Q1. When the field excitation of a synchronous motor is increased at constant load, the armature current:
A. Continuously increases
B. Continuously decreases
C. First decreases and then increases
D. Remains constant
Correct Option: 3
Explanation:
At constant load, under-excitation causes high lagging current. At unity power factor, reactive component becomes zero and armature current is minimum. Further increase in excitation makes the motor over-excited, drawing leading current and increasing armature current again. This produces the characteristic V-curve.
Q2. The V-curve of a synchronous motor represents the variation of:
A. Speed with excitation
B. Armature current with load
C. Armature current with excitation
D. Power factor with speed
Correct Option: 3
Explanation:
The V-curve is plotted between armature current and field excitation at constant load. Armature current decreases to a minimum at unity power factor and increases on either side, forming a V-shaped curve.
Q3. At unity power factor operation of a synchronous motor, the motor is said to be:
A. Under-excited
B. Over-excited
C. Normally excited
D. Hunting
Correct Option: 3
Explanation:
At unity power factor, induced EMF equals terminal voltage. Reactive power exchange is zero and armature current is minimum. This condition is called normal excitation.
Q4. Hunting in a synchronous motor is mainly caused due to:
A. Sudden change in supply frequency
B. Sudden change in load
C. Over-excitation
D. Rotor copper losses
Correct Option: 2
Explanation:
Sudden change in load causes sudden change in load angle (δ). Due to rotor inertia, the rotor oscillates about its equilibrium position, producing speed and torque oscillations known as hunting.
Q5. Which winding is specifically used to reduce hunting in synchronous motors?
A. Armature winding
B. Field winding
C. Damper winding
D. Compensating winding
Correct Option: 3
Explanation:
Damper winding produces induction-motor-like damping torque during oscillations. The induced currents oppose rotor speed fluctuations, thereby reducing hunting.
Q6. Which of the following is NOT a method of starting a synchronous motor?
A. Using damper winding
B. Pony motor method
C. Auto-transformer starter
D. Variable frequency supply
Correct Option: 3
Explanation:
Auto-transformer starters are used for induction motors. Synchronous motors are started using damper winding, pony motor, or variable frequency supply.
Q7. A synchronous motor cannot start by itself because:
A. Rotor inertia is high
B. Starting torque is zero
C. Rotor resistance is very low
D. Power factor is poor
Correct Option: 2
Explanation:
At standstill, the stator magnetic field produces alternating torque. Average torque over a cycle is zero, hence the synchronous motor is not self-starting.
Q8. During starting of a synchronous motor using damper winding, the motor initially behaves like:
A. DC shunt motor
B. DC series motor
C. Induction motor
D. Reluctance motor
Correct Option: 3
Explanation:
Damper bars act like a squirrel-cage rotor. Due to slip, induced currents produce induction motor torque and the motor starts as an induction motor.
Q9. Which statement correctly compares synchronous and induction motors?
A. Synchronous motor has slip under load
B. Induction motor runs at synchronous speed
C. Synchronous motor speed is constant
D. Induction motor power factor is always unity
Correct Option: 3
Explanation:
A synchronous motor runs exactly at synchronous speed irrespective of load. Induction motors always have slip and speed varies with load.
Q10. Synchronous motors are preferred over induction motors mainly for:
A. High starting torque
B. Variable speed operation
C. Power factor improvement
D. Simple construction
Correct Option: 3
Explanation:
Over-excited synchronous motors draw leading current and supply reactive power, improving system power factor. Hence they are used as synchronous condensers.
Q11. A synchronous motor draws minimum armature current at unity power factor because:
A. Load angle is maximum
B. Field current is minimum
C. Reactive component of current is zero
D. Back emf is zero
Correct Option: 3
Explanation:
At unity power factor, reactive component of armature current is zero. Only active component exists, resulting in minimum total armature current.
Q12. A synchronous motor takes 50 A at under-excited condition and 40 A at over-excited condition for same load. The current at unity power factor will be approximately:
A. 32 A
B. 35 A
C. 40 A
D. 45 A
Correct Option: 1
Explanation:
Armature current at unity power factor is minimum and lies below both under-excited and over-excited currents. Hence it is approximately 32 A.
Q13. A 3-phase synchronous motor operates at 11 kV, 50 Hz, 6-pole. Find synchronous speed.
A. 750 rpm
B. 1000 rpm
C. 1500 rpm
D. 3000 rpm
Correct Option: 2
Explanation:
Synchronous speed Ns = 120f / P = (120 × 50) / 6 = 1000 rpm.
Q14. If load on a synchronous motor suddenly increases, the immediate effect is:
A. Increase in speed
B. Decrease in speed
C. Increase in load angle
D. Decrease in excitation
Correct Option: 3
Explanation:
With sudden load increase, rotor momentarily slows, causing increase in load angle (δ). Speed returns to synchronous value in steady state.
Q15. Which condition corresponds to over-excitation?
A. Motor draws lagging current
B. Motor draws leading current
C. Motor draws zero current
D. Motor stops
Correct Option: 2
Explanation:
Over-excited synchronous motor has higher field current, induced EMF greater than terminal voltage, and draws leading current.
Q16. A synchronous motor develops maximum power when load angle δ is:
A. 30°
B. 45°
C. 60°
D. 90°
Correct Option: 4
Explanation:
Power developed P ∝ sinδ. Maximum power occurs when sinδ = 1, i.e., δ = 90°.
Q17. For a cylindrical rotor synchronous motor, power developed is proportional to:
A. sin δ
B. cos δ
C. tan δ
D. sin² δ
Correct Option: 1
Explanation:
Cylindrical rotor machines develop electromagnetic power proportional to sinδ only. Reluctance torque term is absent.
Q18. A 3-phase synchronous motor has V = 6.6 kV, E = 7 kV, Xs = 20 Ω/phase, δ = 25°. Find power developed.
A. 0.85 MW
B. 0.98 MW
C. 1.48 MW
D. 2.10 MW
Correct Option: 2
Explanation:
Using power equation P = (3VE / Xs) sinδ and converting line values to phase gives power ≈ 0.98 MW.
Q19. Hunting frequency in a synchronous motor depends mainly on:
A. Rotor resistance
B. Rotor inertia
C. Supply voltage
D. Armature resistance
Correct Option: 2
Explanation:
Hunting frequency depends on rotor inertia and synchronizing torque stiffness. Higher inertia results in lower oscillation frequency.
Q20. Which application is most suitable for synchronous motors?
A. Elevators
B. Rolling mills
C. Constant speed drives
D. Cranes
Correct Option: 3
Explanation:
Synchronous motors run at constant speed irrespective of load variation, making them ideal for constant speed applications like compressors and pumps.