SSC JE electrical question paper explained 2016-2017

Ques 41.  The unit of retentivity is

  1. Weber
  2. Weber/sq.m
  3. Amper turn/meter
  4. Amper turn

Answer.2. Weber/sq.m

 

Ques 42. Which of the following motors are preferred for tape recorders?

  1. Hysteresis motor
  2. Synchronous motor
  3. Digital Motor
  4. Reluctance Motor

Answer.1. Hysteresis motor

Explanation:

  • Hysteresis motor develops hysteresis torque which is extremely steady both in amplitude and phase.

 

Ques 43. An air gap is usually inserted in magnetic circuits to

  1. Increase M.M.F
  2. Increase the flux
  3. Prevent Saturation
  4. None of the above

Answer.3. Prevent Saturation

Explanation:

  • Saturation is the state reached when an increase in applied external magnetic field H cannot increase the magnetization of the material further, so the total magnetic flux density B levels off.
  • The air gap is mostly used in the magnetic circuit because magnetic saturation causes loss of inductance, increasing current, and power loss in the circuit.
  • implementation of the air gap in a magnetic circuit extends the range before magnetic saturation occurs and increases the saturation current of the magnetic inductor.

 

Ques 44. The speed of the motor for the frequency of 60 Hz and 4 poles motor is?

  1. 3600 rpm
  2. 1800 rpm
  3. 1500 rpm
  4. None of these

Answer.2. 1800 rpm

Explanation:

The speed of the motor is given as

Ns =120f/P

=120 x 60/4

= 1800 rpm

 

Ques 45.  The unit of magnetic flux is

  1. Henry
  2. Weber
  3. Ampere-turn/metre
  4. Ampere/meter

Answer.2. Weber

Explanation:

The SI unit of magnetic flux is the weber (Wb) (in derived units: volt-seconds)

 

Ques 46.  Compensating winding  is connected in

  1. Parallel with Ia
  2. Parallel with If
  3. Parallel with Isc
  4. Series with Ia

Answer.4. Series with Ia

Explanation:

  • In order to neutralize the cross magnetizing effect of armature reaction, a compensating winding is used.
  • The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. The series-connected compensating windings produce a magnetic field, which varies directly with the armature current.

 

Ques 47.  The magnetizing force (H) and magnetic flux density (B) are connected by the relation

  1. B = μrH/μο
  2. B = μοH
  3. B = H/μομr
  4. B = μrHμο/μr

Answer.2. B = μοH

Explanation:

In free space,

B=μoH

Where,

is the magnetic flux density,

is the magnetic field intensity

μo is the permeability of free space

 

Ques 48. Equivalent resistance and capacitance in case of the series combination

  1. .Increase, Decrease
  2. Increase, Increase
  3. Decrease, Decrease
  4. Decrease, Increase

Answer.1. Increase, Decrease

Explanation:

  • In series combination the equivalent resistance is added up R1 + R2 = 2R
  • While in series combination the equivalent capacitance become 1/C1 + 1/C2 = C/2

Therefore for resistance the equivalent resistance increase and for capacitance equivalent capacitance decrease in the case of the series combination.

 

Ques 49. A 0-10 mA PMMC ammeter reads 4 mA in a circuit. Its bottom control spring snaps suddenly. The meter will now read nearly

  1. 10 mA
  2. 8 mA
  3. 2 mA
  4. Zero

Answer.4. Zero

Explanation:

The spring gives controlling torque and is connected in series with the coil. If the spring is snapped, there will be no deflection. Hence the pointer will come in its initial position i.e zero

 

Ques 50. The specific resistance of a wire depends upon

  1. Length
  2. Cross-sectional area
  3. Material
  4. Temperature

Answer.4. Material

Explanation:

Specific Resistance

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

Resistance

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 its 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.

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