EET-203

MEASUREMENTS AND
INSTRUMENTATION

ANUVINDA V
TEXTBOOKS
MODULE 1
 MEASUREMENTS
• Process by which one can convert physical parameters
into meaningful numbers.
• Measurement of a given quantity is an act of comparison
between the quantity(whose magnitude is unknown) and a
predefined standard.
• The numerical measure is meaningless unless followed by
a unit.
• For the result of measurement to be meaningful, there are
two basic requirements,
1)the standard used for comparison purposes must be
accurately defined and should be commonly accepted.
2)the apparatus used and the method adopted must be
provable.
METHODS OF MEASUREMENTS
• Broadly classified into two.
1)Direct method
o The unknown quantity is directly compared against
a standard
o The result is expressed as a numerical number and
a unit.
2)Indirect method
o Use measurement systems
 STANDARDS OF MEASUREMENT
• A standard is defined as something that is set up and
established by an authority as rule of the measure of a
quantity like length, weight, value or quality.
• For example, a meter is a standard established by an
international organization for measurement of length.
• Industry, commerce, international trade in modern
civilization would be impossible without a good system
of standards.
• The role of standards is to achieve uniform, consistent
and repeatable measurements throughout the world.
 STANDARDS OF MEASUREMENT
• Depending upon the degree of accuracy required for the
work, the standards are divided into following four
categories or grades:
1. Primary Standards (Reference Standards).
2. Secondary Standards (Calibration Standards)
3. Tertiary Standards (Inspection Standards).
4.Working Standards (Workshop Measuring Standards).
 STANDARDS OF MEASUREMENT
1. Primary Standards (Master Standards)
o The primary standard is also known as Master
Standard, and is preserved under the most careful
conditions.
o These standards are not commonly in use.
o They are used only after long internals.
o They solely used for comparing the secondary
standards.
o Sometimes it is also called Reference Standards.
 STANDARDS OF
MEASUREMENT
2.Secondary Standards (Calibration Standards)
o The secondary standard is more or less similar to the
primary standard.
o They are nearly close in accuracy with primary
standards.
o The secondary standard is compared at regular
intervals with primary Stands and records their
deviation.
o These Standards are distributed to a number of places
where they are kept under safe custody. They are used
occasionally for comparing the territory standards.
 STANDARDS OF
MEASUREMENT
3.Tertiary Standards (Inspection Standards)
o The Tertiary standard is the first standard to be used
for reference purpose in workshops and laboratories.
o They are used for comparing the working standards.
o These are not used as frequently and commonly as
the working standards but more frequently than
secondary standards.
o Tertiary standards should also be maintained as a
reference for comparison at intervals for working
standards.
 STANDARDS OF
MEASUREMENT
4. Working Standards (Workshop Measuring Standards)
o The working standard is used for actual measurement
in workshop or laboratories by the workers.
o These standards should also be as accurate as
possible to the tertiary standard.
o But sometimes, lower grades of materials can be
used for their manufacturing to reduce cost.
 STATIC CHARACTERISTICS OF
MEASURING INSTRUMENTS
• Static error
• Accuracy
• Precision
• Linearity
• Sensitivity
• Reproducibility
• Drift
• Dead zone
• Resolution
• Scale range and scale span
 True Value of a quantity ????
o The average of an infinite number of measured
values when the average deviation due to the various
contributing factors tends to zero.

While measuring a quantity, we always rely on a set of

readings instead of a single reading. The set of readings
will
be in normal /Gaussian distribution.
 STATIC ERROR
• It is the difference between measured value and true
value of the quantity.

δA is error
Am is measured value of quantity
At is true value of quanti

•δA is also called absolute static error (Eo)

 STATIC ERROR
 Relative Static Error (RSE)
o Ratio of absolute static error (δA) to true value(At).
RSE= Er= δA/ At
% Er = Er *100

 Static Correction
o Difference between true value and measured value of
the quantity.
δC = At – Am = - δA
 PROBLEMS
• A meter reads 127.5 V and the true value of the voltage
is 127.43 V. Determine a)static error, b)static correction
for this instrument.
Answer
a)
=127.50-127.43
= +0.07 V
b) δC= -δA
= -0.07 V
 PROBLEMS
• A thermometer reads 95.45°C and the static correction
given in the correction curve is -0.08°C. Determine the
true value of the temperature.
o Answer
δC= -0.08°C
δA= +0.08°C

At =95.45-0.08
=95.37°C
 PROBLEMS
• A voltage has a true value of 1.50V .An analog indicating
instrument with a scale range of 0-2.50V shows a voltage of
1.46V .What are the values of absolute error and correction.
Express the error as a fraction of the true value and the full
scale deflection (fsd).
Answer

• Absolute error δA= A – A

m t

= 1.46-1.5
= -0.04V
• Absolute correction δC = - δA
= +0.04 V
• %RSE= %Er= (δA/ A ) * 100
t

= -2.67 %
• Relative error (expressed as a % of fsd)=(-0.04/2.5) * 100
= -1.60 %
 ACCURACY AND PRECISION
 Accuracy and precision same???
• In ordinary usage , the distinction between accuracy and
precision is usually very vague.
• But in the field of measurement, they are different.
• Accuracy is the closeness with which an instrument
reading approaches the true value of the quantity being
measured.
• Precision is the capability of an instrument to repeat and
reproduce the same reading.
• Precision does not guarantee accuracy.
• Accuracy leads to precision.
 LINEARITY
• An instrument is considered to be linear if its output is
linearly proportional to the input.
• Linearity usually reported as non linearity, which is the
maximum of the deviation between the calibration curve
and a straight line positioned.
 SENSITIVITY
• Ratio of change in output to change in input.
• In case of measuring instruments high sensitivity is
always preferred for higher accuracy.
 REPRODUCIBILITY AND DRIFT
• Reproducibility is the degree of closeness with which a
given value may be repeatedly measured.
• Perfect reproducibility means that the instrument has no
drift.
• No drift means that with a given input, the measured
value do not vary with time.
• Difference b/w repeatability and
reproducibility
– Repeatability-same meter , same observer,
conducting experiments under same
conditions
– Reproducibility-same meter , different
observer and conducting experiment under
same conditions.
 DEAD ZONE
• This is the minimum value of input beyond which
response will occur in an instrument.
• It is also called as threshold.
 RESOLUTION
• It is the smallest output we can detect in the scale of an
instrument with certainty and clarity.
• Resolution= Full scale deflection/Total no of divisions
• As the number of divisions increases, resolution
increases.
 SCALE RANGE AND SCALE SPAN
• Scale range is the largest reading that an instrument can
read.
• Scale span is the difference between the largest and the
smallest reading of the instrument.
• For example, the largest value an instrument can read is
‘y’ units and smallest value it can read is ‘x’ units.
– Then scale range is y (or x to y)
– -scale span is y-x
 ERRORS IN MEASUREMENT

 Limiting errors(Guarantee errors)

– In order to assure the purchaser of the quality of the
instrument , the manufacturer guarantees certain
accuracy.
– The manufacture has to specify the deviations from
the nominal value of a particular quantity.
– The limits of these deviations from the specified
value are defined as limiting errors or guarantee
errors.
ERRORS IN MEASUREMENT
– If the magnitude of a quantity has a nominal value of
‘As’ and maximum error or limiting error of ‘±δA’ must
have a magnitude ‘Aa’ between the limits As ‘- δA’
and As ‘+δA ‘
– Actual value of quantity Aa=As ± δA
– For example nominal magnitude of a resistor is 100Ω
with a limiting error of ± 10 Ω,then the magnitude of
resistance between the limits;
Aa=100 ±10 Ω or Aa≥90 Ω and Aa≤110
Ω
 ERRORS IN MEASUREMENT
 Relative or fractional limiting error
– It is the ratio of error to the specified(nominal)
magnitude of a quantity.
 PROBLEM
• The value of capacitance of a capacitor is specified as 1
μF±5% by the manufacturer. Find the limits between
which the value of the capacitance is guaranteed.

Solution

=1 X (1±0.05)= 0.95 to 1.05 μF

 TYPES OF ERRORS
• Errors may originate in a variety of ways, and are usually
classified into three main categories
(1) Gross errors
(2) Systematic errors
(3) Random errors(Accidental errors)
 TYPES OF ERRORS
 Gross Errors:
– Gross errors are due to human mistakes in reading or using
instruments & in recording and calculating measurement
results .
– Complete elimination is not possible
– For example, person may read a pressure gauge indicating
1.01 N/m2 as 1.10 N/m2.
– Gross error may be of any amount and therefore their
mathematical analysis is impossible.
– However they can be avoided by adopting two means,
1)Great care should be taken in reading and recording the data
2) Two, three or even more readings should be taken for the quantity
under measurement , preferably by different experimenters
 TYPES OF ERRORS
 Systematic errors
– Systematic errors are divided into three
categories
i)Instrumental errors
ii)Environmental errors
iii)Observational errors
 TYPES OF ERRORS
i)Instrumental errors
– Instrumental errors arise due to inherent
shortcomings in the instrument, misuse of the
instrument and due to loading effects of instrument
– Errors due to inherent shortcomings of instrument
refers to the errors inherent in the instruments
because of their mechanical structure
– They may be due to construction, calibration or
operation of the instruments.
– This cause the instrument to read too low or too high.
– An example is, if the spring of a permanent magnet
instrument become weak, the instrument will always
read high.
 TYPES OF 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) careful recalibration of the
instrument.
– Misuse of instruments /using instruments in an
unintelligent way also may give erroneous results.
– Examples of this misuse of instrument may be failure
to adjust the zero of the instrument, poor initial
adjustments and using leads of too high resistance etc.
– Errors due to loading effect occur due to the use of an
instrument with lower sensitivity with heavier load.
 TYPES OF ERRORS
ii)Environmental errors
–This error is due to the use of an instrument in
different conditions than in which it was assembled &
calibrated.
–These may be due to change in temperature, humidity,
altitude, earth’s magnetic field, gravity, stray
electrostatic & magnetic Fields .
–Can be reduced by taking the following precautions.
a) Using instrument in controlled conditions.
b)Using techniques that eliminate the effects of these
disturbance.
c)If external magnetic field and electrostatic field
effect the reading, provide electrostatic & magnetic
shields.
 TYPES OF ERRORS
iii)Observational errors
– Introduced by the observer and are of the following
types
a)Construction of the Scale : There is a possibility of
error due to the division of the scale not being
uniform and clear.
b) Fitness and Straightness of the Pointer : If the
pointer is not fine and straight, then it always gives
the error in the reading
c)Parallax : Without a mirror under the pointer there
may be parallax error in reading
d)Efficiency or Skill of the Observer : Error in the
reading is largely dependent upon the skill of the
observer by which reading is noted accurately
 TYPES OF ERRORS
 Random errors(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.
– These errors may be either positive or negative .
– These are random error and their magnitudes are not
constant.
 TYPES OF ERRORS
– Persons performing the experiment have no control
over the origin of these errors.
– Generally, these errors may be minimized by taking
average of a large number of readings.
 CALIBRATION OF METERS
• Instrument calibration is one of the primary processes
used to maintain instrument accuracy.
• Calibration is the process of configuring an instrument to
provide a result for a sample within an acceptable range
• Eliminating or minimizing factors that cause inaccurate
measurements is a fundamental aspect of
instrumentation design
• So basically calibration of meters is the process of
design of instruments by referencing standard
instruments which are not used for ordinary purpose
 INSTRUMENTS AND
MEASUREMENT SYSTEMS
HISTORY OF DEVELOPMENT OF
INSTRUMENTS
 Mechanical Instruments
• The first instrument used by mankind were mechanical
in nature
• Very reliable for static and stable condition
• Unable to respond rapidly to measurement of dynamic
and transient condition.
 Electrical Instruments
• More rapid than mechanical instruments.
• But it depends upon a mechanical meter movement as
indicating device.
• Hence these instruments have a limited time response.
• Eg:- enrgy meter
HISTORY OF DEVELOPMENT OF
INSTRUMENTS
 Electronic Instruments
• Since the only movement require in electronic
instrument is the movement of electrons, the
response time is extremely small.
• Eg-CRO
CLASSIFICATION OF INSTRUMENTS
I)Broadly instruments are classified into two category.
1)Absolute instrument:- these instruments give the
magnitude of quantity under measurement in terms
of physical constants of the instrument.
Eg:-Tangent galvanometer.
2)Secondary instruments:- these instruments are
calibrated by comparison with absolute instrument.
Eg:-galvanometer
Secondary instruments have scale and pointer display.
CLASSIFICATION OF INSTRUMENTS
II) Instruments can be classified into two major categories
depending upon the way they present the result of
measurement.
1)Deflection type instrument:-The deflection of
pointer over a calibrated scale gives the quantity
under measurement
Eg:-voltmeter
2)Null type instrument:-A null or zero indication leads
to the determination of magnitude of unknown
quantity
Eg:-potentiometer
CLASSIFICATION OF INSTRUMENTS
III) Secondary instruments can be divided into two
1)Analog instruments:-signals that vary in a continuous
fashion and take on an infinite number of values in any
given range are called analog signals and the
instruments for measuring these signals are called
analog instruments.

2)Digital instruments:-signals which vary in discrete steps

and thus take up only finite different values in a given
range are called digital signals . Instruments used for the
measurement of digital signal are digital instruments
 ANALOG INSTRUMENTS
CLASSIFICATION
I)Analog electrical instruments are classified into three
according to the current that can be measured by them.
1)direct current instrument
2)alternating current instrument
3)both direct and alternating current instrument
(universal instrument)
 ANALOG INSTRUMENTS
CLASSIFICATION
II)Depending upon their operation classified into three
1)Indicating instruments
•Indicate the value of a quantity being measured.
•Pointers moving over the scale give the
indication
•E.g., Ammeters, Voltmeters , wattmeters etc.
2)Recording Instruments
•A continuous record of variations of the electrical
quantity over a long period of time is given by these
instruments
•It has a moving system which carries an inked pen
which rests tightly on a graph chart
•E.g., Graphic recorders and galvanometer recorders
are the examples of these instruments
 ANALOG INSTRUMENTS
CLASSIFICATION
3)Integrating instrument:-
• The total amount of either electricity or electrical
energy supplied over a period of time is measured
by these instruments
• E.g:- Energy meters, Ampere hour meters etc
 ANALOG INSTRUMENTS
CLASSIFICATION
III)Secondary analog instrument may be classified
according to the principle of operation they utilize . The
effects they utilize are,
1)magnetic effect
2)heating effect
3)electrostatic effect
4)electromagnetic effect
5)hall effect
 INDICATING INSTRUMENTS
OPERATING FORCES/TORQUES
• In order to ensure proper operation of indicating
instruments, the following three torques are required
1)Deflecting torque Td or Operating torque
2) Controlling torque Tc or Restoring torque
3) Damping torque
 DEFLECTING TORQUE (Td)
• The deflecting torque is produced by utilizing the
various effects of electric current or voltage(magnetic
effect, induction effect, thermal effect, hall effect etc).
• This causes the moving system to move and hence the
pointer to move from zero position.
• A deflecting system converts an electrical signal to a
mechanical force .
• The method of producing this torque depends upon
the type of instrument
CONTROLLING TORQUE (Tc)
• It is the torque which controls the movement of the
pointer.
• The magnitude of movement of the moving system
would be somewhat indefinite under the influence of
deflecting torque, unless the controlling torque
exerted to oppose the deflecting torque .
• It increases with increase in deflection of moving
system
• Controlling torque is also called restoring or
balancing torque
CONTROLLING TORQUE (Tc)
– The controlling torque serves two functions
• The pointer stops moving beyond the final
deflection
• The pointer comes back to its zero position when
the instrument is disconnected
– In indicating instruments, the controlling torque is
obtained by two methods
i)Spring control
ii)Gravity control
 CONTROLLING TORQUE (Tc)
i)Spring Control
• In the spring control method, a hair spring usually
made of phosphor bronze, attached to the moving
system is used.
 CONTROLLING TORQUE (Tc)
• With the deflection of the pointer, the spring is twisted in
the opposite direction.
• This twist in the spring produces restoring torque which
is directly proportional to the angle of deflection of the
moving system.
• That is for spring control Tc ∝θ
• The pointer comes to a position of rest (or equilibrium)
when the deflecting torque (Td) and the controlling
torque (Tc) are equal .
• Instruments with spring control have a uniform scale
over the entire range of measurement.
 CONTROLLING TORQUE (Tc)
ii)Gravity Control
– In gravity control system, a small adjustable weight is
placed on an arm attached to the moving system.
– The position of weight is adjustable.
– This weight produces a controlling torque due to
gravity.
 CONTROLLING TORQUE (Tc)
 When the pointer is at zero position as shown in fig 1 , the
controlling torque is zero.

 The instruments employing gravity control must be in vertical

position in order that the control may operate.
 CONTROLLING TORQUE (Tc)
 Comparison of spring control and gravity control
 DAMPING TORQUE
• Damping torque is one which acts on the moving system
of the instrument only when it is moving and always
opposes its motion.
• Such a stabilizing or damping torque is necessary to
bring the pointer to rest quickly , otherwise due to inertia
of the moving system the pointer will oscillate about its
final deflected position for some time.
 DAMPING TORQUE
• The function of damping is to absorb the energy from
oscillating system and to bring it to rest in its equilibrium
position
• The damping force can be produced by
i) Air friction damping
ii) Fluid friction damping
iii) Eddy current damping
 DAMPING TORQUE
i) Air friction damping
• Two arrangements of air friction damping are shown
in fig.

Fig 1 Fig 2
 DAMPING TORQUE
• In the first arrangement (fig 1) , a light aluminium piston
is attached to the spindle that carries the pointer and
moves with a very little clearance in a rectangular or
circular air chamber closed at one end
• The cushioning action of the air on the piston damps out
any tendency of the pointer to oscillate about the final
deflected position .
 DAMPING TORQUE
• In the second arrangement (fig 2) one or two light
aluminium vanes are attached to the same spindle that
carries the pointer.
• The vanes are permitted to swing in a sector shaped
closed box that is just large enough to accommodate
the vanes.
• As the pointer moves, the vanes swing in a box
compressing the air in front of them.
• The pressure of compressed air in the vanes provides
the necessary damping force to reduce the tendency of
the pointer to oscillate .
 DAMPING TORQUE
ii) Fluid friction damping
– In this method discs or vanes attached to the spindle
of the moving system are kept immersed in a pot
containing oil of high viscosity
– As the pointer moves, the friction between the oil and
vanes opposes the motion of the pointer and thus
necessary damping is provided
 DAMPING TORQUE
iii)Eddy current damping

• In Eddy current damping, a thin aluminium or

copper disc attached to the moving system is
allowed to pass between the poles of a permanent
magnet.
 DAMPING TORQUE
• As the pointer moves, the disc cuts the magnetic field
and eddy currents are induced in the disc.
• These eddy currents react with the field of the magnet to
produce a force which opposes the motion(Lenz’s law).
• In this way eddy currents provide the damping torque to
reduce the oscillations of the pointer .
AMMETERS AND VOLTMETERS
Ammeters

–Ammeters are used to measure current in amperes and are

connected in series with the circuit whose current is to be
measured.
–The power loss in an ammeter is I R where I is the current to
2 a ,

be measured and Ra is the resistance of the ammeter.

–Ammeters have low electrical resistance so that they cause
small voltage drop and consequently absorb small power.
AMMETERS AND VOLTMETERS
 Voltmeter
– Voltmeters are used to measure
voltage in volts and are connected
in parallel with the circuit whose
voltage is to be measured
– The power loss in a voltmeter is
(V2/RV), where V is the voltage to be
measured and Rv is the resistance
of the voltmeter.
– Therefore voltmeters have a very
high electrical resistance, in order
that the current drawn by them is
small and consequently the power
consumed is small.
 TYPES OF INSTRUMENTS
• The main types of instruments used as ammeters and
voltmeters are
i)Permanent magnet moving coil (PMMC)-dc only
ii)Moving iron- both ac & dc
iii)Electro-dynamometer- both ac & dc
iv)Hot wire- both ac & dc
v)Thermocouple- both ac & dc
vi)Induction-ac only
vii)Electrostatic- both ac & dc
viii)Rectifier - both ac & dc
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
• Permanent Magnet Moving Coil or PMMC Instruments are the
most accurate type for the measurement of DC current or
voltage.

 Construction of PMMC Instruments

• Permanent Magnet Moving Coil or PMMC Instruments
consists of following components.
– Deflecting system

– Magnet System

– Control Spring

– Damping

– Pointer and Scale

 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
Deflecting system:
– The moving coil made up of copper is wound with many turns
on the rectangular Aluminum former.
– This Aluminum former is pivoted on the jewelled bearing.
– The coil can move freely in the magnetic field produced by
the Permanent Magnet System.
– The current or voltage to be measured is passed through the
coil.
– The coil experience a force and provide necessary deflection.
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
Magnet System
– Simple U shaped permanent magnet made of Alcomax or Alnico
is widely use in PMMC instruments.
– Theses magnets have high coercive force and can produce
field of the order of 0.1 to 1 Wb/m2.
Control Spring
– The controlling torque in PMMC Instruments is provided by two
phospher bronze hair springs mounted on the jewel bearing.
– The springs also serve to lead current in and out of the coil.
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
Damping:
– Damping torque in PMMC instruments are produced
by the movement of Aluminum former in the
magnetic field of Permanent Magnet.
– Due to movement of Aluminum former an emf is
induced resulting in eddy current which opposes the
motion thereby provides a damping torque.
Pointer and Scale
– The pointer is carried by the spindle and moves over
a graduated scale.
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
 Principle of operation
– When D.C. supply is given to the moving coil, D.C.
current flows through it.
– When the current carrying coil is kept in the magnetic
field, it experiences a force. This force produces a
torque and the former rotates.
– The pointer is attached with the spindle. When the
former rotates, the pointer moves over the calibrated
scale.
– When the polarity is reversed a torque is produced in
the opposite direction .
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
• The mechanical stopper does not allow the deflection in
the opposite direction.
• The polarity should be maintained with PMMC
instrument.
• If A.C. is supplied, a reversing torque is produced. This
cannot produce a continuous deflection. Therefore this
instrument cannot be used in A.C .
PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
Torque developed
•N–Number of turns of coil
•l, b – the vertical and horizontal length of the side.
•I – current through the coil.
• B–flux density in the air gap
Force produced in the coil F=NBI l sinθ
Field is radial so θ=90
F= NBI l
Td=Force * Distance

Td =NBI l*b= NBIA

T αI ie T = GI
PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
Ƭc = KƟ
Where K = Spring constant
Ɵ = Angular movement of coil.
–At steady state condition, deflecting and controlling torque
shall be equal,
Ƭd = Ƭc ⇒ GI = KƟ
⇒ Ɵ = (G / K)I
–Deflection directly proportional to I Passing through the meter
scale is uniform (linear)
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
Advantages of permanent magnet moving coil instruments
1.The scale is uniformly divided as the current is directly
proportional to deflection of the pointer. Hence it is very
easy to measure quantities from these instruments.
2.Power consumption is also very low in these types of
instruments.
3.A high torque to weight ratio.
4.These are having multiple advantages; a single instrument
can be used for measuring various quantities by using
different values of shunts and multipliers.
5.Damping effective and reliable.
6.No hysteresis loss because of Al former.
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
 Disadvantages of permanent magnet moving coil
instruments
1. These instruments cannot measure AC quantities.
2. The cost of these instruments is high as compared
to moving iron instruments.
3. Error due to ageing of magnets.
 PERMANENT MAGNET MOVING
COIL INSTRUMENT (PMMC)
 Errors in PMMC instruments
– Frictional error.
– Temperature error: As temp increases spring
becomes more flexible and magnets weaker.
– Error owing to weakening of magnet due to ageing.
– Stray magnetic field: usually not affected but due to
presence of iron in working parts effects of external
magnetic field increases.
 MOVING IRON INSTRUMENTS (MI)

• This type of instrument is principally used for the

measurement of alternating currents and voltages.
• This can also be used for DC measurements.
• There are two types of moving iron instruments.
1)Attraction type
2) Repulsion type
MOVING IRON INSTRUMENTS (MI)

Attraction type MI instrument

 MOVING IRON INSTRUMENTS (MI)
• It consist of a flat coil which has a narrow slot like
opening.
• The moving iron is a flat disc or sector eccentrically
pivoted just outside the coil.
• A pointer is attached to the spindle so that it is deflected
with the motion of the soft iron piece.
• When current passes through the coil ,a magnetic field is
developed.
• The moving iron piece moves from a position of weaker
field outside to a position of stronger field inside.
• Or in other words the moving iron is attracted in.
 MOVING IRON INSTRUMENTS (MI)
• The controlling torque is produced by springs but
gravity control can be used for panel type of
instruments which are vertically mounted.
• A piston, attached to the spindle, moves inside an air
chamber and gives the necessary damping.
• If current in the coil is reversed, the direction of
magnetic field also reverses and so does the
magnetism produced in the soft iron piece.
• Hence the direction of the deflecting torque remains
unchanged
• For this reason , such instruments can be used for
both ac and dc measurements .
 MOVING IRON INSTRUMENTS (MI)
Repulsion Type MI Instrument
• In the repulsion type, there are two
vanes inside the coil ,one fixed and
other movable.
• These are similarly magnetized
when the current flows through the
coil and there is a force of repulsion
between the two vanes resulting in
the movement of the moving vane.
• The movable vane is attached to the
spindle which carries the pointer
will move away from the fixed vane
resulting in deflection of the pointer.
• Controlling torque is provided by
spring or gravity.
• Air friction damping is provided.
 MOVING IRON INSTRUMENTS (MI)
• Two different designs are in
common use ,
(i) Radial Vane Type:- In this type, the
vanes are radial strips of iron. The
strips are placed within the coil . The
fixed vane is attached to the coil and
the movable one to the spindle of the
instrument.

(ii)Co-axial Vane Type:- In this type of

instrument, the fixed and moving
vanes are sections of co-axial
cylinders. The fixed and moving
vanes .
MOVING IRON INSTRUMENTS (MI)
 Torque equation of MI instruments
• The expression for torque of a moving iron instrument can
be derived by considering the energy relations when there is
a small increment in current supplied to the instrument.
• This result in a small deflection dθ and some mechanical
work will be done.
• Let Td be the deflecting torque.
• Therefore mechanical work done = torque × angular
displacement

• Due to the change in inductance there will be a change in the

energy stored in the magnetic field.
 MOVING IRON INSTRUMENTS (MI)
• Let I be the initial current, L be the instrument
inductance and θ is the deflection. If the current
increases by dI then it causes the change in deflection
dθ and the inductance by dL.
• In order to involve the increment dI in the current, the
applied voltage must be increase by:
MOVING IRON INSTRUMENTS (MI)
MOVING IRON INSTRUMENTS (MI)
MOVING IRON INSTRUMENTS (MI)

Scale is nonuniform
 MOVING IRON INSTRUMENTS (MI)
 Advantages of MI instruments
• Universal use-can be used for both ac and dc
• Less frictional error-since current carrying part (heavy
part ) is stationary and the moving parts are light in
weight.
• Cheapness
• Robustness
• Reasonably accurate
 Disadvantages of MI instruments
• Have non uniform scale
• Not as sensitive as PMMC instruments
EXTENSION OF RANGE OF METERS-
(PMMC)
 Extension of range of ammeter
• Shunts:-A low resistance called as shunt is connected
in parallel with the ammeter to extent the range of
current measurement.
EXTENSION OF RANGE OF METERS-
(PMMC)
EXTENSION OF RANGE OF METERS-
(PMMC)

• Shunt resistance is made of manganin

• This has least thermoelectric emf
• The change in resistance due to change in
temperature is neglegible
•The general requirements of shunts are
•It should carry current without excessive temperature rise
•The change in resistance due to change in temperature is
negligible
•The resistance of shunt should not vary with time
•Shunt resistance is usually made of manganin.
EXTENSION OF RANGE OF METERS-
(PMMC)
• Voltmeter multipliers
• A large resistance known as multiplier is connected in
series with the voltmeter to extent the range of
voltage measurement.
EXTENSION OF RANGE OF METERS-
(PMMC)
EXTENSION OF RANGE OF METERS-
(PMMC)
• The general requirements of multiplier are
• The change in resistance due to change in
temperature is negligible.
• The resistance of multiplier should not vary with time.
• Multiplier resistance is usually made of manganin and
constantan.
PROBLEMS
PROBLEMS
PROBLEMS