Three Phase System Question and Answers

1. Which of the following are necessary conditions for an entire 3-phase system to be balanced?

1. The line voltages are equal in magnitude.
2. The phase differences between successive line voltages are equal.
3. The impedances in each of the phases are identical.

Select the correct answer using the codes given below:
(a) 1, 2 and 3
(b) 1 and 3
(c) 1 and 2
(d) 2 and 3

2. The instantaneous values of currents in phases R and Y of a 3-phase system are 25 A each. For a phase sequence of BRY, the instantaneous values of current in phase B is

(a) 25 A
(b) 50 A
(c) 12.5 A
(d) 43.3 A

3. The phase sequence of the 3-phase system shown in given Figure below is

(a) RYB
(b) RBY
(c) BRY
(d) YBR

4. The rated voltage of a 3-phase power system is given as

(a) RMS phase voltage
(b) Peak phase voltage
(c) RMS line-to-line voltage
(d) Peak line-to-line voltage

5. Total instantaneous power supplied by a 3-phase AC supply to a balanced R-C load as

(a) Zero
(b) Constant
(c) Pulsating with zero average
(d) Pulsating with zero non-average

6. An alternator is delivering power to a balanced load at unity power factor. The phase angle between the line voltage and the line current is

(a) 90^o
(b) 60^o
(c) 30^o
(d) 0^o

7. E_oa, E_ob and E_oc are three phase voltages while E_ab, E_bc and E_ca are three line voltages of a balanced 3-phse system having a-b-c phase sequence. In relation to E_oc, E_bc would

(a) lag by 30^o.
(b) lead by 30^o.
(c) have the same phase.
(d) have no definite phase relationship.

8. In the balanced 3-phase voltage system generated by a star connected alternator, V_YB lags E_R by

(a) 90^o
(b) 120^o
(c) 60^o
(d) 30^o

9. For a 3-phase load balanced condition, each phase has the same value of …….

(a) impedance.
(b) resistance.
(c) power factor.
(d) all of these.

10. For a delta-connected circuit the correct relationship is

(a) V_L = V_ph
(b) I_ph x √3 = I_L
(c) V_L = I_L x √3
(d) Both (a) and (b)

11. A 3-phase delta-connected symmetrical load consumes P watts of power from a balanced supply. If the same load is connected in star to the same supply, then what is the power consumption?

(a) P/3
(b) P
(c) √3P
(d) 3P

12. A 3-phase 3-wire supply feeds a star connected load consisting of 3 equal resistors. If one of the resistors is to be removed, then what is reduction in power as compared to the original power?

(a) 25% of the original power.
(b) 33.3% of the original power.
(c) 50% of the original power.
(d) 66.6% of the original power.

13. Power factor of an inductive circuit is usually improved by adding a capacitor to it in

(a) series.
(b) parallel.
(c) either in series or parallel.

14. To improve the power factor in three-phase circuits, the capacitor bank is connected in delta to make

(a) capacitance calculation easy.
(b) capacitance very small.
(c) the connection elegant.
(d) the power factor correction more effective.

15. A lamp load is given a supply from 3-phase 4-wire, 230/400 V AC supply system. If a three phase motor is now switched on across the same supply, then neutral current will

(a) increase.
(b) decrease.
(c) remain unchanged.
(d) unpredictable.

16. The power measurement in balanced three-phase circuit can be done by

(a) one wattmeter method only.
(b) two wattmeter method only.
(c) three wattmeter method only.
(d) any one of the above.

17. The power delivered to a 3-phase load can be measured by the use of 2 wattmeter only when the

(a) load is balanced.
(b) load is unbalanced.
(c) 3-phase load is connected to the source through 3-wires.
(d) 3-phase load is connected to the source through 4-wires.

18. While measuring power in a 3-phase load by two wattmeter method. The readings of two wattmeters are equal and opposite when

(a) power factor is unity.
(b) load is balanced.
(c) phase angle is between 60^o and 90^o.
(d) the load is pure inductive.

19. In two wattmeter method of power measurement, one of the wattmeter will show negative reading when the load power factor angle is strictly

(a) less than 30^o
(b) less than 60^o
(c) greater than 30^o
(d) greater than 60^o

20. If the readings of two-wattmeters are equal and positive in two wattmeter method, the load power factor in a balanced 3-phase 3-wire circuit will be

(a) zero.
(b) 0.5
(c) 0.866
(d) unity.

21. The power of a three phase, three wire balanced system was measured by two wattmeter method. The reading of one of the wattmeters was found to be double that of the other. What is the power factor of the system?

(a) 1
(b) 0.866
(c) 0.707
(d) 0.5

22. W₁ and W₂ are the readings of two wattmeters used to measure power of a 3-phase balanced load. The reactive power drawn by the load is

(a) W₁ + W₂
(b) W₁ – W₂
(c) √3(W₁ + W₂)
(d) √3(W₁ – W₂)

23. In a 3-phase supply, floating neutral is undesirable because it may result in ……… across the load.

(a) unequal line voltages
(b) high voltages
(c) low voltages

24. Phase reversal in a 4-wire unbalanced load supplied from a balanced 3-phase supply causes change in

(a) the power consumed.
(b) magnitude of phase currents.
(c) only the magnitude of the neutral current.
(d) magnitude as well as phase angle of the neutral current.

25. Three unequal impedances are connected in delta to a 3-phase, three wire system.

(a) the voltages across the three phases will be different.
(b) both the phase currents and line currents will be unbalanced.
(c) phase currents will be unbalanced but the line currents will be balanced.
(d) none of the above.

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