250+ TOP MCQs on Three-Phase Balanced Circuits and Answers

Network Theory Multiple Choice Questions on “Three-Phase Balanced Circuits”.

1. In a balanced three-phase system-delta load, if we assume the line voltage is V_RY = V∠0⁰ as a reference phasor. Then the source voltage V_YB is?
A. V∠0⁰
B. V∠-120⁰
C. V∠120⁰
D. V∠240⁰

Answer: B
Clarification: As the line voltage V_RY = V∠0⁰ is taken as a reference phasor. Then the source voltage V_YB is V∠-120⁰.

2. In a balanced three-phase system-delta load, if we assume the line voltage is V_RY = V∠0⁰ as a reference phasor. Then the source voltage V_BR is?
A. V∠120⁰
B. V∠240⁰
C. V∠-240⁰
D. V∠-120⁰

Answer: C
Clarification: As the line voltage V_RY = V∠0⁰ is taken as a reference phasor. Then the source voltage V_BR is V∠-240⁰.

3. In a delta-connected load, the relation between line voltage and the phase voltage is?
A. line voltage > phase voltage
B. line voltage < phase voltage
C. line voltage = phase voltage
D. line voltage >= phase voltage

Answer: C
Clarification: In a delta-connected load, the relation between line voltage and the phase voltage is line voltage = phase voltage.

4. If the load impedance is Z∠Ø, the current (I_R) is?
A. (V/Z)∠-Ø
B. (V/Z)∠Ø
C. (V/Z)∠90-Ø
D. (V/Z)∠-90+Ø

Answer: A
Clarification: As the load impedance is Z∠Ø, the current flows in the three load impedances and the current flowing in the R impedance is I_R = V_BR∠0⁰/Z∠Ø = (V/Z)∠-Ø.

5. If the load impedance is Z∠Ø, the expression obtained for current (I_Y) is?
A. (V/Z)∠-120+Ø
B. (V/Z)∠120-Ø
C. (V/Z)∠120+Ø
D. (V/Z)∠-120-Ø

Answer: D
Clarification: As the load impedance is Z∠Ø, the current flows in the three load impedances and the current flowing in the Y impedance is I_Y = V_YB∠120⁰/Z∠Ø = (V/Z)∠-120-Ø.

6. If the load impedance is Z∠Ø, the expression obtained for current (I_B) is?
A. (V/Z)∠-240+Ø
B. (V/Z)∠-240-Ø
C. (V/Z)∠240-Ø
D. (V/Z)∠240+Ø

Answer: B
Clarification: As the load impedance is Z∠Ø, the current flows in the three load impedances and the current flowing in the B impedance is I_B = V_BR∠240⁰/Z∠Ø = (V/Z)∠-240-Ø.

7. A three-phase balanced delta connected load of (4+j8) Ω is connected across a 400V, 3 – Ø balanced supply. Determine the phase current I_R. Assume the phase sequence to be R_YB.
A. 44.74∠-63.4⁰A
B. 44.74∠63.4⁰A
C. 45.74∠-63.4⁰A
D. 45.74∠63.4⁰A

Answer: A
Clarification: Taking the line voltage V_RY = V∠0⁰ as a reference V_RY = 400∠0⁰V, V_YB = 400∠-120⁰V and V_BR = 400∠-240⁰V. Impedance per phase = (4+j8) Ω = 8.94∠63.4⁰Ω. Phase current I_R = (400∠0^o)/(8.94∠63.4^o )= 44.74∠-63.4⁰A.

8. A three-phase balanced delta connected load of (4+j8) Ω is connected across a 400V, 3 – Ø balanced supply. Determine the phase current I_Y.
A. 44.74∠183.4⁰A
B. 45.74∠183.4⁰A
C. 44.74∠183.4⁰A
D. 45.74∠-183.4⁰A

Answer: C
Clarification: Taking the line voltage V_RY = V∠0⁰ as a reference V_RY = 400∠0⁰V, V_YB = 400∠-120⁰V and V_BR = 400∠-240⁰V. Impedance per phase = (4+j8)Ω = 8.94∠63.4⁰Ω. Phase current I_Y = (400∠120^o)/(8.94∠63.4^o)= 44.74∠-183.4⁰A.

9. A three-phase balanced delta connected load of (4+j8) Ω is connected across a 400V, 3 – Ø balanced supply. Determine the phase current I_B.
A. 44.74∠303.4⁰A
B. 44.74∠-303.4⁰A
C. 45.74∠303.4⁰A
D. 45.74∠-303.4⁰A

Answer: B
Clarification: Taking the line voltage V_RY = V∠0⁰ as a reference V_RY = 400∠0⁰V, V_YB = 400∠-120⁰V and V_BR = 400∠-240⁰V. Impedance per phase = (4+j8) Ω = 8.94∠63.4⁰Ω. Phase current I_B = (400∠240^o)/(8.94∠63.4^o) = 44.74∠-303.4⁰A.

10. Determine the power (kW) drawn by the load.
A. 21
B. 22
C. 23
D. 24

Answer: D
Clarification: Power is defined as the product of voltage and current. So the power drawn by the load is P = 3V_PhI_PhcosØ = 24kW.