(1) Philosophy of Measurement-Methods of measurement, Measurement system , Classification of instrument systems, Characteristics of instruments & measurement systems, Errors in measurement & its analysis, Standards. (2)Analog Measurement of Electrical Quantities-Electrodynamic, Thermocouple, Electrostatic & Rectifier type ammeters & voltmeters, Electrodynamic wattmeter, Three Phase wattmeter, Power in three phase systems, Errors & remedies in wattmeter and energy meter.

1. ELECTRICAL MEASUREMENT & MEASURING INSTRUMENTS (NEE-302)
Md Irshad Ahmad
irshad.ahmad@jit.edu.in
Electrical Engineering Department
LECTURE
on
UNIT –I
Philosophy of Measurement-&
Analog Measurement of Electrical Quantities

2. CONTENTS
UNIT- I Philosophy of Measurement-
 Methods of measurement
 Measurement system
1. Essential Requirement of Indicating Instruments
1. Deflecting torque
2. Controlling torque
i. Spring Control method
ii Gravity control method
3. Damping torque or restoring torque
i. Air friction damping
ii. Eddy current damping
 Classification of instrument systems
 Characteristics of instruments & measurement
Systems
 Errors in measurement & its analysis.

3. (2)Analog Measurement of Electrical Quantities-
Electrodynamic, Thermocouple,
Electrostatic & Rectifier type ammeters &
voltmeters,
Electrodynamic wattmeter,
ThreePhase wattmeter
Power in three phase systems
Errors & remedies in wattmeter
Energymeter.
CONTENTS

4. OBJECTIVES
 At the Start of this chapter, students should be able
to:
1. List the functions instruments.
2. Define terms related to the quality of instruments.
3. List and describe categories of standards.
4. Define terms related to error and error analysis.
5. Describe the basic elements of electronic instrument.

5. INTRODUCTION
 Measurement is the process of determining the
amount, degree or capacity by comparison with the
accepted standards of the system units being used.
 Instrumentation is a technology of measurement which
serves sciences, engineering, medicine and etc.
 Instrument is a device for determining the value or
magnitude of a quantity or variable.
 Electronic instrument is based on electrical or
electronic principles for its measurement functions.

6. ELECTRONIC
INSTRUMENT
1) Transducer
- convert a non electrical signal into an electrical signal
2) Signal modifier
- convert input signal into a suitable signal for the indicating
device (e.g amplifier)
3) Indicating device
- indicates the value of quantity being measure (e.g ammeter)
Transducer
Signal
Modifier
Indicating
Device
• Basic elements of an electronics instrument

7. FUNCTIONS
 The 3 basic functions of instrumentation :-
 Indicating – visualize the process/operation
 Recording – observe and save the measurement
reading
 Controlling – to control measurement and process

8. STATIC CHARACTERISTICS
 Accuracy – the degree of exactness (closeness) of
measurement compared to the expected (desired) value.
 Resolution – the smallest change in a measurement variable
to which an instrument will respond.
 Precision – a measure of consistency or repeatability of
measurement, i.e successive reading do not differ.
 Expected value – the design value or the most probable value
that expect to obtain.
 Error – the deviation of the true value from the desired value.
 Sensitivity – ratio of change in the output (response) of
instrument to a change of input or measured variable.

9. Essential requirement of indicating instruments
1. Deflecting torque
2. Controlling torque
3. Damping torque or restoring torque
Deflecting Torque Deflecting torque causes the moving system and pointer of the
instrument to move from its zero position. Production of deflecting torque depends upon
the type of indicating instrument and its principle of operation
Controlling Torque
Controlling torque limits the movement of pointer and ensures that the magnitude of
deflection is unique and is always same for the given value of electrical quantity to be
measured.
Two methods of controlling Torque
i. Spring Control method
ii Gravity control method

10. Spring Control method
1. Two phosphor bronze
hair springs of spiral
shapes are attached to
the spindle of the
moving system of the
instrument.
2. They are wound in
opposite direction
3. Pointer is attached to
the spindle of the
moving system

11. Working of Spring Control Method
1. When the moving system deflected, one
spring gets wounded and the other one
gets unwounded. This results in
controlling torque whose magnitude is
directly proportional to angle of
deflection.
2. Td is directly proportional to current I and Tc is
directly proportional to deflection angle,
at final steady state Td = Tc,
deflection is directly proportional to
current, hence scale is linear

12. Gravity Control Method
1. In gravity control method, a small weight is
attached to the spindle of the moving
system. Due to the gravitational pull, a
control torque (acting in opposite direction
to the deflecting torque) is produced
whenever the pointer tends to move away
from its initial position.
2. In this case, Td is directly proportional to
current I and Tc is directly proportional to
sine of the deflection angle, since Td = Tc,
sine of the deflection is directly
proportional to current, hence scale is non
linear i.e. cramped scale.

13. Damping torque
 Damping torque minimizes the oscillations of the pointer about the final steady state
deflection and makes it steady.. In the absence of this torque, pointer continues
oscillating to its final position after reaching to its final position.
 Depending on the magnitude of damping, it can be classified as underdamped,
overdamped and critcal damp
 1. Air Friction Damping
 2. Eddy current Damping

14. Air Friction damping
 A light aluminum frame is attached to
the moving system. This piston moves in
an air chamber (cylinder) closed at one
end. At the time of oscillation of the
moving system or pointer about its final
steady state, if the piston is moving into
the chamber, the trapped air gets
compressed and the pressure opposes
the motion of the piston (and therefore
the moving system or pointer). Similarly,
if the piston is moving out of the
chamber, the pressure in the closed
chamber falls and becomes less than air
pressure on the outer part of the piston.
Motion is thus again opposed.
Oscillations are damped.

15. Eddy current damping

16. Eddy current damping
 Construction & Working
1. An aluminum frame or damping disc is mounted on the spindle and free
to rotate in the magnetic field provided by damping magnets. Since
damping disc is rotating with spindle, emf is induced in the disc
according to faradays law of electromagnetic induction. Since disc is a
closed circuit, eddy current in the form of concentric circles will be
induced in the damping disc. Interaction between this eddy current
and magnetic field develops a force on the damping disc which
opposes the movement of sheet. And thus provides damping of the
oscillations of the pointer.

17. Construction
1. Instrument consists of a stationary coil in which the current to be
measured is passed.
2. A piece of un-magnetized soft iron which is of oval shape is mounted
rigidly on the spindle. This soft iron piece is free to move about the spindle
and along with the spindle.
Working
1. The current to be measured is flowing in the coil, produces a magnetic
field. Iron piece gets attracted towards centre of the magnetic
field and pointer deflects on the scale.
2. Control torque is provided either by control springs or by gravity control
method
3. Damping is provided by air friction damping
4. The scale is non-linear. Mirror is provided to avoid parallax error.

18. Attraction type M. I. Instruments
 Construction
1. This instrument consists of stationary coil in which current I
that is to be measured is passed
2. A piece of un-magnetised soft iron which is oval in shape is
mounted rigidly on the spindle. This soft iron piece is free
to move about the spindle and along with the spindle. It is
placed closer to the stationary coil as shown in fig.
3. A pointer is fixed on the spindle.

19. Repulsion type M. I. Instruments
Construction
 This instrument consists of two iron vanes, one is attached to the stationary
coil and other one is attached to the movable spindle.
 Both vanes are surrounded by the stationary coil, current to be measured is
passing thorough this coil.
Working
 Current to be measured is passing thorough stationary coil produces magnetic field. Both
the vanes magnetizes with similar polarities.
 As a result a force of repulsion is set up between two vanes.
 This force produces a deflecting torque on the movable vanes, gives deflection on the
scale.
 General Torque equation of M. I. Instruments
d
dL
ITd 2
2
1


20. Permanent Magnet Moving Coil
Instruments
Construction
 It consists of permanent magnet which is stationary.
 Moving system consists of a spindle attached to a rectangular aluminum frame. A coil made
up of thin copper wire is wound over the frame. The current to be measured is passed
through this coil.
 A soft iron core is placed in the in the space within the alluminium frame.
 Two spiral springs are mounted on the spindle to produce control torque. Control spring also
serves an additional purpose & acts as control lead.
 Pointer is mounted on spindle. Mirror is provided below the scale to avoid parallax error. The
spindle is supported by jeweled bearings.

21. Permanent Magnet Moving Coil Instruments
Construction
1. It consists of permanent magnet which is stationary.
2. Moving system consists of a spindle attached to a rectangular aluminum
frame. A coil made up of thin copper wire is wound over the frame. The
current to be measured is passed through this coil.
3. A soft iron core is placed in the in the space within the alluminium frame. This
core is stationary and is provided to reduce the reluctance of the magnetic
path between two poles of the permanent magnet.
4. Two spiral springs are mounted on the spindle to produce control torque. The
control spring also serves an additional purpose and acts as control lead.
Pointer is mounted on spindle. Mirror is provided below the scale to avoid
parallax error. The spindle is supported by jeweled bearings.
Working
1. The current to be measured is passed through moving coil via control springs.
2. A current carrying moving coil is now in a magnetic field. According to
Flemings left hand rule, torque is produced on the coil and coil moves,
pointer deflects.
3. Damping torque is provided by eddy current damping method.
 Torque equation- Deflection is proportional to current

22. Permanent Magnet Moving Coil Instruments
Errors in PMMC Instruments
 Weakening of permanent magnet due to ageing and temperature
effects
 Weakening of springs due to ageing and temperature effects
 Change of resistance of moving coil with temperature.
Merits
 Uniform scale for the instrument
 Power consumption is very low
 A single instrument can be used for different current and voltage
ranges
 The toque-weight ratio is high gives higher accuracy.
Demerits
 This instrument can be used only on DC supply
 The cost of the instrument is more than M.I. Instruments

23. Electrodynamic Instruments
Construction
 Stationary part consists of two fixed coils connected in series as shown in fig.
so that they carry same current.
 The moving system consists of a coil mounted on the spindle which is free to
rotate in the space between the two fixed coils. The coil is made up of thin
copper wire and is air cored to avoid hysterisis.
 Control torque provided by two spiral springs. They also act as connecting
leads for the moving coil. Pointer is mounted on the spindle.
 Mirror is provided to avoid parallax error.
 Damping is provided by air friction damping.

24. Electrodynamic Instruments
Working
 Current to be measured is passed through two stationary
coils which are connected in series, forms magnetic field.
 Current to be measured is also passed through moving coil
via control springs. Now current carrying moving coil is
placed in magnetic field. According to Flemings left hand
rule, force is experienced on the moving coil, gives
deflection of the pointer.
 Torque
Td α i1 x i2
i1- current flowing through fixed coil,
i2 - current flowing through moving coil

25. Electrodynamic Instruments
Merits
 It can be used on a.c as well as d.c.
 It can be used as ammeter voltmeter and wattmeter
 it is also called as dynamometer instruments
Demerits
 Low torque to weight ratio
 More expensive
 Weak magnetic field
 Scale is non-uniform

26. Electrodynamic Instruments
Electrodynamic instruments as voltmeter, ammeter and wattmeter
 When used as an ammeter, the fixed coils are made up of thick conductors to carry the
load current. But since it is not possible with moving coil, it is shunted (connected in
parallel) with suitable resistor
 When used as an voltmeter, all coils are made from less cross section conductor. To
increase instruments impedance. To increase the instruments impedence, a heavy
resistor is connected in series with them
 When used as an wattmeter, fixed coils are used as current coil and moving coil as
pressure coil. Constructionally, fixed coils are made up of thick copper wire and
moving coil with thin conductors

27. Errors

28. TYPES OF STATIC ERROR
 Types of error in measurement:
1) Gross error/human error
2) Systematic Error
3) Random Error
1) Gross Error
- caused by human mistakes in reading/using instruments
- cannot eliminate but can minimize

29. TYPES OF STATIC ERROR
(cont)
2) Systematic Error
- due to shortcomings of the instrument (such as
defective or worn parts)
- 3 types of systematic error :-
(i) Instrumental error
(ii) Environmental error
(iii) Observational error

30. TYPES OF STATIC ERROR
(cont)
(i) Instrumental error
- inherent while measuring instrument
because of their mechanical structure (bearing
friction, irregular spring tension,
stretching of spring, etc)
- error can be avoided by:
(a) selecting a suitable instrument for the
particular
measurement application
(b) apply correction factor by determining
instrumental error
(c) calibrate the instrument against standard

31. TYPES OF STATIC ERROR
(cont)
(ii) Environmental error
- due to external condition effecting the
measurement including surrounding area
condition
such as change in temperature, humidity,
barometer pressure, etc
- to avoid the error :-
(a) use air conditioner
(b) sealing certain component in the
instruments
(c) use magnetic shields
(iii) Observational error
- introduce by the observer
- most common : parallax error and estimation
error (while reading the scale)

32. TYPES OF STATIC ERROR
(cont)
3) Random error
- due to unknown causes, occur when all systematic
error has accounted
- accumulation of small effect, require at high degree
of accuracy
- can be avoided by
(a) increasing number of reading
(b) use statistical means to obtain best
approximation
of true value

33. Dynamic Characteristics
 Dynamic – measuring a varying process condition.
 Instruments rarely respond instantaneously to
changes in the measured variables due to such things
as mass, thermal capacitance, fluid capacitance or
electrical capacitance.
 The three most common variations in the measured
quantity:
 Step change
 Linear change
 Sinusoidal change

34. LIMITING ERROR
 The accuracy of measuring instrument is
guaranteed within a certain percentage (%) of
full scale reading
 E.g manufacturer may specify the instrument to
be accurate at 2 % with full scale deflection
 For reading less than full scale, the limiting error
increases

35. What are thermocouples?
 Thermocouples operate under the principle that a circuit
made by connecting two dissimilar metals produces a
measurable voltage (emf-electromotive force) when a
temperature gradient is imposed between one end and
the other.
They are inexpensive, small, rugged and accurate when
used with an understanding of their peculiarities.
Most frequently used method to measure temperatures
with an electrical output signal.

36. Thermocouples Principle of
Operation
 In, 1821 T. J. Seebeck observed the existence of an
electromotive force (EMF) at the junction formed
between two dissimilar metals (Seebeck effect).
 Seebeck effect is actually the combined result of two other
phenomena, Thomson and Peltier effects.
 Thomson observed the existence of an EMF
due to the contact of two dissimilar metals at
the junction temperature.
 Peltier discovered that temperature gradients
along conductors in a circuit generate an EMF.
 The Thomson effect is normally much smaller
than the Peltier effect.

37. Let’s take a look at this circuit

38. How thermocouples work
 It is generally reasonable
to assume that the emf is
generated in the wires,
not in the junction. The
signal is generated when
dT/dx is not zero.
 When the materials are
homogeneous, e, the
thermoelectric power, is
a function of
temperature only.
 Two wires begin and
end at the same two
temperatures.
E  (T  To)  (T  To )2
Generally, a second order Eqn. is used.

39. Thermocouple Effect
 Any time a pair of dissimilar wires is
joined to make a circuit and a thermal
gradient is imposed, an emf voltage will
be generated.
 Twisted, soldered or welded junctions are
acceptable. Welding is most common.
 Keep weld bead or solder bead diameter
within 10-15% of wire diameter
 Welding is generally quicker than
soldering but both are equally acceptable
 Voltage or EMF produced depends on:
 Types of materials used
 Temperature difference between the
measuring junction and the reference
junction

40. What thermocouple materials should be
used?
 Depends on requirements:
 Temperature range?
 Required accuracy
 Chemical resistance issues
 Abrasion or vibration resistance
 Installation requirements (size of wire)
 Thermal conduction requirements

41. Thermocouple
 The thermocouple is widely used temperature sensor in industry.
 Whenever two different types of metals are connected together, a
thermoelectric potential (sometimes called thermoelectric EMF) is
generated across the two free ends the metals according to the
temperature of the joint. This is known as the thermoelectric effect.
 This thermoelectric effect was discovered by Thomas Johan Seeback
discovered in 1821. This thermoelectric EMF is generated due to the
combination of Peltier effect and Thomson effect.

42. The EMF generated can be approximately expressed by
the relationship:
The values of constants a1, a2, a3, etc. depend on the
metals A and B
A Typical Thermocouple

43. In thermocouple temperature sensor, the temperature of hot junction is
measured in respect of cold junction of vise varsa.
The thermoelectric potential is generated in a thermocouple instrument
is in range of μV.
Hence, the voltmeter connected to measure the thermoelectric potential
is extremely sensitive and the temperature can be read directly from this
voltmeter, if it is calibrated properly in the scale of temperature.
Thermocouples are a very important class of device as they provide the
most commonly used method of measuring temperatures in industry.
The major reasons behind popularity of thermocouple temperature
measurement are;
1. They are very strong and readings are uniform,
2. They can measure wide range of temperatures,
3. Their characteristics are almost linear with a accuracy of ±0.05%.

44. Temperature Characteristics for Some Standard Thermocouple
Materials

45. 2. Electrostatic Type
VoltmeterElectrostatic type instruments are almost
always used as voltmeters and that too
more as a laboratory rather than as
industrial instruments.
The underlying principle of their
operation is the force of attraction
between electric charges on neighboring
plates between which a p.d. is maintained.
There are two general types of such instruments:
1. The quadrant type-used upto 20kV
2. The attracted disc type-used upto 500kV
oThis force gives a rise to a deflecting torque.
oUnless the p.d. is sufficiently large, the force is
small.
oHence, such instruments are used for
measurement of very high voltages.

46. 1. Quadrant Type
Voltmeters
 A light aluminium vane C is mounted on a spindle S and is
situated partially within a hollow metal quadrant B.
 When the vane and the quadrant are oppositely charged by the
voltage under measurement, the vane is further attracted
inwards into the quadrants thereby causing the spindle and
hence the pointer to rotate.

47. 1. Attracted-Disc Type
Voltmeters
 It consists of two discs or
plates C & D mounted
parallel to each other.
 Plate D is fixed and is earthed
while C is suspended by a
coach spring, the support for
which carries a micrometer
head for adjustment.
 When a p.d. is applied
between two plates, then C is
attracted towards D but may
be returned to its original
position by the micrometer
head.

48. 3. Rectifier Type Ammeter
•To deal with current exceeding 5A, use a 5A(1A) meter in
combination with a C.T.
•A.C ammeter is used with a current transformer externally
mounted, and it is noted that primary current of a current
transformer and a full scale of the meter are identical when used.
•The full scale of ammeter are indicated from 5~10000A etc.
•If the circuit voltage exceeds 500V even when the current is less
then 5A use a 5A (1A) meter in combination with a C.T to ensure
insulation.

49.  The scale range of ammeter for
motor circuit is extended up to
150%~500% of rated current, so that
these meters are protected from the
impact of the starting current and
ensured of long life.
 AM expanded scale has the normal
maximum reading (upper limit of the
effective measurement range) at the
app. 65% point on the scale, and the
rest of it (65%~100%) is devoted to
extension reading (tolerance of
extension area : + 5%)
 Angle of deflection: 250o
 ANSI C39.1, C12.10
 Impulse voltage withstand
2.5KV/1min(1.2/50us)
 Readily accessible span and zero
adjustments

50. 4. Rectifier Type Voltmeter
The AC Volt meter is rectifier type and moving iron type.
The types of operating principle of voltmeter are the
rectifier type and moving iron type.
The voltmeter is used with a potential transformer
externally mounted in case of high voltage.
For higher ranges than 300V, use potential transformer
(P.T) with a 150V A.C voltmeter. But the voltmeter of 600V
is able to supply by order of customer.

51.  When lettering the voltage ratio on
voltmeter, the meters for 3-phase 4-
wire are the indicating for line-to-
line voltage by line-to-ground
voltage, and line-to-line voltage by
line-to-line voltage.
 Full scale of voltmeter when use
the P.T, the standard calibration
value of voltmeters are 150V
and/or 300V, but the full scale of
voltmeters are indicated the
150V~69KV etc.
 Angle of deflection: 250o
 ANSI C39.1, C12.10
 Impulse voltage withstand
2.5KV/1min(1.2/50us)
 Readily accessible span and zero
adjustments.

52. 5. Electrodynamometer
 Working Principle:
“When any current carrying conductor is placed
inside a magnetic field, it experiences a mechanical
force and due this mechanical force deflection of
conductor takes place".
 There are Two types of coils present in the
electrodynamometer.
1. Moving Coil 2. Fixed Coil

54. Moving coil:
 Moving coil moves the pointer with the help of
spring control instrument. A limited amount of
current flows through the moving coil so as to
avoid heating.
 So in order to limit the current we have connect the
high value resistor in series with the moving coil.
The moving is air cored and is mounted on a
pivoted spindle and can moves freely.
 In electrodynamometer type wattmeter, moving coil
works as pressure coil.
 Hence moving coil is connected across the voltage
and thus the current flowing through this coil is
always proportional to the voltage.

55. Fixed coil:
 The fixed coil is divided into two equal parts and these
are connected in series with the load, therefore the load
current will flow through these coils.
 Now the reason is very obvious of using two fixed
coils instead of one, so that it can be constructed to
carry considerable amount of electric current.
 These coils are called the current coils of
electrodynamometer type wattmeter.
 Earlier these fixed coils are designed to carry the
current of about 100 amperes but now the modern
wattmeter are designed to carry current of about 20
amperes in order to save power.

56. Control system:
Out of two controlling systems i.e.
1. Gravity control
2. Spring control: Only spring controlled systems are
used in these types of wattmeter. Gravity controlled
system cannot be employed because they will
appreciable amount of errors.
3. Damping system: Air friction damping is used, as eddy
current damping will distort the weak operating
magnetic field and thus it may leads to error.
4. Scale: There is uniform scale is used in these types of
instrument as moving coil moves linearly over a range
of 40 degrees to 50 degrees on either sides.

57. Advantages of Electrodynamometer Type Wattmeter:
Scale is uniform upto certain limit.
They can be used for both to measure ac as well dc quantities as scale
is calibrated for both.
Errors in Electrodynamometer Type Wattmeter:
Errors in the pressure coil inductance.
Errors may be due to pressure coil capacitance.
Errors may be due to mutual inductance effects.
Errors may be due connections.(i.e. pressure coil is connected after
current coil)
Error due to Eddy currents.
Errors caused by vibration of moving system.
Temperature error.
Errors due to stray magnetic field.

58. 5. Three-phase Wattmeter
A dynamometer type three-
phase wattmeter consists of
two separate wattmeter
movements mounted together
in one case with the two
moving coils mounted on the
same spindle.
There are two current coils
and two pressure coils.
A current coil together with
its pressure coil is known as
an element. Therefore, a
three-phase wattmeter has
two elements.
The torque of each element is
proportional to the power
being measured by it.

59.  The total deflecting torque on the moving system
is proportional to the total power.
 Compensation for mutual effects can be done by
using Weston’s method. Resistance R may be
adjusted to compensate for errors caused by
mutual interference.

60. Thank You