# GTU last year question papers -BE- Sem-VIIth -Electrical and Electronics Measuring Instruments-Nov/Dec-2011

GTU last year question papers

GUJARAT TECHNOLOGICAL UNIVERSITY

BE SEM-VII Examination-Nov/Dec-2011

Subject code: 170803

Subject Name: Electrical and Electronics Measuring Instruments

Instructions:

1. Attempt all questions.

2. Make suitable assumptions wherever necessary.

3. Figures to the right indicate full marks.

Q.1 (a) Describe the various operating torque needed for proper

operation of an analog indicating instrument. Draw the neat

sketch & explain in detail of eddy current damping torque in

analog instrument.       03+04

(b) Explain the construction and working of an electrodynamometer

type of wattmeter.       04+03

Q.2 (a) Derive the general torque equation for a moving iron type

instrument. The inductance of a moving iron ammeter is given

by the following expression.

L = (20+10θ – 2θ2)μH.

Where θ is deflection in radians. The spring constant is

24 x 10-6 Nm/rad. Calculate the values of deflections for a

current of 5A.          04+03

(b) Draw the phasor diagram & derive the expression for deflecting

torque & braking torque in single phase induction type energy meter.        02+05

OR

(b) A moving coil voltmeter with a resistance of 20Ω gives a full

scale deflection of 120˚, when a potential difference of 100mv is

applied across it. The moving coil has dimensions of 30mm ×

25mm and is wound with 100 turns. The control spring constant

is 0.375 × 10-6 Nm/deg. Find the flux density in the air gap. Find

also the diameter of copper wire of coil winding if 30% of

instrument resistance is due to coil winding. The specific

resistance of copper = 1.7× 10-8Ω m.                            03+04

Q.3 (a) Derive expression for unknown resistance in case of kelvin’s

double bridge.            07

(b) State the application of schering bridge. Explain its working

with circuit diagram & phasor diagram. Why earth screen is

required in High voltage schering bridge? 02+02+03

OR

Q.3 (a) Describe with the help of neat diagram the loss of charge

method to determine the insulation resistance of a short length

of cable and derive an expression for determination of insulation

resistance. 03+04

(b) The four arm of a maxwell’s capacitance bridge at balance are :

Arm ab, an unknown inductance L1 having an inherent

resistance R1, Amr bc, a non magnetic resistance of 1000Ω, Arm

03+02+02

cd, a capacitor of 0.5 μF in parallel with a resistance of 1000Ω,

Arm da, a resistance of 1000Ω.

Derive the equations of balance for the bridge and determine the

value of R1 & L1 . Draw the phasor diagram of the bridge under balance conditions.

Q.4 (a) A C.T has a primary & 200 turn secondary winding .The

secondary supplies a current of 5A to a non inductive burden of

1 Ω resistance. The requisite flux is set up in the core by an mmf

of 80A. Frequency is 50Hz & net cross section area of the core is 1000 mm2 ?

Calculate the ratio & phase angle of the C.T. Also find the flux

density in the core. Neglect the effect of magnetic leakage , iron

losses and I2R losses. (Page 398 AKS)                                                            02+02+03

(b) Draw the equivalent circuit & phasor diagram of a potential

transformer. Derive the expressions for its ratio and phase angle errors.                        03+04

OR

Q.4 (a) Explain with the help of neat diagram the working of a digital voltmeter.        02+05

(b) Explain with the help of a block diagram the working of a

spectrum analyzer ? Where are spectrum analyzers commonly used ?         04+03

Q.5 (a) Draw the block diagram of storage oscilloscope and explain the

working of each block.           03+04

(b) What is signal conditioning and why is it required ? Draw a

block diagram of a signal conditioning system and explain the

functions of each block.          02+02+03

OR

Q.5 (a) Explain the construction & principle of working of a linear

voltage differential transformer. Explain how the magnitude and

direction of the displacement of core of an L.V.D.T detected.         02+02+03

(b) Explain the terms :

(i) Limiting error (ii) Accuracy (iii) Precision

(iv) Sensitivity (v) Reproducibility (vi) resolution (vii) relative error   01 X 07=07

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