DECEMBER 2024
ELECTRICAL AND ELECTRONIC MEASUREMENT
Time Allowed: 3 Hours Full Marks-60
Answer to Question No. 1 of Group A must be written in the main answer script. In Question No. 1, out of 2 marks for each MCQ, 1 mark is allotted for right answer and 1 mark is allotted for correct explanation of the answer.
Answer any Five (05) Questions from Group-B.
GROUP-A
1. Choose the correct answer from the given alternatives and explain your answer (any ten) 10×2=20
(i) Controlling torque is directly proportional to –
- (a) Angle of deflection of the moving system
- (b) Deflecting torque
- (c) Both
- (c) None of these.
Show Answer
Answer: (a) Angle of deflection of the moving system
Explanation: In an indicating instrument, controlling torque is proportional to the angular displacement of the moving system. At equilibrium, controlling torque balances the deflecting torque.
(ii) The torque which controls the movement of pointer is called-
- (a) Controlling torque
- (b) Deflecting torque
- (c) Both
- (d) None of these.
Show Answer
Answer: (a) Controlling torque
Explanation: Controlling torque opposes the deflection of the moving system and determines the final steady position of the pointer. It also provides a restoring torque when the deflecting torque is removed.
(iii) The pointer (or moving system) returns to its zero position on removing the source producing the deflecting torque. This happens due-
- (a) Controlling torque
- (b) Damping torque
- (c) Balancing torque
- (d) Mass of pointer.
Show Answer
Answer: (a) Controlling torque
Explanation: The controlling system provides a restoring torque that brings the pointer back toward its zero position when the deflecting torque is removed. Damping torque only controls oscillations during movement.
(iv) Tangent Galvanometer is a example of-
- (a) Absolute instrument
- (b) Indicating Instrument
- (c) Recording Instrument
- (d) Integrating Instrument.
Show Answer
Answer: (a) Absolute instrument
Explanation: A tangent galvanometer is an absolute instrument because the current can be determined from the instrument constants and the measured deflection without comparison with a calibrated standard instrument.
(v) Operating torque of an indicating instrument (angle) are-
- (a) Deflecting
- (b) Controlling
- (c) Damping
- (d) a,b & c.
Show Answer
Answer: (d) a,b & c.
Explanation: The operating mechanism of an indicating instrument involves deflecting, controlling, and damping torques. These three torque systems determine deflection, final position, and steady movement of the pointer.
(vi) When a PMMC instrument is connected to AC voltage-
- (a) The instrument damage
- (b) The pointer will not move to all
- (c) The pointer will oscillate and fro
- (d) The instrument will indicate zero.
Show Answer
Answer: (d) The instrument will indicate zero.
Explanation: A PMMC instrument responds to the average value of current and is intended for DC. With symmetrical AC, the torque reverses every half-cycle and the average deflecting torque is zero, so the pointer indicates zero.
(vii) Electro-dynamometer type moving coil instrument are mainly used as-
- (a) Indicating instruments
- (b) Standard instrument for calibration of other instruments.
- (c) Transfer instruments only
- (d) Both as standard instruments and transfer instruments.
Show Answer
Answer: (d) Both as standard instruments and transfer instruments.
Explanation: Electrodynamometer instruments can be used on both AC and DC and are commonly used as transfer instruments. Precision electrodynamometer instruments are also used as standards for calibration.
(viii) The rectifier instrument is not free from-
- (a) Temperature error
- (b) Frequency error
- (c) Wave shape error
- (d) All of these
Show Answer
Answer: (d) All of these
Explanation: Rectifier instruments can have errors due to temperature variation, frequency dependence of the rectifier and waveform differences from the waveform for which the instrument is calibrated. Therefore, all listed errors can occur.
(ix) A laboratory AC voltmeter is most likely to be
- (a) PMMC
- (b) Electrostatic
- (c) Moving Iron
- (d) Induction
Show Answer
Answer: (c) Moving Iron
Explanation: Moving-iron instruments are suitable for both AC and DC and are commonly used as AC voltmeters. PMMC instruments are primarily DC instruments unless used with a rectifier.
(x) Suppose the potential coil and current coil of a wattmeter are changed accidently and supply is given.
- (a) Wattmeter will not read
- (b) Current coil will burn out
- (c) Potential coil burn out
- (d) Nothing will happen.
Show Answer
Answer: (b) Current coil will burn out
Explanation: The current coil has very low resistance and is designed to carry current in series with the load. If it is accidentally connected across the supply like a potential coil, a very large current can flow through it and damage or burn the coil.
(xi) W1 & W2 are the reading of two wattmeters used to measure power of 3-phase balanced load. The reactive power drawn by the load is-
- (a) W1+W2
- (b) W1-W2
- (c) √3(W1-W2)
- (d)√3(W1-W2)
Show Answer
Answer: (c) √3(W1-W2)
Explanation: For a balanced three-phase load measured by the two-wattmeter method, reactive power Q is given by √3(W1−W2). The total active power is W1+W2.
(xii) Kelvin’s double bridge is measure-
- (a) Low resistance
- (b) Medium resistance
- (c) High resistance
- (d) All
Show Answer
Answer: (a) Low resistance
Explanation: Kelvin’s double bridge is designed for accurate measurement of very low resistances. Its additional ratio arms minimize the effect of lead and contact resistance.
(xiii) In a CRO quantity to be measure is applied across-
- (a) Focusing electrodes
- (b) Cathode
- (c) Y-plates
- (d) X-plates
Show Answer
Answer: (c) Y-plates
Explanation: In a CRO, the input signal to be observed is normally applied to the vertical deflection system, which uses the Y-plates. The resulting vertical deflection represents the instantaneous amplitude of the input signal.
(xiv) The source of electron beam in a CRO is-
- (a) Electron gun
- (b) Grid
- (c) Focusing anode
- (d) Vertical deflection plates.
Show Answer
Answer: (a) Electron gun
Explanation: The electron gun in a CRT generates and accelerates the electron beam. The grid controls beam intensity and the focusing and accelerating anodes shape and accelerate the beam further.
(xv) With the help of Anderson bridge we can measure –
- (a) Resistance
- (b) Inductance
- (c) Capacitance
- (d) None of these
Show Answer
Answer: (b) Inductance
Explanation: Anderson bridge is an AC bridge used for precise measurement of unknown self-inductance. It uses a standard capacitor and resistors to establish a balance condition from which inductance is determined.
GROUP B
Answer any Five (05) questions.
2.(a) Describe briefly about indicating instrument. (b) What is Damping and why it is provided in measuring instruments? (c) Why Damping torque is required? 6+1+1
3.(a) Describe briefly about sensitivity and accuracy. (b) Write down the difference between Spring control and Gravity control. 4+4
4.(a) Write down the torque equation of PMMC. (b) Describe briefly the construction and working principal of CT. 3+5
5.(a) Describe briefly the measurement of active power in three-phase circuit for balanced load by two wattmeter method. (b) Two wattmeters are connected to measure power in a 3-φ network. The two readings are 2000 watts and 1000 watts respectively. If another wattmeter be connected such that its current coil is in one phase and potential coil is across the other two phase terminals, what will it read? Also estimate reactive power. 4+4
6.(a) Describe briefly about single phase energy meter. (b) Describe briefly about the working principal and basic block diagram of L-C-R meter. 3+5
7.(a) Describe briefly about the Kelvin’s double bridge. (b) What is shunt type wattmeter? 5+3
8.(a) Describe briefly about digital storage oscilloscope. (b) What is CRT? (c) What is electrostatic deflection? 4+2+2
9.(a) Describe briefly about Schering bridge. (b) Describe briefly the working principal of function generator and basic block diagram. 4+4