1. Open-Circuit Characteristic Fundamentals
Q1. The open-circuit characteristic (OCC) of a DC generator represents the relationship between:
A. Terminal voltage and load current
B. No-load generated EMF and field current
C. Armature current and load torque
D. Speed and frequency
Answer: B
Explanation: The OCC plots the no-load EMF (E₀) versus the field current (I_f), illustrating how the generator’s output voltage builds up as the field excitation increases. This curve is fundamental to understanding self-excitation and voltage regulation in DC generators.
Q2. In a DC generator, the OCC is typically obtained by:
A. Connecting a load to the armature
B. Short-circuiting the armature terminals
C. Exciting the field winding with an external DC supply while the load is open
D. Varying the armature resistance
Answer: C
Explanation: To obtain the OCC, the generator is run under no-load conditions, and the field winding is excited externally. This ensures that the voltage measured is solely due to the generated EMF without any voltage drop from load currents.
Q3. Residual magnetism in a DC generator affects the OCC by:
A. Starting the curve at zero voltage
B. Eliminating the need for external excitation
C. Providing an initial voltage even with very low field current
D. Increasing the slope of the curve
Answer: C
Explanation: Residual magnetism causes a small initial voltage at the armature, even when the field current is nearly zero. This ensures that the generator can begin the voltage build-up process when external excitation is applied.
2. Internal and External Characteristics
Q4. The internal characteristic curve of a DC generator plots:
A. Terminal voltage versus load current
B. Generated EMF under load versus armature current
C. No-load voltage versus field current
D. Armature reaction versus load resistance
Answer: B
Explanation: The internal characteristic shows how the generated EMF (E) decreases from its no-load value due to voltage drops (mainly from armature reaction) as the armature current (I_a) increases.
Q5. Which factor primarily causes the internal characteristic to lie below the OCC?
A. Increased field current
B. Armature reaction and voltage drops across internal resistances
C. Higher rotor speeds
D. Improved efficiency
Answer: B
Explanation: When a DC generator supplies load, the armature reaction demagnetizes the main flux and the voltage drop across the armature resistance causes the actual generated EMF to fall below the no-load value represented by the OCC.
Q6. The external (load) characteristic curve of a DC generator shows the variation of:
A. Terminal voltage with field current
B. Terminal voltage with load current
C. Armature current with speed
D. Load torque with armature reaction
Answer: B
Explanation: The external characteristic considers the additional voltage drops due to the armature and field resistances and plots how the terminal voltage (V) changes with the load current (I_L).
3. Effects of Load and Armature Reaction
Q7. When a DC generator is loaded, the terminal voltage decreases mainly due to:
A. Increased rotor speed
B. Armature reaction and ohmic losses in the windings
C. A rise in field current
D. Enhanced residual magnetism
Answer: B
Explanation: Under load, the voltage drop across the armature resistance and the demagnetizing effect of the armature reaction lower the terminal voltage compared to the no-load condition.
Q8. Armature reaction in a DC generator is:
A. The increase in flux due to armature current
B. The effect of load current that demagnetizes the main field
C. The rise in terminal voltage under load
D. The improvement in efficiency
Answer: B
Explanation: Armature reaction refers to the influence of the armature’s magnetic field on the main field flux. It generally causes a demagnetizing effect that reduces the net flux, resulting in a drop in generated EMF under load.
Q9. A significant drop in the terminal voltage of a DC generator under load is most likely due to:
A. Saturation of the magnetic circuit
B. Increased residual magnetism
C. Voltage drop across internal resistances and armature reaction
D. A decrease in field current
Answer: C
Explanation: As load increases, the increased armature current causes a larger voltage drop across the armature and field resistances, compounded by the demagnetizing effect of armature reaction, thus reducing the terminal voltage.
4. DC Generator Types and Their Characteristics
Q10. A DC shunt generator is characterized by:
A. A series connection of armature and field windings
B. A nearly constant terminal voltage over a wide range of loads
C. A significant drop in voltage at light loads
D. High sensitivity to load variations
Answer: B
Explanation: In a DC shunt generator, the field winding is connected in parallel with the armature. This self-regulating feature allows the terminal voltage to remain nearly constant even when the load changes.
Q11. In a separately excited DC generator, the output voltage is mainly controlled by:
A. Load resistance
B. Armature reaction
C. Field current from an independent DC source
D. The internal armature resistance
Answer: C
Explanation: Since the field winding is supplied by an independent DC source, the output voltage can be precisely controlled by adjusting the field current, making it independent of the load current.
Q12. A DC series generator is often called a constant current generator because:
A. The output voltage remains constant regardless of the load
B. The same current flows through the armature, series field, and load
C. The field current is independent of the load current
D. The armature reaction is negligible
Answer: B
Explanation: In a series generator, the armature, series field, and load are connected in series, meaning that the same current flows through all components. This unique connection results in the generator acting as a constant current source over a range of load resistances.
Q13. In a compound DC generator, combining shunt and series windings results in:
A. Poor voltage regulation at all loads
B. A voltage output that remains constant across all load conditions
C. Improved voltage regulation under heavy loads
D. Increased armature reaction effects
Answer: C
Explanation: Compound generators use both shunt and series windings to balance the effects of voltage drop and armature reaction. Over-compounding can boost the voltage under heavy loads, providing better regulation compared to a pure shunt generator.
5. Magnetization and Saturation
Q14. The linear portion of the magnetization curve of a DC generator exists until:
A. The armature current reaches its maximum
B. The magnetic poles reach saturation
C. The load current becomes zero
D. The generator is disconnected from the field supply
Answer: B
Explanation: Initially, as the field current increases, the magnetic flux and hence the generated voltage increase linearly. However, once the poles approach magnetic saturation, additional increases in field current produce only a marginal increase in voltage.
Q15. Saturation in the magnetic circuit of a DC generator leads to:
A. A steep increase in generated voltage
B. A flat region in the OCC where voltage gain is minimal
C. A decrease in residual magnetism
D. A linear relationship between field current and EMF
Answer: B
Explanation: Once the magnetic circuit saturates, further increases in field current do not significantly increase the flux. This results in the OCC entering a nearly flat region, where the generated EMF rises very little with additional field current.
Q16. In the context of DC generators, residual magnetism is important because it:
A. Eliminates the need for field excitation
B. Ensures that the generator can start building up voltage
C. Increases the overall efficiency
D. Causes a linear OCC from zero current
Answer: B
Explanation: Residual magnetism provides the initial flux necessary for the generator to begin voltage build-up when an external excitation is applied, making it crucial for self-excitation in DC machines.
6. Effects of Speed and Excitation
Q17. If the rotor speed of a DC generator is increased while keeping other parameters constant, the generated EMF will:
A. Decrease due to armature reaction
B. Remain unchanged
C. Increase proportionally
D. Drop to zero
Answer: C
Explanation: According to Faraday’s law of electromagnetic induction, the generated EMF is directly proportional to the rotor speed. Hence, increasing the speed results in a higher EMF.
Q18. In a DC shunt generator, increasing the field resistance beyond a critical value will likely cause:
A. An increase in terminal voltage
B. Failure to build up voltage during self-excitation
C. A linear increase in armature current
D. Improved voltage regulation
Answer: B
Explanation: If the field resistance is too high, the field current becomes insufficient for building up the necessary flux, which can prevent the generator from self-exciting and building up the voltage.
Q19. Adjusting the field current in a separately excited DC generator primarily affects:
A. The armature reaction
B. The load current directly
C. The terminal voltage
D. The armature resistance
Answer: C
Explanation: In a separately excited generator, the field current is controlled by an independent source. Changes in the field current directly affect the magnetic flux and, consequently, the generated EMF and terminal voltage.
7. Advanced Concepts and Practical Considerations
Q20. The voltage drop in a DC generator under load is mainly due to:
A. Increase in field current
B. Decrease in rotor speed
C. Ohmic drops in the armature and field windings
D. Enhancement of residual magnetism
Answer: C
Explanation: When a load is applied, the current flowing through the armature and field windings causes voltage drops across their resistances, which, along with the effect of armature reaction, reduce the terminal voltage compared to the no-load voltage.
Q21. Which of the following best describes the self-regulating property of a DC shunt generator?
A. Terminal voltage remains constant despite changes in load
B. Field current increases linearly with load
C. Armature current remains constant with load variations
D. Generated EMF increases with load current
Answer: A
Explanation: The self-regulating property of a DC shunt generator ensures that any drop in terminal voltage under load leads to a slight increase in field current (due to the parallel connection), which in turn helps maintain nearly constant voltage.
Q22. In a series DC generator, if the load resistance decreases, the load current will:
A. Increase significantly
B. Decrease due to saturation
C. Remain nearly constant
D. Increase and then suddenly drop
Answer: C
Explanation: In a series generator, the load current is nearly constant over a range of load resistances because the series connection forces the same current through the field and armature, making it act as a constant current source.
Q23. The term “compound generator” refers to a DC generator that:
A. Uses only shunt windings
B. Uses only series windings
C. Combines both shunt and series windings to improve voltage regulation
D. Has no armature reaction
Answer: C
Explanation: A compound generator is designed with both shunt and series windings, combining the advantages of each. This design improves voltage regulation across different load conditions by counteracting voltage drops more effectively.
Q24. Which factor does NOT directly affect the open-circuit characteristic of a DC generator?
A. Magnetic circuit design
B. Rotor speed
C. Armature reaction
D. Field excitation level
Answer: C
Explanation: The open-circuit characteristic is measured under no-load conditions; therefore, armature reaction (which occurs under load) does not affect the OCC. The curve is mainly determined by the magnetic circuit design, rotor speed, and field excitation.
Q25. When plotting the internal characteristic from the external load curve, one must account for:
A. The increase in field current
B. The ohmic voltage drops across armature and series resistances
C. The residual magnetism
D. The variation in rotor speed
Answer: B
Explanation: To derive the internal characteristic, the voltage drops due to the armature and series resistances (IaR_a) must be added back to the terminal voltage. This accounts for the losses that cause the generated EMF under load to be lower than the no-load EMF.
8. Detailed Application-Based Questions
Q26. In practical testing of a DC generator, why is it important to obtain the OCC?
A. To measure the load current directly
B. To determine the critical field resistance for self-excitation
C. To find the efficiency at full load
D. To calculate the armature resistance
Answer: B
Explanation: The OCC helps determine the point at which the generator will self-excite and build up voltage. It is also used to identify the critical field resistance that ensures the generator can start up and maintain a stable voltage.
Q27. In a DC shunt generator, if the load increases causing a slight drop in terminal voltage, the effect on the field current is generally:
A. A decrease in field current
B. No change in field current
C. A compensatory increase in field current
D. An unpredictable fluctuation
Answer: C
Explanation: Because the field winding is connected in parallel with the armature, a drop in terminal voltage increases the field current slightly, which helps the generator self-correct and maintain a nearly constant voltage.
Q28. Which of the following is a major disadvantage of the DC series generator?
A. Poor voltage regulation at light loads
B. High cost of field excitation
C. Excessive heat generation in shunt windings
D. Inability to maintain constant current
Answer: A
Explanation: DC series generators tend to have poor voltage regulation at light loads due to the close interconnection of armature and field windings. At light loads, the output voltage can be very sensitive to small changes in load, making it less stable.
Q29. The effect of armature reaction is most pronounced in which part of the generator’s characteristic curves?
A. Open-circuit characteristic
B. Internal and external characteristics under load
C. The residual magnetism zone
D. The saturation region of the OCC
Answer: B
Explanation: Armature reaction primarily affects the generator when it is under load. It causes a reduction in the effective flux, leading to a noticeable drop in both the internal and external characteristic curves relative to the open-circuit condition.
Q30. In a compound generator, if the series field is over-compounded, the terminal voltage under heavy load will:
A. Drop significantly
B. Remain nearly the same as no-load voltage
C. Increase compared to light load voltage
D. Become erratic
Answer: C
Explanation: Over-compounding means that the series field winding produces additional flux with load current. This compensates for the voltage drop due to internal resistance and armature reaction, resulting in an increased terminal voltage under heavy load.
9. Concept Integration and Synthesis
Q31. Which statement best summarizes the relationship between generated EMF and load current in a DC generator?
A. The generated EMF remains constant regardless of load
B. The generated EMF increases with load due to armature reaction
C. The generated EMF under load is lower than the no-load EMF due to voltage drops
D. The generated EMF is independent of field current
Answer: C
Explanation: Under load conditions, the generated EMF decreases relative to the no-load value because of voltage drops across internal resistances and the demagnetizing effect of armature reaction.
Q32. What is the primary reason for the nearly constant voltage output of a DC shunt generator over varying loads?
A. High armature reaction
B. The self-regulating action of the shunt field
C. The series connection of the windings
D. The effect of residual magnetism
Answer: B
Explanation: In a shunt generator, the field winding is connected across the armature, so any drop in terminal voltage tends to increase the field current, which helps maintain the output voltage nearly constant despite load variations.
Q33. In a separately excited DC generator, why is it easier to control the terminal voltage compared to a self-excited generator?
A. The load current is fixed
B. The armature reaction is eliminated
C. The field excitation is independent and adjustable
D. There is no voltage drop across the armature
Answer: C
Explanation: The independent field excitation in a separately excited DC generator allows direct control over the magnetic flux, making it easier to adjust and maintain the desired terminal voltage without the self-regulating effects present in shunt or series generators.
Q34. Which parameter, when increased, will directly shift the OCC of a DC generator upward?
A. Armature resistance
B. Field current
C. Load resistance
D. Armature reaction
Answer: B
Explanation: Increasing the field current increases the magnetic flux in the generator. This, in turn, raises the no-load generated EMF, causing an upward shift in the OCC.
Q35. How does an increase in armature current affect the internal voltage of a DC generator?
A. It increases the internal voltage
B. It has no effect on the internal voltage
C. It decreases the internal voltage due to increased voltage drop
D. It leads to saturation, increasing the voltage slightly
Answer: C
Explanation: An increase in armature current results in a greater voltage drop across the armature resistance as well as enhanced armature reaction, thereby reducing the internal generated voltage compared to the no-load EMF.
10. Problem-Solving and Scenario-Based Questions
Q36. A DC generator is tested and shows an OCC that flattens out beyond a certain field current. This is most likely due to:
A. A malfunction in the armature
B. Magnetic saturation of the poles
C. Excessive armature reaction
D. Low residual magnetism
Answer: B
Explanation: The flattening of the OCC indicates that the magnetic circuit has reached saturation, meaning that further increases in field current do not produce a proportional increase in flux or generated EMF.
Q37. During a load test, a DC generator exhibits a greater drop in terminal voltage than expected. Which of the following could be a contributing factor?
A. Lower than normal armature resistance
B. Increased field excitation due to reduced load
C. High internal resistance and pronounced armature reaction
D. Excessive residual magnetism
Answer: C
Explanation: A larger than expected voltage drop under load can be attributed to higher internal (ohmic) resistance in the armature and field windings as well as a strong demagnetizing effect from armature reaction.
Q38. In a DC shunt generator, if the field resistance is decreased, one can expect:
A. A reduction in terminal voltage due to a drop in field current
B. An increase in terminal voltage due to higher field current
C. No change in terminal voltage
D. A decrease in armature reaction
Answer: B
Explanation: Lowering the field resistance allows more field current to flow, which increases the magnetic flux and, therefore, the generated EMF. This results in an increase in terminal voltage.
Q39. For a series DC generator, what happens to the field excitation when the load current increases?
A. It decreases due to the parallel connection
B. It remains constant because the current is fixed
C. It increases as the same load current flows through the series field
D. It fluctuates unpredictably
Answer: C
Explanation: In a series generator, the field winding is connected in series with the load. Therefore, any increase in load current automatically increases the field excitation, affecting the generated voltage.
Q40. A DC compound generator that is under-compounded will typically show which behavior under heavy load?
A. An increased terminal voltage
B. A decreased terminal voltage (overvoltage)
C. Nearly constant terminal voltage
D. Erratic voltage fluctuations
Answer: B
Explanation: An under-compounded compound generator has a series winding that opposes the shunt field effect. Under heavy load, this can lead to a lower net flux, and hence, a lower terminal voltage, sometimes causing overvoltage at light loads but under voltage under heavy loads.
11. Conceptual Depth and Analysis
Q41. What is the significance of plotting perpendiculars from points on the external load curve to the OCC?
A. To measure the armature resistance directly
B. To determine the internal characteristic of the generator
C. To calculate the field current
D. To establish the rotor speed
Answer: B
Explanation: By projecting points from the external load curve upward to the OCC (and accounting for the voltage drop), one can trace the internal characteristic of the generator. This method helps visualize the effect of armature reaction and internal losses.
Q42. In testing a DC generator, why is it important to compare the OCC with the internal and external characteristics?
A. To verify that the load resistance is within acceptable limits
B. To assess the effects of saturation and armature reaction on voltage regulation
C. To determine the generator’s power factor
D. To establish a relationship between frequency and voltage
Answer: B
Explanation: Comparing these curves allows engineers to see how the ideal no-load voltage (OCC) is degraded by internal voltage drops and armature reaction under load. This comparison is crucial for designing proper voltage regulation and understanding performance.
Q43. Which curve provides direct information about the voltage drop caused by the armature’s internal resistance?
A. Open-circuit characteristic
B. Internal characteristic
C. External characteristic
D. Magnetization curve
Answer: C
Explanation: The external characteristic curve, which plots terminal voltage against load current, directly shows the effect of ohmic voltage drops (including those due to armature resistance) as the generator supplies current to a load.
Q44. The phenomenon where the terminal voltage of a series DC generator initially rises with increasing load current but then falls is mainly due to:
A. A linear increase in residual magnetism
B. The onset of magnetic saturation
C. The increased demagnetizing effect of armature reaction
D. Improved efficiency at higher currents
Answer: C
Explanation: In a series generator, the field current increases with load, causing the terminal voltage to rise initially. However, beyond a certain point, the increased armature current intensifies the demagnetizing armature reaction, leading to a drop in terminal voltage.
Q45. Which parameter is least affected by load variations in a well-designed DC shunt generator?
A. Terminal voltage
B. Field current
C. Armature current
D. Magnetic flux
Answer: A
Explanation: A well-designed DC shunt generator maintains a nearly constant terminal voltage over a wide range of loads due to its self-regulating mechanism, which adjusts the field current in response to voltage changes.
12. Synthesis and Higher-Order Thinking
Q46. If a DC generator exhibits an unexpectedly high voltage drop under load, one potential corrective measure is to:
A. Increase the field resistance
B. Decrease the rotor speed
C. Improve the quality of the armature winding (reduce resistance)
D. Remove residual magnetism
Answer: C
Explanation: Reducing the armature winding resistance will decrease the voltage drop under load. This directly improves the terminal voltage stability and overall efficiency by minimizing ohmic losses.
Q47. Which design modification in a compound DC generator can lead to a “flat” external characteristic curve?
A. Using only a series field winding
B. Perfectly balancing the shunt and series field effects
C. Increasing the rotor speed
D. Reducing the armature reaction
Answer: B
Explanation: When the effects of the shunt and series windings are balanced (flat compounding), the voltage drop due to load is effectively compensated, resulting in a nearly constant terminal voltage over a wide range of loads.
Q48. What is the primary role of field excitation in determining the performance of a DC generator?
A. It sets the maximum possible load current
B. It establishes the magnitude of the generated EMF
C. It determines the armature reaction intensity
D. It fixes the internal resistance
Answer: B
Explanation: Field excitation directly controls the magnetic flux in the generator, which according to Faraday’s law, determines the magnitude of the generated EMF. Therefore, it is the key factor in setting the voltage output.
Q49. In a practical application, why might an engineer choose a compound DC generator over a simple shunt generator?
A. To simplify the design and reduce costs
B. To obtain better voltage regulation under variable load conditions
C. To ensure constant armature current
D. To eliminate the need for external field excitation
Answer: B
Explanation: Compound generators offer improved voltage regulation by combining shunt and series windings. This makes them ideal for applications where the load varies significantly, as they better compensate for voltage drops due to internal losses and armature reaction.
Q50. Which of the following best explains why the internal characteristic curve lies below the open-circuit characteristic in a loaded DC generator?
A. Due to increased field current under load
B. Because of the combined effect of armature resistance and armature reaction
C. Because the generator is operating at a higher speed
D. Due to the presence of residual magnetism
Answer: B
Explanation: When the generator is loaded, voltage drops occur due to the inherent resistance in the armature and the demagnetizing effects of armature reaction. These factors lower the effective generated voltage, causing the internal characteristic to lie below the ideal no-load (open-circuit) curve.