No description available.

The research carried out on the stabilization control and protection of two Van de Graaff electrostatic nuclear accelerators is described in this thesis. The system at Oxford consists of a single ended and a tandem accelerator with two different kinds of stabilizers; the results presented here are directly applicable to most electrostatic accelerators. As an end result of the mathematical analysis, the behaviour of the single ended injector with combined liner and spray control is formulated. The stabilizer designed for the injector, based on the analysis, incorporates automatic switch over to the auxiliary generating voltmeter stabilizer loop. The automatic gain control introduced into the slit amplifiers eliminates the necessity of manual gain adjustments for different operating conditions. The energy resolution of the injector was measured by using alpha capture resonance in [15]N. It was 0.65 keV at 4.46 MeV. The analysis presented here describes the behaviour of the corona stabilizer as used on the tandem. The predicted limitations due to the time delay and dispersion of the ions in high pressure gas are in good agreement with the measured performance figures. The two loop stabilizer with the auxiliary generating voltmeter loop provides facilities for semi-automatic operation. The resolution of the tandem was +/-1 keV by measuring the [28]Si(p,p)[28]Si resonance at 5.83 MeV. Multiple breakdowns on both the injector and the tandem were prevented by the introduction of the protection circuit. The circuit design was based on the analysis of the breakdown behaviour of the generators as described. The predicted performance of the circuit and its effect on the accelerators is well verified by the observed behaviour of the generators after total breakdown. The study of the computer controlled generator operation is described in Chapter 5. For the interfacing the internationally accepted Camac system is recommended.

Magnetic Induction Tomography (MIT) belongs to a group of techniques for the non­ invasive imaging of the passive electrical properties of objects which also includes Electrical Impedance Tomography (EIT). In MIT, arrays of coils are employed to induce eddy currents within an object and to detect the resulting secondary magnetic fields. Conductivity and permittivity distributions are then reconstructed from the collected set of four-terminal transimpedance data. MIT has several potential advantages relative to EIT including (i) errors due to variability of electrode contact impedances and positioning are avoided since the coils in an MIT system are typically rigidly attached to a chassis/screen, and (ii) the magnetic fields employed easily pass through high impedance boundary layers (e.g. the skull in brain imaging applications). Applications of MIT may be broadly divided into two categories: high-conductivity and low-conductivity. The development of MIT for low-conductivity applications (a < 30Sm -1)has been much slower than for high­conductivity ones since the conductivities involved are lower, typically by a factor of ~1x107 resulting in much smaller signals and very challenging instrumentation design. The aim of this work was to develop MIT systems with a performance level approaching that required for low-conductivity medical and industrial applications including the detection of haemorrhagic cerebral stroke and the imaging of multi-phase flows in oil pipelines. The specification, design and performance of three novel low-conductivity MIT systems are described and are discussed in relation to the target applications. The Cardiff Mkl single-frequency MIT system was only the second multi-channel MIT system to be constructed, the first low-conductivity MIT system which allowed measurement of both real and imaginary signal components, and provided a higher measurement precision (by a factor of 6) than the previous system, with a phase noise of 17mO for 30ms time constant. A planar array system is described employing a novel coil geometry which provided a very significant reduction in phase noise and drift through the use of coil orientation. Finally, the Cardiff Mk2 multi-frequency system provided an order of magnitude improvement in measurement precision in comparison to the Cardiff Mk1 system with phase measurement precision of 1.1 m0 - 8m0 over 10MHz -0.5MHz, SNR for in vivo human head measurements estimated at 59d.B - 16dB over the same frequency range and phase drift of < 1OmO over periods of up to 12 hours.

No description available.

The study includes the analysis and investigation of inverter-fed squirrel cage induction machine drives. The particular drive used was a 120° square wave inverter feeding a Tubular Axle Induction Motor developed for rail traction by British Railways. The system and its operating modes, including self-excited braking, are described. The conditions under which self-excited braking can be achieved are investigated both theoretically and experimentally and upper and lower limits to the range of permissible rotor speeds are found by several analytical methods. An original analogue model of the inverter is developed which is suitable for the investigation of inverter firing algorithms. A simpler and more efficient model is developed for the investigation of commonly used inverters. A two axis model of the induction machine is described and used to produce an analogue simulation. State-variable analysis is used to predict the steady-state waveforms and the transfer functions of the system. A simple method of predicting the frequency response of a linear or linearised system is described. Steady-state sinusoidal analysis is used to predict the limits to self-excitation. Results from the various methods of analysis are compared with each other and with results from the real system in both the transient and steady-state modes of operation. The results from the analogue model are found to give best agreement with those from the real system in both modes. Inverter losses are found to affect the boundaries of self-excitation. The possibility of using the analogue model in the development of micro-processor control for the traction system is discussed.

Induction heating and melting applications often require a power source to convert 3-phase mains input power to single-phase output power at a higher and variable frequency. Amongst various power conversion schemes, solid-state power converters using the most modern devices provide the best power control techniques available for this application. In designing for this purpose, careful consideration must be given to the characteristics of the load, which presents a very low power factor and an impedance possibly varying widely as the heating cycle proceeds. From the variety of thyristor commutation techniques commonly employed in high-power inverters, series load commutation is particularly suited to high-frequency applications, as it has an intrinsically high turn-off time for the circuit thyristors (clearly essential at high operational frequencies) and much reduced switching losses. However, series commutation circuits are load sensitive, and therefore require careful design, especially with an induction heating load. Recent developments in power conversion techniques have led to the elimination of the d.c. link in a.c. to a.c. power conversion, enabling both high operational efficiencies and substantial savings in the initial cost of the device to be achieved. This new type of converter (called a cycloinverter) power and frequency control facilities. However, in a cycloinverter, since high-frequency switching is performed simultaneously with rectification, these control schemes are dependent on the operational frequency. The direct power conversion in a cycloinverter causes, unfortunately, distortion currents in the input lines and the output circuit, and it is the designer's task to minimise these undesirable components. The project aims to investigate the potential uses, both of the series inverter in its high-frequency form and of the cycloinverter, as power sources for induction heating. Design criteria are established for each circuit, with consideration given to turn-off time, efficiency, power factor, component ratings and predicted load variations. Computer simulations of the converters are employed to investigate the different voltage and current waveforms in the circuits, and to establish how the performance of each inverter may be optimised and these are verified by results obtained an experimental prototypes.

The research presented in this thesis is aimed at predicting the transient behaviour of attractive a.c. magnetic suspension systems. The non-linear characteristics of the suspension system were linearised by considering small displacement from an equilibrium position. Impedance measurements were taken as the physical basis and transfer functions were developed by either using Suppressed Carrier Modulation Theory (Modulation theory model) or by taking the RMS values of the a.c. quantities (simple model). Open loop transfer functions relating changes in control voltage to changes in magnet airgap were developed. These transfer functions were used in closed loop linear models to obtain stability limits and to predict the transient behaviour of: 1) Non-linear analogue simulation of an idealised onedimensional magnetic suspension system controlled by an ideal, variable amplitude, voltage source - Resistance inductance (RL) circuit. 2) An experimental apparatus (cantilever) where the airgap was measured using a Hall effect device and the control signal modulated using a linear multiplier. The resultant amplitude modulated signal was then amplified using a class AB power amplifier - both RL and LCR tuned circuit systems were investigated. 3) The same experimental apparatus but using thyristor bridge networks instead of the linear modulator and the class AB power amplifier (only the RL circuit was investigated) . The transient responses of these systems were investigated by comparing the closed loop step responses and frequency responses of the linear model and the experimental system. It was found that the gains natural frequencies and damping ratios obtained from the experimental system were mostly within 20% of that value predicted by the linear model. At the lower stability limit gains and natural frequencies obtained from experiment were within 10% of that predicted from the linear model. At the upper limit discrepancies between experiment and theory were observed. Open loop transfer functions were also developed for suspension systems affected by eddy currents. Repulsive as well as attractive suspension systems were analysed.

This thesis describes the research work performed on the positional stabilization of the mass-analysed ion beam, which is produced by the 500 keV, multi-ion, open terminal., electrostatic accelerator at the University of Surrey. This work was essentially an engineering project and was based on the available equipment and limited financial resources of a university department. The position of a mass-analysed ion beam, at either target of the accelerator, depends on the beam energy, and this, in turn, is main.ly a function of the accelerating voltage . At the start of the project, accelerating voltage instabilities in the range of 2.5 X 10-2 to 7.1 X 10-2, and with associated frequencies lying between 0.1 and 2 Hz and at close to 50 Hz, were established; these caused beam movements of between 48 mm and 136 mm at the target located at 5 m from the 45&deg; exit of the mass-analysing magnet. The range of frequencies, over which the accelerator voltage could be corrected, was initially limited to about 5 Hz by the electrostatic voltage supply. By modulating the lower end of a stack of high voltage capacitors, placed between the ion-source terminal and ground, this range was extended to at least 1 kHz. Both the generator and the capacitors were supplied with out-of-balance beam positional information, obtained from two vertical jaws intercepting the sides of the mass-analysed ion beam. The stabilising system, constructed by the author, gave accelerating voltage stabilities between 4 and 75 x 10-5 over a frequency range of d.c. to at least 1 kHz, and so reduced the beam movements at the target of the 45&deg; line to between about 0.076 mm and l.M-3 mm. The beam positional stability was thus improved by a factor of between 33.3 and 1775. The performance of the system was investigated with the aid of mathematical models describi.ng both the frequency response of the system and the beam deflection geometry associated with the analysing magnet. The engineering aspects of the system are also detailed.

This is a thesis about the control of magnetic gears. A decade ago (2001) the first modern prototype magnetic gear box was constructed using rare earth magnets (NdFeB). Magnetic gear boxes have some desirable properties not found in their mechanical gear box counterparts, these include: contact-less torque transmission, lubrication-free, reduced noise and vibration, and non-destructive torque overload capability. Hitherto, no detailed investigation or analysis has been conducted on the effects of using a magnetic gear box in place of a mechanical gear box. As will be demonstrated in this thesis, magnetic gears possess a number of undesirable properties which must be given due consideration when designing speed and position controllers. In particular, unlike mechanical gear boxes, magnetic gear boxes have extremely low torsional rigidity. Furthermore, the torque transfer characteristic is fundamentally nonlinear and magnetic gear boxes have the potential to 'slip'. On the one hand, 'slipping' is a great benefit as a non-destructive 'torque fuse'; but on the other, this represents a consequential loss of control. This thesis examines the control issues that arise through the use of a specially constructed magnetic coupling integrated into an experimental test rig. The development of a linearized mathematical model of the experimental magnetic coupling is used to derive optimized classical controllers for speed and position, demonstrating outstanding theoretical and experimental results. To compensate for the possibility of 'slip', a methodology is presented for the detection and recovery from what is defined as 'pole-slip' in a magnetic coupling. To avert 'pole-slip', a model predictive control (MPC) scheme is developed that prevents over-torque pole-slipping. Feedback linearization is considered for a nonlinear model of the magnetic coupling and nonlinear control laws and state transformations are derived to produce perfect linearization, for both speed and position control, over the entire operating range of the experimental magnetic coupling.

The switched reluctance machine (SRM) is the least expensive machine to produce yet is very reliable. The drive system for an SRM has to be designed so that there is integration between the machine and the converter-controller configuration. This thesis represents a study relating to power electronic converters for single-phase switched reluctance machine drives with emphasis on the cost of implication of the converters. The study of the converters results in the proposal of a new converter concept based on minimising the cost contribution of the dc link capacitor. By reducing the size of the capacitor, smaller and lighter SP-SRM drive is achieved. A detailed analysis and simulation of the proposed converter has been conducted and enabled a design guideline for the proposed converter to be laid out. A model of the SRM with hysteresis controller is developed. Consequently, components have to be chosen carefully to minimise voltage dip, component stress, and energy loss from the freewheeling resistor. To verify the operation of the new converter, tests were conducted on an experimental rig. The proposed converter has the ability to supply double the peak supply of voltage to the machine winding. In addition, the arrangement of the capacitors and diodes in the voltage multiplier means that positive and negative voltage rails can be supplied to the machine, which is very useful in an SP-SRM drive. The high voltage allows the fast magnetisation of the windings and enables the machine to achieve potentially high speed. Through the arrangements of the capacitors and diodes, the converter is able to handle recovered energy form the windings.