The document discusses the classification and principles of measuring instruments, such as electromagnetic, electrostatic, and electrothermal types. It details electromechanical indicating instruments, their construction (like PMMC) and sensitivity considerations, including error reduction and temperature compensation methods. Additionally, it provides assignments on calculating shunt resistance for ammeters and voltmeters based on coil resistance and full-scale deflection values.

1. Classification of Measuring
Instruments
• Quantity measured
• Principle of operation
•Principle of operation
1) Electromagnetic (Force due to B & I)
2) Electrostatic (Force due to E & Q)
3) Electro thermal (due to heating effect)

2. Electromagnetic-Electromechanical
Indicating Instruments
Electromechanical Indicating Instruments
Input is electrical signal
Output is mechanical force
Analog (Output is a continuous
function of time)
Example: PMMC Instruments

3. Permanent Magnet Moving Coil
(PMMC) Instruments
• To measure DC current or DC voltage
• Can be used for measuring AC
currents and voltages by introducing
additional circuit and proper calibration

4. Constructional Features of PMMC
Enameled or silk covered copper wire is used for coil

5. Constructional Features of PMMC

6. 6
Various Forces/ Torques in
Measuring Instruments
• The movement of the pointer in PPMC
is governed by three different torques
1) Deflection Torque
2) Control Torque
3) Damping Torque

7. 7
Deflection Torque

8. 8
Deflection Torque

9. 9
Deflection Torque

10. 10
Control Torque

11. 11
Damping Torque

12. 12
Damping Torque

13. 13
Two methods of supporting the
moving system

14. 14
Two methods of supporting the
moving system
• To increase sensitivity,
the diameter of the hair
spring should be
reduced, but it is
limited, so Taut band is
used
• It is thin, metallic,
ribbon like structure
• It improves sensitivity

15. 15
Effect of temperature and methods to
compensate
• Hair springs, Magnetic field, Coil
resistance are effected
• Series resistance with temperature
coefficient nearly zero (manganin) is
used
• Usually three times the coil resistance
• It is called Swamping resistance

16. 16
Reason for placing hair springs in
opposite direction
F
x = 0x
F
x
CASE-1
H
x=H

17. 17
Reason for placing hair springs in
opposite direction
F
x = 0x
F
x
CASE-2
x=H
H

18. 18
Reason for placing hair springs in
opposite direction
F
x
F
x = 0x
CASE-3

19. 19
Multi Range Ammeter

20. 20
Make before Break

21. 21
Multi Range Ammeter (alternative
design)

22. 22
Multi Range Voltmeter

23. 23
Break before Make

24. 24
Advantages
• Uniform scale
• Power consumption can be made very low
(25 µW to 200 µW)
• Torque to weight ratio can be made high with
a view to achieve high accuracy (typically 2%)
• Single instrument can be used for multi range
ammeters and voltmeters
• Error due to stray magnetic field is very small

25. 25
Limitations
• They are suitable for direct current only
• The instrument cost is high
• Variation of magnet strength with time

26. 26
Errors can be reduced by
• Proper pivoting and balancing weight may
reduce the frictional error
• Considering the aging can reduce errors due
to magnetic decay
• Manganin in series with coil reduces
temperature effects
• Maintaining nominal temperature. The
stiffness of spring, permeability of magnetic
core decreases with increase in temperature

27. 27
Sensitivity
Ammeter Sensitivity:
Full scale current (A)
Voltmeter Sensitivity:
( )Total resistance of the meter /
Full scale reading (V)
kΩ Ω

28. 28
Assignment
Q1) A PMMC instrument has a coil
resistance of 100Ω and gives a full-
scale deflection (FSD) for a current of
500μA. Determine the value of shunt
resistance required if the instrument
is to be employed as an ammeter
with a FSD of 5 A.
Ans: 0.01 Ω

29. 29
Assignment
Q2) A PMMC meter with a coil
resistance 100Ω and a full scale
deflection current of 100μA is to be
used as a voltmeter. The voltmeter
ranges are to be 50 V and 100 V.
Determine the required value of
resistances for each range.
Ans: 0.4999 MΩ, 0.9999 MΩ