## 7 Conclusions  
In this study, energy efficiency indicators were evaluated such as life cycle energy savings, cost savings, and payback period for various motors of enhanced efficiency classes: the IE3, IE4 induction motors, the DOL PMSM, and the DOL SynRM. These parameters are calculated with respect to the basic case of using the IE2 motor in the pump unit. The comparison takes into account not only the efficiency of the motors at different pump loads but also the effect of their power factor on the cable and transformer losses. It is shown that taking this factor into account significantly affects the comparison for the studied motors. For example, it was shown that the DOL PMSM provides the greatest amount of energy savings over the life cycle despite the fact that its efficiency is not highest among the motors under consideration. The analysis takes into account that the motors have different initial costs. The IE4 IM has an increased cost compared to the IE3 IM and the DOL SynRM due to the higher consumption of the main active materials: copper and electrical steel. The DOL PMSMs have the highest initial cost compared to the IMs and the DOL SynRM due to the use of expensive rare-earth magnets in its design. In addition, the rare-earth elements processing from raw ore is associated with significant environmental damage. It is shown that the payback period of all considered energy efficient motors is less than 1 year. The use of an IM IE3 class motor would pay off in a very short term after a new pump unit was put into operation since its payback period is 2.2 month (0.18 years), which is less than that of IE4 IM and DOL PMSM (0.26 and 0.31 years, correspondingly). However, after 4 months (0.33 years) IE4 IM and DOL PMSM turn out to be more cost-effective than IM IE3. Estimated 20 years life cycle cost savings of the IE4 IM and the DOL SynRM motors have similar values: 12.8 and 12.5 k\(\epsilon\), respectively, which is 9.9% and 9.7% higher than that of the IE2 IM. In the case of the IE3 IM, the value of 20 years life cycle cost was only 8.1 k\(\epsilon\) or 6.3% higher than that of the IE2 IM, which is significantly less profitable compared to the IE4 IM and the DOL SynRM. The most profitable in the course of 20 years is the use of the DOL PMSM, as its life cycle savings is 14.2 k\(\epsilon\), which is 10.9% higher than that of IE2-IM. The analysis shows that the DOL SynRM provides energy and cost savings close to the more expensive the IE4 IM and the DOL PMSM. The IE4 IM gives only 0.2% higher savings over 20 years than the DOL SynRM. The DOL PMSM delivers 1.2% higher savings over 20 years than the DOL  
Figure 12: Payback period of various motor types (the case of replacing the motor in an exploiting pump unit): (**a**) case 1 (without cable and transformer losses); (**b**) case 2 (3 pump units considering the cable losses); (**c**) case 3 (3 pump units considering the cable and transformer losses). SynRM. However, the DOL SynRM has the shortest payback period of 0.65 years, in case of a motor replacement in a pump unit in operation. The payback period for the DOL PMSM and the IE4 IM is slightly higher than that of the DOL SynRM and in both cases is about 0.78 years. Based on the results of this study, it can be concluded that, at the present time, the use of IE4 motors is more profitable than the use of motors of classes IE2 and IE3. The cost-effectiveness of the use of direct-on-line synchronous motors of various types is also validated in this study. Conceptual approach, V.K. and V.P. ; data curation V.D. and D.G. ; software V.K. and V.D. ; calculations and modeling, V.K., V.P. and V.D. ; writing of original draft, V.K., V.P., V.D. and D.G. ; visualization, V.K., V.D. and D.G. ; review and editing, V.K., V.P., V.D. and D.G. All authors have read and agreed to the published version of the manuscript. The work was partially supported by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ 2020-0060).