Synchronous Motors MCQ

Test your knowledge with important Synchronous Motors MCQ and their applications. These MCQs are beneficial for competitive exams too. Explore 30 + more Synchronous Motors MCQs at Bissoy

• The size of a synchronous motor decreases with the increase in
  ক

  flux density

  খ

  horse power rating

  গ

  speed

  ঘ

  all of the above
• For power factor correction, synchronous motors operate at
  ক

  no-load and greatly over-excited fields

  খ

  no-load and under-excited fields

  গ

  normal load with minimum excitation

  ঘ

  normal load with zero excitation
• The rotor copper losses, in a synchronous motor, are met by
  ক

  d.c. source

  খ

  armature input

  গ

  motor input

  ঘ

  supply lines
• Riunting in a synchronous motor takes place
  ক

  When supply voltage fluctuates

  খ

  When load varies

  গ

  When power factor is unity

  ঘ

  Motor is under loaded
• An over excited synchronous motor is used for
  ক

  Fluctuating loads

  খ

  Variable speed loads

  গ

  Low torque loads

  ঘ

  Power factor corrections
• In case one phase of a three-phase synchronous motor is short-circuited the motor will
  ক

  Not start

  খ

  Run at $$\frac{2}{3}$$ of synchronous speed

  গ

  Run with excessive vibrations

  ঘ

  Take less than the rated load
• An important advantage of a synchronous motor over wound round induction motor is that
  ক

  its power factor may be varied at will

  খ

  its speed is independent of supply frequency

  গ

  its speed may be controlled more easily

  ঘ

  none of the above
• The power factor of a synchronous motor is better than that of induction motor because
  ক

  stator supply is relieved of responsibility of producing magnetic field

  খ

  mechanical load on the motor can be adjusted

  গ

  synchronous motor runs at synchronous speed

  ঘ

  synchronous motor has large air gap
• As the load is applied to a synchronous motor, the motor takes more armature current because
  ক

  The increased load has to take more current

  খ

  The rotor by shifting its phase backward causes motor to take more current

  গ

  The back e.m.f. decreases causing an increase in motor current

  ঘ

  The rotor strengthens the rotating field causing more motor current
• In a 3-phase synchronous motor, the negative phase sequence exists when the motor is
  ক

  supplied with unbalanced voltage

  খ

  under-loaded

  গ

  over-loaded

  ঘ

  none of the above
• The efficiency of a properly designed synchronous motor will usually fall in range
  ক

  60 to 70%

  খ

  75 to 80%

  গ

  85 to 95%

  ঘ

  99 to 99.5%
• The magnitude of field flux in a 3-phase synchronous motor
  ক

  remains constant at all loads

  খ

  varies with speed

  গ

  varies with the load

  ঘ

  varies with power factor
• In a synchronous motor which loss varies with load ?
  ক

  Windage loss

  খ

  Bearing friction loss

  গ

  Copper loss

  ঘ

  Core loss
• A synchronous machine with low value of short-circuit ratio has
  ক

  lower stability limit

  খ

  high stability limit

  গ

  good speed regulation

  ঘ

  good voltage regulation
• Mostly, synchronous motors are of
  ক

  alternator type machines

  খ

  induction type machines

  গ

  salient pole type machines

  ঘ

  smooth cylindrical type machines
• Change of D.C. excitation of a synchronous motor changes
  ক

  Applied voltage of the motor

  খ

  Motor speed

  গ

  Power factor of power drawn by the motor

  ঘ

  Any of the above
• The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as
  ক

  pull-up torque

  খ

  pull-in torque

  গ

  pull-out torque

  ঘ

  none of the above
• A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have
  ক

  no change of power factor

  খ

  lagging power factor with over-excitation

  গ

  leading power factor with under-excitation

  ঘ

  leading power factor with over-excitation
• A synchronous motor develops maximum power when load angle is
  ক

  45°

  খ

  60°

  গ

  90°

  ঘ

  120°
• In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then
  ক

  the motor is said to be fully loaded

  খ

  the torque generated is maximum

  গ

  the excitation is said to be zero per cent

  ঘ

  the excitation is said to be hundred per cent
• If the field of a synchronous motor is underexcited, the power factor will be
  ক

  zero

  খ

  unity

  গ

  lagging

  ঘ

  leading
• The induced e.m.f. in a synchronous motor working on leading power factor will be
  ক

  more than the supply voltage

  খ

  less than the supply voltage

  গ

  equal to the supply voltage

  ঘ

  none of the above
• When the field of a synchronous motor is under-excited, the power factor will be
  ক

  leading

  খ

  lagging

  গ

  unity

  ঘ

  zero
• A synchronous motor can develop synchronous torque
  ক

  when under loaded

  খ

  while over-excited

  গ

  only at synchronous speed

  ঘ

  below or above synchronous speed
• The shaft of synchronous motor is made of
  ক

  mild steel

  খ

  chrome steel

  গ

  alnico

  ঘ

  stainless steel
• The back e.m.f. in the stator of a synchronous motor depends on
  ক

  number of poles

  খ

  flux density

  গ

  rotor speed

  ঘ

  rotor excitation
• The back e.m.f. of a synchronous motor depends on
  ক

  speed

  খ

  load

  গ

  load angle

  ঘ

  all of the above
• The net armature voltage of a synchronous motor is equal to the
  ক

  vector sum of Eb and V

  খ

  arithmetic sum of Eb and V

  গ

  arithmetic difference of Eb and V

  ঘ

  vector difference of Eh and V
• If one-phase of a 3-phase synchronous motor is short-circuited, motor
  ক

  will refuse to start

  খ

  will overheat in spots

  গ

  will not come upto speed

  ঘ

  will fail to pull into step
• While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by
  ক

  induction motor torque in field winding

  খ

  induction motor torque in damper winding

  গ

  eddy current and hysteresis torque in pole faces

  ঘ

  reHetance motor torque due to saliency of the rotor

  ঙ

  all of the above methods