SSC JE DC Motor & DC Generator Solved Questions (2020)

SSC Topic wise Solved paper / March 15, 2022

Ques.81. The type of DC generator used for arc welding should be an (SSC-2011)

  1. Series generator
  2. Shunt generator
  3. Cumulatively compounded generator
  4. Differentially compounded generator

Answer.4. Differentially compounded generator

Explanation:-

The generator used for welding purposes should have high current and low voltage which can be obtained by the differential compound generator. Differentially compounded DC generator has drooping characteristics that mean if load current increases the net flux will decreases(i.e shunt field and series fields in opposition will increases)due to the demagnetization effect hence the Induced  EMF and terminal voltage decreases.

 

Ques.82. A 4-pole, 1200 rpm DC lap wound generator has 1520 conductors. If the flux per pole is 0.01 weber, the emf of generator is (SSC-2010)

  1. 608 V
  2. 304 V
  3. 152 V
  4. 76 V

Answer.2. 304 V

Explanation:-

EMF Of generator is given by

E = PΦZN/60A

Φ = flux per pole in Weber

P = number of poles

Z = total number of armature conductor

N = rotation speed of the armature in revolution per minute (r.p.m)

A = number of parallel paths (Number of parallel paths in lap winding, A = P)

E = (4 x 0.01 x 1520 x 1200)/(60 x 4)

= 304 V

 

Ques.83. The condition for a maximum power output from dc motor is (SSC-2010)

  1. Eb =V
  2. Eb = V/2
  3. Eb = 0
  4. Eb = V/√2

Answer.2. Eb = V/2

Explanation:-

Condition for maximum power

Mechanical power developed in the motor is

P = EbIa = VIa – Ia2Ra

solution 37

 

Ques.84. A 4-pole generator with 16 coils has a two-layer lap winding. The pole pitch is (SSC-2009)

  1. 32
  2. 16
  3. 8
  4. 4

Answer.3. 8

Explanation:-

Pole pitch = slots/pole

Pole pitch = slots/pole

Number of commutator segments = 16

Number of conductors or coil side = 16 x 2 = 32

Pole pitch = 32/4 = 8

 

Ques.85.  In lap winding, the number of brushes is always (SSC-2009)

  1. Double the number of poles
  2. Same as number of poles
  3. Half the number of poles
  4. Multiple of number of poles

Answer.2. Same as number of poles

Explanation:-

In a lap winding, the number of parallel paths, a, is always equal to the number of poles, p, and also to the number of brushes.

 

Ques.86. When the number of poles and number of armature conductor are fixed in the D.C generator then which winding will give Higher EMF? (SSC-2009)

  1. Wave Winding
  2. Lap Winding
  3. Either Lap or Wave Winding
  4. Depend upon the conductor

Answer.1. Wave Winding

Explanation:-

  • E.M.F Equation of d.c generator isEmf equation of dc generator
  • Now for Lap wound generator  A = P
    Where is A = Number of parallel path
    And P = Number of poles
  • Therefore in Lap wound D.C generator they canceled each other.
  • For Wave wound Generator A = 2
  • Therefore the E.M.F is greater in Wave wound D.C generator than Lap wound

 

Ques.87. The efficiency of the DC motor at maximum power is (SSC-2009)

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

Answer.4. Less than 50%

Explanation:-

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.88. The purpose of providing dummy coils in a generator is (SSC-2009)

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

Answer.1. To provide mechanical balance for the rotor

Explanation:-

What is the dummy coil?

  • Dummy coil is used with wave winding when the requirement of the winding is not met by the standard armature.
  • 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.

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