CHAPTER 4 Experimental Study of Rotor Faults of Induction Motor
4.2 Broken rotor bar fault diagnosis using FFT based power spectrum
4.2.3 Observations and discussion
The induction motor was tested for healthy working condition and for broken rotor bars under the various loading condition. The current measurements were made at no load, half load and full load.
The power spectrums of a healthy 3 induction motor (rating given in Table 4.2)for no load, half load and full load are shown in Figures 4.7 to 4.18. These Figures represent the power spectrum of induction motor. Frequency range is selected from 30Hz to 70 Hz, as it contains the fundamental frequency and almost all the visible sideband frequencies. Some important observations from experimental results are given below:
(i) One broken bar
The power spectrums obtained from the current signal for one broken bar at no load, half load and full load are given in Figures 4.8, 4.12, 4.16. At no load the side bands frequency is very close to fundamental frequency and the amplitudes of the sidebands is quite smaller or negligible as shown in Figure 4.8. It can be observed that the detection of the searched slip frequency sideband at no load is too difficult, since the current in the rotor bars is small. It is also observed from Figure 4.12 that even at half load side band fault frequencies are not visible because again their magnitude is low. Thus, it is slightly difficult to detect broken rotor bar fault at half loaded conditions also.
It is observed that fault frequency side bands for one broken bar are visible only at full load as shown in Figure 4.16. These frequencies are marked as FF (Fault frequency). The magnitude of the fault frequencies is approximately -68dB. The complete observations from power spectrum analysis for one broken bar is given in Table 4.5
Table 4.5: Power spectrum analysis of one broken bar at various loading conditions Fault Frequencies K=1 K=2 Figure no. Load Condition Slip LSB (Hz) USB (Hz) Observations LSB (Hz) USB (Hz) Observations 4.8 No Load 0.01 49 51 Not visible 48 52 Not Visible 4.12 Half Load 0.04 46 54 Visible 42 58 Not visible 4.16 Full Load 0.08 42 58 Visible 34 66 Not Visible (ii) Five broken bars
The power spectrums obtained from the current signal for five broken bars at no load, half load and full load are given in Figures 4.9, 4.13, 4.17. At no load, fault frequencies are not clearly visible because these frequencies are very close to fundamental frequency and their amplitudes are quite smaller or negligible as shown in Figure 4.9. It can be observed from the figure that it may not be possible to detect the broken rotor fault at no load or light load due to small current in the rotor bars. Figure 4.13 shows the power spectrum of motor with five broken bars at half load condition. It is also observed from this figure that even at half load fault frequencies are not clearly visible because again their magnitude is low. Thus, it is slightly difficult to detect broken bar fault at half loaded conditions also. The power spectrum of motor with five broken bar at full load is shown in Figure 4.17. This figure clearly show fault frequencies at 34 Hz, 42 Hz, 58 Hz and 66 Hz which is the indication of broken rotor bar fault. The magnitudes of these fault frequencies are in between -78dB to -60dB. The complete observation from power spectrum analysis for five broken bar are given in Table 4.6.
Table 4.6: Power spectrum analysis of five broken bars at various loading conditions Fault Frequencies
K=1 K=2
Figure no. Load Conditions Slip LSB (Hz) USB (Hz) Observations LSB (Hz) USB (Hz) Observations
4.9 No Load 0.01 49 51 Not visible 48 52 Not Visible 4.13 Half Load 0.04 46 54 Visible 42 58 Not visible 4.17 Full Load 0.08 42 58 Visible 34 66 Visible
(iii) Twelve broken bars
The power spectrums obtained from the current signal for twelve broken bar at no load, half load and full load is given in Figures 4.10, 4.14, 4.18. Figure 4.10 shows power spectrum of motor with 12 broken bars under no load condition. Again, at no load condition, the side band frequencies are very close to fundamental frequency and the amplitudes of the sidebands is quite smaller or negligible. The detection of the searched slip frequency sideband at no load or light load is too difficult. It is also observed from Figure 4.14 that side band fault frequencies are visible at half load condition. The fault frequencies appear at 42Hz, 46Hz, 54 Hz and 58 Hz in the power spectrum which is indication of broken rotor bar fault. Figure 4.18 show the power spectrum of motor with 12 broken bars at full load condition. It is observed from the figure that broken bar fault detection at full load may be performed in more reliable way. The frequency components related to broken bar can be clearly recognized in the current spectrum. The complete observations from power spectrum analysis for 12 broken bars are given in Table 4.7
Table 4.7: Power spectrum analysis of twelve broken bars at various loading conditions Fault Frequencies
K=1 K=2
Figure no. Load Condition Slip LSB (Hz) USB (Hz) Observations LSB (Hz) USB (Hz) Observations
4.10 No Load 0.01 49 51 Not visible 48 52 Not Visible 4.14 Half Load 0.04 46 54 Visible 42 58 Visible 4.18 Full Load 0.08 42 58 Visible 34 66 Visible
The results obtained from the experiments show that the magnitude of the frequency components increases when the number of broken bars increases. Based on the results obtained with the systems it can be stated that this method proven to be adequate for the cases and load conditions considered, as the system was capable to detect the broken rotor bars faults.
Figure 4.7: Power spectrum of healthy motor at no load
Figure 4.9: Power spectrum of faulty motor with 5 broken bars under no load condition
Figure 4.10: Power spectrum of faulty motor with 12 broken bars under no load condition
Figure 4.11: Power spectrum of healthy motor under half load
Figure 4.12: Power spectrum of faulty motor with 1 broken bar under half load
[54
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FF
Hz
[46
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Figure 4.13: Power spectrum of faulty motor with 5 broken bars under half load
Figure 4.14: Power spectrum of faulty motor with 12 broken bars under half load
[46
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FF
Hz
FF[54Hz]
[46
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Figure 4.15: Power spectrum of healthy motor under full load
Figure 4.16: Power spectrum of faulty motor with 1 broken bar under full load
[42
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Figure 4.17: Power spectrum of faulty motor with 5 broken bars under full load
Figure 4.18: Power spectrum of faulty motor with 12 broken bars under full load