The synchronous reactance of a motor mainly accounts for
A) Armature leakage reactance
B) Armature reaction
C) Both A and B
D) None
C
-
The reluctance torque in a salient pole synchronous motor is produced due to
A) Non-uniform air gap
B) Magnetic locking
C) DC excitation
D) Armature current
A
-
The reluctance torque in a synchronous motor is maximum when
A) δ = 45°
B) δ = 90°
C) δ = 0°
D) δ = 60°
B
-
The torque developed in a salient pole motor is the sum of
A) Electromagnetic and reluctance torque
B) Induction and reluctance torque
C) Magnetic and friction torque
D) Load and shaft torque
A
-
For a salient pole synchronous motor, the expression for torque is
A) (EV/Xd) sin δ
B) (EV/Xq) sin δ
C) (EV/Xd) sin δ + (V²/2)(1/Xq – 1/Xd) sin 2δ
D) (E²/Xd) sin δ
C
-
The reluctance torque is proportional to
A) sin 2δ
B) sin δ
C) cos δ
D) tan δ
A
-
The excitation emf of a synchronous motor is directly proportional to
A) Field current
B) Armature current
C) Supply voltage
D) Torque angle
A
-
The stability of a synchronous motor is improved by
A) Increasing excitation
B) Decreasing excitation
C) Increasing armature resistance
D) Increasing load
A
-
The synchronizing power is maximum when
A) δ = 0°
B) δ = 45°
C) δ = 90°
D) δ = 180°
C
-
The synchronizing power coefficient is
A) dP/dδ
B) dE/dI
C) dV/dI
D) dI/dE
A
-
The synchronizing torque per electrical radian is
A) dP/dδ × (1/ωs)
B) dδ/dP × ωs
C) (P × ωs)
D) None
A
-
The synchronous motor is said to be stable if
A) δ < 90°
B) δ > 90°
C) δ = 0°
D) δ = 180°
A
-
The effect of increasing the field current beyond normal excitation is
A) Current becomes leading
B) Current becomes lagging
C) Power factor becomes zero
D) Speed decreases
A
-
The value of maximum torque in a cylindrical rotor motor is
A) (EV/Xs)
B) (EV/Xs) sin δmax
C) (EV/Xs)
D) (3EV/Xs)
A
-
The torque angle δ in synchronous motor varies between
A) 0° and 90°
B) 0° and 180°
C) 0° and 360°
D) 90° and 270°
A
-
When a synchronous motor is loaded, its
A) Speed remains constant
B) Speed decreases
C) Speed increases
D) Stops
A
-
The excitation required for unity power factor operation increases with
A) Load
B) Speed
C) Frequency
D) Supply voltage
A
-
Synchronous motor having no load and over-excitation behaves as
A) Capacitor
B) Inductor
C) Resistor
D) Transformer
A
-
If a synchronous motor is under-excited, its armature current
A) Lags voltage
B) Leads voltage
C) In phase with voltage
D) Is zero
A
-
When excitation is increased, current drawn by synchronous motor
A) Decreases and leads voltage
B) Increases and lags voltage
C) Remains constant
D) None
A
-
For a given torque, the load angle increases when
A) Voltage decreases
B) Excitation decreases
C) Load increases
D) Both A and B
D
-
The efficiency of a synchronous motor is highest when operating at
A) Unity power factor
B) Leading power factor
C) Lagging power factor
D) Zero power factor
A
-
The field losses in a synchronous motor are
A) Copper losses
B) Eddy current losses
C) Hysteresis losses
D) All of the above
D
-
The load angle of a synchronous motor depends upon
A) Shaft load
B) Supply voltage
C) Excitation
D) All of these
D
-
The angle between excitation emf and supply voltage in a synchronous motor is
A) Load angle
B) Torque angle
C) Power angle
D) All of these
D
-
The synchronous motor acts as synchronous condenser when
A) Over-excited at no load
B) Under-excited at full load
C) Normally excited at no load
D) Overloaded condition
A
-
The mechanical output of a synchronous motor increases with
A) Increasing torque angle
B) Increasing excitation
C) Decreasing load
D) Increasing supply voltage
A
-
The reactive component of current drawn by a synchronous motor is controlled by
A) Field excitation
B) Load torque
C) Supply frequency
D) Supply voltage
A
-
The synchronizing torque of a synchronous motor depends on
A) Supply voltage
B) Excitation
C) Synchronous reactance
D) All of these
D
-
The damper winding provides
A) Starting torque and damping
B) Regulating torque
C) Load torque
D) Synchronizing torque
A
-
The purpose of DC excitation is
A) To produce constant magnetic field
B) To produce alternating flux
C) To reduce armature current
D) To control torque
A
-
A synchronous motor takes constant current if operated at
A) Unity power factor
B) Zero power factor
C) Lagging power factor
D) Leading power factor
A
-
A synchronous motor running at constant load, when over-excited,
A) Draws leading current
B) Draws lagging current
C) Draws in-phase current
D) None
A
-
The mechanical power developed by the motor is given by
A) (3EV/Xs) sin δ
B) (EV/Xs) cos δ
C) (V²/Xs) sin δ
D) (3E²/Xs) cos δ
A
-
The excitation emf E leads terminal voltage V by
A) δ
B) 90°
C) 180°
D) 0°
A
-
The speed of a 6-pole synchronous motor supplied at 50 Hz is
A) 500 rpm
B) 1000 rpm
C) 1500 rpm
D) 3000 rpm
B
-
If the load on a synchronous motor is suddenly reduced, the rotor
A) Oscillates about its mean position
B) Stops immediately
C) Accelerates continuously
D) Reverses direction
A
-
A synchronous motor can be synchronized by
A) Lamp method
B) Synchroscope
C) Both A and B
D) None
C
-
The synchronous motor behaves like an inductive load when
A) Under-excited
B) Over-excited
C) At unity excitation
D) None
A
-
The synchronous motor behaves like a capacitive load when
A) Over-excited
B) Under-excited
C) Normally excited
D) None
A
-
The excitation voltage E is numerically equal to
A) Internal generated emf
B) Terminal voltage
C) Armature voltage drop
D) Supply emf
A
-
The reactive power generated by a synchronous motor can be adjusted by
A) Field excitation
B) Load current
C) Supply frequency
D) Torque
A
-
If excitation of a synchronous motor is reduced below normal,
A) Current lags
B) Current leads
C) Power factor improves
D) None
A
-
The stator flux and rotor flux in a synchronous motor are
A) Stationary relative to each other
B) Rotating at different speeds
C) Rotating in opposite directions
D) None
A
-
The input power of a synchronous motor equals
A) Mechanical output + losses
B) Mechanical output – losses
C) Field losses
D) Copper losses
A
-
The maximum power developed by a synchronous motor depends on
A) Excitation and supply voltage
B) Armature resistance
C) Shaft load
D) None
A
-
A synchronous motor draws current at lagging power factor when
A) Under-excited
B) Over-excited
C) Normally excited
D) None
A
-
The angle between excitation emf and supply voltage is known as
A) Load angle
B) Power angle
C) Torque angle
D) All of the above
D
-
The reactive power drawn by a synchronous motor is positive when
A) Motor is under-excited
B) Motor is over-excited
C) Motor is at unity excitation
D) None
A
-
The reactive power drawn by a synchronous motor is negative when
A) Over-excited
B) Under-excited
C) Normally excited
D) None
A
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