Anna University Model Question Paper BE IV sem E&I ELECTRICAL MEASUREMENTS AND INSTRUMENTS
B.E. DEGREE EXAMINATION.
Electronics and Instrumentation Engineering
EI 237 — ELECTRICAL MEASUREMENTS AND INSTRUMENTS
(Common to Instrumentation and Control Engineering)
Time : Three hours Maximum : 100 marks
Answer ALL questions.
PART A — (10 ´ 2 = 20 marks)
- How are smaller currents measured with larger range ammeters?
- What are the advantages of dynamometer type instruments?
- What is meant by kVA rating and kVhr rating?
- How is the creep effect in energy meters avoided?
- Justify the application of potentiometers in instrumentation.
- Comment on the effect of change of input frequency on the characteristics of a current transformer.
- Why are Kelvin’s double bridges used for measuring low resistance?
- What is the need for calibration of voltmeters?
- Give the general equation for balance of a.c. bridges.
- State the principle of Q meters.
PART B — (5 ´ 16 = 80 marks)
- Give the construction and principle of operation of a moving coil ammeter. List their advantages and disadvantages. Explain how measuring range is increased in ammeters. (16)
- (a) Explain 2–wattmeter method of measuring 3–phase power with a neat circuit and give the vector diagram. (16)
(b) (i) Explain briefly on the full load and low load adjustments of energy meters.
(ii) With a neat figure explain the operation of a single–phase energy meter. (8 + 8)
- (a) Explain how potentiometers are used to calibrate ammeters and
resistances. Give the principle of operation of a typical a.c. potentiometer
with a neat figure. (16)
(b) (i) Distinguish between operating conditions of a current transformer and potential transformer with regard to materials used, windings, operation and application.
(ii) A PT rated 33 kV/220 V and a CT of 100/5 A are used to measure the voltage and current in a transmission line. If the voltmeter shows 200 V and the ammeter shows 4 A, determine the voltage and current in the line. (12 + 4)
- (a) Describe Price’s guard wire method of finding a high resistance and state the precautions. (16)
(b) (i) A 4–terminal resistance of approximately 50 mW is measured by using a Kelvin double bridge with the component resistances as given below :
Standard resistor = 100.03 mW
Inner ratio arms = 100.31 W and 200 W
Outer ratio arms = 100.24 W and 200 W
Value of low resistance link = 700 mW
Find the unknown resistance to the nearest of 0.01 W.
(ii) Give the constructional details and state the working of a Megger.
(4 + 12)
- (a) (i) Explain the Maxwell’s bridge method for finding an unknown
(ii) Derive the balance condition of a Schering bridge. (8 + 8)
(b) Write briefly on any TWO of the following :
(i) Compensation methods in watt meters
(ii) Bridge configuration to measure mutual inductance
(iii) Measurement of high resistance by loss of charge method. (8 + 8)