AC CIRCUITS ( THREE PHASE)

Question 1

1. Theree wattmeter method is not used to measure power in a 3 phase circuit because…..

A. It is complicated
B. Generally neutral is not available or delta load cannot be opened
C. It requires three watt meters
D. None of the above

Answer 1

B. Generally neutral is not available or delta load cannot be opened

Question 2

2. The most difficult unbalanced 3 phased load to deal with is ….

A. 4 wire star connected unbalanced load
B. Unbalanced delta - connected load
C. Unbalanced 3 wire, Y- connected load
D. None of the above

Answer 2

C. Unbalanced 3 wire, Y- connected load

Question 3

3. The following are positive phase sequence except
   A. ABC
   B. CAB
   C. BCA
   D. CBA

Answer 3

D. CBA

Question 4

4. In a balanced three phase star connected circuit the line voltages are equal.
   A. To the line current
   B. To the phase voltage
   C. And so are the line currents
   D. But the line currenta are unequal

Answer 4

C. And so are the line currents

Question 5

5. The phase displacement between phasor in polyphase system is always?
   A. 90 degrees
   B. 120 degrees
   C. 360 degrees divided by the number of phases
   D. None of the above

Answer 5

C. 360 degrees divided by the number of phases

Question 6

6. In a star connected system line current is?
   A. 0.707 times the phase current
   B. 1.735 times the phase current
   C. Equal to the phse current
   D. 1.414 times the phase current

Answer 6

C. Equal to the phse current

Question 7

7. Power in a three phase delta system with balance load is equal to?
   A. √3×VL×IL×pf
   B. √3×Vph×Iph×pf
   C. 3×Vph×IL×pf
   D.3×VL×IL×pf

Answer 7

A. √3×VL×IL×pf

Question 8

8. In a delta connected system the libe current is?
   A 1.414 times the phase current
   B. Phasor sum of the two phase currents
   C. Equal to the phase current
   D. 1.732 times the phase current

Answer 8

D. 1.732 times the phase current

Question 9

9. Power in star connected system is?
   A. Equal to that of delta system
   B. √2 times the delta system
   C. √3 times the delta system
   D. 3 times of a delta system

Answer 9

A. Equal to that of delta system

Question 10

10. What is the minimum number of wattmeter required for measuring power of a three phase balanced load.
    A. Two
    B. Four
    C. One
    D. Three

Answer 10

C. One

Question 11

11. What is the minimum number of wattmeters required to measure unbalanced power for a thre phase system?
    A. Two
    B. Four
    C. Theee
    D. One

Answer 11

A. Two

Question 12

12. In two wattmeter method, the reading of the wattmeter will be identical when?
    A. Load in one of the phases is zero
    B. Power factor is unity
    C. Power factor is 0.5
    D. Neutral is carthed

Answer 12

B. Power factor is unity

Question 13

13. Power in a three phase star system is equal to?
    A. √3×VL×IL×pf
    B. √3×Vph×IL×pf
    C. √3×VL×Iph×pf
    D. 3×Vph×Iph×pf

Answer 13

D. 3×Vph×Iph×pf

Question 14

14. In 2 wattmeter method, the reading of one of the wattmeter will be zero when?
    A. Power factor is unity
    B. Power factor is 0.5
    C. Load is one of the phases is zero
    D. A neutral wire is not provided

Answer 14

B. Power factor is 0.5

Question 15

9. For an unbalanced 3 phase load, which connection is suitable?
   A. 3 wire, open delta connection
   B. 3 wire, delta connection
   C. 4 wire, wye connection
   D. 3 wire, wye connection

Answer 15

C. 4 wire, wye connection

Question 16

16. Minimum voltage regulation occurs when the power factor of the load is.
    A. Unity
    B. Lagging
    C. Leading
    D. Zero

Answer 16

C. Leading