Electrical Machines Questions and Answers – Operating Characteristics of DC Generators

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Operating Characteristics of DC Generators”.

1. Which of the following characteristics reveal about the magnetization nature of the machine?
a) No-load characteristics
b) Load characteristics
c) Armature characteristics
d) Both no-load and load characteristics
View Answer

Answer: d
Explanation: Both no-load and load characteristics are required to predict the magnetization of the machine.
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2. Choose the most inappropriate out of the following for the no-load characteristics of the dc generator.
a) It is the open circuit characteristic of the machine
b) It is magnetization characteristic of the machine
c) It is conducted on the unloaded machine
d) None of the mentioned
View Answer

Answer: d
Explanation: All the mentioned nature of the characteristics are correct.

3. The external characteristic is plotted between ____________
a) terminal voltage vs armature current at constant excitation
b) terminal voltage vs field current at constant armature current
c) induced armature emf vs armature current at constant excitation
d) none of the mentioned
View Answer

Answer: a
Explanation: The external characteristic is plotted between terminal voltage and armature current at fixed excitation.
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4. 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?
a) By marking graph for constant terminal voltage
b) By marking graph for constant field current
c) By marking graph for constant armature current
d) By marking graph for constant speed
View Answer

Answer: a
Explanation: Armature characteristic is observed at constant terminal voltage for emf vs armature current.

5. Armature characteristic is also known as _________
a) regulation characteristic
b) magnetization characteristic
c) external characteristic
d) load characteristic
View Answer

Answer: a
Explanation: Armature characteristic is also known as regulation characteristic. It is so called due to the fact that the difference in the terminal voltage helps to determine the voltage deviation.
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6. The air gap line represents _________
a) magnetic behaviour of the air gap of the dc machine
b) magnetic behaviour of the air gap of the induction machine
c) magnetic behaviour of the iron core
d) all of the mentioned
View Answer

Answer: a
Explanation: Air gap line in the magnetization curve represents the ideal nature of the machine considering no saturation for the dc machine.

7. Identify the armature characteristic of the dc generator.
a) The armature characteristic of the dc generator - option a
b) The armature characteristic of the dc generator - option b
c) The armature characteristic of the dc generator - option c
d) The armature characteristic of the dc generator - option d
View Answer

Answer: a
Explanation: At low values of the armature conductor, the increase in the field current is very small to provide IaRa drop. At large armature current,field current increases sharply to compensate for the voltage drop caused by armature reaction.
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8. For a given dc generator, the external characteristic is plotted. Without using further plots, how can we obtain internal characteristic?
a) By adding the IaRa drop to the plot
b) By adding armature reaction
c) By reducing IaRa drop
d) All of the mentioned
View Answer

Answer: a
Explanation: Adding the armature resistance drop we can obtain the external characteristic for a dc generator.

9. Identify the machines by observing their external characteristics for (i) and (ii) respectively.
(i) Find machines by observing their external characteristics for i & ii respectively
(ii) In dc shunt generator voltage drops off much more rapidly with load due to fal in field
a) Separately excited dc generator, shunt generator
b) Shunt generator, separately excited dc generator
c) Differentially compound dc generator, separately excited
d) Series dc generator, shunt generator
View Answer

Answer: a
Explanation: In dc shunt generator, voltage drops off much more rapidly with load due to fal in field current with terminal voltage.
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10. The voltage drop in terminal voltage from no-load to full load in a shunt generator can be compenated using _________
a) aiding series field
b) long-shunt, diferential field
c) aiding shunt field
d) any of the measures
View Answer

Answer: a
Explanation: By aiding the series field the armature reaction drop can be compensated.

11. Mark the correct order for the external characteristics of compound generators from the below graph.
Over compounding provides maximum voltage difference at no load in given figure
a) 1: over compound, 2: level compound, 3: under compound, 4: differential compound
b) 1: under compound, 2: level compound, 3: over compound, 4: differential compound
c) 1: differential compound, 2: level compound, 3: under compound, 4: over compound
d) 1: over compound, 2: differential compound, 3: under compound, 4: level compound
View Answer

Answer: a
Explanation: Over compounding provides maximum voltage difference at no load while the differential compunding has least.

12. The external characteristic for the dc series generator is?
a) The external characteristic for the dc series generator - option a
b) The external characteristic for the dc series generator - option b
c) The external characteristic for the dc series generator - option c
d) The external characteristic for the dc series generator - option d
View Answer

Answer: a
Explanation: There is a linear behaviour till the saturation.

13. Why is armature of a dc machine made of silicon steel stampings?
a) To reduce hysteresis loss
b) To reduce eddy current loss
c) For the ease with which slots can be created
d) To achieve high permeability
View Answer

Answer: a
Explanation: The silicon steel has very high permeability and makes the flux past consistent. Thereby making lesser losses.

14. What losses occur in the teeth of dc generator?
a) To reduce hysteresis loss
b) To reduce eddy current loss
c) To reduce eddy current as well as hysteresis losses
d) To achieve high permeability
View Answer

Answer: c
Explanation: Teeth of the machine has both eddy current losses as well as hysteresis losses.

15. For a 220-V level compound generator the terminal voltage at the half load is?
a) more than 220-V
b) same as no-load voltage
c) more than no-load voltage
d) lesser than no-load voltage
View Answer

Answer: b
Explanation: In a level compound dc generator the terminal voltage remains same as no load voltage at all the loading.

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