# 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.