Asenkron motörde boşta akım denetimi

Abstract

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.

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.