Sincap kafesli asenkron makinenin rotor alan yönlendirmeli kontrolü
Rotor field-orientation control of a squirrel cage induction machine
- Tez No: 46532
- Danışmanlar: PROF.DR. M. EMİN TACER
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 125
Özet
ÖZET Son yıllarda gelişen yarıiletken, mikroelektronik, mikroişlemci ve kontrol teknolojisindeki gelişmeler sonucunda, küçük güçteki asenkron makineler de, konum ve hız kontrolü amaçlanan uygulamalarda kullanılmaya başlanmıştır. Böylece, kontrol açısından bakıldığında ideal bir davranış gösteren d.a. makinelerine benzer bir hız ve moment denetimi asenkron motorlar için de söz konusu olmuştur. Asenkron makinelerin sincap kafesli veya bilezikli olmasına göre geliştirilen en uygun kontrol yöntemi rotor alan yönlendirme yöntemidir. Bu yöntem ile, stator frekansı devamlı olarak rotor halkalanma akısının frekansına ayarlanarak istenen biçimde denetim ile asenkron makinenin karasız durumlara girmesi önlenmiştir. Yöntemin yetersiz kaldığı durum ise, her ne kadar üzerinde çalışmalar yoğun bir şekilde devam ediyorsa da sıcaklık ve doyma ile değişen makine parametrelerinin fiziksel davranışlarının matematiksel modele uyarlanmasında zorluklar ile karşılaşılması d ir. Yöntem bir çok hesaplama gerekmektedir ve hızlı çalışan bir mikroişlemciye ya da DSP ye gerek duyulmaktadır. Tez çalışmasında, yukarıda belirtilen hususlar göz önüne alınarak sincap kafesli asenkron makinenin rotor alan yönlendirmeli denetiminin sayısal benzetişimi gerçekleştirilmiştir. XII
Özet (Çeviri)
SUMMARY ROTOR FIELD-OR1ENTATION CONTROL OF A SGUIRREL-CAGE INDUCTION MACHINE The main goal of this study is to simulate rotor field-orientation for a squirrel-cage inductisn machine. Field-orientation control tecnique is to control an aç machine to obtain the performance characteristics similar to a de machine, providing good efficiency and excellent dynamic performance. in rotor field- orientation, it is essential to have decoupled and independently torque and flux of the induction machine. The field-orientation requires knovvledge of the instantaneous position and magnitude of the rotor flux to achieve decoupling. in the case of induction machines rotor flux-oriented control is usually employed. With rotor flux-oriented control there are two main implementations to obtain the modulus and space angle of the rotor flux-Iinkage space phasor. When direct rotor flux-oriented control is used, these quantities are directly measured ör they are calculated from a flux model. Hovvever, indirect rotor flux-oriented control, the modulus and space angle of the rotor flıoc-linkage space phasor are obtained by utilizing the monitored stator currents and the rotor speed. in indirect field- orientation control of an induction motor drive system, the position of the rotor flux is calculated rather than measured. Since the calculated position and magnitude of the rotor flux are based on knovvledge of the rotor time constant, the accurate information of the rotor time constant is an essential requirement [1,2,3]. in this study the rotor resistance, vvhich varies with temperature, is considered as the most important parameter to effect the performance of the drive system. in high performance electromechanical system requiring four-quadrant operation with a fast torgue response and good performance about zero speed the separately excited de machine has long been used. This is due to their excellent operational properties and control charasteristics; the only essential disadvantage is the mechanical commutator vvhich restricts the power and speed of the machine, increases inertia and axial length and requires periodical maintenance. By using aç machines, fed by variable frequency static power converters, the commutator is eliminated, hovvever at considerable cost and complexity. This is öne of the reasons why rotor field-orientation controlled induction machines are only now becoming an economical alternative to de drives [4]. in 1887 Nikola Tesla (1856-1943) buitt the first induction machine in America, in which a 'two-phase' alternating current and some fbced electromagnets, instead of a permanent magnet, were used to generate a rotating field [5]. in the late of the 19th century, induction machine is the most widely used electrical drive motor and its invention has given a strong impetus tovvards thetransition from de to aç in the field of generation, transmission and distribution of electrical energy. Its main advantage is the elimination of ali sliding electrical contacts, resulting in an exceedingly simple and rugged construction. Induction machines are made in a variety of designs with ratings of a few watts to several megavvatts. Unfortunately the speed of supply-fed induction motors cannot be continuously varied without additional equipment ör vvtthout incurring large povver losses. Even though the problems of efficiently controlling the speed of induction motors have been investigated for decades, ali solutions realizable until a few ego were too complex and costly. it is only due to the progress of semiconductor technology in recent years that static frequency converters can now be built at acceptable price so that the induction machine also has a future in variable speed drives [4,5]. The theory of the induction motor for dynamic conditions is somevvhat involved because of the rotating fields, the spatial relationships of which depend on speed and load; it will only be treated here in simplified form. On the other hand, the equivalent circuit usually derived for steady state operation with sinusoidal voltages proves to be inadequate when dealing with transients ör when the motor is supplied from a static converter. The steady state condition will be treated as a special case of the more general solution. The mathematical model to be used is tailored to the needs of cotrolled drives. Traditionally, induction machines have been used in open-loop constant speed applications vvhere the steady state characteristics are of principal importance. in closed-loop adjustable speed drive applications, a consideration of static as well as dynamic behavior is important. A closed-loop system has the advantages that the output tracks the commanded input, the system response is less sensitive to parameter variation effect, and unvvanted noise and disturbance effects are attenuated. However, the disadvantages include stability problems and the complex design criteria needed to make the system stable, the loss of system gain, and the requirement for precision feedback signals [3,4,5]. The induction motor drive system is basically a multivariable system, and therefore in principle the state variable control theory should be applicable. The dynamic model of the induction motor is nonlinear because of the speed of rotor in the voltage equtations of the stator and rotor, in addition, the parameters of the machine may vary with saturation, temperature and skin effect, adding further nonlinearity to the system. The system is also discrete time because of the sampling nature the converters. If a microcomputer ör other digital circuits are used in the control system, then additional sampling characteristics must be added. The discrete time effect of the converters and controllers can, of course, be neglected if the machine response is sluggish, vvhich is normally the case. Önce, the control structure and the parameter of the controllers are determined on the simulation, the prototype system is designed and laboratory tested with further iteration of the controller parameters. On the other hand, the implementation of the vector controlled induction machine rotor time constant using a powerfull DSP has ben used. The cost of DSP becomes more and more attractive for common applications, the usage of direct control will penetrate low povver equipments in the near future. (n this way, research investigations will be concentrated in the xivintegration of control algorithms for induction machine drives with technology simplification. [6]. A simple and popular open-Ioop volts/herts speed control method for an induction motor system consists of a phase-controlled rectifier w"rth three-phase aç supply, LC filter, and voltage-fed ör current-fed inverter. This is a scaler control which relates to the magnitude control of a variable only, and the command and feedback signals are de quantities which are proportional to the respective variables. This is in contrast to vector control, vvhere both magnitude and phase of a vector variable are controlled, and this is described in the thesis. With open-Ioop voltage control, the aç line voltage fluctuation and impedance drop will cause fluctuation of the air gap flux. This fluctuation can be prevented by providing closed-loop voltage control of the rectifier. If the machine speed does not fail belovv 10%, the stator drop compensation can be ignored. This control scheme has disadvantage, which torque sensttivity with slip ör stator current will vary. If the correct volts/herts ratio is not maintained, the flux may be weak ör may saturate. The stator circuit parameters may also vary due to temperature and saturation, causing drift in the air gap flux. As a result, the machine's maximum torque capability will decrease and transient response vvill deteriorate. The machine terminal voltages and currents can be sensed and torque and flux can be estimated by partial observer. A simple method of flux measurement is the mounting of Hail effect sensors in the machine air gap. A problem here is that the Hail sensor outputs drift with temperature vvhich is difficult to compensate. Alternatively, flux coils may be mounted in the air gap and the correspondingly induced voltages may be integrated to get the flux information. The mounting of external devices, such as Hail sensors ör flux coils, in the air gap is not favored by machine designers[7],. in the scalar control of voltage-fed ör current-fed inverter drives, the voltage ör current and frequency are basic control variables of the induction motor, in a voltage-fed drive, for example, both the torque and air gap flux are functions of voltage and frequency. This coupling effect is responsible for the sluggish response of the induction motor. If the torque is increased by incrementing the frequency (i.e. the slip), the flux tends to decrease. Unlike synchronous motors, rotor flux is not fixed relative to the rotor posrtion and rotor time constant depends on temperature and saturation of the rotor material. Because of this rotor time constant, rotor acts as a lovvpass filter. The delay of the rotor time constant causes the rotor flux to lag behind than the stator field. The difference in rotational speed is referred to as slip. Slip is measured as an angular velocity expressed as a frequency and typically around 3 Hz. This is important in understanding the liminations of the aç induction motor vvithout vector control [7]. There are essentially two general methods of vector control. Öne, called the direct method, was developed by F. Blaschke, and the other, known as the indirect method, was developed by K. Hasse [4,7]. Öne of the prime task in vector control is to decouple the torque and flux based currents from modulus of stator current and keep them in quadrature to öne another at ali times in reference frame that is related to the rotor coordinates. That requires sensing the three phase stator currents. The advantages of vector control include; XV*constant and smooth torque from zero and ali över the complete range of motor speed, *constant horsepovver available above base speed, *increased velocity precision even under varying load conditions, *high motor efficiency due to magnetization current control related to rpm, *fast transient response due to decoupling control, *the conventional stability problem of induction motor, that is by crossing the breakdovvn torque point, does not in vector control, *available four-quadrant operation [7]. Therefore, the vector controlled induction motor drives can be used for high performance applications, servo drives, coilers, hoists, machine tool spindle drives, vvhere traditionally, de machines have been used. But, there are also disadvantages vector controlled drives; *complex control algorithm vvhich limits band width, *high rotor losses can present cooling problems at low speeds. Four internal quantities plus the rotor position of a rotating electrical machine suffice in order to describe its dynamic behaviour. Starting from this fact, first the space-phasor concept is introduced as an excellent analytical tool, the expressions of the main phasors are given (special attention is paid to the analysis and the physical interpretation of the voltage space phasor) and thereafter the formulation of the theory is carried out. it is then shovvn that ali the transient states of a converter-fed symmetrical machine can be represented by a very simple space-phasor diagram, analogous to the öne used for describing its steady state through time phasors. The diagram and the corresponding dynamic equations are deduced in a direct and immediate way, vvhich is very useful for studies of electronically povvered aç machines [8]. Kovacs described the space phasor as a complex representation of fundamental space wave (the space means a quantity in the air gap vvhich takes palace betvveen stator and rotor, such a current density, magnetic flu ete.) and presented voltage equations of the induction machine using space phasors. Kovacs investigated the relations space phasors and their d-q components during a long period of time also [9]. The other investigator J. Stepina had been contributed the space phasor method enormously. Stepina has been noted physical meaning of quanttties of space phasors and their definations in the perspective of space harmonics. He analysed that method mathematically, according to the theory of electrical machines [10]. The steady state values of the main quantities often vary sinusoidally in time. These values can be determined by means of the cartesian projection of an xvioriented segment in the complex plane (time phasor). The study of electrical machines requires the knowledge of certain quantities which are spatially distributed. Space phasors are very suitable to this task. By definition a space phasor is an oriented segment in the complex plane that characterizes at very moment the spatial distribution of an internal machine quantity, provided this distribution is sinusoidal. An internal quantity is a physical quantity which shows at any instant a set of values that can be expressed by a mathematical function, the independent variable of vvhich is a space coordinate. The phasor always points to the positive maximum of the wave and its modulus is equal to the wave's amplitude. Both the wave amplitude and speed may vary in an arbitrary manner. Usually the internal quantity is not bipolar ör not sinusoidal. As a result, every multipolar wave is characterized by a unique phasor. The space-phasor concept is introduced starting from a physical reality, not from a mathematical formula. The basic idea underlying its defination is the follovving; on the analogy of the time phasors, for the space phasors we must first define the physical quantity and only after that we assign a space phasor to it and determine its mathematical expression [8,9,10]. When aç motor are controlled by means of electronic converters, the air gap flux should be maintained constant belovv the base speed in order to obtain high torque throughout the speed range. Thus, the iran saturation is pratically constant and can be accounted for in calculations increasing the actual air gap. So, if the iron losses are negligible, we are very close to the hypothesis of an ideal magnetic circuits. On the other hand, due to the control unit, there are no overcurrents, and the slip is always kept very small, even during the transient conditions. Therefore, the space harmonics have liftle influence on the machine behaviour and suffices to consider the fundamental space vvaves. Likevvise, as there are no signifıcant variations in the slip and flux levels, the machine parameters are constant during the transients, provided the temperature influence can be neglected (this influence, hovvever, and saturation changes in the field vveakening region are to be considered when designing very high performance servo drives). For general studies of rotating electrical machines space phasors show the follovving main advantages; *they ali have a clear physical meaning, as they characterize a well defıned internal quantity (current sheet, average conductor voltage, radial induction yoke flux, ete.), *they can be applied easily to symmetrical machines with ör without field harmonics, *in steady state, space and time phasor diagrams overlap each other, *the instantaneous torque expression is very simple vvhich is a vectorial product of two phasors, *the space phasors allovv a total decoupling of the transient currents system, *the dynamic equations of the machine are obtained in a direct and immediate way, vvhichever the phase number, xvii*the electric equations system obtained is reduced already to its most simple form and no phase transformation ör reduction matrices are required, *the yoke flux, which unlike the leakage fluxes is held pratically canstant in most electronic control applications, is represented by a specifıc phasor vvhich can be taken in many cases as very useful starting points for analyzing control problems, *the space phasors allovv the graphic representation of any transient state by a simple phasorial diagram [8,9,10]. If an a.c. machine is supplied by a converter, time harmonics can be preseni in the voltage, current, and flux linkage vvaveforms. These are undesirable, since they contribute to extra losses, unvvanted torque pulsation and so on. it is possible to obtain information content of these vvavefoms directly from their space phasor loci. A space phasor locus of a particular quantity (in both the steady-state and the transient state) can be obtained by plotting on the two axes of a reference frame the real and imaginary components of the space phasor quantity under consideration. The tip of the space phasor at any time instant will be circle on the complex reference frame. But, there are time harmonics in the quantity, the locus of the space phasor will deviate from the circle. Until recently the cost of the introduction of the variable speed induction motor has been prohibitive and the complexity of control has made its development difficult. Hovvever, the rapid developments in the field of povver electronics, vvhereby berter and more povverful semiconductor devices are available and where the povver devices and circuits are packaged into modular form, and the existence of povverful and inexpensive microprocessors, vvhich allovv the complex control functions of the aç drive to be performed by utilizing softvvare instead of expensive hardware, mean that aç drives employing induction machines can be considered as aconomical alternatives to adjustable speed de drives. Some of the other functional advantages of the application of microprocessors ör digital techniques are; *cost reduction in control electronics, *improved reliability, due to the reduction of the number of components, *standart universal hardvvare is required and the only changes are to the softvvare, vvhich is very flexible and can be easily modified, *very high accuracy, excellent repeatability, linearity, and stability with different setting ranges, *centralized operatör communications, monitoring, and diagnostics (diagnostics programs monitör the operation of the system), *calculating quite complex, high speed arithmetic operations and capability of decision making [3,11]. xviiiThe stator and rotor voltages equations and torgue equation which describe the performance of induction machine can be applied space phasor theory and it is possible to implement vector control by means of space phasors. First of all, the coefficients of the differential equations which are functions of the rotor speed must be done constant. For this reason, in the late of 1920s, R. H. Park introduced the variables of stator to a frame of reference fixed in the rotor. In the late 1930s, H. C. Stanley showed that the time-varying inductances in the voltage equations of a induction machine could be eliminated by transforming the variables associated with the rotor windings to variables associated with fictitious stationary winding. In this case the rotor variables are transformed to a frame of reference fixed in the stator. G. Kron introduced the time-varying inductances of a symmetrical induction machine by transforming both the stator variables and the rotor variables to a reference frame rotating in synchronism with the rotating magnetic field. D. S. Brereton employed the time-varying inductances of a symmetrical induction machine by transforming the stator variables to a reference frame fixed in the rotor. This is essentially Park's transformation applied to induction machines. Park, Stanley, Kron, and Brereton developed changes of variables suited for a particular application. Consequently, each transformation was derived and treated seperately in literature until it was noted in 1965 that all known real real transformations used in induction machine analysis are contained in one general transformation which eliminates all time-varying inductances by referring the stator and rotor variables to a frame of reference which may rotate at any angular velocity or remain stationary. All known real transformations may then be obtained by simply assigning the appropriate speed of rotation to this arbitrary reference frame. A special reference frame fixed to the rotor flux-linkage space phasor is used to implement vector controlled induction machine [12]. The space phasor theory, the reference frame theory and rotor flux- linkage oriented method have been mentioned so far. In this thesis, the digital simulation of the drive system had been applied on INTEL 80X86 computer. Although digital simulation is common because of the easy availability of digital computers, an analog computer can also be used for simulation. Since the machine-converter system is basiclly analog in nature, an analog simulation is easy and convenient and provides easy access to the analog variables. In a hybrid computer, which consists of analog and digital computers, the system can be appropriately partioned for analog and digital simulation. Normally, the converter and the machine are simulated on an analog computer, but the control can be simulated on either an analog or a digital computer. A microcomputer-based control system, which is discrete time in nature and where complex computation and decision-making processes are involved, is simulated in the digital computer. The parity simulation technique has also been used. In this method, analog and digital simulation principles are combined to simulate individual components of the drive system to maintain their topological parity (i.e., a machine simulation looks like a machine at the terminal except that the voltages and currents are XIXappropriately scaled). Such a simulation with modular building blocks can be done quickly by an engineer. Real-time parity simulation of the power components can be driven by breadboard control hardware which subsequently can be used as a protype. In particular, computer results are given the following modes of operation; * balanced conditions (negleting zero-sequence component), * uniform air gap, * linear magnetic circuits (negleting the effects of magnetic saturation of the main flux paths, hysteresis and eddy-current losses, dielectric losses in the electric field), * identical stator windings, distributed so as to produce a sinusoidal rotating field in space with the phases and arranged so that only one rotating field is established by balanced stator currents, * rotor coils or bars arranged so that, for any fixed time, the rotor rotating field can be considered to be a space sinusoidal having the same number of poles as the stator rotating field, * the number of effective turns of stator windings and the number of effective turns of rotor windings are equal, * negleting inverter dead time (switching losses) and no delay due to signal processing, and the effect of space harmonics. * negleting skin and end-effects [13,14]. If field oriented control principles are applied on the saturated model of induction machine, new equations must be obtained, which differ significantly from the well-known model valid when the main flux saturation is absent or negleted. Rotor flux oriented control of voltage-fed saturated induction machine will be studied in the sixth part of the study. New decoupling circuits which are needed under saturated conditions are presented [3,15,16]. Condition monitoring of electrical machines and drive system is a very important factor in achieving efficient and profitable operation of a large variety of industrial processes. Recent trends and advances in electronics have stimulated the development of new types of a real-time monitoring devices. In the past, condition monitoring has been performed using special sensors, some of which are very expensive, yield signal which contain additional unwanted components, are problematic to fit, require the machine to be specially prepared in advance, and whose malfunction can lead to the collapse of the entire system [17]. This study presents the transient and steady state operating conditions of rotor flux field orientation controlled induction machine with space phasors, which are given mathematically, their physical meanings and relations, by using a computer programme in C language. The aim of this thesis will be to find the XXcorrect ratio between computation and sensor elimination without disturbing the dynamic performance of the ac drives In this thesis, first chapter gives a introduction by taking into account the above consideration. In the second chapter contains a step by step the physical and mathematical development of a space-phasor theory. The theory developed covers the operation of a smooth-air-gap machines under linear magnetic condition. In the third chapter, reference frame theory is developed and the voltage equations of stator and rotor are formulated in a general reference frame. In the fourth and fifth chapter, the expression for the electromagnetic torque of induction machine which contains a flux producing current componenet and a torque-producing current is developed similar to the expression for the electromagnetic torque of a separately excited d.c. machine. Rotor field-orientation control of a induction machine is discussed in detail. In the sixth chapter, the effects of a saturation of the main flux paths are discussed in detail by considering the equations which are required rotor field orientation and also the monitoring of various machine parameters is discussed in great detail. Seventh chapter contains results of this study. XXI
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