Preface
xv
Guided Tour
Comprehensive Topical Coverage
∑
∑
∑
13
The text focuses on detailed coverage of electronics
measurement systems and related components
including analysers, data acquisition systems, etc.
In-depth discussion on special-purpose measurements and applications such as magnetic measurements and fibre optic measurements
Dedicated chapter on Sensors and Transducers
13.1
14
14.1
Data Acquisition System
12.1
INTRODUCTION
Magnetic Measurements
INTRODUCTION
12.2
TYPES OF MAGNETIC MEASUREMENTS
Magnetic measurements can be divided into two general classes: direct-current (dc) tests and alternatingcurrent (ac) tests. Although quite different methods and objectives are found, these are the two most
distinctly defined classes of tests. The dc branch may be further subdivided into measurement of field
strength, flux, permeability, B-H curves and hysteresis loops. Such tests are most generally made upon
solid materials, the ac test methods being used chiefly for laminated materials. The ac measurements
are concerned mainly with losses in magnetic materials under conditions of alternating magnetisation.
12.3
THE BALLISTIC GALVANOMETER
The ballistic galvanometer is used to measure the quantity of electricity passed through it. This quantity
in magnetic measurements is the result of an
n emf instantaneously induced in a search coil connected to
the galvanometer terminals, when the magnetic
etic flux interlinking with the search coil is changed. Such a
galvanometer is usually a D’Arsonval type, since this type is least affected by external magnetic fields.
It does not show a steady deflection when in
n use, owing to the transitory nature of the current passing
through, but gives a “throw” which is proportional
ortional to the quantity of electricity instantaneously passed
BASIC COMPONENTS OF DATA ACQUISITION SYSTEMS
The basic elements of a data acquisition system, as shown in the functional diagram (Figure 14.1) are
as follows:
ensors and transducers
Sensors
eld wiring
Field
gnal conditioning
Signal
Data
ata acquisition hardware
18
Fibre Optic Measurements
11
11.1
18.1
INTRODUCTION
Kaol first suggested the possibility that low-loss optical fibres could be competitive with coaxial cable
and metal waveguides for telecommunications applications. It was not, however, until 1970 when
Corning Glass Works announced an optical fibre with loss less than the benchmark level of 10 dB/km.
After that, commercial applications began to be realised. The revolutionary concept which Corning
incorporated and which eventually drove the rapid development of optical fibre communications was
primarily a materials one—it was the realisation that low doping levels and very small index changes
could successfully guide light for tens of kilometres before reaching the detection limit. The ensuing
demand for optical fibres in engineering and research applications spurred further applications. Today,
we see a tremendous variety of commercial and laboratory applications of optical fibre technology.
18.2
HOW DOES AN OPTICAL FIBRE WORK?
The optical fibre works on principles similar to other waveguides, with the important inclusion of a
cylindrical axis of symmetry.
When light is travelling from one medium (n0) into a medium of different density (n1), a certain
amount of incident light is reflected. This effect is more prominent where the light is travelling from a
high-density medium into a lower-density medium. The exact amount of light that is reflected depends
on the degree of change of refractive index and on the angle of incidence. If the angle of incidence
is increased, the angle of refraction is increased at a greater rate. At a certain incident angle ( C), the
refracted ray will have an angle of refraction that has reached 90° (that is, the refracted ray emerges
parallel to the interface). This is referred to as the critical angle. For rays that have incident angles
greater than the critical angle, the ray is internally reflected totally. In theory, total internal reflection
is considered to reflect 100% of the light energy but in practice, it reflects about 99.9% of the incident
ray.
c
Prelims of Purkait.indd xv
= sin
1
n1
n0
OSCILLATORS
Oscillator is the basic element of ac signal sources and generates sinusoidal signals of known frequency
and amplitude. The main applications of oscillators are as sinusoidal waveform sources in electronic
measurement work. Oscillators can ggenerate a wide range
(few Hz to many GHz) as per
g of frequencies
q
the requirement of the application. Although an oscillator
lator can be considered as generating sinusoidal
signal, it is to be noted that it merely acts as an energy
converter. It converts a dc source of supply to
gy converter
alternating current of desired frequency.
Oscillators are generally an amplifier with positivee feedback. An oscillator has a gain equal to or
slightly greater than unity. In the feedback path of thee oscillator, capacitor, inductor or both are used
as reactive components. In addition to these reactive component
components, an operational amplifier or bipolar
transistor is used as amplifying device. No external acc input is req
required to cause the oscillator to work
as the dc supply energy is converted by the oscillator into ac ener
energy.
Magnetic quantities are measured using a variety of different technologies. Each technique has unique
properties that make it more suitable for particular applications. These applications can range from
simply sensing the presence or change in the field to the precise measurements of a magnetic field’s
scalar and vector properties. Magnetic measurements are most difficult to make and essentially less
accurate than electrical measurements for two reasons. First, in magnetic measurement, flux is not
measured as such, but only some effect produced by it, such as the voltage induced by a change of flux.
As a second and greater difficulty, flux paths are not defined as are electrical circuits—difficulties are
definitely faced in precision ac bridges in making the electrical circuits, but the magnetic situation is
radically worse and not subject to the same control.
A data acquisition system is a device or an integra ted system used to collect information about the state
or condition of various parameters of any process. For example, collecting day-to-day temperature of a
particular location can be termed data acquisition. Say, a person recording the level of municipal waterstoring tank into a piece of paper, is actually performing the task of a data acquisition system. With the
advancement of digital electronics, various electronic devices have been developed to perform this kind
of recording or logging job.
Now a days, most data acquisition systems are integrated with computer, sensors, signal conditioning
devices, etc. and the function of these kind of data acquisition systems varies for simple recording of
process parameter to control of industrial system. These kinds of systems basically have a hardware
and a software part. The hardware part consists of a sensor, signal conditioning, analog-to-digital
converter, memory, processor, switches, digital-to-analog converter, etc. and the software part consist,
of operating system, editor, graph display program and data processing software, etc..
A data acquisition system is used in various applications, starting from industry to scientific
laboratories.
The actual definition of a data acquisition system also varies; here is a common definition of data
acquisition system.
“Data acquisition is the process by which physical phenomena from the real world are transformed
into electrical signals that are measured and converted into a digital format for processing, analysing,
and storage by a computer”.
14.2
INTRODUCTION
Signal generators provide a variety of waveforms for testing of electronic circuits at low power levels.
There are various types of signal generators, but the following characteristics are common to all
types:
1. Always a stable generator with desired frequency signals should be generated.
2. Generated signal amplitude should be regulated over a wide range from very small to relatively
large level.
3. Generated signal should be free from any distortions.
There are many variations of the above requirements, especially for specialised signal generators
such as function generators, pulse generators and pulse frequency generators. Sine wave generators,
both in audio and radio frequency ranges are called oscillators. Although, the terminology is not
universal, the term oscillator is generally used for an instrument that provides only a sinusoidal output
signal. The term function generator is applied to an instrument that provides several output waveforms,
including sine wave, square wave, triangular wave and pulse trains as well as amplitude modulation of
the output signal.
13.2
12
Signal Generators
and Analysers
(18.1)
Sensors and Transducers
INTRODUCTION
The physical quantity under measurement makes its first contact in the time of measurement with a
sensor. A sensor is a device that measures a physical quantity and converts it into a signal which can be
read by an observer or by an instrument. For example, a mercury thermometer converts the measured
temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube.
A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For
accuracy, all sensors need to be calibrated against known standards.
In everyday life, sensors are used everywhere such as touch sensitive mobile phones, laptop’s touch
pad, touch controller light, etc. People use so many applications of sensors in their everyday lifestyle that
even they are not aware about it. Examples of such applications are in the field of medicine, machines,
cars, aerospace, robotics and manufacturing plants. The sensitivity of the sensors is the change of
sensor’s output when the measured quantity changes. For example, the output increases 1 volt when the
temperature in the thermocouple junction increases 1°C. The sensitivity of the thermocouple element is
1 volt/°C. To measure very small charges, the sensors should have very high sensitivity.
A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic,
photonic, or photovoltaic that converts one type of energy or physical attribute to another (generally
electrical or mechanical) for various measurement purposes including measurement or information
transfer (for example, pressure sensors).
The term transducer is commonly used in two senses; the sensor, used to detect a parameter in one
form and report it in another (usually an electrical or digital signal), and the audio loudspeaker, which
converts electrical voltage variations representing music or speech to mechanical cone vibration and
hence vibrates air molecules creating sound.
11.2
ELECTRICAL TRANSDUCERS
Electrical transducers are defined as the transducers which convert one form of energy to electrical
energy for measurement purposes. The quantities which cannot be measured directly, such as pressure,
displacement, temperature, humidity, fluid flow, etc., are required to be sensed and changed into
electrical signal first for easy measurement.
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xvi
Guided
Preface
Tour
Illustrations and Diagrams
treated mathematically, so great care should be taken during measurement to avoid this error. Pictorial
illustration of different types of gross error is shown in Figure 1.10.
Over 400 illustrations help clarify the concepts, with
real-life photographs to augment schematic diagrams
wherever necessary.
(b) Gross error will occur in the
measurement result if the pointer
deflects between the two scaling points.
(a) Error will occur in the measurement result if
proper zero setting are not there.
1
1
oss section of a wound-t
wound-type CT. Figure 3.8 (b) shows a photograph of such a wound-type
(b)
(c) Parallax error will occur in the
measurement result if the pointer is not
set in the vertical position.
Figure 1.10
Different types of gross errors
—
2
–
2
Flux
2
(a)
(c)
Permanent magnet moving coil instrument
the cross-section
oss-section of a bar-type CT. Figure 3.10 show photograph of such bar-type CTs.
1.
2.
3.
4.
5.
6.
External shield
Permanent magnet
Moving coil
Control spring
Pointer
Arrangement for zero
balance of the pointer
Figure 3.9
(d)
Cross section of bar-type CT
Photograph of different components of a PMMC instrument
Figure 2.11
Contd
(b)
Figure 3.8
(a) Cross section of wound-type CT (b) Butyl-molded wound primary
type indoor CT (Courtesy of General Electric Company)
(a)
Figure 3.10
(b)
(a) Butyl-molded bar primary-type indoor CT for 200–800 A and 600 V range circuit
(Courtesy of General Electric Company) (b) Single conductor (bar type) primary CT
2. Clamp-on Type
T or Portable Type CTs
Typ
By the use of a construction with a suitably split and hinged core, upon which the secondary winding is
wound, it is possible to measure the current in a heavy-current conductor or bus-bar without breaking
5.
The ageing of the instrument (permanent magnet and control spring) may introduce some
errors.
Example 2.1
Solution
The coil of a PMMC instrument has 60 turns, on a former that is 18 mm wide, the
effective length of the conductor being 25 mm. It moves in a uniform field of flux
density 0.5 Tesla. The control spring constant is 1.5 × 10−6 Nm/degree. Calculate
the current required to produce a deflection of 100 degree.
Total deflecting torque exerted on the coil,
Td Bilnb (N-m)
0.5
i
25
10
−3
60
18
10
The control torque of the springs is
Tc ks
1.5 10–6 100
Tc
At equilibrium, Td
= 0.5
i
10−3
18
1.5
i=
10
0.5 18 10
Example 2.2
Solution
3
10–3
25
6
60
100
25 10
3
Solved Examples
−3
10–6
1.5
100
11.11 mA
60
A PMMC instrument has a coil of dimensions 15 mm 12 mm. The flux density
in the air gap is 1.8 10−3 wb/m2 and the spring constant is 0.14 10−6 N-m/rad.
Determine the number of turns required to produce an angular deflection of 90°
when a current of 5 mA is flowing through the coil.
About 100 Solved Examples are given in the text which
help reinforce the understanding of concepts and illustrate
the way the formulae developed can be used for solving
problems.
Total deflecting torque exerted on the coil,
Td Bilnb (N-m)
1.8 × 10−3 × 5 × 10−3 × 15 × 10−3 × 12 × 10−3 × n
The control torque of the springs is
Tc ks ×
0.14 × 10−6 × 90 × /180
At equilibrium, Td
Tc
1.8 × 10−3 × 5 × 10−3 × 15 × 10−3 × 12 × 10−3 × n
n=
Example 2.3
0.14 10
1.8 10
3
5 10
6
3
90
15 10
0.14 × 10−6 × 90 × /180
/180
3
12 10
3
136
A PMMC voltmeter with a resistance of 20 Ω gives a full-scale deflection of 120°
when a potential difference of 100 mV is applied across it. The moving coil has
dimensions of 30 mm × 25 mm and is wound with 100 turns. The control spring
constant is 0.375 × 10−6 N-m/degree. Find the flux density in the air gap. Find also
the dimension of copper wire of coil winding if 30% of the instrument resistance is
due to coil winding. The specific resistance of copper is 1.7 × 10−8 Ωm.
Prelims of Purkait.indd xvi
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Guided
Preface
Tour
xvii
Exercises
Long-answer Questions
1.
∑
∑
∑
Over 160 Short-answer-type Questions, distributed
over the chapters, enable the student apply the
techniques learned.
Over 119 Long-answer-type Questions, spread
across the text, test the student’s understanding of
the key concepts.
Over 236 Objective-type Questions (with key)
are given in the book which further help drill in the
concepts and tools.
(a) How many operating forces are necessary for successful operation of an indicating instrument?
Explain the methods of providing these forces.
(b) A moving-coil instrument has the following data: number of turns = 100, width of coil = 20 mm,
depth of coil = 30 mm, flux density in the gap = 0.1 Wb/m2. Calculate the deflecting torque
when carrying a current of 10 mA. Also calculate the deflection if the control spring constant is
[Ans. 60 10−6 Nm, 30°]
2 10−6 N-m/degree.
(a) What are the advantages and disadvantages of moving-coil instruments?
(b) A moving-coil voltmeter has a resistance of 200 and the full scale deflection is reached when
a potential difference of 100 mV is applied across the terminals. The moving coil has effective
dimensions of 30 mm × 25 mm and is wound with 100 turns. The flux density in the gap is 0.2
Wb/m2. Determine the control constant of the spring if the final deflection is 100° and a suitable
diameter of copper wire for the coil winding if 20% of the total instrument resistance is due to the
coil winding. Resistivity of copper is 1.7 × 10–8 Ωm.
[Ans. 0.075 × 10–6 Nm/degree; 0.077 mm]
(a) Derive the expression for the deflection of a spring controlled permanent magnet moving coil
instrument. Why not this instrument able to measure the ac quantity?
(b) The coil of a moving coil voltmeter is 40 mm × 30 mm wide and has 100 turns wound on it. The
control spring exerts a torque of 0.25 × 10−3 Nm when the deflection is 50 divisions on the scale. If
the flux density of the magnetic field in the air-gap is 1 Wb/m2, find the resistance that must be put
in series with the coil to give 1 volt per division. Resistance of the voltmeter is 10000 Ω.
[Ans. 14000 Ω]
(a) A moving-coil instrument has at normal temperature a resistance of 10 and a current of 45 mA
gives full scale deflection. If its resistance rises to 10.2 Ω due to temperature change, calculate the
reading when a current of 2000 A is measured by means of a 0.225 × 10−3. A shunt of constant
resistance. What is the percentage error?
[Ans. 44.1 mA, –1.96%]
1 2
) µH, where is the deflection
(b) The inductance of a certain moving-iron ammeter is (8 4
2
2.
3.
4.
in radian from the zero position. The control spring torque is 12 × 10−6 Nm/rad. Calculate the scale
position in radian for current of 5 A.
[2.04 rad]
(a) Discuss the constructional details of a thermocouple-type instrument used at very high frequencies.
Write their advantages and disadvantages.
(b) The control spring of a moving-iron ammeter exerts a torque of 0.5 × 10−6 Nm/degree when the
deflection is 52 . The inductance of the coil varies with pointer deflection according to
deflection (degree)
20
40
60
80
inductance ( H)
659
702
752
792
Determine the current passing through the meter.
[0.63 A]
(a) Describe the constructional details of an attraction-type moving iron instrument with the help of
a neat diagram. Derive the equation for deflection if spring control is used and comment upon the
shape of scale.
(b) Derive a general equation for deflection for a spring-controlled repulsion-type moving-iron
instrument. Comment upon the share of the scale. Explain the methods adopted to linearise the
scale.
5.
6.
EXERCISE
Objective-type Questions
1.
2.
3.
4.
5.
6.
7.
The disadvantages of using shunts for high current measurements are
(a) power consumption by the shunts themselves is high
(b) it is difficult to achieve good accuracy with shunts at high currents
(c) the metering circuit is not electrically isolated from the power circuit
(d) all of the above
The disadvantages of using multipliers with voltmeters for measuring high voltages are
(a) power consumption by multipliers themselves is high at high voltages
(b) multipliers at high voltage need to be shielded to prevent capacitive leakage
(c) the metering circuit is not electrically isolated from the power circuit
(d) all of the above
The advantages of instrument transformers are
(a) the readings of instruments used along with instrument transformers rarely depend on the
of the instrument
(b) due to availability of standardised instrument transformers and associated instrumen
reduction in cost and ease of replacement
(c) the metering circuit is electrically isolated from the power circuit
(d) all of the above
Nominal ratio of a current transformer is
(a) ratio of primary winding current to secondary winding current
(b) ratio of rated primary winding current to rated secondary winding current
(c) ratio of number of turns in the primary to number of turns in the secondary
(d) all of the above
Burden of a CT is expressed in terms of
(a) secondary winding current
(b) VA rating of the transformer
(c) power and power factor of the secondary winding circuit
(d) impedance of secondary winding circuit
Ratio error in a CT is due to
(a) secondary winding impedance
(b) load impedance
(c) no load current
(d) all of the above
Phase-angle error in a CT is due to
(a) primary winding impedance
(b) primary circuit phase angle
1. (d)
8. (d)
15. (d)
2. (d)
9. (d)
16. (d)
3. (a)
10. (d)
17. (b)
4. (b)
11. (d)
18. (a)
5. (b)
12. (b)
19. (c)
6. (d)
13. (c)
20. (a)
7. (c)
14. (b)
Short-answer Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Discuss the advantages of instrument transformers as compared to shunts and multipliers for extension
of instrument range.
Describe with clear schematic diagrams, how high voltage and currents are measured with the help of
instrument transformers.
Draw and explain the nature of equivalent circuit the and corresponding phasor diagram of a current
transformer.
Discuss the major sources of error in a current transformer.
Describe the design and constructional features of a current transformer for reducing ratio error and
phase-angle error.
Explain with the help of a suitable example, the method of turns compensation in a CT to reduce ratio
error.
Why should the secondary winding of a CT never be open circuited with its primary still energised?
Explain how the core of a CT may get permanent magnetisation induced in it. What are the bad effects
of such permanent magnetisation? What are the ways to de-magnetise the core in such situations?
Draw and explain the constructional features of wound-type, bar type, clamp type and bushing type
CTs.
Draw the equivalent circuit and phasor diagram of a potential transformer being used for measurement
of high voltages.
What are the differences between a potential transformer and a regular power transformer?
Describe the methods employed for reducing ratio error and phase angle error in PTs.
Long-answer Questions
1.
2.
Draw and explain the nature of equivalent circuit and corresponding phasor diagram of a current
transformer. Derive expressions for the corresponding ratio error and phase angle error.
(a) What are the sources of error in a current transformer?
(b) A ring-core type CT with nominal ratio 1000/5 and a bar primary has a secondary winding resistance
of 0.8 and negligible reactance. The no load current is 4 A at a power factor of 0.35 when full load
Solved Question Papers
Solved question papers of 9 universities help students
understand university question patterns and prepare for
examinations.
Solved Sample
Question Papers
SOLVED QUESTION PAPER-5
PART-A (10 × 2 = 20)
1. Answer the following
(a) Sketch a simple diagram of an electronic dc voltmeter.
Refer Section 10.6 in Chapter 10 on Electronic Instruments.
(b)
Enumerate application of CRO for measurement of electrical quantities.
Refer Section 9.7 in Chapter 9 on Cathode Ray Oscilloscope.
(c)
How many cycles of a 6 kHz sinusoidal signal appear on the CRO screen if the sweep
frequency is 3 kHz?
Two cycles.
(d)
For what measurement is an LCR meter used?
Self-inductance, capacitance, loss tangent, resistance.
(e)
A wave analyser is used for what type of analysis?
A wave analyser is an instrument designed to measure relative amplitude of single frequency
components in a complex waveform. Basically, the instrument acts as a frequency-selective
voltmeter which is turned to the frequency of one signal while rejecting all other signal
components. The desired frequency is selected by a frequency-calibrated dial to the point of
maximum amplitude. The amplitude is indicated either by a suitable voltmeter or a CRO.
(f)
For what applications can CT and PT be used?
Reducing high current and voltage to smaller values measurable with easily available lowrange ammeters and voltmeters.
(g)
Define a transducer and distinguish between active and passive transducers.
A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic,
photonic, or photovoltaic that converts one type of energy or physical attribute to
another (generally electrical or mechanical) for various measurement purposes including
measurement or information transfer (for example, pressure sensors). An active transducer
is a transducer whose output is dependent upon sources of power, apart from that supplied
by any of the actuating signals, which power is controlled by one or more of these signals.
Passive transducers are those which do not need an external source. Passive transducers
directly produce electric signals without an external energy source.
SOLVED QUESTION PAPER-9
1. Answer any Four:
(20)
(a) Define sensitivity of an analog instrument. For a PMMC instrument with FSD = 100 mA,
find the sensitivity.
Sensitivity of a voltmeter is defined as
S
Total voltmeter resistance in ohm
Full-scale reading in volts
For FSD = 100 mA, sensitivity S = 1/(100
1
I FSD
/V
10–3) = 10
(b)
What is Meggar? Explain its working.
Refer Section 4.4.4 in Chapter 4 on Measurement of Resistance.
(c)
For ADC, define resolution. Given a suitable example.
Refer Section 10.3(a) in Chapter 10 on Electronic Instruments.
(d)
Explain the working principle of a dc motor.
Readers can refer to reference.
(e)
Explain the function of delay line in oscilloscope. What are the types of delay lines?
The delay line is used to delay the incoming vertical signal and synchronise it with the
horizontal signal. Old scopes used a series of LC combinations tuned to get a good waveform
representation. Modern oscilloscopes use two types of delaying methods, namely free running
sweep and triggered sweep.
[For the remaining part of the answer, refer Sections 9.5.1 and 9.5.2 in Chapter 9 on Cathode
ray Oscilloscopes]
2. (a)
What is intensity modulation? For what purpose is it used? Can phase and frequency
be measured using intensity modulation?
10
Readers can refer to reference.
What is Q meter? Explain any one of the types of Q-meter with the help of circuit
diagram.
(10)
In most radio frequency work, it is important to obtain a large ratio of reactance to resistance
in the reactive elements of the circuit. This ratio is called the Q of the circuit.
(b)
Q
XL
R
2 f
L
R
XC
R
1
2 fCR
A high Q is required to obtain good efficiency, good waveform, good frequency stability,
high gain, etc.
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