## 5 Results and Discussion
### Results for the IM Operating at 31 Hz
Finally, the three techniques are used for the current signal when the motor operates at low load. In this case, it is expected that the failure harmonics appear at 29.6 and 32.4 Hz. From Figure 3c, it is clear the existence of the LSH; however, the presence of the RSH is not clear since its energy is low and the spectral leakage around the FFC hides it. In this case, when the NF is used for attenuating the FFC, the result gets even worse (see Figure 3f). It is observed that the spectral leakage associated with the FFC is so severe that the energies of the RSH and the LSH are completely covered; therefore, the presence of the failure harmonics cannot be properly observed. On the other hand, by looking at Figure 3i, it is clear that the FFC has been highly attenuated and it is almost imperceptible in the spectrum. This situation generates that the leakage around the FFC disappears and the LSH and the RSH are easy to observe at their respective frequencies. At this point, it is important to mention that, when the IM operates at high load and medium load, the MCSA is able to perform the identification by its own. However, the MCSA technique presents some problems when the IM operates at low load and the NF (which is probably the most common solution for the issues related with the MCSA technique) cannot solve these problems. In fact, the NF aggravates the situation. In this sense, the GA-MCSA proves to be the best of the three solutions for the analysis of the IM, even when it operates at low load, allowing the identification of the LSH and the RSH in an easy way. To prove the consistency of the results, five tests are carried out for each load level, resulting in a total of 15 tests. The obtained results are used to obtain the boxplots showed in Figure 4.  
Since the results are similar for both failure harmonics, only the results for the LSH are presented. Here, it is important to mention that the amplitude of the failure harmonic is directly proportional to the IM slip. Thus, it is expected that when the slip is high, the amplitude of the failure harmonic is high too, and when the slip decreases, the amplitude of the failure harmonic decreases too. This situation is not present in Figure 4a. When the conventional MCSA is applied, a same amplitude value for the LSH appears when the IM operates at high load and also when it operates at medium load, i.e., there exists an overlap between these two operative conditions. By observing Figure 4a, it can be noticed that the region comprised between the 0.042 and the 0.057 A appears in both: the high load and the medium load condition. The only condition that does not share amplitudes is the low load operation that appears in the range between the 0.012 and the 0.035 A. All these results correspond to the use of the conventional MCSA. Figure 4: Boxplots for the amplitude results of the left side harmonic (LSH) at 31 Hz for the (**a**) common MCSA technique, (**b**) NF-MCSA technique, and (**c**) GA-MCSA technique. When the amplitude of the LSH is analyzed after applying the NF-MCSA methodology (see Figure 4b), it is shown that for the case of the IM operating with s = 6%, the amplitudes fluctuate from 0.032 to 0.065 A; for the case of the IM working at medium load, the amplitudes lay between the 0.032 and the 0.048 A; and finally, for the case of low load operation, the amplitudes appear between the 0.011 and the 0.022 A. Thus, it is observed that all the amplitudes associated with the medium load case could also be related to the high load operation. Just as in the previous case, an overlap between the amplitude values of these two conditions is presented. This situation represents a problem, because, if the amplitude value is intended to be used as an indicator of the fault severity, with this overlap, it is not possible to correctly distinguish the correct result. These errors in the amplitude are related to the existence of spectral leakage in the neighborhood of the FFC.