March 2023
ELECTRICAL AND ELECTRONIC MEASUREMENT
Time Allowed: 2.5 Hours Full Marks: 60
Answer to Question No. 1 of Group A is compulsory and to be answered first. This answer is to be made in separate loose script(s) provided for the purpose. Maximum time allowed is 30 minutes, after which the loose answer scripts will be collected and fresh answer scripts for answering the remaining part of the question will be provided. On early submission of answer scripts of Question No. 1, a student will get the remaining script earlier.
Answer any Five (05) Questions from Group B.
Group A
1. Choose the correct answer from the given alternatives (any twenty): 1×20
i. The spring material used in a spring control device should have the following property:
- a)Should be non-magnetic
- b)Must be of low temperature co-efficient
- c)Should have low specific resistance
- d)All of the above
Show Answer
Answer: d) All of the above
Explanation: Spring-control materials should be non-magnetic, have a low temperature coefficient, and have suitable electrical resistance so that the controlling torque remains stable and reliable.
ii. The device which prevents oscillation of the moving system and enables the latter to reach its final position quickly is called
- a)Deflecting device
- b)Controlling device
- c)Damping device
- d)All of these
Show Answer
Answer: c) Damping device
Explanation: The damping device opposes the motion of the moving system and suppresses oscillations. It allows the pointer to settle quickly at its final steady position.
iii. The use of which type of instruments are merely confined within laboratories as standardizing instruments?
- a)Absolute
- b)Indicating
- c)Recording
- d)Integrating
Show Answer
Answer: a) Absolute
Explanation: Absolute instruments determine a quantity in terms of the instrument constants and are mainly used in laboratories as standards for calibration.
iv. The ratio of the output to input change for a given measuring system is referred to as-
- a)sensitivity
- b)linearity
- c)stability
- d)none of these.
Show Answer
Answer: a) sensitivity
Explanation: Sensitivity is the ratio of change in output to the corresponding change in input of a measuring system.
v. Shunt for extension of range of instrument generally made of –
- a)copper
- b)manganese
- c)nichrome
- d)aluminium
Show Answer
Answer: b) manganese
Explanation: A shunt requires low resistance and good stability. Manganin, a copper-manganese alloy, is commonly used for precision shunts because of its low temperature coefficient.
vi. Meter constant of energy meter is designated by-
- a)rev/kw
- b)rev/kwh.
- c)kw/rev.
- d)kwhr/rev.
Show Answer
Answer: b) rev/kwh.
Explanation: The meter constant of an energy meter is commonly expressed as revolutions per kilowatt-hour, indicating the number of disc revolutions corresponding to one kWh of energy.
vii. The minimum no of wattmeter required to measure power of 3 phase unbalanced star connected load is-
- a)one
- b)two
- c)three
- d)four.
Show Answer
Answer: c) three
Explanation: For an unbalanced three-phase, three-wire star-connected load, three wattmeters are required to measure the total power directly, with one wattmeter associated with each phase.
viii. Difference between the true value and measured value is called-
- a)accuracy.
- b)precision.
- c)error.
- d)sensitivity.
Show Answer
Answer: c) error.
Explanation: The difference between the measured value and the true value is called error. Accuracy describes closeness to the true value, while precision describes repeatability.
ix. A wheatstone bridge cannot be used for precision measurement because error is introduced on account of-
- a)thermo electric emf
- b)resistance of connecting lead
- c)contact resistance
- d)all of the above.
Show Answer
Answer: d) all of the above.
Explanation: Precision low-resistance measurements with a Wheatstone bridge can be affected by thermoelectric EMFs, lead resistance, and contact resistance. These unwanted effects introduce measurement error.
x. CT is basically a which kind of transformer?
- a)Step-down
- b)Step-up
- c)Isolation
- d)None of these
Show Answer
Answer: a) Step-down
Explanation: A current transformer is essentially a step-down transformer in terms of current: it produces a much smaller secondary current proportional to the primary current for measurement and protection.
xi. Range of an ammeter can be extended by :
- a)Shunt
- b)Multiplier
- c)Rectifier
- d)Multiplexer
Show Answer
Answer: a) Shunt
Explanation: A low-resistance shunt is connected in parallel with the ammeter movement so that most of the current bypasses the movement, extending the current range.
xii. Electrodynamometer type of instruments are used commonly for the measurement of
- a)Resistance
- b)Voltage
- c)Current
- d)Power
Show Answer
Answer: d) Power
Explanation: Electrodynamometer instruments are commonly used as wattmeters for measuring electrical power. They can operate on both AC and DC.
xiii. In a single phase power factor meter the phase difference between the currents in the two pressure coils is
- a)Exactly 0°
- b)Approx 0°
- c)Exactly 90°
- d)Approx 90°
Show Answer
Answer: c) Exactly 90°
Explanation: The two pressure-coil currents in a single-phase electrodynamic power-factor meter are arranged to have a phase difference of approximately 90°; among the given choices, the intended answer is exactly 90°.
xiv. A C.R.O can be used to measure
- a)A.C. voltages only
- b)D.C. voltages only
- c)Frequency
- d)Any of the above
Show Answer
Answer: d) Any of the above
Explanation: A CRO can display and measure AC and DC voltage waveforms and can determine frequency from the displayed waveform and time-base scale.
xv. For measuring a very high resistance we should use
- a)Kelvin’s double bridge
- b)Wheat stone bridge
- c)Meggar
- d)None of these
Show Answer
Answer: c) Meggar
Explanation: A Megger is designed to measure very high resistance, particularly insulation resistance, using a high test voltage.
xvi. In which instrument creeping can be observed?
- a)Voltmeter
- b)Ammeter
- c)Wattmeter
- d)Energy meter
Show Answer
Answer: d) Energy meter
Explanation: Creeping is an unwanted slow rotation of the disc when a single-phase induction energy meter is at no-load. It is therefore associated with energy meters.
xvii. Which of the following are integrating instruments?
- a)Voltmeter
- b)Ammeter
- c)ohm-meter
- d)Energy meter
Show Answer
Answer: d) Energy meter
Explanation: An integrating instrument measures the totalized value of a quantity over time. An energy meter integrates electrical power with respect to time to indicate energy consumed.
xviii. The power of a n-phase circuit can be measured by using a minimum of _____ number of wattmeters.
- a)n
- b)n+1
- c)n-1
- d)2n
Show Answer
Answer: c) n-1
Explanation: For an n-phase, n-wire system, the total power can be measured by the minimum n-1 wattmeters using the corresponding generalized wattmeter method.
xix. Capacitor can be measured by:
- a)Kelvin Double Bridge
- b)Wheatstone Bridge
- c)Schering Bridge
- d)Maxwell Bridge
Show Answer
Answer: c) Schering Bridge
Explanation: The Schering bridge is an AC bridge specifically used for measuring capacitance and dielectric loss of capacitors and insulating materials.
xx. In a Kelvin’s double bridge two sets of readings are taken when measuring a low resistance, one with the current in one direction and the other with direction of current reversed. This is done to-
- a)eliminate the effect of contact resistance.
- b)eliminate the effect of resistance of leads.
- c)correct for changes in battery voltage.
- d)eliminate the effect of thermo-electric emf.
Show Answer
Answer: d) eliminate the effect of thermo-electric emf.
Explanation: Reversing the current reverses the polarity of thermoelectric EMFs relative to the measurement, allowing their effect to be cancelled by averaging the two readings.
xxi. CT has two types of error; these are phase angle error and-
- a)creeping error
- b)parallax error
- c)ratio error
- d)none of these
Show Answer
Answer: c) ratio error
Explanation: The two principal errors of a current transformer are ratio error and phase-angle error. Ratio error is the deviation of the actual transformation ratio from the nominal ratio.
xxii. A permanent magnet is used in –
- a)PMMC instrument
- b)MI instrument
- c)EMMC instrument
- d)CRO
Show Answer
Answer: a) PMMC instrument
Explanation: A permanent magnet provides the magnetic field in a permanent-magnet moving-coil (PMMC) instrument. The interaction between this field and coil current produces deflecting torque.
xxiii. The power in a d.c. circuit is measured with the help of an ammeter and a voltmeter. The voltmeter is connected on the load side. The power indicated by the product of readings of two instruments (VI) is-
- a)the power consumed in the load.
- b)the sum of power consumed by load and the voltmeter.
- c)the sum of power consumed by load and the ammeter.
- d)none of the above.
Show Answer
Answer: b) the sum of power consumed by load and the voltmeter.
Explanation: With the voltmeter connected directly across the load, the ammeter measures the total current supplied to both the load and the voltmeter. Thus VI includes the load power plus the voltmeter power.
xxiv. A megger is used for measurement of
- a)low valued resistances.
- b)medium valued resistances.
- c)high valued resistances, particularly insulation resistance.
- d)all of the above
Show Answer
Answer: c) high valued resistances, particularly insulation resistance.
Explanation: A Megger is primarily used for high resistance measurements, especially insulation resistance of electrical equipment, cables, and wiring.
xxv. A C.R.O. can be used to measure
- a)A.C. voltages only
- b)D.C. voltages only
- c)Frequency
- d)Any of the above
Show Answer
Answer: d) Any of the above
Explanation: A CRO can be used to observe AC and DC voltage signals and to measure frequency from the waveform displayed against the calibrated time base.
Group B
2. a) Compare between spring control and gravity control. Which of the two is preferred for portable instruments and why it is provided in measuring instruments? c) Why damping is necessary in indicating instrument? What is critical damping? (5+3)
3. a) Define the terms: Sensitivity, Precision and Accuracy. b) Why damping torque is required? Give in brief the different methods of damping. (2+5)
4. a) Draw the neat labeled diagram the construction of moving iron repulsion type instrument and explain its operation. b) A moving coil instrument has a resistance of 10Ω and gives full scale deflection of 1mA. Show how it can be used to measure voltage up to 250 volt. (5+3)
5. a) Two wattmeters are connected to measure the power consumed by a 3-phase load with power factor 0.4 Total power consumed by the load, as indicated by the two wattmeters is 30 kW. Find the individual wattmeter readings. b) A unity power factor wattmeter has a current range of 5 amp and voltage range of 200 volt. If its full-scale deflection is 500 watt, what should be the value of Multiplying Factor? (5+3)
6. a) What is creeping error? How can it be overcome? b) Mention the different errors which occur in dynamometer type wattmeter. (4+4)
7. a) Derive the equations for balance for an Anderson’s bridge. b) Write down two differences between “T” and “P” instrument. (6+2)
8. a) Draw the block diagram of a basic CRO. b) What are major components of a CRT? (4+4)
9. a) The meter constant of a 230 volt, 10 amp watt-hour meter is 1800 revolutions per kWh. The meter is tested at half load and rated voltage and unity power factor. The meter is found to make 80 revolutions in 138 second. Determine the meter error at half load. b) What are the functions of a spring in P.M.M.C instrument? (6+2)
10. Draw the block diagram of a basic Digital Frequency Meter and write the function of each block. (4-4)