Model Question and Answer - 1 PART A

MODEL QUESTION AND ANSWER

MODEL QUESTION PAPER - I

1. State Kirchhoffs law

KCL:  The algebraic sum of currents meeting at a junction (node) is equal to zero.

KVL: The algebraic sum of EMF plus the algebraic sum of voltage across the impedance in any closed electrical circuit is equal to zero.

2. Write the mesh equations for the circuit shown in Figure.

Ans :

⇒ 7I₁ – 2I₂ = 10

and - 2 I₁ + I 2I₂= 0

3. State superposition theorem.

"In a linear circuit containing more than one source, the current that flows at any point or the voltage that exists between any two points is the algebraic sum of the currents or the voltages that would have been produced by each source taken separately with all other sources removed."

4. State Reciprocity theorem.

Statement: In a linear, bilateral network a voltage source V volts in a branch gives rise to a current I, in another branch. If V is applied in the second branch, the current in the first branch will V be I. This V/I is called transfer impedance or resistance.

5. Define transient time.

The time required for the network to change from one steady state to another steady state is called transient time or period.

6. A resistance of 5Ω and an inductance of 0.03 H are connected in series with a voltage source of 10 V (dc). Find the time constant of the circuit.

Ans: Time constant T = L / R

= 0.03 / 5 sec

= 6 × 10^-3 sec

= 6 ms

7. What are the advantages of 3-phase system over 1-phase system? phase system over 1-phase system?

Advantages of 3ϕ systems:

1. For the same size a 3 phase machine delivers more power than single phase machine. A three phase induction motor delivers 1.5 times the power as single phase motor of same size. Thus 3 phase machines are economical and occupy less space as compared to equivalent single ys niphase machines.

 2. Single phase machine's performance is not satisfactory, for example a single phase induction motor produces pulsating torque and hence is not desirable compared to this a three phase induction motor produces a uniform torque hence its operation is smooth and steady.

3. Parallel operation of three phase alternatives is very smooth compared to single phase alternators.

4. Power transmission by 3 phase system is very economical as it requires less amount of copper for transmitting same amount of power.

5. Single phase induction motors are not self starting, whereas three phase induction motors are self starting. To start single phase induction motor, it is temporarily converted into two phase induction motor.

8. Calculate the power factor if V(t) = V_m sin (ωt - 45°) and I(t) = Im sin (ωt-135°).

Power factor = cos ϕ

= cos ϕ  ( Voltage angle - Current angle)

=cos (-45+135)

= cos 90°

= 0

9. Define Bandwidth.

The bandwidth of a resonant circuit is defined as the difference of the upper half power frequencies and lower half power frequencies.

Bandwidth = f₂ - f₁

f₁ = Lower half power frequency

f₂ = Upper half power frequency

10. Define quality factor of a coil.

The quality factor of a coil is determined by the maximum amount of energy that can be stored in the circuit compared with the energy that is lost during one complete period of the response.

Q = Quality factor

= 2 π Maximum energy stored /  Total energy lost in one period