EX-303 Electrical Measurements and Measuring Instruments - Dec 2024 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. 4 Marks
   aWhat are the different types of errors in measurement? Explain.
   How to answer

   • Define gross, systematic and random errors
   • Sub-types: instrumental, environmental, observational, parallax, limiting errors
   • Explain causes and remedies with examples
2. 5 Marks
   bDescribe the static and dynamic characteristics of measuring instruments.
   How to answer

   • Define static characteristics: accuracy, precision, sensitivity, resolution, linearity, drift
   • Define dynamic characteristics: speed of response, fidelity, lag, dynamic error
   • Compare static vs dynamic with examples
3. 5 Marks
   cAn electric current of 3 ampere is flowing through a resistance of 10 ohms. It was found that the resistance was 0.2% greater than what was specified as rated and the ammeter measurement was 0.5% more than the true value. Determine the relative error in power measurement.
   How to answer

   • Power $P=I^2R$, relative error $\delta P/P = 2\delta I/I + \delta R/R$
   • Given $\delta R/R = 0.2\%$, $\delta I/I = 0.5\%$
   • Compute $2\times0.5+0.2 = 1.2\%$
   • Conclude relative error in power measurement
4. 7 Marks
   aDerive the equation of motion for Ballistic Galvanometer.
   How to answer

   • Torque balance: deflecting, inertia, damping, control torques
   • Differential equation $Jd^2\theta/dt^2 + D d\theta/dt + C\theta = Gi$
   • Undamped and damped cases and ballistic condition
   • Expression for charge / first throw
5. 7 Marks
   bExplain the construction and working of Digital electronic energy meter.
   How to answer

   • Block elements: CT/PT sensors, signal conditioning, ADC, microcontroller, display
   • Working: sampling V-I, multiplication for power, integration for energy
   • Features: digital storage, tariff, anti-tamper
6. 7 Marks
   aExplain the working operation for moving iron instruments and derive it's torque equation.
   How to answer

   • Construction: attraction vs repulsion types, coil, vane, spring, damping
   • Working: magnetisation of soft iron in coil field
   • Torque derivation: $T = \frac{1}{2}I^2 dL/d\theta$
   • Spring control balance for deflection
7. 7 Marks
   bExplain the construction of multi-range voltmeter and Range extension DC voltmeter.
   How to answer

   • Basic DC voltmeter: meter movement + series multiplier
   • Multi-range: tapped / individual multipliers and selector switch
   • Range extension: calculation of series resistance $R_s = V/I - R_m$
   • Loading and sensitivity effects
8. 7 Marks
   aExplain the construction of (i) Current transformer (ii) Potential transformer.
   How to answer

   • CT: toroidal core, primary turns, secondary, burden; open-circuit precaution
   • PT: step-down voltage transformer, primary-secondary, burden
   • Insulation, earthing and connection diagrams
9. 7 Marks
   bWhat are the steps are taken by the minimizing error in PT?
   How to answer

   • Sources of PT error: ratio and phase angle due to $I_0$, burden, impedance
   • Reduce magnetising current, low core loss, low winding resistance/leakage
   • Proper burden, turns compensation, high permeability core
10. 7 Marks
    aDiscuss with circuit and phase diagram, describe the working of three phase AC energy meter.
    How to answer

    • Circuit: two-element induction meter, pressure and current coils
    • Phasor diagram for balanced 3-phase supply
    • Driving, braking torque and disc rotation integration
    • Meter constant and registration
11. 7 Marks
    bExplain the construction and working of maximum demand meter.
    How to answer

    • Purpose: record maximum average load over interval
    • Construction: thermal / Merz-Price indicator, pointer with friction
    • Working: resetting after interval, time lag mechanism
12. 7 Marks
    aExplain the construction and working of three phase energy meter.
    How to answer

    • Elements: electromagnets, aluminium disc, braking magnet, registering train
    • Production of rotating field and eddy currents
    • Torque balance and energy registration
13. 7 Marks
    bA 50A, 230 V meter on full load test makes 61 revolutions in 37 seconds. If the normal disc speed is 520 revolutions per Kwh, find the percentage error.
    How to answer

    • Actual energy = $VI \times t/3600$ kWh
    • Recorded energy = revolutions / meter constant (520 rev/kWh)
    • Percentage error = (recorded-actual)/actual x 100%
    • Substitute 61 rev, 37 s, 50 A, 230 V
14. 7 Marks
    aExplain the construction of three phase electrodynamometer power factor meter.
    How to answer

    • Fixed and moving coils, pressure coil with series resistor/inductor
    • Three-phase connection and resultant deflecting torque
    • Pointer indicates $\cos\phi$ / power factor scale
15. 7 Marks
    bExplain the details in energy meter and also the limitations are details to explain.
    How to answer

    • Details: driving system, moving system, braking, registering, errors/adjustments
    • Limitations: temperature, frequency, waveform, stray field, friction
    • Overload and creeping effects
16. 4 Marks
    aDraw the block diagram of frequency meter and explain it.
    How to answer

    • Blocks: input conditioner, gate, counter, time base, display
    • Unknown frequency vs reference time measurement
    • Range selection and accuracy
17. 5 Marks
    bExplain the construction and working of Earth resistance measurement.
    How to answer

    • Principle: fall of potential / Megger earth tester
    • Construction: hand generator, current and potential electrodes
    • Procedure: 3-terminal measurement, resistance calculation
18. 5 Marks
    cExplain the working of Wheatstone bridge. For which type of resistance measurement it is used.
    How to answer

    • Circuit: four arms P,Q,R,S, galvanometer and battery, balance $P/Q=R/S$
    • Working: null deflection at balance
    • Used for medium resistance measurement