**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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