Measurement and Instrumentation: 2 Marks Important Questions with Answers

Basic Electronics and Electrical Engineering

CHAPTER 6: MEASUREMENT AND INSTRUMENTATION

2 Marks Important Questions with Answers

1. State the requirements of any measuring instruments?

(i) The operation, output and condition of the actual circuit should not be affected / altered with the presence of measuring instruments.

(ii) The quantity to be measured should not affected by the measuring instruments.

(iii) The measuring instruments should consume less power as low as possible.

2. Give the classification of measuring instruments?

Measuring instruments broadly classified as:

(i) Primary Instruments.

(ii) Secondary instruments.

According to nature of reading

(a) Indicating instruments.

(b) Integrating Instruments.

(c) Recording Instruments.

3. Which are the essential torques required in measuring instruments?

(i) Deflecting Torque, T_d.

(ii) Controlling torque, T_C.

(iii) Damping torque.

4. State the various methods of producing deflecting torque in measuring instruments?

(i) Magnetic effect

(ii) Electrodynamic effect

(iii) Electromagnetic effect

(iv) Thermal effect

(v) Chemical effect

(vi) Electrostatic effect

5. Which are two methods of producing, controlling torque in measuring instruments?

(i) Spring control

(ii) Gravity control

6. State the important functions of controlling system in measuring instruments.

(i) It produces a torque equal and opposite to the deflecting torque in order to make the deflection of pointer at a definite magnitude. If this system is absent, then deflection will be come indefinite.

(ii) It brings the moving system back to zero position when the force which causes the movement of the moving system is removed.

7. What are the methods available for providing damping torque in measuring instruments?

(i) Air friction damping

(ii) Fluid friction damping

(iii) Eddy current damping

8. List the possible errors in PMMC instruments?

(i) Errors due to weakening of permanent magnets

(ii) Errors due to weakening of springs

(iii) Change of resistance

9. Why MI instruments can be used for both ac and dc quantities?

The deflecting torque is proportional to the square of the operating current. So, whatever may be the direction of current, the deflecting torque will be produced in forward direction only. Hence MI instruments can be used for both ac and dc measurements.

10. List the possible errors in MI instruments?

(i) Errors with both ac and de

(a) Hysteresis error

(b) Temperature error

(c) Error due to stray magnetic fields.

(ii) Errors with AC only:

(a) Frequency error

(b) Error due to reactance

(c) Error due to reactance

11. How to increase the range of an dc ammeter and dc voltmeter?

By keeping a shunt (low resistance parallel path) across the meter, the range of an ammeter can be extended

By keeping a multiplier (high resistance series path) through the meter, we can extend the range of a voltmeter.

12. Explain the various adjustments required in an energy meter.

(i) Preliminary light load adjustment

(ii) Full load unity power factor

(iii) Lag adjustment

(iv) Light load adjustment

(v) Creep adjustment

13. What is Creep?

In some meters a slow, but continuous rotation is obtained when the potential coils are excited but with no load currents flowing. This is known as creep. This may be due to

(i) Incorrect friction compensation

(ii) Vibration

(iii) Stray magnetic field

(iv) the voltage of the supply circuit is in excess of the normal.

14. Why cannot a moving coil instrument be used in AC circuits?

In moving coil instruments, the operating torque depends on the direction of the operating current. Thus, this type of instruments are polarised. So, if moving coil instrument are connected for AC circuits, then the deflecting torque is a pulsating one and the meter will not read. That is, the meter will read the average value of the measuring quantity.

15. State the principle of Dynamometer type watt meter.

It works on the principle which can be said as "The pressure coil produces a flux which is proportional to the applied voltage. The flux produced by the pressure coil is nearly in phase with the applied voltage. The current coil produces a flux which is in phase with the load current. These two fluxes link each other and a torque is produced on the moving system".

This principle will look similar to the principle of operation of motor.

16. List the essential requirements (torque) of an instrument.

(i) Deflecting torque

(ii) Controlling torque

(iii) Damping torque

17. What are the main parts of the operating mechanism in an energy meter?

(i) Driving system

(ii) Moving system

(iii) Braking system

(iv) Registering (counting) system:

18. What are the coils in wattmeter?

Symbol:

(i) Fixed coil (or) Series coil (or) Current coil (M‒L)

(ii) Moving coil (or) Shunt coil (or) Pressure coil (or) Potential coil (or) Voltage coil (C‒V)

19. State the different type of instruments based on their operating principles.

(i) Moving coil instruments (ii) Moving iron instruments

20. Mention the three torques required for the proper operation of indicating instrument.

(i) Deflecting Torque, T_d; (ii) Controlling torque, T_C; (iii) Damping torque.

21. Define the principle of moving iron instrument for attraction type.

When the moving iron piece is brought near either ends of the current carrying coil winding, the iron piece will be attracted towards that portion where magnetic flux density is more. This movement is used to measure the current and voltage which produces the magnetic field.

22. Define ‒ Transducer.

Transducer is defined as a device that receives energy from one system and transmits it to another, often is a different form.

23. Write the parameters of electrical transducer.

The parameters of electrical transducer are,

(i) Linearity

(ii) Sensitivity

(iii) Dynamic range

(iv) Repeatability

24. List out the advantages of electrical transducers.

The advantages of electrical transducers are,

(i) Electrical amplification and attenuation can be easily done.

(ii) Mass inertia effects are minimized.

(iii) Effects of friction are minimized.

(iv) Using very small power level.

(v) Electrical output can be easily used, transmitted and processed for the purpose of measurement.

(vi) The output can be indicated and recorded remotely at a distance from the sensing medium.

25. List out the types of potentiometer.

The types of potentiometer are

(i) Translatory

(ii) Rotational

(iii) Helipot

26. List out the limitations of thermistor.

The limitations of thermistor are

(i) Non‒linearity in resistance Vs temperature characteristics.

(ii) Unsuitable for wide temperature range.

(iii) Very low excitation current to avoid self‒heating.

(iv) Need of shielded power lines, filters etc., due to high resistance.

27. Write short notes on LVDT.

LVDT (Linear Variable Differential Transformer) converts the mechanical energy into differential electrical energy. It has single primary, winding, and two secondary windings wound on a hollow cylindrical former. A movable soft iron slides within the hollow former and therefore affects the magnetic coupling between the primary and the two secondaries.

28. List out the advantages of LVDT.

The advantages of LVDT are,

(i) High range of displacement measurement.

(ii) Friction and electrical isolation.

(iii) Immunity from external effects.

(iv) High input and high sensitivity.

(v) Ruggedness.

(vi) Low hysteresis and lower consumption.

29. List out the limitations of LVDT.

The limitations of LVDT are

(i) Large displacements are required for appreciable differential output.

(ii) They are sensitive to stray magnetic fields

(iii) Dynamic response is limited.

(iv) Temperature also affects the transducer.

30. List out the two physical parameters in strain guage.

The two physical parameters in strain gauge are,

(i) The change in gauge resistance.

(ii) The change in length.