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

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 (a) Nominal ratio (b) Turns
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)

11. (i) Derive an expression of torque formula for a moving Iron Instrument and comment
on the nature of the scale. (10)
(ii) Meter A has a range of (0–10V) and a multiplier resistance of 18 kO. Meter B has a range
of (0–300V) and a multiplier resistance of 298 kO. Both meter movements have a
resistance of two kO. Which meter has a better accuracy? Why? (6)

12. (a) (i) discuss the errors encountered in electrodynamometer kind instruments. (8)
(ii) A meter having a full scale deflection of one mA and Rm of 300 O is to be used
to measure sinusoidal ac voltage of 0–10V range. calculate the multiplier
resistance that is needed. presume the diode has a forward resistance of 0 O
and a reverse resistance of infinite ohms.(8)
Or
(b) (i) discuss the construction and working principle of an Energy meter for ac circuits.(8)
(ii) An energy meter records 1 unit for every 660 revolutions of the disc. A load of
1000 watt hour is measured using this meter for 12 hours. The disc was obtained rotating at 10.2 revolutions per minute. compute the fault involved in terms of units of energy. (8)

13. (a) (i) define Gall potentiometer with a neat diagram and bring out its salient
features. (8)
(ii) discuss how to calibrate wattmeters using potentiometers. (8)
Or
(b) (i) How do current transformers differ from potential transformers. (4)
(ii) A current transformer with a bar primary has 300 turns in its secondary winding. The
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)

14. (a) (i) Which bridge is used for measuring low resistances? Derive an expression for
finding out the unknown low resistance under balanced condition.(8)
(ii) With neat sketch, discuss the operation of a megger. (8)
Or
(b) (i) discuss the principle of loss of charge method for measurement of high resistance
and derive an expression to obtain out the unknown resistance using this method.(8)
(ii) A 2.5 m F capacitor is charged to a potential of 450 volts. The capacitor is
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)

15. (a) (i) discuss with circuit diagram the bridge circuit that can be used to measure
high voltages. (8)
(ii) Derive the formula of balance of a schering bridge. Draw the phasor diagram under
null condition and discuss how loss angle of capacitor can be computed.(8)
Or
(b) In an ac bridge the arrangement is as follows :
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 bridge. (16)
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Earning:   Approval pending.