In a D.C. generator the critical resistance can be increased by
A. Increasing its field resistance
B. Decreasing its field resistance
C. Increasing its speed
D. Decreasing its speed
Answer: Option C
Solution (By Examveda Team)
Explanation:The critical resistance is the maximum field resistance for a DC generator to begin voltage buildup or excitation.
It is the resistance represented by the tangent to the O.C.C (Open Circuit Characteristic) curve of a DC generator.
The O.C.C curve shows the relationship between the generated EMF and field current at a constant speed.
If we increase the speed of the generator, the generated EMF increases at a given field current.
This means the O.C.C curve shifts upwards.
A steeper O.C.C curve (at higher speed) allows for a higher tangent, hence a higher critical resistance.
Changing the field resistance doesn't directly affect the critical resistance. It affects the operating point after the generator has built up voltage, but not the initial build-up.
Therefore, the correct answer is increasing its speed. Option C is the correct answer.
This Question Belongs to Electrical Engineering >> D.C. Generators
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A. Demagnetization only
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For a self-excited DC generator:
Critical resistance (Rc) is the maximum field circuit resistance above which the generator will fail to build up voltage.
The open-circuit characteristic (OCC) shifts upward when the speed increases because generated emf ∝ speed.
When OCC rises upward, the slope increases, which means the generator can tolerate higher field resistance, so critical resistance increases.
Therefore:
✔ Increasing speed → OCC becomes steeper → critical resistance increases.
speed will increases its Generated EMF
Rc=(Emf/Field Current)
Can any one please explain it breaflie.
if Rfc is increaced more than Rfc critical in speed "x" the voltage will not building up
so we must inceasing the speed to tangent the the new speed curve
any one can explain?