SSC JE electrical question paper explained 2016-2017

Ques 91. The leakage resistance of a 50 Km long cable is 1 MΩ. For a 100 Km long cable it will be

  1. 1 MΩ
  2. 2 MΩ
  3. 0.5 MΩ
  4. None of these

Answer.3. 0.5 MΩ

Explanation:

Leakage resistance of the cable α 1/Length

For 100 km long cable leakage resistance of cable

=50/100 =0.5MΩ

 

Ques 92. A series drawing an armature current of Is is operating under saturated magnetic conditions, the torque developed in the motor is proportional to

  1. 1/Ia
  2. 1/I2a
  3. I2a
  4. Ia

Answer.4. Ia

Explanation:

We know Torque Tα ΦIa As the motor is operating under saturation condition so torque will be independent of flux i.e. T α Ia.

 

Ques 93. The insulation resistance of a cable of length 10 Km is 1 MΩ. Its resistance for 50 Km length will be

  1. 1 MΩ
  2. 0.5 MΩ
  3. 0.2 MΩ
  4. None of these

Answer.2. 0.2 MΩ

Explanation:

Insulation resistance of the cable

R α 1/Length

R2/R1 = L1/L2

Hence, insulation resistance for 50 km length

R2 = (1 × 10)/50

= 0.2 MΩ

 

Ques 94. When a line-to-ground fault occurs, the current in a faulted phase is 100 A. The zero sequence current in the case will be

  1. Zero
  2. 33.3 A
  3. 66.6 A
  4. 100 A

Answer.2. 33.3 A

Explanation:

For LIne to ground fault, we know that

IN = If = 3Io

Where IN = Neutral current

If  = Fault current

Io = zero sequence current

Hence zero sequence current is

100/3 = 33.3 A

 

Ques 95. A 3 phase, 400-V, 50 Hz 4 pole induction motor is fed from a 3-phase supply and runs of 1425 rpm. The frequency of the rotor emf is

  1. 2.5 Hz
  2. 50 Hz
  3. 48 Hz
  4. Zero

Answer.1. 2.5 Hz

Explanation:

Synchronous speed

Ns = 120f/P

= 120 × 50/ 4

= 1500 rpm

Rotor speed = 1425 rpm

Slip = (Ns – Nr)/Ns

= (1500 – 1425)/1500

= 0.05

∴ Frequency of rotor emf

= 0.05 × 50

= 2.5 Hz

 

Ques 96. The field coils of a dynamo have an inductance of 8H and have a resistance of 60 ohms and are connected to 120 V dc source. The steady-state current in the field coiled is

  1. 15 A
  2. 2 A
  3. 0 A
  4. None of these

Answer.2. 2A

Explanation:

Steady-state field current = 120/60 = 2A

 

Ques 97. The surge impedance of a 50 miles long underground cable is 50 ohms. For a 25 miles length, it will be

  1. 25 Ω
  2. 50 Ω
  3. 100 Ω
  4. None of these

Answer.2. 50 ohms

Explanation:

Surge impedance is independent of the length of the line hence the surge impedance will remain 50 ohms

 

Ques 98. In a BJT with α = 0.98, β is equal to

  1. 49
  2. 98
  3. 0.49
  4. 980

 Answer.1. 49 

Explanation:

In CB mode current gain is

Ic/IE = α

In CC mode current gain is

β =IC/IB

and β = α (1 – α)

= 0.98/(1-0.98) = 49

 

Ques 99. Two coils in differential connection have self-inductance of 2 mH and 4 mH and a mutual inductance 0f 0.15 mH. The equivalent inductance of the combination is

  1. 5.7 mH
  2. 5.85 mH
  3. 6 mH
  4. 6.15 mH

Answer.1. 5.7 mH

Explanation:

Total inductance for differential connection is given as

Leq = L1 + L2 + 2M……………..1

= 2 + 4 – 2 x 0.15

= 5.7 mH

 

Ques 100. Find the value of I?

figure 4

  1. 10 A
  2. 13 A
  3. 17/2 A
  4. None of these

Answer.4. None of these

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