IV. Напишите предложения, заменив инфинитивный оборот простым сказуемым по образцу.
Образец. The separately-excited generator is known to be frequently used in automatic motor control systems. – The separately-excited generator is frequently used in automatic motor control systems.
1. Permanent-magnet fields are said to be used only in very small generators. 2. The field coils in a self-excited generator are known to be connected either in series with the armature coils, in parallel with them or partly in series and partly in parallel with the armature coils. 3. Separately-excited d.c. generators are known to have two circuits. 4. The separately-excited field is known to provide a means of controlling the power output of the generator.
V. Выразите согласие или несогласие с данными утверждениями, употребив выражения : You are quite right; I am afraid you are wrong (парная работа).
1. Permanent-magnet fields will be used only in very large machines. 2. The d.c. current in the field coils is called the "excitation current". 3. Separately-excited d.c. generators have one circuit. 4. In a separately-excited d.c. generator, the field is independent of the armature. 5. In a self-excited generator the field coils may be connected only in series with armature coils. 6. With a slight change in the field current, a large change in the load current resulted. 7. In the automatic motor control systems the field current is controlled by an amplifier.
VI. Употребив слова и словосочетания, расскажите о:
1) генераторе с самовозбуждением – self-excited d.c. generator, some of the generator output, field current, the field coils, in series, in parallel, partly, armature coils, is used to supply, may be connected.
2) генераторе с независимым возбуждением – separately-excited d.c. generator, field circuit, field coils, a separate d.c. source, armature circuit, armature coil, the load resistance, has, consisting of, connected across.
Text 8
The Series D.C. Generator
Self-excited generators use part of the generator’s output to supply excitation current to the field. These generators are classified according to the type of the field connection used.
In a "series" generator, the field coils are connected in series with the armature, so that the whole armature current flows, through both the field and the load. If the generator is not connected across a load, the circuit is incomplete and no current will flow to excite the field. The series field winding contains relatively few turns of wire.
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With no load, no current can flow and therefore very little e.m.f. will be induced in the armature – the amount actually produced depending on the strength of the residual magnetism. If a load is connected, current will flow, the field strength will build up, and the terminal voltage will increase. As the load draws more current from generator, this additional current increases the field strength, generating more voltage in the armature winding. A point is soon reached (A) where further increase in load current does not result in greater voltage because the magnetic field has reached saturation point.
The series generator is not used as ordinary generator, but the fact that its output voltage is proportional to its armature current makes it useful for some special applications.
Text 8a
The Shunt D.C. Generator
A shunt d.c. generator has its field winding connected in shunt (or parallel) with the armature. Therefore the current through the field coils is determined by the terminal voltage and the resistance of the field. The shunt field windings have a large number of turns, and therefore require a relatively small current to produce the necessary field flux.
When a shunt generator is started, the build-up time for rated terminal voltage at the brushes is very rapid, since field current flows even through the external circuit is open.
The illustration shows the schematic diagram and characteristic curve for the shunt generator. Observe that over the normal operating region of no load to full load (A-B), the drop in terminal voltage as the load current increases is relatively small. The shunt generator is therefore used where a practically constant voltage is desired, regardless of load changes. If the load current drawn from the generator increases beyond point B, the terminal voltage starts dropping off sharply. The generator should not be run beyond point B.
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Words and Word Combinations
| series generator | генератор с последовательным возбуждением |
| incomplete a | неполный, незаконченный |
| relatively adv | относительно |
| schematic 1. n 2. a | схема; схематичный |
| diagram n | схематичный чертеж |
| actually adv | фактически, на самом деле |
| resid u al a | остаточный, оставшийся |
| build up v | нарастать |
| build-up time | время нарастания |
| draw v | зд. получать |
| additional a | дополнительный |
| further a | дальнейший |
| saturation point | точка насыщения |
| reach v | достигать |
| useful a | полезный |
| ordinary a | обычный |
| shunt generator | генератор с параллельным возбуждением |
| field winding | обмотка возбуждения |
| determine v | определять |
| require v | требовать |
| flux n | поток |
| start v | пускать (машину) |
| rated a | номинальный, расчетный |
| rapid a | быстрый |
| open v | разомкнуть (цепь), отключить |
| characteristic n | характеристика |
| curve n | кривая |
| observe v | замечать |
| region n | зона; полоса; область |
| drop n | падение, понижение |
| drop (off) v | падать, понижаться |
| sharply adv | резко |
E x e r c i s e s
I. Прочитайте слова и определите, к какой части речи они относятся. Переведите их на русский язык :
excitation,connection, incomplete, excite, relatively, few, actually, residual, building, draw (drawn), additional, increase, further, saturation, increase, reach, proportional, useful, determine (determined), require, necessary, flux, rated, schematic, characteristic, curve, region, beyond, sharply, amount.
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