28 + Interview Questions in Synchronous Generators in Electrical Machines Page 1 InterviewSolution

1.	The core losses in synchronous machine is _______ dependent.(a) voltage(b) current(c) temperature(d) insulationThe question was posed to me during an online interview.The question is from Operating Characteristics of Alternators and their Ratings in section Synchronous Generators of Electrical Machines
Answer» Correct answer is (a) VOLTAGE The EXPLANATION: CORE losses depend on the voltage applied to the machine.

Discussion

2.	At zpf lag, the excitation should be increased in order to maintain the armature terminal voltage constant.(a) True(b) FalseI had been asked this question during an interview.Question is from Operating Characteristics of Alternators and their Ratings in portion Synchronous Generators of Electrical Machines
Answer» Correct option is (a) True Best explanation: DUE to demagnetizing armature reaction at ZPF lag in an alternator.

Discussion

3.	The rating of the alternator is decided by _________(a) losses(b) voltage(c) armature current(d) temperatureI got this question in exam.Asked question is from Operating Characteristics of Alternators and their Ratings in chapter Synchronous Generators of Electrical Machines
Answer» CORRECT CHOICE is (a) losses The best EXPLANATION: The losses determine the rating to be used for ALTERNATOR.

Discussion

4.	If the machine was operating at upf, then the new excitation voltage will be _________(a) greater than 1 pu(b) less than 1 pu(c) 1 pu(d) zeroThe question was posed to me in an online interview.This intriguing question comes from Operating Characteristics of Alternators and their Ratings in section Synchronous Generators of Electrical Machines
Answer» Right CHOICE is (a) GREATER than 1 PU To elaborate: The new excitation will be more than 1 pu to compensate for the flux REQUIRED.

Discussion

5.	Alternator compounding characteristic is obtained by _________(a) keeping Vt constant(b) keeping Ef constant(c) keeping Ia constant(d) varying VtThe question was posed to me during an interview.This question is from Operating Characteristics of Alternators and their Ratings in chapter Synchronous Generators of Electrical Machines
Answer» Correct answer is (a) keeping VT CONSTANT For EXPLANATION I would say: Alternator compounding CHARACTERISTIC is obtained by keeping Vt constant.

Discussion

6.	Ideally the voltage regulation of an alternator should be ___________(a) zero(b) infinite(c) 50%(d) 100%I had been asked this question by my school teacher while I was bunking the class.Origin of the question is Voltage Regulation of an Alternator topic in section Synchronous Generators of Electrical Machines
Answer» The CORRECT ANSWER is (a) zero The BEST explanation: Ideally the terminal VOLTAGE and the excitation voltage should be same. So V.R. = E-V/V = 0%.

Discussion

7.	If the machine size increases, the three phase alternator resistance _______ and synchronous reactance _______(a) increases, decreases(b) decreases, increases(c) remains constant, decreases(d) decreases, decreasesThe question was posed to me in an online interview.This question is from Voltage Regulation of an Alternator in portion Synchronous Generators of Electrical Machines
Answer» Right choice is (a) increases, decreases For explanation I would say: If the machine SIZE increases, the COPPER amount will increase but the air gap will ALSO increase having more reluctance so the reactance will reduce.

Discussion

8.	If the emf from the air gap line is the 3-phase alternator is 440V per phase and armature current is 110 A. The synchronous reactance is?(a) 4 ohms(b) 2 ohms(c) 6.92 ohms(d) 2.32 ohmsThis question was posed to me at a job interview.Query is from Voltage Regulation of an Alternator in section Synchronous Generators of Electrical Machines
Answer» CORRECT choice is (a) 4 ohms To ELABORATE: Z = V/I = 440/110 = 4 ohms.

Discussion

9.	In a calculation, the actual voltage regulation is 33.1% while the calculated value of the voltage regulation is 32.9%. This infers that the chosen method has been ___________(a) zpf or asa(b) emf(c) mmf(d) mmf or zpfThe question was posed to me during an interview.I'm obligated to ask this question of Voltage Regulation of an Alternator topic in section Synchronous Generators of Electrical Machines
Answer» The CORRECT ANSWER is (a) zpf or asa To elaborate: As the given value is very CLOSE to ACTUAL, so it must be zpf or asa method of regulation.

Discussion

10.	In a calculation, the actual voltage regulation is 33.1% while the calculated value of the voltage regulation is 25%. This infers that the chosen method has been ___________(a) emf(b) mmf(c) asa(d) zpfThis question was posed to me in an online interview.This is a very interesting question from Voltage Regulation of an Alternator in division Synchronous Generators of Electrical Machines
Answer» The CORRECT answer is (B) MMF Best explanation: As the given value is LESS than actual, so it MUST be mmf method.

Discussion

11.	The preferred order of calculating the voltage regulation is ___________(a) ZPF > ASA > MMF > EMF(b) ZPF > MMF > ASA > EMF(c) ASA > MMF > ASA > EMF(d) EMF > ASA > ZPF > MMFI have been asked this question during an interview.This is a very interesting question from Voltage Regulation of an Alternator in chapter Synchronous Generators of Electrical Machines
Answer» Right CHOICE is (a) ZPF > ASA > MMF > EMF To explain I would say: Emf method is least preferred over the other methods and zpf method gives the ACCURATE RESULTS as it does not alter the values.

Discussion

12.	In a calculation, the actual voltage regulation is 33.1% while the calculated value of the voltage regulation is 56.8%. This infers that the chosen method has been ___________(a) emf(b) mmf(c) asa(d) zpfI got this question during an interview for a job.This key question is from Voltage Regulation of an Alternator in portion Synchronous Generators of Electrical Machines
Answer» Right CHOICE is (a) EMF Easiest EXPLANATION: As the given VALUE is more than ACTUAL, so it must be emf method.

Discussion

13.	Mmf method of voltage regulation is called _______ while the emf method is __________(a) optimistic, pessimistic(b) pessimistic, pessimistic(c) optimistic, optimistic(d) pessimistic, optimisticThe question was posed to me during an interview.This interesting question is from Voltage Regulation of an Alternator topic in division Synchronous Generators of Electrical Machines
Answer» Right choice is (a) OPTIMISTIC, pessimistic The explanation: In the mmf method the calculated VALUE is lesser than the actual value so it is LESS MISLEADING than emf method which predicts a GREATER value.

Discussion

14.	In mmf method ___________(a) all the emf is scaled to mmf(b) only the mmf values are considered neglecting impedance drop(c) all the emf are taken zero(d) emf is converted to saturated impedance dropsThe question was asked during an internship interview.My question comes from Voltage Regulation of an Alternator in section Synchronous Generators of Electrical Machines
Answer» Correct CHOICE is (a) all the EMF is SCALED to mmf To explain: In mmf METHOD all the emf is scaled to mmf

Discussion

15.	Emf method is also known as ___________(a) pessimistic method(b) optimistic method(c) zero power factor method(d) none of the mentionedThe question was posed to me in final exam.Asked question is from Voltage Regulation of an Alternator in division Synchronous Generators of Electrical Machines
Answer» The correct answer is (a) pessimistic method The explanation is: UNSATURATED impedance is greater than SATURATED impedance, so the drop ACCOUNTED will be less than the actual making the voltage REGULATION more than actual.

Discussion

16.	The emf method of the voltage regulation is applicable only to cylindrical rotor alternator due to ___________(a) resultant air gap flux is not affected by angular position of rotor(b) uniform angular position of rotor(c) non uniform angular position of rotor(d) saliency of the poles is a trouble while estimating the emfThe question was posed to me in semester exam.This interesting question is from Voltage Regulation of an Alternator in division Synchronous Generators of Electrical Machines
Answer» The correct answer is (a) resultant air gap FLUX is not AFFECTED by ANGULAR position of ROTOR Easy explanation: It is so because resultant air gap is unaffected by rotor position.

Discussion

17.	Alternator on infinite bus bar has constant ___________(a) terminal voltage and frequency(b) frequency(c) power factor(d) power factor and terminal voltageThis question was addressed to me in class test.This is a very interesting question from Voltage Regulation of an Alternator in section Synchronous Generators of Electrical Machines
Answer» Correct option is (a) terminal VOLTAGE and frequency To EXPLAIN I WOULD say: Infinite bus BAR has CONSTANT voltage and frequency.

Discussion

18.	Voltage regulation for the alternator operating at leading power factor is negative.(a) True(b) FalseThis question was posed to me in homework.My question is taken from Voltage Regulation of an Alternator in portion Synchronous Generators of Electrical Machines
Answer» The correct OPTION is (a) True The best EXPLANATION: VOLTAGE regulation for an alternator operating at leading power factor is negative due to MAGNETIZING nature of armature reaction.

Discussion

19.	Voltage regulation for an alternator operating at leading power factor is negative due to ___________(a) magnetizing nature of armature reaction(b) demagnetizing nature of armature reaction(c) cross-magnetizing nature of armature reaction(d) all of the mentionedI got this question during an interview for a job.This intriguing question originated from Voltage Regulation of an Alternator in portion Synchronous Generators of Electrical Machines
Answer» CORRECT option is (a) MAGNETIZING nature of armature reaction To ELABORATE: FIELD flux gets AIDED by the armature reaction leading to higher Vt.

Discussion

20.	When the load on an alternator is varied, its terminal voltage also changes due to _____________(a) armature resistance(b) leakage reactance(c) armature reaction(d) all of the mentionedThe question was asked during an interview.I need to ask this question from Voltage Regulation of an Alternator topic in division Synchronous Generators of Electrical Machines
Answer» Right answer is (a) armature resistance Easiest explanation: Armature REACTANCE, leakage reactance and armature reaction cause the terminal VOLTAGE to differ from the EXCITATION voltage.

Discussion

21.	Potier method is very accurate due to __________(a) emfs are handled as voltage and mmf as AT(b) emf and mmf are handled as AT(c) emf and mmf are handled as voltage(d) none of the mentionedI got this question during an online interview.My question comes from Phasor Diagram of a Cylindrical Rotor Alternator topic in chapter Synchronous Generators of Electrical Machines
Answer» Correct choice is (a) EMFS are HANDLED as VOLTAGE and MMF as AT Easiest EXPLANATION: The potier’s method is very accurate because emfs are handled as voltage and mmf as AT.

Discussion

22.	The leakage reactance can be determined with lesser accuracy by using Potier triangle.(a) True(b) FalseThe question was posed to me during an interview.The question is from Phasor Diagram of a Cylindrical Rotor Alternator topic in chapter Synchronous Generators of Electrical Machines
Answer» CORRECT option is (b) False To explain: The leakage reactance can be DETERMINED with better ACCURACY by USING POTIER triangle.

Discussion

23.	From the OCC and SCC curves, what does the intercept BC represent?(a) Armature reaction(b) Leakage reactance(c) Armature copper losses(d) Sum of armature reaction and leakage reactanceThis question was addressed to me in an international level competition.Query is from Phasor Diagram of a Cylindrical Rotor Alternator in division Synchronous Generators of Electrical Machines
Answer» The CORRECT option is (b) LEAKAGE reactance The EXPLANATION is: It is leakage reactance which is calculated BASED on potier triangle.

Discussion

24.	The short circuit characteristic of alternator is a straight line due to __________(a) no saturation(b) no armature reaction(c) no eddy current(d) all of the mentionedThis question was addressed to me during an online interview.I'm obligated to ask this question of Phasor Diagram of a Cylindrical Rotor Alternator topic in division Synchronous Generators of Electrical Machines
Answer» The CORRECT answer is (a) no saturation For explanation: At the SHORT CIRCUIT of the alternator is having the LINEAR relation as the armature does not enter the saturation mode due to demagnetizing nature of armature REACTION.

Discussion

25.	From the OCC and SCC curves, what does the intercept AB represent?(a) Armature reaction(b) Leakage reactance(c) Armature copper losses(d) Sum of armature reaction and leakage reactanceThis question was addressed to me in class test.The query is from Phasor Diagram of a Cylindrical Rotor Alternator topic in chapter Synchronous Generators of Electrical Machines
Answer» Correct choice is (a) ARMATURE REACTION Easiest EXPLANATION: It is armature reaction which is calculated based on Potier triangle.

Discussion

26.	Phasor diagram below is best describing the __________(a) Open circuit characteristic of alternator(b) Short circuit characteristic of alternator(c) External circuit characteristic of alternator(d) Internal circuit characteristic of alternatorThis question was posed to me in homework.Question is taken from Phasor Diagram of a Cylindrical Rotor Alternator topic in portion Synchronous Generators of Electrical Machines
Answer» Right answer is (a) Open circuit characteristic of alternator The best EXPLANATION: Graph of EF VS Ff is the open circuit characteristic of the MACHINE.

Discussion

27.	If the resultant flux will not be in phase with the resultant mmf Fr. This is due to __________(a) hysteresis effect(b) armature reaction(c) hysteresis effect and armature reaction(d) eddy currentsThe question was asked in a job interview.This question is from Phasor Diagram of a Cylindrical Rotor Alternator topic in chapter Synchronous Generators of Electrical Machines
Answer» The correct CHOICE is (a) hysteresis effect Explanation: Hysteresis effect CAUSES the flux to be LAGGING in NATURE.

Discussion

28.	The load angle δ is between __________(a) Ef and Ia(b) Ia and field flux(c) Ef and Vt(d) Vt and IaThis question was addressed to me during an interview for a job.This question is from Phasor Diagram of a Cylindrical Rotor Alternator topic in chapter Synchronous Generators of Electrical Machines
Answer» RIGHT answer is (C) Ef and Vt Explanation: It is the angle between EXCITATION VOLTAGE and the TERMINAL voltage.

Discussion

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The rating of the alternator is decided by _________(a) losses(b) voltage(c) armature current(d) temperatureI got this question in exam.Asked question is from Operating Characteristics of Alternators and their Ratings in chapter Synchronous Generators of Electrical Machines

Emf method is also known as ___________(a) pessimistic method(b) optimistic method(c) zero power factor method(d) none of the mentionedThe question was posed to me in final exam.Asked question is from Voltage Regulation of an Alternator in division Synchronous Generators of Electrical Machines

Voltage regulation for the alternator operating at leading power factor is negative.(a) True(b) FalseThis question was posed to me in homework.My question is taken from Voltage Regulation of an Alternator in portion Synchronous Generators of Electrical Machines

The leakage reactance can be determined with lesser accuracy by using Potier triangle.(a) True(b) FalseThe question was posed to me during an interview.The question is from Phasor Diagram of a Cylindrical Rotor Alternator topic in chapter Synchronous Generators of Electrical Machines

The load angle δ is between __________(a) Ef and Ia(b) Ia and field flux(c) Ef and Vt(d) Vt and IaThis question was addressed to me during an interview for a job.This question is from Phasor Diagram of a Cylindrical Rotor Alternator topic in chapter Synchronous Generators of Electrical Machines