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VI. Выразите согласие или несогласие с данными утверждениями (парная работа).
1. The series motor runs very slowly with light loads. 2. If the load is completely removed, the motor will speed dangerously and fly apart. 3. Series motor must always be run under no-load conditions. 4. Series motors are variable-speed motors. 5. Thus the series motor has a low starting torque. 6. If there is an opposing torque on starting, the motor will accelerate furiously.
VII. Переведите слова второго собеседника в диалоге. 1. Would you tell me how does the series motor run with different loads? 2. Он работает очень медленно при больших нагрузках и очень быстро при малых. 1. What will happen (произойдет) if the load completely removed? 2. Он будет опасно набирать скорость и разлетится на части. 1. So, can series motor be run under no-load conditions? 2. Нет, двигатели с последовательным возбуждением никогда не должны работать в условиях без нагрузки. 1. And what can you say about the speed of the series motors? 2. Их скорость значительно изменяется, когда меняется нагрузка.
VIII. Расскажите о вращающем моменте двигателя с последовательным возбуждением, употребив данные слова и словосочетания:
the torque, developed by any d.c. motor, armature current, field strength, very high, motor speed, low or zero, because of its high starting torque, never, unloaded, depends on, is, is starting, must be started.
Text 4 Compound Motors A compound motor is a combination of series and shunt motor. The field consists of two separate sets of coils. One set, whose coils are wound with many turns of fine wire, is connected across the armature as a shunt field. The other set, whose coils are wound few turns of heavy wire, is connected in series with the armature as a series field. The characteristics of the compound motor combine the features of the series and shunt motors. Cumulatively-compound motors, whose series and shunt fields are connected to aid each other, are the most common. In a cumulatively-compound motor, an increase in load decreases the speed and greatly increases the developed torque. The starting torque is also large. Thus the cumulative compound motor is fairly constant-speed motor, with excellent pulling power on heavy loads, and good starting torque. In a differentially-compound motor the series field opposes the shunt field, and the total field is weakened when the load increases. This allows the speed to increase with increased load, up to a safe operating point. The starting torque is very low, and the differentially-compound motor is rarely used.
Words and Word Combinations
E x e r c i s e s I. Прочитайте и переведите следующие слова и словосочетания: compound, combination, wound, characteristics, features, cumulatively, large, excellent, pulling power, differentially, weaken (weakened), safe, rarely, fairly, oppose – opposes. II. Найдите в тексте эквиваленты следующих слов и словосочетаний: виток, тонкая проволока, несколько витков толстой проволоки, двигатель с постоянной скоростью, очень низкий. III. Вставьте нужное слово. 1. A compound motor is a... of series and shunt motor (combination, assembly). 2. The field consists of... separate sets of coils (many, two). 3. In a cumulatively-compound motor, an increase in load... the speed (increases, decreases). 4. In a differentially-compound motor... increases with increased load (torque, speed). 5. The differentially-compound motor is... used (rarely, often). IV. Переведите слова второго собеседника в диалоге. 1. What can you say about a compound motor? 2. Я знаю, что этот двигатель – соединение двигателей с последовательным и параллельным возбуждением. 1. And what are the characteristics of the compound motor? 2. Характеристики двигателя смешанного возбуждения сочетают основные черты двигателей последовательного и параллельного возбуждения. V. Расскажите о: 1) компаундном двигателе с согласно включенными обмотками возбуждения и 2) компаундном двигателе со встречно включенными обмотками возбуждения, употребив данные слова и словосочетания: 1) cumulatively-compound motor, series and shunt fields, each other, the most common, the starting torque, also, large,are connected to aid, are, is; 2) in a differentially-compound motor, series field, shunt field, total field, when the load, the starting torque, very low, opposes, is weakened, increases, is.
Text 5 Back-E.M.F. Remember that in a d.c. motor, as the armature rotates, so the armature coils out the magnetic field and thereby induce a voltage or electromotive force in these coils. Since the induced voltage opposes the applied terminal voltage, it is called the back-e.m.f. This back-e.m.f. depends on the same factors as the generated e.m.f. in the generator – the speed and direction of rotation, and the field strength. The stronger the field and the faster the rotating speed, the larger will be the back-e.m.f. But the back-e.m.f. will always be less than the applied voltage, because of the internal voltage drop caused by the resistance of the armature coils. What actually moves the armature current through the armature coils is the differences between the voltage applied to the motor and the back-e.m.f. The internal resistance of the armature of a d.c. motor is very low, usually less than one ohm. If this resistance were all that limited the armature current, this current would be very high. When the motor is just starting and the back-e.m.f. is too small to limit the current effectively, a temporary resistance called the "starting resistance" must be put in series with the armature in order to keep the current flow within safe limits. As the motor speeds up, the back-e.m.f. increases and this starting resistance can be gradually reduced, allowing a further increase in speed and back-e.m.f. At normal speed, the starting resistance is completely shorted out of the circuit.
Words and Word Combinations
Note. The stronger the field and the faster the rotating, the larger will be the back-e.m.f. – чем сильнее поле и быстрее скорость вращения, тем больше будет противоэлектродвижущая сила.
E x e r c i s e s I. Прочитайте и переведите слова и словосочетания: induce – induced, thereby, oppose – opposes, generated, rotation, strength, always, because of, caused by the resistance, actually, difference, usually, just, too small, in order to, keep, allow – allowing, further.
II. Найдите в тексте английские эквиваленты следующих слов и словосочетаний: чем быстрее скорость вращения, тем больше будет; из-за внутреннего падения напряжения; временное (стартовое) сопротивление; можно постепенно понижать; дальнейшее увеличение скорости; при нормальной скорости.
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