The electromagnetic torque in a synchronous motor is given by
A) (EV/Xs) sin δ
B) (EV/Xs) cos δ
C) (V²/Xs) sin δ
D) (E²/Xs) cos δ
A
-
The maximum torque occurs at
A) δ = 90°
B) δ = 0°
C) δ = 180°
D) δ = 45°
A
-
The power developed in a cylindrical rotor motor is
A) (EV/Xs) sin δ
B) (E²/Xs) sin δ
C) (V²/Xs) cos δ
D) (EV/Xs) cos δ
A
-
In a salient pole machine, the power equation includes
A) Two terms due to direct and quadrature axes
B) One term only
C) No torque component
D) None
A
-
The reluctance power term in salient pole motor is
A) (V²/2)(1/Xq - 1/Xd) sin 2δ
B) (E²/Xd) sin δ
C) (EV/Xs) sin δ
D) (E²/2Xd) cos 2δ
A
-
The electromagnetic torque (Te) of a salient pole motor is proportional to
A) sin δ + (K sin 2δ)
B) cos δ
C) sin δ
D) sin² δ
A
-
The reluctance torque in salient pole motor acts
A) Even with zero excitation
B) Only with excitation
C) Only at no load
D) None
A
-
The damper winding reduces
A) Hunting
B) Speed oscillation
C) Starting difficulty
D) All
D
-
The inertia constant of synchronous motor is measured in
A) MJ/MVA
B) kW
C) Nm/A
D) Wb
A
-
For synchronizing torque to exist, the system must be
A) Stable
B) Electrically coupled
C) Magnetically coupled
D) In synchronism
D
-
The transient stability is concerned with
A) Large disturbance
B) Small disturbance
C) Damping oscillations
D) Constant load
A
-
The synchronizing torque depends on
A) System voltage
B) Reactance
C) Load angle
D) All
D
-
The excitation emf of synchronous motor is induced by
A) Rotor field current
B) Stator current
C) Leakage flux
D) None
A
-
If excitation increases while load remains same, the armature current
A) Decreases
B) Increases
C) Constant
D) Becomes zero
A
-
If field current decreases, the motor power factor becomes
A) Lagging
B) Leading
C) Unity
D) None
A
-
For a given load, under-excitation causes
A) Lagging power factor
B) Overcurrent
C) Both
D) None
C
-
The mechanical output of synchronous motor is
A) Electrical input – losses
B) Electrical output
C) Constant
D) Proportional to excitation
A
-
The synchronous speed (Ns) is given by
A) 120f/P
B) 60f/P
C) f/P
D) 2f/P
A
-
A 6-pole, 50 Hz motor runs at synchronous speed of
A) 1000 rpm
B) 1500 rpm
C) 1200 rpm
D) 3000 rpm
A
-
The field winding resistance is usually
A) High
B) Low
C) Medium
D) Variable
B
-
Damper bars are placed on
A) Pole faces
B) Stator slots
C) Shaft
D) Core
A
-
The power factor correction by synchronous motor improves
A) Line voltage regulation
B) System stability
C) Transmission efficiency
D) All
D
-
When the excitation emf is less than supply voltage, power factor is
A) Lagging
B) Leading
C) Unity
D) None
A
-
When the excitation emf equals supply voltage, power factor is
A) Unity
B) Lagging
C) Leading
D) Zero
A
-
When excitation emf is greater than supply voltage, power factor is
A) Leading
B) Lagging
C) Unity
D) None
A
-
The magnitude of resultant current in a synchronous motor is minimum when
A) Motor operates at unity pf
B) Lagging pf
C) Leading pf
D) None
A
-
The load angle δ changes with
A) Load torque
B) Speed
C) Field current
D) Both A & C
A
-
The phasor diagram of over-excited synchronous motor shows current
A) Leading voltage
B) Lagging voltage
C) In phase with voltage
D) Opposite to voltage
A
-
The synchronizing power per electrical radian is
A) (EV/Xs) cos δ
B) (EV/Xs) sin δ
C) (E²/Xs) sin δ
D) (E²/Xs) cos δ
A
-
The power developed per phase neglecting resistance is
A) (EV/Xs) sin δ
B) (V²/Xs) cos δ
C) (E²/Xs) sin δ
D) None
A
-
When excitation is reduced below normal, the line current
A) Increases and lags
B) Decreases
C) Leads
D) None
A
-
Synchronous motors are generally preferred for loads requiring
A) Constant speed
B) Variable torque
C) Frequent starts
D) High slip
A
-
The main disadvantage of synchronous motor is
A) Not self-starting
B) Variable speed
C) High losses
D) Noisy
A
-
The power factor can be improved by
A) Over-exciting synchronous motor
B) Increasing voltage
C) Increasing load
D) Decreasing frequency
A
-
The damper winding acts as
A) Rotor cage
B) Damping device
C) Starter
D) All
D
-
The phase difference between supply voltage and excitation emf is
A) Load angle
B) Torque angle
C) Power angle
D) All
D
-
The synchronizing torque is proportional to
A) cos δ
B) sin δ
C) δ
D) tan δ
A
-
The reactive power of synchronous motor is controlled by
A) Field current
B) Load current
C) Voltage
D) None
A
-
The load power factor depends on
A) Excitation current
B) Load angle
C) Supply voltage
D) Both A and B
D
-
The phasor representing excitation emf rotates at
A) Synchronous speed
B) Slip speed
C) Zero speed
D) Half synchronous speed
A
-
The magnetic field in stator rotates at
A) Synchronous speed
B) Slip speed
C) Constant
D) None
A
-
The field poles in synchronous motor rotate
A) In synchronism with stator field
B) Faster than stator field
C) Slower than stator field
D) Independently
A
-
The torque angle δ is
A) Angle between V and E
B) Mechanical angle
C) Phase difference between fluxes
D) All
D
-
The no-load current in synchronous motor is
A) Mainly magnetizing
B) Largely active
C) Mostly resistive
D) None
A
-
The starting current of synchronous motor is large because
A) No back emf
B) Field unexcited
C) Both
D) None
C
-
The armature reaction in over-excited condition is
A) Demagnetizing
B) Magnetizing
C) Cross-magnetizing
D) Neutral
B
-
The armature reaction in under-excited condition is
A) Demagnetizing
B) Magnetizing
C) Cross-magnetizing
D) Neutral
A
-
A synchronous condenser operates at
A) No mechanical load
B) Over-excitation
C) Leading current
D) All
D
-
The direction of rotation of synchronous motor is reversed by
A) Reversing any two stator leads
B) Reversing field current
C) Changing frequency
D) None
A
-
The efficiency of synchronous motor increases with
A) Load
B) Excitation
C) Power factor
D) All
D
-
The synchronous reactance Xs is
A) Leakage + armature reaction reactance
B) Armature resistance
C) Inductive reactance only
D) None
A
-
The voltage regulation of synchronous motor is
A) Negative
B) Positive
C) Zero
D) Variable
A
-
The magnetization characteristic curve shows
A) Relation between field current and emf
B) Armature current vs field current
C) Torque vs speed
D) None
A
-
A synchronous motor is said to be hunting when
A) Rotor oscillates about synchronous speed
B) Speed changes permanently
C) Frequency fluctuates
D) None
A
-
Hunting is caused by
A) Sudden load change
B) Supply voltage fluctuation
C) System disturbance
D) All
D
-
The effect of hunting can be minimized by
A) Damper winding
B) Flywheel
C) Both A and B
D) None
C
-
A flywheel attached to motor shaft
A) Increases inertia
B) Reduces hunting
C) Improves stability
D) All
D
-
The voltage regulation of a motor can be improved by
A) Increasing excitation
B) Decreasing resistance
C) Reducing load
D) All
D
-
A synchronous condenser supplies
A) Reactive power
B) Real power
C) Both
D) None
A
-
A synchronous motor is said to be stable when
A) It runs in synchronism
B) δ < 90°
C) Synchronizing torque positive
D) All
D
-
If δ > 90°, the machine
A) Loses synchronism
B) Increases torque
C) Becomes generator
D) None
A
-
The damping torque acts
A) Opposite to speed deviation
B) Along torque
C) Zero at synchronism
D) None
A
-
The synchronous machine connected to infinite bus has
A) Constant voltage and frequency
B) Variable speed
C) Constant torque
D) None
A
-
The frequency of oscillation in hunting depends on
A) Synchronizing torque
B) Inertia constant
C) Both A and B
D) None
C
-
In synchronous motor, torque is independent of
A) Speed
B) Excitation
C) Load angle
D) None
A
-
The excitation control helps in
A) Maintaining power factor
B) Regulating voltage
C) Controlling reactive power
D) All
D
-
The synchronizing power per mechanical degree is
A) (π/180)(EV/Xs) cos δ
B) (180/π)(EV/Xs) cos δ
C) (EV/Xs) sin δ
D) None
A
-
The excitation emf per phase is proportional to
A) Field current
B) Supply voltage
C) Load current
D) None
A
-
The torque developed in synchronous motor is zero if
A) δ = 0°
B) δ = 180°
C) δ = 90°
D) Both A and B
A
-
For constant excitation, torque ∝
A) sin δ
B) cos δ
C) tan δ
D) δ
A
-
The current drawn by synchronous motor at no load is
A) Lagging
B) Leading
C) Depends on excitation
D) None
C
-
A synchronous motor has
A) Zero slip
B) Constant torque
C) High efficiency
D) All
D
-
The field winding losses are
A) I²R losses
B) Iron losses
C) Copper losses
D) Both A & C
D
-
The excitation system type is chosen based on
A) Motor size
B) Speed
C) Control requirement
D) All
D
-
The phasor of excitation emf (E) lags voltage (V) in
A) Under-excited motor
B) Over-excited motor
C) Unity pf
D) None
A
-
The synchronous motor field poles are excited by
A) DC source
B) AC source
C) Variable frequency supply
D) None
A
-
The main losses in a synchronous motor are
A) Copper and iron losses
B) Friction and windage losses
C) Stray losses
D) All
D
-
The line current of synchronous motor is
A) Depends on excitation
B) Depends on load
C) Both
D) None
C
-
In synchronous motor, torque is produced by
A) Magnetic locking
B) Induction
C) Both
D) None
A
-
The torque per ampere of armature current is
A) Proportional to cos δ
B) Proportional to sin δ
C) Constant
D) None
B
-
The excitation emf in the rotor is due to
A) DC field current
B) AC armature flux
C) Induced voltage
D) None
A
-
The constant-speed property of synchronous motor makes it useful in
A) Timing devices
B) Conveyors
C) Rolling mills
D) All
D
-
The synchronization process involves
A) Matching voltage
B) Matching frequency
C) Matching phase sequence
D) All
D
-
A synchronous motor connected to busbars always
A) Runs at synchronous speed
B) Runs slower
C) Runs faster
D) None
A
-
The maximum torque occurs at
A) δ = 90°
B) δ = 0°
C) δ = 45°
D) δ = 120°
A
-
A motor with low SCR has
A) Poor voltage regulation
B) Less stability
C) Smaller air gap
D) All
D
-
High SCR machine is
A) More stable
B) Less voltage variation
C) Larger in size
D) All
D
-
The value of Xd and Xq affect
A) Power angle
B) Reluctance torque
C) Synchronizing torque
D) All
D
-
The reluctance torque acts in
A) Salient pole motor
B) Cylindrical rotor
C) Both
D) None
A
-
The excitation emf is proportional to
A) Field current
B) Speed
C) Flux
D) All
D
-
The current required for synchronizing increases with
A) Excitation
B) Load
C) Voltage
D) None
B
-
The motor may lose synchronism if
A) Load torque > pull-out torque
B) Supply voltage drops
C) Frequency varies
D) All
D
-
To prevent hunting, the motor is provided with
A) Damper winding
B) Flywheel
C) Both
D) None
C
-
When field excitation is lost, synchronous motor runs as
A) Induction motor
B) DC motor
C) Reluctance motor
D) None
A
-
The synchronous motor field winding is supplied through
A) Slip rings and brushes
B) Commutator
C) Solid rotor
D) None
A
-
Brushless excitation eliminates
A) Maintenance of brushes
B) Sparking
C) Wear
D) All
D
-
The direction of torque in synchronous motor depends on
A) Polarity of rotor field
B) Phase sequence of supply
C) Both
D) None
C
-
Synchronous motors are not suitable for
A) Variable speed drives
B) Constant speed drives
C) Power factor improvement
D) Voltage regulation
A
-
When a synchronous motor runs at no load, it takes
A) Leading current
B) Lagging current
C) Unity current
D) Depending on excitation
D
-
The torque developed by synchronous motor is maximum when
A) δ = 90°
B) δ = 0°
C) δ = 180°
D) δ = 45°
A
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