The size of a synchronous motor is determined from which of the following?
AIts kVA rating
BIts kW rating
CIts kVAR rating
DIts power factor rating
Electrical Engineering · Question Bank
Synchronous Motors
81 of 112 questions match · page 2 of 5. Answer in your head first, then check the solution.
A synchronous motor fails to fall into synchronism after applying dc field current. Which one is the probable cause?
ALow field current
BHigh eddy current in the rotor
CToo much field current
DHigh losses in the core
The mechanical power developed by a synchronous motor is independent of which of the following?
AApplied voltage
BField excitation
CTorque angle
DSpeed
Which of the following defines a synchronous condenser?
AIt is an ordinary static capacitor
BIt is a synchronous generator that delivers a leading kVAR to a load
CIt is an over-excited synchronous motor driving a mechanical load
DIt is an over-excited synchronous motor running without a mechanical load
What will happen to the power factor correction capability of a loaded synchronous motor if its mechanical load is increased?
AIt will decrease
BIt will increase
CIt remains the same
DNone of these
When will a synchronous motor deliver reactive power?
AWhen the motor is over-excited
BWhen the motor is under-excited
CWhen the motor is excited normally
DWhen the motor is not excited
What is the operating cost of a leading pf synchronous motor in comparison to the operating cost of a unity pf synchronous motor?
AThe cost is more
BThe cost is less
CThe costs are approximately equal
DThe costs are unpredictable based from the given situations
While running, a synchronous motor is compelled to run at synchronous speed due to which of the following?
ADamper windings in its pole face
BInduced emf in the rotor field windings
CCompulsion due to Lenz’ law
DMagnetic locking between the stator and rotor
Which of the following statements will reverse the direction of rotation of a synchronous motor?
AInterchange any two stator lines
BReduce the dc field excitation
CChange the frequency of the supply voltage
DNone of these, since a synchronous motor always runs in one direction only
How do you call an unexcited single-phase synchronous motor?
AReluctance motor
BInduction motor
CUniversal motor
DRepulsion motor
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
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