JNTU
JNTU B.Tech III MEASUREMENTS AND INSTRUMENTATION Previous Question Paper (February - 2022 - R18)
Answer any five questions
All questions carry equal marks
Main
A moving coil instrument gives a full-scale deflection of 10mA when the potential difference across its terminal is 100mV. Calculate,
i) The shunt resistance for a full-scale deflection of 100A
ii) Find the resistance for full-scale reading with 100V
Also, calculate the power dissipation in each case.
Answer
First, determine the internal resistance of the instrument (Rm):
i) For a shunt resistance (Rsh) to measure 100 A:
Power dissipation in the shunt: Psh = Ish2 * Rsh ≈ (99.99)2 * 0.0010001 ≈ 10 W.
ii) For a series resistance (Rse) to measure 100 V:
Power dissipation in the series resistor: Pse = Im2 * Rse = (0.01)2 * 9990 = 0.999 W.
Derive an equation for the torque developed in PMMC instrument.
Answer
In a PMMC instrument, the deflecting torque (Td) is produced by the interaction of the magnetic field and the current-carrying coil. The force on one side of the coil is F = BIlN, where B is flux density, I is current, l is length of the conductor, and N is the number of turns. For two sides of the coil, the torque is:
Since the area of the coil A = l * d, the equation becomes:
Where:
- B = Flux density in the air gap (Wb/m2)
- A = Area of the coil (m2)
- N = Number of turns
- I = Current in the coil (A)
Thus, Td = G * I, where G = BAN is the displacement constant, showing that the deflecting torque is directly proportional to the current.
A PMMC ammeter has following specification, coil dimension of 1cm×1cm, spring constant is 0.15×10-6 N-m/rad, Flux density is 1.5×10-3wb/m2. Determine the number of turns required to produce a deflection of 90°, when a current of 2mA flows through the coil.
Answer
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With the help of a neat diagram, explain the working of attracted disc type voltmeter.
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Discuss how AC potentiometer can be used for calibration of wattmeter.
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Explain the procedure to calibrate voltmeter and ammeter using DC potentiometer.
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With help of a neat diagram explain the working of coordinate type potentiometer.
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Draw the equivalent circuit diagram and phasor diagram of the current transformer.
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Discuss the construction and Working Principle of Electrodynamometer type 1-φ wattmeter with help of a neat diagram.
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Explain any two errors that occur in electrodynamometer type 1-φ wattmeter and its compensation.
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With help of a neat diagram, explain the construction and working of a three-phase energy meter.
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Two-watt meters are connected to measure the input to a balanced 3 phase circuit indicating 2000W and 500W respectively. Find the power factor of a circuit,
i) When both the reading is positive and
ii) When the latter reading is obtained after reversing the connections to the current coil of the first instrument.
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Draw the circuit of Kelvin double bridge used for measurement of low resistance. Explain its working principle
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An AC bridge is balanced at 2KHz with the following components in each arm: Arm AB=10KΩ, Arm BC=100µF in series with 100KΩ, Arm AD=50KΩ. Find the unknown impendence R ± jX in the arm DC, if the detector is between BD.
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Derive an equation for gauge factor in strain gauge.
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With help of a neat diagram, explain the principle and working of LVDT.
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