*3.4. Research Limitations*

In this section, dealing with the limitations of this research, we make suggestions for future research activities on the theme of changes in the performance of electric motors, which will contribute to research in the area:


## **4. Conclusions**

It is common to read in the technical literature that "SCIMs have hardly changed in the last 100 years". However, current SCIMs are significantly different from the SCIM developed by Mikhail Dolivo-Dobrovolsky's team between 1888 and 1890. Therefore, this statement is only valid when referring to the SCIM's working principle. This research showed significant changes in the design, materials, and components that make up the parts of SCIMs.

The present research analysed the performance levels of SCIMs based on the results of tests carried out at the Laboratory of Electrical Machines of IEE/USP in the period between 1945 and 2020. SCIMs with powers of 3.7 kW, 37 kW, and 150 kW were used in a total of 359 electric motors. Regarding the performance levels, the results showed that the SCIMs presented a similar trend, and it was possible to identify three distinct periods in the historical timeline.

Between 1945 and the mid-1960s, SCIMs showed practically constantly increasing gains in performance. This was due to the various technological innovations in the period, mainly the use of oriented grains in the ferromagnetic material, the use of aluminium in the rotor, essential improvements in the projects, and the ventilation of the SCIMs.

Between the 1960s and 1980s, which was a period of cheap energy, manufacturers built cheap and relatively inefficient SCIMs, minimizing the use of materials such as copper, aluminium, and steel. The production of lower-performance, lower-volume SCIMs was made possible by developing insulating materials (particularly varnishes) that could withstand high temperatures. This allowed SCIMs to be designed with higher losses (particularly Joule losses in the stator winding), since the temperature rise due to losses could be transferred to the housing (the varnish is electrically insulating and thermally conductive) without damaging the insulation or reducing the expected motor life (Figure 18). In this period, the reduction in the performance of SCIMs was so high that, in some cases, the performance reached lower levels than for the SCIMs marketed in 1945.

Although these motors had lower start-up costs than previous designs, they used more energy due to their inefficiency.

From the 1980s to 2020, performance improvement dominated the scene again. The central aspect of this performance variation was the technology and materials used to construct the machines. It was possible to observe that the gains were significantly higher for minor power values, due to the large margin for improvements in materials and projects due to the low technical construction rigour.

The reduction of losses in the SCIMs analysed in the period 1945–2020 was in all cases more than 40% for the three analysed output power values (3.7, 37, and 150 kW) and the four possible speeds (two, four, six, and eight poles). In the case of 150 kW SCIMs with a speed corresponding to four poles, the loss reduction in the period reached 72.9%, showing a significant advance.

The 37 kW SCIMs with a speed corresponding to 2 poles had the highest accumulated efficiency gain in the analysed period. They went from 81.5% average yield in 1945 to 94.5% 75 years later (2020), resulting in an absolute 13% performance gain.

The relationship between the mass and power of SCIMs presented two periods in the analysis performed. The first period was the 94% reduction between 1891 and 1984, from 86 kg/kW to 4.8 kg/kW, due to the various technological innovations discussed in this paper. The second period showed a decrease by 112.5% between 2000 and 2020, from 4.8 kg/kW to 10.2 kg/kW on average, due to the need to resume the performance increase.

In conclusion, continuous performance gains occurred during intense technological innovation, showing the importance of performance legislation for SCIMs. In the 1970s and 1980s, the search for lower-cost SCIM manufacturing reduced the equipment's performance. Thus, the self-regulation of the SCIM market, in terms of performance, did not show positive results in periods of low technological innovation. A return of the performance improvement was observed, mainly by the imposition of performance legislation, motivated by a global need to rationalize the final energy use and by sustainable energy considerations.

**Author Contributions:** Conceptualization, D.F.d.S., F.A.M.S., I.L.S. and A.G.K.; methodology, D.F.d.S., F.A.M.S. and A.G.K.; validation, D.F.d.S., F.A.M.S., H.T. and A.G.K.; investigation, D.F.d.S., F.A.M.S., A.T.d.A. and A.G.K.; resources, F.A.M.S., A.T.d.A., I.L.S. and A.G.K.; data curation, D.F.d.S. and F.A.M.S.; writing—original draft preparation, D.F.d.S., F.A.M.S., I.L.S., H.T., A.T.d.A. and A.G.K.; writing—review and editing, A.T.d.A., F.A.M.S., H.T. and A.G.K.; supervision, H.T. and A.G.K.; funding acquisition, I.L.S. and H.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** National Electric Energy Agency (ANEEL): Project number 00390-1086/2018 (ENEL)— Integrated assessment of distributed generation, demand management, monitoring, quality, and performance of the network, aiming at optimizing investments and tariff regulation in the underground network, and Project number 00061-0054/2016 (CESP)—Analysis of the efficiency of complementary energy storage with hydroelectric plants, using electrochemical and hydrogen storage technologies; National Council for Scientific and Technological Development (CNPq): Project 870814/1999–0, Process 142323/2020–9.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank the National Council for Scientific and Technological Development (CNPq) for the scholarship made available to the first author through the project 870814/1999–0, process 142323/2020–9, so that he could dedicate himself to his PhD research in this and related fields. The authors thank the researchers Walter Aguiar Jr. and Kenny Kawaguchi for their support in creating the figures for this paper, and the researcher Jefferson Oliveira for the discussions on electrical machine projects. Furthermore, the workers who worked since the foundation of the Laboratory of Machines of the IEE/USP and throughout their careers generated the database used in this research. The authors thank the researcher Richardson M Abraham-A for improvements in the translation of the manuscript. Moreover, the teacher Lars Eirik Frantzen made the text more friendly. The first author honors Mário Kawaphara and Mateus Rondina from the Federal University of Mato Grosso, for their teaching on SCIMs. The authors would like to thank the researcher José Roberto Cardoso from the Applied Electromagnetism Laboratory (LMAG/USP) for his contributions to the discussion in section (Reduction of Volume and Losses of Ferromagnetic Materials in SCIMs) of this paper. The authors are grateful for the reviewers' criticisms, which contributed to improving this paper.

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