ELECTRICAL MEASUREMENTS JNTU previous years question papers

ELECTRICAL MEASUREMENTS JNTU previous years question papers

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

1. (a) Explain the working of an quadrant electrometer with a neat diagram.
(b) The spring constant of a 3000 V electrostatic voltmeter is 7.06×10-6Nm/rad. The full scale deflection of the instrument is 800. Assuming the rate of change of capacitance with angular deflection to be constant over the operating range,calculate the total change of capacitance from zero to full scale. [10+6]

2. (a) Explain the disadvantages of shunts and multipliers when used for extension of range. Explain how instrument transformers are a better substitute for shunts and multipliers especially for high range values.
(b) Define the following ratios of instrument transformers. [8+8]
i. Transformation ratio
ii. Nominal ratio
iii. Burden
iv. Ratio correction factor.

3. (a) Explain the construction and principle of operation of a dynamometer type Wattmeter. How it can be made to read d.c as well as A.C?
(b) In a dynamometer type wattmeter the moving coil has 500 turns of mean diameter 3cm. Calculate the torqe if the axis of the field and moving coils are at
i. 30o
ii. 60o
iii. 90o
when the flux density in the field coils is 15mWb/m2, the current in the moving coil is 0.5A and power being measured has a power factor of 0.866. [8+8]

4. (a) Draw a neat sketch showing the construction of a single phase induction type energymeter. Give the theory & operation of the instrument.
(b) An Energymeter is designed to make 200 revolutions of disc for one unit ofenergy. Calculate the number of revolutions made by it when connected to load carrying 80 A at 230v and 0.6 powerfactor for an hour. If it actually makes 500 revolutions, find the percentage error. [10+6]

5. (a) Explain standardizing of A.C. potentiometers and use of transfer instruments in case of a.c. potentiometer.(b) Measurements for the determination of the impedance coil are made on a coordinate type of potentiometer. The results are: Voltage across 1 standard resistance in series with the coil + 0.952V on in phase dial and - 0.340 V on quadrate 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 quadrate dial. Calculate the resistance and reactance of the coil. [8+8]

6. (a) Derive the expression for bridge sensitivity for a Wheatstone bridge with equal arms. Find also the expression for current through the galvanometer for a small unbalance.
(b) The four arms of a wheatstone bridge are as follows: AB=100 BC=1000 CD=4000 and DA=400. The galvanometer has a resistance of 100 a sensitivity of 100 mm/µA and is connected across AC. A source of 4V d.c. is connected across BD. Calculate the current through the galvanometer and its deflection if the resistance of arm DA is changed from 400  to 401
.[10+6]

7. (a) Draw the circuit diagram of Hay’s bridge. Also, draw the pahsor diagram under balance condtions.
(b) Define Quality factor of a coil? Why is Hay’s bridge suited for measurement of inductance of high Q coils?
(c) State the advantages and dis-advantages of Hay’s bridge? [6+5+5]

8. Explain the constructional features of flux meter with a neat diagram. Show that for a flux meter
Nf = K (?2 - ?1)
Where N is the number of turns in the search coil used, f is the change in flux, ?1 is the initial reading in the flux meter and ?2 is the final reading in the flux meter and K is a constant.

FEEL USEFUL PLEASE GIVE +1