Giáo trình Tiếng Anh chuyên ngành điện tử

rushes to rub against the commutator. The rubbing creates friction, and 
the higher the speed, the harder the brushes have to press to maintain good contact. 
Not only does this friction make the motor noisy, but it also creates an upper limit 
on the speed and causes the brushes eventually to wear out and to require 
replacement. The imperfect electric contact also causes electrical noise in the 
attached circuit. These problems vanish when you turn the motor inside out, 
putting the permanent magnets on the inside and the coils on the outside thus 
designing out the need for brushes in a brushless design. However such designs 
need electronic circuits to control the switching of the electromagnets (the function 
that is performed in conventional motors by the commutator). 
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Wound field DC motor 
The permanent magnets on the outside (stator) of a DC motor may be replaced by 
electromagnets. By varying the field current it is possible to alter the speed/torque 
ratio of the motor. Typically the field winding will be placed in series (series 
wound) with the armature winding to get a high torque low speed motor, in parallel 
(shunt wound) with the armature to get a high speed low torque motor, or to have 
a winding partly in parallel, and partly in series (compound wound) to get the best 
of both worlds. Further reductions in field current are possible to gain even higher 
speed but correspondingly lower torque. This technique is ideal for electric traction 
and many similar applications where its use can eliminate the requirement for a 
mechanically variable transmission 
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Unit 9 THE MOVING COIL 
I. Reading and comprehension: 
1.Construction and component 
The essential component of a moving coil meter are a permanent magnet and a 
moving coil. The magnet is U-shaped or semi-circular and is made of a material 
such as Alcomax. Each pole terminates in a soft-iron pole piece shaped and 
positioned as in figure 5.1. 
Figure 5.1 
The moving coil, which gives the instrument its name, is composed of fine copper 
wire wound on a thin rectangular aluminum former. The former is mounted 
centrally on hard steel spindles and can rotate around a fixed cylindrical soft-iron 
core. The core is placed between the pole pieces in such a manner that an annular 
gap is formed between it and the pole pieces. A pointer is attached to the former 
and traverses a linear scale. 
The spindles which bear the moving coil are mounted on jeweled bearing two 
spiral hair springs are attached to the spindles. They are wound in apposition and 
are adjusted so that they balance when the pointer is at the zero mark. 
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2.Operation 
This instrument operates on the principle that when a current carrying conductor 
is placed in a magnetic field, a force is exerted on the conductor which causes it to 
move. 
When the meter is inserted in a live circuit, current flows through the control 
springs into the coil. This sets up a magnetic field around the coil which reacts 
with the radial magnetic field of the permanent magnet. The reaction produces a 
torque which tends to rotate the coil. Since the strength of the permanent magnet’s 
field is uniform, this torque is directly proportional to the current flowing in the 
coil. As the coil rotates, the control springs tighten thus opposing the motion of 
the coil. When the deflecting force of the coil is balanced by the controlling force 
of the springs, the coil comes to rest. The extent of the coil’s movement, and hence 
the size of the current flowing through the coil, is indicated on the scale by the 
pointer. 
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Exercise 1: Describing position 
Say where the following components are located. Use the expressions you learnt 
in Unit 1. 
1. the pole pieces. 
2. the core. 
3. the pointer 
4. the former. 
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5. the springs. 
 Exercise 2: Describing functions 
This table describes the function of the component of the meter. The functions are 
in the wrong order. Write a sentence to describe the function of each component 
using the methods you learnt in unit 2. note that the springs have two functions. 
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Now add part 1 of the reading passage a description of the function of these 
component. Begin like: 
The function of the moving coil meter is to detect the presence of a direct current. 
Its essential components 
Component Function 
1. core 
2. former 
3. spring 
4. bearings 
5. magnet 
To provide controlling torque. 
To reduce friction and wear. 
To produce a powerful uniform magnetic field. 
To carry the coil. 
To serve as leads to carry current to the coil. 
To produce radial filed lines within the gap. 
Exercise 3: phrasing 
Rewrite the following sentences rephrasing the words in italics with expressions 
from the passage which have similar meaning: 
1. The meter is inserted in a circuit with a current flowing through it. 
2. The coil is rotated by turning force. 
3. The strength of the permanent magnet’s field is always the same. 
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4. The turning force varies directly with the current flowing through the 
coil. 
5. The force which rotates the coil is balanced by the force which 
restrains the coil 
II. Use of language: 
1. Cause and effects 1 
Study this sentence 
Insulation breakdown leads to short circuit. 
This sentence contains a cause and effect. We can link a cause and an effect as 
follow 
 Insulation causes short circuit 
 Breakdown results in 
 Produces 
 Leads to 
 Gives rise to 
 Is the cause of 
We can also put the effect first 
 Short circuits are caused by insulation breakdown 
Result from 
Arise from 
 Are the effect of 
Are the result of 
 Are the consequence of 
 Are due to 
cause effect 
effect cause 
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When a cause has several effects or when an effect has a number of possible 
causes, we put can or may before the causative expression. 
Example: 
Sparking MAY be cause by worn brushes 
Or: sparking CAN be caused by a worn commutator. 
Similarly, instead of the cause/effect/result/consequence of, we write one 
cause/effect/result/consequence of. 
Example: 
Worn brushes are ONE cause of sparking 
A worn commutator is ONE cause of sparking. 
Now match these cause and effect pairs. Then link them using the expressions 
given above. Write two sentences for each example, one with the cause first and 
the other with the effect first. 
CAUSE EFFECT 
1. glare arching across the points 
2. eddy currents power losses in transformers 
3. excessive heat serious accidents 
4. faulty soldering breakdown of the motor 
5. sparking discomfort to the eyes 
6. failure of a point capacitor damage to semiconductor 
7. exceeding the motor rating bad joints 
8. faulty earth connections interference in receivers 
 2. Cause and effect chains 
Describe this cause and effect chain. Use different expressions for each link 
Overrunning Excessive Insulation Short Blown a motor heat breakdown circuit 
fuses 
Now rewrite your description starting at the end of the chain and working 
backwards. 
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3. Describing the reception of a signal 
Put these events, which describe the progress of a signal through a receiver, in 
sequence with the help of the diagram. Then link the sentences in pairs using time 
clauses with before and after. 
a. the signal is again amplified. 
b. The desired signal is fed to the acceptor circuit. 
c. The signal is amplified 
d. The signal is fed to a loudspeaker 
e. The signal is mixed with a signal from the oscillator to give a standard 
intermediate frequency 
f. The signal is rectified by the detector. 
3. Short relative clause Study these 
sentences: 
1. A telephone dial consists of small keys 
2. those keys has many characters and numbers We can link them using a 
relative clause: 
1+2. A telephone dial consists of small keys having many characters and numbers. 
A telephone dial consists of small keys WITH many characters and numbers. 
Sometimes we can reduce a relative clause to an adjective Example: 
3. high quality instrument use resistors. 
4. the resistors are wire wound. 
3+4. high quality instruments use resistors WHICH ARE WIREWOUND 
 high quality instruments use WIREWOUND resistors. 
Make this paragraph shorter by reducing the relative clauses. Use all the methods 
of reduction you have learned in this unit. 
 oscillator 
mixer r 
Acceptorr 
Intermediate 
e 
detector 
o
r 
Audio 
a 
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The telephone is an instrument which enables us to transmit speech via wire (wireless 
ness). The body of the telephone contains an induction coil, capacitors, resistors, a 
regulator, which controls the sensitivity of the instrument, and a bell. The handset 
contains a microphone and a receiver which are enclosed by screwed caps at the ends 
of the handset. The bell contains a hammer which is operated by a solenoid. The 
hammer is set between two domes which is eccentrically mounted. The dial is 
mounted on the face of the telephone. It consists of small keys which have characters 
and numbers. When those keys are pressed in, it causes spring contacts to open and 
close a number of times which respond to the number dialed. This transmits pulses 
down the line causing selectors, which connect the calling line to the line which is 
called, to operate. 
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III. Further- reading: 
Oscillator 
The Basics 
One of the most commonly used oscillators is the 
pendulum of a clock. If you push on a pendulum to start it 
swinging, it will oscillate at some frequency -- it will 
swing back and forth a certain number of times per second. 
The length of the pendulum is the main thing that controls 
the frequency. 
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For something to oscillate, energy needs to move back and forth between two 
forms. For example, in a pendulum, energy moves between potential energy and 
kinetic energy. When the pendulum is at one end of its travel, its energy is all 
potential energy and it is ready to fall. When the pendulum is in the middle of its 
cycle, all of its potential energy turns into kinetic energy and the pendulum is 
moving as fast as it can. As the pendulum moves toward the other end of its swing, 
all the kinetic energy turns back into potential energy. This movement of energy 
between the two forms is what causes the oscillation. 
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Eventually, any physical oscillator stops moving because of friction. To keep it 
going, you have to add a little bit of energy on each cycle. In a pendulum clock, 
the energy that keeps the pendulum moving comes from the spring. The pendulum 
gets a little push on each stroke to make up for the energy it loses to friction. 
An electronic oscillator works on the same principle. 
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Symbols : general warnings and safety symbols: Danger/ warning/caution/hazard. 
Specific hazards Flammable Toxic/ poison High voltage 
Safety equipment or help 
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Emergency exit/fire exit Fire alarm Fire extinguisher Hospital 
First aid Emergency stop 
Prohibitons 
No entry No exit No smoking 
 TÀI LIỆU THAM KHẢO 
1. David Bonamy- Christopher Jacques-Technical English – Students’ book. 
2. Eric H.Glendinning John McEwan-Oxford English for Electronics. 
3. Giáo trình tiếng anh chuyên ngành điện- Đại Học Hưng Yên. 
4. Google – data sheet- internet.