Doğrultucudan beslenen doğru akım motorunun davranışının incelenmesi

Abstract

Doğrultucudan beslenen bir doğru akım motorunun davranışı deneysel olarak incelenmiştir. Motor ilk olarak doğru gerilimle, daha sonra Uç fazlı kontrolsüz yarım dalga doğrultucu ile ve son olarak da bir fazlı kontrolsüz köprü doğrultucu ile beslenmiştir. Bu Uç gerilimle beslenen motorun tam yükte, ısınması, verimi ve gürültü sınırı deneysel olarak incelenmiştir. Bu deneylerde kullanılan motor 5.5 kW, 330 V, 39 A, 1500 d/d nominal değerlerdedir. Sonuç olarak elde edilen değerler her Uç durum için karşılaştırmalı olarak verilmiştir.

Numerous textbooks have been written over the last century on the design, theory, and operation of dc machines. One can add little to the analytical approach which has been used for years and with which all electrical engineers are familiar. Perhaps the largest change in the applications of dc machines has come about since the early 1960s. The development of high power, silicon controlled rectifier CSCRD, which is commonly called the thyristor, provided a means to achieve a fast responding, reliable, adjustable dc supply. Consequently, ac-to-dc converters have replaced the dc generator in many applications especially in the area of speed control. In fact, nearly all variable speed dc-drive systems manufactured today are thyristor dc-drive systems consisting of either a single or 3- phase thyristor, phase controlled converter, and a dc motor. The modern electrical engineer working with control system involving electromechanical devices must be aware of the basic characteristics of thyristor dc drives. We can write the field and armature voltage equations of dc machine in generally. r +pL f ^ FF co L r AF r +pL a r AA Where, L _and L are self inductances of the field and armature windings,r and r are the resistance, p is the oper at or d/dt. Direct current Cdc3 motors have been used in -vi- variable speed drives for a long time. The versatile control characteristics of dc motors have contributed to their extensive use in industry. Dc motors can provide high starting torques, which are required for traction drives. Control over a large speed range both below and above the rated speed can be easily achived. The methods of control are simpler and less expensive than those of alternating current CacD motors. Although commutators prohibit their use in certain applications» dc motors play a significant role in many industrial drives. The technology of speed control of dc drives has evolved considerably. The present state of the art of dc motor drives is far advanced compared to what it was just a quarter century ago. During the last century the available power supply was constant voltage direct current, and the use of electric power to any significant extent was confined to larger cities. In industrial plants, located in urban areas, dc motors were used to drive machinary. The dc motors were oparated from a constant volotage bus. Variable speed drives were made possible only by the adjustment of motor field flux. This in turn produced such severe problems as commutator sparking and the consequent limitation on the life of brushes and commutators. In fact, The phenomenon of commutation was not well under stood at the time. As a result, most drives were constant speed drives. Any variables speed drives, achieved by changing field flux, used rugged durable motors that generally had poor mechanical and electical time constant. In phase controlled dc drives an ac- to-dc phase controlled converter is used to control the dc drive motor. The converter changes the ac input voltage to a controllable dc output voltage. In such converters thyristor commutation is easily achieved by a process referred to as natural or line commutation. When an incoming thyristor is turned on, it immediately reverse biases the outgoing thyristor and turns off. Phase controlled converters are therefore simple and less expensive and are axtensively used in industries. Power conversion efficiency in these converters is generally high, above SS95, because of relatively low losses in thyristors. In smaal sizes they are used as speed controllers for fractional horsepower dc motors. Phase controlled converters are broadly classified as single-phase converters and three-phase converters. The converter used for a particular application depends on such factors as supply available Cl# or 340, rating of the drive, need for regeneration, etc. -vii- Semi converters are one quadrant converters, that is, they have one polarity of voltage and current at the dc terminals. Full cover ters are two quadrant converters in which voltage polarity can reverse, but current remains unidirectional because of the unidirectional thyristor. Dual converters can oparate in all four quadrants. Regeneration of power, that is, power from motor to the ac supply, is possible with full converters. Where regeneration is not required, semi converters are used for the sake of economy. A diode is sometimes used across the motors terminals. This diode is referred to as a free wheeling diode. It dissipates the stored energy in the motor inductance when the thyristor blocks. It may also provide protection against transients. In the single phase half wave circuit, the motor current is always discontinuous. This causes the performance of the drive to deteriorate. In other single phase converters, The motor current may or may not be dis continuous. It depends on the operating conditions. Large horse power dc drives take power from three phase sources. In such drives the drive motor is controlled by three phase controlled converters. The ripple frequency of the motor terminal voltage is higher than that of the single phase converters. Consequently the filtering requirements for smoothing out the motor current are less. The motor current is mostly continuous, and therefore the motor performance is better compared to single phase drives. The three phase half wave converter is impratical for most purposes because the supply currents would contain dc components. Semi converters and full converters are most commonly used in practice. Dual converters are used in reversible drives having power ratings as high as several megawatts. In Chapter 4, the behavior of a dc motor examined by means of three different supply voltages. The effect of the changes at the supply voltages, on the motors heating, current, efficiency and also noise were studied. For this purpose, a dc machine with the following data was used, Power : 5.5 kW Armature voltage : 220 V Armature current : 29 A -viii- Rotor speed Field voltage Field current : 1500 min : 220 V : 0.82 A -1 First of all, a pure direct voltages was applied to the terminal at the machine and, heating, efficiency, noise were determined. Then a three-phase half wave uncontrolled rectifier used and the changes on the above parameters were examined.