DC Motors
DC Motors
In general, DC motors are similar to DC generators in construction. They may be described as generators “run backwards.” When current is passed through the armature of a DC motor, a torque is generated by magnetic reaction, and the armature revolves. The action of the commutator and the connections of the field coils of motors are precisely the same as those used for generators. The revolution of the armature induces a voltage in the armature windings. This induced voltage is opposite in direction to the outside voltage applied to the armature, and hence is called back voltage. As the motor rotates more rapidly, the back voltage rises until it is almost equal to the applied voltage. The current is then small, and the speed of the motor will remain constant as long as the motor is not under load and is performing no mechanical work except that required to turn the armature. Under load the armature turns more slowly, reducing the back voltage and permitting a larger current to flow in the armature. The motor is thus able to receive more electric power from the source supplying it and to do more mechanical work.
Because the speed of rotation controls the flow of current in the armature, special devices must be used for starting DC motors. When the armature is at rest, it has virtually no resistance, and if the normal working voltage is applied, a large current will flow, which may damage the commutator or the armature windings. The usual means of preventing such damage is the use of a starting resistance in series with the armature to lower the current until the motor begins to develop an adequate back voltage. As the motor picks up speed, the resistance is gradually reduced, either manually or automatically.
The speed at which a DC motor operates depends on the strength of the magnetic field acting on the armature, as well as on the armature current. The stronger the field, the slower is the rate of rotation needed to generate a back voltage large enough to counteract the applied voltage. For this reason the speed of DC motors can be controlled by varying the field current.
Words and expressions
similar in construction - одинаковые по конструкции
torque - вращающий момент
magnetic reaction - магнитная реакция
revolve - вращать
commutator - коммутатор; переключатель
connection - соединение
field coil - катушка индуктора
revolution of the armature - вращение якоря
induce - индуцировать
back voltage - обратное напряжение
under load - под нагрузкой
constant - постоянный
resistance - сопротивление
adequate - адекватный
gradually reduce - постепенно снижать
manually - в ручную
automatically - автоматически
strength of the magnetic field - сила магнитного поля
rate of rotation - частота вращения
Exercise 1
Ответьте на вопросы:
- Are DC motors similar to DC generators in construction?
- Is torque generated by magnetic reaction when current is passed through the armature of a motor?
- Are the action of the commutator and the connections of motors and generators the same?
- Does the revolution of the armature induce a voltage in the armature windings?
- Is induced voltage opposite in direction to the outside voltage applied to the armature?
- When does the back voltage rise until it is almost equal to the applied voltage?
- Does the armature turn more slowly under load?
- How does back voltage reduce a larger current flow in the armature?
- What devices must be used for starting DC motors?
- Does the speed at which DC motors operate depend on the strength of the magnetic field acting on the armature?
- Is it right that the stronger the field, the slower is the rate of rotation?
12.How can the speed of DC motors be controlled?
Exercise 2
Найдите в тексте английские эквиваленты следующих слов и выражений:
Двигатели и генераторы постоянного тока; конструкция; ток проходит через якорь двигателя; магнитная реакция; обмотка двигателя; индуцирует напряжение; напряжение противоположенное в направлении; обратное напряжение; скорость двигателя; постоянный источник; механическая работа; скорость вращения; поток напряжения; специальные устройства; пуск двигателя; повредить коммутатор и обмотку якоря; набирать скорость; сопротивление постепенно снижается; сила магнитного поля.
Exercise 3
Составьте предложения, используя данные слова и словосочетания:
- Motors: similar; construction; generators; in general.
- Torque; magnetic; generate; relation; armature; revolve.
- Voltage; induce; winding; armature; revolution; voltage.
- Rotate; motor; rapidly; voltage; back; equal; apply; rise.
- Load; slowly; turn; under; armature.
- Speed; control; rotation; current; flow; armature.
- Magnetic; field; speed; operate; motor; depend; act; armature.
- Speed; can be controlled; vary; current; field.
Exercise 4
Переведите на английский язык следующие предложения:
- Двигатели и генераторы постоянного тока, в общем, схожи по своей конструкции.
- Индуцированное напряжение противоположено в направлении внешнему источнику напряжения.
- Скорость двигателя остается постоянной до тех пор пока он не находится под нагрузкой.
- Двигатель получает электрическую энергию от внешнего источника.
- Скорость вращения двигателя контролирует поток напряжения в якоре.
- По мере ускорения двигателя, сопротивление постепенно уменьшается.
- Рабочее напряжение двигателя рано 220 вольт.