Asenkron makinanın vektör denetimine doymanın etkileri

Başlık çevirisi mevcut değil.

Tez No: 75514
Yazar: HÜSEYİN ERCAN
Danışmanlar: PROF. DR. M. EMİN TACER
Tez Türü: Yüksek Lisans
Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1998
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Elektrik Mühendisliği Ana Bilim Dalı
Bilim Dalı: Belirtilmemiş.
Sayfa Sayısı: 170

Özet

ÖZET Bu çalışmada, doymalı asenkron makinanın vektör kontrolü ile incelenmesi amaçlanmıştır. Çeşitli tipte vektör kontrolları için gerekli akı modelleri ve bağımsızlaştırma devreleri verilmiş, kontrol teknikleri blok diyagramlarla gösterilmiştir. Ayrıca MATLAB proramı ile ilk olarak, doymalı ve doymasız asenkron makinanın sayısal simülasyonu yapılmış elde edilen sonuçlar, karşılaştırmalı bir sekide verilmiştir. Simülasyonun ikinci adımında, rotor akısı yönlendirmeli kontrol tekniği, doymalı asenkron makina için sayısal simülasyonu yapılmış, sonuçlar doymalı akı modeli ve liner akı modeli ile karşılaştırmalı olarak verilmiştir. xv

Özet (Çeviri)

SUMMARY VECTOR CONTROL OF INDUCTION MACHINES AND EFFECTS OF MAGNETIC SATURATION The main aim of this thesis is to study vector control of saturated induction machine. In addition to this, the dynamic behaviours of the saturated (main flux saturation) induction machine and rotor flux oriented control will be simulated by means of MATLAB program. In the past d.c. motors were used extensively in areas where variable speed operation was required, since their flux and torque could be controlled easily by the field an armature current. In particular, the separately excited d.c. motor has been used mainly for applications where there was a requirement of fast response and four quadrant operation with high performance near zero speed. However, d.c. motors have certain disadvantages, which are due to the existence of the commutator and the brushes. That is, they require periodic maintenance; they cannot be used in explosive or corrosive environments and they have limited commutator capability under high speed, high voltage operational conditions. These problems can be overcome by the application of alternating currents motors, which can have simple and rugged structure, high maintainability and economy; they are also robust and immune to heavy overloading. Their small dimension compared with d.c. motors allows a.c. motors to be designed with substantially higher output ratings for low weight and low rotating mass. On the other hand induction machines have some disadvantages, the squirrel cage induction machines need complex control algorithms due to their non-linear dynamic model. However, as a result of progress in the field of power electronics and power converters the control of induction machines became more practical. At the same time, the implementation of microprocessor in the digital control circuits has introduced a wide scope of possibilities to overcome the complex dynamics of the machine. Further more, the development of processing integrated circuits is one of the major factor of the rapid evolution of induction machine control. As a result of these developments, when the control of an induction machine needs fast transient responses or torque at standstill (or low speed), the vector control seems the only efficient solution. Therefore, vector control techniques with fast microcomputers or microprocessors have made possible the high-performance application of induction machine. To apply vector control methods; first of all, the mathematical model of an induction machine must be obtained. As it is mentioned above, the dynamic performance of an induction machine is complex because of the coupling effects between the stator and rotor phases where the coupling coefficients vary with rotor XVIposition. Therefore, the mathematical model of the machine can be described by differential equations with time-varying coefficients as a first stage, the mathematical model of a three-phase squirrel-cage induction machine on a b c phase axis will be developed. This model in naturel axis is based on the fallowing assumptions:. Rotor windings or bars are replaced symmetrically on rotor periphery. Symmetrical windings on stator. Negligible cross section of the conductors (skin effect and slotting are not taken into account). Magnetic permeability is infinite. Hysteresis and iron losses are negligible. Concentric stator and rotor with constant air gap of negligible width. Ideal magnetic circuit. Sinusoidal distributed windings According to the above assumptions, the differential equations in other words the mathematical model which describes the dynamic behaviour of the electrical and mechanical parts of induction machine in a b c axis (natural axis) can be written. But these equations have time-varying coefficients because of the sinusoidal variations of mutual inductance with the displacement angle. If the power supply is balanced three-phased, as it is usually true when it is fed by an inverter, two axis, in other words odq theory in normally used for dynamic modelling. In this theory, the time-varying parameters are eliminated and the variables and parameters are expressed in orthogonal or mutually decoupled direct (d) and quadrature (q) axis. From that point of view, by using odq axis theory and invariant of the power, the time-varying differential equations of induction machine are transformed into time invariant equations in odq coordinates, two different transformations are applied to the mathematical model of squirrel-cage induction machine. Since the machine has more than three phases on rotor, it is impossible to use symmetric Park transformation. By applying first transformation, symmetrical component transformation, the model is expressed as a model having three phases on both stator and rotor. But this model includes time-varying coefficients too. Therefore the second transformation must be applied by this way the model of squirrel-cage induction machine is expressed by time invariant coefficients in odq axis. The odq dynamic model of a machine can be expressed either in a stationery or a rotating reference frame. In a stationery reference frame, the reference (sD-sQ) axis are fixed on the stator whereas in a rotating reference frame (rd-rq) these are rotating. The rotating frame may either be fixed on the rotor or move at synchronous speed. There are various implementations of the vector-controlled induction machine, but its common feature that the instantaneous electromagnetic torque of the machine is controlled in a similar way to that of a separately excited d.c. machine operated with a current regulated armature supply. In the separately excited d.c. machine, by assuming that the effects of magnetic saturation can be neglected, the electromagnetic torque is proportional to the product of the excitation flux (field flux) an the armature current, an c is a constant. te=Cfia xvuHowever, the field flux is proportional to the field current and does not depend on the armature current since, due to the action of the commutator, the field flux armature m.m.f. are in space quadrature. Thus if the field flux constant, electromagnetic torque is proportional to the armature current and therefore when the armature current is controlled, there will be quick change in the torque. However, in an induction machine, the space angle between the stator and the rotor fields varies with the load and to be able to obtain a technique of torque control, which is similar to that for the separately excited d.c. machine (assuming that the machine is supplied by a converter on the stator), it is necessary to decouple the stator currents into a torque- and flux-producing stator current components. In special reference frame, fixed to the rotor flux linkage space vector, under linear magnetic condition the instantaneous electromagnetic torque of the induction machine is proportional to the product of the modulus of the rotor flux linkage (which is proportional to the so-called rotor-magnetising current, which is the direct axis stator current component in the special reference frame) and the quadrature-axis stator current component. Thus as in the separately excited d.c. machine the torque is proportional to a flux times a torque producing current. This is utilised in the implementations of the rotor-flux-oriented control, which has also been termed as field-oriented control. If the special reference frame is fixed to the stator flux linkage space vector; by neglecting the effects of magnetic saturation, the instantaneous electromagnetic torque of induction machine is proportional to the product of the modulus of the stator flux linkage space vector, (which is proportional to the so-called stator magnetising current component, which is the direct-axis stator current component) and the torque producing stator current component (quadrature axis stator current component). Thus again the expression of the torque is proportional to a flux times a current, as the expression of the separately excited d.c. machine. When this utilised for the torque control of the induction machine, the so-called stator-flux-oriented control is obtained. Finally, if the special reference frame is oriented with the magnetising flux linkage space vector (or with the magnetising current space vector, which is coaxial with the magnetising flux linkage space vector in a smooth air gap machine), and the effects of magnetic saturation are neglected, the instantaneous electromagnetic torque is proportional to the product of the magnetising flux linkage space vector and the quadrature axis stator current expressed in the magnetising flux oriented reference frame. This is again similar to the expression of the electromagnetic torque of the separately excited d.c. machine, and when torque control is implemented by using this technique, the so-called magnetising-flux-oriented control is obtained. It is follows that when an induction machine is subjected to vector control it will behave similarly to a separately excited d.c. machine in both the steady state and transient state. Thus the same type of control techniques can be used as for the d.c. machine and below base speed the flux is kept at its maximum possible value (it is limited by magnetic saturation), and over base speed it is reduced due to the voltage limitation of the inverter, yielding the flux-weakening mode of operation. If the converter is on the rotor side of the induction machine, then it is also possible to obtain the required decoupling of the rotor currents into a flux- and torque-producing component, respectively, and the fact that in the case the rotor currents can be directly monitored, simplifies the implementation. xvmIn the various implementations of the vector controlled induction machine, different techniques are used to obtain the flux- and torque-producing stator currents components, often referred to as direct and indirect methods. If the flux, or the flux producing current component, which for the magnetising-flux-oriented case is the magnetising flux (and the corresponding current is the magnetising current) are determined by using direct measurement (for example by using Hall elements or search coils) or by utilising computations which use the terminal voltages and currents of the machines, then the direct implementation is obtained. However, in the case of the indirect technique, a rotor position sensor (resolver, encoder, and so on) is employed for the derivation of these quantities. The implementations can also be classified into two main groups depending on the stator supply, which can contain current or voltage sources. If the machine is supplied by impressed stator currents (current sources), for example, if the induction machine is supplied by a current-controlled PWM inverter with fast current control loops, then the stator currents can accurately follow the reference currents and the great simplification can be achieved in the implementation, since the stator equations of the machine can be omitted from the model equations of the drive. In this work, saturated induction machine is supplied by a PWM voltage-source thyristor inverter and indirect method is used to obtain flux- and torque-producing current component. As a result of saturation of the main flux paths, the magnetising inductance and thus also stator and rotor inductances (and the stator and the rotor transient time constants) are not constant, but vary with saturation. The variation of the magnetising inductance will be incorporated into the voltage equations. It will be sown that the voltage equations which are valid under saturated conditions differ from the equations which are valid under linear magnetic conditions. The equations to be derived will be based on space-phasor (or two-axis) theory. Saturation of the main flux paths distorts the flux density distributions, which in the case of linear theory (which neglects saturation) are sinusoidal in space, if sinusoidal m.m.f. distributions are assumed. The resulting space harmonics can, however, be neglected if sinusoidal distributed windings are assumed since only the fundamental sinusoidal component of a flux wave can produce flux linkages with sinusoidally distributed windings, and thus it is possible to use space-phasor and two- axis theories. The effects of main flux saturation are discussed for vector-controlled induction machines with single-cage rotor. The various expressions for the electromagnetic torque will not be derived for the saturated machine, since formally they are the same as in the case of the unsaturated machine, since saturation does not introduce new terms in the expression for the electromagnetic torque. Of course, as a result of saturation the saturation-dependent machine parameters which are present in the different expressions for torque, eg. the magnetising inductance, or the rotor self inductance or the stator self inductance will be different to their unsaturated values and are variables which depend on the machine currents. This thesis consists of seven chapters. In previous part of study, mathematical model of induction machine is presented and space phasor model of induction machine is also obtained. Then, the effects of magnetic saturation are incorporated machine equations. Vector control methods for induction machines are given. And then, flux models are obtained under saturated conditions. Finally, saturated, unsaturated induction machine and rotor-flux-oriented control is simulated for saturated flux and unsaturated flux models (unsaturated induction machine is xixused for both flux models) by means of MATLAB program and results of simulation are also given. The first chapter deals with the general approach of the thesis. The various implementations of vector control methods are mentioned. And effects of magnetic saturation are briefly discussed. In the second chapter, the general dynamic equations of symmetrical induction machine are developed by using the fundamental laws of electromechanical energy conversion theory. The obtained model is very complex and has nonlinearity. This makes the analysis more difficult even for computer analysis. In order to obtain more flexible and simple model space vector theory is used and machine quantities are obtained as a space phasors. In the third chapter, the equations of induction machine are obtained various reference frame (special reference frame is fixed to the stator, rotor, flux linkages etc.) In the fourth chapter, the effects of magnetic saturation are incorporated into induction machine equations (which are obtained under linear magnetic conditions) and under saturated conditions, two-axis model of induction machine also obtained. The equations are first obtained in the general reference frame and then equations are obtained special reference frames. The various expressions for the electromagnetic torque are derived. Furthermore, the equations are given with the current as state variables and these equations are used in computer simulation. In the fifth chapter, rotor-flux-oriented control, stator-flux-oriented control and magnetising-flux-oriented control are introduced under linear magnetic conditions. Flux models and decoupling circuits are also obtained. Block diagrams of rotor-, stator- and magnetising-flux-oriented control is also given. In the sixth chapter, effects of magnetic saturation are incorporated into flux models and new flux models which are valid under saturated conditions are obtained. The various expressions for the electromagnetic torque are not derived for the saturated machine, since formally they are same as in the case of unsaturated machine, since saturation does not introduce new terms in the expression for the electromagnetic torque. In the last chapter, saturated, unsaturated induction machine and rotor-flux- oriented control is simulated for saturated flux and unsaturated flux models (unsaturated induction machine is used for both flux models) by means of MATLAB program and results of simulation are also given. xx

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