EX-303 Electrical Measurements and Measuring Instruments - Dec 2023 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aBriefly define and explain all the static characteristics of measuring instruments.
   How to answer

   • Define static characteristics: accuracy, precision, sensitivity, linearity, resolution, threshold, drift, stability, etc.
   • Give brief explanation of each characteristic
   • Distinguish static vs dynamic characteristics
2. 7 Marks
   bA voltmeter has a true value of $1.50 \text{V}$. An analog indicating instrument with a scale range of $0$-$2.50 \text{V}$ shows a voltage of $1.46 \text{V}$. What are the values of absolute error and correction? Express the error as a fraction of the true value and the full scale deflection.
   How to answer

   • Calculate absolute error = measured - true value
   • Calculate correction = - error
   • Express error as % of true value and % of full scale 2.50V
3. 7 Marks
   aGive the detailed classification of analog instruments along with their operating principle.
   How to answer

   • Classify by operating principle: moving coil, moving iron, electrodynamometer, induction, electrostatic, etc.
   • Classify by effect: magnetic, heating, electrostatic, induction
   • State operating principle for each type
4. 7 Marks
   bThe constants of a galvanometer are: Control constant $= 0.23\times10^{-6}$ N-m/rad, Damping constant $= 5\times10^{-9}$ N-m/rad-s$^{-1}$ and moment of inertia $= 0.18\times10^{-6}$ kg-m$^{2}$. It is proposed to increase the periodic time to $15$ second by attaching small weights on light arms fixed to the coil spindle. Determine by how much these weights must increase the moment of inertia of the coil.
   How to answer

   • Use undamped natural frequency / periodic time formula $T = 2\pi\sqrt{J/K}$
   • Calculate original periodic time from given $J$ and $K$
   • Find required $J$ for $T=15$ s and hence increase in moment of inertia
5. 14 Marks
   Develop the torque expression of a dynamometer instrument. Sketch the nature of scale distribution for a dynamometer instrument used as an ammeter and voltmeter. The instrument is spring controlled.
   How to answer

   • Derive deflecting torque $T_d \propto I_1 I_2 \cos\phi$ for dynamometer
   • Apply spring control $T_c = K\theta$ for balance
   • Show scale is non-uniform / square-law for ammeter and voltmeter and sketch nature
6. 7 Marks
   aDifferentiate between CT and PT. Discuss any method of testing the potential transformer.
   How to answer

   • Define CT and PT with construction and ratios
   • Tabulate differences: function, connection, range, burden, etc.
   • Describe a method of testing potential transformer e.g. comparison method
7. 7 Marks
   bA current transformer having a single turn primary is rated at $500/5$ A, $50$ Hz with an output of $15$ VA. At rated load with non-inductive burden, the in phase and quadrature components of the exciting mmf are $8$A and $10$A respectively. The number of turns in the secondary is $98$ and the resistance and leakage reactance of the secondary winding are $0.35 \Omega$ and $0.3 \Omega$ respectively. Calculate the ratio and phase angle error.
   How to answer

   • Calculate secondary burden impedance from VA and rated current
   • Resolve exciting current into in-phase and quadrature components
   • Compute ratio error and phase angle error using CT phasor relations
8. 7 Marks
   aSketch the circuit diagram for power measurement in a three phase circuit using two wattmeter's and show that the total power is given by the algebraic sum of the wattmeter readings using vector diagrams.
   How to answer

   • Draw two-wattmeter connection to 3-phase 3-wire system
   • Draw phasor/vector diagram for balanced load
   • Prove $W_1+W_2 = \sqrt{3}VI\cos\phi$ = total power
9. 7 Marks
   bExplain with neat diagram the working of single phase energy meter.
   How to answer

   • Show construction: electromagnet, aluminium disc, braking magnet, registering mechanism
   • Explain driving, braking and registering torques
   • Give neat labelled diagram and working
10. 7 Marks
    aExplain the loss of charge method for the measurement of insulation of cables.
    How to answer

    • Describe circuit for loss of charge method with capacitor and voltmeter
    • Give voltage decay relation $V = V_0 e^{-t/CR}$
    • Explain procedure to find insulation resistance of cables
11. 7 Marks
    bA Kelvin bridge is balanced with the following constants: outer ratio arm $100 \Omega$ and $1000 \Omega$: inner arm ratio $99.92 \Omega$ and $1000.6 \Omega$; resistance of link $0.1 \Omega$; standard resistance $0.00377 \Omega$. Calculate the value of unknown resistance.
    How to answer

    • Apply Kelvin bridge balance condition with outer and inner ratio arms
    • Include effect of link resistance $0.1 \Omega$
    • Calculate unknown low resistance from standard $0.00377 \Omega$
12. 7 Marks
    aDescribe the 'Lloyd Fisher Square' method of measuring the iron loss in ferro-magnetic material.
    How to answer

    • Describe Lloyd Fisher square with magnetic square circuit
    • Explain wattmeter and test specimen arrangement
    • State how iron loss is separated/measured
13. 7 Marks
    bDiscuss in brief, the working principle of power factor meter.
    How to answer

    • State types: electrodynamometer / moving iron power factor meter
    • Explain deflecting torque depends on phase angle
    • Describe scale indicating cos phi directly
14. 14 Marks
    Write Technical notes on any two.

    1. a)Testing and calibration of instrument
    2. b)Loading effects due to series and parallel connected instruments
    3. c)PMMC instruments
    4. d)Frequency meter
    How to answer

    • Select any two topics from a-d
    • Give principle, construction/diagram and application for each
    • Mention calibration/loading effects/PMMC/frequency meter key points