Synchronous motors are best suited for
A) Constant speed applications
B) Variable speed drives
C) Starting torque applications
D) Intermittent load drives
A
-
Which of the following is NOT an advantage of a synchronous motor?
A) Constant speed
B) Variable speed operation
C) Power factor correction
D) High efficiency
B
-
The synchronous motor has a unity speed regulation because
A) It runs at constant speed
B) Slip is zero
C) Speed is independent of load
D) All of the above
D
-
Synchronous motors are generally employed in industries where
A) Power factor correction is needed
B) Variable speed is required
C) Frequent starting is necessary
D) Both A and B
A
-
The torque developed by a synchronous motor is
A) Electromagnetic torque
B) Reluctance torque
C) Magnetic torque
D) Both A and B
D
-
Which type of load is best driven by a synchronous motor?
A) Constant torque
B) Variable torque
C) Impact load
D) Intermittent load
A
-
The leading power factor operation of a synchronous motor helps in
A) Improving system power factor
B) Reducing voltage
C) Increasing losses
D) Reducing excitation
A
-
A synchronous condenser operates at
A) No mechanical load
B) Full mechanical load
C) Half load
D) Variable mechanical load
A
-
A synchronous condenser supplies
A) Reactive power only
B) Active power only
C) Both reactive and active power
D) None
A
-
The efficiency of a synchronous motor is maximum at
A) Unity power factor
B) Leading power factor
C) Lagging power factor
D) No-load
A
-
The air-gap flux in a synchronous motor is mainly produced by
A) Field winding current
B) Armature current
C) Both field and armature currents
D) Permanent magnets
A
-
The field excitation in a synchronous motor is adjusted to control
A) Power factor
B) Speed
C) Torque
D) Frequency
A
-
In synchronous motor, when excitation increases, the armature current
A) Decreases then increases
B) Increases
C) Decreases
D) Remains constant
A
-
The armature reaction in a synchronous motor can be
A) Magnetizing
B) Demagnetizing
C) Cross-magnetizing
D) All of these depending on load
D
-
The most efficient method of starting a synchronous motor is
A) Using damper winding
B) Using pony motor
C) Using variable frequency supply
D) All of the above
C
-
A synchronous motor runs at synchronous speed even with load change because
A) Rotor is locked with stator field
B) Slip is zero
C) Frequency is constant
D) All of these
D
-
The load angle (torque angle) δ is the angle between
A) Rotor field and stator field
B) Stator voltage and current
C) Supply voltage and induced emf
D) None
C
-
The condition for stability in synchronous motor operation is
A) dP/dδ > 0
B) dP/dδ < 0
C) δ > 90°
D) δ = 180°
A
-
The rotor speed of a 6-pole synchronous motor on 50 Hz is
A) 1000 rpm
B) 1200 rpm
C) 1500 rpm
D) 3000 rpm
A
-
Synchronous motors can be used to improve the system voltage by
A) Operating over-excited
B) Operating under-excited
C) Operating at unity pf
D) None
A
-
The power factor of a synchronous motor can be controlled by changing
A) Field excitation
B) Load
C) Supply voltage
D) Supply frequency
A
-
Synchronous motors are not self-starting because
A) Rotor cannot develop torque at standstill
B) Stator field rotates too fast
C) No relative speed at start
D) All of these
D
-
A synchronous motor draws current lagging the voltage when it is
A) Under-excited
B) Over-excited
C) Normally excited
D) None
A
-
The most common application of a synchronous motor is
A) Power factor correction
B) Pump drives
C) Fans
D) Lathes
A
-
When the load on a synchronous motor increases, the torque angle
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
A
-
A synchronous motor can be used as a synchronous condenser for
A) Reactive power control
B) Frequency control
C) Speed control
D) None
A
-
The field excitation of a synchronous motor is provided from
A) DC source
B) AC source
C) Induced voltage
D) Permanent magnets only
A
-
The armature of a synchronous motor carries
A) AC current
B) DC current
C) Pulsating current
D) No current
A
-
The power developed by a cylindrical rotor synchronous motor is
A) (EV/Xs) sin δ
B) (E²/Xs) sin δ
C) (V²/Xs) cos δ
D) (EV/Xs) cos δ
A
-
The maximum power developed by a salient pole motor is
A) E/V ratio dependent
B) Depends on both δ and reluctance torque
C) Constant
D) None
B
-
The reluctance torque is present in
A) Salient pole motors only
B) Cylindrical rotor motors
C) Induction motors
D) None
A
-
The reactive power of a synchronous motor depends upon
A) Field excitation
B) Load angle
C) Supply voltage
D) All of these
D
-
The torque developed in synchronous motor depends upon
A) Load angle
B) Excitation voltage
C) Supply voltage
D) All
D
-
The damper winding is short-circuited through
A) End rings
B) Field winding
C) Armature
D) None
A
-
Synchronous motors are not preferred for
A) Variable speed drives
B) Constant speed drives
C) Power factor improvement
D) Voltage regulation
A
-
The relation between mechanical power and torque is
A) P = T × ω
B) P = T/ω
C) P = ω/T
D) P = T²ω
A
-
For a 50 Hz supply, the synchronous speed of a 2-pole motor is
A) 3000 rpm
B) 1500 rpm
C) 1000 rpm
D) 750 rpm
A
-
The excitation of a synchronous motor is adjusted to operate at
A) Required power factor
B) Constant current
C) Constant voltage
D) Constant torque
A
-
The core losses in synchronous motor depend on
A) Supply voltage and frequency
B) Load
C) Excitation
D) None
A
-
The friction and windage losses in synchronous motor are
A) Constant
B) Variable with speed
C) Variable with load
D) Variable with voltage
A
-
The power factor at which a synchronous motor operates depends on
A) Field excitation
B) Load
C) Supply frequency
D) Both A and B
D
-
A synchronous motor at constant load, if excitation is increased,
A) Current decreases and pf becomes leading
B) Current increases and pf becomes lagging
C) Speed increases
D) None
A
-
A synchronous motor working as a condenser has
A) No mechanical output
B) Constant torque
C) Variable speed
D) None
A
-
The direction of rotation of a synchronous motor is reversed by
A) Interchanging any two stator leads
B) Reversing field current
C) Both A and B
D) None
A
-
The field excitation controls
A) Power factor
B) Load
C) Speed
D) Torque
A
-
The synchronous motor speed depends only on
A) Frequency and poles
B) Load
C) Excitation
D) Supply voltage
A
-
For leading power factor, excitation emf is
A) Greater than terminal voltage
B) Less than terminal voltage
C) Equal
D) Zero
A
-
The active power developed by a synchronous motor is
A) (EV/Xs) sin δ
B) (E²/Xs) sin δ
C) (V²/Xs) cos δ
D) (E/Xs)
A
-
The motor loses synchronism when
A) Load torque exceeds pull-out torque
B) Excitation becomes zero
C) Supply fails
D) Any of the above
D
-
The angle between the stator current and excitation emf is
A) Load angle
B) Torque angle
C) Power factor angle
D) Internal angle
C
-
The V-curves are plotted between
A) Armature current and field current
B) Speed and torque
C) Torque and excitation
D) Voltage and current
A
-
For stable operation of a synchronous motor
A) δ < 90°
B) δ = 90°
C) δ > 90°
D) δ = 180°
A
-
The efficiency of a synchronous motor is maximum at
A) Unity pf
B) Lagging pf
C) Leading pf
D) Zero load
A
-
The damper winding helps in
A) Reducing hunting
B) Starting the motor
C) Damping oscillations
D) All of these
D
-
The air-gap flux per pole in a synchronous motor is
A) Constant
B) Variable with load
C) Variable with excitation
D) None
A
-
A synchronous motor operates at
A) Constant speed
B) Variable speed
C) Slip speed
D) Half speed
A
-
The power factor of an over-excited synchronous motor is
A) Leading
B) Lagging
C) Unity
D) Zero
A
-
The reactive power supplied by synchronous condenser depends on
A) Field current
B) Armature voltage
C) System voltage
D) All
D
-
The pull-out torque occurs when δ =
A) 90°
B) 0°
C) 180°
D) 45°
A
-
The pull-out torque of a synchronous motor is proportional to
A) EV/Xs
B) V²/Xs
C) sin δ
D) All
A
-
The main use of salient pole motors is in
A) Low speed drives
B) High speed drives
C) Traction
D) Aircraft
A
-
Cylindrical rotor synchronous motors are used for
A) High-speed applications
B) Low-speed applications
C) Variable torque loads
D) Compressors
A
-
The field winding losses are due to
A) Copper loss
B) Iron loss
C) Hysteresis loss
D) Eddy loss
A
-
Which of the following losses varies with load?
A) Armature copper loss
B) Core loss
C) Friction loss
D) Windage loss
A
-
If supply frequency is doubled, the synchronous speed
A) Doubles
B) Halves
C) Triples
D) Remains constant
A
-
In a synchronous motor, back emf is equal to
A) Induced emf due to excitation
B) Supply emf
C) Load voltage
D) None
A
-
The torque developed by reluctance is proportional to
A) sin 2δ
B) cos 2δ
C) tan δ
D) sin δ
A
-
The reluctance torque is independent of
A) Field excitation
B) Armature current
C) Load
D) None
A
-
The mechanical power developed in a synchronous motor is
A) P = (EV/Xs) sin δ
B) P = V²/Xs
C) P = E²/Xs
D) None
A
-
The phase angle δ is also called
A) Load angle
B) Torque angle
C) Power angle
D) All of these
D
-
The air-gap length in a salient pole machine is
A) Non-uniform
B) Uniform
C) Zero
D) Constant
A
-
The function of slip rings in a synchronous motor is
A) To supply DC excitation
B) To supply AC voltage
C) To reduce losses
D) To maintain power factor
A
-
The efficiency of synchronous motor is generally
A) 90–95%
B) 50–60%
C) 70–80%
D) 99%
A
-
When load increases, δ increases causing
A) More torque
B) More power
C) Both A and B
D) None
C
-
The voltage regulation of a synchronous motor is
A) Negative
B) Positive
C) Zero
D) None
A
-
The synchronous motor has better efficiency than induction motor because
A) No rotor copper losses
B) Constant speed
C) Fixed excitation
D) All
A
-
In a synchronous motor, the current drawn is
A) AC
B) DC
C) Pulsating DC
D) None
A
-
In parallel operation, synchronous machines must have
A) Same voltage, frequency, and phase sequence
B) Same speed
C) Same load
D) None
A
-
When excitation decreases, current
A) Increases
B) Decreases
C) Remains constant
D) Zero
A
-
The power developed in synchronous motor depends on
A) δ
B) V
C) E
D) All
D
-
The field current in a synchronous motor controls
A) Power factor
B) Torque
C) Speed
D) Load
A
-
When synchronous motor works as over-excited, it behaves as
A) Capacitor
B) Inductor
C) Resistor
D) Transformer
A
-
When under-excited, it behaves as
A) Inductor
B) Capacitor
C) Resistor
D) None
A
-
The torque angle δ at light load is
A) Small
B) Large
C) 90°
D) Zero
A
-
The synchronous motor torque equation is
A) T = (EV/Xs) sin δ × (60/2πN)
B) T = P×N
C) T = I²R
D) None
A
-
The armature reaction effect in synchronous motor depends on
A) Power factor
B) Supply voltage
C) Speed
D) Load
A
-
Hunting occurs due to
A) Sudden load change
B) Magnetic saturation
C) Unbalanced supply
D) Rotor design
A
-
To reduce hunting
A) Use damper windings
B) Reduce load
C) Reduce voltage
D) Increase poles
A
-
The synchronous motor has a power factor
A) Adjustable
B) Constant
C) Unity only
D) Always lagging
A
-
In synchronous condenser mode, mechanical losses are
A) Small but constant
B) Zero
C) Large and variable
D) None
A
-
The supply voltage to a synchronous motor is
A) 3-phase AC
B) DC
C) Single-phase
D) Mixed
A
-
The starting current of a synchronous motor is
A) Large
B) Small
C) Zero
D) Depends on excitation
A
-
At unity pf, excitation emf is
A) Equal to terminal voltage
B) Greater
C) Lesser
D) Zero
A
-
The most common method to start large synchronous motors is
A) Using damper windings
B) Using pony motor
C) Using VFDs
D) All
D
-
The reactive power supplied by a synchronous condenser is
A) Adjustable
B) Constant
C) Fixed
D) Zero
A
-
The no-load power factor of a synchronous motor is
A) Leading
B) Lagging
C) Unity
D) Zero
A
-
The input power to a synchronous motor is
A) Electrical
B) Mechanical
C) Both
D) None
A
-
The synchronous motor torque is
A) Electromagnetic
B) Reluctance
C) Both A and B
D) None
C
-
The field current is supplied to rotor through
A) Slip rings and brushes
B) Commutator
C) Armature
D) Shaft
A
-
In synchronous condenser operation, the shaft delivers
A) No mechanical power
B) Mechanical power
C) Constant torque
D) Variable speed
A
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