EX-303 Electrical Measurements and Measuring Instruments - Jun 2025 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
   aExplain the construction and operation of D'Arsonval galvanometer.
   How to answer

   • Definition and type: PMMC / D'Arsonval moving-coil galvanometer
   • Construction details: permanent magnet, soft-iron core, moving coil, springs, pointer/scale
   • Working principle: $F = BIl$ and deflecting torque $T_d = NBIA$
   • Control and damping and operation with diagram
2. 7 Marks
   bGeneralize on how the range of instruments can be extended in PMMC instruments.
   How to answer

   • Need for range extension in PMMC ammeter and voltmeter
   • Ammeter: shunt resistance formula $R_{sh}=R_m/(m-1)$
   • Voltmeter: series multiplier $R_s = R_m(m_v-1)$
   • Multirange arrangement and precautions
3. 7 Marks
   aExplain the construction of

   1. i)Current transformer
   2. ii)Potential transformer
   How to answer

   • Definition and use of instrument transformers
   • CT construction: core, primary turns, secondary, insulation
   • PT construction: core, windings, voltage ratio and burden
   • Diagram and differences CT vs PT
4. 7 Marks
   bGive the constructional details of electro dynamometer type wattmeter with a neat sketch.
   How to answer

   • Fixed and moving coils arrangement
   • Control springs, damping and pointer/scale
   • Connections: current coil and pressure coil
   • Neat labelled sketch and working
5. 7 Marks
   aDiscuss the errors of single phase energy meter.
   How to answer

   • Construction of single-phase induction energy meter
   • List of errors: creeping, friction, temperature, frequency, phase, stray field
   • Cause of each error
   • Compensation/adjustment for each
6. 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

   • Given: $V=230\,V$, $I=50\,A$, $N=61$, $t=37\,s$, $K=520\,rev/kWh$
   • Actual energy $E_a = VI t/3600$ and recorded energy $E_r = N/K$
   • % error $= (E_r-E_a)/E_a \times 100$
   • Numerical value with sign and conclusion fast/slow
7. 7 Marks
   aExplain stray magnetic field errors in electro dynamometer wattmeter.
   How to answer

   • Stray field effect on weak operating field of dynamometer
   • Effect on current coil and pressure coil readings
   • Resulting error in deflecting torque/power
   • Shielding and astatic construction remedies
8. 7 Marks
   bExplain the construction of two element and three element dynamometer type wattmeter.
   How to answer

   • Need for polyphase power measurement
   • Two-element construction for 3-phase 3-wire
   • Three-element construction for 3-phase 4-wire
   • Connections and total power as sum of elements
9. 7 Marks
   aDerive the torque equation for electro dynamometer type wattmeter.
   How to answer

   • Instantaneous currents in fixed and moving coils
   • Instantaneous deflecting torque $T_i \propto i_1 i_2$
   • Average torque for sinusoidal AC with phase $\phi$
   • Final form $T_d \propto VI\cos\phi$ and spring control balance
10. 7 Marks
    bA load draws 10A current from 230V AC mains at 0.75 power factor for half an hour calculate the energy consumed.
    How to answer

    • Given $V=230\,V$, $I=10\,A$, $\cos\phi=0.75$, $t=0.5\,h$
    • Power $P = VI\cos\phi$
    • Energy $W = P \times t$ in Wh/kWh
    • Numerical energy value
11. 7 Marks
    aProve that in a Ballistic Galvanometer, the charge is proportional to first swing of the moving coil.
    How to answer

    • Ballistic galvanometer principle and short charge pulse
    • Equation of motion and first deflection $\theta_1$
    • Relation $Q = K\theta_1$ with damping correction
    • Condition: pulse duration << period
12. 7 Marks
    bCompare flux meter and Ballistic Galvanometer.
    How to answer

    • Construction/principle of flux meter vs ballistic galvanometer
    • Damping, control and scale differences
    • Application: flux vs charge measurement
    • Tabular comparison
13. 7 Marks
    aDescribe the method for determination of B.H. curve of a magnetic material using.

    1. i)Method of reversals
    2. ii)Six point method
    How to answer

    • Principle of B-H curve determination with specimen ring
    • Method of reversals: procedure and $B$ from ballistic throw
    • Six-point method: stepwise magnetisation
    • Plot of $B$ vs $H$ and hysteresis inference
14. 7 Marks
    bDescribe measurement of resistance using Kelvin double bridge method and obtain expression for unknown resistance.
    How to answer

    • Kelvin double bridge circuit and need for low resistance
    • Balance condition derivation
    • Expression for unknown resistance $R_x$
    • Effect of lead/yoke resistance elimination
15. 14 Marks
    Write short notes on:

    1. i)Wheatstone bridge balance
    2. ii)Eddy current damping
    3. iii)Ammeter range extension
    4. iv)Use of megger
    How to answer

    • Wheatstone bridge balance condition $P/Q=R/S$
    • Eddy current damping principle and application
    • Ammeter shunt range extension formula
    • Megger construction and insulation testing use