Q1. An alternator works on the principle of:
A. Self-induction
B. Mutual induction
C. Electromagnetic induction
D. Electrostatic induction
Correct Option: C
Explanation:
An alternator operates on Faraday’s law of electromagnetic induction. When a conductor cuts magnetic flux or when the flux linking a conductor changes, an emf is induced. In alternators, the rotating magnetic field produced by the rotor cuts the stationary armature conductors, generating alternating emf.
Q2. In an alternator, emf is induced because:
A. Rotor current changes
B. Flux linking armature conductors changes
C. Armature resistance is low
D. Rotor speed is constant
Correct Option: B
Explanation:
EMF is induced only when there is a change in magnetic flux linking the armature conductors. Constant flux or constant current alone cannot induce emf unless the flux linkage varies with time.
Q3. Why is the armature winding kept stationary in large alternators?
A. To reduce iron loss
B. To simplify insulation of high voltage
C. To reduce copper loss
D. To reduce friction loss
Correct Option: B
Explanation:
Large alternators generate very high voltages (11–33 kV). Insulating such high voltage on rotating parts is difficult. Keeping the armature stationary simplifies insulation, cooling, and maintenance.
Q4. Which part of an alternator carries the load current?
A. Rotor
B. Field winding
C. Stator armature
D. Shaft
Correct Option: C
Explanation:
The stator armature winding supplies electrical power to the external load and hence carries the load current. The rotor carries only DC excitation current.
Q5. Which part provides mechanical support to armature conductors?
A. Field poles
B. Stator core
C. Rotor shaft
D. Slip rings
Correct Option: B
Explanation:
The stator core provides slots for armature conductors, gives mechanical support, and offers a low-reluctance path for magnetic flux.
Q6. Laminations in stator core are used to:
A. Reduce hysteresis loss
B. Reduce eddy current loss
C. Increase flux
D. Reduce copper loss
Correct Option: B
Explanation:
Laminating the stator core increases resistance to circulating currents, thereby reducing eddy current losses and associated heating.
Q7. Salient pole rotors are mainly used in:
A. High-speed alternators
B. Steam turbine alternators
C. Low-speed alternators
D. Turbo alternators
Correct Option: C
Explanation:
Salient pole rotors are suitable for low-speed alternators such as hydro generators. They have large diameter and many poles required for low-speed operation.
Q8. Cylindrical rotor is preferred for:
A. Hydro generators
B. Diesel generators
C. Steam turbine driven alternators
D. Low frequency machines
Correct Option: C
Explanation:
Cylindrical rotors are used in steam turbine driven alternators because they have high mechanical strength, uniform air gap, and are suitable for very high speeds.
Q9. Damper windings are provided on rotor poles to:
A. Increase output voltage
B. Improve efficiency
C. Reduce hunting
D. Reduce copper loss
Correct Option: C
Explanation:
Damper windings reduce hunting by opposing rotor oscillations during sudden load changes, thereby improving stability of the alternator.
Q10. Damper windings in alternator are similar to:
A. Rotor winding of DC motor
B. Armature winding
C. Squirrel cage winding
D. Field winding
Correct Option: C
Explanation:
Damper windings resemble squirrel cage windings of induction motors. Induced currents in these bars produce damping torque that stabilizes rotor motion.
Q11. The frequency of an alternator is given by:
A. f = PN / 60
B. f = 120N / P
C. f = NP / 120
D. f = 60 / NP
Correct Option: C
Explanation:
The standard frequency equation of an alternator is f = (P × N) / 120, where P is number of poles and N is speed in rpm.
Q12. A 12-pole alternator runs at 500 rpm. Find frequency.
A. 25 Hz
B. 50 Hz
C. 60 Hz
D. 100 Hz
Correct Option: B
Explanation:
f = (P × N) / 120 = (12 × 500) / 120 = 50 Hz.
Q13. For constant frequency operation:
A. Speed must change with load
B. Speed must remain constant
C. Field current must be zero
D. Armature resistance must be zero
Correct Option: B
Explanation:
Frequency depends only on speed and number of poles. Therefore, speed must remain constant to maintain constant frequency.
Q14. A 4-pole alternator produces 50 Hz. Find synchronous speed.
A. 750 rpm
B. 1000 rpm
C. 1500 rpm
D. 3000 rpm
Correct Option: C
Explanation:
Synchronous speed Ns = 120f / P = (120 × 50) / 4 = 1500 rpm.
Q15. If rotor speed increases while field current is constant:
A. Frequency increases
B. Voltage increases only
C. Both voltage & frequency increase
D. Frequency decreases
Correct Option: C
Explanation:
Increase in speed increases rate of flux cutting, increasing induced emf. Since frequency is proportional to speed, both voltage and frequency increase.
Q16. Which loss is absent in rotor of alternator?
A. Copper loss
B. Windage loss
C. Eddy current loss
D. Core loss (in DC field rotor)
Correct Option: D
Explanation:
Rotor carries DC excitation, producing constant magnetic field. Hence hysteresis and eddy current losses are absent in rotor.
Q17. Air-gap in alternator is kept:
A. Very large
B. Zero
C. Uniform
D. Non-uniform
Correct Option: C
Explanation:
Uniform air gap ensures smooth emf waveform, uniform magnetic pull, and reduced vibration.
Q18. A 6-pole alternator operates at 1000 rpm. What is frequency?
A. 40 Hz
B. 50 Hz
C. 60 Hz
D. 100 Hz
Correct Option: B
Explanation:
f = (6 × 1000) / 120 = 50 Hz.
Q19. Which rotor has non-uniform air gap?
A. Cylindrical rotor
B. Turbo rotor
C. Salient pole rotor
D. Smooth rotor
Correct Option: C
Explanation:
Salient pole rotors have projecting poles, resulting in non-uniform air gap.
Q20. Damper winding improves stability by:
A. Increasing torque
B. Reducing oscillations
C. Increasing flux
D. Reducing voltage regulation
Correct Option: B
Explanation:
Damper winding suppresses rotor oscillations during load disturbances and helps maintain synchronism.