No description available.

No description available.

No description available.

No description available.

The plasma opening switch (POS) is used in pulsed power systems where a fast opening and very high current switch is required. Plasma is injected into the switch, which carries a large conduction current before it opens in a process that lasts for a few nanoseconds and transfers the current to a parallel-connected load The conduction and opening times of the switch are dependent on the plasma parameters such as distribution, speed, temperature and species, which are all determined by the plasma source. This thesis begins with a description of the POS, with its conduction and opening mechanisms and the techniques of plasma generation all being considered, before it concentrates on the simple and inexpensive carbon gun. Plasma is normally produced by a pulsed discharge that evolves plasma from the evaporation and ionisation of a carbon based insulator. The first prototype carbon gun discussed in the thesis uses a classical coaxial arrangement that successfully produces dense, fast and hot plasma, although this is only capable of filling a small region with plasma. A number of plasma diagnostic techniques are described, before details are provided of the electrical probes that were used to characterise the plasma In a large POS a well-distributed plasma is obtained by combining a large number of guns in a complex and large system. This restncts the compactness of the POS resulting in a problem for any future commercial applications. A succession of developments to the prototype gun has led to a novel ring-shaped version that produces a much improved distribution of plasma, without the need for additional guns. In this, a pulsed discharge is initiated at a single point and the self-generated magnetic field forces the discharge to spread and to travel around the gun, whilst continuously ejecting plasma into the POS. The ideas and theories that explain how a discharge can be forced to move are described, together with details of the prototype designs. Results are given to confirm the operation of the gun, using high speed photography and electrical probes.

The thesis is concerned with microprocessor-based control of a d.c. drive system using armature voltage control technique with a separately-excited, chopper-fed d.c. motor. The chopper circuit has a full-bridge configuration employing gate-turn-off (GTO) thyristors, and. is capable of four-quadrant operation, making the drive both reversible and regenerative. The control of motor speed is achieved by a completely digital technique, which does not require any analog-to-digital or digital-to-analog converters. A microcomputer, specially designed and built, generates the triggering signals for the GTO's by using the unipolar switching strategy, which is accomplished totally by software using look-up tables.

Brushless dc motor drives have become increasingly popular, following recent developments in rare-earth permanent-magnet materials and the semiconductor devices used to control the stator input power and to sense the rotor position. They are now frequently used in applications such as flight control systems and robot actuators, and for drives which require high reliability, long life, little maintenance and a high torque-to-weight ratio. In many motor drives the presence of torque and speed ripples, especially at low speed, is extremely undesirable. The mathematical model developed in this thesis was used to investigate their occurrence in a typical brushless dc drive system, with the objective of establishing factors which effect their magnitude and ways by which they may be reduced. The model is based on the numerical solution of the differential equations for the system, with those for the motor being formulated in the phase reference frame. Tensor methods are used to account for both the varying topology and the discontinuous operation of the motor arising from changes in the conduction pattern of the inverter supply switches. The thesis describes the design, construction and testing of an experimental voltage source PWM inverter, using MOSFET switching devices, to drive a 1.3 kW 3-phase brushless dc motor. A practical circuit is described which implements current profiling to minimize torque ripple, and the optimum phase current waveforms are established. The effect of changes in the firing angle of the inverter switches on the torque ripple are also examined. Throughout the thesis, theoretical predictions are verified by comparison with experimental results.

The applications of robust squirrel-cage induction motors in variable speed inverter drive systems have increased considerably due to the availability of easily controlled semiconductor switching devices. One problem encountered in inverter drives is the non-sinusoidal nature of the supply voltage, which results in increased motor losses and harmful torque pulsations producing undesirable speed oscillations. The latter effects are negligible at high frequency operation, due to the damping effect of the rotor and load inertia. However, torque pulsations and speed ripple may be appreciable at low frequency, wore they may result in abnormal wear of gear-teeth or torsional shaft failure. Hence, in applications where constant or precise speed control is important, eg; machine tool, antenna positioning, traction drives etc., it is essential to establish a method for determining the magnitudes of these torque pulsations and speed ripple, as a first stage in minimizing or eliminating them. When a voltage source inverter is used in such applications, pulse width modulation (PWM) techniques are usually employed, whereby the quasi square waveshape is modulated so as to minimize or eliminate the low order harmonic voltage components and thereby reduce the torque pulsations. Recent investigations have shown that total elimination of low order components does not produce optimal efficiency or torque pulsations and speed ripple. minimization. This thesis describes new PWM strategies which does not rely on complete elimination of low order harmonics, but on controlling the magnitude and phase of these components to achieve a smooth rotor motion. Initially, a mathematical model for the inverter/induction motor drive was developed, based on numerical integration of the system differential equations. The changing topology of the inverter bridge was simulated using tensor techniques. Then an analytical method, based on harmonic equivalent circuit analysis was proposed for calculating the induction motor pulsating torque components under steady-state operating conditions, in terms of stator and rotor current harmonics. The accuracy of this method was verified by comparing its results with those obtained from the mathematical model developed earlier. This provided an extremely rapid, numerically stable and efficient means for evaluating harmonic current and torque components with balanced non-sinusoidal applied voltages. This method was then used to formulate the torque performance function necessary to determine the new optimal PWM switching strategies. Throughout the work, the predicted performance was extensively validated and supported by practical results obtained from an experimental rig specifically designed to drive the machine under different PWM techniques.

A single doubly-fed induction machine (SDFM) is a wound rotor induction machine with the stator connected to a supply network and the rotor fed by a bi-directional converter. A cascaded doubly-fed induction machine (CDFM) is a. connection of two wound rotor induction machines with the rotors connected electrically and mechanically thus avoiding brushes. One stator is connected to the supply network and the other is fed by a bidirectional converter. Both schemes, the SDFM and the CDFM, have in common that the VA-rating of the power converter is reduced compared to a singly-fed system. This thesis presents investigations of the field oriented control for the SDFM and the CDFM. - After reviewing and categorising doubly-fed machines a thorough steady state analysis and stator flux oriented control treatment of the SDFM is presented. Although the steady state analysis and the field oriented control of a SDFM is well established it is necessary that this is included as foundation for the CDFM control behaviour and for the sensorless control investigations. Steady state analysis of the CDFM highlights similarities to the SDFM. Two different field oriented control schemes are applied to the CDFM. A previously developed combined flux oriented scheme is modified to be applicable to a CDFM consisting of any machine combination. Furthermore, the scheme is simplified by removing a mathematical control extension in the q-axis, which has a stabilising effect on the control performance. Justified by steady state analysis the stator flux oriented control structure initially developed for the SDFM is applied to the CDFM. Two variations of a position sensorless scheme taking advantage of the proportionalitics between stator and rotor quantities are applied to the SDFM. Differentiating the estimated position angle allows the schemes to be extended for speed control purposes. The performance of the scnsorless field oriented control methods are also investigated on the CDFM. Harmonic analysis of the SDFM / CDFM systeme stablishes harmonic sources and harmonic current propagation through the system. A theoretical harmonic current prediction process incorporating simulation and steady state modeling delivers good results. All theoretical investigations are confirmed by experimental results. The experimental realtime controlled drive system consists of two 2.25 kW wound rotor induction machines, a bidirectional IGBT converter and the control hardware comprises two 8OC167 microcontrollers.

The work presented in this thesis is concerned with the analysis, modelling, simulation and control of a surface mounted permanent magnet motor supply by a voltage controlled Pulse-Width Modulation (PWM) inverter. In Chapter 1 an overall description of the design and construction of individual components of the brushless dc drive system is presented along with a review of the general concept of the drive system. This type of machine is compared with other types of machine and the potential advantages of this new concept, both technical and economic, outlined. In Chapter 2 the operation and the control aspects of the brushless dc motor are described, with particular emphasis placed on the basic requirements for the operation, torque production, performance characteristic and control. The high-speed torque control methods are also described and their merits are reviewed. In addition the effects of different parameters of machine design on the torque-speed characteristics are discussed. Chapter 3 elaborates on the analysis and simulation work by presenting a comprehensive analysis which aims to show that direct three-phase representation can be used as an effective tool for performance assessement of brushless dc drive systems operating over a wide speed range. In Chapter 4 the performance of the brushless dc motor supplied by a PWM inverter with a view to improving the high-speed torque performance is investigated. Simulation and analysis of the brushless dc motor is presented in which the actual parameters of the experimental machine are used. The aim of the analysis is to simulate a brushless d. c. drive system operating in closed-loop control modes, which use high speed torque control techniques in conjunction with a PWM control technique. A detailed analytical model which makes possible the use of machine theory for representing the performance of the brushless dc motor is presented in Chapter 5. The method utilizes the phasor diagram, where machine performance in terms of the main control variables such as voltage and phase advance angle is demonstrated. Chapter 5 also presents an analytical expression for the phase-advance angle which yields maximum torque at a given motor speed. An analytical study concerning the optimum phase advance is developed in Chapter 6. In this work two analytical approaches to the problem of obtaining an optimum phase advance angle are presented. Chapter 6 presents a detailed analysis of the shape of the current and back-emf waveforms in a trapezoidal brushless dc motor drive and their effects on the torque/speed performance. Chapter 7 presents the implementation of a microprocessor based system, which can set the phase advance angle to its optimum value at any motor speed. This implementation is done in real time on the protortype drive using a TMS320C30 digital signal processor. Features of the method proposed in this thesis include the estimation algorithms for predicting the time advance. Experimental results on a drive system demonstrate the satisfactory performance of both the hardware and software of the control scheme.