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Three-Phase Circuit - Multiple Choice Questions

Three-Phase Circuit - Multiple Choice Questions Points : three-phase circuit, multiple choice questions and answers, electrical objective type question answers, mcqs 1. For the similar rating size of a 3-phase motor will be single phase motor.
(i) less than that of
(ii) more than that of
(iii) same as that of
(iv) none of the above

2. In a six-phase generator, electrical displacement among different phases or windings is electrical degrees.
(i) 60
(ii) 90
(iii) 120
(iv) 45

3. The resistance among any two terminals of a balanced star-connected load is 12Ω. The resistance of every phase is ……….
(i) 12 Ω
(ii)24 Ω
(iii) 6 Ω
(iv) none of the above

4. In a balanced star-connected system line voltages are ahead of their respective phase voltages.
(i) 30°
(ii) 60°
(iii) 120°
(iv) none of the above

5. To convey similar amount of power over a fixed distance at a given voltage, 3-phase system requires weight of copper.
(i) 3 times
(ii) 3/4th times
(iii) 15 times
(iv) 05 times

6. In a two phase generator, electrical displacement among the two phases or windings is ……. electrical degrees.
(i) 120
(ii) 90
(iii) 180
(iv) none of the above

7. The phase sequence of a three-phase system is RYB. The other potential phase sequence can be
(i) BRY
(ii) YRB
(iii) RBY
(iv) none of the above

8. The torque on rotor of a 3-phase motor is more stable than that of a single motor as
(i) single phase motors are not self-starting
(ii) single phase motors are small in size
(iii) 3-phase power is of constant value
(iv) none of the above

9. The benefit of star-connected supply system is that
(i) line current is equal to phase current
(ii) two voltages can be used
(iii) phase sequence can be easily changed
(iv) it is a simple arrangement

10. Three delta-connected resistors absorb 60 kW while connected to a 3-phase line. If resistors are linked in star, the power absorbed is ……….
(i) 60 kW
(ii) 20 kW
(iii) 40 kW
(iv) 180 kW

11. A 3-phase load is balanced if all the three phases contain the same
(i) impedance
(ii) power factor
(iii) impedance and power factor
(iv) none of the above

12. In a 3-phase system, if instantaneous values of phase R and Y are + 60V and -40 V in that order, then instant voltage of phase B is
(i) -20 V
(ii) 40 V
(iii) 120 V
(iv) none of the above

13. If a balanced delta load has an impedance of (6 +j9) ohms per phase, then, impedance of every phase of equivalent star load is ………………
(i) (6+j9) ohm
(ii) (2+j3) ohm
(iii) (12 +j 18) ohm
(iv) (3 +j 45) ohm

14. Three identical resistances linked in star carry a line current of 12 A. If the similar resistances are connected in delta across the similar supply, line current will be ……….
(i) 12 A
(ii) 4 A
(iii) 8 A
(iv) 36 A

15. If one line conductor of a 3-phase line is cut, the load is then supplied through …….. voltage.
(i) single phase
(ii) two phase
(iii) three phase
(iv) none of the above

16. The algebraic sum of instantaneous phase voltages in a three-phase circuit is equivalent to
(i) zero
(ii) line voltage
(iii) phase voltage
(iv) none of the above

17. A wattmeter measures .......... power.
(i) instantaneous
(ii) apparent
(iii) reactive
(iv) average

18. Three identical resistances, every of 15 Ω, are linked in delta across 400 V, 3-phase supply. The value of resistance in every leg of equivalent star-connected load would be ……..
(i) 15 Ω
(ii) 7.5 Ω
(iii) 5 Ω
(iv) 30 Ω

19. The mainly tricky unbalanced 3-phase load to deal by is ...........
(i) 4-wire star connected unbalanced load
(ii) unbalanced s-connected load
(iii) unbalanced 3-wire, Y..connected load
(iv) none of the above

20. Three-wattmeter method is not use to calculate power in a 3-phase circuit as
(i) it is complicated
(ii) generally neutral is not available or delta load cannot be opened
(iii) it requires three wattmeters
(iv) none of the above

21. The resistance among any two terminals of a balanced delta-connected load is 12 Q. The resistance of every phase is …………
(i) 12 Ω
(ii) 18 Ω
(iii) 6 Ω
(iv) 36 Ω

22. Three identical resistances linked in star use 4000 W. If resistances are linked in delta crossways the same supply, power consumed will be ………..
(i) 4000 V
(ii) 6000 W
(iii) 8000W
(iv) 12000W

23. In a single phase circuit, instantaneous power varies sinusoidally as of zero to a peak value at
(i) the line frequency
(ii) twice the line frequency
(iii) thrice the line frequency
(iv) none of the above

24. At 80°, phase R is - 138 V, and phase Y is + 154 V. The voltage of phase B is
(i) 64 V
(ii) -32 V
(iii) 16 V
(iv) - 16 V

25. For mainly efficient use of power supply equipment, the power factor should be
(i) 1
(ii) 0.707
(iii) 0.62
(iv) 0.85

26. In a three phase circuit, true power
(i) cannot exceed apparent power
(ii) can exceed apparent power
(iii) data insufficient
(iv) none of the above

27, When 3 phase system is impartial, the neutral wire carries
(i) no current
(ii) one-third of current for each phase
(iii) half of current for each phase
(iv) none of above

28. Three identical capacitances, each of 450 μF, are linked in star. The rate of capacitance in each phase of the equal delta-connected load would be ……….
(i) 150 μF
(ii) 450 μF
(iii) 225 μF
(iv) 900 μF

29. In a three-phase circuit, total instantaneous power is equivalent to instantaneous power per phase multiplied by
(i) 3
(ii) 6
(iii) 25
(iv) 15

30. In a 3-phase circuit, the load p.f. is 1. The relation among apparent power (S) and true power (P) is
(i) P=2S
(ii) S=2P
(iii) P = S
(iv) none of the above

31. In a balanced three-phase system, phase voltage is 200 (i) 0.707 lagging
(ii) 0.64 lagging
(iii) 0.85 leading
(iv) 0.5 lagging

32. If wattmeter is usually phased and it indicates reverse reading, current and voltage are
(i) more than 90°outofphase
(ii) less than 90° out of phase
(iii) insufficient data
(iv) none of the above

33. In two-wattmeter method of 3-phase power measurement, load is resistive. The readings of the wattmeters are W1 and W2. Then,
(i) W1 may be greater than W2
(ii) W1 may be less than W2
(iii) W1 = W2
(iv) none of the above

34. In order to calculate power in a 3-phase, 4-wire unbalanced Load, the smallest number of wattmeter required would be ………
(i) 1
(ii) 2
(iii) 4
(iv) 3

35. When the p.f. is better, lagging reactive power supplied to the load
(i) decreases
(ii) increases
(iii) remains unchanged
(iv) none of the above

36. A wattmeter uses
(i) D’Arsonval movement
(ii) electro-dynamometer movement
(iii) hot-wire principle
(iv) none of the above

37. In delta-connected load, every impedance is 15 (i) 0.94
(ii) 0.707
(iii) 0.766
(iv) 1

38. In a single circuit, a wattmeter indicates 200 W. Circuit current is 3 A and the circuit voltage is 115 V. What is the reactive power?
(i) 546 VAR
(ii) 281 VAR
(iii) 342 VAR
(iv) 153 VAR

39. In two wattmeter method, algebraic sum of the readings of two wattmeters will signify true power only if ……………
(i) the load is balanced
(ii) phase sequence remains unchanged
(iii) there is no source unbalance
(iv) neutral wire available does not carry any current

40. Two wattmeter method use to calculate power in a 3-phase system. When both meters are normally phased, the meter readings are 2400 W and 1260 W. That is the load power?
(i) 1140W
(ii) 3660W
(iii) 1830 W
(iv) none of the above

41. A low lagging power factor results in
(i) good voltage regulation
(ii) poor voltage regulation
(iii) insufficient data
(iv) none of the above

42. An a.c. voltmeter linked in series by the coils of a properly phased delta-connected source must indicate
(i) 0 V
(ii) phase voltage
(iii) line voltage
(iv) none of the above

43. The line current in a balanced delta system is 30 A. What is magnitude of load current?
(i) 10 A
(ii) 20 A
(iii) 30 A
(iv) 17.32 A

44. The measured phase power in a circuit is 500 W. The phase current is 5 A and phase voltage is 120 V. The reactive power is
(i) 217.5 VAR
(ii) 152.4 VAR
(iii) 193.6 VAR
(iv) 331.7 VAR

45. A lagging power factor can be better through connecting
(i) an inductor in parallel with load
(ii) a capacitor in parallel with load
(iii) a resistor in parallel with load
(iv) none of the above

46. A 30 MVA. 11 kV 3-phase alternator supplies full-load at a Lagging power factor of 0.6. The fraction increase in earning ability as the power factor is raised to 0.9 becomes.
(i) 25%
(ii) 30%
(iii) 60 %
(iv) 50 %

47. While a capacitor is joined across a single phase motor, active power drawn through the motor
(i) decreases
(ii) increases
(iii) remains same
(iv) data insufficient

48. In a three-phase circuit, if load p.f. is decrease, line current
(i) decreases
(ii) increases
(iii) remains the same
(iv) none of the above

49. What is phase relationship among line currents in a delta system?
(i) 300 between each line current
(ii) 600 between each line current
(iii) 120° between each line current
(iv) 450 between each line current

50. All a.c. equipement is rate in kVA because
(i) kVA>kW
(ii) kVA (iii) of temperature rise limitation
(iv) none of the above

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