Vektör kontrollü yöntemi ile denetime yönelik bir asenkron makina modeli
An Induction machine model for vector control methods
- Tez No: 46433
- Danışmanlar: PROF.DR. ATİLLA BİR
- Tez Türü: Yüksek Lisans
- Konular: Bilgisayar Mühendisliği Bilimleri-Bilgisayar ve Kontrol, Computer Engineering and Computer Science and Control
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 79
Özet
ÖZET Bu çalışmanın ana amacı, vektör kontrolü yöntemi ile denetime yönelik bir asenkron makina' modeli oluşturmaktır. Bu amaçla ilk önce makinanın matematiksel modeli geliştirilmiş ve simülasyon ile dinamik analizi yapılmış tır. İkinci aşamada da elde edilen bu model üzerinde vektör kontrolü yöntemi temel ilke ve esaslarıyla incelenmiştir. - ix-
Özet (Çeviri)
SUMMARY AN INDUCTION MACHINE MODEL FOR VECTOR CONTROL METHODS This thesis aimes at obtaining an induction machi ne model which is adequate to vector control methods. Af ter modelling, the fundamentals of the vector control are reviewed. The three-phase induction motor, also known as the asynchronous motor, is the one most commonly used in dri ve systems. A schematic diagram of the motor, together with the windings, is shown in Fig. I. Stator S is a round cylinder inside which a concentrically placed rotor R rotates. The stator has a three-phase symmetric winding, and because the rotor construction is also symmetric, the motor shows magnetic symmetry. The stator iron and rotor core are both made from magnetic sheet material. Depend ing on the rotor design, there are slip-ring (ring) or wound rotor motors, and squirrel-cage (or cage) motors. Fig. I. Three-phase induction motor design? A, B, C stator windings, a, b, c rotor windings, ft angular speed of the rotor tyM main flux linkage (instantaneous positxon) m The wound rotor induction motor has a three-phase symmetric winding (insulated from the core) in the rotor R. The winding leads are connected with slip rings SR (Fig. II. a) along which (when the rotor is in motion) slide brushes Br mounted in brush-holders in the stator. Thus, alongside the stator winding terminals, those con nected with the brushes constitute an additional three- xphase input which can be used for control purposes. It should be pointed out that, in the general case, the ro tor winding can be supplied with an ac voltave whose pa rameters (Ur,fr) differ from those of the stator winding (Us,fs). «,.', Fig. II. Induction motor types? a) slip-ring (wound motor, b) cage winding of the machine with short-circuited rotor. The squirrel-cage rotor induction motor has a cage winding in the rotor (Fig. II. b). Such a winding consists of bars B placed in rotor grooves with their terminals short-circuited by means of rings RS. The bars and short ing rings are made of copper, brass or aluminium and, as a rule, they are not insulated from the core. This makes it possible to fabricate windings by casting, which makes for very simple, inexpensive and reliable motor designs. It is thanks to these advantages that cage motors have come to be so widespread in industrial drive system app lications. However, its application was limited by the complexity of its control which arises because of the variable-frequency supply, ac signals processing and complex dynamics of the machine where only stator termi nals are available for control. The recent developments in power electronics have solved the variable-frequency supply problem by adequate frequency converter s. On the other hand, the implementa tion of microprocessor in the digital control circuits has introduced a wide scope of possibilities to overcome the complex dynamics of the machine and ac signal proces sing. Many models have been proposed to analyses and de ve lope control algarithms for the induction machine. In general, a model must cover the transient and steady states of the machine. Besides that, it must also be a simple model, as the aim is to obtain an applicable cont rol method. This is the main reason why a model adequate to vector control is considered in this work. The vector control methods are all based on“Field Orientation”principle first introduced by Felix - xi -Blanchke [l], [2]. The aim of field orientation is to cre ate dc machine-like decoupling of flux and torque produc ing currents on rotating field machines. This allowes to control the machine flux and output torque directly and seperately, therefore all the control facilities known for dc machines can be easily applied to the rotating field machines. According to the field orientation theory [l], [2j, if the same vector position of flux and armature current, which are naturally kept perpendicular in dc machines by means of collector-brush mechanicsm, is created on rotat ing field machine at each instant of time through an ex ternal control logic, the torque and flux level of the rotating field machine can be controlled in the same way of a dc machine. As it is clearly seen from the previous fact, keep ing the rotor flux and rotor current perpendicular at each instant of time requires same meaning of current control based on the rotating flux vector and orienting the stator current to the certain direction at each ins tant of time. So the stator current is controlled as a vector. This is why the method is called as vector cont rol. In order to achive field orientation, both rotat ing to stationary and two phase to three phase transfor mation relation are used. Rotating reference frame is coincided with rotating flux vector, then the specified rotating frame components of stator current, determining the flux level and output torque, are converted to the stationary frame and three phase current references for inverter. If this can be ensured satisfactorily, the sta tor current of induction machine can be decoupled so that the flux producing component is onphase with flux vector and torque producing component is perpendicular to it even in the transient state. Obviously, such on orienta tion gives the best torque response, i.e., no fluctuation occurs during the torque changes. By using field orientation principle, the torque control facility of a standard induction machine becomes the same of the high frequency chopper cotrolled sepera tely excited dc machine. While the basic approach remains the same, the field orientation and vector control met hods can be applied other rotating field machines such as synchronous and reluctance machines. Field orientation is based on the knowledge of the rotor flux vector. This makes the flux sensing the key point. During the implementation the special atten tion should be taken for this sensing function. The in correct sensing the flux means that all the other units such as micro computer based controller and inverter are - xli -unable to implement field orientation. In field orrented drives, the sensing rotor flux as a vector (both modulus and angle) is realized two different techniques. The first method is developed by F. Blashke [2]. In this method, flux vector is directly sensed by using Hall probes or test coils mounted on the stator winding of machines. Since the only air gap flux vector is attainable, the rotor flux is then computed using flux linkage equations relating motor and airgap fluxes. Because of the direct sensing algorithm, sensi- vity on parameter variations is at negligible level. How ever there are several limitations to apply direct sens ing method in practical using. The most important one is the neccesity for spesific sensors that must be located on stator windings. Such devices are not being contained in standard machines, therefore each motor has to be con sidered sperately. Beside this, another difficulty is to obtain harmonic- free, easy proccesable signals from the sensors. As it is well known airgap, flux vector is not pure sinusoid but actually contains considerably harmo nics. Using harmonic filters may couse frequency depen dent phase shifts. If the Hall probes are used the tempe rature dependent output voltage level of this elements becomes a problem which is hard to presisely compansate 1. Another alternative, the flux sensing coils also pre sents some pratical problems originatied by the airgap flux fluctuations. In test coil mounting method, the in tegration of coil voltages may have some errors, especi ally when the motor speed is low. However, if this practical limitations can be sol ved the direct flux sensing and field orientation will give the best performance obtained so far. The second medhod suggested by K. Hasse [l] is ba sed on calculating the rotor flux vector from dynamic machine equations for field orientation constraints. This technique is only valid when the parameter changes are succesfully reflected to the computation circuits. As a comparison, even the direct method has the best accurarcy, under the practical considerations the directly sensing rotor flux vector and direct field orien tation is impratical for the industrial using. In practi cal applications the indirect field orientation and flux sensing is widely used with additional parameter estima tion equipment. Especially with the help of today's high speed, high performance single chip microcontrollers, even the most complex estimation calculations can be per formed on the background of main control program. So in thesis, indirect field orientation is used. - xiii -Beside, the sensing flux correctly, supplying three phase currents according to references has particu lar importance. Any kind of inverter circuit permitting instant current control can be used in a field oriented drive system. The output power and switching frequency determine the compatibility of an inverter circuit to the drive system. The thesis consists of six chapters. In previous parts of the study, modelling of induction machine is presented and the background of two axis theory is revi sed, then the dynamic analysis of a uncontrolled machine is performed. At the fifth and following chapters the vector control methods of induction machines are given. The general contents of each chapter is given in the fol lowing. The first chapter deals with the general approach of the thesis and the historical developments of vector control methods. In second chapter, the general dynamic equations of symmetrical induction machine are developed by using the fundamental laws of electromechanical energy conver- tion theory. The obtained model, in terms of phase cur rents and flux linkages presents highly nonlinearity. This makes the analysis more difficult and time consum ing even the computer is used. In order to obtain more flexible and simple model the equations are transformed by using symmetrical Park's transformation. The new model is obtained in d-q axis, rotating an arbitrary angular velocity. But in simulation, this d-q axis rotates synch ronously with stator voltage frequency. After Park trans formation, these equations are rewritten in terms of per unitized quantities. In this model, d-q axis stator cur rents-rotor fluxes and rotor speed are chosen as indepen dent state variables and d-q axis stator voltages and load torque are selected as input variables. This choice for state variables causes significant flexibility in the model. In fourth chapter, the types of inverter circuits are discussed. In fifth chapter, the vector control and field orientation are introduced. It is shown that, the reali zation of fast and linear torque control depends on some conditions which have to be satisfied at each instant of time. By using this principles, the conventional steady state equivalent circuit is analized with respect to tor que control. In the last chapter, the results obtained through out the thesis are given and final conclutions are made. - xiv -
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