Railways NTPC (Technical Ability) Electrical Machine Question Bank

done Electrical Machine Total Questions - 50

• question_answer1)
  For an induction motor, operating at a slip s, the ratio of gross power output to air gap power is equal to
  A)
                                                                               \[{{(1-s)}^{2}}\]                         done clear
  B)
                                                                                      \[(1-s)\]   done clear
  C)
                                                                                  \[\sqrt{(1-s)}\]                         done clear
  D)
                                                                                  \[(1-\sqrt{s})\]   done clear
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• question_answer2)
  Which three phase connection can be used in a transformer to introduce a phase difference of 30° between its output and corresponding input time voltages?
  A)
   Star - Star                     done clear
  B)
   Star - Delta done clear
  C)
   Delta - Delta done clear
  D)
   Delta - Zigzag done clear
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• question_answer3)
  The equivalent circuit of a transformer has leakage reactance?s                                                                   \[{{X}_{1}},\,\,X{{'}_{2}}\] and magnetizing reactance                                                                              \[{{X}_{M}}\] , Their magnitudes satisfy
  A)
                                                           \[{{X}_{1}}>>X{{'}_{2}}>>{{X}_{M}}\]             done clear
  B)
                                                            \[{{X}_{1}}=X{{'}_{2}}<<{{X}_{M}}\] done clear
  C)
                                                      \[{{X}_{1}}\approx X{{'}_{2}}>>{{X}_{M}}\]             done clear
  D)
                                                        \[{{X}_{1}}\approx X{{'}_{2}}<<{{X}_{M}}\] done clear
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• question_answer4)
  A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is
  A)
   magnetizing                   done clear
  B)
   demagnetizing done clear
  C)
   cross-magnetizing           done clear
  D)
   ineffective done clear
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• question_answer5)
  For a single phase capacitor start induction motor which of the following statements is valid?
  A)
   The capacitor is used for power factor improvement. done clear
  B)
   The direction of rotation can be changed by reversing the main winding terminals. done clear
  C)
   The direction of rotation cannot be changed. done clear
  D)
   The direction of rotation can be changed by interchanging the supply terminals. done clear
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• question_answer6)
  In a transformer, zero voltage regulation at full load is
  A)
   not possible done clear
  B)
   possible at unity power factor load done clear
  C)
   possible at leading power factor load done clear
  D)
   possible at lagging power factor load done clear
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• question_answer7)
  The DC motor, which can provide zero-speed regulation at full load without any controller, is
  A)
   series                done clear
  B)
   shunt done clear
  C)
   cumulative compound done clear
  D)
   differential compound done clear
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• question_answer8)
  A 4-point starter is used to start and control the speed of a
  A)
   dc shunt motor with armature resistance control done clear
  B)
   dc shunt motor with field weakening control done clear
  C)
   dc series motor done clear
  D)
   dc compound motor done clear
  View Solution play_arrow

• question_answer9)
  A negative sequence relay is commonly used to protect
  A)
   an alternator                 done clear
  B)
   a transformer done clear
  C)
   a transmission line          done clear
  D)
   a bus bar done clear
  View Solution play_arrow

• question_answer10)
  The slip of an induction motor normally does not depend on
  A)
   rotor speed                    done clear
  B)
   synchronous speed done clear
  C)
   shaft torque                   done clear
  D)
   core-loss component done clear
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• question_answer11)
    Leakage flux in an induction motor is
  A)
   flux that leaks through the machine done clear
  B)
   flux that links both stator and rotor windings done clear
  C)
   flux that links none of the windings done clear
  D)
   flux that links the stator winding or the rotor winding but not both done clear
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• question_answer12)
  In dc machines, the armature windings are placed on the rotor because of the necessity for
  A)
   electromechanical energy conversion done clear
  B)
   generation of voltage done clear
  C)
   commutation done clear
  D)
   development of torque done clear
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• question_answer13)
  m normal dc machines operating at full-load conditions, the most powerful electromagnet is
  A)
   field winding done clear
  B)
   armature winding done clear
  C)
   inter pole winding done clear
  D)
   inter pole and compensating windings together done clear
  View Solution play_arrow

• question_answer14)
  In a synchronous generator operating at zero power factor lagging, the effect of armature reaction is
  A)
   magnetizing done clear
  B)
   demagnetizing done clear
  C)
   cross-magnetizing done clear
  D)
   both magnetizing and cross-magnetizing done clear
  View Solution play_arrow

• question_answer15)
  In double delta transformation, a double delta refers to the case where there are two delta transformations in
  A)
    Parallel done clear
  B)
   Series done clear
  C)
   Both series and parallel done clear
  D)
   Neither series nor parallel done clear
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• question_answer16)
  Most familiar application of zig-zag transformer is as
  A)
   Ground reference on an ungrounded system done clear
  B)
   Converting single phase to two phase done clear
  C)
   Reducing harmonics done clear
  D)
   All of these done clear
  View Solution play_arrow

• question_answer17)
  In Scott connection, the voltage across the teaser leads the mains by
  A)
   30 degree                      done clear
  B)
   60 degree done clear
  C)
   90 degree                      done clear
  D)
   120 degree done clear
  View Solution play_arrow

• question_answer18)
  The load carried by                                                                                    \[V-V\] connection is
  A)
   47.7% of the original load done clear
  B)
   57.7% of the original load done clear
  C)
   67.7% of the original load done clear
  D)
   87.7% of the original load done clear
  View Solution play_arrow

• question_answer19)
  The most commonly used connections for power systems as a step - up and step - down transformers are
  A)
   Star - delta, star - star   done clear
  B)
   Delta - star, star - delta done clear
  C)
   Star - star, delta - delta done clear
  D)
   Star - delta, delta - star done clear
  View Solution play_arrow

• question_answer20)
  For large low voltage transformers, the most commonly used connections are
  A)
   Star - star connection   done clear
  B)
   Delta - delta connection done clear
  C)
   V connection           done clear
  D)
   All of these done clear
  View Solution play_arrow

• question_answer21)
  Which test is also called as back to back test?
  A)
   Retardation test done clear
  B)
   Sumpner's test done clear
  C)
   Field test                       done clear
  D)
   Voltage drop test done clear
  View Solution play_arrow

• question_answer22)
  Open circuit test is to be performed on transformer to find
  A)
   Constant losses  done clear
  B)
   Copper losses done clear
  C)
   Both [a] and [b] done clear
  D)
   None of these done clear
  View Solution play_arrow

• question_answer23)
  In washing machines, most commonly used motors are
  A)
   Split phase induction motors done clear
  B)
   Slip ring induction motors done clear
  C)
   Capacitor start induction motors done clear
  D)
   Shaded pole induction motors done clear
  View Solution play_arrow

• question_answer24)
  The phase shift in a star-star connected three phase transformer is
  A)
   0 degree                        done clear
  B)
   30 degree done clear
  C)
   60 degree                      done clear
  D)
   120 degree done clear
  View Solution play_arrow

• question_answer25)
  In a single phase induction motor, the motor speed on no load is
  A)
   Almost equal to its synchronous speed done clear
  B)
   Less than its synchronous speed done clear
  C)
   Greater than synchronous speed done clear
  D)
   None of these done clear
  View Solution play_arrow

• question_answer26)
  In refrigerators the commonly used motors are
  A)
   Split phase induction motors done clear
  B)
   Capacitor induction motors done clear
  C)
   Shaded pole induction motors done clear
  D)
   None of these done clear
  View Solution play_arrow

• question_answer27)
  The constant voltage transformers are most commonly used in
  A)
   Personal computers        done clear
  B)
   Arc welding done clear
  C)
   Power distribution          done clear
  D)
   Electric cars done clear
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• question_answer28)
  For the parallel operation of three phase transformers, which among the following connection is not applicable?
  A)
                                                                                \[\Delta -\Delta\]  to\[YY\]            done clear
  B)
   \[YA\]to                                                                                    \[A-Y\] done clear
  C)
   \[YY\]to\[YY\]   done clear
  D)
   None of these done clear
  View Solution play_arrow

• question_answer29)
  The spit phase induction motor has
  A)
   Low starting current and high starting torque done clear
  B)
   Moderate starting current and moderate starting torque done clear
  C)
   Low starting current and moderate starting torque done clear
  D)
   Moderate starting current and low starting torque done clear
  View Solution play_arrow

• question_answer30)
  If field current is decreased in shunt dc motor, the speed of the motor
  A)
   remains same.   done clear
  B)
   increases.     done clear
  C)
   decreases.                     done clear
  D)
   none of the above. done clear
  View Solution play_arrow

• question_answer31)
  Eddy current loss will depends on
  A)
   Frequency                     done clear
  B)
   Flux density done clear
  C)
   Thickness                      done clear
  D)
   All of the above done clear
  View Solution play_arrow

• question_answer32)
  Thin laminations are used in a machine in order to reduce
  A)
   Eddy current losses         done clear
  B)
   Hysteresis losses done clear
  C)
   Both 1 and 2                 done clear
  D)
   Copper losses done clear
  View Solution play_arrow

• question_answer33)
  Hysteresis loop represents the area of
  A)
   Copper loss                   done clear
  B)
   Eddy current loss done clear
  C)
   Hysteresis loss   done clear
  D)
   Total iron losses done clear
  View Solution play_arrow

• question_answer34)
  Hysteresis loss will depends on
  A)
   Frequency                     done clear
  B)
   Flux density done clear
  C)
   Thickness                      done clear
  D)
   All of the above done clear
  View Solution play_arrow

• question_answer35)
  The emf induced in the dc generator armature winding is
  A)
   AC                               done clear
  B)
   DC done clear
  C)
   AC and DC                   done clear
  D)
   None of the above done clear
  View Solution play_arrow

• question_answer36)
  A DC generator without commentator is a
  A)
   AC generator                 done clear
  B)
   DC motor done clear
  C)
   DC generator                 done clear
  D)
   Induction motor done clear
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• question_answer37)
  In DC generators brushes are used for
  A)
   Collecting of current with minimum sparking done clear
  B)
   Collecting of voltage done clear
  C)
   Reduce eddy current loss done clear
  D)
   Convert ac armature current in to dc done clear
  View Solution play_arrow

• question_answer38)
  Which of the bearings and their uses are correct
  A)
   Ball bearings                                                                                     \[\to\] small machines done clear
  B)
   Roller bearings                                                                                     \[\to\] large machines done clear
  C)
   Both A and B done clear
  D)
   Neither 1 nor 2 done clear
  View Solution play_arrow

• question_answer39)
  Which of the following windings are necessary in case call dc machines?
  A)
   Closed winding  done clear
  B)
   Lap winding done clear
  C)
   Wave winding    done clear
  D)
   Open type winding done clear
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• question_answer40)
  DC machine windings are
  A)
   Full pitched                   done clear
  B)
   Short pitched done clear
  C)
   Either of these   done clear
  D)
   None of the above done clear
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• question_answer41)
  Inter pole winding is connected in____?
  A)
   Series with armature    done clear
  B)
   Series with main poles done clear
  C)
   Parallel with armature   done clear
  D)
   Parallel with main poles done clear
  View Solution play_arrow

• question_answer42)
  Equalizer rings are used in which of the followings?
  A)
   Lap winding                  done clear
  B)
   Wave winding done clear
  C)
   Both 1 and 2                 done clear
  D)
   None of the above done clear
  View Solution play_arrow

• question_answer43)
  For a dc machine shunt resistance and armature resistance values are
  A)
   High and high    done clear
  B)
   High and low done clear
  C)
   Low and low                 done clear
  D)
   Low and high done clear
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• question_answer44)
  The effect of _____ on main field flux is armature reaction?
  A)
   Armature mmf   done clear
  B)
   Armature current done clear
  C)
   Armature flux                done clear
  D)
   All of the above done clear
  View Solution play_arrow

• question_answer45)
  Shunt field of DC generators consists of?-?number (turns and ???- conductors respectively
  A)
   Large and thick  done clear
  B)
   Large and thin done clear
  C)
   Less and thick   done clear
  D)
   Less and thin done clear
  View Solution play_arrow

• question_answer46)
  No load saturation characteristics are plotted between
  A)
   No load voltage and field current done clear
  B)
   No load voltage and armature current done clear
  C)
   Short circuit current and field current done clear
  D)
   Short circuit current and armature current done clear
  View Solution play_arrow

• question_answer47)
  Cross magnetization affects which of the following?
  A)
   Commutation done clear
  B)
   Reduction in main field flux done clear
  C)
   Reduces the terminal voltage done clear
  D)
   Both 2 and 3 done clear
  View Solution play_arrow

• question_answer48)
  Series field of DC generators consists of ___number of turns and ____ conductors respectively
  A)
   Large and thick  done clear
  B)
   Large and thin done clear
  C)
   Less and thick   done clear
  D)
   Less and thin done clear
  View Solution play_arrow

• question_answer49)
  DC Motor Yoke is generally made of
  A)
   Wood               done clear
  B)
   Copper done clear
  C)
   Aluminium                    done clear
  D)
   Steel done clear
  View Solution play_arrow

• question_answer50)
  The transformer noise is mainly because of
  A)
   Cooling oil                    done clear
  B)
   Sinusoidal current done clear
  C)
   Magnetic flux                done clear
  D)
   All of the above done clear
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