Li, Jiaqi
(2021)
Vibration analysis and investigation of synchronous reluctance motor.
PhD thesis, University of Nottingham.
Abstract
In the last decade, the interest in synchronous reluctance motors has increased considerably due to several key advantages in comparison to other types of motors. It offers good torque density, wide speed range, and can deliver higher efficiency when compared to induction motor for the same frame size. Even though the torque density of synchronous reluctance is lower than the permanent magnet synchronous motors, it is compensated by a lower cost of the rotor that is free from active excitation. As for synchronous reluctance motors, their electromagnetic torque is entirely developed by the reluctance torque. Hence, the rotors of these machines are usually designed as a relatively complex structure, made by flux barriers and iron paths forming an anisotropic structure to produce high reluctance torque. This leads to a new challenge with respect to the mechanical performance of the synchronous reluctance motor, as the retaining iron ribs, if not designed well can lead to a fragile rotor and excessive vibrations.
In order to investigate the relations between the geometrical parameters and the mechanical performance of the synchronous reluctance motors, this thesis takes a 5 kW synchronous reluctance motor as a case study to develop the research, and then propose a 15 kW synchronous reluctance prototype to validate the analyzing methods. The main contributions of the thesis are shown below.
In Chapter 2, an analytical model based on the magnetic equivalent circuit is proposed. This model studies the magnetic characteristics of the synchronous reluctance motor, followed by the analytical derivation of the air-gap flux density, electromagnetic torque and force. A sensitivity analysis is conducted to investigate the impact of the key geometrical parameters on the electrical performance, from where the air-gap flux density, electromagnetic torque and force are highlighted.
In Chapter 3, the mechanical characteristics of the synchronous reluctance motor are analysed by performing the modal analysis, stress analysis, and vibration analysis, from where the modal shape, natural frequency, stress tensor tensile, and vibration level are presented, respectively. There are several sensitivity studies following each of the analysis, that is the impacts of the motor geometry on the mechanical characteristics are explored and analysed.
In Chapter 4, the impact of permanent magnets inserted into the rotor structure are investigated. The equivalent magnetic circuit is developed by adding the influence of permanent magnet, therefore the magnetic characteristics of the permanent magnet assisted synchronous reluctance motor are calculated. Then, the mechanical characteristics of permanent magnet assisted synchronous reluctance motor are analysed through the modal analysis, stress analysis, and vibration analysis, followed by their corresponding sensitivity analysis, in order to deeply dig out the influence of permanent magnet. Furthermore, the vibration level of permanent magnet assisted synchronous reluctance motor is compared to that of synchronous reluctance motor by considering the electromagnetic force harmonics.
In Chapter 5, the synchronous reluctance motor is studied in case of eccentric rotor position. The air-gap length of synchronous reluctance motor is usually very small for getting a better electrical performance. Hence, a tiny eccentricity, within the permissible manufacturing tolerances, will have a significant effect on the motor performance. The analytical model is developed by applying the eccentric condition, from where the air-gap distribution is uneven. Then, the electrical performance and mechanical characteristics are analysed in case of eccentricity.
In Chapter 6, a 15 kW synchronous reluctance motor is analysed by using the same analytical model and Finite-Element Analysis (FEA) methods, then it is prototyped to experimentally validate the analytical modelling and FEA research method. This prototype is operated under various power mode, including light load (5 kW) condition, mild load (10 kW) condition, and nominal load (15 kW) condition. Then the electrical and mechanical characteristics of this prototype are tested and compared with the FEA results.
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