The power factor of a synchronous motor can be controlled by
A) Field excitation
B) Supply voltage
C) Load current
D) Speed
A
-
The synchronous motor acts as a synchronous condenser when operating
A) Over-excited and unloaded
B) Under-excited and loaded
C) At unity pf
D) Overloaded
A
-
Synchronous condensers are mainly used for
A) Power factor improvement
B) Voltage regulation
C) Speed control
D) Frequency control
A
-
The efficiency of a synchronous motor is given by
A) Output power/Input power × 100
B) Input/Output × 100
C) Losses/Input × 100
D) Output/Losses × 100
A
-
In synchronous motor, mechanical losses are due to
A) Friction and windage
B) Field losses
C) Armature losses
D) Stray load losses
A
-
Electrical losses in synchronous motor include
A) Copper and iron losses
B) Friction losses
C) Mechanical losses
D) All of these
A
-
A synchronous condenser supplies leading reactive power by
A) Over-excitation
B) Under-excitation
C) Increasing load
D) None
A
-
The advantage of using a synchronous condenser over capacitor bank is
A) Adjustable power factor
B) Lower losses
C) Smaller size
D) Fixed voltage
A
-
The reactive power capacity of a synchronous condenser depends on
A) Field current
B) Shaft power
C) Supply frequency
D) Speed
A
-
When excitation is reduced, the synchronous motor
A) Draws lagging current
B) Draws leading current
C) Draws zero current
D) Speed increases
A
-
The reactive current component in synchronous motor varies with
A) Excitation
B) Load
C) Speed
D) None
A
-
The mechanical output power of synchronous motor
A) Remains constant for constant load
B) Varies with excitation
C) Depends on power factor
D) Varies with field current
A
-
A synchronous motor running at half voltage
A) Cannot start
B) Will pull out of step
C) May run but with low torque
D) Runs normally
C
-
The armature reaction in synchronous motor is
A) Demagnetizing or magnetizing
B) Cross-magnetizing only
C) Constant
D) None
A
-
In over-excited condition, armature reaction is
A) Magnetizing
B) Demagnetizing
C) Cross-magnetizing
D) None
A
-
In under-excited condition, armature reaction is
A) Demagnetizing
B) Magnetizing
C) Neutral
D) None
A
-
The core loss of synchronous motor depends on
A) Supply voltage and frequency
B) Load
C) Excitation
D) Power factor
A
-
Field copper loss depends upon
A) Field current
B) Armature current
C) Supply voltage
D) Frequency
A
-
The damper winding helps to
A) Start the motor and prevent hunting
B) Increase torque
C) Improve pf
D) Reduce voltage
A
-
In synchronous motor, torque developed is proportional to
A) sin δ
B) cos δ
C) δ
D) tan δ
A
-
The load angle δ is
A) Angle between E and V
B) Angle between I and V
C) Angle between rotor and stator fields
D) Both A and C
D
-
A synchronous motor running with leading current can
A) Improve system pf
B) Deteriorate pf
C) Not change pf
D) None
A
-
When a synchronous motor runs at lagging power factor, it behaves as
A) Inductive load
B) Capacitive load
C) Resistive load
D) None
A
-
The value of δ for maximum torque is
A) 90°
B) 60°
C) 45°
D) 30°
A
-
Synchronizing power increases with
A) Increase in excitation
B) Increase in load angle
C) Decrease in voltage
D) None
A
-
The voltage regulation of a synchronous motor is
A) Negative
B) Positive
C) Zero
D) Infinite
A
-
For constant excitation, an increase in load
A) Increases δ
B) Decreases δ
C) Keeps δ constant
D) None
A
-
The motor loses synchronism when
A) Load angle exceeds 90°
B) Field current exceeds limit
C) Supply drops
D) Speed increases
A
-
The main reason for loss of synchronism is
A) Sudden overload
B) Over-excitation
C) Under-excitation
D) High pf
A
-
The critical excitation is the value for which
A) pf = 1
B) pf = 0
C) Load = 0
D) Torque = 0
A
-
A synchronous motor operating at leading power factor
A) Supplies reactive kVAR
B) Absorbs reactive kVAR
C) Supplies active power
D) None
A
-
The current drawn at unity power factor is
A) Minimum
B) Maximum
C) Constant
D) Zero
A
-
The main losses in synchronous motor are
A) Stator copper, iron, friction and windage losses
B) Armature and rotor losses
C) Iron only
D) None
A
-
The starting torque of synchronous motor using damper winding is due to
A) Induction motor action
B) Magnetic locking
C) Synchronizing torque
D) None
A
-
The direction of rotation can be reversed by
A) Interchanging any two supply leads
B) Reversing field polarity
C) Both A and B
D) None
A
-
During synchronizing, voltage magnitude and phase sequence must be
A) Same
B) Opposite
C) Random
D) None
A
-
The power developed in a synchronous motor per phase is
A) (EV/Xs) sin δ
B) (E²/Xs) sin δ
C) (V²/Xs) cos δ
D) (EV/Xs) cos δ
A
-
If load increases suddenly, the rotor
A) Lags behind stator field
B) Leads stator field
C) Speed changes
D) None
A
-
The motor returns to synchronism after load removal due to
A) Damper winding
B) Increased excitation
C) Reduced field
D) None
A
-
A synchronous motor can be used as synchronous condenser because
A) It can draw leading current
B) It runs at synchronous speed
C) It maintains constant torque
D) It is self-starting
A
-
For good voltage regulation, the excitation should be
A) Adjusted according to load
B) Constant
C) Maximum
D) Minimum
A
-
The excitation emf is induced due to
A) Field current
B) Armature reaction
C) Load current
D) None
A
-
The load sharing between parallel synchronous motors depends on
A) Excitation and load angle
B) Speed
C) Frequency
D) Voltage only
A
-
The synchronous motor power input =
A) Output + losses
B) Output – losses
C) Mechanical input
D) None
A
-
The field excitation controls
A) Power factor
B) Speed
C) Load
D) Frequency
A
-
For a given load, as field current increases,
A) pf changes from lag to lead
B) pf changes from lead to lag
C) Speed increases
D) δ increases
A
-
Synchronous motor current locus for constant load is
A) Circle
B) Ellipse
C) Straight line
D) Parabola
A
-
The excitation voltage is proportional to
A) Field current
B) Supply voltage
C) Load current
D) None
A
-
The mechanical angle corresponding to one electrical cycle in 4-pole machine is
A) 180°
B) 90°
C) 360°
D) 720°
A
-
When a synchronous motor falls out of step
A) It stops rotating synchronously
B) Speed remains constant
C) Current becomes zero
D) None
A
-
The torque-speed characteristic of synchronous motor is
A) Flat line (constant speed)
B) Hyperbolic
C) Parabolic
D) Drooping
A
-
The pull-out torque is proportional to
A) 1/Xs
B) Xs
C) E²
D) sin δ
A
-
The effect of armature resistance is to
A) Reduce maximum torque
B) Increase torque angle
C) Both A and B
D) None
C
-
When excitation increases, torque for constant power increases due to
A) Improved pf
B) Reduced current
C) Both
D) None
C
-
A synchronous motor cannot start without
A) Auxiliary means
B) Load
C) DC supply
D) Excitation
A
-
If field excitation fails during running, the motor
A) Runs as induction motor
B) Stops
C) Speeds up
D) Burns
A
-
Field excitation in synchronous motor is adjusted to control
A) Power factor
B) Speed
C) Torque
D) Frequency
A
-
The constant losses of synchronous motor include
A) Iron and friction losses
B) Copper losses
C) Load losses
D) Stray losses only
A
-
The efficiency of synchronous motor improves with
A) Load
B) Speed
C) Over-excitation
D) Frequency
A
-
The power input to motor equals
A) Power output + losses
B) Power output – losses
C) Field loss only
D) None
A
-
A synchronous motor draws lagging current when
A) Under-excited
B) Over-excited
C) Unity excitation
D) None
A
-
A synchronous motor draws leading current when
A) Over-excited
B) Under-excited
C) Unity excitation
D) None
A
-
The torque angle increases linearly with
A) Load
B) Speed
C) Field current
D) Voltage
A
-
For constant load, as field current decreases,
A) Armature current increases
B) Power factor decreases
C) Both A and B
D) None
C
-
The synchronous motor gives constant speed due to
A) Magnetic locking
B) Constant torque
C) Damper winding
D) Excitation
A
-
The reactive power supplied by synchronous motor can be
A) Controlled continuously
B) Fixed
C) Zero
D) None
A
-
The field current of synchronous motor at unity pf is
A) Normal
B) Maximum
C) Minimum
D) Zero
A
-
At lagging pf, field current is
A) Less than unity pf current
B) More than unity pf current
C) Same
D) None
A
-
The magnetizing current in synchronous motor is supplied by
A) DC field
B) AC armature
C) Both
D) None
A
-
The excitation emf per phase is
A) Proportional to field flux
B) Inversely proportional
C) Constant
D) None
A
-
The power developed internally is
A) P = (EV/Xs) sin δ
B) P = (E²/Xs) sin δ
C) P = (V²/Xs) cos δ
D) P = (EV/Xs) cos δ
A
-
For constant excitation, torque ∝
A) sin δ
B) cos δ
C) δ
D) tan δ
A
-
The excitation emf lags supply voltage in
A) Motor
B) Generator
C) Both
D) None
A
-
When operated as generator, the excitation emf leads
A) Supply voltage
B) Armature current
C) Torque
D) None
A
-
The armature current at unity pf is
A) Minimum
B) Maximum
C) Constant
D) Zero
A
-
The curve between field current and armature current for constant load is
A) V-shaped
B) Straight line
C) Hyperbolic
D) None
A
-
The inverted V-curve shows
A) pf vs field current
B) Torque vs current
C) Voltage vs excitation
D) None
A
-
When operated at leading pf, it acts as
A) Synchronous condenser
B) Inductive load
C) Resistive load
D) None
A
-
The shaft torque is proportional to
A) Synchronous power
B) Load angle
C) Both
D) None
C
-
Torque produced depends on
A) δ and excitation
B) Frequency
C) Speed
D) None
A
-
Synchronizing torque is given by
A) dP/dδ
B) P × δ
C) V × I × sin δ
D) None
A
-
The synchronizing power is zero when
A) δ = 90°
B) δ = 0°
C) δ = 180°
D) None
A
-
The phasor E leads V in
A) Generator
B) Motor
C) Both
D) None
A
-
The power factor can be made leading or lagging by
A) Varying excitation
B) Changing supply voltage
C) Changing load
D) Changing poles
A
-
Torque developed is zero when
A) δ = 0° or 180°
B) δ = 45°
C) δ = 60°
D) None
A
-
The synchronizing torque helps in
A) Restoring synchronism
B) Increasing speed
C) Reducing torque
D) None
A
-
The constant torque line on phasor diagram corresponds to
A) Circle
B) Ellipse
C) Straight line
D) None
C
-
The excitation emf is adjusted by
A) Field current
B) Armature current
C) Supply voltage
D) None
A
-
The armature current leads voltage in
A) Over-excited motor
B) Under-excited motor
C) Unity pf
D) None
A
-
The torque developed depends on
A) Product of E and V and sin δ
B) Sum of E and V
C) Ratio of E to V
D) None
A
-
Synchronous motor torque equation neglecting resistance is
A) P = (EV/Xs) sin δ
B) P = (V²/Xs) sin δ
C) P = (E²/Xs) cos δ
D) None
A
-
The internal emf E is
A) Generated by DC excitation
B) Induced by armature reaction
C) Due to supply
D) None
A
-
The rotor position determines
A) Load angle
B) Excitation emf
C) Supply voltage
D) None
A
-
The power factor can be improved by
A) Increasing field current
B) Reducing field current
C) Increasing load
D) None
A
-
The speed regulation of synchronous motor is
A) Zero
B) Small positive
C) Negative
D) Infinite
A
-
When load torque increases,
A) δ increases
B) Speed constant
C) Both
D) None
C
-
For a given excitation, δ increases with
A) Load
B) Supply voltage
C) Speed
D) None
A
-
When excitation decreases, pf becomes
A) Lagging
B) Leading
C) Unity
D) None
A
-
The mechanical output torque =
A) (3EV/Xs) sin δ / ω
B) (EV/Xs) sin δ
C) (3EV/Xs)
D) None
A
-
When supply frequency increases, synchronous speed
A) Increases
B) Decreases
C) Constant
D) None
A
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