Lappeenraman teknillinen korkeakoulu Lappeenranta University of Technology 011 i Pyrh on en ANALYSIS AND CONTROL OF EXCITATION, FIELD WEAHENING AND STABILITY IN DIRECT TORQUE CONTROLLED ELECTRICALLY EXCITED SYNCHRONOUS MOTOR DRIVES Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in the Auditorium in the Students’ Union Building at Lappeenranta University of Technology, Lappeenranta, Finland on the I Ithof December, 1998, at noon Tieteellisia julkaisuja Research papers 74 ISBN 95 1-764-274-1 ISSN 0356-8210 Lappeenrannan teknillinen korkeakoulu Monistamo 1998 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. 1 ABSTRACT Lappeenranta University of Technology Research papers 74 Olli Pyrhonen Analysis and Control of Excitation, Field Weakening and Stability in Direct Torque Controlled Electrically Excited Synchronous Motor Drives Lappeenranta 1998 ISBN 95 1-764-274-1 UDK 621.313.32 : 621.3.077 Key words: synchronous machines, synchronous motor drives, direct torque control, excitation, field weakening, stability control Direct torque control (DTC) is a new control method for rotating field electrical machines. DTC controls directly the motor stator flux linkage with the stator voltage, and no stator current controllers are used. With the DTC method very good torque dynamics can be achieved. Until now, DTC has been applied to asynchronous motor drives. The purpose of this work is to analyse the applicability of DTC to electrically excited synchronous motor drives. Compared with asynchronous motor drives, electrically excited synchronous motor drives require an additional control for the rotor field current. The field current control is called excitation control in this study. The dependence of the static and dynamic performance of DTC synchronous motor drives on the excitation control has been analysed and a straightforward excitation control method has been developed and tested. In the field weakening range the stator flux linkage modulus must be reduced in order to keep the electro motive force of the synchronous motor smaller than the stator voltage and in order to maintain a sufficient voltage reserve. The dynamic performance of the DTC synchronous motor drive depends on the stator flux linkage modulus. Another important factor for the dynamic performance in the field weakening range is the excitation control. The field weakening analysis considers both dependencies. A modified excitation control method, which maximises the dynamic performance in the field weakening range, has been developed. In synchronous motor drives the load angle must be kept in a stabile working area in order to avoid loss of synchronism. The traditional vector control methods allow to adjust the load angle of the synchronous motor directly by the stator current control. In the DTC synchronous motor drive the load angle is not a directly controllable variable, but it is formed freely according to the motor’s electromagnetic state and load. The load angle can be limited indirectly by limiting the torque reference. This method is however parameter sensitive and requires a safety margin between the theoretical torque maximum and the actual torque limit. The DTC modulation principle allows however a direct load angle adjustment without any current control. In this work a direct load angle control method has been developed. The method keeps the drive stabile and allows the maximal utilisation of the drive without a safety margin in the torque limitation. 11 ACKNOWLEDGEMENTS I took special interest in electrical drives and in the DTC method already during the years 1990 and 1993, when I was working at the research department in ABB Industry Oy. These years of working experience turned out to be very valuable for all later research work, because during that time the DTC method has been intensively developed for asynchronous motor drives by the company’s research team. Additionally, my working in the industry branch gave me a good general survey of the large application area of electrical drives. The research work of this study has been camed out during the years 1995 and 1998 in the Laboratory of Electrical Engineering at Lappeenranta University of Technology, where I have worked as a laboratory manager and senior researcher. The research work introduced in this thesis is part of a larger research project, in which the different aspects of the DTC method for synchronous motor drives are studied. I would lie to thank professor Jarmo Partanen, the head of the Institute of Electrical Engineering and the supervisor of this work. It was his activity and enthusiasm, which made this research project possible in the first place. His has encouraged and helped throughout the work. Special thanks are due to professor Juha Pyrhonen, my brother, whose knowledge of electrical machines has been extremely valuable also for this project. His contribution to the revision of the manuscript has been of immense importance. I wish to express my sincere thanks to the pre-examiners of this work, professor Mats Al&la, Lund Institute of Technology, Sweden and docent Janne Vglsniinen, Tellabs Oy, for their valuable comments and corrections. The “red pen boutique” of professor Pekka Eskelinen has given an important contribution to the scientific style of the manuscript. He revised the text not only once, but twice. I am most gratehl to him for his valuable help as well as for his encouragement. I also would like to thank Mrs Julia Parkkila for her contribution to improve the English language of the manuscript. The good working atmosphere in the synchronous motor drive research team has been of great importance to me. I would like to thank the members of the team as well as the other staff in the Laboratory of Electrical Engineering. I wish also to express my thanks to my former boss, professor Martti Harmoinen, and to my former colleagues at the research team in ABB Industry Oy, for giving me an interesting career start. I am obliged to the NO Foundation, the Lauri ja Lahja Hotisen Siizitio and the Sahkoinsinooriliitto for the financial support. I also thank ABB Industry Oy for helping our research team to construct the test equipments for the laboratory tests. My parents, Raili and Jorma, have given me all the best for a good start in life, they have encouraged and supported me also during the recent years. They deserve my special thanks. Most of all, I am indebted to my wife Kaisa for her love and patience, and to my children, Aino, Lauri and Eeva, for giving me strength and motivation for this work. Lappeenranta, December 1998 OIli Pyrhonen ... u1 CONTENTS ABSTRACT ........_._... ........... ................................................ ...................... 1 ACKNOWLEDGEMENTS .. .. .. .. , .. .. .. ................................................... 11... CONTENTS .. .. ... .. .. .. ... .. ... .. .. .. .... .. .............................................. 111 NOMENCLATURE .. .. .. ........................................ ............................. V 1 INTRODUCTION ............................................... 1 1.1 History of speed controlled AC motor drives. ................. 1 1.2 Functional principle of DTC .. .. .. .. .. .. .. .. ........................................... 2 1.3 DTC for electrically excited synchronous mot 8 1.4 Outline of the thesis..._. .. .. .. .. .. .. .. .. .. .. .. .. ......................................... 12 2 ANALYSIS OF STATIC AND DYNAMIC PERFORMANCE ........... 13 2.1 Maximal static torque. ............................................. ......................... 13 2.2 Maximal dynamic torq 17 2.2.1 Stator current, stator flux linkage and fie1 18 2.2.2 Maximal dynamic torque in the theoretic 21 2.2.3 Transient analysis with derived operator inductances ......................... 24 3 EXCITATION CONTROL OF DTC SYNCHRONOUS MOTOR DRIVES ... ___.___. __ 35 3.1 Reactive power compensation .................................... ............................... 35 3.2 Effect of magnetic saturation on the calculation of the excitation curve.. .. .. .. .. .. .. 36 3.3 Combined open loop and feedback control .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 39 3.4 Reaction excitation control in DTC .... .. .. .. .. .. .. .. .. .. .. .. .. ..................... 42 4 FIELD WEAKENING CONTROL OF DTC SYNCHRONOUS MOTOR DRIVES ........... 46 4.1 Voltage Reserve in field weakening ................................................... ................. 47 4.2 Relation between voltage reserve and excitation voltage ................................... 50 4.3 DTC modulation in field weakening range .................................................... 53 4.4 DTC stability control in field weakening range .. .. .. .. .. .. .. .. .. ....................... 61 4.4.1 Indirect load angle control ................................. ................................ 61 4.4.2 Direct load angle control ................ .................................................. 64 5 SIMULATION AND TEST RESULTS ................................................................. 67 5.1 Description of the simulation method ...................................... .... ...................... 67 5.2 Simulation results of excitation control .. .. ..................................................... 69 5.2.1 Combined excitation control in the n eed range .............. .............. 69 5.2.2 Excitation control in the field