SSC JE 2013 electrical question paper with solution

24 Comments / SSC previous year solved electrical papers / April 10, 2022

Ques 90. The commutator in a dc machine acts as

  1. A mechanical inverter
  2. A mechanical rectifier
  3. Current controller
  4. Either (A) or (B)

Answer.4. Either (A) or (B)

Explanation:

  • The commutator is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
  • In a motor, the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced.

Therefore commutator work as the mechanical rectifier and current controller.

 

Ques 91. The purpose of using a dummy coil in a dc machine is to

  1. Eliminate harmonics developed in the machine
  2. Eliminate armature reaction
  3. Bring mechanical balance to the armature
  4. Bring mechanical balance to the body of the motor

Answer.3. Bring mechanical balance of the armature

Explanation:

  • A Dummy coil is used with wave winding when the requirement of the winding is not met by the standard armature.
  • The Dummy coil  is not connected to the commutator so they do not influence the electrical characteristics of the winding
  • Dummy coil ends are cut short and taped.
  • Their main use is to provide mechanical balance for the rotor because the rotor having some slots without winding would be out of balance mechanically.

 

Ques 92. An inductor with ferromagnetic core supplied from a sinusoidal voltage source with frequency ‘f’. The current drawn by the inductor will be

  1. Sinusoidal with frequency ‘f’
  2. Sinusoidal with frequency ‘2f’
  3. A sawtooth wave
  4. Non-sinusoidal with frequency ‘f’

Answer.4. Non-sinusoidal with frequency “f”

Explanation:

Inductors with ferromagnetic cores have additional energy losses due to hysteresis and eddy currents in the core, which increase with frequency. At high currents, iron core inductors also show a gradual departure from ideal behavior due to nonlinearity caused by magnetic saturation of the core

If the current through a ferromagnetic core coil is high enough that the magnetic core saturates, the inductance will not remain constant but will change with the current during each half of the period to provide the maximum flux density, therefore, the inductor with a ferromagnetic core end up having non-sinusoidal excitation current.

 

Ques 93. For a 6-pole dc machine with wave wound armature, the number of brushes required is

  1. 2
  2. 4
  3. 6
  4. 12

Answer.1. 2

Explanation:

In wave winding the number of brushes is always 2 irrespective of the number of poles.

 

Ques 94.  The function of interpoles in a dc machine are to

  1. Reduce field winding heating
  2. Improve commutation
  3. Compensate for air-gap variation
  4. Reduce losses

Answer.2. Improve Commutation

Explanation:

  • Interpoles are small poles compared to the main poles and are placed in the Interpolar region between the main poles and they are connected in series with the armature winding.
  • Their purpose is to counterbalance the armature reaction due to the flow of current in the armature winding.
  • Interpole’s primary functions are
    1. It produces a counter flux on the coil which is undergoing commutation to nullify the REACTANCE VOLTAGE.
    2. It also nullifies the armature flux in the inter-polar region automatically.

 

Ques 95. The commutator segments of the dc machine are made of

  1. Tungsten
  2. Hard-drawn copper
  3. Soft copper
  4. Electrolytic copper

Answer.2. Hard-Drawn copper

Explanation:

Commutators require periodic maintenance such as brush replacement, therefore, to reduce wear and tear commutator segment s of dc machines are made up of hard-drawn copper.

 

Ques 96. Which of the following is a speed control method for a three-phase squirrel cage induction motor?

  1. Plugging method
  2. Star-delta switch method
  3. Pole-changing method
  4. Centrifugal clutch method

Answer.3. Pole-changing method

Explanation:

  • The pole changing method is used in squirrel cage IM because like slip ring IM we can’t add external resistance in the rotor to reduce speed.
  • In this method, it is possible to have one or two speeds by changing the number of poles. This is possible by changing the connection of the stator winding with the help of simple switching.

 

Ques 97. In the circuit, as shown, the voltage measured between A, B is found to be 70 V. Value of M is

Ques 98.  Two coupled coils, connected in series, have an equivalent inductance of 16 mH or 8 mH depending on the connection. The mutual inductance between the coils is

  1. 12 mH
  2. 8 √2 mH
  3. 4 mH
  4. 2 mH

Answer.4. 2mH

Explanation:

The overall inductance of 2 coil L1 and L2 connected in series with mutual inductance aiding self-inductance L1 with mutual inductance opposing  self-inductance L2  then the mutual inductance M is given as

1/2 (L1 – L2)

1/2 (16 – 8)

= 2mH

Ques 99. Tesla is the unit of

  1. Electric flux density
  2. Magnetic flux intensity
  3. Electric field intensity
  4. Magnetic flux density

Answer.4. Magnetic Flux Density

Explanation:

  • The tesla (symbol T) is a derived unit of magnetic flux density (informally, magnetic field strength).
  • One tesla is equal to one weber per square meter.

 

Ques 100. Which one of the following is a valid value of the coefficient of coupling between two inductors?

  1. 1.414
  2. 0.9
  3. 1.732
  4. 17.32

Answer.2. 0.9

Explanation:

The value of the coefficient of coupling is always greater than 0 and less than 1, or 0% and 100% respectively. A coefficient of coupling of 0 would represent no coupling, and 1 would represent perfect coupling.

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