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With the introduction of high speed semiconductor power devices and the increased application of adjustable speed drives (ASDs) for efficient speed control of ac motors, there has been a growing number of costly motor-drive related process failures. It has been found that the high dv/dt and high switching frequency have caused premature motor insulation failures due to motor terminal over-voltages (exacerbated by longer cable lengths). It is also acknowledged that high dv/dt and high frequency common-mode voltages generated by pulse-width modulated (PWM) inverters contribute significantly to electromagnetic interference (EMI) and may also cause damaging bearing and leakage currents. In response to these problems, a variety of mitigation techniques have been proposed in the past. However, the known solutions usually address these problems one at a time and some of the mitigation techniques are not highly effective. The major objective of this research is to search for solutions to these ASD application issues with an emphasis on solving all of the problems at the source. Therefore, theoretical analysis of all the above adverse effects are presented and the existing mitigation techniques are evaluated in this dissertation. It is found that common-mode voltage is the major cause of both bearing currents and the conducted EMI, thus the research is focused on new inverter topologies and control strategies in order to eliminate the common-mode voltage. To achieve the goal of common-mode voltage cancellation, a novel dual-bridge inverter (DBI) is proposed and studied. The DBI employs an additional inverter output stage to drive a standard three-phase dual-voltage induction motor and is controlled to generate balanced excitation of the motor resulting in a zero common-mode voltage. It is shown through experimental results that the motor bearing current is eliminated and the conducted EMI is significantly reduced. In addition to the DBI, multilevel inverter topologies have also been studied. It has been found in this research that with proper selections of the switching states, certain multilevel PWM inverters will not generate common-mode voltages. This new control method is verified in simulation by using both sine-triangle intersection PWM (SPWM) and voltage space-vector modulation (SVM). / Graduation date: 1999

Thesis (M.S.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains x, 100 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 82-84).

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Thesis (Ph. D.)--Oregon State University, 2004. / Printout. Includes bibliographical references (leaves 75-79). Also available on the World Wide Web.

Student Number : 9811492X - MSc dissertation - School of Electrical and Information Engineering - Faculty of Engineering and the Built Environment / This dissertation discusses the development of an ATP model of a network to aid measurement techniques in a harmonic evaluation. A theoretical back- ground discussion of various pieces of equipment and their signi#12;cance to har- monics is included. National Electricity Regulator (NRS 048) standards are discussed with refer- ence to performing a basic investigation and short comings. A test study was performed on the Brandspruit Mine in Secunda. ATP models are developed for equipment relevant to the test case, these in- clude AC{AC converters, AC{DC converters, three phase transformers and cables. Finally the measured test case is compared to simulation results and conclusions drawn.

The awareness of electric power quality has increased over the past decade as electronic equipment has become more susceptible to power disturbances. The most disruptive power disturbances are voltage sags and momentary interruptions and their effect on adjustable speed drives (ASDs) is studied in this thesis. Several solutions have been suggested to provide only voltage sag ride-through to ASDs, but most solutions focus on ASDs with passive rectifiers since they hold the largest share of the market. This thesis focuses on ASDs with active rectifiers, which is an emerging and growing market due to the advantages of four quadrant operation and reduced harmonics offered. A solution is presented which provides an ASD with an active rectifier with the capability to ride through the most common sags in order to reduce the frequency at which the ASD trips and thereby increase its reliability. In order to provide ASDs with the capability to ride through momentary interruptions, it is necessary to interface an energy storage system to the ASD. Flywheels, ultra-capacitors and batteries are evaluated for use in an energy storage system to provide voltage sag and momentary interruption ride-through and a detailed comparison of six systems based on these technologies is presented. The interface circuit between the energy storage system and ASD has a significant influence on the performance of the energy storage system and therefore interface circuits to ASDs with passive and active rectifiers are studied. The addition of an ultra-capacitor energy storage system to an ASD with an active rectifier in order to provide ride-through of deeper sags and momentary interruptions is studied and a fuzzy logic controller is designed to enhance system performance. Initially, no communication between the ASD and the ultra-capacitor system is assumed and the ultra-capacitor system can therefore be added as a retro-fit to an existing ASD. It is, however, foreseen that the market for ASDs with ride-through capability of voltage sags and momentary interruptions will grow and the concepts for an integrated design of an ASD and an energy storage system are presented. / Graduation date: 1999

The continuing desire of industry to further improve process efficiency, through tighter control and energy conservation, has prompted users to pay closer attention to Adjustable Speed Drives (ASDs). The conventional ASDs consist of induction or synchronous motors controlled by power electronic controllers through the adjustment of supply frequency and line voltage. The drawback of these conventional ASDs lies in the high cost of the power electronic controllers which have the same rating as that of the machine itself. The Brush less Doubly-Fed Machine (BDFM) ASD has proven, both analytically and experimentally, to provide a cost effective and a wide range of precise speed control. The experimental BDFM prototypes built to date were designed and constructed individually based on designers' experience with self-cascaded machines. The success with these prototypes has promoted the idea of standardizing the design procedure for all future BDFMs. This thesis offers a general design procedure for the BDFM, which can serve as a first step in standardizing the manufacturing process of this machine. The procedure is presented in the form of a demonstration, by applying it to the design of a 60-hp, 600 to 900 r/min, 460-volts BDFM pump drive to replace the currently utilized conventional 60-hp wound rotor induction motor ASD. An ideal design, which determines machine details such as physical dimensions, slot specifics and conductor details based on conservative magnetic and electric loading assumptions, is one form of the design procedure. The other form, the practical design, involves utilizing a specified physical dimensions and slot details to determine the associated conductors' details and to insure the compliance of machine loadings with up-to-date industrial standards. In both procedures, the design will be made to satisfy, if not to exceed, the existing conventional drive performance. / Graduation date: 1994