## 5 Results and Discussion
### Results for the IM Operating at 20 Hz  
Another test is performed for the IM operating at 20 Hz. The same three methodologies of the previous section are applied: the MCSA, the NF-MCSA, and the GA-MCSA. The conventional MCSA and the GA-MCSA techniques are applied without any modification. However, in the case of the NF-MCSA methodology, it is necessary to design a completely different NF. The filter used in the new test considers a notch frequency and a 0.8 Hz bandwidth. The spectra for the three methodologies at the three load levels are presented in Figure 5.  
This time, the conventional MCSA presents difficulties for properly identifying the fault condition even when the motor operates at high load. It is observed in Figure 5a that the RSH is high enough to be detected at first sight; on the other hand, the LSH is merged on the spectral leakage and it cannot be appreciated in the figure. In this particular case, the NF is able to solve the issues related to the MCSA, allowing the proper identification of both failure harmonics (see Figure 5d). Notwithstanding, it is observed the effect of the leakage in the neighborhood of the FFC in the range between 18 and the 22 Hz. The GA methodology allows to perform a better attenuation of the FFC and provides the smallest range of spectral leakage of the three methodologies, and the most important part is that both failure harmonics can be easily appreciated in the spectrum of Figure 5g, even when a smallest number of data points are used. For the case of the IM operating at medium load, the MCSA can identify the LSH by itself, but the spectral leakage generates a detrimental effect, which causes the RSH to appear as a gap instead of a peak (see Figure 5b). Here, the remaining two methodologies (the NF and the GA) solve the problem, allowing the proper identification of both failure harmonics (Figure 5e,h respectively). However, it is observed that the GA is able to perform a bigger attenuation than the NF. The worst of all the presented cases occurs when the IM operates at 20 Hz and low load. In this case, the MCSA cannot provide a reliable result because the amplitudes of the failure harmonics are so low that they are completely covered by the spectral leakage that surrounds the FFC (Figure 5c). This time, the NF is not effective for solving the problem, by looking at Figure 5f, it is clear that the failure harmonics are not visible in the spectrum; therefore, it can be said that the NF-MCSA methodology experiences problems when the operation frequency and the load are low. Meanwhile, the GA-MCSA methodology can perform a correct suppression of the FFC, which allows observing the failure harmonics as two small peaks at their respective frequencies (see Figure 5i). In this sense, it can be said, that the only methodology that delivers reliable results, regardless of the operating conditions of the IM, is the GA-MCSA. It must be said that in this work, the proposed methodology is compared with the use of a static NF. This technique is selected because it is very common, its implementation is simple, and it provides  
Figure 5: Spectrum for the motor current operating at 20 Hz using (**a**) MCSA at high load, (**b**) MCSA at medium load, (**c**) MCSA at low load, (**d**) NF-MCSA at high load, (**e**) NF-MCSA at medium load, (**f**) NF-MCSA at low load, (**g**) GA-MCSA at high load, (**h**) GA-MCSA at medium load, and (**i**) GA-MCSA at low load. acceptable results in many cases. However, there are some other alternatives similar to NF that could have also been used. For instance, a more robust approach consists in the use of adaptive NF. These type of filters contain a preprocessing stage to estimate the required frequency for properly tuning the NF [36]. The estimated value is selected as the central frequency and it is considered to be the only frequency that can pass through the filter. Notwithstanding, this is the ideal behavior, but in real conditions, a narrow frequency band is suppressed.