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Using Passive Elements and Control to Implement Single-To Three-Phase

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Using Passive Elements and Control to Implement Single-To Three-Phase LINEAR LIBRARY C01 0072 5350 1111111111111111 University of Cape Town Electrical Engineering Dept Power Electronics Group ' Using Passive Elements and Control to Implement Single- to Three-Phase Conversion University of Cape Town Prepared by: Stuart Marinus University of Cape Town South Africa Prepared for: Mr. M. Malengret Department of Electrical Engineering University of Cape Town Due Date: 30 Septem her 1999 Acknowledgments I wish to thank the following people for their invaluable contribution towards this project: Mr M. Malengret, my thesis supervisor, for helping me a great deal with the initial formulation of ideas and making it possible for me to complete my Master of Science Degree at the University of Cape Town. My parents, Shirley and Andy, for sacrificing so much for me. Their unconditional love and belief in me has made it possible for me to get this far. Caroline, for her support, understanding and thoughtfulness. Dan Archer, who gave up much of his time, knowledge and experience on a daily basis, especially in the design of the saturable-core reactors and switch-mode power supplies. Clive Granville for his advice and excellent technical assistance. The research group: Huey, Dave, Elvis, Dan, Sven and Kurt, and staff and students of the Power Machines Laboratory- Chris, Clive, Brian, Colin and Phineas who were always ready and willing to help when needed and who made my time spent at UCT very enjoyable. II Terms of Reference This thesis was commissioned and supervised by Mr Malengret of the Electrical Engineering Department at the University of Cape Town in partial fulfilment of the requirements for a MSc Degree in Electrical Engineering. Mr Malengret's requirements were: 1. To do a literature review of methods of single- to three-phase conversion already devised. 2. To purchase and then determine through testing the equivalent circuit parameters of a l 5kW, 3~, 380 V L-L, delta connected, four-pole, 50Hz, squirrel-cage induction motor. 3. To do a literature review into saturable-core reactors and to obtain a better understanding of their working. 4. To design a robust single- to three-phase converter, based on the theory researched, that utilises only passive components. The possibility of using the saturable-core reactor is suggested. The aim of this would be to demonstrate the viability of running the 15 kW, three-phase, induction motor from a single-phase supply. The motor is to run a centrifugal water pump. 5. To devise a stand alone controller that is capable of maintaining a steady, well balanced, three-phase supply to the motor, without requiring any external speed monitoring devices, such as tacho-generators, to be attached to the motor-pump system. 6. To test the converter through analysing its performance. I have attempted to address all of these issues, with the design of a converter that utilises the saturable-core reactor and passive components. III Declaration This thesis could not have been carried out without the help of many people, all of whom have been mentioned in the Acknowledgements. A breakdown of the work done, on various aspects of the thesis, is given below for clarity: • Single- to Three-Phase Converter Review. Much of this comes from preliminary work done by M. Malengret towards his Ph.D. with contributions by the author. • Load Balancing Review. Largely from literature review with additions by M. Malengret. • Calculating Motor Characteristics. Entirely authors own work, guided by literature example. • Calculating Compensating Elements. Entirely authors own work, using derived formulas. • Saturable-Core Reactor Review. Largely from literature review. • Design and Construction of Saturable Reactors. Actual construction was done by MLT Drives, specifications by author, initial calculations resulted from a joint venture between author and Dan Archer, testing and experimental determination of air-gap by author. • Converter Design. Initial ideals formulated in discussion between author and M. Malengret. All other work contributed by author. • Control Theory. Entirely authors own work. IV • Controller Simulation and Design and Construction of Controller Circuitry. Entirely authors own work. • Design and Construction of Switch-Mode Power Supplies. Controller circuit design and printed circuit board (PCB) provided by Dan Archer. PCB population and power supply construction done by author. • Experimental Construction, Testing and Results, Conclusions and Recommendations. Entirely authors own work. • Final working state of the project. The final state of the project was as follows: • The converter functions, producing balanced phase currents over nearly the full slip range of the motor (from start up to near no load speed.) • Recommendations were made, however, they could not be implemented owing to lack of time. Signature: --------- Date: ---------- V Synopsis The most costly element of countrywide electrification is that of distribution. If only one phase is transmitted instead of three, then the overall cost of distribution is considerably lessened. However, the problem is that the majority of large machines used in industrial and farming equipment, rely on three-phase power. This is typical of the situation as found in rural and developing areas where the cost of distributing three-phase power is prohibitively high but three-phase machines are in demand. As a result of this, there is a growing need to perform single- to three-phase conversion. For any balanced three-phase load, such as an induction motor, it is shown that a two­ element reactive phase converter can offer exact phase balance. The principal is illustrated below in Fig 0.1. ,. Ih~~~~Ph~~~.l()~g ......• I~ C 0 M p E N s A T 0 R Figure 0.1 Reactive phase-shift, two-element, converter. The two compensating elements, Za and Z~, are found to be inductive and capacitive, and therefore act only as energy storage units, wasting no power. With the use of the saturable reactor, it is found that this proposed converter topology is realisable, however two main problems are encountered. Firstly, producing a continuously variable source of power capacitance proves to be difficult, especially when working with high voltages. VI Secondly, many control strategies for the compensating elements involve instability. These problems are addressed and suggested solutions made. As a single- to three-phase conversion technique, this is attractive, not only because of its simplicity, but also from a harmonic perspective. If truly passive components are utilised, the total harmonic distortion of this converter is near zero. VII Table of Contents ACKNOWLEDGMENTS ................................................................................. II TERMS OF REFERENCE .............................................................................. Ill DECLARATION ............................................................................................. IV SYNOPSIS ..................................................................................................... VI LIST OF ILLUSTRATIONS .......................................................................... XIII 1 INTRODUCTION .................................................................................. 1 1.1 The Need for Single to Three-Phase Conversion ................................................................... 1 1.2 The Objectives of this Thesis ................................................................................................... 2 2 REVIEW OF SINGLE-PHASE TO THREE-PHASE CONVERTERS ... 3 2.1 Introduction .............................................................................................................................. 3 2.2 Reactance Compensator Converters ...................................................................................... 3 2.2.1 Capacitor Phase-Shift Method ......................................................................................... 3 2.2.2 Autotransformer type Converter. ...................................................................................... 4 2.2.3 Two-Element Converter. .................................................................................................. 5 2.3 The Rotary Converter ............................................................................................................. 8 2.3 .1 Introduction ...................................................................................................................... 8 2.3.2 Construction ..................................................................................................................... 9 2.3.3 Operation .......................................................................................................................... 9 2.3.4 Output Characteristics .................................................................................................... 10 2.3.5 Ratings ............................................................................................................................ 11 2.3.6 Cost, Size and Weight. ................................................................................................... 12 2.3.7 Advantages and Disadvantages ...................................................................................... 13 2.4 Semiconductor Converters .................................................................................................... 14 2.4.1 Diode Rectifier and Three-Phase Full Bridge Inverter. .................................................. 14 2.4.2 Converter with
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