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51.	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 commentI had been asked this question in an online quiz.This intriguing question comes from No Load with On Load Characteristics of DC Generator topic in portion DC Generator of DC Machines
Answer» Correct choice is (c) N2 >> N1 To elaborate: 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.

52.	Which of the following characteristic lies above of all others?(a) Differential compound(b) Under compound(c) Level compound(d) Over compoundThis question was posed to me by my college director while I was bunking the class.The above asked question is from Characteristics of Separately Excited Generators topic in division DC Generator of DC Machines
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53.	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 determinedThe question was asked in final exam.My query is from No Load with On Load Characteristics of DC Generator topic in section DC Generator of DC Machines
Answer» Right answer is (b) Ea α φ 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.

54.	If load characteristics are drawn on OCC itself, we get curve ________(a) Above OCC(b) On OCC(c) Below OCC(d) Intersecting OCCThis question was posed to me in an internship interview.This is a very interesting question from No Load with On Load Characteristics of DC Generator topic in division DC Generator of DC Machines
Answer» Right option is (c) Below OCC Easy 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.

55.	Which of the following is not the operating characteristics of Dc generator?(a) No-load characteristics(b) Load characteristics(c) External characteristics(d) Internal characteristicsThis question was posed to me in an international level competition.Asked question is from No Load with On Load Characteristics of DC Generator in chapter DC Generator of DC Machines
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56.	External characteristic differ from an internal characteristic in separately excited DC generator by _________(a) Ia Ra(b) If Ra(c) IL RL(d) If RfThe question was posed to me in class test.Question is from Characteristics of Separately Excited Generators topic in chapter DC Generator of DC Machines
Answer» Right option is (a) Ia *Ra To EXPLAIN: INTERNAL characteristic is DRAWN across armature generated voltage, it doesn’t ACCOUNT the presence of armature resistance. External characteristic takes into account the presence of armature resistance as it is drawn across terminal voltage.

57.	By changing the polarity of the armature of failed self-exciting machine, machine can be started.(a) True(b) FalseI got this question in a national level competition.The query is from Characteristics of Self Excited Generator topic in division DC Generator of DC Machines
Answer» The correct ANSWER is (a) True 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.

58.	External characteristics for all separately DC generators (except series generator) can be summarized as ____________(a) Parabola opening in +x direction(b) Parabola opening in -x direction(c) Parabola opening in +y direction(d) Parabola opening in -y directionThe question was asked in unit test.My doubt stems from External and Armature Characteristics of DC Generator topic in portion DC Generator of DC Machines
Answer» The correct choice is (b) Parabola OPENING in -x direction The explanation: All parabolas for SEPARATELY EXCITED DC generator, shunt generator, compound generator are parabolas which are opening in -x directions, only centres of all parabolas lie at different places DEPENDING on the type of generator.

59.	what is the first step in drawing the armature characteristic?(a) If is made 0(b) Field current is adjusted to give Voc equal to rated value(c) Switch connecting armature circuit with external circuit is made open(d) Cannot be determinedI have been asked this question during an online exam.This intriguing question originated from External and Armature Characteristics of DC Generator topic in chapter DC Generator of DC Machines
Answer» The correct ANSWER is (C) Switch connecting armature circuit with external circuit is MADE open The explanation is: While drawing armature characteristic, switch of armature circuit is made open so that armature current is set to 0. Then, field current is adjusted to give VOC equal to RATED value. Switch its then closed.

60.	The variation in terminal voltage of DC shunt generator with respect to variation in separately excited DC generator is ___________(a) Much rapid(b) Much slower(c) Remains constant(d) Can’t sayI got this question during an interview for a job.The above asked question is from Characteristics of Separately Excited Generators topic in chapter DC Generator of DC Machines
Answer» The correct option is (a) MUCH rapid To explain I WOULD say: The terminal voltage DROPS off much more rapidly with load in a shunt generator than in a separately-excited generator because of fall in field current with terminal voltage. The EXTERNAL characteristic is a double-valued curve with a certain IL (MAX).

61.	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-zeroThis question was addressed to me during an online exam.The query is from Characteristics of Self Excited Generator topic in portion DC Generator of DC Machines
Answer» Right CHOICE is (d) Armature resistance is non-zero The explanation is: 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.

62.	In an OCC at If =0, graph starts from origin.(a) True(b) FalseI have been asked this question during an online interview.The origin of the question is No Load with On Load Characteristics of DC Generator in division DC Generator of DC Machines
Answer» The correct choice is (b) False The best 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.

63.	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 curveI had been asked this question in an online interview.Asked question is from Characteristics of Self Excited Generator in chapter DC Generator of DC Machines
Answer» The CORRECT choice is (b) Straight line PASSING through the origin The best I can explain: 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.