This set of DC Machines Assessment Questions and Answers focuses on “No Load with On Load Characteristics of DC Generator”.
1. Which of the following is not the operating characteristics of Dc generator?
a) No-load characteristics
b) Load characteristics
c) External characteristics
d) Internal characteristics
View Answer
Explanation: The relationship between various parameters has to be presented graphically because of the magnetic saturation effect. Four characteristics of importance are the following: 1) No load characteristics 2) Load characteristics 3) External characteristics 4) Armature characteristics.
2. Characteristics drawn at Ia = 0 is also called as ____________
a) Magnetization characteristics
b) Non-magnetization characteristics
c) Anti-magnetization characteristics
d) Cannot be determined
View Answer
Explanation: With Ia = 0 (no load) at constant n, it is the presentation of Vt (=Ea) vs If. This is the most important characteristic as it reveals the nature of the magnetization of the machine. It is easy to determine as the generator is on no load and so only low rated prime mover will serve the purpose. It is commonly called the open–circuit/magnetization characteristic.
3. Open circuit characteristics (OCC) is generally drawn across __________
a) Ea vs If, Ia=constant (not equal to rated)
b) Ea vs If, Ia=0
c) Ea vs If, Ia=constant
d) Ea vs If, Ia=constant (rated)
View Answer
Explanation: Open circuit characteristics is also called as no-load characteristics or magnetization characteristics. No load clearly states that armature current will equal to 0. Thus, OCC is drawn at Ea vs If, Ia=0.
4. Characteristics of a DC generator drawn across Vt vs If at rated armature current and constant speed, is called as ____________
a) Load characteristics
b) No-load characteristics
c) External characteristics
d) Armature characteristics
View Answer
Explanation: Since we have Ia value which is equal to rated i.e. non-zero, it is indeed not a no-load characteristic. Axes given are Vt and If, hence it is not an armature characteristic. Thus, it’s called as load characteristic or magnetization characteristic on load.
5. In an OCC at If =0, graph starts from origin.
a) True
b) False
View Answer
Explanation: As the machine would have been previously subjected to magnetization, a small residual voltage would be present with field unexcited. As will be seen practically, this is necessary for generator to self-excite. So, graph will start from just above the origin on Voc axis.
6. While conducting OCC, in order to avoid hysteresis loop, in which direction If should be increased?
a) -ve direction
b) +ve direction
c) In any direction
d) In both direction there exists hysteresis loop
View Answer
Explanation: In conducting the OCC test, If must be raised gradually only in the forward direction otherwise the curve would exhibit local hysteresis loops. In OCC at If =0 there exists small residual voltage shown by non-zero Voc.
7. Air gap line is drawn at iron _________
a) Saturated
b) Unsaturated
c) Moderately saturated
d) Variable saturation
View Answer
Explanation: The extension of the liner 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, the iron being unsaturated in this region consumes negligible ampere-turns; in any case the effect of iron is also linear here.
8. If suppose OCC is conducted at speed n1, where n1< nrated, OCC will lie ____________
a) Above OCC at nrated
b) On OCC at nrated
c) Below OCC at nrated
d) Can’t comment by only speed information
View Answer
Explanation: For a less speed than the rated one, residual voltage appearing at terminal call Voc will also be less than Voc at rated value, it will vary in parallel manner but will never intersect OCC at rated speed.
9. Ea can be determined using __________
a) No-load characteristics
b) Load characteristics
c) Cannot be determined
d) Above OCC
View Answer
Explanation: Under load conditions Ea cannot be determined from the OCC for If in the saturation region because of the demagnetizing effect of armature reaction. We must therefore determine experimentally the equivalent demagnetizing ampere-turns ATd due to armature reaction under actual load conditions.
10. If load characteristics are drawn on OCC itself, we get curve ________
a) Above OCC
b) On OCC
c) Below OCC
d) Intersecting OCC
View Answer
Explanation: Since on load operation of a DC machine, we’ll get terminal voltage less than the terminal voltage obtained in OCC, graph will start from below OCC. On load, the effect of armature reaction will draw load characteristics parallel to the OCC below it, causing no intersection.
11. Load characteristics drawn at Ra =0 and Ra not equal to 0, will lie _____
a) Above
b) On
c) Below
d) Intersecting
View Answer
Explanation: Load characteristics with at Ra =0 will lie below the load characteristics drawn at Ra not equal to 0. To the load characteristic we add IaRa drop to get Ea induced emf with load. Thus, it will lie above.
12. OCC is drawn at two different speeds both less than rated speed. OCC drawn at speed N1 lies below OCC drawn at speed N2. Which of the following relation is correct?
a) N2 = N1
b) N2 < N1
c) N2 >> N1
d) Can’t comment
View Answer
Explanation: As a speed on which OCC is taken decreases, the residual voltage appearing on Voc axis also decrease and OCC starts from below, compare to first one. Thus, N1 is less comparatively, as its OCC lies below than the OCC drawn at other speed.
13. Why No-load or load characteristics are also called as magnetization characteristics?
a) Ea α If
b) Ea α φ
c) If α φ
d) Cannot be determined
View Answer
Explanation: As the generated voltage in the armature in the case of DC generator is proportional to terminal voltage, which also proportional to magnetic flux, as seen by residual voltage appearing at 0 field current. No-load and load characteristics are called as magnetization curves.
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