Asenkron motorde boşta akım denetimi
No load current control of induction motors
- Tez No: 21767
- Danışmanlar: PROF. DR. İLHAMİ ÇETİN
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Akım denetimi, Asenkron motorlar, Elektrik motorları, Current control, Induction motors, Electric motors
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Bu çalışmada, elektrik motorleri içinde kullanım payı en yüksek olan asenkron motorlerin denetimde de kullanılmalarına ilişkin yeni bir yaklaşım olan boşta akım denetimi incelenmiştir. Bu nedenle, öncelikle simertrili asenkron makineye ilişkin genel denklemler, uzay göstericileri yöntemi ile ikinci bölümde verilmiştir. Ayrıca bu denklemlerin bilgisayar ile çözümü yapılmış. asenkron makineye ilişkin büyüklüklerin değişimleri çizilmiştir. Üçüncü bölümde, asenkron motorun sürekli i si etmesindeki devre ve diyagramları kullanılarak hız ayar yöntemleri ve kullanılan frekans dönüştürücü türleri verilmiştir. Dördüncü bölümde, asenkron makinenin denetiminde kullanılan farlı yöntemler karşılaştırmalı olarak incelenmiştir. Beşinci bölümde bu çalışmanın temelini oluşturan boşta akım denetimi ayrıntılı olarak ele alınmış, sürekli işletmede geçerli eşdeğer devre ve diyagramlar kullanılarak yöntem incelenmiştir. Asenkron motörlerde boşta akımın denetimine ilişkin iki farklı yöntem verilmiştir. -iv-
Özet (Çeviri)
SUMMARY NO LOAD CURRENT CONTROL OF INDUCTION MOTORS Adjustable speed ac drives are visible gaining momentum for industrial applications. The trends in the present technology convey the belief that ac drives will find wide acceptance in the near future. Historically, ac machines have been with us for nearly a century. During these years, analysts, designers, and experimenters have studied ac machines extensively. Before the advent of the era of power semiconductor devices, ac machines were commonly accepted for fixed speed applications, in which the speed was governed by the frequency of sinusoidal voltage waves. The complex and expensive techniques of speed control, such as pole changing, Scherbius, and Kramer methods, were known long ago, but were not favored in industrial applications. From the beginning, dc motors have been considered workhorses in industry for variable speed applications. Though the control principle and the converter equipments are somewhat simple, the dc machine is expensive compared to the simple and rugged cage type induction motor. In addition, the principle problem of a dc machine is that its commutators and brushes make it unreliable, unsuitable to operate in dusty and expolosive environment, and require frequent maintenance. Maintenance causes difficulty when interruptions can not be tolerated or when the motor is used at inaccessible locations. Such deficiencies obviously can not be tolerated in many applications. Although the ac machines, especially the cage type induction motor, is more rugged and reliable, as well as less expensive and more efficient, the cost of the converter and the control is considerably higher, which makes the ac drive system more expensive than the dc drive system. In addition, the control of ac drives is very complex and needs intricate signal processing to obtain the comparable performance of ac drive. The research and development efforts in ac drives technology have been focused recently on solving the above problems. The cost and performance of ac machines are expected to remain pratically unaltered in the near future. The price of discrete thyristors has been stable, though the performance, including voltage and current ratings, is continuously improving. Recently, the power transistor ratings have improved significantly and are showing a trend of lower prices favoring application in low to medium power ac drives. Although the price of transistor per kilowatt of power handling capasity is somewhat higher at present than that of thyristor, the elimination of forced commutation requirement and reduced snubber -v-rating reduce the converter cost, resulting in greater efficiency. Another major factor in ac drives technology is the availability of microprocessors for the control of ac drive systems. Microprocessors operate at an adequately high clock frequency to complate their calculations in sufficient time to directly control the firing of the power semiconductors in a three phase bridge circuit operating from the utility supply frequency. These rapid technical advancements and declining prices for power semiconductors and microprocessors, coupled with a demand for a high efficiency, adjustible speed control for both existing and newly installed equipment, have led to the world wide application of adjustible frequency controllers for ac motors. In this thesis, a new control strategy for induction motors is studied. First of all, the mathematical model of the induction machine is given by making use of the below assumptions. The air gap is smoot along the stator and rotor periphery The distribution of the field and of the mmf respectively along the air gap are sine waves and that the individual phase windings are symmetrically distributed along the circumference of the stator iron The magnetic circuit is ideal and the saturation effect is not present With these assumptions and by making use of the complex space phasors, the voltage equations of the symmetrical induction machine can be given in the view of any reference frame as, d* + - - s s- s U = R İ + -jr-^. + j U, * df y = R i + r + j C<o - oD *.r r- r dt k -Vi- -rEquation of mechanical motion is do _ J = -|- k C ** i ) - M dt 2“S ”S L Here J and Ml are the total inertia and load torque respectively of the system transfer ed to the rotor Here, there is no restriction for the reference frame. In the choice of the coordinate system, one is not limited to reference frames whose angular relocity is either the velocity of the rotor or zero. In fact, the frame may rotate at any general speed co. If the reference frame is at rest C fixed to the stator } cok = 0 ;if the frame fixed to the rotor, wk = os ;and for example tok = w± if the reference frame is rotating at synchronous speed. The torque and power equations are invariant against coordinate transformation. Generally, the conditions of operation will determine the most convenient reference frame for analysis or simulation purposes. If, for example, the stator voltages are unsymmetrical or discontinous and the rotor applied voltages symmetrical or zero; the stationary reference frame should be used to simulate the performance of the induction machine. If, on other hand, the extarnal rotor circuits are unsymmetrical but the applied stator voltages are symmetrical, then reference frame fixed on the rotor is most convenient. Either the stationary or synchronously rotating reference frame is generally used to analyze balanced or symmectrical condi ti ons. By using digital computer, mathematical model equations of the induction machine can be solved by separating real and imaginary parts and the dynamic conditions can be analyzed. Phase quantities are solved and their instantaneous variations with time are obtained. In the variable frequency oparation of the induction machine, the air gap flux wave is the main concern of the control strategy and has an important effect on the dynamic behavior of the machine. The control methods have to keep the air gap flux wave constant within the full operating area. For this purpose, there are several methods developed. These are -vii-1. U/f control 2. Stator current control 3. Field oriented control 4. No load current control The first method is known as voltage-frequency method. In the variable frequency operation the air gap flux can be kept constant by keeping the U/f ratio constant. When the frequency changed, the voltage must be changed, otherwise saturation or decrease in the torque will occur. According to the simplicity of the method, it has poor dynamic response when the high dynamic working condition desired. In the low frequency operation, the voltage drop across the stator resistances needs compensation. This is the main disadvantage of the method. In the second method, the air gap flux is kept constant by keeping the stator current within the allowable range. This method can not be operated open loop, closed loop control of the current is essential. The field oriented control is much more sophisticated than the others and gives high dynamic response. The torque and the air gap flux can be controlled independently. Although this method improves the dynamic capability of the induction machine as a servomotor, it has some important disadvantages such as measuring or calculating air gap flux, coordinate transformation. Air gap flux can be measured directly from the air gap of the machine by using Hall elements or coils. But Hall elements are fragile and semiconductor elements so they are sensitive to mechanical stress and temperature, and the deviation from the real quantities must be compensated. In addition, placing such measuring equipment in the motor is not well come by the designers. To cope with these problems; air gap flux can be calculated by using the dynamic model equations and the phase quantities of the induction motors. But this makes the control structure more comlex and expensive. Moreover, the parameters of the machine are dependent to the temperature. The resulting controller becomes too complex and expensive -viiiAs an alternative, no load current of the induction machine can be used the control quantity in the variable frequency operation. The no lead current consists of two components, magnetising and iron currents. Iron current is much more less than magnetising current so, the magnetising current can be taken approximately as no load current The no load current is determined from the no load test of the machine. It is unpossible to measure this current while the load torque exists. But on the other hand, in the variable frequency operation, a control signal associated with the magnetising current of the machine can be measured from the converter as explained later. The importance of the control of the magnetising current is on the improvement of the dynamic responses. The no load current of a given induction motor can be solved from the dynamic equations and its instantaneous variation with time be drawn from the stand still to the rated speed CFig. ID. ioA Fig. 1. Instantaneous variation of no load current From the figure, The amplitude of the no load current is exponanti onal 1 y increasing untill the rated speed reached. Than it stays constant. When the load torque applied to the shaft of the motor increased, the amplitude of the no load current tends to decrease. After the breakdown torque reached, this decrease will result in the unstable operation of the motor. The voltage drops across the stator resistances are responsible for this decrease. In the normal operation from the utility grid system, the decrease takes place and the well known torque -ix-versus speed characteristic is obtained. To keep the no load current constant by adjusting the voltage within the frequency converter, will result in the improvement of the torque speed curve. Because, the torque produced in the motor is proportional to the square of the no load current CFig. 2D. n CWnl, t lo-safe Usf» n [l/nin3 Fig. 2. Torque speed characteristic In the figure, the torque curves are given for various frequencies, and for comparison, characteristics are also given. control, the linear portion of five times the rated value and seven times greater than the rated torque. The stalling torque is kept constant in the no load current control as in the U/f control technique. versus speed characteristic i nput vol tages and U/f torque versus speed In the no load current the curves reach about the stalling torque is In this thesis, two method is given to control the no load cur ret constant in each working points. One is, the calculation of the stator voltage by using the parameters of the motor from the T equivalent circuit. Another method is the measurement a signal from the converter which is proportional with the magnetising current. This is based on the measurement of the energy stored in the magnetising inductance by means of the freewhelling diodes and producing a control signal related to the magnetising current. -x-
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