How do you call an unexcited single-phase synchronous motor?
AReluctance motor
BInduction motor
CUniversal motor
DRepulsion motor
Electrical Engineering · Question Bank
Synchronous Motors
112 questions · page 4 of 6. Answer in your head first, then check the solution.
For the same horsepower output, what is the efficiency of a synchronous motor in comparison to an induction motor?
AIt has lesser efficiency
BIt has more efficiency
CThey have approximately the same efficiencies
DAny one of these
Which one is the effect of the armature reaction in a synchronous motor at rated voltage and zero power factor leading?
ADemagnetizing
BMagnetizing
CCross magnetizing
DBoth magnetizing and cross magnetizing
What is the speed regulation of a synchronous motor?
AZero
BNegative number
CPositive number
DUndefined
In a synchronous motor, synchronizing power is needed in which of the following conditions?
AWhen rotor speed is equal to synchronous speed
BWhen rotor speed falls below the synchronous speed
CWhen rotor speed exceeds the synchronous speed
DWhen rotor speed either exceeds or falls below the synchronous speed
The armature current of a synchronous motor is minimum at which of the following power factor operation?
ALeading pf
BLagging pf
CUnity pf
DAny of these
An over-excited synchronous motor takes what type of current?
AUnity pf
BLagging pf
CLeading pf
DUnity, lagging or leading depending on the load
In a synchronous motor, as load is applied the motor takes more armature current. Why?
AThe rotor strengthens the rotating field causing more motor current
BThe rotor by shifting its phase backward causes the motor to take more current
CThe increased load has to take more current
DThe back emf decreases causing an increase in motor current
An over-excited synchronous motor is behaving like which of the following circuit elements?
ACapacitor
BInductor
CResistor
DSeries RLC
What is the purpose of the damper windings in a synchronous motor?
ATo provide starting torque only
BTo reduce the noise
CTo reduce the eddy current loss
DTo prevent hunting and provide the starting torque
A synchronous motor is cheaper than a 3-phase induction motor for which of the following load conditions?
ALow speed, high output
BHigh speed, high output
CHigh speed, low output
DNone of these
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
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
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