SSC JE 2009 Electrical question paper with Solution

SSC previous year solved electrical papers / April 8, 2022

Ques 41. Which among the following magnetic materials has the highest energy product to make it a permanent magnet?

  1. Alnico✔
  2. Ferrite
  3. Samarium cobalt
  4. Cobalt – Iron alloy

The material which we choose for making magnet should have high coercivity. The high coercivity ensures that magnetism in the material stays for a longer time. As steel and alnico have high coercivity, therefore, they are used for making permanent magnets.

 

Ques 42. Which bridge is used to determine the audio frequency?

  1. Anderson bridge
  2. Desanty bridge
  3. Wien bridge✔
  4. Campbell bridge

220px Mostek Wiena.svg

The Wien bridge is one of many common bridges. Wien’s bridge is used for the precision measurement of capacitance in terms of resistance and frequency. It was also used to measure audio frequencies.

 

Ques 43. The direction of rotation of the 3-Φ induction motor is clockwise when it is supplied with the 3-Φ sinusoidal voltage having a phase sequence of A-B-C. For counterclockwise rotation, the phase sequence of the power supply should be

  1. B-C-A
  2. A-C-B✔
  3. C-A-B
  4. Any of the above

The direction of rotation of a 3 phase induction motor can be reversed by interchanging any two of the three motor supply lines.

The phase sequence of the three-phase voltage applied to the stator winding is A-B-C. If this sequence is changed to A-C-B, it is observed that the direction of rotation of the field is reversed i.e., the field rotates counterclockwise rather than clockwise. However, the number of poles and the speed at which the magnetic field rotates remain unchanged.

 

Ques 44. A single-phase transformer with a 2-kVA rating has a 480-V primary and a 120-V secondary. Determine the primary and secondary full-load currents of the transformer.

  1. 4.17 A and 16.67 A✔
  2. 10 A and 13. 2 A
  3. 6 A and 16.67 A
  4. Insufficient Data

Primary full-load current = VA rating / Primary Voltage

Primary full-load current = ( 2 kVA * 1 000 ) / 480 V = 4.17 A

Secondary full-load current = VA rating / Secondary Voltage

Secondary full-load current = ( 2 kVA * 1 000 ) / 120 V = 16.67 A

 

Ques 45.  The reactance offered by a capacitor to alternating current of frequency 50 Hz is 20 Q. If the frequency is increased to 100 Hz, reactance becomes _____ ohms.

  1. 5
  2. 7
  3. 10✔
  4. 15

Capacitive reactance

Xc = 1/2πf

Therefore XC1/XC2 = 2πf2/2πf1

= 20/XC2 =100/50

= XC2 = 10

 

Ques 46. The efficiency of a DC motor at maximum power is

  1. 90%
  2. 100%
  3. Around 80%
  4. Less than 50%✔

Why efficiency of D.C motor is below 50%

  • The DC motor develops maximum power when the Back EMF is half the applied voltage.
    Eb = V/2.
  • Practically it is not possible to develop an exact 50% of maximum power because in that case, the current would be much beyond the rated current of the motor.
  • Some of the energy is wasted in the form of heat and other losses.
  • Therefore motor efficiency is below 50%.

 

Ques 47. The purpose of providing dummy coils in a generator is

  1. To provide mechanical balance for the rotor✔
  2. To reduce eddy’s current loss
  3. To reduce hysteresis loss
  4. To increase the efficiency of the Generator

What is dummy coil?

  • 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 48. In repulsion motor direction of rotation of the motor

  1. Same as that of brush shift✔
  2. Independent of brush shift
  3. Opposite to that of brush shift
  4. None of the above

  • The direction of rotation is determined by the position of the brushes with respect to the magnetic field of the stator.
  • If the brushes are shifted clockwise from the main magnetic axis, the motor will rotate in a clockwise direction.
  • If the brushes are shifted counterclockwise from the main magnetic axis, the motor will rotate in a counter-clockwise direction

 

Ques 49. Stones are provided in the substation to

  1. To provide insulation
  2. To avoid Fire accidents during leakage of transformer oil
  3. To avoid growing plants and weed
  4. All of the above✔

  • Oil leakage takes place during operation or when changing the oil in the transformer. This oil spillage can catch fire is dangerous to the switchyard operation. So Stones is provided to protect from fire when oil spillage takes place.
  • On plain grounds when grass grows it will form moisture and cause damage to transmission lines and also cause current leakage. Stones eliminate the growth of small weeds and plants or grass inside the Substation.

 

Ques 50. Find the total diameter of the ACSR conductor with 2 layers and the diameter of each strand is 3?

  1. 9✔
  2. 10
  3. 5
  4. 15

The diameter of the ACSR  conductor can be calculated by

D = (2x – 1)d

where
x = number of layer
d = diameter of each strand

Therefore, the total diameter of the ACSR conductor

D = (2×2 -1)3
D = 9

For SSC JE 2017  Electrical paper with complete solution Click Here

For SSC JE 2015  Electrical paper with complete solution Click Here

For SSC JE 2014 (Evening shift)  Electrical paper with complete solution Click Here

For SSC JE 2014 (Morning shift)  Electrical paper with complete solution Click Here

For SSC JE 2013 Electrical paper with complete solution Click Here

For SSC JE 2012 Electrical paper with complete solution Click Here

For SSC JE 2011 Electrical paper with complete solution Click Here

For SSC JE 2010 Electrical paper with complete solution Click Here

For SSC JE 2009 Electrical paper with complete solution Click Here

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