SSC JE 2014 Electrical question paper with solution(Morning-Shift)

SSC previous year solved electrical papers / March 25, 2022

Ques 60. The total capacitance between the points L and M in the figure is:

numerical 60

  1. 4.05 μF
  2. 1.45 μF
  3. 1.85 μF
  4. 2.05 μF✔

Total capacitance between point L and N

=2 + 1 = 3μF

Now total capacitance between L and O

= 1 + (3 x 2)/(3 + 2) = 2.2μF

Total capacitance Between L and M

= 1 + (2.2 x 2)/(2.2 + 2) = 2.05μF

 

Ques 61. EMF induced in a coil rotating in a uniform magnetic field will be maximum when the:

  1.  Rate of cutting flux by the coil sides is minimum
  2.  Flux linking with the coil is maximum
  3.  Rate of change of flux linkage is minimum
  4.  Rate of change of flux linkage is maximum✔

The rate of change of flux linkage is directly proportional to the induced EMF in a coil. Hence, the EMF is maximum when the rate of change of flux linkage is maximum.

 

Ques 62. If resistance is 20Ω and inductance is 2H in an RL series circuit, then time constant of this circuit will be

  1. 100s
  2. 0.001s
  3. 0.1s✔
  4. 10s

Time constant of RL series circuit is given by

τ = L/R

τ = 2/20 = 0.1 sec

 

Ques 63. When the rotor of a three-phase induction motor is blocked, the slip is

  1. 1✔
  2. 0
  3. 0.1
  4. 0.5

When the rotor of a three-phase induction motor is blocked then the rotor speed “Nr” = 0

and

s = Ns – Nr/Ns

= Ns – 0/Ns = 1

 

Ques 64. The positive, negative and zero sequence impedances of 3-phase synchronous generator are j 0.5 pu, j 0.3 pu and j 0.2 pu respectively. When the symmetrical fault occurs on the machine terminals. Find the fault current. The generator neutral is grounded through reactance of j0.1 pu

  1. -j 3.33 pu
  2. -j 1.67 pu✔
  3. -j2.0 pu
  4. -j 2.5 pu

For symmetrical fault

If = E/(Zi +Zn)

Where E = Pre fault voltage Which is equal to 1

Zi = 0.5j & Zn = 0.1 j

If = 1/(0.5j + 0.1j)

If = -j 1.67

 

Ques 65. Transient current in RLC circuit is oscillatory when the value of R is

  1. More than 2√c/√L
  2. Less than 2√L/√C
  3. Less than 2√C/√L
  4. More than √L/√C

Ques 66. For average values of load current, current chopping occurs more frequently in

  1.  VCB’s
  2. OCB’s
  3. ACB’s✔
  4. SF6 CB’s

Current Chopping in the circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero. Current Chopping is mainly observed in Vacuum Circuit breakers and Air Blast Circuit breakers.

For average values of load current, current chopping occurs more frequently in the Air circuit breaker.

In an air blast breaker, a high-velocity jet of air is made to flow through the arcing contacts that extinguish the arc much before natural ac current 0 is reached, the air blast breaker retains the same extinguishing power irrespective of the nature of the fault, consequently, the blast of air sweeps out the ionized particles in between the contacts thereby building the resistance and hence ultimately extinguishing the arc.

 

Ques 67. A BJT is said to be operating in the saturation region if

  1. Both the junctions are forward biased✔
  2. Both the junctions are reverse biased
  3. B-E junction is reverse biased and B-C junction is forward biased
  4. B-E junction is forward biased and B-C  junction is reverse biased

In the saturation mode, both the junctions of the transistor (emitter to base and collector to base) are forward biased. In other words, if we assume two p-n junctions as two p-n junction diodes, both the diodes are forward biased in saturation mode.

 

Ques 68. The mutual inductance between two unity coupled coils of 9 H and 4 H will be

  1. 36 H
  2. 2.2 H
  3. 6 H✔
  4. 13 H

Mutual inductance

M = K√L1L2

For mutual inductance between two unity coupled coils, K =1

M = √L1L2 = √9 x 4 = 6H

 

Ques 69. Determine the voltage at point C shown below with respect to ground

numerical 69

  1. 80 V
  2. 120 V
  3. 40 V✔
  4. 70 V

I = V/R = 120/(100 + 50) = 120/150 = 4/5 A

VCB = 50 x 4/5 = 40 V

Now voltage at ‘c’ w.r.t to ground = Vc

VCB = Vc – Vb

Vc = VCB – VB

= 40 + 0 = 40 V

 

Ques 70. The efficiency normally obtained in a circuit under the conditions of maximum power transfer is

  1. 100%
  2. 25%
  3. 50%✔
  4. 75%

Maximum Power Transfer theorem

solution 3

The maximum power is transferred when the load resistance RL is equal to internal resistance Rth or the equivalent resistance Rth.

RL = Rth

\({I_L} = \frac{{{V_{th}}}}{{{R_{th}} + {R_L}}}\)

\({P_L} = Load\;power = I_L^2{R_L}\)

\({P_L} = {\left( {\frac{{{V_{th}}}}{{{R_{th}} + {R_L}}}} \right)^2} \times {R_L}\)    –(1)

\({P_i} = Input\;power = {V_{th}} \times {I_L}\)

\({P_i} = {V_{th}}\left( {\frac{{{V_{th}}}}{{{R_{th}} + {R_L}}}} \right)\)    –(2)

For RL = Rth,

Equation-(1) becomes:

\({P_L} = {\left( {\frac{{{V_{th}}}}{{{R_{th}} + {R_{th}}}}} \right)^2} \times {R_L}\)

\(= \frac{{V_{th}^2}}{{4{R_{th}}}}\)

Similarly, Equation-(2) becomes:

\({P_i} = {V_{th}}\left( {\frac{{{V_{th}}}}{{{R_{th}} + {R_{th}}}}} \right)\)

\(= \frac{{V_{th}^2}}{{2{R_{th}}}}\)

Power Transfer Efficiency will be:

\(\frac{{{P_L}}}{{{P_i}}} = \frac{{V_{th}^2 \times 2{R_L}}}{{4{R_L} \times V_{th}^2}}\)

\(= \frac{1}{2} = 0.5 = 50\%\)

Under this condition, the same amount of power is dissipated in the internal resistance Rth and hence the efficiency is 50%.

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