## 4 Results and Discussion
### Preprocessing of Stator Current
#### 4.1.1 Extraction of Fault Frequency Band  
The faulty current with a peak value of 280A was taken as an example for analysis to find the optimal decomposition layer in our paper. Both the fault current and normal current were processed by WPMD in four layer, five layers and six layers, and finally \(2^{4}\), \(2^{5}\) and \(2^{6}\) frequency bands were obtained, respectively. Considering that the current fault frequency was mainly distributed in the low frequency component around the current fundamental frequency, it was only necessary to analyze the first four frequency bands obtained by WPMD under normal and faulty conditions. The results obtained under the three decomposition layers are shown in Figures 12-14, respectively.  
Figure 11: The experimental scheme.  
Figure 14: The first four frequency bands from the six-layer decomposition: (**a**) the current under normal working conditions; and (**b**) the current under faulty working conditions.  
Figure 12: The first four frequency bands from the four-layer decomposition: (**a**) the current under normal working conditions; and (**b**) the current under faulty working conditions.  
Figure 13: The first four frequency bands from the five-layer decomposition: (**a**) the current under normal working conditions; and (**b**) the current under faulty working conditions.  
Comparing Figure 12a,b, Figure 13a,b, it can be found that when the current was decomposed into four layers and five layers, the first frequency band of the current under normal and faulty conditions is significantly different, which indicates that the fault signal is concentrated in the first frequency band. Comparing Figure 14a,b, it can be found that when the current is decomposed into six layers, the first two frequency bands of the fault current have changed compared to the normal current, which indicates the distribution of fault signals in the first two frequency bands. Tables 2-4 are the ranges of the first four frequency bands obtained when the current was decomposed in four layers, five layers and six layers, respectively. It can be seen from Table 2, that when the current was decomposed in four layers, the fault frequency range was 0-312.5 Hz. It can be seen from Table 3, that when the current was decomposed in five layers, the fault frequency range was 0-156.3 Hz, which was shortened by half compared to the four-layer decomposition, thereby improving the accuracy of extracting the fault frequency band. It can be seen from Table 4, that when the current was decomposed in six layers, the fault frequency band included 0-78.2 Hz and 78.2-156.3 Hz. At this time, because the fault signal was decomposed into two frequency bands, it was not conducive to the extraction of the fault frequency band.  
Based on the above analysis, the following conclusions can be drawn: when the number of decomposition layers is less than five, the signal decomposition is not accurate enough, at this time, and the extracted fault frequency band may be mixed with the impact signal of the coal flow and scraper chain; when the amount of decomposition layers is more than five layers, it will cause the dispersion of the fault frequency band. When the number of decomposition layers is equal to five layers, all the fault information is concentrated in one frequency band, and the impact interference of coal flow and scraper can be removed to the greatest extent. Therefore, the optimal number of decomposition layers of the current signal is five layers. The first frequency band signal obtained when the current was decomposed in five layers was the fault frequency band signal we expected to extract.