DC generator MCQ
70 Most Important Qustion of DC
Category 1: Construction and Components (10 Questions)
1. What is the main function of a DC generator?
a) Convert electrical energy to mechanical energy
b) Convert mechanical energy to electrical energy
c) Store electrical energy
d) Regulate voltage
Answer: b) Convert mechanical energy to electrical energy
Explanation: A DC generator uses mechanical energy from a prime mover to produce electrical energy through electromagnetic induction, delivering DC output.
2. Which part of a DC generator creates the magnetic field?
a) Armature
b) Field poles
c) Commutator
d) Brushes
Answer: b) Field poles
Explanation: Field poles, with their windings, generate the magnetic field needed to induce voltage in the armature.
3. What material is used for the yoke in a DC generator?
a) Copper
b) Aluminum
c) Cast iron or steel
d) Plastic
Answer: c) Cast iron or steel
Explanation: The yoke, made of cast iron or steel, provides structural support and a path for magnetic flux due to its ferromagnetic properties.
4. Why is the armature core laminated in a DC generator?
a) To increase strength
b) To reduce eddy current losses
c) To improve cooling
d) To increase flux
Answer: b) To reduce eddy current losses
Explanation: Laminations reduce eddy currents by breaking their paths, minimizing energy loss in the core.
5. What does the commutator do in a DC generator?
a) Increases voltage
b) Converts AC to DC
c) Regulates current
d) Stores energy
Answer: b) Converts AC to DC
Explanation: The commutator reverses the alternating current induced in the armature to produce direct current output.
6. What material are brushes made of in a DC generator?
a) Copper
b) Carbon
c) Aluminum
d) Steel
Answer: b) Carbon
Explanation: Carbon brushes are used for their conductivity and durability, ensuring good contact with the commutator.
7. Where is the armature winding located in a DC generator?
a) Stator
b) Rotor
c) Yoke
d) Field poles
Answer: b) Rotor
Explanation: The armature, where voltage is induced, is placed on the rotor, which rotates in the magnetic field.
8. What is the role of interpoles in a DC generator?
a) Increase voltage
b) Reduce sparking at brushes
c) Improve cooling
d) Increase flux
Answer: b) Reduce sparking at brushes
Explanation: Interpoles counteract armature reaction, improving commutation and reducing brush sparking.
9. What do pole shoes do in a DC generator?
a) Support the armature
b) Distribute magnetic flux evenly
c) Connect brushes
d) Reduce losses
Answer: b) Distribute magnetic flux evenly
Explanation: Pole shoes spread the magnetic field uniformly, ensuring efficient voltage induction.
10. Which component connects the armature to the external circuit?
a) Yoke
b) Commutator
c) Brushes
d) Field winding
Answer: c) Brushes
Explanation: Brushes maintain electrical contact with the rotating commutator, transferring current to the external circuit.
Category 2: Principles and EMF Generation (10 Questions)
11. On which principle does a DC generator work?
a) Electromagnetic induction
b) Coulomb’s law
c) Ohm’s law
d) Faraday’s law of electrolysis
Answer: a) Electromagnetic induction
Explanation: Electromagnetic induction, per Faraday’s law, induces voltage when conductors move in a magnetic field.
12. What is the EMF equation of a DC generator?
a) E = PΦZN / (60A)
b) E = PΦZN / (120A)
c) E = PΦZA / (60N)
d) E = PΦZN / (A)
Answer: a) E = PΦZN / (60A)
Explanation: EMF (E) depends on poles (P), flux per pole (Φ), conductors (Z), speed (N in RPM), and parallel paths (A).
13.What is residual magnetism in a DC generator?
a) Magnetism in the yoke
b) Magnetism remaining in field poles after de-excitation
c) Magnetism in the armature
d) Magnetism in brushes
Answer: b) Magnetism remaining in field poles after de-excitation
Explanation: Residual magnetism is the small magnetic field left in the poles, crucial for initiating voltage in self-excited generators.
14. The generated EMF in a DC generator is directly proportional to:
a) Number of poles
b) Speed of rotation
c) Field current
d) Both b and c
Answer: b) Speed of rotation
Explanation: Per the EMF equation, EMF increases directly with rotational speed (N).
15. Why is the induced EMF alternating in a DC generator?
a) Armature rotates in a magnetic field
b) Field current is alternating
c) Brushes are misaligned
d) Load varies
Answer: a) Armature rotates in a magnetic field
Explanation: The rotating armature induces an alternating EMF, which the commutator converts to DC.
16. What is the critical speed of a DC generator?
a) Speed for maximum power
b) Minimum speed for voltage buildup
c) Speed for maximum efficiency
d) Maximum safe speed
Answer: b) Minimum speed for voltage buildup
Explanation: Below critical speed, the EMF is too low to initiate voltage buildup.
17. What determines the flux per pole in a DC generator?
a) Armature current
b) Field current
c) Load resistance
d) Speed of rotation
Answer: b) Field current
Explanation: Field current through the field windings produces the magnetic flux.
18. In a lap-wound DC generator, the number of parallel paths equals:
a) Number of poles
b) Number of conductors
c) Number of brushes
d) Twice the number of poles
Answer: a) Number of poles
Explanation: Lap winding has parallel paths equal to the number of poles (A = P).
19. In a wave-wound DC generator, the number of parallel paths is:
a) 1
b) 2
c) Number of poles
d) Half the number of poles
Answer: b) 2
Explanation: Wave winding always has two parallel paths, regardless of pole count.
20. What affects the voltage regulation of a DC generator?
a) Armature reaction
b) Load resistance
c) Speed variations
d) All of the above
Answer: d) All of the above
Explanation: Armature reaction, load changes, and speed variations impact terminal voltage.
Category 3: Types of DC Generators (10 Questions)
21. Which DC generator has its field winding in parallel with the armature?
a) Series generator
b) Shunt generator
c) Compound generator
d) Separately excited generator
Answer: b) Shunt generator
Explanation: Shunt generators have field windings connected across the armature, ensuring constant voltage.
22. What defines a series DC generator?
a) Field winding in series with armature
b) Constant field current
c) High voltage regulation
d) Low output current
Answer: a) Field winding in series with armature
Explanation: The field winding carries the full armature current, varying with load.
23. Which DC generator is used for battery charging?
a) Series generator
b) Shunt generator
c) Compound generator
d) Separately excited generator
Answer: b) Shunt generator
Explanation: Shunt generators provide stable voltage, ideal for battery charging.
24. A compound DC generator combines features of:
a) Series and shunt generators
b) Shunt and separately excited generators
c) Series and separately excited generators
d) All generator types
Answer: a) Series and shunt generators
Explanation: It has both series and shunt field windings for balanced voltage regulation.
25. In a cumulative compound DC generator, the fields:
a) Aid each other
b) Oppose each other
c) Are only series
d) Are only shunt
Answer: a) Aid each other
Explanation: Series and shunt fields reinforce each other, improving voltage stability.
26. A differential compound DC generator is used for:
a) Constant voltage
b) Arc welding
c) Battery charging
d) Motor starting
Answer: b) Arc welding
Explanation: Its drooping voltage characteristic suits welding’s high-current, low-voltage needs.
27. What does a separately excited DC generator require?
a) Separate DC source for field excitation
b) High load current
c) Constant speed
d) Commutator
Answer: a) Separate DC source for field excitation
Explanation: The field winding is powered by an external DC source, not the armature.
28. In a shunt generator, the field current comes from:
a) The armature
b) An external source
c) The load
d) The commutator
Answer: a) The armature
Explanation: The shunt field is connected across the armature terminals.
29. Which generator has poor voltage regulation under heavy loads?
a) Series generator
b) Shunt generator
c) Compound generator
d) Separately excited generator
Answer: a) Series generator
Explanation: Voltage increases with load, leading to poor regulation.
30. What is the main advantage of a compound DC generator?
a) High efficiency
b) Stable voltage under varying loads
c) Low cost
d) Simple construction
Answer: b) Stable voltage under varying loads
Explanation: Series and shunt fields balance voltage across load changes.
A DC Generator works on electromagnetic induction. In this MCQ guide, we cover all important DC generator questions for electrical diploma exams.
Category 4: Characteristics and Performance (15 Questions)
31. What does the open-circuit characteristic (OCC) show?
a) Load current vs. Terminal voltage
b) Field current vs. Generated EMF
c) Armature current vs. Speed
d) Voltage vs. Power output
Answer: b) Field current vs. Generated EMF
Explanation: OCC plots EMF against field current under no-load conditions.
32. What does the external characteristic of a DC generator show?
a) Field current vs. EMF
b) Terminal voltage vs. Load current
c) Speed vs. Voltage
d) Efficiency vs. Load
Answer: b) Terminal voltage vs. Load current
Explanation: It shows how terminal voltage changes with load current.
33. What does armature reaction cause in a DC generator?
a) Increased voltage
b) Distorted magnetic field
c) Reduced mechanical losses
d) Increased efficiency
Answer: b) Distorted magnetic field
Explanation: Armature current creates a field that distorts the main magnetic field, affecting commutation.
34. Why does voltage drop in a shunt generator under load?
a) Armature reaction and IR drop
b) Increased field current
c) Reduced speed
d) Flux saturation
Answer: a) Armature reaction and IR drop
Explanation: Armature reaction and resistance losses cause voltage reduction.
35. When is the efficiency of a DC generator maximum?
a) Copper losses equal iron losses
b) Variable losses equal constant losses
c) Armature current is zero
d) Field current is maximum
Answer: b) Variable losses equal constant losses
Explanation: Efficiency peaks when copper (variable) losses equal iron and mechanical (constant) losses.
36. Where do copper losses occur in a DC generator?
a) Field windings
b) Armature windings
c) Both a and b
d) Commutator
Answer: c) Both a and b
Explanation: Copper losses (I²R) occur in the resistive armature and field windings.
37. What causes iron losses in a DC generator?
a) Eddy currents and hysteresis
b) Copper resistance
c) Brush friction
d) Load variations
Answer: a) Eddy currents and hysteresis
Explanation: Eddy currents and hysteresis in the core cause iron losses.
38. What are mechanical losses in a DC generator?
a) Friction and windage
b) Copper losses
c) Iron losses
d) Armature reaction
Answer: a) Friction and windage
Explanation: Friction in bearings and air resistance (windage) cause mechanical losses.
39. What is the critical resistance of a shunt generator?
a) Maximum field resistance
b) Minimum resistance for voltage buildup
c) Armature resistance
d) Load resistance
Answer: b) Minimum resistance for voltage buildup
Explanation: Critical resistance allows enough field current for voltage buildup.
40. What does the load characteristic of a series generator show?
a) Constant voltage
b) Increasing voltage with load
c) Decreasing voltage with load
d) Zero voltage at full load
Answer: b) Increasing voltage with load
Explanation: Voltage rises with load due to increased field current.
41. How is voltage regulation of a DC generator defined?
a) (No-load voltage – Full-load voltage) / Full-load voltage
b) (Full-load voltage – No-load voltage) / No-load voltage
c) (No-load voltage – Full-load voltage) / No-load voltage
d) (Full-load voltage – No-load voltage) / Full-load voltage
Answer: a) (No-load voltage – Full-load voltage) / Full-load voltage
Explanation: It measures the percentage voltage drop from no-load to full load.
42. Why does a shunt generator have better voltage regulation than a series generator?
a) Constant field current
b) Lower armature resistance
c) Higher speed
d) Stronger flux
Answer: a) Constant field current
Explanation: The shunt field current is load-independent, stabilizing voltage.
43. How is armature reaction minimized in a DC generator?
a) Increasing field current
b) Using interpoles
c) Reducing load
d) Increasing speed
Answer: b) Using interpoles
Explanation: Interpoles counteract armature reaction, improving commutation.
44. Why is residual magnetism necessary?
a) For voltage buildup in self-excited generators
b) To increase efficiency
c) To reduce armature reaction
d) To stabilize load current
Answer: a) For voltage buildup in self-excited generators
Explanation: Residual magnetism starts the voltage generation process.
45. What determines the maximum power output of a DC generator?
a) Speed and load
b) Field current only
c) Armature resistance only
d) Brush quality
Answer: a) Speed and load
Explanation: Power output depends on voltage and current, influenced by speed and load.
Category 5: Applications and Miscellaneous (15 Questions)
46. What are common applications of DC generators?
a) AC power systems
b) Battery charging and welding
c) High-voltage transmission
d) Household appliances
Answer: b) Battery charging and welding
Explanation: DC generators provide stable DC for battery charging and arc welding.
47. Why is a shunt DC generator used for constant voltage applications?
a) High current output
b) Stable voltage output
c) Variable speed
d) Low power
Answer: b) Stable voltage output
Explanation: Shunt generators maintain stable voltage across loads.
48. What is the main disadvantage of a series DC generator?
a) Low efficiency
b) Poor voltage regulation
c) High cost
d) Complex construction
Answer: b) Poor voltage regulation
Explanation: Voltage varies significantly with load, making it unstable.
49. What is a differential compound generator used for?
a) High voltage stability
b) Arc welding
c) Battery charging
d) Motor starting
Answer: b) Arc welding
Explanation: Its drooping voltage suits welding’s high-current needs.
50. Why are brushes replaced periodically in a DC generator?
a) They wear out due to friction
b) They lose conductivity
c) They accumulate dust
d) They overheat
Answer: a) They wear out due to friction
Explanation: Brushes wear down from contact with the commutator.
51. What insulates commutator segments?
a) Copper
b) Mica
c) Plastic
d) Aluminum
Answer: b) Mica
Explanation: Mica provides high dielectric strength to insulate segments.
52. What material are field poles made of?
a) Soft iron
b) Copper
c) Aluminum
d) Plastic
Answer: a) Soft iron
Explanation: Soft iron’s high permeability creates a strong magnetic field.
53. What causes sparking at the brushes?
a) Armature reaction
b) High speed
c) Low field current
d) High load resistance
Answer: a) Armature reaction
Explanation: Armature reaction distorts the field, causing poor commutation and sparking.
54. What is the purpose of compensating windings?
a) Increase voltage
b) Reduce armature reaction
c) Improve efficiency
d) Stabilize speed
Answer: b) Reduce armature reaction
Explanation: Compensating windings counteract armature reaction effects.
55. Where are separately excited DC generators typically used?
a) High-power industrial systems
b) Laboratory and testing equipment
c) Household power supply
d) AC motor drives
Answer: b) Laboratory and testing equipment
Explanation: They offer precise voltage control for testing purposes.
56. What controls the speed of a DC generator?
a) Prime mover
b) Load current
c) Field current
d) Armature resistance
Answer: a) Prime mover
Explanation: The prime mover (e.g., turbine) sets the rotational speed.
57. What determines the critical field resistance of a shunt generator?
a) Armature current
b) Load resistance
c) Field winding resistance
d) Speed of rotation
Answer: c) Field winding resistance
Explanation: It’s the maximum resistance allowing voltage buildup.
58. How is the voltage of a DC generator controlled?
a) Load resistance
b) Field rheostat
c) Armature speed
d) Brush position
Answer: b) Field rheostat
Explanation: A rheostat adjusts field current, controlling flux and voltage.
59. What winding is common in small DC generators?
a) Lap winding
b) Wave winding
c) Series winding
d) Compound winding
Answer: b) Wave winding
Explanation: Wave winding is simpler for small generators with fewer poles.
60. What is the typical efficiency range of a DC generator?
a) 50-60%
b) 70-85%
c) 90-95%
d) 95-100%
Answer: b) 70-85%
Explanation: Efficiency is reduced by copper, iron, and mechanical losses.
Category 6: Numerical and Analytical Concepts (15 Questions)
61. A 4-pole DC generator with 480 conductors, 0.02 Wb flux per pole, at 1200 RPM (lap-wound) produces an EMF of:
a) 192 V
b) 384 V
c) 96 V
d) 576 V
Answer: a) 192 V
Explanation: E = PΦZN / (60A), A = P = 4 (lap winding). E = (4 × 0.02 × 480 × 1200) / (60 × 4) = 192 V.
62. The voltage drop due to armature resistance depends on:
a) Field current
b) Armature current
c) Load voltage
d) Speed
Answer: b) Armature current
Explanation: Voltage drop = I_a × R_a, where I_a is armature current.
63. The power output of a DC generator is:
a) V × I
b) V / I
c) I²R
d) V²/R
Answer: a) V × I
Explanation: Power is the product of terminal voltage and load current.
64. If armature resistance is 0.1 Ω and current is 50 A, the voltage drop is:
a) 5 V
b) 10 V
c) 2 V
d) 20 V
Answer: a) 5 V
Explanation: Voltage drop = I_a × R_a = 50 × 0.1 = 5 V.
65. The copper loss in an armature with 0.1 Ω resistance and 50 A current is:
a) 250 W
b) 500 W
c) 100 W
d) 1000 W
Answer: a) 250 W
Explanation: Copper loss = I²R = 50² × 0.1 = 250 W.
66. A 4-pole lap-wound generator with 120 slots and 4 conductors per slot has how many conductors?
a) 480
b) 240
c) 960
d) 120
Answer: a) 480
Explanation: Total conductors = Slots × Conductors per slot = 120 × 4 = 480.
67. At 1200 RPM, a DC generator completes one revolution every:
a) 0.05 s
b) 0.02 s
c) 0.01 s
d) 0.1 s
Answer: a) 0.05 s
Explanation: Time per revolution = 60 / RPM = 60 / 1200 = 0.05 s.
68. If field current is 2 A and field resistance is 50 Ω, the field voltage is:
a) 100 V
b) 200 V
c) 50 V
d) 25 V
Answer: a) 100 V
Explanation: Field voltage = I_f × R_f = 2 × 50 = 100 V.
69. The frequency of induced EMF in a 4-pole generator at 1200 RPM is:
a) 40 Hz
b) 20 Hz
c) 80 Hz
d) 60 Hz
Answer: a) 40 Hz
Explanation: Frequency = (P × RPM) / 120 = (4 × 1200) / 120 = 40 Hz.
70. The torque required to drive a DC generator depends on:
a) Power output
b) Load current
c) Speed
d) Both a and b
Answer: d) Both a and b
Explanation: Torque (T = P / ω) depends on power (V × I) and speed.
10 Additional Important MCQs on DC Generators with Answers and Explanations
1. What is the purpose of the equalizer ring in a lap-wound DC generator?
a) To increase voltage output
b) To balance current in parallel paths
c) To reduce magnetic flux
d) To improve cooling
Answer: b) To balance current in parallel paths
Explanation: In lap-wound generators, equalizer rings connect points of equal potential across parallel paths to balance current distribution and prevent overloading.
2. The back EMF in a DC generator is:
a) Equal to the terminal voltage
b) The voltage induced in the armature
c) The voltage across the load
d) The voltage drop in the armature
Answer: b) The voltage induced in the armature
Explanation: Back EMF is the voltage induced in the armature due to rotation in the magnetic field, opposing the applied voltage in a motor but equivalent to generated EMF in a generator.
3. In a DC generator, the brush contact loss is proportional to:
a) Armature current
b) Field current
c) Speed of rotation
d) Magnetic flux
Answer: a) Armature current
Explanation: Brush contact loss occurs due to resistance at the brush-commutator interface, proportional to the armature current flowing through it.
4. A 6-pole DC generator with 720 conductors, 0.015 Wb flux per pole, and 1000 RPM (wave-wound) produces an EMF of:
a) 180 V
b) 360 V
c) 90 V
d) 540 V
Answer: a) 180 V
Explanation: For wave winding, A = 2. Using E = PΦZN / (60A), E = (6 × 0.015 × 720 × 1000) / (60 × 2) = 180 V.
5. The main purpose of a diverter in a compound DC generator is to:
a) Increase field current
b) Control series field current
c) Reduce armature reaction
d) Improve efficiency
Answer: b) Control series field current
Explanation: A diverter (shunt resistor) across the series field adjusts the series field current to fine-tune voltage regulation.
6. The saturation region in the OCC of a DC generator indicates:
a) Constant voltage output
b) Maximum flux with increasing field current
c) Minimum armature reaction
d) Linear increase in EMF
Answer: b) Maximum flux with increasing field current
Explanation: In the saturation region, the magnetic core reaches maximum flux, and further field current increases yield minimal EMF gains.
7. If a DC generator’s speed increases by 20%, the generated EMF:
a) Increases by 20%
b) Decreases by 20%
c) Remains constant
d) Doubles
Answer: a) Increases by 20%
Explanation: EMF is directly proportional to speed (E ∝ N). A 20% speed increase results in a 20% EMF increase.
8. The stray losses in a DC generator include:
a) Copper and iron losses
b) Mechanical and brush contact losses
c) Hysteresis and eddy current losses
d) Miscellaneous unaccounted losses
Answer: d) Miscellaneous unaccounted losses
Explanation: Stray losses are minor, unclassified losses (e.g., due to flux leakage), not covered by copper, iron, or mechanical losses.
9. A shunt generator fails to build up voltage if:
a) Field resistance is above critical resistance
b) Armature resistance is too high
c) Load current is too high
d) Speed is too high
Answer: a) Field resistance is above critical resistance
Explanation: If field resistance exceeds critical resistance, insufficient field current prevents voltage buildup.
10. The typical use of a level-compounded DC generator is:
a) Arc welding
b) Constant voltage supply
c) High-current applications
d) Variable speed drives
Answer: b) Constant voltage supply
Explanation: Level-compounded generators maintain nearly constant voltage across loads, ideal for stable power supplies.