Q1. In a salient pole synchronous generator, armature reaction produces:
A. Only demagnetizing effect
B. Only cross-magnetizing effect
C. Both demagnetizing and cross-magnetizing effects
D. Only magnetizing effect
Correct Option: C
Explanation:
In a salient pole machine, the air-gap is non-uniform. Due to this non-uniform air-gap, armature reaction splits into two components depending on load angle: one component produces demagnetizing effect and the other produces cross-magnetizing effect. Hence, both effects are present simultaneously.
Q2. Why salient pole alternators are used for low-speed applications?
A. High efficiency
B. Large diameter and short axial length
C. Better voltage regulation
D. Lower synchronous reactance
Correct Option: B
Explanation:
Salient pole alternators have a large diameter and short axial length, allowing accommodation of a large number of poles. This construction makes them suitable for low-speed prime movers such as hydro turbines and diesel engines.
Q3. In a salient pole synchronous generator, power developed depends upon:
A. Only excitation emf
B. Only load angle
C. Load angle and reluctance torque
D. Speed only
Correct Option: C
Explanation:
The power equation of a salient pole generator contains two terms: excitation power (dependent on load angle) and reluctance power (due to saliency). Hence, power depends on both load angle and reluctance torque.
Q4. The power developed by a cylindrical rotor alternator is given by:
A. P = (EV/Xs) sinδ
B. P = (EV/Xd) cosδ
C. P = (E²/Xs) sinδ
D. P = V²Xs
Correct Option: A
Explanation:
In cylindrical rotor alternators, the air-gap is uniform, so only excitation power exists. The developed power is given by P = (EV/Xs) sinδ.
Q5. Which condition must be satisfied for synchronizing an alternator?
A. Equal power factor
B. Equal voltage, frequency and phase sequence
C. Equal speed only
D. Equal load sharing
Correct Option: B
Explanation:
For proper synchronizing, the incoming alternator must have the same RMS voltage, frequency, phase sequence, and zero phase angle difference with the bus-bars to avoid heavy circulating currents.
Q6. Synchronizing current is maximum when:
A. Voltages are equal
B. Phase angle difference is zero
C. Voltage difference is maximum
D. Frequency is same
Correct Option: C
Explanation:
Synchronizing current depends on voltage difference and phase angle difference. When the voltage difference between the alternator and bus-bars is maximum, the circulating synchronizing current is maximum.
Q7. Synchronizing power is proportional to:
A. Rotor speed
B. Load current
C. Cosine of load angle (cosδ)
D. Sine of load angle
Correct Option: C
Explanation:
Synchronizing power is the rate of change of developed power with respect to load angle. Since P = (EV/Xs) sinδ, synchronizing power Ps = dP/dδ = (EV/Xs) cosδ.
Q8. When alternator is connected to infinite bus-bars:
A. Speed changes with load
B. Terminal voltage changes
C. Frequency remains constant
D. Excitation has no effect
Correct Option: C
Explanation:
Infinite bus-bars maintain constant voltage and constant frequency. When connected, alternator speed and frequency remain fixed, while excitation controls reactive power only.
Q9. Synchronizing torque in an alternator is due to:
A. Armature resistance
B. Damper winding
C. Change in load angle
D. Field current only
Correct Option: C
Explanation:
Synchronizing torque is produced due to change in load angle between rotor magnetic field and stator rotating magnetic field. It acts as a restoring torque to maintain synchronism.
Q10. Effect of increase in load on synchronizing power is:
A. Increases
B. Decreases
C. Becomes zero
D. Remains constant
Correct Option: B
Explanation:
As load increases, load angle increases and the slope of the power-angle curve reduces. Hence, synchronizing power decreases with increase in load.
Q11. An alternator has synchronizing power coefficient of 500 kW per electrical radian. Find synchronizing power for a displacement of 5°.
A. 21.8 kW
B. 43.6 kW
C. 87.2 kW
D. 174.4 kW
Correct Option: B
Explanation:
Synchronizing power Ps = K × Î´. Here δ = 5° = 0.0873 rad. Ps = 500 × 0.0873 ≈ 43.6 kW.
Q12. Which torque tends to maintain synchronism?
A. Shaft torque
B. Reluctance torque
C. Synchronizing torque
D. Mechanical torque
Correct Option: C
Explanation:
Synchronizing torque is a restoring torque that acts whenever the rotor deviates from synchronous speed, thereby maintaining synchronism with the rotating magnetic field.
Q13. If excitation of alternator connected to infinite bus-bars is increased:
A. Speed increases
B. Power output increases
C. Reactive power increases
D. Frequency increases
Correct Option: C
Explanation:
In infinite bus operation, speed and frequency remain constant. Increasing excitation increases internal EMF, causing the alternator to supply more reactive power (VARs).
Q14. In salient pole machine, second component of power is due to:
A. Armature reaction
B. Reluctance
C. Mechanical losses
D. Eddy current losses
Correct Option: B
Explanation:
The second term in the power equation of a salient pole machine arises due to saliency of poles and is called reluctance power.
Q15. An alternator operates at synchronous speed of 1500 rpm. Find electrical angular velocity.
A. 157 rad/s
B. 314 rad/s
C. 78.5 rad/s
D. 628 rad/s
Correct Option: B
Explanation:
Electrical angular velocity ω = 2Ï€N/60. For N = 1500 rpm, ω = (2Ï€ × 1500)/60 = 314 rad/s.
Q16. Synchronizing power becomes zero when:
A. Load angle is zero
B. Load angle is 90°
C. Voltage is zero
D. Speed is zero
Correct Option: B
Explanation:
Synchronizing power Ps ∝ cosδ. At δ = 90°, cos90° = 0, hence synchronizing power becomes zero, indicating stability limit.
Q17. Which factor improves stability of alternator?
A. High armature resistance
B. High synchronizing power
C. Low excitation
D. Low speed
Correct Option: B
Explanation:
Higher synchronizing power provides stronger restoring torque for small disturbances, thereby improving stability of the alternator.
Q18. If maximum power of alternator is 1000 kW, find power at δ = 30°.
A. 250 kW
B. 500 kW
C. 707 kW
D. 866 kW
Correct Option: B
Explanation:
For cylindrical rotor alternator, P = Pmax sinδ. At δ = 30°, sin30° = 0.5, so P = 1000 × 0.5 = 500 kW.
Q19. Salient pole alternator power equation includes:
A. One term only
B. Two terms
C. Three terms
D. Four terms
Correct Option: B
Explanation:
Salient pole alternator has excitation power and reluctance power components. Therefore, its power equation consists of two terms.
Q20. An alternator has synchronizing torque coefficient of 200 Nm per electrical radian. Find torque for 10° displacement.
A. 17.4 Nm
B. 34.9 Nm
C. 69.8 Nm
D. 139.6 Nm
Correct Option: B
Explanation:
Torque Ts = Kt × Î´. Here δ = 10° = 0.1745 rad. Ts = 200 × 0.1745 ≈ 34.9 Nm.