SSC JE 2009 Electrical question paper with Explained Solution

Ques 1. The value of current I flowing in the 1Ω resistor in the circuit shown in the figure below will be

10A

6A

5A✔

0

When only voltage source is present then voltage across 1Ω resistor is

V = 5V

I = V/R = 5/1 = 5A

Ques 2. In the figure shown below, if we connect a source of 2V, with internal resistance of 1Ω at AA’ with positive terminal at A, then the current through R is

2A

1.66 A

1 A

0.625 A✔

Connect a source of 2V, with internal resistance of 1Ω at AA’

Applying Nodal analysis at C and B

Solving equation 1 & 2

V_{C} = 1.125 Volt

V_{B} = 1.25 Volt

Current through resistor R

I_{R} = V_{B}/2 = 1.25/2 = 0.625 A

Ques 3. The curve representing Ohm’s law is

Linear✔

Hyperbolic

Parabolic

Triangular

Ohm’s law of constant proportionality state that for a given conductor the ratio of voltage to current is constant i.e if the temperature, area, the length did not change the resulting current is directly proportional to the variation in the applied voltage. Hence the graph between voltage and current is linear. R(constant) = V/I

★Ohms law does not apply to all material it only applied to “linear” or “Ohmic Resistor”.

Ques 4. The specific resistance of a conductor depends upon

Dimension of the conductor

Composition of conductor material✔

Resistance of the conductor

Both (a) and (b)

For any given material at a certain given temperature, the resistance is given as

where ρ is a constant depending on the nature of the material of the conductor and is known as its specific resistance or resistivity.

Specific Resistance depends only on the temperature and material of the conductor but not on the dimensions of the conductor, on which resistance depends, and mechanical deformation such as stretching, etc. As ρ depends only on the material of a conductor at a given temperature, hence it is a characteristic constant.

Ques 5. The superposition theorem is essentially based on the concept of

Reciprocity

Linearity✔

Duality

Non-linearity

Superposition theorem

In any Linear and bilateral network consisting of linear and bilateral energy sources (e.g. generators), the current flowing at any point in the network is the algebraic sum of the currents those flow with each source were considered separately, with all the remaining sources replaced at a time by their internal impedances.

The theorem is hence based on the linearity property and Ohm‘s law. The fundamental concept is that in linear impedance, for an increase in voltage across it, the current through it increases which is independent of the magnitude of the original current ﬂowing through it.

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Ques 6. If a 500 KVA, 200 Hz transformer is operated at 50 Hz, its KVA rating will be

2000 kVA

125 kVA✔

250 kVA

1000 kVA

For the same load

S ∝ induced EMF

S ∝ 4.44fNΦ

∴ S ∝ f

S_{1}/S_{2} = f_{1}/f_{2}

S_{2} = S_{1}f_{2}/f_{1}

= 500 x 50/200 =125 KVA

Ques 7. The angle between induced emf and terminal voltage on no-load for a single phase alternator is

180°

90°

0°✔

270°

The generated voltage of the alternator is

V = E – Ia(Ra + JXs)

Where E = Induced EMF

Ia = Armature current

Ra = Armature resistance

Xs = leakage reactance of the armature winding

For no-load armature current Ia = 0

∴ V = E (δ = 0°)

So at no load, the angle between induced emf and terminal voltage is 0°

Ques 8. A salient pole synchronous generator connected to an infinite bus power will deliver maximum power at a power angle of

δ = 0°

δ = 90°✔

δ = 45°

δ = 30°

Power delivered by synchronous generator

For Pmax sinδ = 1

δ = 90°

Pmax = VE/X

Ques 9. Starting torque of the synchronous motor is

Very low

Zero✔

Very high

Half-full load torque

The starting torque of the synchronous motor is zero.

Consider the rotor is at rest and the stator field is rotating at the synchronous speed at 100% slip. Suppose the N pole of the rotating field in the stator is approaching at S pole of the rotor then the magnetic force will tend to turn the rotor in the direction opposite to the rotating field When the same N pole of the rotating field has just passed the rotor S pole, the magnet force will tend to pull the rotor in the same direction as the rotating field. These opposite interactions balance out, producing a zero net torque on the rotor.

Ques 10. The power factor at which the transformer operates

Is unity

is 0.8 lag

Is 0.8 lead

Depends upon the power factor of the load✔

The operating power factor in the case of a transformer depends upon the load supplied by the transformer. For example, if the transformer is supplying lamp load, the operating power factor is unity, whereas, in most industries, a large percentage of the load consists of an induction motor.

These machines operate with a lagging power factor hence the power factor of the transformer is lagging, Moreover, the magnitude of the lagging power factor also depends on the nature of the inductive load being supplied by the transformer Thus the transformer rating is always specified in terms of volt-ampere (VA) or Kilo-Volt-ampere KVA.