EX-303 Electrical Measurements and Measuring Instruments - Jun 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
   aEnumerate the dynamic characteristics of a measuring system.
   How to answer

   • Define dynamic characteristics as time-varying response
   • List speed of response, measuring lag, fidelity, dynamic error
   • Brief meaning of each with example
   • Distinguish from static characteristics
2. 7 Marks
   bTwo resistors have the following ratings: $R_1 = 200 \Omega \pm 5\%$, $R_2 = 200 \Omega \pm 5\%$ Determine the magnitude of resultant resistance and the limiting errors in percentage and in ohm if the above resistances are connected in Series and Parallel.
   How to answer

   • Compute series resultant $R_s = R_1+R_2$
   • Compute limiting error in series in ohm and percent
   • Compute parallel resultant $R_p = R_1R_2/(R_1+R_2)$
   • Compute limiting error in parallel in ohm and percent
3. 7 Marks
   aExplain the constructional features & principle of operation of moving iron instruments.
   How to answer

   • State attraction and repulsion types principle
   • Describe fixed coil, moving iron, spindle, spring, damping
   • Explain deflecting torque $\propto I^2$ operation
   • Mention scale non-uniform and AC/DC use
4. 7 Marks
   bA moving coil meter gives full scale deflection with a current of 5mA, if the coil of the instrument has a resistance of $10 \Omega$. Show how it can be adopted to work

   1. i)as an ammeter with a range of 0 to 10 A
   2. ii)as Voltmeter with a range of 0 to 100 V
   How to answer

   • Compute meter voltage $V_m = I_m R_m$
   • Calculate shunt $R_{sh}=V_m/(I-I_m)$ for 0-10 A
   • Calculate multiplier $R_{se}=V/I_m - R_m$ for 0-100 V
   • Show connection diagrams for ammeter and voltmeter
5. 7 Marks
   aDescribe the construction and working of a ballistic galvanometer. Explain the difference in constructional details of a ballistic galvanometer and a d'Arsonval galvanometer.
   How to answer

   • Describe construction: coil, suspension, magnet, damping
   • Explain ballistic operation for charge measurement
   • Differentiate inertia, damping, time period from d'Arsonval
   • State uses for flux and capacitance measurement
6. 7 Marks
   bWhen a current of 0.001A is passed through a ballistic galvanometer, it gives a steady deflection of 50 scale divisions. When a capacitor charged to a potential of 100V d.c is discharged through the galvanometer, it gives a maximum throw of 220 scale divisions. The periodic time of swing is 4 second. Find the value of capacitance. The value of logarithmic decrement is 0.2.
   How to answer

   • Relate steady deflection to current sensitivity
   • Relate first throw to charge sensitivity and capacitance
   • Apply correction for logarithmic decrement
   • Substitute $T=4\,s$, throws 50 and 220 div, $V=100\,V$ to get $C$
7. 7 Marks
   aDraw the equivalent circuit and phasor diagram of a potential transformer. Derive the expression for its ratio and phase angle errors.
   How to answer

   • Draw equivalent circuit referred to primary/secondary
   • Draw phasor diagram with $I_0$, $I_c$, $I_m$ components
   • Derive ratio error and phase angle error expressions
   • Explain effect of burden and no-load current
8. 7 Marks
   bA potential transformer with ratio 1000/100 V, has the following constants: Primary resistance $94.5 \Omega$, Secondary resistance $0.86 \Omega$, Primary reactance $66.2 \Omega$, Total equivalent reactance $110 \Omega$. Magnetizing current 0.02A at 0.4 pf. Calculate:

   1. i)The phase-angle error at no-load between primary and secondary voltages
   2. ii)The load in VA at unity pf at which the phase angle will be zero.
   How to answer

   • List given $R_p, R_s, X_p, X_{eq}, I_0, \phi_0$
   • Calculate no-load phase angle error from $I_0$ components
   • Set total phase angle to zero for unity pf load
   • Solve for burden VA
9. 7 Marks
   aDescribe the construction and working principle of single phase wattmeter.
   How to answer

   • Describe pressure coil, current coil, disc, braking magnet
   • Explain deflecting torque proportional to $VI\cos\phi$
   • Explain braking and registering mechanism
   • Show connection diagram
10. 7 Marks
    bA 240V, 5 A, single phase energy meter has a registration constant of 1200 revolutions per kWh. It is tested by means of a 240V, 5A wattmeter having 500 scale divisions which can be read to 0.1 division and a stop watch which can be read to 0.01 second and which has negligible error. When tested at full load the meter makes 40 revolutions in 99.8s. If the human error in timing be taken as $\pm 0.05$s, estimate the limits within which the error of the meter may lie. The wattmeter is accurate to within 0.05 percent of its full scale reading.
    How to answer

    • Compute true energy from wattmeter reading and time
    • Compute recorded energy from revolutions and meter constant
    • Include timing error $\pm 0.05\,s$ and wattmeter 0.05% error
    • Estimate upper and lower limits of meter error
11. 7 Marks
    aExplain why Kelvin's double bridge is superior to Wheatstone bridge for the purpose of low resistance measurement. Give theory of the bridge and necessary arrangement in order that greatest precision possible may be obtained.
    How to answer

    • State limitation of Wheatstone bridge for low R: lead and contact resistance
    • Describe Kelvin double bridge with two sets of ratio arms
    • Derive balance condition $R_x = (P/Q)R$
    • Explain procedure for greatest precision
12. 7 Marks
    bA multimeter having a sensitivity of $2000 \Omega/V$ is used for the measurement of voltage across a circuit having an output resistance of $10 \Omega$. The open circuit voltage is 6V. Find the reading of the multimeter when it is set to its 10V scale. Find the percentage error.
    How to answer

    • Compute meter resistance on 10V range from sensitivity
    • Compute parallel combination with 10 ohm source
    • Find indicated voltage by divider
    • Compute percentage error vs 6V open circuit
13. 7 Marks
    aDescribe the method for measurement of B-H curve of a magnetic material of bar form.
    How to answer

    • Describe bar specimen with magnetizing winding and search coil
    • Explain step-by-step reversal and fluxmeter/galvanometer readings
    • Plot $B$ vs $H$ from $B=\phi/A$, $H=NI/l$
    • Mention precautions
14. 7 Marks
    bExplain how transformer steel available in strips of 25 cm length and 5 cm width is tested for iron losses by the wattmeter method.
    How to answer

    • Describe Epstein square / strips assembly 25cm x 5cm
    • Explain wattmeter connection with pressure coil and current coil
    • Explain separation of iron loss measurement
    • State precautions and calculations
15. 14 Marks
    Write Technical Notes in the following. (Any two)

    1. a)Megger
    2. b)Synchronoscope
    3. c)Tri-vector meter
    How to answer

    • Megger: principle, hand generator, ratio meter for insulation resistance
    • Synchronoscope: synchronizing alternators, phase and frequency indication
    • Tri-vector meter: measures kW, kVAr, kVAh/kWh for 3-phase