evaluating performance of a polyphase induction motor, the motor including a number of pole pairs, by using an electrical signature of the motor, comprising the steps of:
sensing an instantaneous current signal supplied to the motor as a function of time for a period of time for at least one electrical phase of the motor;
generating a Fourier transform on the sensed current signal over at least a portion of the period of time to provide a current signal spectra;
locating a maximum frequency peak in a predetermined shaft sideband frequency range of the current signal spectra;
locating a frequency peak indicative of line frequency from the current signal spectra; and
determining a shaft speed of the motor from a difference between the located shaft sideband frequency maximum peak and the located line frequency peak, taking into account the number of pole pairs of the motor.
The present invention includes a second method of evaluating performance of a polyphase induction motor, the motor including a number of pole pairs, by using an electrical signature of the motor, comprising the steps of:
sensing an instantaneous current signal supplied to the motor as a function of time for a period of time for at least one electr
Desai Binish Praful
Dowling Martin Joseph
Lang George Fox
Ricciuti, Jr. Louis
Assouad Patrick
Crane Nuclear, Inc.
Womble Carlyle Sandridge & Rice
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