# Anna University Coimbatore 2007 B.E Electronics and Instrumentation Engineering Electrical instruments and measurements - Question Paper

Wednesday, 16 January 2013 01:10Web

MODEL PAPER

B.

**E.**DEGREE exam.

4th Semester

Instrumentation Engineering

IT 238 — ELECTRICAL INSTRUMENTS AND MEASUREMENTS

**Time :**3 hours

**Maximum :**100 marks

ans ALL ques..

**PART A**— (10 ´ two = 20 marks)

**1.**What is the purpose of controlling torque in indicating instruments?

**2.**The inductance of moving Iron instrument is provided by L = (10 + 5Q - Q2 )m H , where Q is

the deflection in radians from zero position. The spring constant is 12 ´ 10–6 N–m/radian.

Estimate the deflection for a current of 5A.

**3.**elaborate the likely errors in an Energy meter?

**4.**What is the special feature of wattmeter that is suitable for low power factor circuits?

**5.**How potentiometers are standardised?

**6.**describe the subsequent terms related to Instrument transformers

**(**Nominal ratio

**a)****(**Turns

**b)**ratio.

**7.**When is contact and head resistances are important?

**8.**Why the voltmeter–Ammeter method is unsuitable for the measurement of low–resistance?

**9.**What kind of Bridge is used for the measurement of frequency?

**10.**elaborate the various sources of errors in a.

**c.**measurements?

**PART B**— (5 ´ 16 = 80 marks)

**1**

**1.****(**Derive an expression of torque formula for a moving Iron Instrument and comment

**i)**on the nature of the scal

**e.**(10)

**(**Meter A has a range of (0–10

**i****i)****V)**and a multiplier resistance of 18 kO. Meter B has a range

of (0–300

**V)**and a multiplier resistance of 298 kO. Both meter movements have a

resistance of two kO. Which meter has a better accuracy? Why? (6)

**1**

**2.****(**

**a)****(**discuss the errors encountered in electrodynamometer kind instruments. (8)

**i)****(**A meter having a full scale deflection of one mA and Rm of 300 O is to be used

**i****i)**to measure sinusoidal ac voltage of 0–10V rang

**e.**calculate the multiplier

resistance that is neede

**d.**presume the diode has a forward resistance of 0 O

and a reverse resistance of infinite ohms.(8)

Or

**(**

**b)****(**discuss the construction and working principle of an Energy meter for ac circuits.(8)

**i)****(**An energy meter records 1 unit for every 660 revolutions of the disc. A load of

**i****i)**1000 watt hour is measured using this meter for 12 hours. The disc was obtained rotating at 10.2 revolutions per minut

**e.**compute the fault involved in terms of units of energy. (8)

**1**

**3.****(**

**a)****(**define Gall potentiometer with a neat diagram and bring out its salient

**i)**features. (8)

**(**discuss how to calibrate wattmeters using potentiometers. (8)

**i****i)**Or

**(**

**b)****(**How do current transformers differ from potential transformers. (4)

**i)****(**A current transformer with a bar primary has 300 turns in its secondary winding. The

**i****i)**resistance and reactance of the secondary circuit are 1.5 O and 1.0 O respectively.

With five A flowing in the secondary winding, the magnetising mmf is 100 amperes

and Iron loss is 1.2 Watts. Determine the ratio and phase angle fault. (12)

**1**

**4.****(**

**a)****(**Which bridge is used for measuring low resistances? Derive an expression for

**i)**finding out the unknown low resistance under balanced condition.(8)

**(**With neat sketch, discuss the operation of a megger. (8)

**i****i)**Or

**(**

**b)****(**discuss the principle of loss of charge method for measurement of high resistance

**i)**and derive an expression to obtain out the unknown resistance using this method.(8)

**(**A 2.5 m F capacitor is charged to a potential of 450 volts. The capacitor is

**i****i)**disconnected from the supply and the potential across the capacitor is

observed using an electrostatic voltmeter. After 15.2 minutes the voltage has

fallen to 280

**V.**This test is repeated using a resistance 'R' in parallel with the

capacitor. But now it took 10.8 minutes for the potential to fall from 450 V to

280

**V.**Determine the value of 'R'.(8)

**1**

**5.****(**

**a)****(**discuss with circuit diagram the bridge circuit that can be used to measure

**i)**high voltages. (8)

**(**Derive the formula of balance of a schering bridg

**i****i)****e.**Draw the phasor diagram under

null condition and discuss how loss angle of capacitor can be computed.(8)

Or

**(**In an ac bridge the arrangement is as follows :

**b)**AB is an inductive coil of Inductance 'L' and effective resistance 'R'. BC is a non–reactive

resistor of 100 O CD and DA are non–reactive resistors of 200 O every. CE is a

loss–free capacitor one m F . DE is a non-reactive resistance of 500 O.

Under balance condition, a supply of 10 V at 50 Hz conneced across AC and a vibration

galvanometer ranging from B and

**E.**compute the value of 'R' and 'L'.Draw the phasor diagram for the balanced bridg

**e.**(16)

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Earning: Approval pending. |