5.1. Case 1
To prove the validity of the novel extraction method, in this section a case is presented where bearing composite faults were separated and diagnosed by the new method for wind turbines from the National Renewable Energy Laboratory (NREL). For this case, a 750-KW wind turbine gearbox with a high-speed shaft bearing was provided; the model was the SKF32222 J2 tapered roller bearing. The gearbox body was installed with the roller bearing in the radial position of the high-speed shaft. The sampling frequency was set to 40 kHz. The rotating speed of the shaft was 1800 r/min. After the end of the experiment, the gearbox was disassembled and the high-speed shaft bearing had suffered severe wear damage and overheating. The high-speed shaft frequency of the gearbox
, the high-speed gear meshing frequency
, the medium-speed gear meshing frequency
, the inner ring defect frequency
of the high-speed rolling bearing, and the holder defect frequency
are all displayed in
Table 1.
The kurtosis values (See
Table 2) of the sliding Teager kurtosis time series were calculated according to the deconvolved inner ring and holder signals. These kurtosis values were obtained for different sliding length L conditions. They are represented using K1 and K2, respectively. K1 represents the kurtosis value of the bearing inner ring source signal. K2 is the kurtosis value of the bearing hold source signal.
The original signal is indicated in
Figure 2a when the rotating speed was about 30 Hz in the high-speed shaft. The envelope spectrum and 1.5-dimensional Teager kurtosis spectrum of composite signal are presented in
Figure 2b and
Figure 3b. In
Figure 2b, the high-speed shaft frequency components
and
, high-speed gear meshing frequency
, and the medium-speed gear meshing frequency
are found. However, there were no prominent defect frequency components.
The measured original signal (see
Figure 2a) was analyzed. The MOMEDA algorithm was first utilized to deconvolute the original signal, and the deconvolution period was set to the inner ring fault period
. Then, 1.5-dimensional Teager kurtosis spectrum analysis was performed. The deconvoluted time signal and its 1.5-dimensional Teager kurtosis are indicated in
Figure 3a,b. The sliding length L was set to 2 because the kurtosis value (K1) (See
Table 2) of the sliding Teager kurtosis time series for inner ring fault signal was the largest.
Figure 3b displays the result of 1.5-dimensional Teager kurtosis spectrum analysis when the sliding window length was 2. From
Figure 3b, it can be seen that the fault frequency
and
of the bearing inner ring were accurately extracted. Noise was suppressed. There was no other defect frequency component, and the fault frequency of bearing
was effectively separated and presented at the same time using the novel extraction method.
The analysis results of holder fault signal are displayed in
Figure 4. MOMEDA algorithm was first used to deconvolute the original signal, and the deconvolution period was set to the holder failure period
. Then, 1.5-dimensional Teager kurtosis spectrum analysis was performed. The deconvoluted time signal and its 1.5-dimensional Teager kurtosis are demonstrated in
Figure 4a,b. The sliding length L was set to 2 because the kurtosis value (K2) (see
Table 2) of the sliding Teager kurtosis time series for the holder fault signal was the largest.
Figure 4b displays the result of 1.5-dimensional Teager kurtosis spectrum analysis when the sliding window length was 2. In
Figure 4b, the fault frequency
of the bearing holder and harmonics were accurately extracted. Noise was suppressed, and there was no other defect frequency component. According to the above analysis results, it can be concluded that the bearing inner ring and holder were faulty at the same time, and the two faults were separated and diagnosed effectively by the novel method.
Envelope spectrum analysis was performed directly on the deconvolved signals in
Figure 3a and
Figure 4a. The analysis results are demonstrated in
Figure 5a,b. The fault frequency of the bearing inner ring
is extracted in
Figure 5a, and the fault frequency
of bearing holder is also demonstrated in
Figure 5b. However, the characteristics are not clear and prominent due to background noise as compared with
Figure 3b and
Figure 4b. The two faults were separated and detected effectively by combining MOMEDA with the 1.5-dimensional Teager kurtosis spectrum, as demonstrated in
Figure 3b and
Figure 4b. The synthetical method can effectively eliminate redundant interference lines and suppress background noise.
In addition, for the sake of illustrating the superiority of the novel method, the measured signals were analyzed and compared by the spectral kurtosis (SK) and MED method referred to in [
15]. The corresponding kurtogram is shown in
Figure 6. There are two resonant frequency bands. The resonant frequency band A was from 10,000 Hz to 11,250 Hz and the resonant frequency band B was from 17,500 Hz to 18,750 Hz. The original signal was filtered with band A and band B to obtain the filtered signal, and the filtered signal was handled by the MED method.
Figure 7a,c,e present the analysis of frequency band A, and
Figure 7b,d,f present the analysis of frequency band B. As can be concluded from
Figure 7c,d, the fundamental frequency components of the inner race fault were prominent, but the background noise interference was very large and the cage fault feature could not be displayed, even in enlarged spectrums as demonstrated in
Figure 7e,f. The SK and MED method could not separate the composite fault features, but the novel method was able to eliminate interference successfully and extract fault features, as demonstrated in
Figure 3b and
Figure 4b. The comparison between MED and the proposed method is presented in
Table 3.
5.2. Case 2
Another case is presented where bearing composite faults were separated and diagnosed by the novel method in wind turbine from a wind farm set in Hebei, China. The wind turbine used a three-stage gearbox. The first stage was a planetary gear, and the second and the third stages (middle stage and high speed stage) were parallel helical gears. The acceleration measurement was adopted during vibration testing.
Figure 8 illustrates the structural diagram of wind turbine drive system. The condition monitoring system (CMS) was applied to obtain data on the gearbox faults. The structure sketch and the exact placement of the sensor of a gearbox for wind turbine are shown in
Figure 8. There were seven sensors installed in the gearbox system. The fault occurred in the seventh sensor, as shown in
Figure 8. That is, the experimental data were measured by the vibration acceleration sensor on the generator. Output shaft frequency was
Hz. The bearing model was SKF 6330M.C3 (deep groove ball bearing). The feature frequencies of the SKF 6330M.C3 bearing are illustrated in
Table 4.
The test picture is displayed in
Figure 9. A transducer in the gearbox is shown in
Figure 9a. The bearing fault on inner ring is presented in
Figure 9b. The time domain waveform and envelope spectrum of the fault bearing vibration signal are presented in
Figure 10. As shown in
Figure 10b, the fundamental frequency of bearing inner and outer rings defect frequency could be extracted, but their harmonic frequencies could not be presented. Therefore, the diagnosis of bearing fault type is difficult to achieve.
Table 5 displays kurtosis values of the sliding Teager kurtosis time series for the deconvolved outer ring and inner signals. These kurtosis values are obtained for different sliding length L conditions. They are represented using K1 and K2, respectively. K1 is the kurtosis value of outer ring source signal. K2 is the kurtosis value of inner ring source signal.
The measured original fault signal (see
Figure 10a) was analyzed. The MOMEDA algorithm was first utilized to deconvolute the original signal, and the deconvolution period was set to the inner ring fault period
. Then, 1.5-dimensional Teager kurtosis spectrum analysis was performed. The deconvoluted time signal and its 1.5-dimensional Teager kurtosis are displayed in
Figure 11a,b. The sliding length L was set to 2 because the kurtosis value (K1) (See
Table 5) of the sliding Teager kurtosis time series for the inner ring fault signal was the largest when L = 2.
Figure 11b displays the result of 1.5-dimensional Teager kurtosis spectrum analysis when the sliding window length was 2. From
Figure 11b, it can be seen that the inner race failure frequency, its doubling frequency component, and the modulation frequency components of the characteristic frequency were clearly extracted. From the spectrum, it can be concluded that the noise was evidently suppressed. There was no other defect frequency component, and the feature frequency of bearing inner ring fault was effectively separated and presented at the same time utilizing the novel method.
The analysis results from the bearing outer ring fault are displayed in
Figure 12. The MOMEDA algorithm was first utilized to deconvolute the original signal, and the deconvolution period was set to the outer ring failure period
. Then, 1.5-dimensional Teager kurtosis spectrum analysis was performed. The deconvoluted time signal and its 1.5-dimensional Teager kurtosis are displayed in
Figure 12a,b. Sliding length L was set to 3 because the kurtosis value (K2) (see
Table 4) of the sliding Teager kurtosis time series for the outer ring fault signal was the largest when L = 3. The result of 1.5-dimensional Teager kurtosis spectrum analysis is presented in
Figure 12b when the sliding window length was 3. In
Figure 12b it can be seen that the fault frequency
of outer ring and its harmonic frequencies were accurately extracted. The spectrum shows that the noise was suppressed, and there was no other defect frequency component. According to the above analysis results, it appears that bearing inner and outer rings were faulty at the same time, and the two faults were separated and detected effectively by utilizing the novel method. These diagnosis results are consistent with the actual situation.
Envelope spectrum analysis was performed directly on the deconvolved signals in
Figure 11a and
Figure 12a, and the results are illustrated in
Figure 13a,b. The failure frequency of bearing inner ring
is presented in
Figure 13a. Besides, the outer ring failure frequency
and its double frequency component are with lower amplitudes in
Figure 13a. This means that the separation of the inner race failure feature was insufficient. The envelope spectrum of the outer race fault is displayed in
Figure 13b. It was found that the amplitudes of the extracted outer race fault
and 2
were all low. The fault features in
Figure 13b were not clear due to background noise as compared with
Figure 11b. The two faults were separated and detected effectively when combining MOMEDA with the 1.5-dimensional Teager kurtosis spectrum, as illustrated in
Figure 11b and
Figure 12b. The synthetical method can effectively eliminate redundant interference and extract the fault characteristics.
The kurtogram of measured original fault signal is presented in
Figure 14. Only one resonant frequency band, marked as band C, was observed in the kurtogram. The MED analysis result is displayed in
Figure 15. As shown in
Figure 15b, only the inner race fault characteristic frequency
and its double component could be extracted, but no relevant component of outer race fault was found. The spectrum was interspersed with a number of interference lines. It is hard to decide the type of bearing compound fault accurately, but the novel method is capable of eliminating interference and extracting fault features effectively, as displayed in
Figure 11b and
Figure 12b. The comparison between MED and the proposed method for case 2 is presented in
Table 6.