Alternating - current motors (b)

When a coil of wire is rotated in a magnetic field, the current changes its direction every half turn. There are two alterna­tions of current for each revolution of a bipolar machine. As previously noted, this alternating current is rectified by the use of a commutator in a direct-current generator. In an alternatingcurrent generator also termed an alternator, the current induced in the armature is led out through slip rings or collector rings.

The magnetic field – the coils of wire and iron core – are called simply the field of the generator. The rotating loop in which the voltage is induced is called the armature.

The rotating – armature type of generator is generally used only on small machines, whereas large machines almost without exception are buit with rotating fields.

The stator in the wound-rotor motor is identical to the stator in the squirrel-cage motor. The basic difference in the two motors lies in the rotor winding.

In the squirrel-cage motor, the rotor winding is nearly always self-contained; it is not connected either mechanically or electri­cally to the outside power-supply or control circuit. However, in wound-rotor motors, the rotor winding consists of insulated coils of wire that are not permanently short-circuited, but are con­nected in regular succession to form a definite polar area having the same number of poles as the stator. The ends of these rotor windings are brought out to collector rings, or slip rings.

The wound-rotor motor is often used in cranes, hoists, and elevators. These devices are operated intermittently and for short periods of time, where exact speed regulation and loss in effi­ciency are of little consequence.

3. Find the answers to the questions.

1. What machine is called a motor?

2. What are the main parts of the motor?

3. How are direct-motors classified as?

4. What is the difference between d.c. motors and a.c. motors?

5. Where can wound-rotors be used for?

4. Complete the sentences:

1. ... consists of an electromagnet, an armature, and a commutator witch its brushes.

2. A machine that converts electrical energy into mechanical energy is ... .

3. ... incorporates both the series-wound and the shunt-­wound windings.

4. The rotating – armature type of generator is ... .

5. The ... is often used in cranes, hoists, and elevators.

6. ... can be used to start extremely heavy loads.

5. Match the following English words with their Ukrainian equivalents:

1. current a. перетворювати

2. convert b. арматура

3. armature c. щітка

4. magnet d. колектор

5. brush e. статор

6. commutator f. генератор

7. indicate g. виявлять, повідомлять

8. generator h. магніт

9. stator i. струм

10. induce j. викликати

6. Translate and memorise the following words and word combinations.

series-wound, shunt-wound, compound-wound, permanent-magnet, the field current, shunt-wound windings, the field coils, a rotating armature, member.

II Retell the text “Motors”.

III Rendering.

1. Read the text and translate it into Ukrainian using new words in their specialized meanings below.

1. generator – генератор

2. armature – якір, арматура

3. commutator – коммутатор, колектор

4. compound – змішаний

5. starting – включення, запуск

6. conductor – провідник