TW201740673A - Method for determining characteristic feature quantity of three-phase induction motor - Google Patents

Abstract

This method, for determining a characteristic feature quantity of a three-phase induction motor required for calculation of a deterioration parameter used in diagnostics of a rotary mechanical system for which the three-phase induction motor is the drive source, involves: a first step for performing frequency analysis of an operation-time current signal of the three-phase induction motor 12; a second step in which the sidebands SA, SB present in a frequency region A on the low frequency side and a frequency region B on the high frequency side, said regions being centered on the power frequency spectrum SL, are detected from the frequency analysis; and a third step in which the absolute value of the difference between the power frequency fline and the peak position frequency of either of the sidebands SA, SB is set as the actual rotation frequency fr of the three-phase induction motor 12, wherein the actual rotation frequency fr is set as a first characteristic feature quantity.

Description

Method for determining the intrinsic feature quantity of a three-phase induction motor

The present invention relates to a method of determining a characteristic feature of a three-phase induction motor necessary for calculating a deterioration parameter when a deterioration parameter is used to diagnose a state of a rotary machine system using a three-phase induction motor as a drive source, wherein The degradation parameter is obtained by analyzing the current signal of the three-phase induction motor during operation.

Heretofore, the state of the rotary mechanical system that diagnoses the three-phase induction motor as the drive source is performed by paying attention to the deterioration parameter of the vibration generated by the rotary mechanical system, which is defined as the time domain and the frequency (the number of vibrations, respectively). ) Both sides of the domain. However, when diagnosing a rotating mechanical system existing in a remote area or a state of a rotating mechanical system that is not accessible, the vibration of the rotating mechanical system cannot be directly measured. Here, the current spectrum (spectral pattern) obtained by performing frequency analysis on the current signal of the three-phase induction motor can be used to determine the degradation parameter, and the rotating machine can be rotated by paying attention to the change of the deterioration parameter. State diagnosis (for example, refer to Non-Patent Document 1). [Prior Technical Literature] [Non-Patent Literature]

[Non-Patent Document 1] 豊田利夫 "Diagnosis of Current Ambient Analysis MCSA による Motor Retraction Machine" Takada Technical News, 2010, Vol. 20, p. 3-6

[Summary of the Invention] [Problems to be Solved by the Invention] For example, a current spectrum obtained by analyzing the frequency of a current signal during operation of a three-phase induction motor of a pole number p, as shown in FIG. 4, at a power supply frequency f _line The two sides of the spectrum (the frequency of the commercial power source) (the low-frequency side and the high-frequency side centered on the spectrum of the power supply frequency f _line ) are known to have sidebands caused by the polar pass frequency f _pp . Here, the power spectrum magnitude of the frequency f _line I _line (referred to herein as a current value of the power peak, likewise hereinafter) of the electrode caused by the size of the I _pole frequency f _pp of the sideband ratio I _line / I _pole as Deteriorating the parameter, and determining whether the rotor bar of the three-phase induction motor is damaged by the change of the degradation parameter.

Here, at the pole passing frequency f _pp , the actual rotational frequency f _r of the rotor of the three-phase induction motor (after which the actual rotational frequency f _r represents the actual rotational frequency f _{r of the} rotor), the synchronous rotational frequency f of the three-phase induction motor _x and the relationship between the number of poles p of the three-phase induction motor have a relationship of f _pp = (f _x - f _r )p. Further, the number of poles p is determined by the specifications of the three-phase induction motor, and the synchronous rotation frequency f _x can be obtained by using the power source frequency f _line (the frequency of the commercial power source) and the number of poles p. Therefore, if the energy is measured by the actual rotation frequency f _r , the sideband caused by the pole passing frequency f _pp in the current spectrum can be specified, and the correct size of the sideband can be determined, and the correct degradation parameter can be obtained. However, if the actual rotation frequency f _{r of the} three-phase induction motor is unknown, if the estimated value (for example, the previously measured value or the value described in the specification) is used, the correct degradation parameter cannot be obtained, and the result cannot be obtained. Properly diagnose the problem of the state of the rotating mechanical system.

The present invention has been made in view of such circumstances, and an object is to provide an intrinsic feature quantity of a three-phase induction motor necessary for calculating a deterioration parameter when a deterioration parameter is used to diagnose a state of a rotary machine system using a three-phase induction motor as a drive source. The method is determined, wherein the degradation parameter is obtained by analyzing a current signal when the three-phase induction motor is operated. [How to solve the problem]

According to the above object, the method for determining the inherent characteristic quantity of the three-phase induction motor according to the present invention is to determine the inherent characteristics of the necessary three-phase induction motor when diagnosing the state of operation of the rotary mechanical system using the three-phase induction motor as a drive source. The method includes the following steps: measuring the current signal of the three-phase induction motor during operation, and performing frequency analysis of the current signal during the operation; and the second step, from the current spectrum obtained by the frequency analysis, obtaining the three-phase induction motor power spectrum of a sideband: sideband sideband S _a and S _B, S _a is the sideband to obtain the maximum height peak is present in the frequency domain a is obtained within the frequency-domain a The difference between the power supply frequency f _line and the synchronous rotation frequency f _x of the three-phase induction motor is used as the lower limit frequency, and the lower limit frequency is added to the value set in the range of 0.01 to 4 Hz as the upper limit frequency. The sideband S _B is obtained by finding a peak having a maximum height in the frequency domain B, and the frequency domain B is a value obtained by adding the aforementioned synchronous rotation frequency f _x to the power supply frequency f _line as an upper limit frequency, and The Frequency minus limit value is set at a range 0.01 ~ 4Hz frequency values of the lower limit frequency; and a third step, to the sideband S _A, the peak position of the power-source frequency of frequency f _line according to any one of the S _B The absolute value of the difference is the actual rotational frequency f _{r of the} three-phase induction motor, and the actual rotational frequency f _{r is} used as the first inherent feature amount. [Effects of the Invention]

In the method for determining the intrinsic feature quantity of the three-phase induction motor according to the present invention, the actual rotation frequency f of the three-phase induction motor is determined because the current signal during operation is measured from the state at the time of diagnosis of the operation of the rotary machine system. _r , and the actual rotational frequency f _{r is} used as the first inherent characteristic quantity of the three-phase induction motor. Therefore, the determined actual rotational frequency f _r accurately reflects the rotational state of the three-phase induction motor at the time of diagnosis. Therefore, when the actual rotation rotation frequency f _{r is} used to constitute the deterioration parameter for judging the state of the three-phase induction motor during operation, the deterioration parameter can be used to correctly evaluate the state of the three-phase induction motor, and the state diagnosis of the rotary machine can be realized. Increased accuracy. Further, since the actual rotational frequency f _{r of the} three-phase induction motor participates in the generation of the sideband associated with the mechanical configuration of the rotary mechanical system, the deterioration parameter reflecting the mechanical configuration of the rotary mechanical system can be accurately obtained.

In the following, the embodiments of the present invention will be described with reference to the accompanying drawings in order to explain the invention. In one embodiment of the present invention, the method for determining the intrinsic feature quantity of the three-phase induction motor is to determine the state of the rotary machine system in which the three-phase induction motor is used as the drive source when the deterioration parameter is used to determine the state of operation of the rotary machine system. A method of degrading an inherent characteristic quantity of a three-phase induction motor necessary for a parameter, wherein the degradation parameter is obtained by analyzing a running current signal of a three-phase induction motor. Hereinafter, it will be explained in detail.

The method for determining the inherent feature quantity of the three-phase induction motor includes: in the first step, measuring the current signal of the three-phase induction motor during operation, performing frequency analysis of the current signal during operation, and performing logarithmic conversion on the value of the obtained current spectrum; In the second step, as shown in FIG. 1, the sideband of the power supply spectrum S _L of the three-phase induction motor is obtained from the logarithmically converted current spectrum: the sideband S _A and the sideband S _B , the sideband S _A It is obtained by finding a peak existing in the maximum height of the frequency domain A. The frequency domain A is a difference between the power supply frequency f _line and the synchronous rotation frequency f _x of the three-phase induction motor as the lower limit frequency, and the lower limit frequency is added. The upper side is set to a frequency value in the range of 0.01 to 4 Hz. For example, a value obtained by adding 2 Hz is used as an upper limit frequency, and the sideband S _B is obtained by finding a peak having a maximum height in the frequency domain B, and the frequency domain B is obtained by The power frequency f _line plus the synchronous rotation frequency f _x is used as the upper limit frequency, and the upper limit frequency is subtracted from the frequency value set in the range of 0.01 to 4 Hz, for example, the value after subtracting 2 Hz as the lower limit frequency; and the third step the sideband S _a, S _B according to any one of, for example, With S _A, the absolute value of the difference between the position of the peak frequency of the power frequency f _line S _B in one of the higher peak height (the current power value larger one) of the three-phase induction motor, the actual rotational frequency f _r, and The actual rotational frequency f _{r is taken} as the first inherent characteristic amount of the three-phase induction motor.

In the first step, as shown in FIG. 2, for the wiring of any one of the motor wirings 13 of the three-phase induction motor 12 in the switchboard 11 of the rotary mechanical system 10, for example, a non-contact clamp current is used. The sensor 14 detects the current signal during operation, and the current signal during the operation is an analog signal. Moreover, in the case where the switchboard 11 is close to the rotary machine system 10, a non-contact clamp-type current sense can also be used for the cable connected to any one of the power cables 15 of the three-phase induction motor 12. The detector detects the current signal during operation. The operational current signal detected by the clamp-type current sensor 14 is input to the A/D converter 16 to be converted into a digital signal, and input to the frequency analysis for performing fast Fourier transform (an example of frequency analysis) via the transmission line 17. 18. In this way, the current spectrum of the current signal during operation can be obtained.

As shown in FIG. 2, the second step is to input the spectrum data (the relationship between the display frequency and the current power value (peak height) of the current spectrum of the current signal output from the frequency analyzer 18 to the first. The data processing program 19 is executed by the first data processing program 19. In the second step, the power supply spectrum S _L of the three-phase induction motor 12 is first obtained from the current spectrum. Here, since the power supply frequency f _line is the commercial power supply frequency for supplying the power to the three-phase induction motor 12 (50 Hz in East Japan and 60 Hz in West Japan), the frequency value can be obtained from the spectrum data of the current spectrum to be consistent with the commercial power supply frequency. The spectrum data is obtained, and the current power value included in the spectrum data is taken as the current power value of the power spectrum S _L (the above is Process 1).

Next, since the synchronous rotation frequency f _{x of} the three-phase induction motor 12 can be calculated from f _x = 2f _line /p in the case where the number of poles of the three-phase induction motor 12 is p, in the current spectrum, from the presence of In the spectrum data of the frequency domain A as follows, the spectrum data whose current power value is the largest (the peak of the maximum height, the same applies hereinafter) is extracted, wherein the frequency domain A is the synchronous rotation of the power frequency f _line and the three-phase induction motor 12. f _line -f _x the difference between the frequency f _x, i.e. f _line -2f _line / p as the lower limit frequency and the lower limit of the frequency value plus 2Hz, i.e. f _line -2f _line / p + 2 as the upper limit frequency. Next, the frequency and current value of the power spectrum of the extracted information are included as frequency and current value of the power of the sidebands S _A (2 or more to process). Further, in the current spectrum, the spectrum data having the largest current power value is extracted from the spectrum data existing in the frequency domain B, wherein the frequency domain B is added to the synchronous frequency f _x by the power supply frequency f _line The value f _line +f _x , that is, f _line +2f _line /p is taken as the upper limit frequency, and the upper limit frequency is subtracted by 2 Hz, that is, f _line +2f _line /p-2 is taken as the lower limit frequency. Next, the frequency and the current power value included in the extracted spectrum data are respectively taken as the frequency of the sideband S _B and the current power value (the above is Process 3). Further, the first data processing program 19 is configured by, for example, mounting a program for executing the processes 1 to 3 on a personal computer. Here, the value of the commercial power source frequency and the number of poles p of the three-phase induction motor 12 is input to the first data processing program 19 in advance.

As shown in FIG, 2 is a third step from the first data processing program data S _A sideband of the output 19 (current frequency f _A power value and the power value corresponding to the current sideband S _A) and the sideband S The data of _B (the current power value of the sideband S _B and the frequency f _{B of the} corresponding current power value) are input to the second data processing program 20 and executed by the second data processing program 20. In the third step, the current power value of the sideband S _{A and} the current power value of the sideband S _{B are} compared, and a sideband having a large current power value is specified, and the peak position frequency f _{Smax of} the specified sideband is obtained. . Then, the absolute value |f _Smax -f _line | of the difference between the peak position frequency f _Smax and the power supply frequency f _line is taken as the actual rotational frequency f _{r of the} three-phase induction motor 12. For example, at a current value of power sideband current power ratio S _A S _B sideband case of larger _{value, f r = f line -f A} , the sideband power current value S _B S _A sideband ratio of In the case where the current power value is larger, f _r =f _B -f _line . Then, the obtained actual rotation frequency f _r is output by the second data processing program 20. (Process 4). Here, the second data processing program 20 is configured by, for example, a program for executing the process 4 on a personal computer.

In the present embodiment, the method for determining the intrinsic feature quantity of the three-phase induction motor further includes: in the fourth step, the difference between the synchronous rotation frequency f _x and the actual rotation frequency f _r is taken as the slip frequency f _s ; In the self-current spectrum, the sideband of the power spectrum is obtained: the sideband S _PA (peak position frequency f _PA ) and the sideband S _PB (peak position frequency) which exist on the low frequency side and the high frequency side due to the pole passing frequency f _pp f _PB ), the pole passing frequency f _pp is defined as the product of the slip frequency f _s and the pole number p of the three-phase induction motor 12; in the sixth step, it is found that exists between the sideband S _PA and the sideband S _A The peak of the maximum height is taken as the sideband S _LA , and the peak of the maximum height between the sideband S _PB and the sideband S _B is obtained as the sideband S _LB ; in the seventh step, the sidebands S _LA , S _LB For example, the absolute value of the difference between the peak position frequency of the side having the higher peak height among the sidebands S _LA and S _LB and the power source frequency f _line is taken as the rotor bar slip frequency of the three-phase induction motor 12. f _rs; and a second step 8, the rotor bar slip frequency f _rs slip rate f _s by dividing the obtained three-phase induction H number of motor rotor 12, and the guide number of the rotor 2 h inherent feature quantity of up to 12 as a three-phase induction.

As shown in FIG. 2, the fourth step to the eighth step are obtained by the second step of the spectrum data of the current spectrum of the current signal obtained during the first step (output from the frequency analyzer 18). The data of the sidebands S _LA and S _LB obtained by the data processing program 19 and the synchronous rotation frequency f _{x of the} three-phase induction motor 12 and the actual rotation obtained by the third step (obtained from the second data processing program 20) The frequency f _{r is} input to the third data processing program 21 and executed by the third data processing program 21.

In the fourth step, the difference f _x -f _{r is obtained} as the slip frequency f _s from the synchronous rotation frequency f _x and the actual rotation frequency f _r respectively obtained by the first and second data processing programs 19 and 20 (Process 5 ). In the fifth step, as shown in FIG. 3, the sideband S _PA (peak frequency position f _PA ) and the sideband S _PB (peak frequency position f _PB ) existing at the following frequency position due to the pole passing frequency f _pp are obtained. , it is a three-phase induction motor supply current of frequency f _line 12 as the center of the spectral peaks, to the low frequency side and high frequency side by a frequency, respectively, from a pole frequency f _pp of the position, wherein the pole frequency f _pp is by It is defined as the product of the slip frequency f _s and the number of poles p of the three-phase induction motor 12 . In the sixth step, the maximum height of the peak is determined is present between the sideband sideband S _PA and S _A (i.e., from the spectral data is present between the sideband sideband S _PA and S _A, a current is drawn The power value is the largest spectrum data), and the frequency and current power values of the extracted spectrum data are respectively used as the peak frequency f _LA and the current power value of the sideband S _LA , and are found in the sideband S _PB and the side The peak with the maximum height between S _B (that is, the spectrum data with the largest current power value extracted from the spectral data existing between the sideband S _PB and the sideband S _B ), and the extracted spectral data The frequency and current power values are used as the peak frequency f _LB and the current power value of the sideband S _LB (above, process 6).

In the seventh step, any one of the sidebands S _LA and S _LB , for example, compares the current power value of the sideband S _{LA with} the current power value of the sideband S _LB to specify a sideband having a large current power value. And find the peak position frequency of the specific sideband. Then, the absolute value of the difference between the obtained peak position frequency and the power supply frequency f _line is taken as the rotor bar slip frequency f _{rs of the} three-phase induction motor 12. For example, when the current power value of the sideband S _LA is larger than the current power value of the sideband S _LB , f _rs =f _line -f _LA , the current power value of the sideband S _LB is lower than the current power of the sideband S _LA In the case of a large value, f _rs =f _LB -f _line (Process 7). In the eighth step, the value obtained by dividing the rotor bar slip frequency f _rs obtained in the seventh step by the slip frequency f _s (=f _x -f _r ) obtained in the fourth step is obtained. The integer value closest to the obtained value is taken as the rotor guide number h of the three-phase induction motor 12 (Process 8). Here, the third data processing program 21 can be configured by, for example, mounting a program for executing the processes 5 to 8 on a personal computer.

As described above, the current spectrum obtained by performing the frequency analysis of the current signal during the operation of the three-phase induction motor 12 can determine the three-phase induction by using the method of determining the inherent characteristic quantity of the three-phase induction motor of the present invention. The actual rotational frequency f _{r of the} motor 12 is used as the first inherent characteristic amount of the three-phase induction motor 12. As a result, the pole passing frequency f _pp (=(f _x -f _r )p) can be obtained using the actual rotational frequency f _r , the synchronous rotational frequency f _x of the three-phase induction motor 12, and the number of poles p of the three-phase induction motor 12 . And can be from the current spectrum in the sideband of the specific frequency of the pass frequency f _pp . Thereby, the correct size (current power value) of the sideband of the _pole pass frequency f _pp can be known as 20 logI _pole (here, I _pole is the current power value of the sideband caused by the pole pass frequency f _pp , the pole pass frequency f _pp is obtained by performing fast Fourier transform on the current signal of the three-phase induction motor 12, for example, using the size of the sideband of the frequency f _pp 20 pi I _pole and the spectrum of the power supply frequency f _line 20 logI _line (in Therefore, I _line is the current power value of the current spectrum of the power frequency, and the power frequency is obtained by performing fast Fourier transform on the current signal of the three-phase induction motor 12, and 20logI _line -20logI _pole is formed, that is, 20log (I _line /I _pole ), by using this as a deterioration parameter, it is possible to diagnose whether the rotor bar of the three-phase induction motor 12 is damaged by high-accuracy by the change of the deterioration parameter.

Moreover, for the current spectrum of the current signal during the operation of the three-phase induction motor 12, the number of rotor bars of the three-phase induction motor 12 can be determined by using the method of determining the inherent characteristic quantity of the three-phase induction motor of the present invention. This is taken as the second inherent characteristic amount of the three-phase induction motor 12. As a result, the rotor bar slip frequency f _rs (=(f _x -f _r ) can be obtained using the rotor bar number h, the actual rotation frequency f _r , and the synchronous rotation frequency f _{x of the} three-phase induction motor 12 . h), and from the power spectrum, the following sideband is specified: the sideband is the sideband of the power spectrum, and is the sideband caused by the rotor bar slip frequency f _rs . Thereby, the correct size of the sideband caused by the rotor bar slip frequency f _rs can be known as 20logI _rs (I _rs is the current power value of the sideband caused by the rotor bar frequency f _rs , the rotor bar frequency f _Rs is obtained by performing fast Fourier transform on the current signal of the three-phase induction motor 12, for example, the size of the sideband caused by the rotor bar frequency f _rs is 20 logI _rs and the spectrum of the power supply frequency f _line I _line The 20logI _line -20logI _rs , that is, 20log (I _line /I _rs ), can be used as a degradation parameter to diagnose whether the load torque of the three-phase induction motor 12 changes with high accuracy by the change of the deterioration parameter. .

The present invention has been described above with reference to the embodiments, but the present invention is not limited by the configuration described in any of the above embodiments, and includes other implementations that can be considered within the scope of the matters described in the scope of the invention patent application. Type, variation. Furthermore, the combination of the present embodiment and the constituent elements included in the other embodiments or variations is also included in the present invention. For example, although the power supply frequency f _line of the power spectrum of the current spectrum is used as the fundamental frequency for supplying the alternating current to the power source of the three-phase induction motor, the power supply frequency f _{line of the} current spectrum may be used as the harmonic frequency. Thereby, even if the frequency of the sideband associated with the mechanical structure of the rotating mechanical system exceeds the fundamental frequency of the power source, the sideband can be detected and the mechanical construction associated with the rotating mechanical system can be correctly judged. Deterioration parameter. Further, the present invention is applicable even if the power source supplying the alternating current to the three-phase induction motor is the inverter power source.

10‧‧‧Rotating Machinery System
11‧‧‧Distribution panel
12‧‧‧Three-phase induction motor
13‧‧‧Motor wiring
14‧‧‧Clamp-type current sensor
15‧‧‧Power cable
16‧‧‧A/D converter
17‧‧‧Transmission line
18‧‧‧Frequency analyzer
19‧‧‧1st data processing procedure
20‧‧‧2nd data processing procedure
21‧‧‧3rd data processing procedure
P‧‧‧ poles
S _L ‧‧‧Power Spectrum
S _A ‧‧ ‧frequency domain A sideband
S _B ‧‧ ‧frequency domain B sideband
S _PA ‧‧‧ pole low frequency sideband caused by frequency f _pp
S _PB ‧‧‧Very high frequency sidebands caused by frequency f _pp
S _LA ‧‧‧ Maximum peak between sideband S _PA and sideband S _A (sideband)
S _LB ‧‧‧ Maximum peak between sideband S _PB and sideband S _B (sideband)
f _L ‧‧‧Power frequency
f _A ‧‧‧peak frequency of frequency domain A
f _B ‧‧ ‧ peak frequency of frequency domain B
f _x ‧‧‧Synchronous rotation frequency
f _r ‧‧‧ actual rotation frequency
f _s ‧‧‧ slip frequency
f _pp ‧‧‧polar pass frequency
f _rs ‧‧‧Rotor bar slip frequency
I _line ‧‧‧The frequency spectrum of the power frequency f _line
I _pole ‧‧‧ pole pass spectrum size of f _pp
I _rs ‧‧‧The size of the rotor bar slip frequency f _rs

1 is a schematic view showing a sideband of a power supply spectrum and an actual rotation frequency of a current spectrum of a current signal during operation of a three-phase induction motor. Fig. 2 is a view showing the configuration of a device used in a method for determining the intrinsic feature quantity of a three-phase induction motor according to an embodiment of the present invention. 3 is a schematic view showing a sideband of a power spectrum of a current spectrum of a current signal and a slip frequency of a rotor bar during operation of a three-phase induction motor. 4 is a schematic diagram of a current spectrum obtained by frequency analysis of a current signal during operation of a three-phase induction motor.

S _L ‧‧‧Power Spectrum

S _A ‧‧ ‧frequency domain A sideband

S _B ‧‧ ‧frequency domain B sideband

f _line ‧‧‧Power frequency

f _A ‧‧‧peak frequency of frequency domain A

f _B ‧‧ ‧ peak frequency of frequency domain B

f _x ‧‧‧Synchronous rotation frequency

Claims (3)

A method for determining an intrinsic feature quantity of a three-phase induction motor, which is a method for determining an intrinsic feature quantity of a three-phase induction motor necessary for diagnosing a state of operation of a rotary machine system, the three-phase induction system The motor is a driving source, and the method comprises: a first step of measuring a current signal of the three-phase induction motor during operation, and performing frequency analysis of the current signal during the operation; and a second step, analyzing the current spectrum from the frequency analysis , the sideband to obtain the power spectrum of the three-phase induction motor: sideband S _a, S _B, S _a of the sidebands is obtained in the presence of the maximum peak height a obtained within the frequency domain, the frequency domain a The difference between the power supply frequency f _line and the synchronous rotation frequency f _x of the three-phase induction motor is used as the lower limit frequency, and the lower limit frequency is added to the value set in the range of 0.01 to 4 Hz as the upper limit frequency. The sideband S _B is obtained by finding a peak having a maximum height in the frequency domain B, and the frequency domain B is a value obtained by adding the aforementioned synchronous rotation frequency f _x to the power supply frequency f _line as an upper limit frequency, and The upper a limit frequency minus a value set to a frequency value of a range of 0.01 to 4 Hz as a lower limit frequency; and a third step, a peak position frequency of any one of the sidebands S _A and S _B and the power supply frequency f _line The absolute value of the difference is the actual rotational frequency f _{r of the} three-phase induction motor, and the actual rotational frequency f _{r is the} first inherent feature amount. The method for determining the inherent feature quantity of the three-phase induction motor according to claim 1, further comprising: the fourth step, using the difference between the synchronous rotation frequency f _x and the actual rotation frequency f _r as the slip frequency f _s ; In the fifth step, the sidebands of the power spectrum are obtained from the current spectrum: the sidebands S _PA and S _PB respectively present on the low frequency side and the high frequency side due to the pole passing frequency f _pp , and the pole passing frequency f _pp is is defined as the product of the slip frequency f _s and the number of poles p of the three-phase induction motor; a sixth step of determining the maximum height peak is present in between the sidebands S _PA and the sideband side as S _a With S _LA , the peak of the maximum height between the sideband S _PB and the sideband S _B is obtained as the sideband S _LB ; in the seventh step, any of the aforementioned sidebands S _LA and S _LB The absolute value of the difference between the peak position frequency and the aforementioned power supply frequency f _line is taken as the rotor bar slip frequency f _{rs of the} aforementioned three-phase induction motor; and in the eighth step, the rotor bar slip frequency f _{rs is} divided by the aforementioned turn rate f _s as the difference between the rotor bar of the three-phase induction motor , And the number of rotor bars to a second inherent feature quantity. The method of claim 1 or 2, wherein the frequency domain A has a lower limit frequency of the frequency domain A plus a value of 2 Hz as an upper limit frequency of the frequency domain A; the frequency domain B takes the value obtained by subtracting 2 Hz from the upper limit frequency of the frequency domain B as the lower limit frequency of the frequency domain B.