250+ TOP MCQs on Star to Delta and Delta to Star Transformation and Answers

Network Theory Multiple Choice Questions on “Star to Delta and Delta to Star Transformation”.

1. If a resistor Z_R is connected between R and N, Z_BR between R and B, Z_RY between R and Y and Z_BY between B and Y form a delta connection, then after transformation to star, the impedance at R is?
A. (Z_BRZ_BY)/(Z_RY+Z_BY+Z_BR)
B. (Z_RYZ_BR)/(Z_RY+Z_BY+Z_BR)
C. (Z_RYZ_BY)/(Z_RY+Z_BY+Z_BR)
D. (Z_RY)/(Z_RY+Z_BY+Z_BR)

Answer: B
Clarification: After transformation to star, the impedance at R is
(Z_RYZ_BR)/(Z_RY+Z_BY+Z_BR).

2. If a resistor Z_R is connected between R and N, Z_BR between R and B, Z_RY between R and Y and Z_BY between B and Y form a delta connection, then after transformation to star, the impedance at Y is?
A. (Z_RY)/(Z_RY+Z_BY+Z_BR)
B. (Z_BY)/(Z_RY+Z_BY+Z_BR)
C. (Z_RYZ_BY)/(Z_RY+Z_BY+Z_BR)
D. (Z_RYZ_BR)/(Z_RY+Z_BY+Z_BR)

Answer: C
Clarification: After transformation to star, the impedance at Y is
(Z_RYZ_BY)/(Z_RY+Z_BY+Z_BR).

3. If a resistor Z_R is connected between R and N, Z_BR between R and B, Z_RY between R and Y and Z_BY between B and Y form a delta connection, then after transformation to star, the impedance at B is?
A. (Z_BRZ_BY)/(Z_RY+Z_BY+Z_BR)
B. (Z_RYZ_BY)/(Z_RY+Z_BY+Z_BR)
C. (Z_BY)/(Z_RY+Z_BY+Z_BR)
D. (Z_BR)/(Z_RY+Z_BY+Z_BR)

Answer: A
Clarification: After transformation to star, the impedance at Y is
(Z_BRZ_BY)/(Z_RY+Z_BY+Z_BR).

4. If the resistors of star connected system are Z_R, Z_Y, Z_B then the impedance Z_RY in delta connected system will be?
A. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_B
B. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_Y
C. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_R
D. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/(Z_R+Z_Y)

Answer: A
Clarification: After transformation to delta, the impedance Z_RY in delta connected system will be (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_B.

5. If the resistors of star connected system are Z_R, Z_Y, Z_B then the impedance Z_BY in delta connected system will be?
A. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/(Z_B+Z_Y)
B. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_B
C. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_Y
D. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_R

Answer: D
Clarification: After transformation to delta, the impedance Z_BY in delta connected system will be (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_R.

6. If the resistors of star connected system are Z_R, Z_Y, Z_B then the impedance Z_BR in delta connected system will be?
A. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_Y
B. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/_R
C. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_B
D. (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/(Z_B+Z_R)

Answer: A
Clarification: After transformation to delta, the impedance Z_BR in delta connected system will be (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_Y.

7. A symmetrical three-phase, three-wire 440V supply is connected to star-connected load. The impedances in each branch are Z_R = (2+j3) Ω, Z_Y = (1-j2) Ω, Z_B = (3+j4) Ω. Find Z_RY.
A. (5.22-j0.82) Ω
B. (-3.02+j8) Ω
C. (3.8-j0.38) Ω
D. (-5.22+j0.82) Ω

Answer: C
Clarification: Z_RY = (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_B
= (3.8-j0.38) Ω.

8. A symmetrical three-phase, three-wire 440V supply is connected to star-connected load. The impedances in each branch are Z_R = (2+j3) Ω, Z_Y = (1-j2) Ω, Z_B = (3+j4) Ω. Find Z_BY.
A. (-5.22+j0.82) Ω
B. (5.22-j0.82) Ω
C. (3.8-j0.38) Ω
D. (-3.02+j8) Ω

Answer: B
Clarification: Z_BY = (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_R
= (5.22-j0.82)Ω.

9. A symmetrical three-phase, three-wire 440V supply is connected to star-connected load. The impedances in each branch are Z_R = (2+j3) Ω, Z_Y = (1-j2) Ω, Z_B = (3+j4) Ω. Find Z_BR.
A. (5.22-j0.82) Ω
B. (-3.02+j8) Ω
C. (-5.22+j0.82) Ω
D. (3.8-j0.38) Ω

Answer: B
Clarification: Z_BR = (Z_RZ_Y + Z_YZ_B + Z_BZ_R)/Z_Y
= (-3.02+j8) Ω.

10. If a star connected system has equal impedances Z₁, then after converting into delta connected system having equal impedances Z₂, then?
A. Z₂ = Z₁
B. Z₂ = 2Z₁
C. Z₂ = 3Z₁
D. Z₂ = 4Z₁

Answer: C
Clarification: If a star connected system has equal impedances Z₁, then after converting into delta connected system having equal impedances Z₂, then Z₂ = 3Z₁.