Q1. For an alternator supplying a purely inductive load, the armature reaction is:
A. Purely demagnetising
B. Purely magnetising
C. Purely cross-magnetising
D. Cross + magnetising
Correct Option: A
Explanation:
For a purely inductive load, armature current lags terminal voltage by 90°. In this condition, armature MMF directly opposes the main field MMF, causing a reduction in flux. Hence, armature reaction is purely demagnetising.
Q2. Synchronous reactance concept is mainly introduced to account for:
A. Copper loss only
B. Core loss only
C. Armature reaction + leakage flux
D. Skin effect
Correct Option: C
Explanation:
Synchronous reactance represents the combined effect of armature reaction and leakage reactance. It simplifies alternator analysis and is not a physical reactance.
Q3. A 3-phase alternator has synchronous reactance Xs = 15 Ω and armature resistance is negligible. If it supplies rated current at 0.8 lagging PF, the regulation will be:
A. Zero
B. Negative
C. Positive
D. Infinite
Correct Option: C
Explanation:
Lagging power factor load produces demagnetising armature reaction, causing terminal voltage to drop. Hence voltage regulation is positive.
Q4. In the vector diagram of a loaded alternator, the angle between E and V mainly depends on:
A. Speed
B. Load current magnitude
C. Power factor of load
D. Armature resistance only
Correct Option: C
Explanation:
The phase angle between generated EMF (E) and terminal voltage (V) depends on the phase angle of armature current, which is governed by load power factor.
Q5. An alternator has Ra = 0.3 Ω and Xs = 12 Ω per phase. If phase current is 20 A, find the magnitude of internal impedance drop.
A. 6 V
B. 24 V
C. 240 V
D. 241 V
Correct Option: C
Explanation:
Impedance Z = √(0.3² + 12²) ≈ 12 Ω. Voltage drop = I × Z = 20 × 12 = 240 V.
Q6. Negative voltage regulation in an alternator occurs when it supplies:
A. Lagging PF load
B. Unity PF load
C. Leading PF load
D. Short circuit load
Correct Option: C
Explanation:
With leading power factor load, armature reaction is magnetising and reactive drop subtracts from terminal voltage, causing voltage rise on load and negative regulation.
Q7. Which statement is TRUE regarding synchronous reactance?
A. It is a physical coil
B. It is independent of load
C. It varies with saturation
D. It equals armature resistance
Correct Option: C
Explanation:
Synchronous reactance varies with load due to change in armature reaction and magnetic saturation of the alternator core.
Q8. If an alternator supplies current at leading PF, the generated emf E will be:
A. Greater than terminal voltage
B. Equal to terminal voltage
C. Less than terminal voltage
D. Zero
Correct Option: C
Explanation:
For leading PF load, reactive voltage drop subtracts from terminal voltage, making generated emf less than terminal voltage.
Q9. Why armature resistance is neglected in most alternator voltage regulation calculations?
A. It is zero
B. It is frequency dependent
C. It is very small compared to Xs
D. It causes instability
Correct Option: C
Explanation:
Armature resistance is very small compared to synchronous reactance, so its voltage drop contribution is negligible.
Q10. An alternator has Xs = 20 Ω and Ra = 1 Ω. For Ia = 10 A, percentage contribution of Xs in impedance drop is approximately:
A. 50%
B. 80%
C. 95%
D. 99%
Correct Option: D
Explanation:
Z ≈ √(1² + 20²) ≈ 20 Ω. Contribution of Xs ≈ (20/20)×100 ≈ 99%.
Q11. At unity PF load, armature reaction flux:
A. Strengthens main flux
B. Weakens main flux
C. Distorts main flux
D. Cancels main flux
Correct Option: C
Explanation:
At unity PF, armature MMF is perpendicular to main field MMF, causing distortion without change in magnitude of flux.
Q12. If Xs >> Ra, then voltage regulation depends mainly on:
A. Armature resistance
B. Speed
C. Power factor
D. Frequency
Correct Option: C
Explanation:
When synchronous reactance dominates, voltage regulation is mainly affected by load power factor.
Q13. Which method of voltage regulation gives optimistic (higher than actual) results?
A. Direct loading
B. MMF method
C. ZPF method
D. Synchronous impedance method
Correct Option: D
Explanation:
Synchronous impedance method assumes armature reaction to be fully reactive, leading to overestimation of voltage regulation.
Q14. An alternator delivers rated load at 0.9 leading PF and shows −5% regulation. This implies:
A. Terminal voltage decreases
B. Terminal voltage increases
C. Generated emf equals V
D. Armature reaction is demagnetising
Correct Option: B
Explanation:
Negative regulation means terminal voltage rises on load, which occurs for leading PF due to magnetising armature reaction.
Q15. In synchronous reactance method, armature reaction is assumed to be:
A. Purely magnetising
B. Purely demagnetising
C. Equivalent to leakage reactance
D. Zero
Correct Option: C
Explanation:
Armature reaction is treated as an equivalent reactance and combined with leakage reactance to form synchronous reactance.
Q16. An alternator has per phase impedance (1 + j10) Ω. If Ia = 10 A, magnitude of voltage drop is:
A. 10 V
B. 100 V
C. 101 V
D. 110 V
Correct Option: C
Explanation:
|Z| = √(1² + 10²) ≈ 10.05 Ω. Voltage drop = 10 × 10.05 ≈ 101 V.
Q17. Increase in excitation at constant load causes terminal voltage to:
A. Decrease always
B. Increase always
C. Remain unchanged
D. Become zero
Correct Option: B
Explanation:
Increasing excitation increases field current and main flux, thereby increasing generated EMF and terminal voltage.
Q18. An alternator supplies leading PF load. The synchronous reactance drop will:
A. Add to V
B. Subtract from V
C. Be zero
D. Reverse direction
Correct Option: B
Explanation:
For leading PF, reactive voltage drop subtracts from terminal voltage.
Q19. Why synchronous reactance is not constant practically?
A. Due to frequency change
B. Due to magnetic saturation
C. Due to copper loss
D. Due to armature resistance
Correct Option: B
Explanation:
Magnetic saturation changes flux level with load, altering armature reaction and hence synchronous reactance.
Q20. In an alternator on load, generated emf is greater than terminal voltage because:
A. Armature resistance drop
B. Synchronous reactance drop
C. Mechanical losses
D. Core losses
Correct Option: B
Explanation:
Generated EMF must overcome the reactive drop IaXs. Hence E = V + jIaXs, making E greater than V.