**MODEL PAPER**

**B.E. DEGREE EXAMINATION.**

**Fourth Semester**

**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)

Or

** **(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)

Or

** **(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)

Or

** **(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

inductance.

** ** (ii) Derive the balance condition of a Schering bridge. (8 + 8)

Or

** **(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)

** **

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