Anna University Electrical Measurements and Instruments Question Paper



IV Semester E & I Engineering


(Common to Instrumentation and Control Engineering)

Time : Three hours                                                                        Maximum : 100 marks

Answer ALL questions.

PART A — (10 ´ 2 = 20 marks)

1. How are smaller currents measured with larger range ammeters?

2. What are the advantages of dynamometer type instruments?

3. What is meant by kVA rating and kVhr rating?

4. How is the creep effect in energy meters avoided?

5. Justify the application of potentiometers in instrumentation.

6. Comment on the effect of change of input frequency on the characteristics of a current transformer.

7. Why are Kelvin’s double bridges used for measuring low resistance?

8. What is the need for calibration of voltmeters?

9. Give the general equation for balance of a.c. bridges.

10. State the principle of Q meters.

PART B — (5 ´ 16 = 80 marks)

11. 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)

12. (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)

13. (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)

14. (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)

15. (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)

Anna University Electrical Measurements and Instruments Question Paper            


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