Урок 9. Unit 9.

1. Прочитайте и запомните следующие слова и словосочетания:

the starting torque – пусковой момент;

a series motor – двигатель последовательного возбуждения;

field coils – катушки обмотки возбуждения, обмотки возбуждения;

times – умноженный на;

to be at rest – находиться в покое;

CEMF (counter electromotive force) – противоэдс;

to gain speed – наращивать скорость;

to race – разгоняться;

to crank the engine – заводить двигатель (рукояткой);

are set back of – сдвинуты назад;

to be laminated – иметь слоистую, пластинчатую структуру (состоять из пластин);

to prevent eddy current loss – потери на вихревые токи;

depending upon the connection – в зависимости от соединения;

in relation to – относительно;

a differential compound motor – электродвигатель смешанного возбуждения со встречновключенными обмотками;

a cumulative compound motor – электродвигатель смешанного возбуждения с согласно включенными обмотками;

2. Прочитайте текст 9 “Series Motors”. Найдите в тексте ответы на следующие вопросы.

• When and where are series motors used?

• What is the difference between AC and DC motors?

• What is the difference between compound and shunt motors?

5. SERIES MOTORS.

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. 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 the torque. If an 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 the 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 the automobile engine starter, where the high torque is needed for cranking the engine. Hoists, streetcars 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.

Compound motor. 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 differential compound motor, the series field opposes the connected shunt field. Therefore, this motor operates at a practically constant field. 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 strong starting torque, but it will also have poor speed regulations. 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. Найдите в тексте предложения, где говорится о:

- двух типах двигателей;

- о скорости работы дифференциального компаунд-двигателя;

- о последствиях увеличения тока якоря;

- о преимуществах двигателя с последовательным соединением обмоток;

- о происхождении названия кумулятивный компаун-двигатель.

4. Выберите всю информацию и расскажите о работе двигателя.

5. Переведите текст на русский язык.