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Title: Investigation of vibration in switched reluctance machines
Author: Dexter, James
ISNI:       0000 0004 9358 274X
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2020
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The popularity of magnet-less electrical machines has increased due to the volatility of permanent magnet prices, prompting a rise in interest of switched reluctance machines (SRMs). SRMs are suitable for a wide range of applications, but limited by high torque ripple, acoustic noise and vibration. This thesis systematically investigates electromagnetically excited stator vibrations in a 6-stator-pole/4- rotor-pole (6s/4r) SRM. With the aid of ANSYS Workbench finite element (FE) package, a series of electromagnetic and mechanical FE models are developed to investigate the vibration response. The 2D and 3D FE mechanical models are progressively built from a simple laminated annular ring to the final 6s/4r prototype with housing, and verified with experimental hammer impact tests at each stage of assembly. The mechanical connections of various SRM parts are investigated. It is found that the bolted connection of the endcaps to the housing frame has a significant influence on the natural modes and frequencies of the SRM, e.g. introducing new dual vibration modes of order 2. This phenomenon has not been previously covered, and in industrial applications this may cause unexpected and unwanted resonant frequencies, resulting in potential for worsened acoustic noise and vibration. The influence of various control parameters is also investigated. The 6s/4r SRM is systematically simulated by FE models under different load conditions, turn-off and turn-on angles, in terms of electromagnetic forces and dominant vibration modes. It is shown that the load and turn-off angle have a significant impact on vibration response, while turn-on angle has minimal influence. The FE results are verified experimentally in single pulse voltage mode control. As the turn-off of current in each phase is the dominant source of acoustic noise and vibration in SRMs, the influence of current profile is investigated by two methods, in time domain and frequency domain, respectively. In the time domain, the current profile is defined by a discrete series of points. With reference to the baseline unipolar square wave current profile, a continuous slope differential and a current step are introduced at turn-off. It is shown that significant reduction in vibration response can be achieved by increasing the duration of the continuous slope due to reduced severity of radial force change. It is also found that a current step reduces vibration response but the duration of the step has no influence. In the frequency domain, the baseline unipolar square wave current profile is decomposed by the Fourier transform into a series of harmonic components. Through progressive reintroduction of harmonic contents it is found that with low order current harmonics only, the vibration response is significantly reduced, albeit with electromagnetic performance affected. The largest influence on vibration response is due to the amplitude of the first current harmonic amplitude. It is also found that the 2nd current harmonic amplitude can improve the electromagnetic torque but has minimal effect on vibration response. By operating the motor under current control with harmonic components, the vibration response can be reduced whilst simplifying the control strategy and hardware, lowering cost which is a critical factor for industrial applications.
Supervisor: Zhu, Z. Q. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available