The synchronous motor is synchronized with the supply when
A) Rotor speed equals synchronous speed
B) Slip = 1
C) Slip = 0.5
D) Rotor frequency = supply frequency
A
-
The synchronizing power is maximum when
A) δ = 0°
B) δ = 90°
C) δ = 45°
D) δ = 180°
A
-
The synchronous motor may fall out of step when
A) Load exceeds pull-out torque
B) Frequency varies suddenly
C) Voltage drops
D) Any of the above
D
-
The static stability limit is given by
A) (EV/Xs) cos δ
B) (EV/Xs) sin δ
C) (EV/Xs)
D) None
A
-
The dynamic stability of a synchronous motor depends on
A) Damping torque
B) Synchronizing torque
C) Inertia constant
D) All
D
-
Synchronizing power coefficient is proportional to
A) cos δ
B) sin δ
C) tan δ
D) δ
A
-
The synchronizing torque helps to
A) Maintain constant speed
B) Restore synchronism after disturbances
C) Reduce field current
D) None
B
-
The pull-out torque occurs at
A) δ = 90°
B) δ = 0°
C) δ = 45°
D) δ = 180°
A
-
The reluctance torque is present in
A) Salient pole machines
B) Non-salient machines
C) Cylindrical rotor only
D) None
A
-
The reluctance torque is proportional to
A) sin 2δ
B) cos δ
C) sin δ
D) δ
A
-
The synchronizing power is given by
A) (EV/Xs) cos δ
B) (V²/Xs) cos δ
C) (EV/Xs) sin δ
D) (E²/Xs) sin δ
B
-
The synchronizing torque per mechanical degree is obtained by
A) Multiplying synchronizing power by 180/πNs
B) Dividing by ωs
C) Both A and B
D) None
A
-
The excitation system of a synchronous motor is designed for
A) DC excitation
B) AC excitation
C) Pulse excitation
D) None
A
-
Brushless excitation in synchronous motors is achieved using
A) Rotating rectifiers
B) Slip rings
C) Commutators
D) Capacitors
A
-
The field current of a synchronous motor is normally supplied by
A) DC generator
B) Exciter or rectifier
C) Inverter
D) Induction motor
B
-
The short-circuit ratio (SCR) is defined as
A) Ratio of field current for rated voltage to that for rated armature current
B) Ratio of synchronous reactance to resistance
C) Ratio of open-circuit to short-circuit current
D) None
A
-
Higher SCR in a synchronous motor means
A) Better voltage regulation
B) Greater stability
C) Larger air-gap
D) All
D
-
Low SCR implies
A) Poor voltage regulation
B) Less stability
C) Smaller air-gap
D) All
D
-
The synchronous motor stability improves with
A) Larger air-gap
B) Higher SCR
C) Lower reactance
D) All
D
-
The synchronizing torque increases with
A) Voltage
B) Excitation
C) Frequency
D) All
D
-
When a synchronous motor is over-excited, it operates as
A) Capacitor
B) Inductor
C) Resistor
D) Transformer
A
-
The phase angle between supply voltage and induced emf is
A) Load angle
B) Torque angle
C) Power angle
D) All
D
-
A synchronous motor working on leading pf is used for
A) Improving power factor
B) Speed control
C) Load torque variation
D) Voltage reduction
A
-
When load torque increases,
A) δ increases
B) δ decreases
C) δ constant
D) Speed decreases
A
-
The efficiency of a synchronous motor is high because
A) No rotor copper loss
B) No slip
C) Constant speed
D) All
D
-
The no-load current in a synchronous motor is
A) Small
B) Large
C) Depends on excitation
D) Zero
A
-
If a synchronous motor draws lagging current, it behaves as
A) Inductive load
B) Capacitive load
C) Resistive load
D) Generator
A
-
The field winding is located on
A) Rotor
B) Stator
C) Shaft
D) Frame
A
-
The armature winding is located on
A) Stator
B) Rotor
C) Shaft
D) End ring
A
-
The torque developed in synchronous motor is zero when
A) δ = 0°
B) δ = 90°
C) δ = 45°
D) δ = 30°
A
-
The synchronous reactance includes
A) Armature leakage reactance
B) Armature reaction reactance
C) Both A and B
D) None
C
-
The open-circuit characteristic (OCC) is plotted between
A) Field current and induced emf
B) Armature current and voltage
C) Speed and torque
D) Power and excitation
A
-
The short-circuit characteristic (SCC) is plotted between
A) Field current and short-circuit armature current
B) Voltage and current
C) Speed and torque
D) None
A
-
The Potier triangle method is used to determine
A) Voltage regulation
B) Armature reaction
C) Leakage reactance
D) All
D
-
The method of synchronizing a synchronous motor includes
A) Dark lamp method
B) Bright lamp method
C) Synchroscope method
D) All
D
-
When synchronizing lamps are dark, it indicates
A) Correct phase sequence and same frequency
B) Opposite phase
C) Wrong frequency
D) Wrong sequence
A
-
A synchroscope indicates
A) Phase difference between incoming and bus voltage
B) Frequency difference
C) Both A and B
D) None
C
-
For correct synchronization,
A) Voltages must be equal
B) Frequencies must be same
C) Phase sequence same
D) All
D
-
If frequency of incoming machine is higher, synchroscope rotates
A) Fast in the forward direction
B) Reverse direction
C) Stops
D) Flickers
A
-
Hunting can cause
A) Mechanical stress
B) Electrical oscillations
C) Noise
D) All
D
-
The damper winding prevents
A) Hunting
B) Vibration
C) Starting issues
D) All
D
-
If field excitation is lost suddenly, motor
A) Runs as induction motor
B) Stops
C) Burns
D) Overspeeds
A
-
The synchronous motor continues to run even if field fails because
A) Induced currents in damper winding
B) Residual magnetism
C) Supply voltage
D) None
A
-
The torque developed under field failure is
A) Induction torque
B) Reluctance torque
C) Synchronizing torque
D) None
A
-
The field failure protection relay trips motor by
A) Detecting low field current
B) Detecting overcurrent
C) Detecting overspeed
D) None
A
-
The excitation system controls
A) Power factor
B) Armature current
C) Voltage
D) All
D
-
Over-excitation leads to
A) Leading power factor
B) Lagging power factor
C) High current
D) None
A
-
Under-excitation leads to
A) Lagging power factor
B) Leading power factor
C) High voltage
D) None
A
-
The core loss in synchronous motor is
A) Proportional to V² and f
B) Proportional to I²
C) Constant
D) Negligible
A
-
The copper loss depends on
A) Armature current
B) Field current
C) Both A and B
D) None
C
-
Damper winding bars are made of
A) Copper or brass
B) Aluminum
C) Iron
D) Carbon
A
-
Damper winding helps during
A) Starting and hunting
B) Load variation
C) Excitation failure
D) None
A
-
A synchronous motor can start by
A) Using damper winding
B) Auxiliary motor
C) Variable frequency drive
D) All
D
-
During starting, the field winding is
A) Short-circuited
B) Energized
C) Open
D) Disconnected
A
-
A synchronous motor has maximum efficiency at
A) Full load and unity pf
B) No load
C) Half load
D) Leading pf
A
-
The load angle δ is proportional to
A) Load torque
B) Field current
C) Voltage
D) Frequency
A
-
The power developed by reluctance torque is proportional to
A) sin 2δ
B) sin δ
C) cos δ
D) tan δ
A
-
The pull-out torque is independent of
A) Load
B) Excitation
C) Supply voltage
D) Speed
D
-
The constant losses in a synchronous motor are
A) Iron, friction and windage losses
B) Copper losses
C) Eddy current
D) Hysteresis only
A
-
Synchronous motor can be used as
A) Synchronous condenser
B) Compensator
C) Power factor corrector
D) All
D
-
When synchronous motor is running under-excited, it behaves as
A) Inductive load
B) Capacitive load
C) Resistive
D) None
A
-
For unity pf, excitation emf equals
A) Supply voltage
B) Higher than supply
C) Lower than supply
D) None
A
-
At constant load, if excitation decreases, current
A) Increases
B) Decreases
C) Constant
D) Zero
A
-
The per-unit synchronous reactance affects
A) Voltage regulation
B) Stability
C) Power factor
D) All
D
-
The constant-speed characteristic is called
A) Mechanical characteristic
B) Electrical characteristic
C) Speed-torque curve
D) None
A
-
The stability of a synchronous motor improves with
A) Higher inertia
B) Lower reactance
C) High SCR
D) All
D
-
A synchronous condenser is operated
A) Without mechanical load
B) At no load
C) At over-excitation
D) All
D
-
The starting torque of synchronous motor is produced by
A) Damper winding
B) Field winding
C) Both
D) None
A
-
The field excitation voltage is generally
A) 110 V DC
B) 220 V DC
C) 24 V DC
D) 12 V DC
B
-
When connected to an infinite bus, speed of synchronous motor
A) Remains constant
B) Varies slightly
C) Increases
D) Decreases
A
-
When δ exceeds 90°, the motor
A) Loses synchronism
B) Speeds up
C) Reduces load
D) None
A
-
The motor power factor is
A) Lagging when under-excited
B) Leading when over-excited
C) Unity when normally excited
D) All
D
-
A synchronous motor with field current zero acts as
A) Induction motor
B) Transformer
C) Generator
D) None
A
-
For constant excitation, armature current depends on
A) Load
B) Supply voltage
C) Speed
D) None
A
-
The excitation is adjusted for
A) Desired pf
B) Constant voltage
C) Constant torque
D) None
A
-
At starting, field winding is kept short to
A) Avoid high voltage
B) Prevent damage
C) Induce starting torque
D) All
D
-
The synchronous impedance is
A) √(Ra² + Xs²)
B) Ra + jXs
C) Ra - jXs
D) jXs
A
-
The induced emf in armature is
A) Proportional to field current
B) Inversely proportional to speed
C) Constant
D) None
A
-
The synchronous motor efficiency is given by
A) Output/Input × 100
B) Input/Output × 100
C) Loss/Input × 100
D) Output/Loss × 100
A
-
The excitation current controls
A) Reactive power
B) Active power
C) Torque
D) None
A
-
The field losses increase with
A) Field current²
B) Armature current²
C) Supply voltage²
D) Frequency²
A
-
The synchronizing torque acts
A) To maintain synchronism
B) To oppose disturbance
C) In direction of restoring
D) All
D
-
Synchronizing torque is proportional to
A) cos δ
B) sin δ
C) tan δ
D) δ
A
-
The steady-state stability limit is given by
A) (EV/Xs)
B) (EV/Xs) sin δ
C) (EV/Xs) cos δ
D) None
C
-
A synchronous motor runs at
A) Constant synchronous speed
B) Variable speed
C) Slip speed
D) Half speed
A
-
The stator field rotates at
A) Synchronous speed
B) Variable speed
C) Slip speed
D) Zero speed
A
-
The damper winding reduces
A) Hunting
B) Oscillations
C) Vibration
D) All
D
-
Synchronous motor operation at unity pf is obtained when
A) Induced emf equals supply voltage
B) Field current optimum
C) Load moderate
D) All
D
-
When δ = 90°, motor delivers
A) Maximum torque
B) Zero torque
C) Half torque
D) Minimum torque
A
-
In a synchronous motor, load angle is
A) Directly proportional to load
B) Inversely proportional
C) Constant
D) None
A
-
For zero excitation, motor acts as
A) Induction motor
B) Capacitor
C) Generator
D) None
A
-
The excitation current of motor increases when
A) Power factor changes from lag to lead
B) Load decreases
C) Speed changes
D) Voltage increases
A
-
A synchronous motor can improve system
A) Power factor
B) Stability
C) Voltage regulation
D) All
D
-
The rotor excitation ensures
A) Synchronism
B) Magnetic coupling
C) Torque production
D) All
D
-
The mechanical input of synchronous motor is
A) Zero
B) Electrical
C) DC
D) None
A
-
The armature current lags voltage in
A) Under-excited condition
B) Over-excited condition
C) Unity pf
D) None
A
-
Synchronous motor torque-speed characteristic is
A) Vertical line
B) Horizontal line
C) Linear
D) Parabolic
A
-
When δ increases, developed torque
A) Increases up to 90°
B) Decreases after 90°
C) Both A and B
D) None
C
-
Synchronous motor can be used for
A) Power factor correction
B) Voltage stabilization
C) Constant speed drive
D) All
D
-
The excitation control regulates
A) Power factor and reactive power
B) Speed
C) Torque
D) Frequency
A
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