73 + Interview Questions in Basic Concepts In Rotating Machines in Electrical Machines Page 2 InterviewSolution

51.	In a two phase cylindrical rotor synchronous machine, the mutual inductance Mαβ between α phase and β phase winding is __________(a) Mmax(b) 0(c) Mmin(d) any of the mentionedI have been asked this question in homework.I need to ask this question from Elementary Synchronous Machines in chapter Basic Concepts in Rotating Machines of Electrical Machines
Answer» The correct option is (B) 0 Explanation: Since the axes of phase α and phase β WINDINGS are NORMAL to each other and AIR gap is uniform, the mutual inductance Mαβ between α,β winding is zero.

Discussion

52.	In a two phase cylindrical rotor synchronous machine, the self inductances Lα, Lβ for phases α,β respectively are __________(a) constant and equal(b) constant and unequal(c) varying and equal(d) varying and unequalThis question was posed to me in examination.Query is from Elementary Synchronous Machines in section Basic Concepts in Rotating Machines of Electrical Machines
Answer» Right answer is (a) constant and equal For explanation I would SAY: Constant and equal, as the reluctance OFFERED to the fluxes PRODUCED by PHASE α and β winding doesn’t vary with rotor MOVEMENT.

Discussion

53.	The following torque expression: Te=-isirMmaxsinθr is applicable in a single phase cylindrical rotor synchronous machine when the rotor is __________(a) revolving(b) stationary(c) revolving or stationary(d) none of the mentionedThe question was asked during an online interview.Query is from Elementary Synchronous Machines in portion Basic Concepts in Rotating Machines of Electrical Machines
Answer» Correct OPTION is (c) revolving or stationary To explain I would say: The TORQUE expression is applicable WHETHER the ROTOR is revolving or stationary.

Discussion

54.	In a single phase cylindrical rotor synchronous machine, the torque Te tends to ____________ the space angle θr.(a) reduce(b) increase(c) maintain constant(d) all of the mentionedThis question was posed to me at a job interview.This is a very interesting question from Elementary Synchronous Machines in chapter Basic Concepts in Rotating Machines of Electrical Machines
Answer» The CORRECT answer is (a) reduce To EXPLAIN: Wfld(is,IR,θr)=1/2 is^2Ls+1/2 ir^2Lr+ isirMmaxcosθr Torque, Te=∂Wfld(is,ir,θr)/∂θr= -isirMmaxsinθr Negative sign indicates that the torque Te TENDS to reduce angle θr.

Discussion

55.	Which of the following equation best represents the mutual inductance between stator and rotor in a single phase cylindrical rotor synchronous machine? (where θr= space angle between stator and rotor field axis)(a) Msr=0(b) Msr=Mmax(c) Msr=Mmaxcosθr(d) Msr=MmaxsinθrThe question was posed to me by my college director while I was bunking the class.Query is from Elementary Synchronous Machines topic in portion Basic Concepts in Rotating Machines of Electrical Machines
Answer» The correct choice is (c) MSR=Mmaxcosθr The best explanation: In a single PHASE cylindrical ROTOR synchronous machine, the Msr is MAXIMUM, when θr=0, and is zero when θr=90° and COSINE function represents it.

Discussion

56.	Consider a single phase synchronous machine of cylindrical rotor type. A single phase supply is given to the stator winding, and a voltmeter is connected across rotor winding. For what value of θr the voltmeter reads a zero?(a) 0°(b) 30°(c) 45°(d) 90°This question was addressed to me during an internship interview.This interesting question is from Elementary Synchronous Machines in division Basic Concepts in Rotating Machines of Electrical Machines
Answer» The CORRECT option is (d) 90° The explanation is: When θr=90°, the voltmeter reading is zero, because the stator winding AXIS is perpendicular to the ROTOR winding axis, and the mutual inductance Msr is zero.

Discussion

57.	Consider a single phase synchronous machine of cylindrical rotor type. A single phase supply is given to the stator winding and a voltmeter is connected across rotor winding. For what value of θr, the voltmeter reads maximum?(a) 90°(b) 0°(c) 45°(d) 30°I have been asked this question in homework.Enquiry is from Elementary Synchronous Machines topic in chapter Basic Concepts in Rotating Machines of Electrical Machines
Answer» Right answer is (b) 0° To ELABORATE: When θr=0, the flux linkages with the rotor are maximum, and this is INDICATED by a maximum VOLTMETER READING.

Discussion

58.	Electromagnetic torque in rotating electrical machinery is present when ________(a) stator winding alone carries current(b) rotor winding alone carries current(c) air gap is uniform(d) both stator and rotor windings carry currentI have been asked this question during an online exam.Question is from Doubly Excited Magnetic Systems in portion Basic Concepts in Rotating Machines of Electrical Machines
Answer» The correct choice is (d) both STATOR and rotor windings carry current The explanation: Electromagnetic TORQUE = isirdMsr/dθr and if either is or ir = 0, then Te=0.

Discussion

59.	Singly and doubly excited magnetic systems applications are respectively ________(a) loud speakers and tachometers(b) synchronous motors and moving iron instruments(c) DC shunt machines and solenoids(d) reluctance motors and synchronous motorsThis question was addressed to me during an internship interview.This is a very interesting question from Doubly Excited Magnetic Systems in portion Basic Concepts in Rotating Machines of Electrical Machines
Answer» The CORRECT OPTION is (d) RELUCTANCE motors and SYNCHRONOUS motors Explanation: Reluctance motors can work on single excitation, and synchronous motors REQUIRE double excitation.

Discussion

60.	In a doubly excited magnetic system with salient pole type stator and rotor, if the rotor is not allowed to move, then the equation for magnetic field stored energy in establishing the currents from zero to is and ir is __________(a) Wfld=1/2 is^2Ls+1/2 ir^2Lr(b) Wfld=1/2 is^2Ls+Mrs is ir(c) Wfld=1/2 is^2Ls+1/2 ir^2Lr+Mrs is ir(d) Wfld=1/2 ir^2Lr+Mrs is irI had been asked this question during a job interview.I want to ask this question from Doubly Excited Magnetic Systems in chapter Basic Concepts in Rotating Machines of Electrical Machines
Answer» The CORRECT option is (c) Wfld=1/2 is^2Ls+1/2 IR^2Lr+Mrs is ir The best EXPLANATION: As the rotor is not allowed to move, dWmech=0, thus dWelec=0+dWfld=isdΨs+irdΨr, if in this equation, we introduce the self and mutual inductance terms, (Ψs=Lsis) and integrate the resulting equation from 0 to is, 0 to ir and 0 to IRIS, the respective terms, finally we getWfld=1/2 is^2Ls+1/2 ir^2Lr+Mrs is ir.

Discussion

61.	Most of the electromagnetic energy conversion devices belong to __________(a) singly excited magnetic systems(b) doubly excited magnetic systems(c) multiply excited magnetic systems(d) both doubly excited magnetic systems and multiply excited magnetic systemsI had been asked this question in my homework.My question is taken from Doubly Excited Magnetic Systems in chapter Basic Concepts in Rotating Machines of Electrical Machines
Answer» Right choice is (d) both doubly EXCITED magnetic systems and multiply excited magnetic systems Explanation: Widely used motor/generators such as SYNCHRONOUS and induction machines BELONG to doubly excited systems, and MANY other PRACTICAL applications require multiply excited magnetic systems.

Discussion

62.	Which of the following are examples of doubly-excited magnetic systems?(a) Synchronous Machines(b) Loudspeakers and Tachometers(c) D.C Shunt Machines(d) All of the mentionedThis question was posed to me at a job interview.Query is from Doubly Excited Magnetic Systems in chapter Basic Concepts in Rotating Machines of Electrical Machines
Answer» The correct choice is (d) All of the mentioned Easiest EXPLANATION: All of the above APPLICATIONS REQUIRE two INDEPENDENT sources of excitation.

Discussion

63.	Which of the following are applications of singly excited magnetic systems?(a) electromagnets, relays(b) moving-iron instruments(c) reluctance motors(d) all of the mentionedI had been asked this question in an interview for job.Question is taken from Reluctance Motor in section Basic Concepts in Rotating Machines of Electrical Machines
Answer» The CORRECT choice is (d) all of the mentioned To EXPLAIN: All of the APPLICATIONS mentioned above needs singly excited magnetic systems.

Discussion

64.	Single phase reluctance motors are extensively used in ________(a) grinder applications(b) driving electric clocks and other timing devices(c) welding applications(d) lifts/ elevatorsI got this question in a job interview.This question is from Reluctance Motor in section Basic Concepts in Rotating Machines of Electrical Machines
Answer» Right option is (B) DRIVING electric clocks and other timing devices The best I can explain: The single phase reluctance motors operate at CONSTANT synchronous speed, in case the supply frequency remains constant, and hence timing devices MOSTLY USE these motors.

Discussion

65.	The single phase reluctance machine acts as a generator when angle δ is _______(a) positive(b) negative(c) zero(d) any of the mentionedI have been asked this question during an interview.My question is taken from Reluctance Motor in section Basic Concepts in Rotating Machines of Electrical Machines
Answer» The CORRECT CHOICE is (b) NEGATIVE To explain I would say: If δ is positive, then the MACHINE acts as a motor and if δ is negative, the machine acts as a generator.

Discussion

66.	For a given reluctance motor, Rld and Rlq are ________(a) constant(b) varying(c) zero(d) all of the mentionedI had been asked this question during an online interview.My question is based upon Reluctance Motor topic in division Basic Concepts in Rotating Machines of Electrical Machines
Answer» The correct answer is (a) constant To explain I WOULD say: Rld and RLQ are constant for a GIVEN motor, as they depend on the GEOMETRY of the MAGNETIC circuit.

Discussion

67.	For a reluctance motor, the maximum average torque occurs when δ = __________(a) 45°(b) 90°(c) 0°(d) 180°I have been asked this question in final exam.The origin of the question is Reluctance Motor in portion Basic Concepts in Rotating Machines of Electrical Machines
Answer» Correct OPTION is (a) 45° Best EXPLANATION: The equation for average TORQUE is Te(av)=1/8 ∅max^2(Rlq – RLD)sin⁡(2δ), and it is maximum when δ=45°.

Discussion

68.	Reluctance motor can produce torque at ________(a) any speed less than synchronous speed(b) synchronous speed only(c) any speed greater than synchronous speed(d) all of the mentionedI have been asked this question in a job interview.My question is based upon Reluctance Motor topic in section Basic Concepts in Rotating Machines of Electrical Machines
Answer» Correct choice is (b) synchronous speed only Easiest explanation: Te(av) = -1/4 ∅max^2(Rlq – RLD)(1/2 sin⁡(-2δ)), w = synchronous speed if wr≠w then Te(av)=0, andif wr=w then Te(av)≠0, as the LAST TERM in the EQUATION 1/2 sin⁡(-2δ) does not vary with time, andhence average torque is not EQUAL to zero.

Discussion

69.	The variation of reluctance Rl with space angle θr depends on the shape of __________(a) stator poles(b) rotor poles(c) stator or rotor poles(d) both stator and rotor polesI have been asked this question in an interview for internship.I'd like to ask this question from Reluctance Motor topic in chapter Basic Concepts in Rotating Machines of Electrical Machines
Answer» Correct choice is (d) both STATOR and rotor poles Explanation: ALSO, the variation is ASSUMED to be a FUNCTION of SPACE angle θr.

Discussion

70.	The reluctance offered to the stator flux by two small air gaps in series with high permeability iron, in reluctance machine is minimum, when the space angle θr = ___________(a) 0°(b) 45°(c) 90°(d) 270°This question was posed to me during an interview for a job.The doubt is from Reluctance Motor in division Basic Concepts in Rotating Machines of Electrical Machines
Answer» Right OPTION is (a) 0° Best explanation: Only when θr = 0°, the long ROTOR axis is coincident with the STATOR d-axis, and THUS reluctance is minimum.

Discussion

71.	The reluctance offered to the stator flux by two very large air gaps in series with high permeability iron, in reluctance machine is maximum, when the space angle θr = ___________(a) 0°(b) 45°(c) 90°(d) 180°I had been asked this question by my college professor while I was bunking the class.My question comes from Reluctance Motor in portion Basic Concepts in Rotating Machines of Electrical Machines
Answer» The correct choice is (C) 90° Easiest explanation: Only when θr = 90°, the long rotor axis is PERPENDICULAR to the stator d-axis, and THUS reluctance is MAXIMUM.

Discussion

72.	Space angle, θr is measured between stator d-axis and _______________(a) quadrature axis(b) direct d-axis(c) long rotor axis(d) none of the mentionedI got this question in an international level competition.The origin of the question is Reluctance Motor in portion Basic Concepts in Rotating Machines of Electrical Machines
Answer» Right OPTION is (C) LONG ROTOR axis Best EXPLANATION: Space angle is measured between stator direct axis and long rotor axis.

Discussion

73.	What is the angle between stator direct axis and quadrature axis?(a) 90°(b) 0°(c) 45°(d) 60°This question was addressed to me during an interview.The query is from Reluctance Motor topic in section Basic Concepts in Rotating Machines of Electrical Machines
Answer» The correct ANSWER is (a) 90° For explanation: In RELUCTANCE MOTOR, DIRECT axis is HORIZONTAL axis and quadrature axis is at 90° to the direct axis.

Discussion

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For a given reluctance motor, Rld and Rlq are ________(a) constant(b) varying(c) zero(d) all of the mentionedI had been asked this question during an online interview.My question is based upon Reluctance Motor topic in division Basic Concepts in Rotating Machines of Electrical Machines

For a reluctance motor, the maximum average torque occurs when δ = __________(a) 45°(b) 90°(c) 0°(d) 180°I have been asked this question in final exam.The origin of the question is Reluctance Motor in portion Basic Concepts in Rotating Machines of Electrical Machines

What is the angle between stator direct axis and quadrature axis?(a) 90°(b) 0°(c) 45°(d) 60°This question was addressed to me during an interview.The query is from Reluctance Motor topic in section Basic Concepts in Rotating Machines of Electrical Machines