The V-curve of a synchronous motor shows relation between
A) Armature current and field current
B) Torque and speed
C) Voltage and current
D) Torque and power
A
-
The shape of a V-curve is
A) V-shaped
B) U-shaped
C) Parabolic
D) Linear
A
-
The minimum armature current on the V-curve corresponds to
A) Unity power factor
B) Leading power factor
C) Lagging power factor
D) Zero power factor
A
-
The inverted V-curve shows variation of
A) Power factor with field current
B) Torque with field current
C) Speed with field current
D) Efficiency with field current
A
-
Over-excitation in a synchronous motor causes it to
A) Supply reactive power
B) Absorb reactive power
C) Draw more current
D) Slow down
A
-
Under-excitation in a synchronous motor causes it to
A) Absorb reactive power
B) Supply reactive power
C) Speed up
D) Overheat
A
-
The hunting in a synchronous motor is caused by
A) Sudden load changes
B) Voltage fluctuations
C) Speed variation
D) Supply frequency change
A
-
The oscillations in hunting are of
A) Mechanical nature
B) Electrical nature
C) Magnetic nature
D) All of these
A
-
Hunting can be minimized by
A) Damper windings
B) Increasing load
C) Reducing excitation
D) None
A
-
The frequency of hunting oscillations depends upon
A) Moment of inertia
B) Synchronizing power
C) Both A and B
D) None
C
-
Damping torque in a synchronous motor is provided by
A) Damper winding currents
B) Field winding
C) Armature current
D) None
A
-
Damper windings are placed on
A) Pole faces of rotor
B) Stator slots
C) Shaft
D) End rings
A
-
The damper winding acts like
A) Squirrel cage winding
B) Slip ring winding
C) Compound winding
D) Shunt winding
A
-
The main disadvantage of hunting is
A) Mechanical stress on shaft
B) Increased losses
C) Instability
D) All of these
D
-
Synchronizing torque is proportional to
A) cos δ
B) sin δ
C) dP/dδ
D) tan δ
C
-
The synchronizing torque is maximum at
A) δ = 0°
B) δ = 90°
C) δ = 45°
D) δ = 60°
A
-
A synchronous motor running at no load with normal excitation draws
A) Small current at unity power factor
B) Large current at lagging pf
C) Large current at leading pf
D) Zero current
A
-
For a given excitation, if supply voltage decreases, armature current
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
A
-
The torque angle increases when
A) Load increases
B) Load decreases
C) Excitation increases
D) Supply voltage increases
A
-
A synchronous motor cannot start itself because
A) No starting torque
B) Speed depends on load
C) High inertia
D) High synchronous reactance
A
-
The maximum power developed by a cylindrical rotor synchronous motor is
A) EV/Xs
B) (EV/Xs) sin δ
C) V²/Xs
D) E²/Xs
A
-
The motor runs stably only when
A) δ < 90°
B) δ > 90°
C) δ = 0°
D) δ = 180°
A
-
In a synchronous motor, torque is directly proportional to
A) sin δ
B) cos δ
C) tan δ
D) δ
A
-
The excitation required for unity power factor increases with
A) Load
B) Speed
C) Frequency
D) Supply voltage
A
-
A synchronous motor draws leading current when
A) Over-excited
B) Under-excited
C) Normally excited
D) None
A
-
The synchronous motor running at zero power factor leading acts as
A) Synchronous condenser
B) Inductive load
C) Resistive load
D) Transformer
A
-
In power factor correction, synchronous motor operates
A) Over-excited
B) Under-excited
C) At unity pf
D) None
A
-
Synchronous motor speed is determined by
A) Supply frequency
B) Number of poles
C) Both A and B
D) Load
C
-
For constant frequency, synchronous speed varies with
A) Number of poles
B) Supply voltage
C) Excitation
D) Load
A
-
The motor torque equation is derived from
A) Power angle characteristic
B) Current locus
C) Voltage triangle
D) Phasor diagram
A
-
For a lagging power factor, excitation emf is
A) Less than terminal voltage
B) Greater than terminal voltage
C) Equal to terminal voltage
D) Infinite
A
-
When supply frequency increases, synchronous speed
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
A
-
The synchronous motor is suitable for
A) Constant speed drives
B) Variable speed drives
C) Low torque drives
D) Small machines only
A
-
The load angle δ increases with
A) Load torque
B) Supply voltage
C) Field current
D) None
A
-
The direction of rotation of synchronous motor can be changed by
A) Interchanging any two stator leads
B) Reversing field current
C) Both A and B
D) None
A
-
The starting torque of a synchronous motor is
A) Zero
B) Maximum
C) Depends on excitation
D) Depends on load
A
-
The magnitude of armature current at constant load varies with
A) Field current
B) Supply voltage
C) Speed
D) None
A
-
The rotor copper losses in a synchronous motor occur due to
A) Field excitation current
B) Armature current
C) Supply current
D) None
A
-
When supply frequency is increased keeping voltage constant,
A) Torque decreases
B) Torque increases
C) Torque constant
D) Speed decreases
A
-
The damper winding currents are induced by
A) Relative motion between stator field and rotor
B) Field excitation
C) Armature reaction
D) DC current
A
-
A synchronous motor can operate at leading power factor when
A) Over-excited
B) Under-excited
C) Normally excited
D) None
A
-
Synchronous motors are generally not used for
A) Variable speed drives
B) Power factor correction
C) High power constant speed
D) Compressor drives
A
-
The phasor representing excitation emf lies
A) Ahead of V by δ
B) Behind V by δ
C) In phase with V
D) None
A
-
The maximum torque occurs when
A) δ = 90°
B) δ = 60°
C) δ = 30°
D) δ = 0°
A
-
For constant load, if excitation decreases,
A) Power factor becomes lagging
B) Power factor becomes leading
C) Speed changes
D) None
A
-
When excitation increases beyond normal,
A) Power factor becomes leading
B) Power factor becomes lagging
C) Load decreases
D) None
A
-
The synchronizing power per phase is
A) (EV/Xs) cos δ
B) (V²/Xs) cos δ
C) (EV/Xs) sin δ
D) (E²/Xs) sin δ
B
-
The pull-out torque is
A) Maximum torque developed by motor
B) Starting torque
C) Torque at unity pf
D) Zero load torque
A
-
The synchronous motor can operate between
A) Lagging and leading power factors
B) Zero and unity power factor only
C) Only lagging pf
D) None
A
-
The mechanical characteristic of a synchronous motor is
A) Constant speed
B) Variable speed
C) Drooping speed
D) Linear
A
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