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Файл:Professional English for Electrical Engineers. Part 1. Учебное пособие
.pdf
Electromagnetic meter movements
PMMC movement
A polarity-insensitive type of meter movement
A full-scale deflection current
Rotate
Fraction
Exercise 3. Find the sentences in the text where these words and phrases were
used. Comment on them.
Exercise 4. Insert the missing words.
• A “……” is the display mechanism of a meter.
• Electromagnetic movements work on the principle of a ……… ……… being
generated by electric ……. through a wire. Examples of electromagnetic meter
movements include the ………., Weston, and iron-vane designs.
• Electrostatic movements work on the principle of ………. force generated by
an electric ……… between two …….
• Cathode Ray Tubes (CRT’s) use an electrostatic field to bend the path of an
electron ……, providing indication of the beam’s position by light created when the beam
strikes the end of the glass …….
Exercise 5. Define the omitted words and compose 5 sentences with them.
Exercise 6. Match the parts of the sentences below.
Usually this “readable form” is visual: capable of moving some kind of
motion of a pointer on a scale, a series indicating mechanism.
of lights arranged to form
Older designs of meters are mechanical (relatively) large quantities of
in nature, voltage, current, or resistance are
the same.
In either case, the principles applied in a magnetic field perpendicular to
adapting a display unit the axis of electron flow.
to the measurement of
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The display mechanism of a meter is full-scale indication at some
often referred to as a movement, specified amount of voltage or
current.
Most mechanical movements are based resulting in a “full-count”
on the principle of electromagnetism: indication on the numerical
that electric current through a conductor display: when the digits cannot
produces display a larger quantity.
While most mechanical meter movements are a “bargraph,” or some sort of
based on electromagnetism (electron flow display composed of
through a conductor creating a perpendicular numerical figures.
magnetic field), a few are based on electrostatics:
If a voltage is applied between two conductive borrowing from its mechanical
surfaces across an air gap, there will be a nature to move a pointer along
physical force attracting the two surfaces together a scale so that a measured value
may be read.
In electromagnetic movements, this will be using some kind of pointer device
the “full-scale deflection current” to show quantity of measurement.
In digital “movements,” it is the amount of voltage necessary to rotate the needle so
that it points to the exact end
of the indicating scale.
The task of the meter designer is to take a given that is, the attractive or repulsive
meter movement and design the necessary force generated by electric charges
external circuitry for across space.
Exercise 7. Compose a dialogue explaining the principles of mechanical and
digital meters.
Exercise 8.
a. Make an outline of the text “What is a Meter?”
b. Retell the text “What is a Meter?” according to the written outline.
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3.2 Ammeters
Before you start
1. What do ammeters measure?
2. How do ammeters work?
Exercise 9. Read and translate the text.
A magnetic compass doesn’t make a very convenient meter. It has to be lying flat,
and the coil has to be aligned with the compass needle when there is no current. But of
course, electrical and electronic devices aren’t all oriented so as to be aligned with the
north geomagnetic pole! But the external magnetic field doesn’t have to come from the
earth. It can be provided by a permanent magnet near or inside the meter. This supplies a
stronger magnetic force than does the earth’s magnetic field, and therefore makes it
possible to make a meter that can detect much weaker currents.
Such a meter can be turned in any direction, and its operation
is not affected. The coil can be attached directly to the meter
pointer, and suspended by means of a spring in the field of the
magnet. This type of metering scheme, called the D’Arsonval
movement, has been around since the earliest days of
electricity, but it is still used in some metering devices today.
The assembly is shown in Figure 34. Figure 34
This is the basic principle of the ammeter.
A variation of the D’Arsonval movement can be obtained by attaching the meter
needle to a permanent magnet, and winding the coil in a fixed form around the magnet.
Current in the coil produces a magnetic field, and this in turn generates a force if the coil
and magnet are aligned correctly with respect to each other. This works all right, but the
mass of the permanent magnet causes a slower needle response. This type of meter is also
more prone to overshoot than the true D’Arsonval movement; the inertia of the magnet’s
mass, once overcome by the magnetic force, causes the needle to fly past the actual point
for the current reading, and then to wag back and forth a couple of times before coming to
rest in the right place.
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It is possible to use an electromagnet in place of the permanent magnet in the meter
assembly. This electromagnet can be operated by the same current that flows in the coil
attached to the meter needle. This gets rid of the need for a massive, permanent magnet
inside the meter. It also eliminates the possibility that the meter sensitivity will change in
case the strength of the permanent magnet deteriorates (such as might be caused by heat or
by severe mechanical vibration). The electromagnet can be either in series with, or in
parallel with, the meter movement coil.
Sometimes, it is desirable to have an ammeter that will allow for a wide range of
current measurements. The full-scale deflection of a meter assembly cannot easily be
changed, because that would mean changing the number of coil turns and/or the strength
of the magnet. But all ammeters have a certain amount of internal resistance. If a resistor,
having the same internal resistance as the meter, is connected in parallel with the meter,
the resistor will draw half the current. Then it will take twice the current through the
assembly to deflect the meter to full scale, as compared with the meter alone. By choosing
a resistor of just the right value, the full-scale deflection of an ammeter can be increased
by a large factor, such as 10, or 100, or 1000. This resistor
must be capable of carrying the current without burning up.
It might have to draw practically all of the current flowing
through the assembly, leaving the meter to carry only 1/10,
or 1/100, or 1/1000 of the current. This is called a shunt
resistance or meter shunt (figure 35). Meter shunts are used Figure 35
when it is necessary to measure very large currents, such as hundreds of amperes. They
also allow microammeters or milliammeters to be used in a versatile multimeter, with
many current ranges.
Exercise 10. Find the synonyms in the text “Ammeters” for the following
words.
Pointer
Move around
Adjust
Flat
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Inactivity
Press out
Drop
Bending
Lying
Bypass
Exercise 11. Explain the meaning of the synonyms found in the text.
Exercise 12. Define whether these statements are true or false. Correct the false
ones.
1. A magnetic compass makes a very convenient meter.
2. The coil can be attached indirectly to the meter pointer, and suspended by means
of a spring in the field of the current.
3. This type of metering scheme, called the D’Arsonval movement, has been around
since the earliest days of electricity, but it is still used in some metering devices today.
4. This electromagnet can be operated by the same voltage that flows in the needle
attached to the meter coil.
5. It also eliminates the possibility that the meter sensitivity will change in case the
strength of the permanent magnet deteriorates (such as might be caused by heat or by
severe mechanical vibration).
6. The electromagnet can be either in series with, or in parallel with, the meter
movement coil.
7. The full-scale deflection of a meter assembly can easily be changed, because that
would mean changing the number of coil turns and/or the strength of the magnet.
8. If a resistor, having the same internal resistance as the meter, is connected in
parallel with the meter, the resistor will draw half the current.
9. By choosing a resistor of just the right value, the full-scale deflection of an
ammeter cannot be increased by a large factor, such as 30, or 300, or 3000.
10. Meter shunts are used when it is necessary to measure very large currents, such
as hundreds of amperes.
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Exercise 13. Compose 7 questions on the text “Ammeters”.
Exercise 14. Answer the following questions. Discuss them with your partner.
1. What the least accuracy rating should a milliammeter possess to provide the
measurement ratio error of current in a range from 0.5 mA to 2 mA being less than 2%?
2. While testing an ammeter with the measurement limit 2,5A, the followings values
of basic error are obtained: 0.02 A; 0.015 A; 0.05 And; 0.25 A; 0.01 A. Does this device
correspond to the accuracy rating 1.5?
3. Decide between two ammeters to measure a current 2 A with minimum error, if
the first ammeter has the measurement limit 2.5 A and accuracy rating 1.5 while the
second one has 5 A and 0.5/0.2, respectively.
Exercise 15. Translate the following sentences into English.
1. Для проведения измерений, производится последовательное включение
амперметра в электрическую цепь с тем участком, где необходимо измерить силу
тока. Чтобы увеличить пределы измерений, производится включение амперметра
через шунт или трансформатор.
2. Наиболее распространенной является схема амперметра, где движущаяся
стрелка совершает поворот на такой угол наклона, который пропорционален
величине измеряемой силы.
3. По своему действию все амперметры разделяются на электромагнитные,
магнитоэлектрические, тепловые, электродинамические, детекторные,
индукционные, фото- и термоэлектрические.
4. Во время работы магнитоэлектрического амперметра, создается крутящий
момент, через взаимодействие между полем в постоянном магните и током,
проходящим через обмотку рамки.
5. Токи, проходящие через катушки, взаимодействуют между собой, в
результате чего происходит отклонение подвижной катушки, с которой соединяется
стрелка.
6. В электрическую цепь амперметр включается последовательно с тем
участком электрической цепи, силу тока в котором измеряют; для увеличения
предела измерений — с шунтом или через трансформатор.
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7. Положительные стороны магнитоэлектрического амперметра заключаются
в том, что для него характерна очень высокая чувствительность и очень малая
потребляемая мощность.
8. Электромагнитные амперметры представляют собой особый механизм с
неподвижной катушкой, по которой протекает электрический ток, а также имеется
специальные сердечники – один или несколько, установленных непосредственно на
оси.
9. Электродинамические амперметры основаны на взаимодействии магнитных
полей токов, которые протекают по подвижной и неподвижной катушкам, в этих
амперметрах в основном используются параллельное и последовательное
включение этих катушек.
10. Ферродинамический амперметр состоит из замкнутого магнитопровода из
ферромагнитного материала, центрального сердечника, а также неподвижной
катушки, они применяются в основном в сфере безопасности и обороны благодаря
своей высокой точности измерения.
Exercise 16.
a. Make an outline of the text “Ammeters”.
b. Retell the text “Ammeters” according to the written outline.
3.3 Voltmeters
Before you start
1. What do voltmeters measure?
2. How do voltmeters work?
Exercise 17. Read and translate the text.
Current as we know consists of a flow of charge carriers. Voltage, or electromotive
force (EMF), or potential difference, is the “pressure” that makes current possible. Given a
circuit whose resistance is constant, the current that flows in the circuit is directly
proportional to the voltage placed across it. Early electrical experimenters recognized that
an ammeter could be used to measure voltage, because an ammeter is a form of constantresistance circuit. But ammeters have low internal resistance. They are designed that way
77
deliberately. They are meant to be connected in series with other parts of a circuit, not
right across a power supply. But if you place a large resistor in series with an ammeter,
and then connect the ammeter across a battery or other type of power supply, you no
longer have a short circuit. The ammeter will give an indication that is directly
proportional to the voltage of the supply. The smaller the full-scale reading of the
ammeter, the larger the resistance that is needed to get a meaningful indication on the
meter. Using a microammeter and a very large value of resistance in series, a voltmeter
can be devised that will draw only a little current from the source.
A voltmeter can be made to have various ranges for the full-scale reading, by
switching different values of resistance in series with the microammeter. The internal
resistance of the meter is large because the values of the resistors are large. The greater the
supply voltage, the larger the internal resistance of the meter, because the necessary series
resistance increases as the voltage increases.
A completely different type of voltmeter uses the effect of electrostatic deflection,
rather than electromagnetic deflection. Remember
that electric fields produce forces, just as do magnetic
fields. Therefore, a pair of plates attracts or repels
each other if they are charged. The electrostatic
voltmeter takes advantage of the attractive force
between two plates having opposite electric charge, or
having a large potential difference. Figure 36 is a
simplified drawing of the mechanics of an
electrostatic voltmeter. It draws almost no current
from the power supply. The only thing between the
plates is air, and air is a nearly perfect insulator. Figure 36
The electrostatic meter can indicate ac voltage as well as dc voltage. The construction
tends to be fragile, however, and mechanical vibration can influence the reading.
Exercise 18. Find the antonyms in the text “Voltmeters” for the following
words.
Relaxation
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Indirectly
External
Decrease
Tiny
Disadvantage
Disgusting
Extra
Exclusive
Common
Exercise 19. Explain the meaning of the antonyms found in the text.
Exercise 20. Match the parts of the sentences below.
Given a circuit whose resistance is constant, you no longer have a short circuit.
Early electrical experimenters recognized having opposite electric charge, or
hat an ammeter could be used to measure having a large potential difference.
voltage,
But if you place a large resistor in series with the larger the resistance that is
an ammeter, and then connect the ammeter needed to get a meaningful
across a battery or other type of power supply, indication on the meter.
The smaller the full-scale reading of the ammeter, the current that flows in the circuit
is directly proportional to the
voltage placed across it.
Using a microammeter and a very large value and mechanical vibration can
of resistance in series, influence the reading.
The greater the supply voltage, a voltmeter can be devised that
will draw only a little current from
the source.
A completely different type of voltmeter just as do magnetic fields.
uses the effect of electrostatic deflection,
79
Remember that electric fields produce forces, because an ammeter is a form of
constant-resistance circuit.
The construction tends to be fragile, however, rather than electromagnetic
deflection.
The electrostatic voltmeter takes advantage the larger the internal resistance of
of the attractive force between two plates the meter, because the necessary
series resistance increases as the
voltage increases.
Exercise 21. Solve the problems. Discuss them with your partner.
1. Draw diagram of ratio error dependences for all the voltmeter measurement
ranges: 20, 40, 50 V; the voltmeter having an accuracy rating equal to 2.0 / 1.5. Determine
the ratio error for each measurement range while measuring 20 V.
2. Draw dependences of absolute, ratio and reduced errors of a voltmeter with the
measuring range from 50 to 200 V; the accuracy rating of device being 2.0.
3. 200 V voltage is measured by a voltmeter. The voltmeter measurement limit is
300 V. From the below proposed list of the accuracy ratings, do choose the answer with
the least measurement error: 1) 1,5; 2) 1.0; 3) 1,5/1,5.
Exercise 22. Translate the following sentences into English.
1. Аналоговый электронный вольтметр измерительный прибор,
представляющий собой сочетание электронного преобразователя, выполненного на
лампах, полупроводниковых элементах, интегральных микросхемах, и
магнитоэлектрического измерителя.
2. Электронные вольтметры постоянного тока по сравнению с
магнитоэлектрическими вольтметрами имеют очень большое входное
сопротивление (порядка 5-10 МОм) и высокую чувствительность.
3. Усилитель постоянного тока служит для повышения чувствительности
вольтметра, является усилителем мощности, необходимым для приведения в
действие магнитоэлектрического измерителя.
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