D.C. Generators MCQ Questions & Answers | Electrical Engineering

The two D.C. shunt generators are running in parallel, so they share the total load current of 1000 A based on their induced e.m.fs. and internal characteristics.

Let I₁ and I₂ be the currents supplied by generators 1 and 2 respectively.

For generator 1 (E₁=270 V): Bus bar voltage V = E₁ - I₁ × 0.02

For generator 2 (E₂=265 V): Bus bar voltage V = E₂ - I₂ × 0.02

Since both are connected to the same bus bars, their bus bar voltages must be equal, so:

E₁ - 0.02 × I₁ = E₂ - 0.02 × I₂

Rearranging gives: 0.02(I₂ - I₁) = E₂ - E₁ = 265 - 270 = -5

So: I₂ - I₁ = -5 / 0.02 = -250 ⇒ I₂ = I₁ - 250

Also, total current constraint gives: I₁ + I₂ = 1000

Substituting I₂: I₁ + (I₁ - 250) = 1000 ⇒ 2I₁ - 250 = 1000 ⇒ 2I₁ = 1250 ⇒ I₁=625 A

Then I₂=625-250=375 A

Finally, bus bar voltage V can be found using either generator (using generator 1):

V = E₁ - I₁ × 0.02 = 270 - (625×0.02) = 270 -12.5= 257.5 V

When DC generators are required to be run in parallel, compound generators are preferred due to their better voltage regulation and stability in load sharing.

Step 1: Understanding Different Types of DC Generators
- Shunt Generators: They have poor voltage regulation under varying loads, which makes them less suitable for parallel operation.
- Series Generators: They cannot be used in parallel because their terminal voltage varies significantly with load changes.
- Compound Generators: These have both shunt and series windings, providing better voltage regulation and allowing them to share loads effectively in parallel operation.

Step 2: Why Compound Generators Are Preferred?
- They maintain a stable voltage when loads change.
- They ensure equal load sharing between generators.
- They prevent overloading of a single generator when connected in parallel.

Step 3: Analyzing the Given Options
- Option A: Shunt generators (Not preferred due to poor voltage regulation under varying loads.)
- Option B: Series generators (Not suitable as they do not work effectively in parallel operation.)
- Option C: Compound generators (Correct, as they provide better voltage stability and load sharing.)
- Option D: None of the above (Incorrect, as compound generators are the correct choice.)

Conclusion: For parallel operation, compound generators are preferred because of their good voltage regulation and stable load sharing.

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.