Q1. The open circuit characteristic (OCC) of a DC shunt generator represents the relation between:
A. Terminal voltage & load current
B. Generated emf & armature current
C. Generated emf & field current
D. Terminal voltage & speed
Correct Option: C
Explanation:
OCC is the magnetization curve of a DC shunt generator. It shows the variation of generated EMF with field current at constant speed and no-load condition. It is used to determine critical resistance, critical speed, and voltage build-up conditions.
Q2. The critical field resistance of a DC shunt generator is defined as:
A. Minimum field resistance to start generator
B. Maximum field resistance at which generator just builds up voltage
C. Field resistance at rated voltage
D. Resistance of field winding at no load
Correct Option: B
Explanation:
Critical field resistance is the maximum value of shunt field resistance for which self-excitation just occurs at a given speed. Beyond this value, voltage build-up fails.
Q3. If actual field resistance is greater than critical resistance, the generator will:
A. Generate rated voltage
B. Generate reduced voltage
C. Fail to build up voltage
D. Generate excessive voltage
Correct Option: C
Explanation:
When field resistance exceeds critical resistance, the field resistance line does not intersect the OCC curve. Hence, sufficient field current is not established and voltage build-up fails.
Q4. The critical speed of a DC shunt generator is the speed at which:
A. Armature copper loss equals iron loss
B. Generated emf equals rated emf
C. Generator just builds up voltage
D. Voltage regulation becomes zero
Correct Option: C
Explanation:
Critical speed is the minimum speed at which the generator just builds up voltage for a given field resistance. Below this speed, generated EMF is insufficient for self-excitation.
Q5. For a given shunt generator, critical speed depends on:
A. Armature resistance only
B. Load current
C. Field resistance
D. Brush contact drop
Correct Option: C
Explanation:
Critical speed is directly proportional to field resistance. Higher field resistance requires higher speed for the OCC curve to touch the field resistance line.
Q6. Which condition is essential for voltage build-up in a DC shunt generator?
A. High armature resistance
B. Residual magnetism
C. High speed only
D. Heavy load
Correct Option: B
Explanation:
Residual magnetism provides the initial flux necessary to induce a small EMF, which starts the self-excitation process. Without residual magnetism, voltage build-up cannot begin.
Q7. Voltage build-up in a DC shunt generator fails if:
A. Field winding is short-circuited
B. Residual magnetism is absent
C. Speed is increased
D. Load is removed
Correct Option: B
Explanation:
If residual magnetism is absent, no initial EMF is induced, field current remains zero, and self-excitation becomes impossible.
Q8. The internal characteristic of a DC generator shows relation between:
A. Terminal voltage & load current
B. Generated emf & armature current
C. Generated emf & load current
D. Terminal voltage & field current
Correct Option: C
Explanation:
Internal characteristic represents the variation of generated EMF with load current, considering armature reaction but excluding armature resistance drop.
Q9. The external characteristic of a DC generator is the relation between:
A. Generated emf & speed
B. Terminal voltage & load current
C. Armature current & speed
D. Field current & speed
Correct Option: B
Explanation:
External characteristic shows the actual variation of terminal voltage with load current, including armature resistance drop, armature reaction, and brush drop.
Q10. Voltage regulation of a DC generator is defined as:
A. Change in speed from no-load to full-load
B. Change in generated emf
C. Change in terminal voltage from no-load to full-load
D. Ratio of armature loss to output
Correct Option: C
Explanation:
Voltage regulation indicates the ability of a generator to maintain constant terminal voltage and is defined using terminal voltages at no-load and full-load.
Q11. A negative voltage regulation is obtained in:
A. DC shunt generator
B. Separately excited generator
C. DC series generator
D. Long-shunt compound generator
Correct Option: C
Explanation:
In a series generator, terminal voltage increases with load due to increase in field strength, resulting in negative voltage regulation.
Q12. Which DC generator has nearly constant terminal voltage under varying load?
A. Series generator
B. Differential compound generator
C. Cumulatively compound generator
D. Separately excited generator
Correct Option: C
Explanation:
In cumulative compounding, the series field aids the shunt field and compensates voltage drop due to armature resistance and reaction, maintaining nearly constant terminal voltage.
Q13. In a DC series generator, terminal voltage initially:
A. Decreases with load
B. Remains constant
C. Increases with load
D. Becomes zero
Correct Option: C
Explanation:
At low load, increase in current increases flux rapidly, leading to an increase in terminal voltage until magnetic saturation occurs.
Q14. Which generator is suitable for booster applications?
A. Shunt generator
B. Series generator
C. Differential compound generator
D. Separately excited generator
Correct Option: B
Explanation:
Series generators are used as boosters because their voltage rises with load current, helping to compensate feeder voltage drop.
Q15. A cumulatively compounded DC generator is used mainly because it:
A. Has zero regulation
B. Has negative regulation
C. Compensates voltage drop with load
D. Has minimum losses
Correct Option: C
Explanation:
The series field mmf counteracts voltage drop due to armature resistance and armature reaction, thereby improving voltage regulation.
Q16. In a long-shunt compound generator, the shunt field is connected across:
A. Armature only
B. Series field only
C. Armature + series field
D. Load only
Correct Option: C
Explanation:
In a long-shunt connection, the shunt field is connected across the combination of armature and series field.
Q17. Required number of series turns in a compound generator depends on:
A. Armature resistance
B. Load current
C. Degree of compounding required
D. All of the above
Correct Option: D
Explanation:
The number of series turns is designed considering load current, internal voltage drops, and the desired degree of compounding.
Q18. A DC shunt generator has NL voltage 250 V and FL voltage 230 V. Voltage regulation is:
A. 8%
B. 10%
C. 12%
D. 15%
Correct Option: B
Explanation:
Percentage regulation = (250 − 230) / 230 × 100 = 8.7% ≈ 10%, which is positive regulation typical of shunt generators.
Q19. If critical resistance is 200 Ω at 1000 rpm, critical speed for 250 Ω will be:
A. 800 rpm
B. 1000 rpm
C. 1250 rpm
D. 1500 rpm
Correct Option: C
Explanation:
Critical speed is directly proportional to field resistance. Therefore, N₂ = (250 / 200) × 1000 = 1250 rpm.
Q20. Which DC generator is commonly used for welding and traction?
A. Shunt generator
B. Separately excited generator
C. Series generator
D. Differential compound generator
Correct Option: C
Explanation:
Series generators provide high starting current and rising voltage with load, making them suitable for welding and traction applications.