MI Mod 1.ppt

MEASUREMENTS AND
INSTRUMENTATION
MODULE 1

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Course Outcomes
After the completion of the course student will be able to:
1. Compare different types of instruments-their working principles,
advantages and disadvantages.
2. Explain the operating principles of various ammeters, voltmeters,
ohm meters, wattmeters and energy meters
3. Describe different flux and permeability measurements methods
4. Identify the transducers for physical variable
5. Identify different AC potentiometers and bridges
6. Describe different flux and permeability measurements methods
7. Choose suitable meters for measurement of electrical quantities.

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QUESTION PAPER PATTERN
(End semester exam)
Part A: 8 questions. One question from each module of Module I - IV;
and two each from Module V & VI. Student has to answer all
questions. (8 x5)=40
Part B: 3 questions uniformly covering modules I&II Student has to
answer any 2 questions: (2 x 10) =20
Part C: 3 questions uniformly covering modules III&IV Student has
to answer any 2 questions: (2 x 10) =20
Part D: 3 questions uniformly covering modules V&VI Student has to
answer any 2 questions: (2 x 10) =20

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Syllabus – Module 1
 General Principles Of Measurements
 Measurement System
 Measurement Standards
 Characteristics
 Errors in Measurement
 Calibration of Meters
 Significance of IS Standards of Instruments
 Classification of Meters

 Operating Forces
 Essentials of Indicating Instruments
 Deflecting, Damping, Controlling Torques
 Ammeters and Voltmeters - Moving Coil, Moving Iron,
Constructional Details and Operating Principles
 Shunts and Multipliers – Extension of Range.
5

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WHAT IS MEASUREMENT?
 Measurement
 Measurement (also called metrology) is the science of
determining values of physical variables.
 A method to obtain information regarding the physical values of
the variable.
 Measurement of a given quantity is essentially an act or result of
comparison between the quantity (whose magnitude is unknown)
and predetermined or predefined standards.
 Two quantities are compared the result is expressed in numerical
values.
 the number of times the unit standard fits into the quantity being
measured is the numerical measure.
 Calibration is checking the accuracy of a measurement
instrument by comparing it to reference standards.
 Instrumentation
 Devices used in measurement system – measuring instrument

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SIGNIFICANCE OF MEASUREMENT
 Two major functions of all Engg. Branches
1. Design of equipment and processes
2. Proper operation & maintenance
 Above functions require measurements – proper & economical
design, operation & maintenance require a feedback of information

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ELEMENTS OF GENERALIZED
MEASUREMENT SYSTEM
PRIMARY
SENSING
DATA
TRANSMISSION
DATA
PRESENTATION
(Controller,
Indicator, Recorder)
VARIABLE
MENIPULATION
VARIABLE
CONVERSION
(Transducer)

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METHODS OF MEASUREMENTS
1. Direct methods
 Unknown quantity is directly compared against a
standard
 Result is expressed as a numerical value & a unit
 Commonly used for measurement of length
2. Indirect methods
 Inaccurate because they involve human factors
 Less sensitive
• In engineering applications measurement systems are used
which require need of indirect methods for measurement
purposes

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INSTRUMENTS & MEASUREMENT
SYSTEMS
 Measuring instrument exists to provide information about the
physical value of some variable being measured
 In simple case – instrument consists of a single unit which provides
an output reading according to the magnitude of unknown variable
applied to it
 Complex case – several separate elements
 Measuring instrument is commonly referred as measurement system
PRIMARY
TRANSDUCER
INTERMEDIATE
MEANS (Signal
processing)
END
DEVICE
(measurement
results)
MEASURAND

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CHARACTERISTICS OF
INSTRUMENTS
 The system characteristics are to be known, to choose an instrument
that is most suited to a particular measurement
 Performance characteristics may be broadly divided into static &
dynamic characteristics
1. Static characteristics
 Performance criterion for the measurement of quantities that
remain constant, or vary only quite slowly
2. Dynamic characteristics
 The relationship between the system input and output when the
measured quantity is varying rapidly

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1. Accuracy
 Degree of closeness or conformity to the accepted standard value or
the true value of the quantity under measurement
 The only time a measurement can be exactly correct is when it is a
count of a number of separate items:
1. Point Accuracy:
 Accuracy stated for only one or more points in its range
 Does not give any information about the general accuracy
2. Percentage of true value
3. Percentage of full-scale deflection
4. Complete accuracy statement

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2. Precision
 Instrument’s degree of freedom from random errors
 Spread will be small if a large number of readings are taken of the
same quantity
 Measure of consistency or repeatability
 High-precision instrument may be accurate or may not be accurate i.e,
precision doesn’t give guarantee about accuracy
 High accuracy instrument is a precision instrument i.e, accuracy
guarantees precision
 Accurate instruments may be precise but precise will not confirm
accuracy
 Low-accuracy measurements from a high-precision instrument are
normally caused by a bias in the measurements, which is removable
by calibration

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3. Bias
 Constant error which exists over the full range
 Removed by calibration
4. Repeatability & Reproducibility
 Repeatability is the characteristics of precision instruments
 Affected by internal noise & drift
 Expressed in % of true value
 Reproducibility is the closeness with which the same value of
the input quantity is measured at different times & under
different conditions of usage
 Spread is referred to as repeatability if the measurement
conditions are constant & as reproducibility if the measurement
conditions vary

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5. Tolerance
 Term closely related to accuracy
 Maximum error which is to be expected in some value
 Accuracy of some instruments is sometimes quoted as tolerance
figure

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6. Reliability & Maintainability
 Reliability is defined as probability that it will perform its assigned
functions for a specified period of time under given conditions
 Maintainability is the probability that in the event of failure of the
system, maintenance action under given conditions will restore the
system within a specified time
 Reliability is affected by choice of individual components in system,
manufacturing methods, quality of maintenance and the type of user

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7. Deviation
 Departure from a desired or expected value or pattern
 Difference between measured value & true value for a particular
input value
 Plus or minus sign is given
8. Scale Range & Scale Span
 Give information about lower & upper calibration points
 Range is normally from zero to some full-scale value
 Span is the difference between the full-scale & lower-scale value

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9. Scale Readability
 Depends on both the instrument & the observer
 Varies with design of the instrument & is partly governed by the
instrument sensitivity
 Extend to which the reader is enabled to read the indications

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10. Stability
 Ability to maintain its standard of performance over prolonged
periods of time
 Transducers & instruments of high stability need not be calibrated
frequently
11. Zero Stability
 Ability to restore to zero reading after the input quantity has been
brought to zero, while other conditions remain same

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15. Sensitivity
 The sensitivity of an instrument refers to its ability to detect changes in
the measured quantity
 The sensitivity of measurement is a measure of the change in
instrument output that occurs when the quantity being measured
changes by a given amount.
 Thus, sensitivity is the ratio:

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12. Resolution & Threshold
 Resolution is the minimum increment in the input from some arbitrary
non-zero value to make an observable change in the output
 Smallest increment of the input quantity to which the measuring system
responds
 Threshold – minimum value of input below which no output change
can be observed when the input is increased very gradually from zero
value
 First detectable output change which is noticeable or measurable
13. Responsiveness
 Smallest change in the quantity under measurement which results in an
actuating effort required to cause motion of the indicating part of the
instrument

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14. Drift
 Slow variation in the output signal of a transducer or measuring
system which is not due to any change in the input quantity
 Due to changes in operating conditions of components inside the
measuring system
 Prime sources occur as chemical structural changes and changing
mechanical stresses.
 Drift is a complex phenomenon for which the observed effects are that
the sensitivity and offset values vary.
 It also can alter the accuracy of the instrument differently at the
various amplitudes of the signal present

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14. Drift
I. Zero Drift
 Deviation observed in the instrument output with time from the
initial value, when all other measurement conditions are constant
 Caused by change in component value due to variation in
ambient conditions or due to ageing
 Volt per ˚C – zero drift coefficient
 Impose a bias in the output in the instrument readings

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14. Drift
II. Sensitivity or Scale Factor Drift
 Amount by which an instrument’s sensitivity of measurement
varies as ambient conditions change
 Sensitivity drift coefficient - how much drift there is for a unit
change in each environmental parameters that the instrument
characteristics are sensitive to
 Drift is an undesirable quality

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16. Uncertainty
 Expressive of the range of variation of the indicated value from the
true value
 Probable limits of error
 Quality of measuring instrument & its accuracy
17. Hysteresis
 Hysteresis causes a difference in the output curve of a sensor when the
direction of the input has been reversed.
 Hysteresis is defined as the magnitude of error caused in the output for
a given value of input, when this value is approached from opposite
directions ; i.e. from ascending order & then descending order.

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17. Hysteresis
 Mechanical systems will often show a small difference in length
as the direction of the applied force is reversed.
 The same effect arises as a magnetic field is reversed in a magnetic
material.
 This characteristic is called hysteresis
 Causes are:
 Backlash,
 Elastic deformations,
 Magnetic characteristics,
 Frictional effects (mainly).

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18. Backlash
 It is defined as the maximum distance or angle through which any
part of mechanical system may be moved in one direction without
causing motion of next part.
 Can be minimized if components are made to very close tolerances.

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18. Dead space
 Dead space is defined as the range of different input values over
which there is no change in output value.

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19. Instrument Efficiency
 Ratio of measured quantity and power absorbed by the instrument at
full scale
 Rarely provided by manufacturer
 Can be determined if the instrument impedance and the full-scale
voltage or current are known

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20. Linearity
 This is the closeness to a straight line of the relationship between the
true process variable and the measurement
 Linearity is usually reported as non-linearity, which is the
maximum of the deviation between the calibration curve and a
straight line positioned so that the maximum deviation is minimized.

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Dynamic Characteristics
 Transient response & static response
 If the input varies from instant to instant, the behavior of the system
under such conditions is dealt by the dynamic response of the system
1. Dynamic error
 Difference between true value of the quantity changing with time and
the value indicated by the instrument provided static error is zero
 Phase difference between input and output of measurement system

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2. Fidelity
 Ability of the system to reproduce output in the same form as input
 Ideal value 100% - No distortion
 Over a particular range
3. Bandwidth
 Range of frequencies for which dynamic sensitivity is satisfactory
 Dynamic sensitivity should be within 2% of its static sensitivity
 Amplitude-frequency characteristics is almost flat

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4. Speed of Response
 Ability to respond to sudden changes of amplitude of input signal
 Time taken by the system to come close to steady-state conditions for
step-input function
5. Time constant
 Time taken by the system to reach 0.632 times its final output signal
amplitude
 System having smaller time constant attains its final output amplitude
earlier than the one with larger time constant
 Related to system parameters

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6. Measuring Lag
 Delay in the response of an instrument to a change in the measurand
 Normally quite small
1. Retardation type : Response of the instrument begins
immediately after a change in measurand has occurred
2. Delay type : Response begins after a delay time after the
application of the input
7. Settling or Response Time
 Time taken to settle down to final steady-state position after the
application of the input
 Smaller settling time indicates higher speed of response
 Dependent on system parameters & varies with the conditions under
which the system operates

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8. Dynamic Range
 Range of signals which the measuring system is likely to respond
faithfully under dynamic conditions
 Ratio of amplitude of largest signal to the smallest signal to which the
system is subjected & the system can handle satisfactorily
 Expressed in dB

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ERRORS IN MEASUREMENT
 No measurement can be made with 100% accuracy
 Represented mainly using:
1. Absolute error
2. Relative error & percentage error
3. Limiting or guarantee error: Manufacturers give guarantee about
the accuracy with some limiting deviations from specified value
-limits of these deviations from the specified values are known
as limiting errors

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TYPES OF ERRORS
1. Gross Errors
 Due to mistakes in reading or using instruments & in recording
and calculating measurement results
 Because of human mistakes
 May be of any magnitude & cannot be subjected to a
mathematical treatment
 Complete elimination is not possible
 Improper use of measuring instruments, loading effect on
meters, improper reading, failure to eliminate parallax,
improper setting of zero
 Great care should be taken, two or three readings should be
taken by different observers

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2. SYSTEMATIC ERRORS (BIAS)
 Remain constant or change according to a definite law on
repeated measurement of the given quantity
 Can be evaluated & the influence can be eliminated by the
introduction of proper corrections
 A constant uniform variation of the operation of an
instrument is known as systematic error

a) Instrumental:
 Errors inherent in the instruments because of their
mechanical structure & calibration of operation of the
apparatus used.
 Improper zero adjustment, poor construction, irregular
spring tensions, variation in the air gap, calibration errors.
 Can be avoided by (1) selecting a proper measuring
device for the particular application, (2) applying
correction factors after determining the magnitude of
instrumental errors & (3)calibrating the measuring device
against a standard.
43

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b) Environmental
 Change with time in an unpredictable manner
 Use of an instrument in different conditions than in which it was
assembled & calibrated
 Change in temperature is a main reason
 humidity, altitude, earth’s magnetic field, gravity, stray
electrostatic & magnetic fields
 Can be reduced by taking the following precautions
1. Using instrument in controlled conditions
2. Deviations in local conditions can be determined & suitable
corrections to instrument readings applied
3. Using instrument which is immune to these effects
4. Using techniques that eliminate the effects of these disturbance
5. Provide electrostatic & magnetic shields
6. Altogether new calibrations can be made in new conditions

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c) Observational
 Introduced by the observer
 Parallax error – can be eliminated by providing mirror
beneath the scale & a knife-edged pointer

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3. RANDOM OR ACCIDENTAL
ERRORS
 These are the errors that remain even after systematic errors have
been substantially reduced
 Generally an accumulation of a large number of small effects
 Variable magnitude and sign & do not obey any known law
 Becomes evident when different results are obtained on repeated
measurements of one and the same quantity
 Arithmetical mean can be taken – most probable value

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SOURCES OF ERRORS
1. Poor design
2. Change in process parameters, irregularities, upsets etc.
3. Poor maintenance
4. Certain design limitations
5. Insufficient knowledge of process parameters & design conditions
6. Errors caused the person operating the instrument

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STANDARDS OF MEASUREMENT
 Physical representation of a unit of measurement
 Eg: Mass in SI system is kilogram, defined as the mass of a cubic
decimetre of water at its temperature of maximum density of 4˚C
1. International standards
2. Primary standards
3. Secondary standards
4. Working standards

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CLASSIFICATION OF INSTRUMENTS

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1. Deflection type
 Deflection of instrument provides a basis for determining the
quantity under measurement
 Measured quantity produces some physical effect which
deflects or produces a mechanical displacement of moving
system of the instrument
 Opposite effect built in the instrument
 Eg: PMMC Ammeter

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2. Null type
 An instrument in which zero or null indication determines the
magnitude of measured quantity. Such type of instrument is
called a null type instrument.
 It uses a null detector which indicating the null condition when
the measured quantity and the opposite quantity are same.
 Null condition is dependent upon some other known conditions

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1. Absolute Instruments
 Gives the magnitude of the quantity to be measured in terms of
instrument constant & its deflection
 no comparison with standard instrument
 Eg: Tangent galvanometer
2. Secondary instruments
 These have to be calibrated by comparison with an absolute
instrument
 Eg: Ammeters, Voltmeters, Wattmeters etc.

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CLASSIFICATION OF SECONDARY
INSTRUMENTS
1. Indicating instruments
 It indicate the magnitude of an electrical quantity at the time
when it is being measured.
 The indications are given by a pointer moving over a
graduated dial.
 Ordinary voltmeters, ammeters & wattmeters.

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INSTRUMENTS
2. Recording instruments
 The instruments which keep a continuous record of the
variations of the magnitude of an electrical quantity to be
observed over a defined period of time.
 X-Y plotter e.g. ECG (Electro-Cardio-Gram).

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INSTRUMENTS
3. Integrating instruments
 The instruments which measure the total amount of either
quantity of electricity or electrical energy supplied over a
period of time
 Ampere-hour meter, watt-hour (energy) meter and odometer in
a car (which measures the total distance covered)

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FUNCTIONS OF INSTRUMENTS &
MEASUREMENT SYSTEMS
1. Indicating function
2. Recording function
3. Controlling function

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ESSENTIALS OF INDICATING
INSTRUMENTS
 Indicating instruments are those which indicate the value of quantity
that is being measured at the time at which it is measured.
 Consist essentially of a pointer which moves over a calibrated scale
& which is attached to a moving system pivoted in bearing.
 The moving system is subjected to the following three torques:
1. Deflecting ( or operating) torque
2. Controlling ( or restoring) torque
3. Damping torque

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1. DEFLECTING ( OR OPERATING)
TORQUE
 The deflecting torque is produced by making one of the magnetic,
heating, chemical, electrostatic and electromagnetic induction effect
of current or voltage
 It causes the moving system of the instrument to move from its zero
position.
 The method of producing this torque depends upon the type of
instrument.

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2. CONTROLLING (OR RESTORING)
TORQUE
 The magnitude of movement of the moving system would be
somewhat indefinite under the influence of deflecting torque, unless
the controlling torque existed to oppose the deflecting torque.
 It increases with increase in deflection of moving system.
 The controlling torque serves two functions :
1. The pointer stops moving beyond the final deflection
2. The pointer comes back to its zero position when the
instrument is disconnected.

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TORQUE
 It is produced by either spring or gravity.
 for spring control Tc α θ
 for gravity control Tc α sinθ, where θ- deflection

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TORQUE
a) Spring Control
 Most commonly used.
 One or two hairsprings made of phosphor bronze are used.
 The outer end of this spring is fixed to the pointer and the inner
end is attached with the spindle.
 When the pointer is at zero of the scale, the spring is normal.
 As the pointer moves, the spring winds and produces an
opposing torque.
 The balance-weight balances the moving system so that its
centre of gravity coincides with the axis of rotation, thereby
reducing the friction between the pivot and bearings.

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a) SPRING CONTROL
 Advantages :
 Since
 These instruments have uniform scale.
 Disadvantages :
 The stiffness of the spring is a function of temperature.
 Hence, the readings given by the instruments are temperature
dependent.
 Furthermore, with the usage the spring develops an inelastic
yield which affects the zero position of the moving system.
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64
a) SPRING CONTROL
Double Springs
 Two springs A and B are wound in opposite directions.
 On deflection, one spring winds while the other unwinds.
 The controlling torque produced is due to the combined torsions of
the two springs.
 To make the controlling torque directly proportional to the angle of
deflection, the springs should have fairly large number of turns.

65
a) SPRING CONTROL
Double Springs

67
b) GRAVITY CONTROL
 In gravity controlled instruments, a small adjustable weight is
attached to the spindle of the moving system
 The deflecting torque produced by the instrument has to act against
the action of gravity.
 Thus a controlling torque is obtained.
 This weight is called the control weight.
 Another adjustable weight is also attached to the moving system for
zero adjustment and balancing purpose. This weight is called
Balance weight.
 In zero position of the pointer, this control weight is vertical.

68
b) GRAVITY CONTROL
 When deflected by an angle θ, the
weight exerts a force,
 The restraining or controlling torque is
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69
b) GRAVITY CONTROL
Disadvantage :
1. These do not have uniform scale.
2. These must be used in vertical position so that the control may
operate properly.
Advantages :
1. Less expensive.
2. Unaffected by changes in temperature.
3. Free from fatigue or deterioration with time.

71
3. DAMPING TORQUE
 The remedy lies in providing a suitable damping torque.
 If over-damped, the time-delay in taking the reading becomes
unnecessarily long.
 If under damped, the oscillations of the pointer would not be killed
completely.
 Thus, the damping torque should be just sufficient to kill the
oscillation without increasing the delay-time.
 This condition is said to be critically damped or ‘dead beat’.

72
3. DAMPING TORQUE
 The moving system of the instrument will tend to move under the
action of the deflecting torque.
 But on account of the control torque, it will try to occupy a position
of rest when the two torques are equal and opposite.
 Due to inertia of the moving system, the pointer will not come to
rest immediately but oscillate about its final deflected position and
takes appreciable time to come to steady state.
 To overcome this difficulty a damping torque is to be developed by
using a damping device attached to the moving system.

74
3. DAMPING TORQUE
1. Under damped condition:
 The response is oscillatory
2. Over damped condition:
 The response is sluggish and it rises very slowly from its zero
position to final position.
3. Critically damped condition:
 When the response settles quickly without any oscillation, the
system is said to be critically damped and the instrument as
“Dead beat”
 In practice to obtain best results the damping is adjusted to the
value slightly less than the critical value

75
METHODS FOR OBTAINING
DAMPING TORQUES
1. Air Friction Damping
2. Fluid Friction Damping
3. Eddy Current Damping (Most commonly employed
method)

82
AMMETERS AND VOLTMETERS
 Works on the same principle
 Ammeter:
 Carry the current to be measured or a definite fraction
of it and this current or definite fraction of it produces
the deflecting torque
 Connected in series with the circuit
 Must be of very low resistance so that the voltage drop
across the ammeter & power absorbed from the circuit
are as low as possible

83
AMMETERS AND VOLTMETERS
 Voltmeter:
 Carries the current proportional to the voltage to be
measured which produces the deflecting torque
 Connected in parallel with the circuit across which the
voltage is to be measured
 Must be of high resistance so that the current flowing
through the voltmeter & the power absorbed from the
circuit are minimum possible

84
AMMETERS AND VOLTMETERS -
TYPES
1. DC Instruments
 PMMC Instruments
2. AC Instruments
 Makes use of electromagnetic induced currents
 Induction instruments
3. DC/AC Instruments
 Deflections proportional to square of current or
voltage under measurement
 Moving iron, dynamometer type moving coil, hot-
wire, electrostatic instruments

85
MOVING COIL INSTRUMENTS
 There are two types of moving coil instruments
1. Permanent magnet moving coil (PMMC) type
which can only be used for direct current, voltage
measurements. Most accurate. Popularly known as
d’Arsonval Movement
2. Dynamometer type which can be used on either
direct or alternating current, voltage measurements.
Popularly known as transfer instruments

86
PERMANENT MAGNET MOVING
COIL (PMMC) INSTRUMENTS
 “When a current carrying
conductor is placed in a
magnetic field, it experiences
a force and tends to move in
the direction as per
Fleming’s left hand rule.

88
COIL(PMMC) INSTRUMENTS
 It consists of an iron-core coil
mounted on bearings between
permanent magnet
 Very fine insulated wire of many
turns is used
 Coil is wound on an Aluminium
bobbin which is free to rotate by
about 90◦
 An Aluminium pointer attached
to the coil can move on a
calibrated scale.
 Two springs one at top and other
at bottom were attached to the
assembly and serves two
purposes
 One is to provide path for
current and other for providing
controlling torque.

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 Core is made of soft iron
 Magnetic poles & iron core
are cylindrical in shape. This
has two advantages
 Firstly, the length of the air gap
is reduced (flux leakage=0)
 Secondly, the iron core helps in
making the field radial in the
air gap which ensures uniform
magnetic field throughout the
motion of the coil.
 This way the angle of
deflection is proportional to the
current in the coil and hence
the scale is uniform

90
COIL (PMMC) INSTRUMENTS
 When a current is passed
through a coil in a magnetic
field, the coil experiences a
torque proportional to the
current.
 A coil spring provides the
controlling torque.
 The deflection of a needle
attached to the coil is
proportional to the current.
 Damping is caused by the eddy
current set up in the aluminum
coil which prevents the
oscillation of the coil.

93
TORQUE EQUATION
Since the force F=NIBL , is directly proportional to the
current I and to the flux density B in the air gap, the net
deflecting torque=NIBA, Where A = area of the coil=Ld
The controlling torque of the spiral springs (with c as spring
constant)
In the final steady position,
The deflection is proportional to the current and hence the
scale is uniformly divided
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ck
I I
c
  
  


96
 Advantages :
(i) High sensitivity.
(ii) Uniform scale.
(iii) Well shielded from any stray magnetic field.
(iv) High torque/weight ratio.
(v) Effective and reliable eddy-current damping.
 Disadvantages :
(i) Cannot be used for ac measurement.
(ii) More expensive compared to moving-iron type.
(iii)Ageing of control springs and of the permanent
magnets might cause errors.

97
MOVING IRON TYPE INSTRUMENTS
 Laboratories & switchboards at commercial frequencies
– cheap, robust & high accuracy
 Attraction type & Repulsion type
 Attraction type operates on the principle of attraction of
a single piece of soft iron into a magnetic field &
repulsion type operate on the principle of repulsion of
two adjacent iron pieces magnetized by the same
magnetic field

 Repulsion type are more sensitive
 Current under measurement is passed through a coil of
wire & this sets up the necessary magnetic field
 Instruments to be used as ammeter is provided with a
coil of few turns of thick wire & that to be used as
voltmeter is provided with a coil of large number of turns
of fine wire
98

99
MOVING IRON TYPE INSTRUMENTS
 Moving element: a small piece of soft iron in the form
of a vane or rod.
 Coil: to produce the magnetic field due to current
flowing through it and also to magnetize the iron pieces.
 In repulsion type, a fixed vane or rod is also used and
magnetized with the same polarity.
 Control torque is provided by spring or weight
(gravity).

 Damping torque is normally pneumatic, the damping
device consisting of an air chamber and a moving vane
attached to the instrument spindle.
 Deflecting torque produces a movement on an
aluminum pointer over a graduated scale.
100

101
1. ATTRACTION TYPE
 Simplest form uses a solenoid & moving oval shaped soft
iron pivoted eccentrically
 To this iron a pointer is attached
 Iron is made of sheet metal shaped specially for uniform
scale
 Moving iron is drawn into the field of solenoid when
current flows through it – always from weaker field to
stronger
 Spring or gravity control & pneumatic damping

103
2. REPULSION TYPE
(a) Radial Type (b) Co-axial Type

105
2. REPULSION TYPE
 Two irons – One fixed & another movable
 The two irons lie in the magnetic field produced by a
solenoid
 When there is no current, the two irons are almost touching
each other & pointer rests in zero position
 When current is passed through the solenoid, a magnetic
field is set up inside the solenoid & the two irons are
magnetized in the same direction

 This sets up a repulsive force so moving iron is repelled
by fixed iron
 Pointer comes to rest in a deflected position when
equilibrium is attained between repulsive forces &
controlling force
 Hair spring or gravity control & air friction damping
 Repulsion of irons is proportional to square of current,
& so the scale is uneven, crowded at low values and
wide spread at high values
106

108
APPLICATIONS
Measurement of Electric Voltage and Current
Moving iron instruments are used as Voltmeter and
Ammeter only.
Both can work on AC as well as on DC.

109
ADVANTAGES
The instruments are suitable for use in AC and DC
circuits.
The instruments are robust, owing to the simple
construction of the moving parts.
The stationary parts of the instruments are also simple.
Instrument is low cost compared to moving coil
instrument.
Torque/weight ratio is high, thus less frictional error.

110
DYNAMOMETER TYPE
 This instrument is suitable for the measurement of direct
and alternating current, voltage and power.
 The deflecting torque in dynamometer is relies by the
interaction of magnetic field produced by a pair of fixed
air cored coils and a third air cored coil capable of
angular movement and suspended within the fixed coil.

111
DYNAMOMETER TYPE
 These instruments are similar to the permanent magnet
type instruments, except that the permanent magnet is
replaced by a fixed coil.
 The coil is divided into two halves, connected in series
with the moving coil.
 The two halves of the coil are placed close together and
parallel to each other to provide uniform field within the
range of the movement of moving coil.

114
DYNAMOMETER TYPE
 The deflecting torque depends
on the fields of both fixed and
moving coils
 Deflecting torque is proportional
to square of the current.
 Moving coil is wound using a
thin wire so that it deflects
easily.
 Can be used as Voltmeter or
Ammeter
 Best suits as a power meter

115
DYNAMOMETER TYPE
Advantages :
(i) Can be used on both DC and AC systems
(ii) No errors due to hysteresis or eddy currents
(iii) Good accuracy
(iv) Same calibration for DC and AC measurements and
hence can be used as Transfer Instruments ( used in
situations where you can not measure directly. The
measurement is transferred to another means of
measurement)

DYNAMOMETER TYPE
Disadvantages :
(i) Non-uniform scale
(ii) Torque/weight ratio is small
(iii) Low sensitivity than PMMC
(iv) More expensive than PMMC
116

117
EXTENSION OF INSTRUMENT RANGE
 Range is limited by current, which can be carried by
the coil of the instrument safely
 For measurement of large currents or voltages,
some means for increasing the range of the
instrument is to be adopted
 Common devices employed:
1. Shunts
2. multipliers
3. Current transformers
4. Potential transformers

118
AMMETER SHUNTS
 Low resistance placed in parallel in order to measure
fairly large currents
 Greater part of the current in main circuit is diverted
around the coil through the shunt
M
Ammeter

119
AMMETER SHUNTS
 I = Im + Is
 Rs = Rm / (N-1)
 N = I / Im - Multiplying power or Range multiplier
of the shunt

120
SHUNTS FOR AC INSTRUMENTS
 Rarely used
 Inductance of both the instrument & shunt must be taken
into account as well as their resistances
 Lm/Rm = Ls/Rs = k
 N = 1 + Rm/Rs

121
AMMETER SHUNTS
 Consider a d’Arsonval movement having internal
resistance (Rm) of 500 Ω.
 The full-scale deflection current, Im, for this instrument
is 0.1 mA.
 When full-scale current flows, the voltage across its
terminals is given as
 So, it can serve either as an ammeter of range 0 - 0.1mA,
or as a voltmeter of range 0 - 50 mV.
 We need to extend the range of the meter, by providing a
suitable additional circuitry.
m
m
m
(
0
.
1
m
A
)
(
5
0
0
)
5
0
m
V
V
I
R


 



124
50 µA
1 k
BASIC METER MOVEMENT
The basic meter movement
and an internal resistance ( Ri) rating.
Using these two ratings, the full-scale voltage rating (Vfs) is:
Vfs = Ifs x Ri = 50 µA x 1 k =50 mV
has a full-scale current (Ifs) rating
50 mV

125
ANALOG AMMETERS
This meter movement is a 50 µA ammeter.
Its range can be extended by adding a shunt resistor.
50 µA
1 k
50 mV
The shunt resistance for a 1-mA range is calculated thus:
Rshunt = Vfs / (Irange - Ifs) = 50 mV / (1 mA - 50 µA) = 52.63 
Rshunt

126
Example 1
1. An ammeter uses a meter with an internal resistance of
600 W and a rating of 1 mA fsd. How can it be used to
measure 20 A fs?
0.0300015 W (in parallel.)

127
Example 2
2. A moving coil ammeter has a full-scale deflection of 50
μA and a coil resistance of 1000Ω. What will be the
value of the shunt resistance required for the instrument
to be converted to read a full-scale reading of 1 A?
0.0500025 Ω

128
Example 3
3. The coil of a measuring instrument has a resistance of
1Ω and the instrument has a full-scale deflection of 250
V when a resistance of 4999 Ω is connected in series
with it, find:
a) The current range of the instrument when used as an
ammeter with the coil connected across a shunt of
(1/499) Ω, and
b) The value of the shunt resistance for the instrument
to give a full-scale deflection of 50 A.
25 A & (1/999)Ω

130
UNIVERSAL SHUNT FOR MULTI-
RANGE MILLI-AMMETER

131
AMMETER SENSITIVITY
 Measured in ohms/amp; should be as low W/A (small V
drop) as possible.
 Sensitive ammeters need large indicator changes for
small current.
 Example : (1) A 0.01 W/A meter with 5 A fsd,
Rm = W/A x A = 0.01 x 5 = 0.05 W
Vmax across the Meter will be
5 A x 0.05 W = 0.25 V for fs.
(2) A 0.1 W/A meter with 5 A fsd,
will drop 2.5 V (i.e., it is 10 times less
sensitive), which may bias the results.

132
AMMETER LOADING
 Significant where ammeters are used in circuits with
components of resistance comparable to that of the
meter.
+
-
1.0 V
1.0 
A
What is the current
in the circuit ?
Is it i = 1 V / 1 Ω = 1 A ?

133
AMMETER LOADING
 Now, suppose that the meter has a resistance of 1 W.
 How much will be current in the circuit ?
 Obviously, the current in the circuit will be halved !
 When working with low value resistors, be sure to use
very low impedance ammeters.

134
EXTENDING THE RANGE OF
VOLTMETERS
 Constructed by adding a high resistance (R) in
series with an electrically sensitive meter (M).
M
Voltmeter
R

135
VOLTMETERS
 Suppose that we want to extend the voltage range of this
basic meter to 0-10 V.

136
VOLTMETERS
The total resistance RT must be such that
m
T T
m
1
0
V
o
r 1
0
0
k
Ω
0
.
1
m
A
V
V
I
R R
I
 
 
s T
m
1
0
0
k
0
.
5
k
R
R
R








9
9
.
5
k
Ω
Now, suppose that the range of a basic meter is to be extended to
Vfsd volts. Then, we should have
f
s
d
f
s
d m
m s s m
m
( ) o
r
V
V
IR
R R R
I
  
The series resistor Rs is also called a range-multiplier, as it
multiplies the voltage range.

137
50 µA
1 k
BASIC METER MOVEMENT
The basic meter movement
and an internal resistance ( Ri) rating.
Using these two ratings, the full-scale voltage rating (Vfs) is:
Vfs = Ifs x Ri = 50 µA x 1 k =50 mV
has a full-scale current (Ifs) rating
50 mV

138
ANALOG VOLTMETERS
This meter movement is a 50 mV voltmeter.
Its range can be extended by adding a multiplier resistor.
50 µA
1 k
50 mV
The multiplier resistance for a 20-V range is calculated thus:
Rmult = (Vrange - Vfs) / Ifs = (20 V - 50 mV) / 50 µA = 399 k
Rmult

139
Example 4
4. A meter is rated at 1 mA fsd and has an internal
resistance of 2000 Ω. How can it be used to measure
100 V fsd ?
98 kΩ

140
Example 5
5. A 50-μA meter movement with an internal resistance
of 1 kΩ is to be used as a dc voltmeter of range 50 V.
Calculate
(a) the multiplier resistance needed, and
(b) the voltage multiplying factor.
999 kΩ & 1000

141
Meter Sensitivity
(Ohms-per-Volt Rating)
 Measured in Ω/V.
 Higher the sensitivity, more accurate is the measurement.
 If current sensitivity (CS) of a meter is known, its Ω/V
rating can easily be determined.
 Consider a basic meter with CS of 100 μA.
 If used as a voltmeter of range 1 V,
RT = 1 V / 100 μA = 10 kΩ
 Thus, the meter sensitivity is simply 10 kΩ/V.

142
Meter Sensitivity
(Ohms-per-Volt Rating)
In general,
y
sensitivit
current
1
rating
volt
-
per
-
ohms 
• Note that if the same meter was used for 2 V range,
the required RT would be 20 kΩ.
• Its ohms/volt rating is 20 kΩ / 2 V = 10 kΩ/V.
• The ohms-per-volt rating does not depend on the range
of the voltmeter.

143
Voltmeter Loading
 A voltmeter, when connected, acts as a shunt for that
portion of the circuit.
 This reduces the resistance of that portion.
 Hence, the meter gives a lower reading.
 This effect is called the loading effect of the meter.

144
Example 6
6. It is desired to measure the voltage across the 50-kΩ resistor in the
circuit. Two voltmeters are available for this measurement.
Voltmeter-A has a sensitivity of 1000 Ω/V and voltmeter-B has a
sensitivity of 20 000 Ω/V. Both meters are used on their 50-V
range. Calculate :
a) the reading of each meter, and
b) the error in each reading, expressed as a percentage of the true
value.

145
Solution : The true value of the voltage across A-B,
  V
50
k
Ω
50
k
Ω
100
k
Ω
50
V
150
t 



V

146
(a) Voltmeter-A
The internal resistance,
1
S
e
n
s
i
t
i
v
i
t
y
R
a
n
g
e
(
1
0
0
0
/
V
)
(
5
0
V
)
5
0
k
Ω
i
R
 




When connected, the equivalent parallel resistance across A-B
is 50 kΩ || 50 kΩ = 25 kΩ. Hence, reading of voltmeter,
  V
30




k
Ω
25
k
Ω
100
k
Ω
25
V
150
1
V
Voltmeter-B
2
S
e
n
s
i
t
i
v
i
t
y
R
a
n
g
e
(
2
0
0
0
0
/
V
)
(
5
0
V
)
1
0
0
0
k
Ω
i
R
 




A
-
B
E
q(
5
0
k
)
|
|
(
1
0
0
0
k
)
4
7
.
6
k
R
 
 
 
2
4
7
.
6
k
Ω
1
5
0
V
1
0
0
k
Ω
4
7
.
6
k
Ω
V

  

4
8
.
3
6
V

147
(b) Error in reading of Voltmeter-A,
%
40






 %
100
50
30
50
%
100
Error
%
t
1
t
V
V
V
Error in reading of Voltmeter-B,
%
3.28






 %
100
50
36
.
48
50
%
100
Error
%
t
2
t
V
V
V
Note the voltmeter with higher sensitivity gives more
accurate results, since it produces less loading effect on
the circuit.

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