Методическое пособие для студентов iv курса очного и заочного обучения специальности 26. 05. 07 "Эксплуатация судового электрооборудования и средств автоматики"
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UNIT III. D.C. MOTOR 1. Read and remember the following words and word combinations.
2. Read and translate the text. D.C. MOTOR When a D.C. machine is operating in a generator mode, the conductors of the armature winding across which the current is flowing interact with the magnetic flux of the poles. As a result, the electromagnetic force opposing the armature rotation is produced. In order to overcome this force, the external force should be exerted upon the generator armature. If the external force is removed, the polarity of the poles is maintained and the current of the same direction is passed through the armature winding, the electromagnetic force will preserve the same direction. When acted upon by the electromagnetic force, the armature will rotate in the direction opposing that of the generator rotation, and hence the D.C. machine will change over to a motor mode. The property by virtue of which any D.C. machine may work either in a generator mode or in a motor mode is called convertibility. While starting the D.C. motor, its armature is stationary (n = 0) and hence the counter e.m.f. E = 0. The starting armature current is many times larger than the rated current, which may be explained by a small quantity of the armature resistance. Such a current is dangerous for the armature winding and the machine as a whole. In order to limit the starting current, an additional resistor (the starting rheostat) is placed in series with the armature circuit. As the speed of rotation increases, the counter e.m.f. rises, but the armature current rapidly decreases. As the armature current decreases, the armature resistance is reduced. Towards the end of starting, the starting rheostat should be completely withdrawn. There exist three methods of D.C. motor speed regulation: 1) by varying the connected voltage, owing to which stepless regulation can be accomplished (the method is used in adjustable-potential systems); 2) by varying the resistance in the armature circuit; the regulation is step-by-step, not economical; it is obtained by putting an adjusting rheostat in series with the armature circuit; 3) by varying the magnetic flux; the regulation is stepless, wide- ranged; it is attained by changing the field current by means of an adjusting rheostat placed in the field circuit. In order to alter the direction of the motor rotation, it is necessary to change the current direction either in the armature or in the field winding. If the current direction in both of them is varied, the reversal will never take place. In practice the change in the current direction of the armature usually applies. 3. Match the following English and Russian word combinations. a) to interact with the magnetic flux, to overcome the force, to maintain, to preserve the same direction, speed regulation, motor mode, to accomplish the regulation, to limit the starting current, to alter the direction b) преодолеть силу, выполнять регулировку, сохранять полярность, взаимодействовать с магнитным потоком, двигательный режим, сохранять одно и то же направление, ограничить пусковой ток, менять направление, регулирование частоты вращения 4. Answer the questions. 1. How is the electromagnetic force produced? 2. What should be done to overcome the electromagnetic force? 3. What are the conditions under which the electromagnetic force preserves the same direction? 4. How will a D.C. machine change over to a motor mode? 5. What kind of property is called convertibility? 6. How is the counter e.m.f. induced in the armature winding? 7. In what way is the counter e.m.f. defined? 8. Why is the starting armature current dangerous for the armature winding and the machine as a whole? 9. What should be done in order to limit the starting current? 10. What should be done to the starting rheostat towards the end of starting? 11. What are three methods of the D.C. motor speed regulation? 12. Which methods are used to attain stepless regulation? 13. Which method is used to accomplish step-by-step regulation? 14. What is to be done to alter the direction of the motor rotation? 15. Does the change in the current direction of the armature or of the field winding apply in practice? 5. Open the brackets using the correct verb form. Translate the sentences. 1. If the external force is removed, the electromagnetic force . . (preserve) the same direction. 2. If we don't change the field current, we . . . (not accomplish) stepless wide-ranged speed regulation. 3. If the armature rotates in the direction opposing that of the generator rotation, the D.C. machine . . . (change over) to a motor mode. 4. When the armature is acted upon by the electromagnetic force, it . . . (rotate) in the direction opposing that of the generator rotation 5. If we don't change the current direction either in the armature in the field winding, the reversal . . . (not take place). 6. As soon as we place a starting rheostat in series with the armature circuit, we . . . (limit) the starting current. 7. If we don't maintain the polarity of the poles, the electromagnetic force . . . (not preserve) the same direction. 8. When the speed of rotation increases, the counter e.m.f. . . . (rise) but the armature current . . . rapidly (decrease). 6. Complete the sentences using one of these verbs in Present Simple Passive.
1. The electromagnetic force opposing the armature rotation .... 2. When the D.C. motor armature is rotating, the counter e.m.f . . . in the armature winding. 3. If the external force . . . , the polarity of the poles . . . and the current of the same direction ...the armature winding, the electromagnetic force will preserve the same direction. 4. The counter e.m.f. ...as much the same way as the generator e.m.f. 5. The armature current and the speed of the armature rotation . . . from the following equations. 6. In order to limit the starting current, an additional resistor . . . in series with the armature circuit. 7. A step-by-step speed regulation ...by putting an adjusting rheostat in series with the armature circuit. 8. If the magnetic flux . . . , a stepless wide-range regulation .... 9. Towards the end of starting the starting rheostat . . . completely . . . from the armature circuit. 7. Speak about D.C. motor. |