# GTU previous question papers -BE- Sem-III -Electrical & Electronics Measuring Instruments -March 2010

GTU previous question papers

GUJARAT TECHNOLOGICAL UNIVERSITY

B.E. Sem-III Remedial Examination March 2010

Subject code: 130903

Subject  Name: Electrical & Electronics Measuring Instruments

Instructions:

1. Attempt all questions.

2. Make suitable assumptions wherever necessary.

3. Figures to the right indicate full marks.

Q.1 (a) Derive an expression for the resistance required in galvanometer circuit to make it dead beat, assuming the damping to be entirely electromagnetic.

(b) Explain the special features incorporated in an electrodynamometer type wattmeter so that it can be used for low power factor applications.

Q.2 (a) With a suitable circuit diagram explain the construction and write steps that are used for measurement of unknown emf by Crompton’s potentiometer.

(b) 1. Explain the construction of a ballistic galvanometer. How can a ballistic galvanometer be calibrated using a mutual inductance?

2. Measurements for the determination of the impedance of a coil are made on a coordinate type of potentiometer. The results are : Voltage across 1 Ω standard resistance in series with the coil + 0.952 V on inphase dial and 0.340 V on quadrature dial.Voltage across 10:1 potential divider connected to the terminals of the coil: + 1.35 V on inphase dial and + 1.28 V on quadrature dial.

Calculate the resistance and reactance of the coil.

OR

(b) Explain the construction and operation of a Drysdale phase shifting transformer. Obtain the necessary expression for the correct magnitudes and the phase relationship in terms of circuit parameters.

Q.3 (a) Describe the construction and working of a PMMC instrument. Mention the method of damping used in this instrument.

(b) 1. A moving coil instrument whose resistance is 25 Ω gives a full scale deflection with a current of 1 mA. This instrument is to be used with a manganin shunt to extend its range to 100 mA. Calculate the error caused by a 100 C rise in temperature when :

(i) Copper moving coil is connected directly across the manganin shunt.

(ii) A 200 Ω manganin resistance is used in series with the instrument moving

coil.

The temperature co-efficient of copper is 0.004/ 0 C and that of manganin

0.00015/ 0 C.

2. Explain the method for measurement of reactive power in three phase circuit using a single electrodynamometer type wattmeter.

OR

Q.3 (a) 1. Derive a general equation for deflection for a spring controlled Electrostatic instrument.

2. The current coil of a wattmeter is connected in series with an ammeter and an Inductive load. A voltmeter and voltage coil are connected across a 100 Hz supply. The ammeter reading is 4.5 A and the voltmeter and wattmeter readings are respectively 240 V and 23 W .The inductance of voltage circuit is 10 mH and its resistance 2000 Ω ,if the voltage drop across the ammeter and the current coil are negligible, what is the percentage error in the wattmeter reading?

(b) Explain the circuit diagram and operation of an Electronic Voltmeter using a differential amplifier. Derive its equivalent circuit and find the expression for the current flowing through the meter.

Q.4 (a) What are the various types of errors encountered in electrical measurements? Explain any two in detail.

(b) Explain the constructional details and working of a three phase electrodynamometer type power factor meter .Prove that the displacement of moving system is equal to the phase angle of the system

OR

Q.4 (a) Describe the construction and characteristics of an unsaturated Weston cells used as an emf standard. What are the precautions taken while using this cell?

(b) What is synchronizing? Describe the construction and working of a Weston type synchroscope.

Q.5 (a) Using expression for torque in single phase induction type meters, Show that the total no of revolutions made by its disc during a particular time is proportional to the energy consumed.

(b) A correctly adjusted, 1 O 230 V, induction watt hour meter has a meter constant of 600 revolutions per kwh, determine the speed of the disc for a current of 10 A at a power factor of 0.8 lagging. If the lag Adjustment is altered so that phase angle between voltage flux and applied voltage is 870.calculate the error introduced at (i) unity p.f.(ii) 0.5 p.f. lagging. (iii) 0.866 pf lagging. Comment upon the result.

OR

Q.5 (a) Explain the various factors which are taken in to consideration while selecting an electronic type analog voltmeter

(b) Give reason of the following statements.

1. Why does the rotating disc of an induction type energy meter carry a small hole?

2. Why should the Pressure coil of an induction type energy meter be highly inductive?

3. In case of energy meters, aluminum disc is preferred over copper disc even though copper is better conductor.