# Source & Delta Star Transformations MCQ (Interview-Exam) Question-Answer

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## Source & Delta Star Transformations MCQ (Interview-Exam) Question-Answer

Q.1 A voltage source connected in series with a resistor can be converted to a?

A. Current source in parallel with a resistor

B. Cannot be modified

C. Current source in series with a resistor

D. Voltage source in parallel with a resistor

Ans : Current source in parallel with a resistor

Q.2 Calculate the total current in the circuit.

A. 4.3mA

B. 1.3mA

C. 2.3mA

D. 3.3mA

Ans : 3.3mA

Explanation: The 9mA source connected in parallel to the 5 kohm resistor can be converted to a 45V source in series with a 5 kohm resistor. Applying mesh analysis, we get:
I=(45-3)/(5+4.7+3) = 3.3mA.

Q.3 Once the circuit is transformed to a voltage source where will the resistance be connected?

A. In parallel with the voltage source

B. Resistance is multiplied by 10 and connected in series with the source

C. In series with the voltage source

D. The resistance is removed from the circuit

Ans : In series with the voltage source

Q.4 Find the value of voltage once source transformation is applied to the circuit.

A. 30V

B. 70V

C. 10V

D. 50V

Ans : 50V

Explanation: Using ohm’s law, we can use the relation: V=IR.
Thus V=10*5 = 50V.

Q.5 Once the circuit is transformed into a current source where will the resistance be connected?

A. In parallel with the current source

B. Resistance is multiplied by 10 and connected in parallel with the source

C. In series with the current source

D. The resistance is removed from the circuit

Ans : In parallel with the current source

Q.6 What will the value of the current be once source transformation is applied to the circuit?

A. 20A

B. 40A

C. 10A

D. 30A

Ans : 10A

Explanation: I=V/R.
I=220/22=10A.

Q.7 A source transformation is_________.

A. Bilateral

B. Cannot be determined

C. Unilateral

D. Unique

Ans : Bilateral

Q.8 A current source connected in parallel with a resistor can be converted to a?

A. Voltage source in series with a resistor

B. Voltage source in parallel with a resistor

C. Current source in series with a resistor

D. Cannot be modified

Ans : Voltage source in series with a resistor

Q.9 If there are 3 10V sources connected in parallel then on source transformation__________.

A. The effect of only one source is considered

B. The effect of only 2 sources is considered.

C. The effect of all the sources is considered

D. The effect of none of the sources is considered

Ans : The effect of only one source is considered

Q.10 In source transformation________.

A. Current sources remain the same

B. Resistances remain the same

C. Voltage source remains the same

D. Both voltage and current source remain the same

Ans : Resistances remain the same

Q.11 The value of the 3 resistances when connected in star connection is_________.

A. 3.55ohm, 4.33ohm, 5.67ohm

B. 4.53ohm, 6.66ohm, 1.23ohm

C. 2.32ohm,1.22ohm, 4.54ohm

D. 2.78ohm, 1.67ohm, 0.83ohm

Ans : 2.78ohm, 1.67ohm, 0.83ohm

Explanation: R1=10*5/(10+5+3) = 2.78 ohm
R2=10*3/(10+5+3) = 1.67 ohm
R3=5*3/(10+5+3) = 0.83 ohm.

Q.12 Which, among the following is the right expression for converting from delta to star?

A. R1=Ra/(Ra+Rb+Rc), R2=Rb/(Ra+Rb+Rc), Rc=/(Ra+Rb+Rc)

B. R1=Ra*Rb*Rc/(Ra+Rb+Rc), R2=Ra*Rb*Rc/(Ra+Rb+Rc), R3=Ra*Rb*Rc/(Ra+Rb+Rc)

C. R1=Ra*Rb/(Ra+Rb+Rc), R2=Rb*Rc/(Ra+Rb+Rc), R3=Rc*Ra/(Ra+Rb+Rc)

D. R1=Ra*Rb*Rc/(Ra+Rb+Rc), R2=Ra*Rb/(Ra+Rb+Rc), R3=Ra/(Ra+Rb+Rc)

Ans : R1=Ra*Rb/(Ra+Rb+Rc), R2=Rb*Rc/(Ra+Rb+Rc), R3=Rc*Ra/(Ra+Rb+Rc)

Q.13 Star connection is also known as__________.

A. Mesh connection

B. Neither Y-connection nor mesh connection

C. Y-connection

D. Either Y-connection or mesh connection

Ans : Y-connection

Q.14 Find the equivalent star network.

A. 1.2ohm, 1.2ohm, 1.2ohm

B. 4.5ohm, 4.5ohm, 4.5ohm

C. 2.3ohm, 2.3ohm, 2.3ohm

D. 3.3ohm, 3.3ohm, 3.3ohm

Ans : 1.2ohm, 1.2ohm, 1.2ohm

Explanation: R1=R2=R3= 3.6*3.6/(3.6+3.6+3.6)=1.2 ohm.

Q.15 Rab is the resistance between the terminals A and B, Rbc between B and C and Rca between C and A. These 3 resistors are connected in delta connection. After transforming to star, the resistance at B will be?

A. Rab/(Rab+Rbc+Rca)

B. Rab/(Rab+Rbc+Rca)

C. Rac/(Rab+Rbc+Rca)

D. Rbc*Rab/(Rab+Rbc+Rca)

Ans : Rbc*Rab/(Rab+Rbc+Rca)

Q.16 Rab is the resistance between the terminals A and B, Rbc between B and C and Rca between C and A. These 3 resistors are connected in delta connection. After transforming to star, the resistance at A will be?

A. Rab/(Rab+Rbc+Rca)

B. Rac/(Rab+Rbc+Rca)

C. Rab*Rac/(Rab+Rbc+Rca)

D. Rbc*Rac/(Rab+Rbc+Rca)

Ans : Rab*Rac/(Rab+Rbc+Rca)

Q.17 Find the current in the circuit.

A. 0.65A

B. 0.87A

C. 0.54A

D. 0.67A

Ans : 0.54A

Explanation: I=V/R. Total resistance R = 20+40=60ohm. V=120V. I=120/60=2A.

Q.18 Rab is the resistance between the terminals A and B, Rbc between B and C and Rca between C and A. These 3 resistors are connected in delta connection. After transforming to star, the resistance at C will be?

A. Rab/(Rab+Rbc+Rca)

B. Rab/(Rab+Rbc+Rca)

C. Rac/(Rab+Rbc+Rca)

D. Rbc*Rac/(Rab+Rbc+Rca)

Ans : Rbc*Rac/(Rab+Rbc+Rca)

Explanation: I=V/R. Total resistance R = 20+40=60ohm. V=120V. I=120/60=2A.

Q.19 If a 4ohm, 3ohm and 2ohm resistor is connected in delta, find the equivalent star connection.

A. 8/9ohm, 4/3ohm, 7/3ohm

B. 8/9ohm, 5/3ohm, 2/3ohm

C. 8/9ohm, 4/3ohm, 2/3ohm

D. 7/9ohm, 4/3ohm, 2/3ohm

Ans : 8/9ohm, 4/3ohm, 2/3ohm

Explanation: R1=4*3/(2+3+4)
R2=2*3/(2+3+4)
R3=2*4/(2+3+4).

Q.20 If a 6 ohm, 2ohm and 4ohm resistor is connected in delta, find the equivalent star connection.

A. 2ohm, 4ohm, 7ohm

B. 1ohm, 2ohm, 2/3ohm

C. 1ohm, 2ohm, 3ohm

D. 5ohm, 4ohm, 2ohm

Ans : 1ohm, 2ohm, 2/3ohm

Explanation: R1=2*6/(2+6+4)
R2=2*4/(2+6+4)
R3=4*6/(2+6+4).

## Source Transformations:

• Definition: Source transformations are a technique used to simplify complex electrical networks by converting voltage sources into equivalent current sources, or vice versa.
• Voltage Source to Current Source: A voltage source in series with a resistor can be transformed into an equivalent current source in parallel with a resistor, and vice versa.
• Purpose: Source transformations are used to simplify circuit analysis by converting sources to their equivalent forms, often making calculations more straightforward.
• Application: Commonly applied in circuit analysis to simplify complex networks, especially in the analysis of circuits containing multiple sources.

## Delta-Star (Δ-Y) Transformations:

• Definition: Delta-star transformations are a method used to simplify electrical networks that contain delta (Δ) and star (Y) configurations of resistors or impedances.
• Transformation: In delta-star transformations, resistors or impedances connected in a delta configuration are converted into an equivalent star configuration, and vice versa.
• Purpose: The main purpose of delta-star transformations is to simplify circuit analysis by converting between different configurations, making calculations easier.
• Application: Delta-star transformations are commonly used in circuit analysis, especially in solving problems involving balanced three-phase circuits, where delta and star configurations are frequently encountered.
• Mathematical Relationship: The transformation involves establishing relationships between the resistors or impedances in the delta and star configurations using specific formulas, often based on the circuit topology and resistor values.

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