Direct Current (DC) Generators
Direct-Current (DC) Generators
If an armature revolves between two stationary field poles, the current in the armature moves in one direction during half of each revolution and in the other direction during the other half. To produce a steady flow of unidirectional, or direct, current from such a device, it is necessary to provide a means of reversing the current flow outside the generator once during each revolution.
In older machines this reversal is accomplished by means of a commutator, a split metal ring mounted on the shaft of the armature. The two halves of the ring are insulated from each other and serve as the terminals of the armature coil. Fixed brushes of metal or carbon are held against the commutator as it revolves, connecting the coil electrically to external wires. As the armature turns, each brush is in contact alternately with the halves of the commutator, changing position at the moment when the current in the armature coil reverses its direction. Thus there is a flow of unidirectional current in the outside circuit to which the generator is connected. DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and commutator that occurs at high voltage. The highest potential commonly developed by such generators is 1500 V. In some newer machines this reversal is accomplished using power electronic devices, for example, diode rectifiers.
Modern DC generators use drum armatures that usually consist of a large number of windings set in longitudinal slits in the armature core and connected to appropriate segments of a multiple commutator. In an armature having only one loop of wire, the current produced will rise and fall depending on the part of the magnetic field through which the loop is moving. A commutator of many segments used with a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field, and as a result the current delivered by the armature windings is virtually constant. Fields of modern generators are usually equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. Sometimes smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.
DC generators are commonly classified according to the method used to provide field current for energizing the field magnets. A series-wound generator has its field in series with the armature, and a shunt-wound generator has the field connected in parallel with the armature. Compound-wound generators have part of their fields in series and part in parallel. Both shunt-wound and compound-wound generators have the advantage of delivering comparatively constant voltage under varying electrical loads. The series-wound generator is used principally to supply a constant current at variable voltage. A magneto is a small DC generator with a permanent-magnet field.
Words and expressions
stationary field poles - стационарные магнитные
полюсы
half of each revolution - половина каждого вращения
steady flow - стабильный поток
unidirectional - однонаправленный
each revolution - каждое вращение
metal ring - металлическое кольцо
shaft of the armature - ось якоря
two halves of the ring - две половины кольца
serve as the terminal - выполняют роль клеммы
armature coil - обмотка якоря
brush - щетка
connecting the coil - соединяя обмотку
external wire - внешний провод
reverses its direction - обратно направленный
outside circuit - внешняя цепь
sparking between brushes - искра между щетками
highest potential - самый высокий потенциал
power electronic devices - силовые электронные приборы
diode rectifier - диодный выпрямитель
drum armature - барабанный якорь
longitudinal slits - продольные шлицы, щели
loop - контур, виток
high-intensity area - высоко интенсивный участок
electromagnetic poles - электромагнитное поле
interpoles - промежуточный (добавочный)
полюс
distortion in magnetic flux - искажение (смещение) в
магнитном потоке
series-wound generator - генератор последовательного
возбуждения
shunt-wound generator - генератор параллельного
возбуждения
connected in parallel - соединенный параллельно
compound-wound generator - генератор со смешанным
возбуждением
constant voltage - постоянное напряжение
electrical load - электрическая нагрузка
permanent-magnet field - постоянное магнитное поле
Exercise 1
Ответьте на вопросы:
- How does the current in the armature move if it revolves between two stationary field poles?
- What should be done to produce a steady flow of direct current from generator?
- Is in older machines the flow of direct current provided by means of a commutator?
- Is there a flow of unidirectional current in the outside circuit to which the generator is connected?
- Are DC generators usually operated at fairly low voltages?
- Why DC generators usually operate at low voltages?
- Do drum armatures usually consist of a large number of windings?
- Why the fields of modern generator equipped with four or more electromagnetic poles?
- 9. How DC generators are commonly classified?
- Is the series-wound generator used mainly to supply a constant current at variable voltage?
Exercise 2
Найдите в тексте английские эквиваленты следующих слов и выражений:
Два полюса; напряжение в якоре; выражение; стабильный поток; направленный в одном направлении; реверсировать поток напряжения; достигать посредством; металлическое кольцо; изолированный друг от друга ; служит в качестве клеммы; большое количество обмоток; продольные щели; контур провода; внешняя цепь; высокоинтенсивный участок поля; современные генераторы обычно оборудованы; классифицировать; электрические нагрузки.