Design, analysis and optimization of line-start permanent-magnet synchronous motors: simultaneous electromagnetic and thermal analysis

Author: Mousalreza Faramarzi Palangar

Faramarzi Palangar, Mousalreza, 2021 Design, analysis and optimization of line-start permanent-magnet synchronous motors: simultaneous electromagnetic and thermal analysis, Flinders University, College of Science and Engineering

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Future energy concerns and global economic challenges are encouraging the world to undertake energy conservation projects. A significant way to address these concerns is to increase the energy efficiency of electric appliances. Since electric machines account for roughly 45% of all industrial electricity usage, an immense amount of energy saving can be accomplished by increasing the efficiency of electric motors. Induction motors are used in industry because of advantages like self-starting capability, affordable manufacturing cost and maintenance. However, intrinsic drawbacks of induction motors like comparatively low efficiency and power factor are not easy to overcome, even though induction motor performance has improved dramatically over the years.

Manufacturers of electric motors worldwide are gradually focusing on alternative electric machine technology to meet more rigorous energy efficiency requirements. Hence, line-start permanent magnet synchronous motor (LSPMSM) machinery has gained substantial recognition in comparison with other motor types. This type of motor has been made very appealing by significant benefits like self-starting, high efficiency and power factor. Extensive literature research on LSPMSMs has been undertaken, concentrating primarily on the development of rotor configurations, developing the steady-state analytical model, and using the transient time-step finite element (FE) approach for synchronization evaluation. Due to a hybrid LSPMSM rotor including both an induction cage and a permanent magnet, torque mechanisms in transient starting and steady-state operating conditions vary. Finite-element analysis (FEA) is commonly used to determine the LSPMSM's synchronization capability. However, this form of verification strategy is costly in terms of calculation. Hence, motor designers and engineers are interested in using a fast and reliable alternative design and optimization approach like analytical methods. Hence, it would be of great significance to develop a design and optimization methodology that allows motor designers to study transient and steady-state performance with high accuracy and low computation time.

This study presents a strategy to design an optimum line-start permanent magnet synchronous motor (LSPMSM) with improved performance in both transient (dynamic operation to reach synchronous speed) and steady-state (operating with constant synchronous speed). A mathematical design and optimization method, based on the developed machine sizing equations of induction motors (IMs) and permanent magnet (PM) motors, is proposed for the design of an optimum LSPMSM. The rotors of the IM and IPM are combined to create a hybrid rotor including an induction cage and permanent magnet for a LSPMSM. To verify the proposed mathematical method, a three-phase, 4-pole 4-kW LSPMSM is selected as a case study. A second case study of a three-phase, 1-kW, 8-pole LSPMSM is studied for further verification. The initial designs of the IM, IPM motor and LSPMSM are analyzed using FEM to verify the proposed analytical design and analysis method. The IM and IPM are then analytically optimised using a genetic algorithm (GA) for the transient improvement (through maximizing the starting torque) and the steady-state performance improvement (via maximizing efficiency), respectively. The rotor cage bar dimensions and PM size are selected as optimisation variables in optimizing the IM and the IPM. Combining the rotors of the optimised IM and IPM yields the optimum hybrid rotor for the LSPMSM. To present a comparative study between the proposed optimisation method and FEM optimisation, the 2D design of an initial LSPMSM design is optimised based on FEM in Ansys/Maxwell. The designed LSPMSM meets the super-premium efficiency (IE4) standards, which outperforms the benchmark IM standard of premium efficiency (IE3). Also, the designed LSPMSM has the capability of starting directly whilst the benchmark PM motor requires an external driver to start.

In addition, this study presents a novel analytical thermal analysis model based on a lumped-parameter model of the LSPMSMs. Hence, a lumped-parameter thermal circuit is proposed for LSPMSMs based on the developed thermal model of an IM. To verify the proposed analytical thermal model, a 3-phase, 4-pole 3-kW IM is selected as a case study incorporated with thermal experimental test results from a 3-kW commercial IM to validate the results of the proposed thermal model. The performance of the proposed thermal model of the LSPMSM is verified using 3D FEM-based thermal analysis. In this section of the thesis (thermal analysis and modeling), the LSPMSMs are researched to discover the achievable maximum output power in the same frame size (3-kW and 4-kW commercial induction motors) with successful synchronization and safe operation in terms of temperature rise.

In summary, the imperative of this study proposes a novel analytical electromagnetic and thermal design, analysis and optimization platform of line-start permanent magnet synchronous motors (LSPMSMs). The main contributions made in this thesis are: (a) simultaneous starting torque and efficiency improvements of the LSPMSM designed based on the commercial IM via implementing optimization using FEM techniques; (b) comparing the performance of two different optimization approaches (gradient-free and gradient-based approaches) in the context of electric machines with a focus on the IMs and LSPMSMs; (c) developing an analytical design, analysis and optimization platform for the LSPMSMs using machine sizing techniques of IMs and permanent magnet (PM) motors; (d) proposing an analytical thermal model and analysis of the LSPMSMs based on lumped-parameter network.

Keywords: Line-start permanent magnet synchronous motor, Optimization, Transient improvement, Steady-state improvement, Analytical thermal Analysis, Finite-element method

Subject: Engineering thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Amin Mahmoudi