JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year I Semester ELECTRICAL AND ELECTRONIC MEASUREMENTS Previous Question Paper (November/December - 2017 - R16)
Part A is compulsory. Part B consists of 5 Units. Answer any one full question from each unit.
Part A
List the sources of errors in Permanent Magnet Moving Coil type of Instruments.
Answer
- Weakening of permanent magnets: Caused by aging, temperature effects, or mechanical shock.
- Changes in spring resistance: Due to fatigue or temperature variations affecting the restoring torque.
- Temperature effects on coil resistance: Variations in copper coil resistance affect the current and sensitivity.
- Friction: Mechanical friction at the pivots and jewels.
What are the properties of spring material used in indicating instruments?
Answer
- Non-magnetic: Must not be affected by external magnetic fields.
- Low resistivity: To conduct current if used as a lead.
- High fatigue strength: To maintain calibration over long periods without permanent deformation.
- Low temperature coefficient of elastic modulus: To ensure the restoring torque remains constant despite ambient temperature changes.
‘Phase angle error’ in a Potential transformer is negative in a particular measuring scenario. What does it mean? Explain.
Answer
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Why the secondary of current transformer is never kept open-circuited?
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How can the error caused by the inductance of the pressure coil, compensated in a low power factor Electrodynamometer type wattmeter?
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What are the main components of an Induction type of Energy meter?
Answer
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List some of the problems associated in measurement of low resistance values.
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How is the effect of Thermo-Electric EMFs eliminated when taking the readings with Kelvin’s double bridge?
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Define Transducers.
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Explain principle of operation of LVDT.
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Part B
Derive the torque of a moving iron instrument. Explain briefly the various errors in the moving iron instrument.
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Explain principle, construction and working of PMMC instruments. Derive an expression for the deflection.
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What is the transformation ratio of a Potential Transformer? Explain the design considerations that are to be taken to minimize the errors in Potential transformer.
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Explain the principle and operation of DC Crompton’s potentiometer.
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Why is the measurement of power in circuits with low power factor by ordinary electrodynamometer wattmeters is difficult and inaccurate? What special features are to be incorporated in the above type of wattmeters to make it a ‘low power factor type wattmeter’? Explain.
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Explain the purpose of lag adjustment devices in an Induction type of energy meter. Explain the different ways of achieving it.
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Anderson’s Bridge is used for precise measurement of self Inductance of a Coil. With the help of a circuit and its phasor diagram, derive an expression for and explain how its self inductance and the associated resistance can be measured.
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Describe with the help of diagram, the loss of charge method for determining the insulation resistance of a length of cable.
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The value of a high resistance is measured by loss of charge method. A capacitor having a capacitance of 2.5µF is charged to a potential of 500 V D.C and is discharged through the high resistance. An electrostatic voltmeter, kept across the high resistance, reads the voltage as 300 V at the end of 60 seconds. Calculate the value of high resistance.
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Explain the constructional details and working principle of Piezo electric transducers with a neat sketch.
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Water flows steadily along a horizontal pipe of cross section area of 30 cm2. The static pressure in the pipe is 1.20×105 pa and the total pressure is 1.28×105 pa. Calculate the flow velocity and the mass of water per second flowing through the pipe. take density of water = 1000 kg/m3.
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