Spectral analysis of induction motor current to detect rotor faults with reduced false alarms
First Claim
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1. A method of detecting rotor faults in an induction motor, wherein the motor includes a rotor having a multiplicity of conductive bars and a stator drawing motor current from a power supply, the method comprising the steps of:
- A. deriving a current signal indicative of said motor current;
B. sampling said current signal at a predetermined sampling rate to obtain a time succession of current values over a predetermined sampling period;
C. transferring said current values to a frequency spectrum;
D. determining the fundamental frequency of said motor current from said frequency spectrum;
E. scanning said frequency spectrum in a lower sideband of said fundamental frequency for current peaks exceeding a threshold amplitude;
F. ascribing the frequency of each said current peak as a candidate indicating the slip frequency of the motor;
G. checking predetermined harmonic sidebands of said frequency spectrum at frequencies predicted from each said slip frequency candidate for matching current peaks until all but one of said slip frequency candidates are disqualified; and
H. conducting a rotor fault analysis of said frequency spectrum using the determined fundamental frequency and the qualified remaining one of said slip frequency candidates as the motor slip frequency.
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Abstract
Pursuant to detecting rotor faults in an induction motor by analysis of a frequency spectrum of the current drawn by the motor under test, wherein success of the fault analysis requires an accurate determination of the fundamental frequency of the motor current and the motor slip frequency, method and apparatus are disclosed for determining the fundamental and slip frequency values solely from analysis of the motor current spectra.
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Citations
17 Claims
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1. A method of detecting rotor faults in an induction motor, wherein the motor includes a rotor having a multiplicity of conductive bars and a stator drawing motor current from a power supply, the method comprising the steps of:
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A. deriving a current signal indicative of said motor current; B. sampling said current signal at a predetermined sampling rate to obtain a time succession of current values over a predetermined sampling period; C. transferring said current values to a frequency spectrum; D. determining the fundamental frequency of said motor current from said frequency spectrum; E. scanning said frequency spectrum in a lower sideband of said fundamental frequency for current peaks exceeding a threshold amplitude; F. ascribing the frequency of each said current peak as a candidate indicating the slip frequency of the motor; G. checking predetermined harmonic sidebands of said frequency spectrum at frequencies predicted from each said slip frequency candidate for matching current peaks until all but one of said slip frequency candidates are disqualified; and H. conducting a rotor fault analysis of said frequency spectrum using the determined fundamental frequency and the qualified remaining one of said slip frequency candidates as the motor slip frequency. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. Apparatus for detecting rotor faults in an induction motor having a rotor including a plurality of conductive bars and a stator drawing motor current from a power source, said apparatus including:
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A. a current sensor for deriving a current signal indicative of said motor current; B. sampling means for storing a time series of current values over a predetermined sampling period; C. transform means for generating a frequency spectrum of said current values; D. first means for determining the fundamental frequency of said motor current from said frequency spectrum of said current values; E. second means for determining the slip frequency of the motor from said frequency spectrum of said current values; and F. third means for analyzing said frequency spectrum of said current values at certain fundamental and harmonic sideband frequencies determined from said fundamental and slip frequencies to identify the presence and severity of any rotor faults. - View Dependent Claims (11, 12, 13, 14)
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15. A method for detecting rotor faults in an induction motor having a rotor including a plurality of conductive bars and a stator drawing motor current from a power supply, said method comprising the steps of:
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A. deriving a current signal indicative of motor current; B. sampling said current signal at a predetermined sampling rate to obtain a time succession of current values over a predetermined sampling period; C. transforming said current values to a frequency spectrum; D. determining the fundamental frequency of said motor current from said frequency spectrum of current values; E. analyzing said frequency spectrum of current values to identify the true slip frequency of the motor; and F. conducting a rotor fault analysis of said frequency spectrum of current values using the fundamental frequency value determined in step D and the true slip frequency value identified in step E. - View Dependent Claims (16, 17)
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Specification