JNTU
JNTU B.Tech III ELECTRONIC MEASUREMENT AND INSTRUMENTATION Previous Question Paper (March - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the principle construction and working of PMMC type meters with neat sketch and derive the expressions for its deflecting torque and controlling torque.
Answer
A Permanent Magnet Moving Coil (PMMC) instrument operates on the principle of electromagnetic torque produced by the interaction of a current-carrying coil placed in a constant magnetic field. It consists of a lightweight rectangular coil wound on a metallic frame, suspended between the poles of a permanent magnet. When current flows through the coil, a force is exerted on the sides of the coil, creating a deflecting torque that causes the pointer to move across a calibrated scale.
The deflecting torque (Td) is given by:
where N is the number of turns, B is the flux density, I is the current, l is the length, d is the width of the coil, and G = NBld is the displacement constant.
The controlling torque (Tc) is provided by spiral springs, which also serve as current leads to the coil. It is proportional to the angular deflection (θ):
where k is the spring constant. At steady state, Td = Tc, leading to the deflection equation:
This shows that the deflection is directly proportional to the current, resulting in a linear scale.
A 500 Volts voltmeter is accurate within +/- 1% at full scale. Calculate the limiting error when the instrument is used to measure a voltage of 200 Volts.
Answer
Given the full-scale reading (Afs) = 500 V and the percentage limiting error at full scale = 1%. The absolute limiting error (δA) is:
When measuring a voltage (Am) of 200 V, the relative limiting error (εr) is calculated as:
The limiting error when measuring 200 V is 2.5%.
Explain various methods with neat diagrams for the extension of ranges of Volt meters and Ammeters using series and shunt resistors.
Answer
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Describe the principle and working of attractive disc type Electrostatic Volt meters with a neat sketch.
Answer
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Explain the construction and working of a Potential Transformer with neat sketch and derive the expressions for its Ratio and Phase angle errors.
Answer
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Explain the working of a polar type A.C. Potentiometer with a neat sketch.
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Describe the working of a three element dynamo-meter type watt meter with a sketch and derive the expressions for its deflecting and controlling torques.
Answer
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Explain the principle and working of single phase induction type energy meter with a neat sketch and derive the expressions for braking and driving torques.
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Discuss the principle and working of Kelvin’s double bridge for measurement of low resistance with a sketch and mention the necessary conditions for balancing the bridge.
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Explain the working of Schering’s bridge for measurement of unknown capacitance and loss angle with a neat diagram and derive the expressions.
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Describe the classification, selection and characteristics of transducers with specific examples.
Answer
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Describe the methods for measurement of force and torque using strain gauge transducers with neat sketches.
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