الفهرس | Only 14 pages are availabe for public view |
Abstract The direct current machine was manufactured before the begining of this century when the available power supply was direct current.Today, almost all electrical power-supply networks are ac systems, and it is accepted practice in industry to employ ac motors wherever they are inherently suitable or can be given appropriate characteristics by means of power electronic devices. Yet there remain important fields of application in which de machines offer economic and technical advantages. Separately excited de motor drives are still chosen in the industrial field for their simplicity in control especially when the wide range and accurate speed control are necessary. On the other hand the de motor requires special measures for starting, except for the smallest motor. The commutator of de motor consists of large number of copper segments separated by thin sheets of mica insulation . This increases the construction cost of the de motor and reduces the power/weight ratio. DC motor requires periodic maintenance, therefore it cannot be used at all in certain environmental conditions e.g. in submerged pump and in some chemical/gases plants. The maximum voltage between the segments of a commutator is about 20 volts for successful commutation. Therefore the total armature voltage is limited to a maximum of about 1500 volts. But the advantages of de motor are enough to make these disadvantages insignificant. The tremendous progress in semiconductor technology have made solid state de motor drives suitable for many practical applications. Such drives has a lot of advantages. Among of these are very low speed operation down to 1 r.p.m , fast response, high efficiency and cheapness except for power ratings below 10kw. |