Hi there, in this article we have listed 150+ important synchronous motor MCQ which will be useful for your competitive exam, interview, viva, and other exams. The questions and answers are all collected from different exams and interviews of previous years.
These listed MCQs are more than enough for your conceptual clarity and preparation about the synchronous motor.
Top 100 MCQs on synchronous motor
Q1. The speed of a synchronous motor can be varied by varying its
(A) excitation.
(B) supply voltage.
(C) supply frequency
(D) load.
Q2. For power factor correction, synchronous motors operate at
(A) no-load and greatly over-excited fields
(B) no-load and under-excited fields
(C) normal load with minimum excitation
(D) normal load with zero excitation
Q3. A synchronous motor can be started by
(A) pony motor
(B) D.C. compound motor
(C) providing damper winding
(D) any of the above
Q4. A three-phase synchronous motor will have
(A) no slip-rings
(B) one slip-ring
(C) two slip-rings
(D) three slip-rings
Q5. The direction of rotation of a synchronous motor can be reversed by reversing
(A) Current to the field winding
(B) Supply phase sequence
(C) polarity of rotor poles
(D) None of the above
Q6. In a synchronous motor, the breakdown torque is
(A) directly proportional to the applied voltage
(B) directly proportional to the square of the applied voltage
(C) inversely proportional to the applied voltage
(D) none of the above
Q7. A synchronous motor develops maximum power when the load angle is
(A) 45°
(B) 60°
(C) 90°
(D) 120°
Q8. What happens if the field winding of the synchronous motor is short-circuited?
(A) First, starts as induction motor then run as synchronous motor
(B) Not start
(C) Motor will burn out
(D) Run as induction motor
Q9. A synchronous motor which works on a leading power factor and does not drive a mechanical load is called as
(A) static condenser
(B) condenser
(C) synchronous condenser
(D) none of the above
Q10. If the field winding of an unloaded salient pole synchronous motor is open-circuited the motor will
(A) Burn
(B) Not start
(C) Run as induction Motor
(D) Run as a variable reluctance motor
Q11. The speed regulation of a synchronous motor is
(A)100%
(B) 50%
(C) 25%
(D) 0%
Q12. The maximum power developed in a synchronous motor will depend on
(A) the rotor excitation only
(B) the supply voltage only
(C) the rotor excitation and supply volt-age both
(D) the rotor excitation, supply voltage and maximum value of coupling angle (90°)
Q13. The negative phase sequence in a three-phase synchronous motor exists when the motor is
(A) Supplied with an unbalanced voltage
(B) Supplied with an unbalanced load
(C) Unbalanced system fault
(D) All of the above
Q14. The armature current of the synchronous motor
(A) has large values for low excitation only
(B) has large values for high excitation only
(C) has large values for low and high excitation
(D) any of the above
Q15. In 3 -Φ synchronous motor if one of the phases is short-circuited the motor will
(A) Run as before
(B) Overheated and eventually burn
(C) Not start
(D) Burn
Q16. If the synchronous motor, properly synchronized to the supply is running on no load and is having negligible loss then
(A) the stator current will be zero
(B) the stator current will be very small
(C) the stator current will be very high
(D) the back e.m.f. will be more than the supply voltage
(E) none of the above
Q17. For a synchronous motor, the breakdown torque will be proportional to
(A) Applied voltage V
(B) V2
(C) 1/V
(D) 1/V2
Q18. The coupling angle or load angle of synchronous motor is defined as the angle between the
(a) rotor and stator teeth
(b) rotor and the stator poles of opposite polarity
(c) rotor and the stator poles of the same polarity
(d) none of the above
Q19. In a synchronous motor, during hunting when the rotor speed exceeds the synchronous speed then damper bar develop
(A) Induction generator torque
(B) Harmonic
(C) DC motor torque
(D) Synchronous motor torque
(E) Show Explanation
Q20. A synchronous motor can be used as a synchronous capacitor when it is
(A) under-loaded
(B) over-loaded
(C) under-excited
(D) over-excited
Q21. A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased
(a) power factor as well as armature current will decreases
(b) power factor, as well as armature current, will increase
(c) power factor will increase but armature current will decrease
(d) power factor will decrease and armature current will increase
Q22. Mostly, synchronous motors are of
(a) alternator type machines
(6) induction type machines
(c) salient pole type machines
(d) smooth cylindrical type machines
Q23. The synchronous motor is not inherently self-starting because
(a) the force required to accelerate the rotor to the synchronous speed in an instant is absent
(b) the starting device to accelerate the rotor to near synchronous speed is absent
(c) a rotating magnetic field does not have enough poles
(d) the rotating magnetic field is produced by only 50 Hz frequency currents
Q24. As the load is applied to a synchronous motor, the motor takes more armature current because
(a) the increased load has to take more current
(b) the rotor by shifting its phase backward causes the motor to take more current
(c) the back e.m.f. decreases causing an increase in motor current
(d) the rotor strengthens the rotating field causing more motor current
Q25. Synchronous motor always runs at
(a) the synchronous speed
(b) less than synchronous speed
(c) more than synchronous speed
(d) none of the above
Q26. An over-excited synchronous motor takes
(a) leading current
(b) lagging current
(c) both (a) and (b)
(d) none of the above
Q27. The working of a synchronous motor is similar to
(a) gear train arrangement
(b) transmission of mechanical power by shaft
(c) distribution transformer
(d) turbine
(e) none of the above
Q28. The minimum armature current of the synchronous motor corresponds to operation at
(a) zero power factor leading
(b) unity power factor
(c) 0.707 power factor lagging
(d) 0.707 power factor leading
Q29. In a synchronous motor, the magnitude of stator back e.m.f. £& depends on
(a) d.c. excitation only
(b) speed of the motor
(c) load on the motor
(d) both the speed and rotor flux
Q30. In a synchronous motor, at no-load condition, and with normal excitation the armature current drawn by a synchronous motor is
(A) Zero
(B) Lagging applied voltage by 90°
(C) Leading Applied voltage by 90°
(D) In phase with applied voltage
Q31. The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as
(a) locked-rotor torque
(b) synchronous torque
(c) pull up torque
(d) reluctance torque
Q32. Exciters of synchronous machines are
(a) d.c. shunt machines
(b) d.c. series machines
(c) d.c. compound machines
(d) any of the above
Q33. In a synchronous motor, hunting occurs due to______
(A) Over excitation
(B) Periodic variation
(C) Over-loading for long periods
(D) Small and constant load
Q34. If the load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is
(a) 2
(b) 3
(c) 4
(d) 6
Q35. For V-curves for a synchronous motor the graph is drawn between
(a) field current and armature current
(b) terminal voltage and load factor
(c) power factor and field current
(d) armature current and power factor
Q36. The back e.m.f. of a synchronous motor depends on
(a) speed
(b) load
(c) load angle
(d) all of the above
Q37. A synchronous motor can operate at
(a) lagging power factor only
(6) leading power factor only
(c) unity power factor only
(d) lagging, leading, and unity power factors
Q38. In a synchronous motor which loss varies with load?
(a) Windage loss
(b) Bearing friction loss
(c) Copper loss
(d) Core loss
Q39. A synchronous motor can be made self-starting by providing
(a) damper winding on rotor poles
(b) damper winding on the stator
(c) damper winding on the stator as well as rotor poles
(d) none of the above
Q40. The oscillations in a synchronous motor can be damped out by
(a) maintaining constant excitation
(b) running the motor on leading power factors
(c) providing damper bars in the rotor pole faces
(d) oscillations cannot be damped
Q41. The shaft of the synchronous motor is made of
(a) mild steel
(b) chrome steel
(c) alnico
(d) stainless steel
Q42. When the field of a synchronous motor is under-excited, the power factor will be
(a) leading
(b) lagging
(c) unity
(d) zero
Q43. The speed regulation of a synchronous motor is always
(a) 1%
(b) 0.5%
(c) positive
(d) zero
Q44. The percentage slip in the case of a synchronous motor is
(a) 1%
(b) 100%
(c) 0.5%
(d) zero
Q45. The operating speed of a synchronous motor can be changed to a new fixed value by
(a) changing the load
(b) changing the supply voltage
(c) changing frequency
(d) using brakes
Q46. A synchronous motor will always stop when
(a) supply voltage fluctuates
(b) load in motor varies
(c) excitation winding gets disconnected
(d) supply voltage frequency changes
Q47. When the load on an over-excited or under excited synchronous*motor is increased, the rate of change of its armature current as compared with that of power factor is
(a) more
(b) less
(c) equal
(d) twice
Q48. The rotor copper losses, in a synchronous motor, are met by
(a) d.c. source
(b) armature input
(c) motor input
(d) supply lines
Q49. The maximum power developed in a synchronous motor occurs at a coupling angle of
(a) 30°
(b) 60°
(c) 90°
(d) 180°
Q50. When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor
(a) remains the same as the original value
(b) decreases to half the original value
(c) tends to become zero
(d) increases to two times the original value
Q51. In which of the following motors the stator and rotor magnetic field rotate at the same speed?
(a) Universal motor
(b) Synchronous motor
(c) induction motor
(d) Reluctance motor
Q52. Synchronous motors are
(a) not-self starting
(b) self-starting
(c) essentially self-starting
(d) none of the above
Q53. The standard full-load power factor ratings for synchronous motors are
(a) Zero or 0.8 leading
(b) unity or 0.8 lagging
(c) unity or 0.8 leading
(d) unity or zero
Q54. A synchronous motor running with normal excitation adjusts to load increases essentially by an increase in
(a) back e.m.f.
(b) armature current
(c) power factor
(d) torque angle
Q55. A synchronous motor has a better power factor as compared to that of an equivalent induction motor. This is mainly because
(a) synchronous motor has no-slip
(b) stator supply is not required to produce a magnetic field
(c) mechanical load on the rotor remains constant
(d) synchronous motor has large airgap
Q56. A synchronous motor working at leading power factor can be used as
(a) voltage booster
(b) phase advancer
(c) noise generator
(d) mechanical synchronizer
Q57. Slip rings are usually made of
(a) carbon or graphite
(b) brass or steel
(c) silver or gold
(d) copper or aluminum
Q58. An over-excited synchronous motor is used for
(a) fluctuating loads
(b) variable speed loads
(c) low torque loads
(d) Power factor corrections
Q59. When the voltage applied to a synchronous motor is increased, which of the following will reduce?
(a) Stator flux
(b) Pull in torque
(c) Both (a) and (b)
(d) None of the above
Q60. The efficiency of a properly designed synchronous motor will usually fall in a range
(a) 60 to 70%
(6) 75 to 80%
(c) 85 to 95%
(d) 99 to 99.5%
Q61. To limit the operating temperature an electrical machine should have proper
(a) voltage rating
(b) current rating
(c) power factor
(d) speed
Q62. Slip-rings in a synchronous motor carry
(a) direct current
(b) alternating current
(c) no current
(d) all of the above
Q63. A synchronous machine with a large air gap has
(a) a higher value of stability limit
(6) a small value of inherent regulation
(c) a higher synchronizing power which makes the machine less sensitive to load variations
(d) all of the above
Q64. The armature current of the synchronous motor has higher values for
(a) high excitation only
(b) low excitation only
(c) both (a) and (b)
(d) none of the above
Q65. In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately
(a) one-third
(b) twice
(c) thrice
(d) six times
(e) nine times
Q66. The angle between the rotating stator flux and rotor poles is called _____ angle.
(a) torque
(b) obtuse
(c) synchronizing
(d) power factor
Q67. Which of the following methods is used to start a synchronous motor?
(a) Damper winding
(b) Star-delta starter
(c) Damper winding in conjunction with star-delta starter
(d) Resistance starter in the armature circuit
Q68. When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop
(a) inductor motor torque
(b) induction generator torque
(c) synchronous motor torque
(d) d.c. motor toque
(e) none of the above
Q69 An important advantage of a synchronous motor overwound round induction motor is that
(a) its power factor may be varied at will
(b) its speed is independent of supply frequency
(c) its speed may be controlled more easily
(d) none of the above
Q70. The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of
(a) zero degree
(b) two degrees
(c) five degrees
(d) ten degrees
Q71. The power factor of a synchronous motor is unity when
(a) the armature current is maximum
(b) the armature current is minimum
(c) the armature current is zero
(d) none of the above
Q72. Change of D.C. excitation of a synchronous motor changes
(a) applied voltage of the motor
(b) motor speed
(c) power factor of power drawn by the motor
(d) any of the above
(e) all of the above
Q73. While starting a synchronous motor by induction motor action, the field winding is usually
(a) connected to D.C. supply
(b) short-circuited by low resistance
(c) kept open-circuited
(d) none of the above
Q74. Which of the following motors will be used in electric clocks?
(a) D.C. shunt motor
(b) D.C. series motor
(c) A.C. induction motor
(d) A.C. synchronous motor
Q75. If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from the constant voltage supply, its field excitation is increased, then its power factor
(a) become more
(b) become less
(c) remain constant
(d) none of the above
Q76. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at the lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will
(a) remain same
(b) go down
(c) improve
(d) none of the above
Q77. 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
(a) induction motor torque in field winding
(b) induction motor torque in damper winding
(c) eddy current and hysteresis torque in pole faces
(d) reHetance motor torque due to saliency of the rotor
(e) all of the above methods
Q78. Armature of a synchronous machine is
(a) of reducing number of slip rings on the rotor
(b) armature is associated with large power as compared to the field circuits
(c) of difficulty of providing high voltage insulation on the rotor
(d) all of the above reasons
Q79. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to
(a) increase in both armature current and power factor angle
(b) increase in back e.m.f. but decrease in armature current
(c) increase in both armature current and power factor which is lagging
(d) increase in torque angle but a decrease in back e.m.f.
Q80. For a synchronous motor, the maximum value of torque developed at an angle of
(A) 120 degree
(B) 45 degree
(C) 0 degree
(D) 90 degree
Q81. The construction of a synchronous motor resembles
(a) a series motor
(b) an induction motor
(c) an alternator
(d) a rotary converter
Q82. If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will
(a) stop
(b) run as induction motor
(c) function as static condenser
(d) burn with dense smoke
Q83. What happens to the synchronous motor when the excitation of an unloaded salient-pole synchronous motor suddenly gets disconnected?
(A) Run as a variable reluctance motor
(B) Run as the induction motor
(C) Motor will stop
(D) None of the above
Q84. A synchronous motor working at leading power factor can be used as
(A) Synchronous induction motor
(B) Static condenser
(C) Synchronous condenser
(D) None of the above
Q85. Which of the following motor is not self-starting?
(A) Wound rotor induction motor
(B) Squirrel cage induction motor
(C) DC series motor
(D) Synchronous motor
Q86. The duration of sudden short-circuit test on a synchronous motor is usually about
(a) one hour
(b) one minute
(c) one second
(d) none of the above
Q87. The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as
(a) pull-up torque
(b) pull-in torque
(c) pull-out torque
(d) none of the above
Q88. A synchronous machine with a low value of short-circuit ratio has
(a) lower stability limit
(6) high stability limit
(c) good speed regulation
(d) good voltage regulation
(e) none of the above
Q89. Which of the following motor can operate on lagging as well as leading power factor?
(a) DC series motor
(b) Wound rotor induction motor
(c) Squirrel cage induction motor
(d) Synchronous motor
Q90. A synchronous motor is switched on to supply with its field windings short-circuited. It will run as
(a) DC series motor
(b) Induction motor
(c) First induction motor then runs as a synchronous motor
(d) None of the above
Q91. In a synchronous motor, the magnitude of back emf depends on________
(a) Flux density
(b) DC excitation
(c) Load on the motor
(d) Speed of the rotor
Q92. The main reason for embedding the damper winding in the pole face is to
(a) Eliminate losses on account of air friction
(b) Reduce bearing friction
(c) Eliminate hunting and provide starting torque
(d) Eliminate air friction
Q93. A 3 phase, salient pole synchronous motor is fed from an infinite bus and is running at no load. Now if the field current of the motor is reduced to zero then the
(a) Motor will run at synchronous speed
(b) Motor will stop
(c) Motor will run at low speed
(d) Motor will burn
Q94. Which of the following losses, in a synchronous motor, does not vary with load?
(a) Windage loss
(b) Copper losses
(c) Any of the above
(d) None of the above
Q95. The size of a synchronous motor decreases with the increase in
(a) flux density
(b) horse power rating
(c) speed
(d) all of the above
Q96. Which of the following losses is not dissipated by the stator core surface in a synchronous motor?
(a) Eddy current losses in the conductors
(b) Iron losses in the stator
(c) Copper losses in the slot portion of the conductors
(d) Windage losses
(e) None of the above
Q97. The maximum value of torque that a synchronous motor, can develop without losing its synchronism, is known as
(A) Pull out torque
(B) Breaking torque
(C) Slip torque
(D) Non-synchronizing torque
Q98. What is the ratio of no-load speed to full load speed of a 200 kVA, 12 poles, 2200 V, 3 phase, 60 Hz synchronous motor?
(A) Infinite
(B) 1
(C) 1.1
(D) 1.21
Q99. In a synchronous motor if the back emf generated in the armature at no load is approximately equal to the applied voltage, then the
(A) The torque generated is maximum
(B) Motor is fully loaded
(C) Excitation is 100%
(D) No Excitation at all
Q100. In a synchronous motor, the armature current has large values for
(A) High excitation
(B) Low excitation
(C) Both high and low excitation
(D) None of the above
More Synchronous motor MCQ
Q1. The construction of a synchronous motor resembles which of the following machine
(a) Differential compound motor
(b) Alternator
(c) DC series motor
(d) Induction motor
Q2. A synchronous motor connected to infinite bus bars has a constant full load, 100% excitation, and unity power factor. On changing the excitation only, the armature current will have
(a) no change of power factor
(b) lagging power factor with over-excitation
(c) leading power factor with under-excitation
(d) leading power factor with over-excitation
Q3. Which of the following motors is non-self starting?
(a) D.C. series motor
(b) Synchronous motor
(c) Squirrel cage induction motor
(d) Wound round induction motor
Q4. In a synchronous motor it the back e.m.f. generated in the armature at no-load is approximately equal to the applied voltage, then
(a) the motor is said to be fully loaded
(b) the torque generated is maximum
(c) the excitation is said to be zero percent
(d) the excitation is said to be hundred percent
Q5. In a synchronous motor, the damping winding is generally used to
(a) prevent hunting and provide the starting torque
(b) reduce the eddy currents
(c) provide starting torque only
(d) reduce noise level
(e) none of the above
Q6. If the field of a synchronous motor is under excited, the power factor will be
(a) zero
(b) unity
(c) lagging
(d) leading
Q7. The back e.m.f. in the stator of a synchronous motor depends on
(a) number of poles
(b) flux density
(c) rotor speed
(d) rotor excitation
(e) none of the above
Q8. The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as
(a) slip torque
(b) pull-out torque
(c) breaking torque
(d) synchronising torque
Q9. In a synchronous motor, the armature current has large values for
(a) high excitation only
(b) low excitation only
(c) both high and low excitation
(d) none of the above
Q10. The synchronous motors are not self-starting because
(A) The direction of torque on the rotor reverses after every half cycle.
(B) Slip is not present in synchronous machine
(C) Starting winding is not present in the synchronous machine
(D) DC excitation is used
Q11. Due to which one of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current?
(A) Low short circuit ratio
(B) High field current
(C) High core losses
(D) Low field current
Q12. The net armature voltage of a synchronous motor is equal to the
(a) vector sum of Eb and V
(b) arithmetic sum of Eb and V
(c) arithmetic difference of Eb and V
(d) vector difference of Eh and V
Q13. The ratio of starting torque to running torque in a synchronous motor is
(a) zero
(b) one
(c) two
(d) infinity
Q14. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
(a) load on the motor
(b) d.c. excitation only
(c) both the speed and rotor flux
(d) none of the above
Q15. A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed
(a) the motor will stop
(b) the motor continue to run in the same direction
(c) the winding of the motor will burn
(d) the motor will run in the reverse direction
(e) none of the above
Q16. The magnitude of field flux in a 3-phase synchronous motor
(a) remains constant at all loads
(b) varies with speed
(c) varies with the load
(d) varies with power factor
Q17. The torque angle, in a synchronous motor, is the angle between
(a) the supply voltage and the back e.m.f.
(b) magnetizing current and back e.m.f.
(c) the rotating stator flux and rotor poles
(d) none of the above
Q18. Hunting in a synchronous motor cannot be due to
(a) windage friction
(b) variable load
(c) variable frequency
(d) variable supply voltage
Q19. By which of the following methods the constant speed of a synchronous motor can be changed to a new fixed value?
(a) By changing the supply frequency
(b) By interchanging any two phases
(c) By changing the applied voltage
(d) By changing the load.
Q20. In a synchronous motor, V-curves represent the relation between
(a) armature current and field current
(b) power factor and speed
(c) field current and speed
(d) field current and power factor
Q21. In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be
(a) 3000 r.p.m.
(b) 1500 r.p.m.
(c) 750 r.p.m.
(d) none of the above
Q22. When a synchronous motor is connected to an infinite bus, while. operating on the leading power factor.
(A) Excitation voltage will be independent of the supply voltage.
(B) Excitation voltage will be more than the supply voltage.
(C) Excitation voltage will be less than the supply voltage.
(D) Excitation voltage will be equal to the supply voltage.
Q23. Which of the following losses is not dissipated by the stator core surface in a synchronous motor?
(A) Eddy current losses in the conductors
(B) Iron losses in the stator
(C) Windage losses.
(D) Copper losses in the slot portion of the conductors
Q24. The direction of rotation of the synchronous motor can be reversed by reversing
(A) Field winding
(B) The polarity of the rotor poles
(C) Supply phase sequence
(D) None of the above
Q25. In synchronous motor out of the following losses, which one will have the highest proportion?
(A) Stator copper losses
(B) Iron losses
(C) Eddy current losses
(D) Friction and windage losses
Q26. In a 3-phase synchronous motor, the negative phase sequence exists when the motor is
(a) supplied with unbalanced voltage
(b) under-loaded
(c) over-loaded
(d) none of the above
Q27. In a synchronous motor, damper windings are provided on
(a) stator frame
(b) rotor shaft
(c) pole faces
(d) none of the abovE
Q28. The induced e.m.f. in a synchronous motor working on leading power factor will be
(a) more than the supply voltage
(b) less than the supply voltage
(c) equal to the supply voltage
Q29. The speed of a synchronous motor
(A) Increases as load increases
(B) Adjusts itself to new equilibrium speed whenever load changes.
(C) Always remains constant
(D) Reduces as load increases
Q30. In a synchronous motor if the saturation is neglected, then the short circuit ratio (SCR) will be related to the synchronous reactance (xd) as
(A) SCR = 1/(Xd)
(B) SCR = 1/(Xd)2
(C) SCR = Xd
(D) SCR = (Xd)2
Q31. Which of the following devices can be used as a phase advancer?
(A) Synchronous motor working at lagging power factor
(B) Synchronous motor working at leading power factor
(C) Squirrel cage induction motor
(D) Slip ring induction motor
Q32. In a synchronous machine is called a doubly excited machine because
(A) It can be over-excited
(B) It needs twice the normal exciting current
(C) It has two sets of rotor poles
(D) Both its rotor and stator are excited
Q33. The maximum speed variation in a 3-phase synchronous motor is
(A) 5%
(B) Zero
(C) 10%
(D) 3%
Q34. If the field of a synchronous motor is underexcited, the power factor will be
(A) Lagging
(B) Leading
(C) Zero
(D) None of the above
Q35. Which of the following is an unexcited single-phase synchronous motor?
(A) Reluctance motor
(B) Repulsion motor
(C) A.C. series motor
(D) Universal motor
Q36. The damper winding in a synchronous motor is provided for
(A) Starting torque only
(B) Reduce eddy currents
(C) Prevent hunting and provide the starting torque.
(D) Reduce the noise level
Q37. When the field of a synchronous motor is over excited, the power factor will be
(A) Leading
(B) Lagging
(C) Zero
(D) Unity
Q38. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then
(a) it runs at a slower speed
(b) the motor stops
(c) it continues to run at the same speed
(d) it runs at a very high speed
Q39. In which of the following motors the stator and rotor fields rotate simultaneously?
(a) D.C. motor
(b) Reluctance motor
(c) Universal motor
(d) Synchronous motor
(e) Induction motor
Q40. The speed of a synchronous motor
(a) increases as the load increases
(b) decreases as the load decreases
(c) always remains constant
(d) none of the above
Q41. A rotory converter can also be run as a
(a) d.c. shunt motor
(b) d.c. series motor
(c) d.c. compound motor
(d) induction motor
(e) synchronous motor
Q42. The maximum speed variation in a 3-phase synchronous motor is
(a) 10 per cent
(b) 6 per cent
(c) 4 per cent
(d) 2. per cent
(e) zero
Q43. Which of the following resistances can be measured by conducting an insulation resistance test on a synchronous motor?
(a) Phase to Phase winding resistance
(b) Stator winding to earthed frame
(c) Rotor winding to earthed shaft
(d) All of the above
Q44. Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current?
(a) High field current
(b) Low short circuit ratio
(c) High core losses
(d) Low field current
Q45. In a synchronous motor, the maximum power developed depends on all of the following except
(a) rotor excitation
(b) maximum value of coupling angle
(c) direction of rotation
(d) supply voltage
Q46. A Synchronous motor can operate at
(A) Unity power factor
(B) Leading power factor
(C) Lagging power factor
(D) Leading as well as lagging power factor
Q47. The armature current of a synchronous motor has large values for
(A) High excitation only
(B) Both low and high excitation
(C) Low excitation only
(D) None of the above
Q48. A synchronous motor is a useful industrial machine due to its property of
(A) Improving the power factor
(B) Speed is constant
(C) Can always be adjusted to operate at the unity power factor
(D) All of the above
Q49. Synchronous capacitor is
(A) An ordinary static capacitor bank
(B) An over-excited synchronous motor driving mechanical load
(C) An over-excited synchronous motor without mechanical load
(D) All of the above
Q50. A synchronous motor is said to be ‘floating‘ when it operates at
(A) High load and variable supply voltage
(B) Pulsating load
(C) Varying load
(D) No load and without losses
Q51. A 3 phase synchronous motor is running clockwise. In case the direction of its field current is reversed then
(A) Winding of the motor will burn
(B) Motor will run in the reverse direction
(C) Motor will continue to run in the same direction
(D) Motor will stop
Q52. The rotor copper losses of the synchronous motor are met by
(A) Armature input
(B) Dc source
(C) Ac supply mains
(D) None of the above
Q53. If the load angle of 4 poles synchronous motor is 8o electrical its value in mechanical degree is
(A) 2
(B) 8
(C) 10
(D) 4
Q54. In a synchronous motor, the synchronizing power comes into action when
(A) Rotor speed is less than synchronous speed
(B) Rotor speed is more than synchronous speed
(C) Rotor speed is equal to synchronous speed
(D) Rotor speed is either less or more than synchronous speed
Q55. The magnitude of field flux in a 3-phase synchronous motor
(A) Remains constant at all loads
(B) Varies with speed
(C) Varies with load
(D) Varies with power factor
Q56. When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way
(a) augments it directly
(b) directly opposes it
(c) cross-magnetises it
(d) none of the above
Q57. Stability of a synchronous machine
(a) decreases with increase in its excitation
(b) increases with increase in its excitation
(c) remains unaffected with increase in excitation
(d) any of the above
Q58. The power factor of a synchronous motor is better than that of induction motor because
(a) stator supply is relieved of responsibility of producing a magnetic field
(b) mechanical load on the motor can be adjusted
(c) synchronous motor runs at synchronous speed
(d) synchronous motor has large air gap
Q59. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor
(a) will become more
(b) will become less
(c) will remain unchanged
(d) none of the above.
Q60. A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
(a) speed of the motor is reduced
(b) power factor is decreased
(c) Eb (back e.m.f.) becomes less than V (applied voltage)
(d) Er (net resultant voltage) in armature is increased
(e) none of the above
Q61. If one-phase of a 3-phase synchronous motor is short-circuited, motor
(a) will refuse to start
(b) will overheat in spots
(c) will not come upto speed
(d) will fail to pull into step
Q62. Synchronous motors are generally not self-starting because
(a) the direction of rotation is not fixed
(b) the direction of instantaneous torque reverses after half cycle
(c) startes cannot be used on these machines
(d) starting winding is not provided on the machines
Q63. In case one phase of a three-phase synchronous motor is short-circuited the motor will
(a) not start
(b) run at 2/3 of synchronous speed
(c) run with excessive vibrations
(d) take less than the rated load
Q64. A pony motor is basically a
(a) small induction motor
(b) D.C. series motor
(c) D.C. shunt motor
(d) double winding A.C./D.C. motor
Q65. A synchronous motor can develop synchronous torque
(a) when under loaded
(b) while over-excited
(c) only at synchronous speed
(d) below or above synchronous speed
Q66. Under which of the following conditions hunting of synchronous motor is likely to occur ?
(a) Periodic variation of load
(b) Over-excitation
(c) Over-loading for long periods
(d) Small and constant load
Q67. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected
(a) the motor stops
(b) it runs as a reluctance motor at the same speed
(c) it runs as a reluctance motor at a lower speed
(d) none of the above
Q68. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as
(a) V
(b) V312
(c) V2
(d) 1/v
Q69. The power developed by a synchronous motor will be maximum when the load angle is
(a) zero
(b) 45°
(c) 90°
(d) 120°
