# Operating Characteristics of DC Generator MCQ || DC Generator Operating Characteristics Questions and Answers

1. Which of the following characteristics reveal about the magnetization nature of the DC Generator?

3. Armature characteristics

Explanation:

No-load characteristics: It is also known as magnetic characteristics or open-circuit characteristics (O.C.C.). It shows the relation between the no-load generated emf in the armature (Eo ) and the field current (i.e., exciting current) If, at a specified speed.

2. To determine the relation between different quantities of a DC generator the Important Characteristics of DC generator are

2. External Characteristics
3. Internal Characteristics
4. All of the above

Answer: 4. All of the above

Explanation:

To determine the relationship between different quantities of a DC generator, the following are the important characteristics of DC generators:

No-load characteristics: It is also known as magnetic characteristics or open-circuit characteristics (O.C.C.). It shows the relation between the no-load generated emf in the armature (Eo ) and the field current (i.e., exciting current) If, at a specified speed.

External characteristics: It is also called performance characteristics. It shows the relation between the terminal voltage (V) and the load current.

Internal Characteristics: It is also known as total characteristics. It gives the relation between the emf actually induced in the armature and the armature current.

3. Total/internal characteristic of DC generators is described as the ______

1. Relation between the emf and the field current
2. Relation between the terminal voltage and the armature current
3. Relation between the on-load generated emf and armature current
4. Relation between the terminal voltage and field current

Explanation:

• An internal characteristic curve of the DC generator shows the relation between the on-load generated emf (Eg) and the armature current (I.
• The on-load generated emf Eg is always less than E0 due to the armature reaction.
• Eg can be determined by subtracting the drop due to the demagnetizing effect of armature reaction from the no-load voltage (E0).
• So that the internal characteristic curve lies below the O.C.C. curve.

4. Out of the following Choose the most inappropriate for the no-load characteristics of the dc generator.

1. It is the open-circuit characteristic of the machine
2. It is a magnetization characteristic of the machine
3. It is conducted on the unloaded machine
4. It is de-magnetization characteristic of the machine

Answer: 4. It is de-magnetization characteristic of the machin

Explanation:

• Open circuit characteristic is also known as magnetic characteristic or no-load saturation characteristic.
• This characteristic shows the relation between generated emf at no load (E0) and the field current (If) at a given fixed speed.
• The O.C.C. curve is just the magnetization curve and it is practically similar for all types of generators.
• The data for O.C.C. curve is obtained by operating the generator at no load and keeping a constant speed.
• Field current is gradually increased and the corresponding terminal voltage is recorded.

5. Which of the following DC generator will have negligible terminal voltage at no load?

1. Series
2. Shunt
3. Compound
4. None of the above

Explanation:

In DC Series generator, the armature winding is connected in series with the field winding so that the field current flows through the load as well as the field winding. The field winding is a low resistance thick wire of few terms. DC series generator will have negligible terminal voltage at no load.

The flux developed by the series field winding is directly proportional to the current flowing through it. But it is only true before magnetic saturation, after saturation flux becomes constant even if the current flowing through it is increased.

6. The external characteristic of DC Generator is plotted between ____________

1. Terminal voltage vs armature current at constant excitation
2. Terminal voltage vs field current at constant armature current
3. Induced armature emf vs armature current at constant excitation
4. None of the mentioned

Answer: 1. Terminal voltage vs armature current at constant excitation

Explanation:

External characteristics of DC Generator (V/I):

• This shows the relation between terminal voltage (V) and load current (I).
• Terminal voltage (V) is less than the generated emf (E) due to the voltage drop in the armature circuit.
• Therefore external characteristic curve lies below the internal characteristic curve.
• It is also called “Performance characteristic” or “load characteristic”.

7. Which of the following generator will be preferred if they are required to be run in parallel?

1. Series generators
2. Shunt generators
3. Shunt and series generators
4. Compound generators

Explanation:

DC shunt generator:

• For stable parallel operation, the most suitable type of DC generator is a shunt generator as it has slightly drooping characteristics.
• If there is any tendency for a generator to supply more or less than its proper share of load it changes system voltage which certainly opposes this tendency.
• This restores the original division of load. Thus the shunt generators automatically remain in parallel, once they are paralleled.

8. A student forgot to mark the x-y axes in his experiments but he just noted down the cause and the effect for each. How will he conclude about the armature characteristic out of the all plotted graphs?

1. By marking graph for constant terminal voltage
2. By marking graph for constant field current
3. By marking graph for constant armature current
4. By marking graph for constant speed

Answer: 1. By marking graph for constant terminal voltage

Explanation:

Armature Characteristic is the presentation of  Armature current Ia vs Field current If with terminal voltage Vt held constant (at rated value) and generator run at constant n and load varied. It reveals the armature reaction effect on the flux/pole. It is also called a regulation characteristic.

9. The internal characteristics of a dc generator is plotted between the

1. Armature current and voltage generated after armature reaction
2. Field current and voltage generated at no load
3. Field current and voltage generated on load
4. Armature current and voltage generated at the output terminals

Answer: 1. Armature current and voltage generated after armature reaction

Explanation:

Internal characteristics of DC Generator (E/I):

• An internal characteristic curve shows the relation between the on-load generated emf (Eg) and the armature current (I.
• The internal characteristics of a dc generator is plotted between the armature current and voltage generated after the armature reaction
• The on-load generated emf Eg is always less than E0 due to the armature reaction.
• Eg can be determined by subtracting the drop due to the demagnetizing effect of armature reaction from the no-load voltage (E0).
• So that the internal characteristic curve lies below the O.C.C. curve.

10. Armature characteristic is also known as _________

1. Regulation characteristic
2. Magnetization characteristic
3. External characteristic

Explanation:

Armature Characteristic is the presentation of  Armature current Ia vs Field current If with terminal voltage Vt held constant (at rated value) and generator run at constant n and load varied. It reveals the armature reaction effect on the flux/pole. It is also called a regulation characteristic.

11. In dc generator, the characteristics­ which give the variation of armature generated e.m.f. (E with field current (If), for zero armature current and constant speed is called _____ characteristic.

3. Armature
4. External

Explanation:

Open Circuit Characteristic of DC generator (O.C.C.) (E0/If):

• Open circuit characteristic is also known as magnetic characteristic or no-load saturation characteristic.
• This characteristic shows the relation between generated emf at no load (E0) and the field current (If) at a given fixed speed.
• The O.C.C. curve is just the magnetization curve and it is practically similar for all types of generators.
• The data for the O.C.C. curve is obtained by operating the generator at no load and keeping a constant speed.
• Field current is gradually increased and the corresponding terminal voltage is recorded.

12. The air gap line in DC Generator represents _________

1. Magnetic behaviour of the air gap of the dc machine
2. Magnetic behaviour of the air gap of the induction machine
3. Magnetic behaviour of the iron core
4. All of the mentioned

Answer: 1. Magnetic behaviour of the air gap of the dc machine

Explanation:

The air gap line gives the relationship between MMF and flux density in the air gap. (Induced voltage is proportional to flux density). At higher field current levels, the iron in the machine saturates and the percentage of field mmf applied to the air gap is reduced.

The air gap line in the magnetization curve represents the ideal nature of the machine considering no saturation for the dc machine. The extension of the linear portion of the magnetization curve is known as the air-gap line as it represents mainly the magnetic behaviour of the machine’s air gap.

13. . For a given dc generator, the external characteristic is plotted. Without using further plots, how can we obtain internal characteristics?

1. By adding the IaRa drop to the plot
3. By reducing IaRa drop
4. All of the mentioned

Answer: 3. By reducing IaRa drop

Explanation:

Internal characteristics of DC Generator(E/I:

• An internal characteristic curve shows the relation between the on-load generated emf (Eg) and the armature current (I.
• The on-load generated emf Eg is always less than E0 due to the armature reaction.
• Eg can be determined by subtracting the drop due to demagnetizing effect of armature reaction from no-load voltage (E0).
• So the internal characteristic curve lies below the O.C.C. curve.
• The internal characteristic can be obtained from external characteristics if we know the values of the winding resistances because the armature reaction effect is included in both the characteristics.

14. The voltage drop in terminal voltage from no-load to full load in a shunt generator can be compensated using _________

1. Aiding series field
2. Long-shunt, differential field
3. Aiding shunt field
4. Any of the measures

Explanation:

• The cause of voltage drop in the terminal voltage from no load to full load in a shunt generator can be partial/fully/overcompensated by use of an aiding series field (cumulative compound, which can be connected in a long and short shunt.
• The aiding ampere-turns of the series field automatically increase with the load, compensating for the armature voltage drop.

15. The armature of a dc machine made of silicon steel stampings?

1. To reduce hysteresis loss
2. To reduce eddy current loss
3. For the ease with which slots can be created
4. To achieve high permeability

Answer: 4. To achieve high permeability

Explanation:

• The core of the armature is made up of a large number of high permeability silicon-steel laminated stampings which are insulated from each other by varnish. The laminations help reduce eddy currents in the core. The armature core is a cylindrical structure.

16. What losses occur in the teeth of dc generator?

1. Hysteresis loss
2. Eddy current loss
3. Eddy current as well as hysteresis losses
4. Achieve high permeability

Explanation:

• The magnetic material between slots of the DC generator are the teeth.
• The teeth cross-section influences significantly the performance characteristics of the machine and parameters such as armature coil inductance, magnetic saturation in teeth, eddy-current loss in the stator poles and the cost and complexity of laying armature winding.
• Increase if flux density under one half of the pole and a decrease on the other causes iron loss in armature teeth to increase as the loss is proportional to the square of flux density

17. For a 220-V level compound generator the terminal voltage at the half load is?

1. More than 220-V