Model Question Paper with Answer - 2 (PART A)

MODEL QUESTION PAPER - II

1. How are the following affected by change of frequency?

(a) Resistance, (b) Inductive reactance

Ans:

(a) Resistance is independent of frequency. That is, it is not affected by frequency.

(b) Inductive reactance X_L =  2π ƒ L

.X_L ∞ f

That is, inductive reactance changes proportional to frequency.

2.Define 'Nodal analysis' of a circuit.

Ans: Nodal analysis is the method of solving a given network. It is used to find the voltages for given currents. If the network consists of n nodes, one of them is taken as reference node. Then other node voltages are considered as unknown voltages. There are (n - 1) such voltages. At each node of these nodes, KCL is applied. The equations obtained are known as node equations. They are solved to find the node voltages with reference to the reference voltage.

3. Define 'Mesh analysis' of a circuit.

Ans: It is one of the methods to solve an electrical network. Here we have to assign loop currents to all independent loops and apply KVL to each loop. Then we have to solve the equations for the loop currents. From this, branch currents can be calculated.

4. Give a delta circuit having resistors, write the required expressions to transform the circuit to a star circuit.

Solution:

R_A = R_AB × R_CA / ∑R_AB

R_B = R_AB × R_CA / ∑R_AB

and  R_C = R_BC × R_CA  / ΣR_AB

5. Determine the Laplace transform for the unit step function u (t).

Ans:

f(t) = u(t)

Here u(t) = 1 for t > 0

α = 0 for t < 0

 Lu(t) = 1 /s

16. Write down the condition for critically damped response of a series RLC circuit excited by a sinusoidal AC source.

Ans: (R/2L)² = (1/LC)

7. Three inductive coils each having resistance of 10Ω and reactance of j12Ω are connected in star across a 400V, 3 ϕ, 50 Hz supply. Calculate phase voltage.

Ans: Unless otherwise stated, for a star connection, given voltage is considered as line voltage.

So, Line voltage = 400 volts.

Phase voltage = Line voltage / √3

= 400 / √3 = 231 volts

8. A three phase motor can be regarded as a balanced Y load. A three phase motor draws 5.6 kW when the line voltage is 220V and the line current is 18.2A. Determine the power factor of the motor.

Ans:

V_L = 220 Volts

I_L = 18.2 A

Real Power P = 5600 Watts

But P = √3 V_LI_L COS ϕ

Substituting the values, we get

Power factor = cos ϕ = 0.807

9. Define self inductance

The self inductance of a coil is defined as flux linkage in that coil per 1 ampere current in the same coil. It is also defined as the weber turns per ampere current in the coil. L is measured in Henry.

10. Define Mutual inductance

The mutual inductance between 2 coils is defined as the weber turns in one coil per ampere current in other coil. It is measured in henrys.