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THEORETICAL QUESTIONS

1.      State the advantages of AC polyphase supply system over single-phase system.

2.      Why is the number of phases in a polyphase system always three rather than any other number?

3.      Explain clearly what is meant by ‘phase sequence’ of 3-phase voltages.

4.      What are the two systems in which three-phases can be connected? What are the advantages and disadvantages of each system?

5.      What are the advantages of inter-connecting the 3-phases?

6.      Derive an expression for power in a 3-phase star-connected system in terms of (i) phase values and (ii) line values of voltages and currents.

7.      Drive the numerical relationship between line and phase currents for a balanced 3-phase delta-connected load.

8.      Deduce an expression for power in a 3-phase balanced load circuit. Show that it is the same irrespective of the load being connected in star or delta.

9.      Compare the star and delta connections in a 3-phase system.

10. Enumerate various methods for 3-phase power measurement, and describe in detail two wattmeters method for 3-phase power measurement.

11. Prove that the power in a balanced 3-phase circuit can be deduced from the readings of two wattmeters. Draw the relevant connection diagram and vector diagram.

12. Discuss the two-wattmeters method for power measurement in 3-phase system and obtain a relation for the power factor.

13. Derive an expression for star-connected impedances equivalent to three delta-connected impedances.

14. What does the following signify in case of measurement of balanced 3-phase power by two-wattmeters method:

i) When the reading of one wattmeter is zero.

ii) When the readings of two wattmeters are equal.

iii) When the readings of two wattmeters are equal and opposite.

In each case give reasons to your answer.

15. Explain what will be the consequences of disconnecting the neutral in a, 3-phase, 4-wire system.

16. What are the disadvantages of having a low lagging power factor in a system.

17. Enumerate the various equipments which can be employed to improve the power factor.