## 4 Results and Discussion
### Fault Characteristic Frequency Extraction and Fault Analysis  
Figures 17-22 are the BA spectrum and the bispectrum slice (BS) of the fault band envelope of the currents with peak values of 90A, 128A, 166A, 204A, 242A and 280A, respectively. By comparing the BA spectra under different load conditions, it can be found that there are obvious characteristic frequencies of gear failure (3.4 Hz, 26.4 Hz, 73.3 Hz and 96.6 Hz) in each figure. As mentioned in  
Figure 16: Time–frequency analysis of the load current with a peak value of 280A: (**a**) the original current; and (**b**) the preprocessed signal. Figure 15: Fault frequency band signals and their envelope under time-varying load conditions: (**a**) the fault frequency band signals; and (**b**) the envelope of the fault frequency band signals. Section 3.1, the characteristic frequency of the gear failure is \(|50\pm n23.3|\) Hz, and these four peaks correspond to the characteristic frequencies when n is equal to 1 or 2, respectively. The location of these fault frequencies is marked in the figure. Because of the symmetry of the BA results, only half of the area was marked. The BS spectrum was obtained by taking a section along the distribution of the fault frequency in the BA spectrum. The purpose of the slice was to observe the change in the fault characteristic frequency more conveniently. Figure 19: Characteristic frequency of the gear failure under a current peak of 166A: **(a)** the result of the BA; and **(b)** the BS. Figure 17: Characteristic frequency of the gear failure under a current peak of 90A: **(a)** the result of the BA; and **(b)** the bispectrum slice (BS). In the process of the load current peak changing from 90A to 280A, the magnitude of the fault frequency in the BA spectrum did not change, but the amplitude of the fault frequency increased continuously. The reducer of the scraper conveyor always keeps a relatively constant output speed during operation, and the gear failure frequency can be controlled by the motor speed, so the gear failure frequency does not change with the load. The increasing amplitude of the fault characteristic  
Figure 21: Characteristic frequency of the gear failure under a current peak of 242A: (**a**) the result of the BA; and (**b**) the BS. Figure 22: Characteristic frequency of the gear failure under a current peak of 280A: (**a**) the result of the BA; and (**b**) the BS. frequency indicates that the load condition had obvious influence on the fault information of the gear in the current. Figure 23 is the change in the fault frequency amplitude under the time-varying load conditions. From Figure 23, it can be seen that the peak value of the fault characteristic frequency increases with the increase in the load. This shows that when the load increases, the fault frequency component in the current is stronger. However, the characteristic frequency amplitude can only qualitatively analyze the strength of the fault information in the current, and cannot quantitatively analyze the impact of the load from the energy perspective. Gear failure can cause the energy distribution in the current to be concentrated, which reduces the BE of the current. Therefore, using BE as an indicator can achieve the quantitative analysis of gear failures. The BE under time-varying load conditions is shown in Figure 24, and the BE under different load conditions was 10.84, 14.71, 10.65, 10.49, 10.35 and 10.15, respectively. Obviously, the BE decreases with the increase in the load. The smaller the BE is, the more the energy in the current is concentrated. The gear fault can make the energy in the current mainly concentrated in the local frequency band, so the smaller BE indirectly reflects that the fault signal in the current is more intense as the load increases. In addition, a gear fault diagnosis method based on current spectrum analysis is proposed in reference [31], which is compared with the method proposed in our paper.