22 + Interview Questions in Electromechanical Energy Conversions in Electrical Machines Page 1 InterviewSolution

1.	The velocity of the dc machine at the final state is ________(a) E/Bl(b) Bl/E(c) EBl(d) None of the mentionedI had been asked this question in an interview for job.This intriguing question comes from Torque in DC Machines in section Electromechanical Energy Conversions of Electrical Machines
Answer» CORRECT option is (a) E/Bl Best explanation: The VELOCITY at STEADY state is E/Bl.

Discussion

2.	For a universal motor, the rated supply current is 10A giving a rated torque of 100 N-m. If a current of 5A is applied, what will be generated torque vary to?(a) 250 N-m(b) 400 N-m(c) 500 N-m(d) 1000 N-mThis question was addressed to me in quiz.The doubt is from Torque in DC Machines topic in division Electromechanical Energy Conversions of Electrical Machines
Answer» CORRECT OPTION is (b) 400 N-m The best I can explain: The torque is proportional to the SQUARE of the current. So the NEW torque will be100*(2^2) = 400 N-m.

Discussion

3.	Pulsating torque frequency in universal motor is ________(a) 2f(b) f(c) f/2(d) 4fI got this question in class test.Origin of the question is Torque in DC Machines in chapter Electromechanical Energy Conversions of Electrical Machines
Answer» CORRECT choice is (a) 2f Best explanation: The TORQUE REVERSES its DIRECTION in the negative cycle of the ac supply and thus it always UNIDIRECTIONAL.

Discussion

4.	DC Motor torque depends on ________(a) geometry(b) magnetic properties(c) any of the mentioned(d) both geometry and magnetic properties of the structureI got this question by my school teacher while I was bunking the class.Origin of the question is Torque in DC Machines in chapter Electromechanical Energy Conversions of Electrical Machines
Answer» The correct answer is (d) both geometry and MAGNETIC properties of the structure Explanation: The torque will indeed depend on the geometry as WELL the magnetic materials USED in the CONSTRUCTION of the dc machine. Though this remains constant with the machine.

Discussion

5.	If the torque induced is zero in the dc machine, it can be said that ____________(a) current is zero(b) flux can be zero(c) current or flux=0(d) any of the mentionedI had been asked this question in an interview for job.I'm obligated to ask this question of Torque in DC Machines in chapter Electromechanical Energy Conversions of Electrical Machines
Answer» RIGHT option is (a) current is zero The best I can explain: The TORQUE at steady state of OPERATION is zero as the emf induced is equal to the source voltage. Also the the very much starting the current will be zero which will not let the emf to get induced.

Discussion

6.	If the torque is in the direction of rotation, the DC machine acts as ________(a) generator(b) motor(c) amplidyne(d) any of the mentionedThis question was addressed to me in quiz.This interesting question is from Torque in DC Machines topic in section Electromechanical Energy Conversions of Electrical Machines
Answer» The CORRECT OPTION is (a) generator Best explanation: The ELECTROMAGNETIC torque generated in the DC generator is in the direction in which the prime mover is rotated.It is produced due to the ACTION of armature current and stator field.

Discussion

7.	The cogging torque is absent in the permanent magnet dc machine is due to ________(a) non-magnetic nature of rotor(b) magnetic nature of rotor(c) absence of ac supply(d) any of the mentionedI have been asked this question in an online quiz.The above asked question is from Torque in DC Machines topic in portion Electromechanical Energy Conversions of Electrical Machines
Answer» Right CHOICE is (a) non-magnetic nature of rotor Easy EXPLANATION: It is DUE to the fact that no field POLES are induced in PMDC to create magnetic flux leading to harmonics.

Discussion

8.	The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What is the steady state angular velocity at no-load?(a) 480 rad/s(b) 960 rad/s(c) 320 rad/s(d) 490 rad/sThe question was posed to me in an internship interview.The query is from Torque in DC Machines in section Electromechanical Energy Conversions of Electrical Machines
Answer» Right OPTION is (a) 480 rad/s For explanation I would say: V = V/(2RlB) = 120/(20.000510.25) = 480 rad/s.

Discussion

9.	The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What will be the magnitude of the following current at t=0+?(a) 400A(b) 200A(c) 0(d) any of the mentionedI have been asked this question at a job interview.The above asked question is from Torque in DC Machines in section Electromechanical Energy Conversions of Electrical Machines
Answer» RIGHT ANSWER is (a) 400A Easiest EXPLANATION: I = (V-E)/R = (120-0)/0.3 = 400 A.

Discussion

10.	The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. Suddenly the switch is closed at t=0, what is observed in the circuit?(a) Current will flow but zero induced EMF(b) Current will not flow and zero induced EMF(c) Current will not flow but EMF is induced(d) Current will flow and EMF will also be inducedThe question was posed to me in an international level competition.This question is from Torque in DC Machines topic in section Electromechanical Energy Conversions of Electrical Machines
Answer» Correct OPTION is (a) CURRENT will FLOW but ZERO induced EMF To elaborate: Initially the induced emf in the armature is zero. But the current will flow as there is terminal source voltage.

Discussion

11.	The torque induced beyond the pole shoes in the DC machine is?(a) 0(b) 2/pi phi i(c) 4/pi phi i(d) none of the mentionedI have been asked this question in an international level competition.This key question is from Torque in DC Machines topic in division Electromechanical Energy Conversions of Electrical Machines
Answer» Right option is (a) 0 Easiest EXPLANATION: The MAGNETIC FIELD vanishes at the pole terminals and thus the cross PRODUCT with the magnetic field yields zero.

Discussion

12.	What is the principle of torque production in a dc machine?(a) Lorentz’s law(b) Lenz’s law(c) Faraday’s law(d) Self inductanceThis question was addressed to me during an online interview.My enquiry is from Torque in DC Machines topic in chapter Electromechanical Energy Conversions of Electrical Machines
Answer» CORRECT choice is (a) Lorentz’s law To EXPLAIN: Though all the mentioned laws contribute towards the torque GENERATION, but PRIMARILY its the Lorentz’s forces which INITIATE and define the torque direction.

Discussion

13.	Which of the following cannot reduce the terminal voltage of a dc shunt generator?(a) Commutation(b) Armature reaction(c) Armature ohmic losses(d) Any of the mentionedThis question was addressed to me in exam.The above asked question is from EMF Generated in the Armature in section Electromechanical Energy Conversions of Electrical Machines
Answer» Right choice is (a) Commutation The explanation is: Commutation ideally does not reduce the terminal VOLTAGE of a dc machine. If it TAKES place PROPERLY at DESIRED TIMINGS, the losses can be avoided. Armature reaction, ohmic losses contribute to the losses in the terminal voltage.

Discussion

14.	For a 4 pole 200 V, 20 A dc generator running at the rated rpm with no load will be ________(a) zero no load losses(b) 4000 W no load losses(c) negligible no load losses(d) none of the mentionedThe question was asked during a job interview.I need to ask this question from EMF Generated in the Armature topic in section Electromechanical Energy Conversions of Electrical Machines
Answer» The correct CHOICE is (a) zero no LOAD LOSSES Explanation: As there is no load and assuming no armature resistance, the no load losses should ideally be zero.

Discussion

15.	For a four pole, 200 V dc shunt generator having armature resistance of 1 ohms. The current flowing in the armature is 10 A. What is the back emf generated if the terminal voltage of 220 V supply is fed?(a) 230 Volts(b) 210 Volts(c) 190 Volts(d) 200 VoltsThe question was asked in final exam.My question is taken from EMF Generated in the Armature topic in section Electromechanical Energy Conversions of Electrical Machines
Answer» Correct ANSWER is (a) 230 Volts Easiest EXPLANATION: E = V+Ira = 220+101 = 230 Volts.

Discussion

16.	For a four pole, 16 slot armature with two coil-side/slot having wave connected progressive winding has single turn coils has simplex winding. The number of current paths is?(a) 2(b) 6(c) 3(d) 4I have been asked this question by my school principal while I was bunking the class.Question is from EMF Generated in the Armature topic in portion Electromechanical Energy Conversions of Electrical Machines
Answer» The CORRECT answer is (a) 2 The EXPLANATION is: The NUMBER of PARALLEL PATH for a wave wound machine is equal to two.

Discussion

17.	A duplex lap wound armature is used in six pole dc machine with six brush set. The number of current paths in this machine is?(a) 12(b) 6(c) 3(d) 2The question was asked in an interview.Enquiry is from EMF Generated in the Armature in portion Electromechanical Energy Conversions of Electrical Machines
Answer» Correct ANSWER is (a) 12 The best I can explain: The number of PARALLEL paths for a LAP wound machine is number of parallel paths. As it is a DUPLEX winding, it will have 6*2 = 12 parallel paths.

Discussion

18.	For a four pole, 16 slot armature with two coil-side/slot having lap connected progressive winding has single turn coils. The same machine is then wave wound, the emf induced in the lap wound dc machine to that of wave wound will be ________(a) 2(b) 0.5(c) 1(d) 4I had been asked this question in unit test.The query is from EMF Generated in the Armature topic in chapter Electromechanical Energy Conversions of Electrical Machines
Answer» Correct choice is (b) 0.5 To EXPLAIN I WOULD SAY: E = 1/(P/2) = 1/(4/2) = 1/2 V.

Discussion

19.	If the armature current is by mistake increased to 50% for the dc machine then ________(a) the induced emf remains unchanged(b) the induced emf doubles(c) the induced emf halves(d) none of the mentionedThe question was posed to me in an online interview.Query is from EMF Generated in the Armature in section Electromechanical Energy Conversions of Electrical Machines
Answer» Correct OPTION is (a) the INDUCED emf REMAINS unchanged Easy EXPLANATION: Emf is proportional to speed of rotation and flux.

Discussion

20.	If the speed of rotation for a four pole, 220V dc machine is 1 m/s, flux density as 2T having length of bore as 0.1m and radius of 0.5m. The emf induced under pole faces will be ________(a) 0.2 V(b) 0.1 V(c) 0.2*pi V(d) 220 VI got this question by my school principal while I was bunking the class.My doubt is from EMF Generated in the Armature in portion Electromechanical Energy Conversions of Electrical Machines
Answer» Right ANSWER is (a) 0.2 V To elaborate: E = BLv = 20.11 = 0.2 VOLTS.

Discussion

21.	The induced emf in the armature of a 4-pole dc machine is ________(a) directly proportional to speed and field strength applied to it(b) directly proportional to speed(c) directly proportional to field strength applied to it(d) inversely proportional to speed and inversely proportional to field strength applied to itThe question was posed to me by my school teacher while I was bunking the class.Asked question is from EMF Generated in the Armature topic in section Electromechanical Energy Conversions of Electrical Machines
Answer» RIGHT OPTION is (a) directly proportional to speed and field STRENGTH applied to it The explanation is: Emf is proportional to speed of rotation and flux.

Discussion

22.	If the speed of rotation of a separately excited dc machine is halved and the filed strength is also reduced to half of the original. Then the emf induced in the armature _____________(a) reduces to 1/4th of original(b) reduces to half of original(c) remains same(d) doublesThe question was posed to me by my school principal while I was bunking the class.This question is from EMF Generated in the Armature in section Electromechanical Energy Conversions of Electrical Machines
Answer» The correct CHOICE is (a) reduces to 1/4th of original Explanation: EMF is PROPORTIONAL to SPEED of ROTATION and flux.

Discussion

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The velocity of the dc machine at the final state is ________(a) E/Bl(b) Bl/E(c) EBl(d) None of the mentionedI had been asked this question in an interview for job.This intriguing question comes from Torque in DC Machines in section Electromechanical Energy Conversions of Electrical Machines

Pulsating torque frequency in universal motor is ________(a) 2f(b) f(c) f/2(d) 4fI got this question in class test.Origin of the question is Torque in DC Machines in chapter Electromechanical Energy Conversions of Electrical Machines