Урок 8. Unit 8.

1. Прочитайте и запомните следующие слова и словосочетания.

a single-turn loop – одновитковая обмотка, контур;

a commutator – коллектор (электрической машины);

a brush assembly – узел щеткодержателя, щеточный узел;

flux lines – линии магнитного потока;

a permanent magnet motor – двигатель с постоянными магнитами;

fractional horse power – маломощный;

a wound armature – якорь с простой обмоткой;

a torque – крутящий момент, вращающий момент;

a shunt motor – двигатель параллельного (шунтового) возбуждения;

DC input terminals – терминалы ввода постоянного тока;

supply voltage – напряжение (источника) питания;

field coils – катушка обмотки возбуждения, обмотка возбуждения, катушка

электромагнита, индукторная катушка, поляризующая обмотка;

a winding – обмотка;

an EMF (an electromotive force) – ЭДС, электродвижущая сила;

a counter electromotive force – противоэдс;

resistance – (активное) сопротивление;

applied voltage – приложенное напряжение;

induced voltage – наведенное напряжение;

a series motor – двигатель последовательного возбуждения;

a starting torque – пусковой момент;

to be at rest – находиться в покое;

to gain speed – набирать, увеличивать скорость;

to race – набирать скорость, разгоняться;

to disintegrate – разрушаться, распадаться;

to crank the engine – приводить в рабочее состояние, заводить двигатель;

to be set back of – сдвигать назад от геометрической нейтрали;

a field core – сердечник обмотки возбуждения;

laminated – слоистый, многослойный, листовой, пластинчатый;

eddy current losses – потери на вихревые токи;

a starter – пусковое устройство;

a compound motor – двигатель постоянного тока смешанного возбуждения,

компаундированный двигатель;

a differential compound motor – двигатель смешанного возбуждения со

встречно включенными обмотками;

a cumulative compound motor – двигатель смешанного возбуждения с

согласно включенными обмотками;

a stabilized shunt motor – стабилизированный двигатель параллельного

возбуждения;

a shunt field – параллельная обмотка возбуждения;

a series field - последовательная обмотка возбуждения, поле

последовательной обмотки;

to interact – взаимодействовать;

to magnify – увеличивать;

clockwise – по часовой стрелке;

to insert – вставлять;

2. Прочитайте текст 8 “Motors”. Найдите в тексте предложения, в

которых глагол-сказуемое выражен глаголом в страдательном залоге и объясните особенности перевода таких предложений.

5. MOTORS

A basic DC motor consists of poles which set up a magnetic field, an armature made of a single-turn loop and a commutator, and a brush assembly. Each coil side of the loop lies in the magnetic field.

When a DC voltage is applied to the brushes, a current flows around the loop. The magnetic field produced by the flow of current interacts with the magnetic field produced by the poles of the magnet. The resultant magnetic field is represented by flux lines.

Small permanent magnet motors are used to power toys, movie cameras, and many other devices. The permanent magnet limits their size. This means their power will be limited to fractional horse power. They work only on DC. What would happen if AC were applied?

The permanent magnet motor has a permanent magnet for a fixed magnetic field. It has a commutator, brushes and a wound armature. Not only does the permanent magnet motor show the basic principles of a DC motor in its simplest form, it also has practical applications.

The loop in the motor rotates clockwise under the influence of the magnetic field. It may be noted that the force acting in one direction (on one side of the loop) and the force acting in the other direction (on the other side of the loop) combine to cause the coil to turn on its axis. The loop thus acts as if it were a lever with a turning force, or torque, at each end. Because of this lever arrangement, the force at each end is magnified by the distance from the center. Thus, the torque is equal to the combined force on the two sides of the loop, multiplied by the distance of the conductors from the axis about which the loop rotates. The greater the torque, the more pool the motor has to drive a mechanical load.

Structurally, a shunt motor is the same as a shunt generator. The field coils of a shunt motor are connected directly across the DC input terminals, and when the supply voltage remains constant, the current through the field coils remain constant. As a result, the magnetic field is also constant and the torque of the shunt motor varies only with the current through the armature windings.

Whenever an armature rotates in a magnetic field, an EMF is induced in the armature coils. During rotation, the armature windings cut the lines of force and an EMF is induced in the windings. Therefore there’s a motor and generator action at the same time. The direction of the induced EMF opposes the applied EMF, and for this reason is known as a counter electromotive force, or CEMF.

In a shunt motor, the amount of current that flows through the armature depends upon the resistance between the applied voltage and the induced voltage (CEMF). The CEMF, in turn, depends solely upon the armature speed because the armature coils are fixed to the armature core and the field does not vary.

A series motor is structurally the same as a series generator. The series motor is adapted for giving a very high starting torque. Actually, the torque of this motor varies approximately as the square of the current. Remember, in a series motor, current flows through its series-connected armature and field coils. If the armature current is doubled, the flux is also doubled. Hence, the torque, which is proportional to the current times the flux, is increased many times.

When an armature is at rest, the armature current (and therefore the torque) is at a maximum, because no CEMF is generated in the coils, and the current is limited only by the field coils. As the armature gains speed, the CEMF increases, decreasing the armature current and torque. If additional load is applied to the motor, the armature slows down, the generated CEMF is decreased, a greater current flows, and a greater torque is produced. The speed of the motor is controlled by the load and, if the load is removed, the motor will race dangerously until centrifugal force causes the armature to disintegrate.

Series motors are generally used only where the load is constantly applied, and a good starting torque is required. An example of this is an automobile engine starter, where high torque is needed for cranking the engine. Hoists, street cars, and other devices use a series motor.

One of the advantages of a series motor is its capability for operating on either DC or AC. On DC the brushes are set back of the neutral plane to compensate for armature action. For AC operation, both the field and the armature change polarity at the same time. The brushes are set in the vertical or neutral plane; the field core must be laminated to prevent eddy current losses.

A starting box is generally used with large series motors to limit current flow through the armature and field coils when starting the motor. This starter has a rheostat which can be connected in series with the motor windings. All the resistance is inserted in the circuit when the motor is being started, and the value of resistance is reduced gradually as the speed of the motor increases.

A compound motor differs from the stabilized shunt type by its more predominant series field. Like compound generators, compound motors may be divided into two classes, differential and cumulative, depending upon the connection of the series field in relation to the shunt field.

In the differential compound motor, the series field opposes the connected shunt field. Therefore, this motor operates at a practically constant speed. As the load increases, the armature current increases to provide more torque. The series magnetomotive force increases, thus weakening the shunt field and reducing the counter electromotive force in the armature, without causing a reduction in speed.

The cumulative compound motor is connected so that its series and shunt fields aid each other. From this comes its name, cumulative compound motor. A motor thus connected will have a very strong starting torque, but it will also have poor speed regulation. Motors of this type are used for machinery where speed regulation is not necessary, but where great torque is desired to overcome sudden application of heavy loads.

3. Подберите к следующим словам соответствующие английские

эквиваленты:

работа (оборудования) – work, job, operation;

сопротивление – inductance, resistance, capacitance;

поле – flux, field, pole;

принцип – principal, personnel, principle;

катушка – coil, current, winding;

обмотка – coil, current, winding;

якорь – core, armature, coil;

сердечник – core, armature, coil;

сила – field, flux, force;

нагрузка – loss, load, line;

потери – loss, load, line;

4. Найдите в тексте предложения, где говорится:

1) о применении двигателей с постоянным магнитом;

2) об особенностях двигателей с параллельным и последовательным

возбуждением;

3) о скорости работы дифференциального компаунд-мотора (двигателя);

4) о последствиях увеличения тока якоря;

5) о преимуществах и сфере применения двигателей с последовательным соединением;

6) о возникновении названия кумулятивный компаунд-двигатель;

7) о средствах ограничения прохождения тока через якорь и катушки.

8) об особенностях применения двигателя смешанного возбуждения с согласным включением обмоток;

5. Выберите термины, которые можно использовать для объяснения

работы двигателей.

6. Ответьте на следующие вопросы.

1) What are the main characteristics of a basic DC motor?

2) When are series and shunt motors used?

3) What is the difference between DC and AC motors?

4) What is the difference between compound and shunt motors?

7. Переведите текст на русский язык.