This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Characteristics of Self Excited Generator”.
1. What is Self-excitation in DC shunt generator?
a) Field winding is connected in series of armature
b) Field winding is connected in parallel of armature
c) Field winding is not connected to the armature
d) Field Winding is not excited
View Answer
Explanation: Rather than arranging a separate dc source for excitation purposes, practical generators are always excited from their own armature terminals, this method of excitation being known as self-excitation. A self-excited generator with such connection known as a shunt generator.
2. For a self-excited DC shunt generator Ia = _____________
a) IL – If
b) IL + IL
c) – IL – IL
d) – IL + IL
View Answer
Explanation: For a self-excited DC shunt generator field winding is connected in parallel with armature winding so that, armature current splits into load current and field current. Load current will be given out as the machine is working as a generator.
3. For self-excited DC generator, field circuit curve in the OCC will be __________
a) Straight line not passing through the origin with +ve slope
b) Straight line passing through the origin
c) Straight line not passing through the origin with -ve slope
d) Increasing curve
View Answer
Explanation: Field is connected such that this current increases the field mmf and therefore the induced emf, the machine will continuously build up. For the field circuit V = If *Rf, which is a straight-line relationship, called the Rf -line, in V-If plot.
4. Intersection of Rf line and magnetization curve will give _____________
a) No load terminal voltage
b) Field current at no load
c) Both a and b
d) Not specific data
View Answer
Explanation: When OCC is plotted against terminal voltage and field current where, field resistance line is also drawn, an intersection point will arise when OCC almost becomes constant due to saturation effect. So, corresponding value field current and no-load voltage can be observed from x and y axis resp.
5. What will happen if field resistance is decreased?
a) No load voltage will decrease
b) No load voltage will increase
c) No change in no load voltage
d) Field current will decrease
View Answer
Explanation: When field resistance is decreased, it will increase field current by Ohm’s law. Thus, field resistance line will lie below the older line of field resistance. Eventually, it’ll intersect OCC afterwards causing increase in no-load voltage.
6. No-load voltage can’t be defined where _____________
a) Field resistance line lies below the magnetization linear line
b) Field resistance line intersects the magnetization linear line
c) Field resistance line coincides the magnetization linear line
d) Defined everywhere
View Answer
Explanation: We get no-load voltage when magnetization curve intersects with field resistance line. If that intersection is not occurring, we’ll not get no load voltage. Thus, when both lines coincide we get undefined value of no-load voltage.
7. Machine does not excite to an appreciable value when ___________
a) Field resistance is less than critical resistance
b) Field resistance is more than critical resistance
c) In both cases a and b
d) Depends on other parameters
View Answer
Explanation: The no-load voltage is undefined for a field resistance whose line coincides with the linear portion of the magnetization curve. With field resistance even slightly more than this value, the machine does not excite to any appreciable value.
8. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in field resistance values?
a) Rf1 < Rf2
b) Rf1 = Rf1
c) Rf1 > Rf1
d) Can’t determine from no-load voltage
View Answer
Explanation: When field resistance value increases the line of field resistance shifts towards more +ve slope, proving that field current decreases. This line will now intersect OCC at lower regions indicating lower values of terminal voltage.
9. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in speed if field resistance is kept same for both cases?
a) N1 < N2
b) N1 = N1
c) N1 > N1
d) Can’t determine from no-load voltage
View Answer
Explanation: For Field resistance kept constant, field resistance line will intersect OCC at different points as speed variation may shift OCC up or down. When speed is increased OCC shifts up indicating that no-load voltage will also rise up.
10. DC generator will not self-excite in __________
a) Positive feedback
b) Negative feedback
c) Zero Feedback
d) Doesn’t depend on feedback
View Answer
Explanation: The field connection to the armature is such that the induced emf due to the residual magnetism tends to destroy the residual magnetism, is called negative feedback. In the absence of residual magnetism machine will fail to self-excite.
11. Which of the following is not a reason, behind failing of self-exciting machine?
a) Residual magnetism is absent
b) Negative feedback
c) Field resistance value is more than critical resistance value
d) Armature resistance is non-zero
View Answer
Explanation: In case a and b, machine will tend to destroy the residual magnetism present in a core, which will not excite machine. When field resistance value is more than critical value as seen from OCC, machine will not excite to appropriate value.
12. By changing the polarity of the armature of failed self-exciting machine, machine can be started.
a) True
b) False
View Answer
Explanation: By changing the polarity of field connections to the armature negative feedback can be turned into positive feedback, thus it will help to build up the voltage and machine will act good in self-excitation mode.
13. Which of the following will be useful to generate positive feedback?
a) Increasing the field resistance
b) Increasing the armature resistance
c) Decreasing the armature resistance
d) Changing the shaft direction
View Answer
Explanation: Negative feedback can be remedied simply by reversing the field connection to the armature or reversing the direction of rotation. For large dc generators with permanent connections and a fixed direction of rotation, the problem is overcome by temporarily exciting the field from a battery source.
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