When is the rotor of a synchronous motor excited with direct current?
AWhen the rotor approaches its synchronous speed
BWhen the rotor approaches half of its synchronous speed
CAt its starting instant
DAfter the load is carried by the rotor
Electrical Engineering · Question Bank
Synchronous Motors
81 of 112 questions match · page 3 of 5. Answer in your head first, then check the solution.
Most of the synchronous motors are what type of power factor motors?
AUnity pf
B0.8 leading pf
C0.5 leading pf
D0.8 lagging pf
What do you call the angle between the synchronously rotating stator flux and the rotor poles of a synchronous motor?
ASynchronizing angle
BPower factor angle
CTorque angle
DSlip angle
What will happen to the pull out torque of a synchronous motor if the field excitation is increased?
AIt will increase
BIt will decrease
CIt will remain the same or unaffected
DIt will drop gradually to zero
In a synchronous machine, when the rotor speed becomes more than the synchronous speed during hunting, the damping bars will develop which of the following torque?
AInduction motor torque
BInduction generator torque
CSynchronous motor torque
DDC motor torque
A synchronous motor is connected to a supply line with its field winding shorted on themselves. What will happen?
AThe motor will not start
BThe motor will overheat and get burn
CThe motor will start and run as an induction motor
DThe motor will start as an induction motor and then run as a synchronous motor
What causes hunting in a synchronous motor?
AThe load fluctuates
BThe supply frequency varies
CBoth A and B
DNone of these
What is the reason why a synchronous motor with salient poles will still operate even if the field current is reduced to zero?
AIt is due to the rotating magnetic field of the stator
BIt is due to the rotating magnetic field of the rotor
CIt is due to the interlocking action between stator and rotor fields
DIt is due to the magnetization of the rotor poles by the stator magnetic field
Which of the following is used to reverse the direction of rotation of a synchronous motor?
AReverse the supply phase sequence
BReverse the polarity of the rotor poles
CReverse the current to the field windings
DAny of these
When a synchronous motor is on no-load, which one is true about the axes of its stator poles and its rotor poles?
AThe stator and rotor poles nearly coincide
BThe stator poles are slightly ahead of the rotor poles
CThe stator poles are slightly behind the rotor poles
DEither B or C, depending on the rotor excitation
A synchronous condenser is an over-excited synchronous motor running at which of the - following conditions?
AThe motor is running at rated load
BThe motor is running without a load
CThe motor is running at unity pf load
DThe motor is running below synchronous speed
A synchronous motor will be more stable in its operation when it is operated at which of the following power factor conditions?
AUnity pf
BLagging pf
CLeading pf
DEither A or
The negative phase sequence in a three-phase *synchronous motor exists in which of the following conditions?
AMotor is underload
BMotor is hot due to overloads
CMotor is under-excited
DMotor is supplied with unbalanced voltages
The armature current of a synchronous motor has large values at what level of excitation?
ALow excitation only
BHigh excitation only
CBoth low and high excitation
DNone of these
What will happen if a unity pf synchronous motor ' is operated at a leading pf?
AIt will produce more than the rated horsepower output
BIt will produce less than the rated horsepower output
CIt will draw more than the rated stator current
DIt will draw less than the rated stator current
When the rotor of a synchronous motor is running at synchronous speed, the voltage induced in the damper windings is how much?
AThe voltage is zero
BThe voltage is maximum
CThe voltage is minimum
DThe voltage is almost half of the supply voltage
When starting a synchronous motor, what will you do with the field winding?
AIt will be left open
BIt will be short circuited
CExcite the windings with a three-phase ac supply
DExcite the windings with dc supply
Where are the damper windings in a synchronous motor placed?
AIn the armature
BIn the pole face
CIn the rotor shaft
DIn the stator frame
What will be the excitation voltage of a synchronous motor when it is connected to an infinite bus while operating on a leading power factor?
AExcitation voltage will be less than the supply voltage
BExcitation voltage will be more than the supply voltage
CExcitation voltage will be equal to the supply voltage
DNo excitation voltage is applied
Which one is the best application of a three-phase synchronous motor?
AHigh speed load requirements
BHigh load torques are driven
CSpeed regulation is required
DConstant speed is absolutely maintained
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