**6. Conclusions**

This review presents the discussion of several works in the state of the art referring to the following aspects: the efficiency of electric machines (motors and drives); the power quality; the relationship between the power quality and electric machines affecting the efficiency; the techniques for power quality disturbances detection, classification, and mitigation; and the techniques for PQD analysis in motors and drives. The discussion of the works related to electrical machines and energy efficiency allows to conclude that there exists a mutual relation between motors and drives with the power quality. For example, the efficacy of the PQ of a power source is reduced by the disturbances induced by electrical equipment connected to the grid, but also, once the grid is contaminated with electric disturbances, they reduce the performance of motors and drives. Is worth to highlight that in the literature, several works have been developed with the purpose to detect, classify, and mitigate the affectations generated by the PQDs. There are several methodologies; the firsts of them were designed only for the detection task, and they were based mainly in spacetransform techniques. Later, the integration of artificial intelligence techniques arrived; this brings out the opportunity for performing the classification of the PQDs. The most recent strategies combine the aforementioned techniques to define well-structured approaches for feature extraction, dimensionality reduction and classification. Alternative methodologies such as novelty detection and heuristic techniques have also been addressed, making a discussion about their characteristics which make them potential solutions to give accurate and reliable results to problems where the reported methodologies cannot. For example,

by performing the detection, identification and classification of power disturbances not considered ye<sup>t</sup> by the standards, or other types of disturbances different from those tackled by the reported works. Additionally, in the fault conditions monitoring, in motors and its drives, these approaches can be explored for detecting and classifying several faults or their combinations. By the other side, the heuristic schemes can be adopted to give support to the novelty detection methodologies, by selecting the parameters that play a key role in the performance of such methodologies. Additionally, the heuristic approaches could be used for estimating the values of parameterized models that describe the power disturbances and the fault conditions, or their combinations, respectively. Mitigation of PQDs is still an area of opportunity, since few works have handled this issue but only for limited power disturbances. The alternative methodologies proposed in this review could be opportune options for proposing strategies to meet this goal. For instance, novelty detection can provide accurate information about the anomalies in the grid, or in a motor, in order to develop mitigation strategies. One example about mitigation of power disturbances could be developed through the use of heuristic techniques, by defining a parameterized model capable of generating the opposite signal that mitigates (attenuating or minimizing) the effects of the disturbances (or their combinations). Finally, the studies of PQD affecting the efficiency of motors and drives the analysis considered until now limits to some disturbances such as voltage sags, voltage unbalance, and harmonic content. Here, the well-structured approaches could be useful to this matter. Regulatory agencies are introducing energy efficiency requirements and the electric machine must meet these restrictions. Therefore, it is important that the new lines of investigation look towards solutions to mitigate the PQDs in order to rise the electric machines efficiency that in consequence will increase the power grid efficiency.

**Author Contributions:** A.-D.G.-A. performed the investigation of the majority of the revised works in the state of the art and wrote most of the paper; A.-Y.J.-C. carried out the research of several complementary works added to the paper from the literature, also revised and corrected most of the paper sections; R.-A.O.-R. conceived and developed the idea of this review, and proofread most of the paper; M.D.-P. made the discussion of several papers included in this review and wrote some sections of the paper; J.-A.A.-D. conceived and developed the idea of this research, performed papers analysis, and wrote some of the paper sections; A.K. conceived several ideas for this research and performed the finals proofreads of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
