D_C_ Generators — Part 1 MCQs with Explanations

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.

Commutator bars: In both lap and wave windings of a DC generator, the number of commutator bars is equal to the number of armature conductors. Reason: Each armature conductor is connected to a commutator bar, and the commutator ensures the reversal of current direction as the armature rotates. This reversal is necessary for maintaining unidirectional current output in the external circuit. Lap winding: In lap winding, the number of commutator bars is the same as the number of armature coils, which corresponds to the number of armature conductors. Wave winding: Similarly, in wave winding, the number of commutator bars also equals the number of armature conductors, ensuring proper commutation for each conductor. Thus, the correct relationship is that the number of commutator bars equals the number of armature conductors.

A D.C. generator can be considered as a rotating amplifier because it converts mechanical energy into electrical energy by electromagnetic induction. The generated voltage is proportional to the speed of rotation and the magnetic flux, thereby amplifying the mechanical input into an electrical output. This amplification property is why it is often compared to an amplifier in the electrical domain. Other options, such as rectifier, prime mover, and power pump, do not match the fundamental operation of a D.C. generator. Rectifier: Converts AC to DC, not applicable here. Prime mover: Provides mechanical energy to drive the generator but is not the generator itself. Power pump: Refers to fluid systems and is unrelated to D.C. generators. Thus, the correct analogy for a D.C. generator is a rotating amplifier .

Here's an explanation to help you understand the question: First, let's define pole pitch . It is the distance between the center of two adjacent poles in a DC generator. To calculate the pole pitch, we need to know the total number of slots or coil sides around the armature and the number of poles . We are given: * Number of poles (P) = 4 * Number of coils = 16 * Two layer winding means each coil has two coil sides. Since it's a two-layer winding, the total number of coil sides is twice the number of coils: 16 coils * 2 coil sides/coil = 32 coil sides. In a lap winding, the number of coil sides equals the number of armature conductors or slots. So, we have 32 slots. The formula for pole pitch is: Pole Pitch = (Total Number of Slots) / (Number of Poles) So, Pole Pitch = 32 / 4 = 8 Therefore, the correct answer is Option B: 8 .

D.C. series generator has its field winding connected in series with the armature, so the field current increases with load current. Because of this, the terminal voltage rises with increase in load , which makes it unsuitable for constant-voltage applications. However, this characteristic is highly suitable for traction loads such as electric locomotives and trams, where heavy starting current and varying load conditions are required. Hence, a D.C. series generator is mainly used to supply traction load .

Shunt generators are most suited for stable parallel operation due to their drooping voltage characteristics. Option A: Rising voltage characteristics Rising voltage characteristics are not suitable for parallel operation as they could cause instability, leading to one generator taking more load than others. Option B: Identical voltage characteristics Identical voltage characteristics are desirable for parallel operation, but in practice, it's challenging to achieve exact identical voltage, making this option less feasible. Option C: Drooping voltage characteristics Drooping voltage characteristics are ideal for parallel operation as they ensure that as the load increases, the voltage decreases slightly, allowing for automatic load sharing between generators. Option D: Linear voltage characteristics Linear voltage characteristics do not provide the necessary voltage drop needed for stable load sharing in parallel operation. Conclusion: Shunt generators with drooping voltage characteristics are most suited for stable parallel operation because they facilitate effective load sharing between generators.

Voltage regulation in a D.C. generator refers to the change in terminal voltage from no-load to full-load, expressed as a percentage of the full-load voltage. A lower percentage of voltage regulation indicates better performance because it means the generator maintains a more constant voltage under varying load conditions. Option A: 100% regulation 100% regulation would mean that the voltage drops to zero under full load, which is highly undesirable. Option B: Infinite regulation Infinite regulation implies a complete loss of voltage under load, which is not practical or acceptable. Option C: 50% regulation 50% regulation indicates a significant drop in voltage under load, which is also not preferred as it would lead to poor performance. Option D: 1% regulation 1% regulation is ideal as it indicates very little voltage drop under load, ensuring that the generator provides a stable voltage output. Conclusion: Among the given options, 1% regulation is preferred because it reflects the best performance in maintaining a constant voltage.

In a D.C. generator, the magnetic field is essential for the generation of electricity through electromagnetic induction. Most commonly, this magnetic field is produced by electromagnets which are created by field windings supplied with direct current. However, in small D.C. generators or specific applications, permanent magnets can also be used to produce the magnetic field. Therefore, depending on the type and size of the generator, both electromagnets and permanent magnets may be used . Hence, the correct answer is Option C: Both (A) and (B)

Copper is the correct answer to “The field coils of D.C. generator are usually made of”. This question is standard in competitive examination syllabi for D_C_ Generators.

Differentially compounded long shunt is the correct answer to “A cumulatively compounded long shunt generator when operating as a motor would be”. This question is standard in competitive examination syllabi for D_C_ Generators.

Cross-magnetization as well as demagnetizing is the correct answer to “If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be”. This question is standard in competitive examination syllabi for D_C_ Generators.

all above is the correct answer to “In a D.C. machine, short-circuited field coil will result in”. This question is standard in competitive examination syllabi for D_C_ Generators.

Relative rotation between field and armature is the correct answer to “Eddy currents are induced in the pole shoes of a D.C. machine due to”. This question is standard in competitive examination syllabi for D_C_ Generators.

Lap winding is the correct answer to “Welding generator will have”. This question is standard in competitive examination syllabi for D_C_ Generators.

Reduces generator e.m.f. is the correct answer to “The demagnetizing component of armature reaction in a D.C. generator”. This question is standard in competitive examination syllabi for D_C_ Generators.

Lap wound is the correct answer to “Equalizer rings are required in case armature is”. This question is standard in competitive examination syllabi for D_C_ Generators.

Lines of force is the correct answer to “According to Fleming's right-hand rule for finding the direction of induced e.m.f., when middle finger points in the direction of induced e.m.f., forefinger will point in the direction of”. This question is standard in competitive examination syllabi for D_C_ Generators.

Shunt field is the correct answer to “Following energized winding of a D.C. machine should not be opened as it would produce high inductive voltage which may be dangerous to personnel and may cause its own insulation failure.”. This question is standard in competitive examination syllabi for D_C_ Generators.

the same drooping voltage characteristics is the correct answer to “The essential condition for parallel operation of two D.C. generators is that they have”. This question is standard in competitive examination syllabi for D_C_ Generators.

Faraday's law of electromagnetic induction is the correct answer to “A D.C. generator works on the principle of”. This question is standard in competitive examination syllabi for D_C_ Generators.

Decrease is the correct answer to “In a D.C. generator in case the resistance of the field winding is increased, then output voltage will”. This question is standard in competitive examination syllabi for D_C_ Generators.

Speed is the correct answer to “In case of D.C. machines, mechanical losses are primary function of”. This question is standard in competitive examination syllabi for D_C_ Generators.

Series generator is the correct answer to “Which of the following generator will have negligible terminal voltage while running on no-load”. This question is standard in competitive examination syllabi for D_C_ Generators.

Just ahead of magnetic neutral axis is the correct answer to “To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so as to bring them”. This question is standard in competitive examination syllabi for D_C_ Generators.

More than no load terminal voltage is the correct answer to “In over compounded generator, full load terminal voltage is”. This question is standard in competitive examination syllabi for D_C_ Generators.

Equal to no-load terminal voltage is the correct answer to “In a level compounded D.C. generator, full load terminal voltage is”. This question is standard in competitive examination syllabi for D_C_ Generators.

Counter sunk screws is the correct answer to “In D.C. generators the pole shoes are fastened to the pole core by”. This question is standard in competitive examination syllabi for D_C_ Generators.

Over compound generator is the correct answer to “Which generator would you prefer for feeding long D.C. transmission lines”. This question is standard in competitive examination syllabi for D_C_ Generators.

Low voltage, high current is the correct answer to “In D.C. generators, lap winding is used for”. This question is standard in competitive examination syllabi for D_C_ Generators.

Where low voltage and high currents are involved is the correct answer to “Copper brushes in D.C. machine are used”. This question is standard in competitive examination syllabi for D_C_ Generators.

240 V is the correct answer to “A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed increases to 1200 r.p.m., the generated e.m.f. will be nearly”. This question is standard in competitive examination syllabi for D_C_ Generators.

Cross-magnetizing is the correct answer to “Armature reaction of an unsaturated D.C. machine is”. This question is standard in competitive examination syllabi for D_C_ Generators.

3 : 1 is the correct answer to “Two generators A and B have 6 poles each. Generator A has wave wound armature while generator B has lap wound armature. The ratio of the induced e.m.f. is generator A and B will be”. This question is standard in competitive examination syllabi for D_C_ Generators.

Only if the resistance of the field circuit is less than critical value is the correct answer to “A shunt generator can self-excite”. This question is standard in competitive examination syllabi for D_C_ Generators.

Only if the load current is not zero is the correct answer to “A series generator can self-excite”. This question is standard in competitive examination syllabi for D_C_ Generators.

reduce eddy current loss is the correct answer to “The armature of D.C. generator is laminated to”. This question is standard in competitive examination syllabi for D_C_ Generators.

Carbon is the correct answer to “The material for commutator brushes is generally”. This question is standard in competitive examination syllabi for D_C_ Generators.

Along neutral axis is the correct answer to “Armature coil is short circuited by brushes when it lies”. This question is standard in competitive examination syllabi for D_C_ Generators.

All of the above is the correct answer to “Satisfactory commutation of D.C. machines requires”. This question is standard in competitive examination syllabi for D_C_ Generators.