VII. Explain each of the methods of connection.
Examples: A is bolted to B. – A is connected to B with bolts.
A is welded to B. – A is connected to B by welding.
A is fixed to B. – no specific method is given.
1. screwed 6. glued
2. soldered 7. riveted
3. attached 8. welded
4. wired 9. brazed
5. bonded 10. nailed
Text 2.
I. Read the statements given below and if you think the statement is true agree to it saying “That’s right”. If you think it is not true, disagree “That’s wrong” and make the necessary corrections.
a. Magnetic field can be produced by electric current.
b. Electromagnet can not change its poles.
c. Armature is a term used only in civil engineering.
d. Brush belongs to parts of an electric motor.
e. The drive shaft transmits rotary motion from one motor to another.
II. Read the text and say if you are right or wrong.
Vocabulary
loop [lu:p] – петля; замкнутая цепь, схема
armature ['ɑ:məʧə] – якорь (электрической машины)
to repel [rɪ'pel] – отталкивать
to reverse [rɪ'vɜ:s] – изменять (на прямо противоположное)
commutator ['kɔmjuteɪtə] – коллектор
Brush – щетка
bearing ['bɛərɪŋ] – подшипник
drive shaft – приводной вал
to attract [ə'trækt] – притягивать
to harness ['hɑ:nɪs] – использовать
Operating Principles of an Electric Motor
In an electric motor an electric current and magnetic field produce a turning movement. This can drive all sorts of machines, from wrist-watches to trains.
An electric current running through a wire produces a magnetic field around the wire. If an electric current flows around a loop of wire with a bar of iron through it, the iron becomes magnetized. It is called an electromagnet; one end becomes a north pole and the other a south pole, depending on which way the current is flowing around the loop.
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If you put two magnets close together, like poles – for example, two north poles – repel each other, and unlike poles attract each other.
In a simple electric motor, like the one shown in Fig. 1, a piece of iron with loops of wire round it, called an armature, is placed between the north and south poles of a stationary magnet, known as the field magnet. When electricity flows around the armature wire, the iron becomes an electromagnet.
Fig. 1
The attraction and repulsion between the poles of this armature magnet and the poles of the field magnet make the armature turn. As a result, its north pole is close to the south pole of the field magnet. Then the current is reversed so the north pole of the armature magnet becomes the south pole. Once again, the attraction and repulsion between it and the field magnet make it turn. The armature continues turning as long as the direction of the current, and therefore its magnetic poles, keeps being reversed.
To reverse the direction of the current, the ends of the armaturewire are connected to different halves of a split ring called a commutator. Current flows to and from the commutator through small carbon blocks called brushes. As the armature turns, first one half of the commutator comes into contact with the brush delivering the current, and then the other. This results in a continual reversal of the direction of the current through the armature.
To harness the power of an electric motor, the armature is fixed to a drive shaft. The rotary motion of the armature generated in the motor is then transferred via the drive shaft to a machine.
III. Answer the following questions:
1. Why does an iron bar become magnetized if you pass electric current through a length of the surrounding wire?
2. What determines the polarity of an electromagnet?
3. What is the rotating component of an electric motor called?
4. How does the commutator reverse the flow of electrical current through the armature?
5. What is the function of a brush?
6. Why must the direction of the flow of the current be reversed in order for an electric motor to function? What would happen if the direction remained the same?
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7. How is the power of an electric motor applied in practice?
IV. Match each of these figures with the correct description, A, B, C, or D. One of the descriptions does not match any of the figures. (The figures are in the correct sequence, but the descriptions are not.)
Motor run on direct current
A
The armature turns a quarter of a turn. Then electric contact is broken because of the gap in the commutator, but the armature keeps turning because there is nothing to stop it.
В
When current flows, the armature becomes an electromagnet. Its north pole is attracted by the south pole and repelled by the north pole of the field magnet.
С
When a universal motor is run on direct current, the magnetic poles in the armature change while those of the field magnet remain constant.
D
When the commutator comes back into contact with the brushes, current flows through the armature in the opposite direction. Its poles are reversed and the turn continues.
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