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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Observer based control of an magnetorheological damper

Eroglu, Mehmet Ali January 2013 (has links)
Magnetorheological (MR) fluids enable the rapid and continuous alteration of flow resistance via the application of a magnetic field. This unique characteristic can be utilised to build semi-active dampers for a wide variety of vibration control systems, including structural, automotive, and bridge applications. However, the non-linear behaviour of smart fluid dampers makes the objective of achieving a desired control force very difficult. Most of the control algorithms proposed to overcome this problem require the measurement of both the MR damper force and the states of the system, which increases the complexity of the system. In this thesis, this problem is overcome by developing a non-linear observer. A further aim of this thesis is to investigate the effective control techniques for broadband excited observer-based MR vibration systems. Through an extensive series of numerical and experimental investigations, the general single-degree-of-freedom and tuned mass damper problems are presented. In an experimental case study, the hardware-inthe-loop-simulation method is adopted, which provides an excellent means to bridge the gap between theory and practice when the behaviour of a specific component is complex. Here, the vibration absorber with controllable MR damper is physically tested, whilst the remainder of the structure is simulated in real-time. The results demonstrate that the chosen control strategy can provide significant performance benefits when compared to more commonly used strategies and equivalent passive systems.
22

Variable frequency cycloconvertor drives and doubly-fed induction motors

Phillips, B. W. January 1968 (has links)
No description available.
23

An investigation into the mechanical and electrical losses of permanent magnet disc machines

Lamperth, Michael January 2000 (has links)
The subject of the thesis is the investigation of the loss-mechanisms in high-speed permanent magnet disc generators. Such electrical machines can be directly coupled to gas turbine engines and run at speeds of up to 60000 revolutions per minute. The work focuses particularly on the losses occurring in the stator of the machine, which are: 1. Ohmic resistance losses 2. Eddy current losses 3. Circulating current losses 4. Pumping losses of coolant fluid Numerical modelling was used to gain a theoretical understanding of the loss-mechanisms involved. Validation of these results was carried out by means of experiments conducted on several purposely built test rigs. The results of the experiments were then correlated to the numerical solutions so that the prediction of the losses for different geometries can be obtained with a high degree of certainty. For the numerical analysis, commercial software was used for thermal and flow problems, but for the electro-magnetic problems a new computer code using the object oriented language C++ was developed. Besides this, the magnitude and relation of eddy-currents and circulating currents in Litz cables was investigated in depth by modelling the Litz cables macroscopic properties in detail, which allowed the investigation of the effect of the cable bundling configuration. It was shown that the design of these machines is always bound onto the operating conditions as the magnitude of the different losses do not allow the ideal combination of properties to be found for a range of load points but only for given operating point.
24

Non-linearities in metal contacts at microwave frequencies

Arazim, Farrokh January 1978 (has links)
No description available.
25

Structural and magnetic properties of the geometrically frustrated 3d and 5d s = ½ Double Perovskites Sr₂CuWO₆, Ba₂YWO₆ and LaSrMgWO₆

Burrows, Oliver James January 2017 (has links)
Double perovskites with a single s = ½ magnetic ion and rocksalt order can show geometric frustration, due to arrangement of electron spins such that they cannot satisfy all nearest-neighbour antiferromagnetic interactions simultaneously. This can give rise to exotic magnetic states at low temperature. Compounds with unpaired 4d and 5d electrons have in this respect been studied far less than 3d compounds. Here, results of studies on the compounds Sr2CuWO6, Ba2YWO6 and LaSrMgWO6 are presented. The synthesis of the previously reported compound Ba2Y1WO6 (W5 +, 5d1) was attempted by many methods, but yttrium deficiencies were present in all samples. This led to the series Ba2YxWO6 with ⅔ ≤ x ≤ 0.8, dependent on the synthesis conditions. The x = ⅔ compound is known to exist as a metastable cubic phase and an 18H rhombohedral thermodynamic phase. The one-third vacant B sites on the x = ⅔ cubic material are doped with lithium, resulting in the Ba2Y2/3LixWO6 compound. This thesis focuses on the new x = 0.75 phase, and characterises its structural and magnetic properties. The Jahn-Teller distorted Sr2CuWO6, with the Cu2+ (3d9) magnetic ion, has an elongated c axis leading to separation of ab planes. This compound had been proposed as a pseudo-2D spin liquid model candidate, following a lack of evidence of transition to long-range order in SQUID magnetometry and heat capacity. However, recent μSR measurements did show a transition to long-range ordered state at 24K. This thesis details further bulk and local probe measurements which indicate that the low-temperature state is type-2 antiferromagnetic, and which point to a thermally activated spin-liquid-like state which occurs between 24K and ∼100K. La0.5Sr1.5MgWO6, has also been synthesised. The 1:1 compound LaSrMgWO6 had previously been reported as W5+ and described as “pseudo-cubic”: X-ray and neutron diffraction studies here characterise the low-temperature structure within the P2₁/n monoclinic space group, and suggest that no ordering of the atoms on the A site is observed.
26

Electronic structure studies of exotic phenomena using magnetic Compton scattering

Kersh, David Alexander January 2016 (has links)
The presented thesis investigates the spin densities of three exotic magnetic materials: Co2MnSi, a proposed half-metallic material which has gathered much interest in recent years due to the very high spin polarisation measured in thin films. CeB6, a Kondo material with a very complex phase diagram where the nature of the origin of the magnetism remains controversial and finally Ca3Co2O6, a complicated low-dimensional system where the electronic structure of the magnetic sites is poorly studied and controversial. These studies would not be possible were it not for the improved statistical quality and greatly enhanced experimental capacity, achieved by upgrading the experimental setup on the BL08W beamline at the SPring-8 synchrotron. The primary technique used to investigate the magnetic properties of these materials was Magnetic Compton Scattering. This technique directly probes the spin-dependent electron momentum density of a material, enabling an isolated measurement of its spin moment. In addition, through comparison with theoretical models, the technique can be used to gain insight into the electronic structure of the material, and determine which bands contribute to the magnetism in the system. These theoretical models were calculated using ab initio methods such as Density Functional Theory (DFT). In addition to magnetic Compton scattering, a range of complementary experimental techniques have been used to provide further useful analysis for these materials. This includes techniques such as SQuID magnetometry, Energy Dispersive X-ray Spectroscopy, Powder X-ray Diffraction and Laue Diffraction. The upgrade to the BL08W beamline at the SPring-8 synchrotron in Hyogo, Japan was prompted by technical issues which were present at the ID15 beamline at ESRF in Grenoble, France. Modelling the stray field effects proved the presence of such a powerful magnet would not impact the electron beam during operation, ensuring the magnet could be used on the beamline without affecting other experiments. Upgrading the 3 T cryostat at SPring-8 to the 9 T Oxford Instruments Spectromag magnet improved the experimental capacity - the range of physical phenomenon which can be measured, considerably. The study of Co2MnSi marks one of the first bulk investigations of the potential half-metal. A single crystal was measured along the [100], [110] and [111] directions using magnetic Compton scattering. Modelling of the system was performed using the ELK DFT code which calculated the system to be a half-metal with a 5uB spin moment. Comparing the experimental and theoretical profiles yielded good agreement, with the theoretical profiles very accurately modelling the broadness and tails of the MCPs but with deviations at low momentum. Comparing the anisotropies found very good agreement between the experimental and theoretical profiles. Characterisation of the material was performed using SQuID magnetometry, Energy Dispersive X-ray Spectroscopy and Powder X-ray Diffraction. The SQuID work found the sample to saturate at 5uB, in very good agreement with the theoretical calculation and literature. Studying the stoichiometry of the system, the EDX suggested a small Co-Si disorder, with the excess Si occupying the Co sites. Finally, Powder XRD using a Co and Cu source was used to probe the disorder in the sample by comparing the differences in the anomalous scattering from the two sources. Due to uorescence and a poor background:noise ratio, this study remained inconclusive. However, the good agreement between the DFT work and the measured magnetic Compton Profiles, the anisotropies and the SQuID work contribute strong evidence for the half-metallicity of Co2MnSi. A sample of CeB6 was measured along the [100] and [110] directions using magnetic Compton scattering. A small anisotropy between the two directions was found which prompted a further investigation. DFT calculations were capable of reproducing an anistropy but were inadequate in describing the shapes of the MCPs. The 4f profiles of the Ce ion were calculated using the GAMESS code. None of the calculated orbitals were found to be in good agreement with the experimental data. A calculation which combined the Ce 4fx(z2-y2) orbital with the calculated 2p orbitals of the B6 octahedra found excellent agreement with the experimental data. A fixed-spin moment calculation performed using DFT where a small moment was allocated to the B sites did not improve agreement with the experimental data. This result gives strong indication that the spin-density of CeB6 requires both a Ce and B moment to be described adequately. The final experimental chapter measured a sample of Ca3Co2O6 along the c axis using magnetic Compton scattering. Modelling the CoO6 trigonal site in GAMESS gave nonphysical results for describing the data. This lack of physicality was reproduced in the Co3O12 and Co5O18 calculations. Removing the O atoms from the calculation improved agreement with the experimental data considerably, finding the orbital contributions to be physical. The results of this work suggest Ca3Co2O6's electron momentum density originates entirely from the Co trigonal site in the 3+ state, with no spin-density originating from the O sites.
27

Magnetic noise studies in advanced metal particle tapes

Ardeleanu, Paul Claudiu January 2003 (has links)
The aim of this research was to achieve two objectives: assembly of magnetisation profiles and noise studies on flexible media. Magnetic profiles of the media were established using anhysteretic techniques derived from Alternating Current (AC) bias analogue recording. The intention of this investigation was to establish processes that could be used to generate non-uniform magnetisation states in tapes, which could be used for spatial analysis of noise sources. In practice, as the project progressed, a rapid decrease in magnetic coating thickness of tapes without a similar reduction in write head gap length prevented this approach and modelling studies were substituted as an alternative. The second and principal objective of this research was the investigation of remanent magnetic noise as a function of Direct Current Demagnetisation (DCD) and Isothermal Remanent Magnetisation (IRM) processes. These measurements were based around a reel to reel tester and a Linear Tape-Open (LTO) drive provided by Hewlett-Packard and were made in the range appropriate to data densities and data rates of the LTO generation one system. Data was recorded by capturing the time series using a digital storage scope, and FYI' analysis of the time series was used to obtain the noise power spectra. Noise data was analysed and, in particular, the iniestigation addressed the structure of noise sources in terms of their spatial distribution in the tape. The majority of the experimental studies were made in terms of noise related to uniform rernanent states of the tapes as these were considered to be related more directly to the media noise without complications of signal dependent features generated by sharp transitions in magnetic state. However, for completeness and also to start to relate findings to practical applications, some tone noise studies were made towards the end of the study along with dc remanent studies of states generated with a standard write head rather than a magnetiser.
28

Self-sensing permanent magnet servo motors

Bottomley, Jack S. January 2014 (has links)
The use of Permanent Magnet Synchronous Machines (PMSMs) has become widespread across numerous applications and industries. Their high power density, efficiency and accuracy of control make them excellent choices, leading them to become the industrial standard. Two issues concerning PMSMs use in recent years have been associated with the elevated cost of rare earth materials required for the Permanent Magnet (PM) rotor poles and the reliance on a direct rotor position sensor such as an encoder. PMSMs require an accurate rotor position feedback within the control scheme, traditionally provided by an encoder or resolver. These devices are excellent at providing the realtime rotor position accurately but have a negative impact on the machine as a whole. Their use increases the size, weight and cost of the electrical machine, while reducing reliability and often limiting use in extreme environments. This has created motivation for sensorless control of PMSMs, which removes the need for a position sensor. Sensorless control can be categorized into two distinctive aspects. The first is the control scheme and focuses on how position dependent properties can be used to estimate rotor position. The second, which has had less focus, is the machine design. This is focused on the ability of a machine to act as a position sensor with clear position dependent properties. Self-sensing machine design is the common term applied to this field since in essence the machine acts as its own position sensor. This thesis is concerned with self-sensing oriented design. The work presented is focused on PMSMs with inset rotor topologies. A methodology was developed to assess the position tracking capability of a machine and incorporated within a traditional machine design optimization routine. The conceptual design of the machine emphasized a generic geometrical topology, accounting for practical material selections and construction techniques. This ensured the design outcome had widespread implications, as opposed to a novel machine design with limited commercial relevance.
29

A comparison of saliency based sensorless control techniques for a PM machine

Duan, Yu January 2012 (has links)
This thesis analyzes saliency-based sensorless control methods for AC surface mounted permanent magnet machines (PMSM), because PMSMs have features that make them attractive for use in industrial drives: small size, high efficiency, low maintenance, high dynamics, and high power density. The thesis focuses on four different HF injection sensorless methods, which utilize resistance and inductance based saliencies for position estimation: the measurement axis method, the eddy current resistance based saliency tracking method, the eddy current inductance based saliency tracking method, and the PWM switching frequency injection method. The emphasis is in the comparison of the four HF saliency tracking methods under various conditions such as steady state, load impact, speed reversal, and zero and low speed operation. The amplitude and frequency of the injection signals are also compared to choose the best HF injection signal for the four saliency tracking methods. The best sensorless control method using eddy current resistance based saliency is introduced and the experimental results confirm the expected advantages for this sensorless application. This thesis also describes the development and enhancement of current derivative measurement for saliency tracking methods, which uses the stator current transient response to the voltage vectors contained in the fundamental PWM sequence. Due to the HF switching oscillations caused by the switching of the IGBT and parasitic capacitance, the accuracy of the current measurement is reduced and requires a minimum vector time of approximately 6µs. A signal processing algorithm is proposed which uses current samples during the high frequency current oscillations, and can potentially reduce this minimum pulse time.
30

Design and modelling of permanent magnet machine's windings for fault-tolerant applications

Arumugam, Puvaneswaran January 2013 (has links)
The research described in this thesis focuses on the mitigation of inter-turn short-circuit (SC) faults in Fault tolerant Permanent Magnet (FT-PM) machines. An analytical model is proposed to evaluate the inter-turn SC fault current accounting for the location in the slot of the short-circuited turn(s). As a mitigation strategy to SC faults at the design stage, a winding arrangement called VSW (Vertically placed Strip Winding) is proposed and analysed. The proposed analytical model is benchmarked against finite element (FE) calculation and validated experimentally. The results demonstrate that the proposed winding arrangement in the slot improves the fault tolerance (FT) capability of the machine by limiting the inter-turn SC fault current regardless the fault location in the slot. Electromagnetic and thermal studies are conducted to verify the merits and drawbacks of the proposed winding compared to the conventional winding using round conductors (RCW). The study shows that the proposed winding scheme, in addition to being fault-tolerant, has an improved bulk radial conductivity, can achieve a good fill factor, but has a significantly higher frequency-dependent AC copper loss. To predict the AC losses an analytical model based on an exact analytical 2D field solution is proposed. This model consists of first solving the two-dimensional magneto-static problem based on Laplace’s and Poisson’s equations using the separation of variables technique. Then, based on that solved solution, by defining the tangential magnetic field (Ht) at the slot opening radius, Helmholtz’ equation is solved in the slot sub-domain. Subsequently, an FE and MATLAB® coupled parametric design is undertaken to maximise the VSW wound machine’s efficiency whilst maintaining its FT capability. The proposed analytical models for prediction of the SC fault current and AC copper losses are integrated into the coupled optimisation. It is shown that the effective losses of the VSW can be minimised through the parametric design while maintaining the required level of machine performance. Using an existing FT-PM machine of which the rotor is kept unchanged two stators were designed, manufactured and wound with RCW and VSW respectively and experimental tests are carried out to validate the analytical models and the new winding concept.

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