Yabancı uyartımlı ile fırçasız DC motorların hız ve tork karakteristiklerinin karşılaştırılması

Abstract

Fırçalı DC (BDC) motorlar, oyuncaklardan araba koltuklarına kadar uzanan geniş bir uygulama alanına sahiptir. Bu motorlar, ucuz ve çalıştırılması kolaydır. Piyasada tüm boyut ve biçimlerde bulunabilmektedir.Yabancı uyartımlı DC motorlar, düşük hızlarda yüksek tork kapasitesine sahip olmaları nedeniyle tercih edilirler. Bu motorların dış kaynaklarla, hem armatür, hem de alan akımlarının kontrol edilebilmeleri nedeniyle, en esnek bir şekilde kontrolü mümkündür. Armatür kaynağı için, kontrollü doğrultucu ya da kıyıcı gereklidir. PI veya PID gibi kontrolörlerle hız kontrolü sağlanmaktadır. Daha hassas kontrol istendiğinde, adaptif kontrol teknikleri kullanılır.Fırçasız doğru akım motorları (BLDC motorlar), sahip oldukları avantajları dolayısıyla günümüzde daha çok tercih edilir duruma gelmiştir. Avantajlarının başlıcaları; yüksek verim, güvenilir çalışma ortamı, daha az bakım, sessiz çalışma, kolay soğutma, uzun ömür ve kolay kontrol edilebilme şeklinde belirtilebilir. Bununla beraber; karmaşık bir kontrol yapısına sahip olmaları, pahalı bir sistem oluşu, rotor pozisyonunun algılanabilmesi için pozisyon sensörlerine ihtiyaç duyması gibi dezavantajlara da sahiptir. Pozisyon sensörlerinin kullanılmadığı durumlarda ilave algoritmalar gerekir. Ancak günümüzde gelinen noktada geliştirilen yöntemlerle, bu motorların dezavantajları önemsiz duruma gelmeye başlamış ve kullanımları artmıştır.Otomotiv sektörü, uzay ve bilgisayar teknolojileri, tıp elektroniği, askeri alanlar, robotik uygulamalar ve ev ürünlerinde sıkça kullanılmakta ve kullanım alanları gittikçe genişlemektedir.Bir BLDC motor, üç faz sargılı stator, sabit mıknatıslı rotor, geri besleme üniteleri (Hall sensörleri v.b.), evirici ve sürücü katmanı ile denetleyici yapılarından oluşmaktadır. Stator sargılarının enerjilendirilmesi rotor pozisyonuna göre yapılır. Rotor konumu algılayıcılar ile belirlenir. Bunun dışında, sürücü için akım veya gerilim bilgileri de ölçülerek kullanılmalıdır. Hız ve konum denetimi için en çok Hall ya da optik sensörler kullanılır. Rotor pozisyonunun sensörsüz olarak belirlendiği teknikler de giderek yaygınlaşmaktadır. Sensörsüz motorlar, sensörlü motorlar kadar yüksek hızlara ve ivmelere ulaşamazlar.Fırçasız DC motorunun elektromanyetik yapısı, sürekli mıknatıslı senkron motorlara benzemekle birlikte; stator hava aralığında endüklenen zıt-emk, sinüzoidal olmayıp trapezoidal (yamuk) şeklindedir. Fırçasız DC motorlar, rotorun yapısına göre üçe ayrılır. Bunlar dış rotorlu, disk tipi ve iç rotorlu yapılardır. Bunun dışında BLDC motorlar sensörlü ve sensörsüz olarak da ikiye ayrılır.Fırçasız DC motorlarda motorun akımı, torku, rotor konumu ve hızı gibi parametreler çeşitli kontrol yöntemleri kullanılarak kontrol edilir. Bu kontrol şu şekilde olmaktadır; denetleyicinin ürettiği kontrol sinyali, seçilen kontrol algoritmasına göre PWM sinyallerinin durumunu kontrol eder. Bu şekilde denetleyici tarafından motor parametreleri kontrol edilir ki, denetleyici hem yazılım, hem de donanım yapılarından oluşur.BLDC motorların kontrolünde yapılarının basitliği nedeniyle ve birçok uygulamalarda yeterli verimi karşılaması nedeniyle klasik denetleyiciler (PI ve PD tipi) kullanılır. Ancak denetlenecek sistemin modeline ihtiyaç duymaları ve en uygun kazanç değerlerinin deneme yanılmayla belirlenmesi dezavantaj oluşturmakta; sinüzoidal ve ani değişimlerdeki performansları yetersiz olmaktadır. Dolayısıyla, PI ve PD tipi denetleyiciler hassasiyet aranmayan uygulamalarda sıkça kullanılmaktadır. Çok hassas denetim gerektiren uygulamalarda ise modern denetim teknikleri tercih edilmektedir. Günümüzde modern denetim tekniklerine, bulanık mantık, yapay sinir ağları, genetik algoritma, sinirsel bulanık denetleyiciler örnek verilebilir.Bu tezde, önce, fırçalı ve fırçasız DC motorların yapıları, çeşitleri, çalışma prensipleri, kullanım alanları, kontrol biçimleri ve ekipmanları hakkında literatüre dayalı açıklayıcı bilgiler verilmiştir. Daha sonra, yabancı uyartımlı DC motorların ve BLDC motorların matematiksel modelleri oluşturulmuş; bu modellerin Matlab/Simulink Programı kullanılarak Simulink modelleri elde edilmiş, PI kontrolü eklenmiş ve step (birim basamak) cevapları incelenmiştir. Son olarak da, bu cevaplar yorumlanmış ve karşılaştırılmıştır.Anahtar Kelimeler: Fırçalı DC motor, yabancı uyartımlı DC motor, fırçasız DC motor,motor kontrol, PI kontrol.

Brushed DC motors have been widely used in a variety of applications, for example toys, car seats, electric trains, electric vehicles etc. These motors are cheap. They provides easy controllability and high performance. The most flexible control is obtained by separately excited DC motor in which the armature and field circuits are provided with seperate sources. The separately excited DC motors have been preferred because of have a high torque in underspeed. For the armature source a controlled rectifier or chopper is required. Conventional controllers such as PI and PID have been applied to control the speed of DC motors. The disadvantages of using conventional controllers are that they are sensitive to variation in the motor parameters and load disturbance. In addition, it is difficult to tune PI or PID gains to eliminate and reduce the overshoot and load disturbance. Nowadays, researchers applied adaptive control techniques for DC motor speed control to achieve parameter insensitivity and fast speed response.Nowadays, brushless DC motors have been preferred more than the other electric motors because of their advantages. Principal advantages; high efficiency, high reliability, less maintenance, silent operation, being easily cooled, long life (no brush and commutator erosion) and being easily controlled. Unfortunately, BLDC motors have disadvantages that have a control system more complexity, expensive system and require position sensors to sensing rotor position. Sensorless control contains higher requirements for control algorithms and more complicated electronics. But, nowadays, the disadvantages of BLDC motors have arrived not important because of the development of BLDC control methods.Nowadays, especially automotive sector and all industries are needed precise and at the same time low cost, reliable and low maintenance velocity variables. Accordingly, in order to provide for the desired specifications, selection of motor becomes important. Classical (brush) DC motors have start up moment, high efficiency and linear caracteristic of speed-moment. These characteristics are desired for servo systems. However, friction and arc formed due to brush and collectors, efficiency of motor is negatively affected. Also, due to abrasion and heating, frequent failures occur. DC motors eliminated from the disadvantages mentioned above are made almost ideal designs that require less maintenance with higher efficiency. BLDC motors designed with this idea have linear speed-moment relation. Start up moment being directly with the motor current, makes control easier compared to other motors. High moments can be produced at small sizes. Which means they have high moment-volume ratio (require less copper for BLDC motors). They require less maintenance because of no brush and collector, they can be used in danger zones. These motors have been widely used in a variety of applications in automobile industry (hybrid vehicles), space and computer technology, medical electronic, military areas, industrial automation, robotic applications and household products.The BLDC motor is an AC synchronous motor with permanent magnets on the rotor (moving part) and windings on the stator (fixed part). Permanent magnets create the rotor flux and energized stator windings create electromagnet poles. The rotor is attracted by the energized stator phase. By using the appropriate sequence to supply the stator phases, a rotating field on the stator is created and maintened. This action of the rotor, chasing after the electromagnet poles on the stator, is the fundamental action used in synchronous permanent magnet motors. The lead between the rotor and the rotating field must be controlled to produce torque and this synchronization implies knowledge of the rotor position.BLDC motor is defined the shape of the back-EMF of the synchronous motor. Both BLDC and PMSM (Permanent Magnet Synchronous Motor) have permanent magnets on the rotor, but differ in the flux ditributions and back-EMF profiles. The back-EMF is trapezoidal in BLDC motor case and sinusoidal in the PMSM motor case. BLDC motor is composed of a permanent magnetic rotor and three stator coils. Besides, It's used to operate inverter and driver circuit and controller. The rotor position must be known to energized stator coils. The rotor position is determined by the sensors. Generally, the Hall effect position sensors are used to detect rotor position. Sometimes, optic sensors are used too. Besides, for driver, current and phase informations are also measured to controlled the motor. The rotor position is usually sensed by sensors, but there are applications that require sensorless control. Benefits of the sensorless solution are elimination of the position sensor and its connections between the control unit and the motor. The sensorless rotor position technique detects the zero crossing points of back-EMF induced in the motor windings. Using sensorless control have been widely increased recently. Sensorless control of BLDC motors can't achieve high speed and acceleration according to motor control with sensors.No sensor studies, Kalman Filter Theory is used as a stronger method. In this method, a mathematical model that contains position, speed and back-EMF values is used. In prediction stage, the change in the motor status at any point in time is predicted by using this method. The predicted back-EMF is compared with measured value and the difference is used for optimizing the motor operation. By using Kalman Method, position and speed of motor can be predicted not only at zero crossings but also at any given time. Therefore more accurate commutation and so higher effiency can be obtained.There are three types of brushless DC motors called inrunner, outrunner and disc type. The inrunner motor has permanent magnetes located on the inside of the stationary electromagnets. Inrunner motors are good when high speed are needed. They are more efficient than outrunner motors the faster they spin. Inrunner motors are low torque than outrunner motors. An outrunner motor has the permanent magnets located on the outside of the stationary electromagnets. Outrunner motors spin slower but output more torque. Disc type brushless DC motors can prefer low power and low speed applications. If we need low speed but high power, we should choose inrunner motor to has high number of poles.Parameters in brushless DC motors, such as motor current, torque, rotor position and speed are controlled using various control methods. Control signal is produced by the controller, controls the status of PWM signals selected according to control algorithm. By means of this method, motor parameters that consist of both software and hardware structures, are controlled by the controller. Torque of BLDC motors generally are controlled by controls of stator currents.PWM process is obtaining voltage at different impulse width by switching a fixed source and therefore voltage control at very wide ranges can be obtained. Impulse width obtained depends on the total of duration the switch remains on and off, the duration the switch remains on. This is provided by changing the switch off duration or period. In BLDC motor applications, control has become increasingly important besides motor design. BLDC motors are controlled by the fundamental power electronic circuits. However, It's necessary that many applications is implemented by the developed control algorithms. Increase in microprocessor capabilities, applicability of obtained mathematical models have made easy designing digital controllers for these models. Due to the improvements mentioned above, technologically advanced and economical solutions are now possible for industrial needs. Classic controllers for example PI and PD type controllers are used for controlling BLDC motors due to their simplicity of structure and enough efficiency in most applications, in general. However, requiring the model of the system to be controlled and determining the optimum gain values by trial and error method are among the disadvantages of this method, together with lack of performans during sinusoidal and instantaneous system changes. Therefore, PI and PD type controllers are commonly used for applications that do not require high precision. For applications that require very high precision, modern control methods are preferred. Fuzzy logic, artificial neeural network, genetic algorithm, neural fuzzy controllers are examples of modern control techniques. Fuzzy logic is the most convenient control method for conditions where classical logic is not enough. Especially, if mathematical model of a system is not constructed or is very difficult to construct, and it is a non-linear system, fuzzy logic control method where human perception and experiences are utilized is preferred.In this thesis, firstly, general information was given about brush and brushless DC motors and their control including basic structures, characteristics, types, working principles, control logic and control methods commonly used. Then, the mathematical models of separately excited DC motors and BLDC motors were obtained and then, simulink models of this models were obtained by using Matlab/Simulink Software. Obtained simulink models were simulated and simulation results were illustrated. Then, PI controllers were added this simulink models. Added simulink models were simulated and simulation results were illustrated. Ultimately, all simulation results were interpreted and compared. Keywords: Brushed DC motor, separately excited DC motor, brushless DC motor, motor control, PI control.