Anna University Electrical Instruments and Measurements Question Paper
IV Semester Instrumentation Engineering
Electrical Instruments and Measurements
Time : Three hours Maximum : 100 marks
Answer ALL questions.
PART A — (10 ´ 2 = 20 marks)
1. What is the purpose of controlling torque in indicating instruments?
2. The inductance of moving Iron instrument is given by , 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. What are 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. Define the following 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 type of Bridge is used for the measurement of frequency?
10. What are the different sources of errors in a.c. measurements?
PART B — (5 ´ 16 = 80 marks)
11. (i) Derive an expression of torque equation 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 k?. Meter B has a range of (0–300V) and a multiplier resistance of 298 k?. Both meter movements have a resistance of 2 k?. Which meter has a better accuracy? Why? (6)
12. (a) (i) Explain the errors encountered in electrodynamometer type instruments. (8)
(ii) A meter having a full scale deflection of 1 mA and Rm of 300 ? is to be used to measure sinusoidal ac voltage of 0–10V range. Compute the multiplier resistance that is required. Assume the diode has a forward resistance of 0 ? and a reverse resistance of infinite ohms. (8)
(b) (i) Explain the construction and working principle of an Energy meter for ac circuits. (8)
(ii) An energy meter records one 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 found rotating at 10.2 revolutions per minute. Calculate the error involved in terms of units of energy. (8)
13. (a) (i) Describe Gall potentiometer with a neat diagram and bring out its salient features. (8)
(ii) Explain how to calibrate wattmeters using potentiometers. (8)
(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 ? and 1.0 ? respectively. With 5 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 error. (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, explain the operation of a megger. (8)
(b) (i) Explain the principle of loss of charge method for measurement of high resistance and derive an expression to find out the unknown resistance using this method. (8)
(ii) A 2.5 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) Explain with circuit diagram the bridge circuit that can be used to measure high voltages. (8)
(ii) Derive the equation of balance of a schering bridge. Draw the phasor diagram under null condition and explain how loss angle of capacitor can be calculated. (8)
(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 ? CD and DA are non–reactive resistors of 200 ? each. CE is a loss–free capacitor 1 . DE is a non-reactive resistance of 500 ?.
Under balance condition, a supply of 10 V at 50 Hz conneced across AC and a vibration galvanometer between B and E. Calculate the value of ‘R’ and ‘L’.
Draw the phasor diagram for the balanced bridge. (16)