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 IV. TYPES OF D.C. MOTORS1. Read and remember the following words and word combinations.
2. Read and translate the texts. 1. SHUNT-WOUND MOTORS This term is referred to a particular type of D.C. motor having the field winding connected in parallel with the armature winding. This type of motor exhibits the following properties: 1. With the load increase from zero to its rated value the speed of rotation is altered moderately - by 1 - 5% of its rated value; predominantly at the expense of the voltage drop in the armature. Consequently, the speed characteristic curve is "rigid", i.e., slightly inclined. With the change in the load the speed of rotation of a shunt-wound motor is kept constant. This can be explained by the fact that the field current and hence the flux remain unchangeable. 2. The motor may be started idle. Stepless speed regulation within wide limits is accomplished by putting both a regulating rheostat in the field current and an additional resistor in the armature circuit. 3. The starting torque of the motor is rather small, directly proportional to the armature current; this being so, the current consumed from the mains is proportional to the load on the shaft. The motor torque characteristic makes a straight line. By virtue of the fact that a shunt-wound motor features a small starting torque, a "rigid" speed characteristic curve and a stepless speed regulation, it finds application in electric drives of fans, pumps, compressors and other mechanisms. 2. SERIES-WOUND MOTORS A series-wound motor possesses a field winding connected in series with the armature circuit. A series-wound motor manifests the following properties: 1. While running idle or at load which is 20 - 25% less than the filed one, the motor tends to overspeed. The reason is that at light loads the current value both in the armature circuit and in the field winding is small and, therefore, the flux and the voltage drop in the field winding are both moderate and, as a consequence, the speed of rotation is significant. With the load increase the current value in the field winding and thus the flux rise. Moreover, the internal voltage drop grows and consequently the speed of rotation abruptly drops off. The motor speed characteristic makes up a "mild" curve, i.e., considerably inclined. The motor is only started under load. 2. The starting torque of a series motor is large, proportional to the square value of the armature current. 3. The regulation of the speed of rotation is accomplished by several methods, but the one which is economically attractive is performed by shunting the field winding, i.e., placing an adjusting rheostat in its parallel. As a result, the speed of rotation shows an increase when compared to the rated one. A series-wound motor is used where a large starting torque is required. They drive haulage mechanisms (starters, turning gears etc.) 3. COMPOUND-WOUND MOTORS This term relates to a specific type of D.C. motor which has two field windings: the shunt one and the series one. According to the method of the field winding connection the above mentioned motors may be either differential compound or cumulatively compound. A differential compound motor has the fluxes directed in opposition. It features a small starting torque and, therefore, hasn't found application on shipboard. A cumulative compound motor has the fluxes that add up. This type of D.C. motor is subclassified into series-compound and shunt-compound. Series-compound motors are those provided with a predominant series field winding and an auxiliary shunt field winding. In properties, they are quite similar to series-wound motors which manifest a considerable starting torque. Nevertheless, series-compound motors while running idle don't tend to overspeed because of their shunt field winding. They are used for driving windlasses, capstans, cranes etc. Shunt-compound motors are those which have the shunt field winding as predominant and the series field winding as auxiliary. In properties, they are similar to shunt-wound motors, but exhibit a considerable starting torque. Shunt-compound motors are in use in electric drives of lathes, pumps etc. 3. Match the following English and Russian word combinations. a) speed of rotation, rated value, to start the motor idle, to exhibit the property, to tend to overspeed b) запускать двигатель вхолостую, скорость вращения, стремиться уйти в разнос, проявлять свойство, номинальное значение 4. Answer the questions. 1. What is the term of "shunt-wound motor" referred to? 2. How is the speed of rotation of a shunt-wound motor altered with the load increase? 3. Is it possible to start a shunt-wound motor idle? 4. How is stepless speed regulation of a shunt-wound motor accomplished? 5. What can you say about the starting torque of a shunt-wound motor? 6. Where exactly on board ship do shunt motors find application? 7. What do you call "series-wound motor"? 8. Why does a series-wound motor tend to overspeed? 9. What can you say about the starting torque of a series-wound motor? 10. What is shunting? Is shunting economically attractive? 11. What mechanisms do series-motors drive? 12. What do you call "compound-wound motor"? 13. What is the difference between a cumulative compound motor and a differential compound motor? 14. What type of compound motor is called "series-compound motor"? Where is it used? 15. What type of compound motor is called "shunt-compound motor"? Where is it used? 5. Put the verbs in brackets in the correct form - Present Simple Active or Present Simple Passive. 1. Fig 1 ... (illustrate) the diagram of a shunt-wound motor. 2. This type of D.C. motor . . . (exhibit) the following properties. 3. The speed of rotation of a shunt-wound motor . . . (keep) constant. 4. The field current and hence the flux . . . (remain) unchangeable. 5. Stepless speed regulation within wide limits . . . (accomplish) by putting a regulating rheostat in the field circuit and an addition resistor in the armature circuit. 6. The motor torque characteristic . . . (make) a straight line. 7. While running idle a series motor . . . (tend) to overspeed. 8. The motor . . . only . . . (start) under load. 9. The speed regulation of a series motor . . . (perform) by shunting the field winding. 10. A series motor . . . (use) where a large starting torque . . . (require). 11. A differential compound motor . . . (feature) a small starting torque. 12. This type of D.C. motor . . . (subclassify) into series- compounded and shunt-compounded. 13. Series-compound motors . . . (provide) with a predominant series field winding and an auxiliary shunt field winding. 14. Series-compound motors . . . (not tend) to overspeed while running idle. 15. Shunt motors . . . (find) application in the electric drives of fans, pumps, compressors etc. 6. Speak about types of D.C. motors. |