Fırçasız doğru akım motorlarının simülasyonu

Savur, Bülent
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Bu çalışmada fırçasız doğru akım motorlarının besleme şemalarına göre sırayla bir, iki ve üç faş için denklemleri çıkarılmış ve simülasyon çalışmaları yapılmıştır. Birinci bölümde, sürekli mıknatıslı fırçasız doğru akım motorlarının kullanım alanları belirtilmiş ve sıra¬ dan fırçalı tip doğru akım motorlarına olan üstünlükleri gösterilmiştir. ikinci bölümde, sürekli mıknatıslı fırçasız doğru akım motorlarını meydana getiren temel elemanların ayrı ayrı tanıtımı yapılmış ve denklemleri çıkarılmıştır. Sonraki bölümlerde bir faslı, iki faslı ve üç faslı sürekli mıknatıslı fırçasız doğru akım motorlarının matematiksel modeli elde edilmiş ve simülasyon çalışmaları yapılmıştır, ilgili bölümlerde kullanılan bilgisayar programları ve paket programlar ekler halinde verilmiştir. Son bölümde ise elde edilen sonuçlar belirtilmiştir.
The digital eornputer simulation for modes of operation is eonveniently obtained from the differential eguations whieh deseribe the brussless de motor in an arbitrary refference frame. For simulaton study, a model of the system is required and the cornplexity of the model will depend on the goal of simulation study. The analytical study of the systejjıs is difficult and therefore computer simulation study beeame essential. The dynamic response of the system can be simulated by state-spaee equations. in these thesis, this method will be used to introduce the dynaosic model of the brussless de motor. A brushless de motor eomprises four basic elemeııts: Eleetronic comautator, rotor position sensor, the stator and the rotor. in a brushless de jaaehine, the rules of the stator and rotor are reversed when compared to a convantional de machine. The co»ffiutator/brush system in a de motor performans two important funetions: (a) it senses the position of the rotor eoils relative to the pole pieces, by virtue of its physical position. (b) it reverses the rotor eoil eurrent at the appropriate insant of time These two actions ensure that the »îotor torque is always unidirectional. it is feasible to substitute each eommutator segment by a semieonductional switch, so that the motor eoil eurrent can be individually switched. The motors are not simply ae motors powered by an eleetronic inverter, as they require a position feedback at some kind so that the input voltage waveforms are kept Vi in the proper timing with the rotor position. Although generally brusshless do motors, they are available in a wide range of designs from the most sophistical type used in space applications, to high- volume, low-cost devices used industrial equipment. The accuracy of the position data which would be required to implement the commutation scheme described suggests the use of an optically encoded disk as a position sensor. The optical sensors would work at all speeds, require no initialising data at start-up. The speed could be simply calculated by differentiation of the position data. In the third chapter, a model of a permanent- magnet ac machine and dc-to-ac inverter which comprise a single- phase brushless dc motor has been developed. Central to this model is the the form of the stator winding flux linkage due to the permanent magnets. A mathematical model of the ac machine, consisting of equational for the electrical and mechanical dynamics, has been established. These equations are expressed in state model form and are combined with a model of the inverter to form a computer simulation of the drive system. The characteristics of brushless dc motors depend largely on their feeding scheme. The two feeding schemes provide very different operation conditions for the motor. These are two- phase feeding and three-phase feeding. In the fourth chapter, two phase brushless dc motor has been given. The ûc input current la is maintained at a constant value by the current regulator which controls the chopper duty cycle. The inverter operates as a comutator feeding the dc current into two phases of the motor, the third phase being in open circuit. The rotating field is created by commutating the dc current from phase to phase at intervals equivalent to 60 electrical degrees. The commutation is synchronised to the rotor position sensor signals. The motor phase current is a 120" square wave having amplitude equal to the dc input current Id. This latter is used as control variable for the the motor torque. Vii Since a synchronisation signal is needed only at intervals of 60°, the position sensor can be made very simple. It may be a simple sector encoder, using optoelectronic or Hall sensors, which provides square wave signals. The Main advantages of the two-phase feeding scheme are simple position sensor, simple control (dc input current control), and only one current sensor is needed. This scheme suffers from several drawbacks as follows: inherent torque pulsation due to commucation between motor phase currents, limited resolution for position control (in a direct drive system, the position resolution is 60°), and limited flexibility of torque ripple control. As will be seen later, the dc input current can be modulated to compansate for torque ripple. A high- resolution position sensor is then required. In the chapter five, three phase brusshless dc motor has been given. The PM synchronous motor is fed by a three-phase pulsewidth modulated (PWM) inverter. The current in motor phases are controlled by three current controllers. Reference input signals for the current controllers are locked in phase and frequency to the position sensor output signals- Several techniques of current control in a such system have been studied and described in the literature. In this scheme, a high-resolution position sensor is required to generate the waveform of the current reference while their amplitude is used as control variable for the motor torque. Note that arbitrary waveforms other than sinusoid can be generated. The advantages of this feeding scheme are high resolution for position control (suitable for direct- drive systems) and high flexibility of torque ripple control. The drawbacks of the three-phase feeding scheme are as a follows: a high- resolution position sensor is required (usually resolver or absolute encoder) which is more expensive than the simple position encoder used in the two- phase feeding scheme (this is not really a disavantages because the same position sensor can be used for position control); complex control; and three current sensors are required. Vtti The diferential equations have time - varying coefficients because of the sinusoid variations of mutual inductance with the displacement angle. If the power supply is balanced three-phase, as is usually true when fed by an inverter, the two- axis or in other words d-q theory is normal ly used for dynamic modeling. In this theory, the time- varying pareme ters are eliminated, and the variables and parameters are expressed in orthogonality or mutually decoupled direct ( d ) and quatrature(q) axis. From that point of view, by making use of the d-q or symmetrical Park transformation technique and in variance of the power, the time varying differential equations of the brushless dc motor are transformed into time invariant equations in (d-q-o) coordinates. The (d-q) dynamic model of a machine can be expressed in the either a stationary or a rotating referance frame. It is convenient here to summarise the advantages of these motors : (a) Long life, high reliability, fast response. (b) Small brushless d.c. motors are capable of efficiencies in excess of the usual 30 to 50 percent obtained from conventional motors with powers of the order of a few watts. Brushless d.c. motors can be designed for a rated power of up to 20 W with efficiencies reaching 80 percent. (c) Little or no maintenance, due to the elimination of the mechanical commutator and brushes. (d) The presence of the brush and the commutator itself in a conventional d.c. motor causes electrical and mechanical noise. However, brushless d.c. motors are much quieter and emit very little electrical noise. (e) A tachogenerator is not required for speed control. (f) Very high speeds and high power densities possible. Speeds up to 100.000 rev/min have been claimed. (g) Speed of motor not limited by the supply. Wide speed range. (h) Good speed regulation; good wow and flutter characteristic possible. (i) Conventional d.c. motor characteristics without the limitations of a mechanical commutator. iX (j) Simple speed control. It is convenient here to summarise the disadvantages of these motors : (a) Expensive because at present production quantities have been rather small. This will reduce with increasing demand. (b) Needs a rotor position sensor to fire semiconductor switching. (c) Difficulties with starting and low-speed running can arise with thyristor commutator, but not usually a problem with a transistor commutator. (d) Overload capacity low; about 150 to 200 pex'cent. This system is becoming increasingly attractive in servo and variable-speed applications since it can produce a torque-speed characteristic similar to that of a permanent magnet dc motor while avoiding the problems of brushes and mechanical commutation. There are two types of voltage source inverters which are generally used to drive the permanent magnet synchronous machine; the 120" and the 180*. The 120" inverter system offers advantages for certain applications. In particular, the inherently larger timing tolerances of the 120° inverter prevent inverter malfunction due to "shoot through". Also, with the 120" inverter it is possible to program the switching of the inverter using the back erof of the machine as reference. This avoids the use of optical encoders or Hall effect devices where upon the electronics necesarry to develop the position information can be located with the inverter, away from the actual machine.
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1993
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1993
Anahtar kelimeler
Benzetim, Doğru akım motorları, Simulation, Direct current motors