**5. Conclusions**

The line-start synchronous motor has drawn the attention of scientists and industry professionals as a possible replacement of three-phase squirrel cage induction motors, especially in constant speed applications, due to their high efficiency and good power factor. The strict regulations of the EU market regarding usage of IE3 efficiency class of motors (that can be achieved with three-phase squirrel cage motors with numerous modifications that require more material with high quality and low losses) increases the interest for these line-start synchronous motors, which can easily achieve IE4 efficiency class. Among the line-start synchronous motors, there are various topologies which require different designs of the rotors and produce different motor operating characteristics. The authors have chosen to analyze the line-start synchronous motor with an interior permanent magne<sup>t</sup> asymmetric array topology as this configuration needs more detailed analysis and modification of rotor design, due to the availability of the space in the rotor to place magnets with sufficient dimensions, in order to achieve good efficiency, power factor and overloading capability of the motor. Starting from the three-phase squirrel cage motor of 2.2 kW, the first model (BM) is obtained by placing the magnets inside the

rotor without any other modifications in the motor. The obtained BM model has high efficiency and power factor but relatively low overloading capability. The second model (M1) with modified rotor slots and magne<sup>t</sup> dimensions has high efficiency and power factor, improved overloading capability of the motor but relatively high consumption of permanent magne<sup>t</sup> material. Therefore, more modifications of motor design were needed that included modification of the air gap length, magne<sup>t</sup> width and thickness and the number of the conductors per slot. In order to determine the best combination of these four parameters that produce the high power factor and efficiency, with good overloading capability and low consumption of permanent magne<sup>t</sup> material, the optometric analysis was run and more than 25,257 combinations were solved. This analysis is a useful tool as the increase in one parameter can improve one operating characteristic and worsen the other, and vice versa. When four different parameters are varied within certain boundaries simultaneously, without optometric analysis, it is very difficult to determine which value each of these parameters should have that will produce a model of the motor which will satisfy four various operating characteristics regarding optimal or improved operation in comparison to the starting model. Among these numerous combinations, the M2 model, with a sufficiently high power factor, efficiency and overloading capability (higher efficiency and overloading factor than models BM and M1) and considerably lower consumption of permanent magne<sup>t</sup> material is chosen as the optimal solution. The impact of each varied parameter on motor operating characteristics is analyzed, providing detailed insight into which design guidelines should be followed for obtaining satisfactory design of the motor. The chosen model (M2) was analyzed with FEM for the magnetic flux density distribution and with dynamic models for obtaining the transient characteristics. The M2 model has some areas of stator yoke with high flux density which can be improved by increasing the stator diameter, subject to further research. The dynamic behavior of M2 is satisfactory since the motor reaches the synchronous speed and continues with stable operation.

The careful analysis of each parameter and its impact on motor operating characteristics can significantly improve the motor operation, leading to the cost effective design of the motor. The proposed model is based on computer analysis and simulations. Its prototyping is highly affected by various manufacturing details such as obtaining a good slot fill factor or vibrations and noise as a result of the air gap length, which can have a significant impact on the final outcomes of this analysis.

**Author Contributions:** Conceptualization, V.S.; methodology, V.S., P.J. and D.M.; software, V.S.; validation, V.S., D.M. and P.J.; formal analysis, V.S.; investigation, V.S.; resources, V.S.; data curation, V.S.; writing—original draft preparation, V.S.; writing—review and editing, P.J.; visualization, V.S.; supervision, P.J.; project administration, D.M.; funding acquisition, D.M. and P.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This publication was created thanks to support under the Operational Program Integrated Infrastructure for the project: International Center of Excellence for Research on Intelligent and Secure Information and Communication Technologies and Systems-II. stage, ITMS code: 313021W404, co-financed by the European Regional Development Fund.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
