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.
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Words and Word Combinations
| remember v | помнить |
| thereby adv | таким образом, с помощью (посредством) этого |
| applied p.p. | приложенный (о напряжении) |
| terminal voltage | конечное напряжение, напряжение на зажимах |
| back-e.m.f. | противоэлектродвижущая сила |
| actually adv | фактически, на самом деле |
| difference n | разность |
| internal a | внутренний |
| just adv | только (что) |
| effectively adv | эффективно |
| temporary a | временный |
| starting resistance | пусковое сопротивление |
| put v | вводить |
| keep v | сохранять, держать, поддерживать |
| within prp | внутри, в |
| gradually adv | постепенно |
| allow v | допускать |
| completely adv | совершенно, полностью |
| short out of | зд. выводить из |
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 :
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чем быстрее скорость вращения, тем больше будет; из-за внутреннего падения напряжения; временное (стартовое) сопротивление; можно постепенно понижать; дальнейшее увеличение скорости; при нормальной скорости.
III. Выберите из правого столбика правильные окончания предложений.
| 1. As the armature in a d.c. motor rotates, so the armature coils cut the magnetic field and ... | the speed and direction of rotation |
| 2. This back-e.m.f. depends on ... | because of the internal voltage drop |
| 3. The back-e.m.f. will always be less than the applied voltage ... | in order to keep the current flow within safe limits |
| 4. The starting resistance must be put in series with the armature ... | the starting resistance can be gradually reduced |
| 5. As the motor speeds up ... | thereby induce a voltage |
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