## 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.