EX-303 Electrical Measurements and Measuring Instruments - Jun 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. 6 Marks
   aDescribe construction and working of a ballistic galvanometer. Explain the difference constructional details of a ballistic galvanometer and a d'Arsonval galvanometer.
   How to answer

   • Construction of ballistic galvanometer: moving coil, suspension, mirror, damping
   • Working: charge measurement from first swing proportional to quantity of electricity
   • Constructional differences vs d'Arsonval: moment of inertia, damping, suspension, time period
2. 8 Marks
   bThe discharge of a capacitor through a ballistic galvanometer produces a damped frequency of 125 Hz and successive swings of 120, 96 and 76.8 mm. calculate the damping ratio. Also determine the logarithmic maximum decrement and period of un-damped and damped oscillations.
   How to answer

   • Given damped frequency and successive amplitudes
   • Calculate logarithmic decrement from amplitude ratio
   • Calculate damping ratio from decrement
   • Determine damped and undamped periods
3. 7 Marks
   aThe inductance of a 25 A electrodynamic ammeter changes uniformly at the rate of $0.0035$ $\mu$H/degree. The spring constant is $10^6$ N-m/degree. Determine the angular deflection at full scale.
   How to answer

   • Electrodynamic ammeter deflecting torque $T_d = I^2 dL/d\theta$
   • Controlling torque $T_c = K \theta$
   • Equate at equilibrium to find $\theta$
   • Substitute $I=25$ A, $dL/d\theta$, $K$
4. 7 Marks
   bExplain how an electrodynamometer type instrument is able to measure the true r.m.s. value of a voltage or current irrespective of its waveform.
   How to answer

   • Deflecting torque proportional to product of coil currents
   • For voltage/current measurement coils in series, torque proportional to $i^2$
   • Inertia averages torque over cycle giving mean-square value
   • Calibration in r.m.s. independent of waveform
5. 8 Marks
   aExplain the effect of secondary burden on the ratio and phase errors of a current transformer.
   How to answer

   • Define ratio error and phase angle error of CT
   • Effect of burden impedance on secondary voltage and exciting current
   • Increase in burden increases ratio and phase errors
   • Compensation and rated burden concept
6. 6 Marks
   bExplain in details the effect of opening the secondary circuit of a current transformer when the primary winding is energized.
   How to answer

   • Normal CT operation with low secondary impedance
   • Open secondary: magnetizing current equals primary current
   • High flux, saturation, very high secondary voltage
   • Core heating, insulation danger and safety precautions
7. 7 Marks
   aA 3 phase 500 V motor load has a power factor of 0.4. Two wattmeter's connected to measure the input. They show the input to be 30 kW. Find the reading of each instrument.
   How to answer

   • Total power $P = W_1+W_2 = \sqrt{3}V_L I_L \cos\phi$
   • Find line current from $P$, $V$, $\cos\phi$
   • Use $\tan\phi = \sqrt{3}(W_1-W_2)/(W_1+W_2)$
   • Solve for $W_1$ and $W_2$
8. 7 Marks
   bDescribe the constructional details of an Electrodynamometer type wattmeter. Derive the Ferrodynamic expression for torque when the instrument is used on A.C.
   How to answer

   • Construction: fixed and moving coils, air core, spring control
   • Instantaneous torque with AC currents
   • Average deflecting torque derivation
   • Ferrodynamic modification with iron core
9. 8 Marks
   aExplain the special features incorporated in an electrodynamometer type of wattmeter so that it can be used for low power factor applications.
   How to answer

   • Problem of low power factor: small deflecting torque, phase error
   • Compensating coil and low pressure coil current
   • Use of high resistance pressure circuit
   • Special design for small angle between voltages and currents
10. 6 Marks
    bDescribe a circuit for testing of a single phase induction type energy meter at different loads and power factors.
    How to answer

    • Phantom loading / test circuit with separate voltage and current supply
    • Variation of load current and power factor
    • Measurement of energy and error calculation
    • Circuit diagram with wattmeter as reference
11. 7 Marks
    aWhat are the different factors which affect the precision measurement of medium resistances with Wheatstone bridge? Explain how their effects are minimized/eliminated.
    How to answer

    • Sources of error: lead resistance, thermo-emf, heating, sensitivity
    • Effect of ratio arm inaccuracy and contact resistance
    • Minimization by high sensitivity galvanometer, thick leads
    • Use of Carey-Foster and guarded measurements
12. 7 Marks
    bWhat are the different problems associated with measurement of low resistances? Explain the principle of working a Kelvin's Double Bridge and explain how the effect of contact resistance and resistance of leads is eliminated.
    How to answer

    • Problems in low resistance: contact and lead resistance comparable
    • Kelvin double bridge principle with two sets of ratio arms
    • Balance condition independent of yoke resistance
    • Elimination of contact and lead effects
13. 6 Marks
    aExplain different method of earth resistance measurement.
    How to answer

    • Fall of potential method and three-electrode method
    • Use of earth tester / Megger
    • Electrode spacing and soil conditions
    • Precautions for accurate measurement
14. 8 Marks
    bExplain the Method of Reversals for experimental determination of hysteresis loop of a magnetic specimen.
    How to answer

    • Ballistic galvanometer method for B-H loop
    • Procedure of reversals of magnetizing current
    • Measurement of flux change and H steps
    • Plotting hysteresis loop from data
15. 14 Marks
    Write a short notes on (any two):

    1. i)Frequency Meter
    2. ii)Ratio Meter
    3. iii)Digital Voltmeter
    4. iv)Lloyd Fischer Square for Measurement of Power Loss
    How to answer

    • Select any two instruments/topics
    • Principle, construction and working for each
    • Diagram and applications
    • Advantages/limitations