Q1. Which DC generator provides nearly constant terminal voltage over a wide range of load current?
A. Series generator
B. Shunt generator
C. Cumulative compound generator
D. Differential compound generator
Correct Option: C
Explanation:
A cumulative compound generator has both shunt and series field windings. The series field aids the shunt field as load increases, compensating for armature resistance and brush drop. This maintains nearly constant terminal voltage over a wide load range, which is why it is commonly used in practice.
Q2. In a DC shunt generator, increase in load current causes terminal voltage to:
A. Increase
B. Decrease slightly
C. Remain absolutely constant
D. Increase sharply
Correct Option: B
Explanation:
In a shunt generator, increase in load current increases armature current, causing higher armature resistance drop and brush drop. Since flux remains nearly constant, the terminal voltage decreases slightly.
Q3. Differential compound generators are rarely used because they:
A. Have low efficiency
B. Give unstable voltage regulation
C. Require high excitation current
D. Produce excessive losses
Correct Option: B
Explanation:
In differential compounding, the series field opposes the shunt field. As load increases, flux decreases, causing a sharp reduction in generated EMF and terminal voltage. This unstable voltage characteristic makes such generators unsuitable for practical use.
Q4. The brush contact drop in a DC machine is approximately:
A. Proportional to armature current
B. Proportional to speed
C. Nearly constant per brush
D. Dependent on load type
Correct Option: C
Explanation:
The contact resistance between the carbon brush and commutator produces an almost constant voltage drop, typically about 1 V per brush, independent of load current and speed.
Q5. Total brush drop in a DC generator with two carbon brushes is approximately:
A. 0.5 V
B. 1 V
C. 2 V
D. 4 V
Correct Option: C
Explanation:
Each carbon brush contributes nearly 1 V drop. With two brushes (positive and negative), the total brush drop is approximately 2 V.
Q6. Generated EMF of a DC generator does NOT depend on:
A. Flux per pole
B. Speed
C. Number of armature conductors
D. Armature resistance
Correct Option: D
Explanation:
Generated EMF is given by E = PΦZN / 60A. It depends on number of poles, flux per pole, total conductors, speed, and parallel paths. Armature resistance affects terminal voltage, not the generated EMF.
Q7. For the same speed and flux, EMF of lap-wound armature compared to wave-wound armature is:
A. Same
B. Double
C. Half
D. Zero
Correct Option: C
Explanation:
Lap winding has more parallel paths than wave winding. Since generated EMF is inversely proportional to the number of parallel paths, lap-wound armatures produce lower EMF under identical speed and flux conditions.
Q8. Wave winding is preferred in DC generators mainly because it:
A. Gives higher current rating
B. Gives higher voltage rating
C. Reduces copper loss
D. Improves commutation
Correct Option: B
Explanation:
Wave winding always has only two parallel paths, which results in higher voltage and lower current capacity. Hence, it is preferred for high-voltage DC generators.
Q9. A 6-pole wave-wound DC generator has 720 conductors, flux per pole 0.025 Wb and speed 600 rpm. Generated EMF is:
A. 300 V
B. 375 V
C. 450 V
D. 540 V
Correct Option: B
Explanation:
Using E = PΦZN / 60A, where P = 6, Φ = 0.025 Wb, Z = 720, N = 600 rpm, and A = 2 (wave winding), the generated EMF comes out to be 375 V.
Q10. If speed of a DC generator is doubled while flux remains constant, generated EMF will:
A. Remain same
B. Become half
C. Become double
D. Become four times
Correct Option: C
Explanation:
Generated EMF is directly proportional to speed. Therefore, doubling the speed while keeping flux constant will double the generated EMF.
Q11. Which loss in a DC generator is independent of load current?
A. Armature copper loss
B. Field copper loss
C. Stray load loss
D. Brush contact loss
Correct Option: B
Explanation:
In a shunt generator, the field current remains nearly constant irrespective of load. Hence, field copper loss is considered a constant loss and is independent of load current.
Q12. Iron losses in a DC generator depend mainly on:
A. Load current
B. Armature resistance
C. Speed and flux density
D. Terminal voltage
Correct Option: C
Explanation:
Iron losses consist of hysteresis loss, which depends on flux density, and eddy current loss, which depends on both speed and flux density.
Q13. Stray load losses in DC machines are primarily due to:
B. Bearing friction
C. Windage
D. Field resistance
Correct Option: A
Explanation:
Armature reaction under load distorts the main flux, causing additional iron and copper losses, which are collectively known as stray load losses.
Q14. Mechanical input power to a DC generator is equal to:
A. Electrical output power only
B. Electrical output power + total losses
C. Electrical output − losses
D. Copper losses only
Correct Option: B
Explanation:
Mechanical input power supplies the electrical output as well as all losses such as copper losses, iron losses, and mechanical losses.
Q15. Which of the following is NOT a mechanical loss in a DC generator?
A. Windage loss
B. Friction loss
C. Bearing loss
D. Eddy current loss
Correct Option: D
Explanation:
Eddy current loss is an iron loss. Mechanical losses include windage, friction, and bearing losses only.
Q16. Maximum efficiency of a DC generator occurs when:
A. Total losses are minimum
B. Output power is maximum
C. Variable losses equal constant losses
D. Copper losses are zero
Correct Option: C
Explanation:
The condition for maximum efficiency is achieved when variable losses (armature copper loss) are equal to constant losses (iron, mechanical, and field copper losses).
Q17. A DC generator has constant losses of 800 W. For maximum efficiency, armature copper loss should be:
A. 400 W
B. 600 W
C. 800 W
D. 1600 W
Correct Option: C
Explanation:
At maximum efficiency, variable losses must equal constant losses. Hence, armature copper loss should be 800 W.
Q18. At no-load condition, efficiency of a DC generator is:
A. Maximum
B. Zero
C. Equal to full-load efficiency
D. Negative
Correct Option: B
Explanation:
At no load, the output power is zero while losses still exist. Therefore, efficiency becomes zero.
Q19. If flux is reduced by 10% and speed is increased by 10%, generated EMF will:
A. Increase
B. Decrease
C. Remain approximately same
D. Become zero
Correct Option: C
Explanation:
Generated EMF is proportional to the product of flux and speed (ΦN). A 10% decrease in flux and a 10% increase in speed nearly cancel each other, keeping EMF approximately constant.
Q20. At light load, efficiency of DC generator is poor mainly because of:
A. High copper loss
B. High stray loss
C. Dominance of constant losses
D. Armature reaction
Correct Option: C
Explanation:
At light load, output power is small while constant losses remain significant, resulting in low efficiency.
Q21. A DC generator supplies 100 A at 220 V. Total losses are 2 kW. Efficiency is:
A. 88%
B. 90%
C. 91.60%
D. 95%
Correct Option: C
Explanation:
Output power = 220 × 100 = 22 kW. Input power = 22 + 2 = 24 kW. Efficiency = 22 / 24 = 91.6%.
Q22. Which loss varies as square of load current?
A. Iron loss
B. Mechanical loss
C. Armature copper loss
D. Field copper loss
Correct Option: C
Explanation:
Armature copper loss is given by I²R and hence varies as the square of load current.
Q23. Lap winding is preferred for high current DC generators because it:
A. Produces higher EMF
B. Has more parallel paths
C. Reduces iron loss
D. Improves speed regulation
Correct Option: B
Explanation:
Lap winding provides more parallel paths, allowing the armature to carry higher current safely.
Q24. Increasing number of poles in a DC generator will:
A. Decrease EMF
B. Increase EMF
C. Not affect EMF
D. Reduce efficiency
Correct Option: B
Explanation:
Generated EMF is directly proportional to the number of poles. Therefore, increasing poles increases the EMF.
Q25. Which loss is absent in an ideal DC generator?
A. Copper loss
B. Iron loss
C. Mechanical loss
D. All of the above
Correct Option: D
Explanation:
An ideal DC generator assumes no copper losses, no iron losses, and no mechanical losses.
Q26. Condition for maximum efficiency of a DC generator is independent of:
A. Load current
B. Constant losses
C. Armature resistance
D. Speed
Correct Option: D
Explanation:
The condition for maximum efficiency depends on the equality of losses and is independent of speed.
Q27. Which loss exists even when DC generator runs at no load?
A. Armature copper loss
B. Field copper loss
C. Load loss
D. Stray loss
Correct Option: B
Explanation:
The field winding remains energized even at no load. Hence, field copper loss always exists.
Q28. Increase in flux density mainly increases which loss?
A. Copper loss
B. Iron loss
C. Mechanical loss
D. Brush loss
Correct Option: B
Explanation:
Higher flux density increases hysteresis and eddy current losses, both of which are iron losses.
Q29. Terminal voltage of a DC generator is less than generated EMF mainly due to:
A. Armature resistance drop
B. Brush contact drop
C. Both A and B
D. Field resistance
Correct Option: C
Explanation:
Terminal voltage is reduced due to voltage drop across armature resistance and brush contact drop.
Q30. Output voltage of a DC generator is controlled primarily by adjusting:
A. Speed
B. Field current
C. Load resistance
D. Armature resistance
Correct Option: B
Explanation:
Output voltage depends on generated EMF, which is controlled by adjusting the field current and hence the flux.