BEEE

1. ELECTRICAL MEASUREMENT
&
MEASURING INSTRUMENTS

2. Classification of Measuring Instruments
• The instrument used for measuring the physical and electrical quantities is
known as the measuring instrument. The term measurement means the
comparison between the two quantities of the same unit. The magnitude of
one of the quantity is unknown, and it is compared with the predefined value.
The result of the comparison obtained regarding numerical value.
• The measuring instrument categorized into three types;
1. Electrical Instrument
2. Electronic Instrument
3. Mechanical Instrument
• The mechanical instrument uses for measuring the physical quantities. This
instrument is suitable for measuring the static and stable condition because
the instrument is unable to give the response to the dynamic condition.
• The electronic instrument has quick response time. The instrument provides
the quick response as compared to the electrical and mechanical instrument.

3. • The electrical instrument is used for measuring electrical quantities likes
current, voltage, power, etc. The ammeter, voltmeter, wattmeter are the
examples of the electrical measuring instrument
• The ammeter measures the current in amps; voltmeter measures voltage
and Wattmeter are used for measuring the power.
• The classification of the electric instruments depends on the methods of
representing the output reading

5. • Absolute Instrument
The absolute instrument gives the value of measures quantities
regarding the physical constant.
The physical constant means the angle of deflection, degree and
meter constant. The mathematical calculation requires for
knowing the value of a physical constant.

6. • Secondary Instrument
In the secondary instrument, the deflection shows the
magnitude of the measurable quantities. The calibration of the
instruments with the standard instrument is essential for the
measurement. The output of this type of device is directly
obtained, and no mathematical calculation requires for knowing
their value.
Digital Instrument
The digital instrument gives the output in the numeric form. The
instrument is more accurate as compared to the analogue
instrument because no human error occurs in the reading.

7. • Analog instrument
The instrument whose output varies continuously is known as the
analogue instrument. The analogue instrument has the pointer which
shows the magnitude of the measurable quantities. The analogue
device classifies into two types.
Null Type Instrument
In this instrument, the zero or null deflection indicates the magnitude
of the measured quantity. The instrument has high accuracy and
sensitivity. In null deflection instrument, the one known and one
unknown quantity use. When the value of the known and the
unknown measuring quantities are equal, the pointer shows the zero
or null deflection. The null deflection instrument is used in the
potentiometer and in galvanometer for obtaining the null point.

8. • Deflection Type Instrument
The instrument in which the value of measuring quantity is
determined through the deflection of the pointer is known as
the deflection type instrument. The measuring quantity deflects
the pointer of the moving system of the instrument which is
fixed on the calibrated scale. Thus, the magnitude of the
measured quantity is known.
The deflection type instrument is further sub-classified into
three types.
1. Indicating Instrument
2. Integrating Instrument
3. Recording Instrument

9. Indicating Instrument –
The instrument which indicates the magnitude of the measured
quantity is known as the indicating instrument. The indicating
instrument has the dial which moves on the graduated dial. The
voltmeter, ammeter, power factor meter are the examples of the
indicating instrument.
Integrating Instrument –
The instrument which measures the total energy supplied at a
particular interval of time is known as the integrating instrument. The
total energy measured by the instrument is the product of the time
and the measures electrical quantities. The energy meter, watt-hour
meter and the energy meter are the examples of integrating
instrument

10. Recording Instrument –
The instrument records the circuit condition at a particular
interval of time is known as the recording instrument. The
moving system of the recording instrument carries a pen which
lightly touches on the paper sheet. The movement of the coil is
traced on the paper sheet. The curve drawn on the paper shows
the variation in the measurement of the electrical quantities.

11. Principles of operation of Electrical
Instruments
• Electrical measuring instruments work on the
principle of operation of a different effect such
as Magnetic effect, Electro-dynamic effect,
Electromagnetic Induction, thermal Effect,
Chemical Effect, Electrostatics Effect etc.
• The principle of operation of the different
electrical instrument is given in the table below-

12. Principles of operation of Electrical
Instruments

13. Characteristic Of Electrical Measuring
Instruments
Accuracy:
It is desirable quality in measurement. It is defined as the degree
of the closeness with which instrument reading approaches the
true value of the quantity being measured. Accuracy can be
expressed in three ways
 Point accuracy
 Accuracy as the percentage of scale of range
 Accuracy as percentage of true value.

14. Sensitivity:
It is also desirable quality in the measurement. It is
defined as the ratio of the magnitude response of
the output signal to the magnitude response of the
input signal.
Precision:
It is a measure of the reproducibility of the
measurements i.e., given a fixed value of quantity,
precision is a measure of the degree of agreement
within a group of measurements. The term precise
means clearly or sharply defined

15. Stability:
The ability of a measuring system to maintain standard of
performance over prolonged periods of time. Zero stability
defines the ability of an instrument restore to zero reading after
the input quantity has been brought to zero, while other
conditions remain the same.

16. 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
2. Moving Iron Instruments
Attraction type M. I. Instruments
Repulsion type M. I. Instruments
3. Permanent Magnet Moving Coil Instruments

17. Essential requirement of
indicating instruments
 Deflecting Torque
 Controlling Torque
 Damping Torque or Restoring Torque

18. Deflecting system:
• The deflection produced by the operating torque is proportional to the
magnitude of the electrical quantity such as current, voltage, etc. being
measured.
• The deflecting torque causes the moving mechanism to move from its
initial zero position.
Controlling system:
• Torque produced by the controlling system is in opposition to the
deflecting torque.
• Pointer comes to rest when deflecting and controlling torques are equal.
Damping system:
• To minimize the oscillations in the deflecting system.
• Air friction damping, Eddy current damping
Essential features of measuring Instruments

19. 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

20. 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.

21. Two methods of
controlling Torque
 Spring Control method
 Gravity control method

22. 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

23. 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 proportional to
directly
current I and Tc is
proportional to deflection
at final steady state Td
directly
angle,
= Tc,
deflection is directly proportional
to current, hence scale is linear

24. 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.

25. 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 under
damped, over damped and critical damp

26. Damping
Methods
Air Friction Damping
Eddy current Damping

27. 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
pointer). Similarly, if the piston
moving out of the chamber,
therefore the moving system or
is
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.

28. Eddy current damping

29. 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.

30. Moving Iron Instruments

31. 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.

32. 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 thestationary
coil as shown in fig.
3. A pointer is fixed on the spindle.

33. 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.

34. Repulsion type M. I.
Instruments
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
Td 
1
I 2 dL
2 d

36. 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 aluminumframe.
• Two spiral springs are mounted on the spindle to produce control torque. Control
spring also serves an additional purpose & acts as controllead.
• Pointer is mounted on spindle. Mirror is provided below the scale to avoid
parallax error. The spindle is supported by jeweled bearings.

37. Permanent Magnet Moving
Coil Instruments
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.
Mechanical force experienced by the coil
F = NBIL newton
Deflecting Torque Td α I
Controlling torque Tc α θ
At equilibrium Td = Tc
Therefore, θ α I
Torque equation -Deflection is proportional to current
Hence, Scale is Uniform (Linear)

38. 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

39. Extension of meter range: DC Ammeters
Shunt Resistor
m
sh
R
I  I

Im Rm
Shunt is a very low resistance connected across the basic meter.
Rm= internal resistance of the basic meter.
Rsh = Resistance of the shunt
Im= full scale deflection of basic meter.
I = Current to be measured.

40. Extension of meter range:
Voltmeter multipliers
• Multiplier is a very high resistance
in series with the basic meter
• Basic dc voltmeter circuit
• Voltmeter sensitivity :
m
m
m
s
I
I
R  R

V

V  I m Rm
V
I fsd
S 
1 