Q1. Voltage regulation of an alternator is defined as:
A. Change in terminal voltage from no-load to full-load at constant excitation
B. Change in excitation at constant terminal voltage
C. Ratio of synchronous reactance to resistance
D. Ratio of induced emf to terminal voltage
Correct Option: A
Explanation:
Voltage regulation indicates the change in terminal voltage when load varies from no-load to full-load, keeping speed and excitation constant. It is given by %VR = (E₀ − V)/V × 100.
Q2. Voltage regulation of an alternator at leading power factor is generally:
A. Positive
B. Zero
C. Negative
D. Infinite
Correct Option: C
Explanation:
At leading power factor, armature reaction is magnetizing. Terminal voltage rises on load, making E₀ < V and hence voltage regulation becomes negative.
Q3. Which method gives maximum value of voltage regulation?
A. ZPF Method
B. MMF Method
C. Synchronous Impedance Method
D. Direct Load Test
Correct Option: C
Explanation:
The synchronous impedance (EMF) method assumes armature reaction to be purely reactive, which overestimates voltage drop and hence gives maximum voltage regulation.
Q4. Which method gives most accurate voltage regulation?
A. EMF Method
B. MMF Method
C. Potier Method
D. Synchronous Reactance Method
Correct Option: C
Explanation:
Potier (ZPF) method separates leakage reactance and armature reaction effects, closely representing actual operating conditions and giving most accurate results.
Q5. Synchronous impedance of an alternator is obtained from:
A. Load test only
B. OC test only
C. SC test only
D. OC and SC tests
Correct Option: D
Explanation:
Synchronous impedance Zs is calculated using open-circuit voltage from OC test and short-circuit current from SC test as Zs = Eoc/Isc.
Q6. An alternator has Zs = 20 Ω and Ra = 4 Ω. Synchronous reactance Xs is:
A. 16 Ω
B. 19.6 Ω
C. 20 Ω
D. 24 Ω
Correct Option: B
Explanation:
Xs = √(Zs² − Ra²) = √(400 − 16) = √384 ≈ 19.6 Ω.
Q7. Why does EMF method overestimate voltage regulation?
A. Neglects armature resistance
B. Assumes constant speed
C. Assumes armature reaction as purely reactive
D. Ignores leakage flux
Correct Option: C
Explanation:
EMF method treats armature reaction as completely reactive, whereas in practice it has magnetizing and demagnetizing components, leading to overestimation.
Q8. In MMF method, which components are added vectorially?
A. EMF and terminal voltage
B. Field MMF and armature reaction MMF
C. Current and voltage
D. Reactance and resistance
Correct Option: B
Explanation:
In MMF method, field MMF for no-load emf and armature reaction MMF at load are added vectorially to obtain resultant excitation.
Q9. Which test is NOT required in Potier (ZPF) method?
A. Open Circuit Test
B. Short Circuit Test
C. Zero Power Factor Test
D. Load Test
Correct Option: B
Explanation:
Potier method uses OC test and ZPF test. Short-circuit test is not required because leakage reactance is obtained from Potier triangle.
Q10. The Potier triangle is used to separate:
A. Copper loss and iron loss
B. Leakage reactance and armature reaction
C. Synchronous reactance and resistance
D. Field and armature currents
Correct Option: B
Explanation:
Potier triangle separates voltage drop due to leakage reactance from that due to armature reaction.
Q11. An alternator operating at leading power factor may show:
A. High positive regulation
B. Zero regulation
C. Negative regulation
D. Maximum regulation
Correct Option: C
Explanation:
At leading PF, terminal voltage rises on load due to magnetizing armature reaction, resulting in negative regulation.
Q12. At zero power factor lagging load, voltage regulation is:
A. Minimum
B. Maximum
C. Zero
D. Negative
Correct Option: B
Explanation:
At ZPF lagging, armature reaction is fully demagnetizing, causing maximum voltage drop and hence maximum regulation.
Q13. Armature reaction in an alternator at unity power factor is:
A. Demagnetizing
B. Magnetizing
C. Cross-magnetizing
D. Zero
Correct Option: C
Explanation:
At unity PF, armature MMF is perpendicular to main field MMF, causing cross-magnetizing effect.
Q14. In EMF method, voltage regulation depends on:
A. Only resistance
B. Only reactance
C. Power factor
D. Frequency only
Correct Option: C
Explanation:
Voltage regulation strongly depends on load power factor. Lagging PF increases regulation, while leading PF may make it negative.
Q15. An alternator delivers rated current at leading PF. Which current is reduced?
A. Armature current
B. Load current
C. Field current
D. Line current
Correct Option: C
Explanation:
Leading PF load produces magnetizing armature reaction, reducing excitation requirement and hence field current.
Q16. Percentage voltage regulation of the given alternator using EMF method is:
A. 5.5%
B. 10.2%
C. 12.8%
D. 15.4%
Correct Option: A
Explanation:
Using EMF method, %VR = (E − V)/V × 100 ≈ 5.5% for the given data.
Q17. Which method requires ZPF characteristic?
A. EMF Method
B. MMF Method
C. Potier Method
D. Direct Method
Correct Option: C
Explanation:
Only Potier method requires Zero Power Factor characteristic to construct Potier triangle.
Q18. Which method treats armature reaction as:
A. Resistance
B. Reactance
C. MMF
D. Constant voltage
Correct Option: C
Explanation:
MMF method treats armature reaction as magnetomotive force rather than reactance.
Q19. In Potier method, leakage reactance drop is assumed to be:
A. Constant
B. Zero
C. Variable
D. Negligible
Correct Option: A
Explanation:
Potier method assumes leakage reactance drop remains constant over load range.
Q20. Which method is best suited for large alternators?
A. EMF method
B. MMF method
C. Potier method
D. Direct load test
Correct Option: C
Explanation:
For large alternators, direct loading is impractical. Potier method provides reliable and accurate voltage regulation results.