13-10-2012, 12:47 PM
ELECTRICAL MEASUREMENTS III B.Tech I Semester (R07) Supplementary Examinations
ELECTRICAL MEASUREMENTS.pdf (Size: 59.67 KB / Downloads: 40)
Answer any FIVE questions
All questions carry equal marks
1.Explain the construction and working of an attracted disc type Kelvin absolute electrometer?
2.(a) Mention the causes of errors in the current transformers
(b)A 100/5, 50Hz current transformer has a bar primary and a rated secondary burden of 12.5VA. The secondary winding has 196 turns and a leakage inductance of 0.96mH.With a purely resistive burden at rated full load, the magnetization mmf is 16A and the loss excitation requires 12A.Find the ratio and phase angle errors.
3. (a) Show with the help of a neat diagram how you would extend the range of a wattmeter.
(b) For a certain load, one of the wattmeter reads 20kW and the other 5kW after the voltage of this transformer has been reversed. Calculate the power and power factor of the load.
4.Derive the expression for deflecting torque in a 1- Φ induction type energy meter. Show that the deflection is maximum when the phase angle between two fluxes is 900and when the disc is purely non-inductive.
5. (a) Draw and explain the circuit diagram of a basic slide wire D.C potentiometer.
(b) During the measurement of low resistance using a potentiometer, the following readings were obtained.
Voltage drop across the low resistance under test = 0.45 V
Voltage drop across the 0.1ohm standard resistance = 1.15 V
Calculate the value of unknown resistance, current and the power lost in it.
6. (a) Explain how insulation resistance of a cable can be measured with a help of loss of charge method?
(b) The following results were obtained by loss of charge method of testing cable:
Discharged immediately after charging the deflection = 200 divisions;
Discharged 30seconds after charging the deflection = 125 divisions;
Discharged 30seconds after charging, when in parallel with a resistance of 10MΩ, the deflection = 100 divisions. Calculate the insulation resistance of the cable.
7. Draw the circuit diagram of Maxwell’s Inductance bridge. Also, draw the phasor diagram under the balanced condition. Write the advantages and disadvantages of the same bridge.
8. Explain the methods of measurement of core losses