122 + Interview Questions in Synchronous Machines in Electrical Machines Page 3 InterviewSolution

101.	In the design of electromagnetic devices, the maximum value of MMF is limited from the considerations of ____________(a) temperature rise(b) limitation on flux density in teeth(c) torque production rise(d) power ratingI have been asked this question in an interview.I need to ask this question from Production of Torque in Non-Salient Pole Machines in section Synchronous Machines of Electrical Machines
Answer» Right answer is (a) TEMPERATURE RISE The EXPLANATION is: The maximum value of MMF is limited from a consideration of temperature rise.

Discussion

102.	Load angle δ is the angle between ____________(a) stator MMF Fs and resultant MMF FR(b) stator MMF Fs and rotor MMF Fr(c) rotor MMF Fr and resultant MMF FR(d) stator MMF Fs and resultant MMF FR (or) rotor MMF Fr and resultant MMF FRI got this question by my school teacher while I was bunking the class.I'd like to ask this question from Production of Torque in Non-Salient Pole Machines in section Synchronous Machines of Electrical Machines
Answer» RIGHT answer is (d) stator MMF Fs and resultant MMF FR (or) rotor MMF Fr and resultant MMF FR For explanation I WOULD say: The ELECTRICAL space angle δs between Fs and FR ; and δr between Fr and FR are CALLED LOAD angles.

Discussion

103.	Torque angle λ is the angle between ____________(a) stator MMF Fs and resultant MMF FR(b) stator MMF Fs and rotor MMF Fr(c) rotor MMF Fr and resultant MMF FR(d) any of the mentionedI got this question in homework.This interesting question is from Production of Torque in Non-Salient Pole Machines topic in portion Synchronous Machines of Electrical Machines
Answer» Right answer is (b) stator MMF Fs and rotor MMF Fr Explanation: The ELECTRICAL space angle λ between stator and rotor MMFs is called the TORQUE angle.

Discussion

104.	In practice, electromagnetic torque acting on the stator _________(a) rotate in opposite direction to rotor(b) is transmitted to ground(c) makes it rotate in same direction as of rotor(d) none of the mentionedI got this question in an online interview.This question is from Production of Torque in Non-Salient Pole Machines in section Synchronous Machines of Electrical Machines
Answer» RIGHT option is (b) is transmitted to GROUND The BEST explanation: As STATOR is FIXED, Teis transmitted to ground through its foundation.

Discussion

105.	Which of the following statements are correct regarding the torque equation in non salient pole machines, Te = -(P/2)(μoΠrl/g)(FsFr)sinλNm?(a) stator experiences less torque than rotor and in opposite directions(b) rotor experiences less torque than stator and in same directions(c) stator and rotor experiences equal and opposite torques(d) stator and rotor experiences equal torque and in same directionsThis question was addressed to me by my college director while I was bunking the class.My question comes from Production of Torque in Non-Salient Pole Machines topic in chapter Synchronous Machines of Electrical Machines
Answer» CORRECT option is (c) stator and ROTOR EXPERIENCES equal and OPPOSITE torques The EXPLANATION: Both stator and rotor experience equal and opposite torques.

Discussion

106.	The torque equation for non salient pole machine is Te = -(P/2)(μoΠrl/g)(FsFr)sinλNm. What does the negative sign in the torque expression indicates?(a) stator and rotor MMF tend to reduce angle λ(b) stator and rotor MMF tend to increase angle λ(c) stator tend to reduce the air gap g(d) rotor tend to reduce the air gap gThe question was posed to me during an interview.The query is from Production of Torque in Non-Salient Pole Machines in chapter Synchronous Machines of Electrical Machines
Answer» The correct OPTION is (a) STATOR and rotor MMF tend to REDUCE angle λ The best I can EXPLAIN: The torque acts in such a direction as to align the two MMFs Fs and FR and thereby reduce the angle λ between them.

Discussion

107.	The electromagnetic torque in non salient pole machines depends on _________(a) number of poles(b) machine dimensions at the air gap(c) peak value of stator and rotor MMF(d) any of the mentionedThe question was asked during an online interview.The origin of the question is Production of Torque in Non-Salient Pole Machines topic in section Synchronous Machines of Electrical Machines
Answer» Right CHOICE is (d) any of the mentioned The BEST I can explain: Te also DEPENDS on the sine of the angle between the axis of the two fields(or MMFs) Te = -(P/2)(μoΠrl/g)(FSFR)sinλNm.

Discussion

108.	Machine A has 60° phase spread and machine B has 120° phase spread. Both the machines have uniformly distributed winding. The ratio of distribution factors of machine A to machine B is ____________(a) 0.866(b) 1.1(c) 1.55(d) 1.155I got this question in a national level competition.Question is from EMF Polygon in division Synchronous Machines of Electrical Machines
Answer» RIGHT OPTION is (C) 1.55 To ELABORATE: KDA=(sinσ/2)/σ/2, σ=60° ⇒ kdA=0.9556 kdB=(sinσ/2)/σ/2, σ=120° ⇒ kdB=0.827 thus, kdA/kdB=1.155.

Discussion

109.	Which among the given harmonics are called belt harmonics?(a) 5, 7, 11, 13(b) 3, 6, 9, 12(c) 5, 6, 11, 12(d) 7, 11, 13, 15The question was posed to me in exam.The origin of the question is EMF Polygon topic in portion Synchronous Machines of Electrical Machines
Answer» The correct option is (a) 5, 7, 11, 13 Easiest explanation: The ODD harmonics of the order 5, 7, 11, 13 etc are called belt harmonics.

Discussion

110.	For eliminating n^th harmonic from the EMF generated in the phase of a 3-phase alternator, the chording angle should be ______________(a) n∗full pitch(b) (1/n)∗full pitch(c) (2/n)∗full pitch(d) (3/n)∗full pitchI have been asked this question during an interview.I'm obligated to ask this question of EMF Polygon topic in chapter Synchronous Machines of Electrical Machines
Answer» RIGHT OPTION is (a) n∗full pitch Best EXPLANATION: kpn=cosnε/2=0 ⇒ nε/2=90° ⇒ ε=180°/n=full pitch/n.

Discussion

111.	A 6-pole alternator with 36 slots carries a 2-phase distributed winding. Each coil is short pitched by one slot. The winding factor is given by _____________(a) cot15°/3√2(b) cot15°/4(c) cot15°/2√2(d) cot15°/4This question was addressed to me during a job interview.I want to ask this question from EMF Polygon topic in portion Synchronous Machines of Electrical Machines
Answer» The correct choice is (a) cot15°/3√2 Best EXPLANATION: Slots PER pole = 36/6=6, Q=6/2=3 ⇒ γ=180°/6=30°, coil is short PITCHED by one slot ⇒ ε=γ=30° and Kp=cosε/2=cos15° and Kd=sin(γq/2)/q(sinγ/2)=sin45/(3sin15) Kw=Kp∗Kd=cot15°/3√2.

Discussion

112.	A 3-phase, 4-pole alternator has 48 stator slots carrying a 3-phase distributed winding. Each coil of the winding is short chorded by one slot pitch. The winding factor is given by ______________(a) (cos7.5)/16(b) (cot7.5)/8(c) 1/(8sin7.5)(d) (cot7.5)/16The question was asked by my school teacher while I was bunking the class.My question is taken from EMF Polygon topic in section Synchronous Machines of Electrical Machines
Answer»

Discussion

113.	The chording angle for eliminating 5^th harmonic should be ____________(a) 30°(b) 34°(c) 36°(d) 35°I have been asked this question in an interview for internship.Enquiry is from EMF Polygon topic in section Synchronous Machines of Electrical Machines
Answer» Right ANSWER is (c) 36° To explain I would say: To ELIMINATE 5^th HARMONIC kp5 must be zero, kpn = cosnε/2 ⇒ kp5 = cos5ε/2=0=cos90° ⇒ 5ε/2=90° ⇒ ε = 36°.

Discussion

114.	In 48 slot, 4-pole,3 phase alternator, the coil-span is 10. Its distribution and pitch factors are respectively ____________(a) 0.9717,0.966(b) 0.9822, 0.9814(c) 0.9577, 0.9814(d) 0.9577, 0.966The question was posed to me in an online quiz.My question is from EMF Polygon topic in chapter Synchronous Machines of Electrical Machines
Answer» Correct option is (d) 0.9577, 0.966 For explanation I would say: Slots per POLE = 48/4=12, for a COIL span of 10 slots, the coil is short pitched by 2 slots and the chording angle is ε=2γ and γ=180/12 ⇒ ε = 30° ⇒ Kp=cosε/2=0.9659. We know, KD=(sin(qγ/2))/qsin(γ/2), here q=48/4∗3 = 4 ⇒ Kd=0.957662.

Discussion

115.	If a stator has 48 slots, 6 poles and 3 phase with narrow spread winding, then the third harmonic belt factor is ____________(a) 0.9556(b) 0.6407(c) 0.1944(d) None of the mentionedThe question was posed to me in an online quiz.My enquiry is from EMF Polygon topic in portion Synchronous Machines of Electrical Machines
Answer» Right option is (b) 0.6407 Explanation: Q= (s/m)/p=K.Sk/k.Pk=Sk/Pk for fractional SLOT winding where k is the HIGHEST common FACTOR between s/m and P, here q= 48/3*6 =16/6= 8/3 = Sk/Pk The distribution factor for fractional slot winding’s is obtained by replacing q by Sk and kdn = (sinnγ/2)/(Sksinnγ/2Sk), if n=3, kd3=0.6407.

Discussion

116.	The pitch factor, in rotating electrical machinery, is defined as the ratio of resultant EMF of a _________(a) full-pitched coil to that of a chorded coil(b) full-pitched coil to the phase EMF(c) chorded coil to the phase EMF(d) chorded coil to that of a full-pitched coilThe question was posed to me in an interview for internship.My question comes from EMF Polygon in chapter Synchronous Machines of Electrical Machines
Answer» RIGHT OPTION is (d) chorded COIL to that of a full-pitched coil To explain: Kp = RESULTANT EMF of a chorded coil/Resultant EMF of a full pitched coil = cosε/2.

Discussion

117.	A 3-phase machine has integral slot winding with fundamental distribution factor kd1, the distribution factor for n^th harmonic kdn is ____________(a) less than kd1(b) more than kd1(c) equal to kd1(d) depends upon the number of slots and polesThe question was posed to me during an interview.This intriguing question originated from EMF Polygon in section Synchronous Machines of Electrical Machines
Answer» Right choice is (a) less than kd1 The explanation is: KDN=(sinqnγ/2)/(qsinnγ/2) and examples will show that kdn is less than kd1. This has the effect of reducing the n^th harmonic EMF in COMPARISION with FUNDAMENTAL EMF. This is CERTAINLY an advantage of distributing the winding in slots.

Discussion

118.	The armature winding of a 2-pole, 3-phase alternator for each phase is distributed in a number of slots per phase. The RMS value of the voltage per phase is less than the RMS value of the voltage per coil multiplied by the number of coils in series because the _________(a) RMS value of the voltage in different coils of the phase is different(b) equal RMS voltages in different coils of the phase has mutual phase difference(c) maximum value of the induced voltage in different coils of the phase are different(d) different coils of the phase pass through different saturated regions of the magnetic circuitThe question was asked during a job interview.My doubt stems from EMF Polygon in division Synchronous Machines of Electrical Machines
Answer» The CORRECT choice is (b) EQUAL RMS VOLTAGES in different coils of the phase has mutual phase difference Easy explanation: The phase displacement will be GIVEN by SLOT angular pitch γ.

Discussion

119.	In a machine with coil span 9, if a coil has its one coil-side in slot 1, then its other coil-side must be in ____________(a) slot 9(b) slot 8(c) slot 10(d) slot 2I have been asked this question in final exam.Query is from EMF Polygon topic in section Synchronous Machines of Electrical Machines
Answer» The correct OPTION is (c) SLOT 10 Easiest explanation: The other coil-side MUST be in slot 10=1+coil span=1+9.

Discussion

120.	The belt factor is defined as the ratio of __________(a) arithmetic sum of coil EMF to the phasor sum of coil EMFs(b) phasor sum of EMF per coil to the arithmetic sum of EMF per coil(c) phasor sum of coil EMFs to the arithmetic sum of coil EMFs(d) phasor sum of coil EMFs to per phase voltageI have been asked this question during an online interview.Question is from EMF Polygon topic in chapter Synchronous Machines of Electrical Machines
Answer» Correct choice is (C) phasor sum of coil EMFs to the ARITHMETIC sum of coil EMFs Easy explanation: Belt FACTOR or distribution factor or breadth factor = KD= phasor sum of coil EMFs/arithmetic sum of coil EMFs.

Discussion

121.	What is the coil span for a 2 pole 18 slot machine?(a) 9(b) 4.5(c) 18(d) 6This question was posed to me in an international level competition.The doubt is from EMF Polygon in portion Synchronous Machines of Electrical Machines
Answer» RIGHT option is (a) 9 To explain: COIL span = number of slots/number of POLES = 18/2 = 9.

Discussion

122.	The angular slot pitch, γ is given by __________(a) γ = πP/Total number of slotsElect radians(b) γ = 180P/Total number of slotsElect degrees(c) γ = 180/Slots per poleElect degrees(d) Any of the mentionedThe question was asked in examination.Query is from EMF Polygon topic in portion Synchronous Machines of Electrical Machines
Answer» The correct option is (d) Any of the mentioned The explanation: Angle γ is the angle between ADJACENT SLOTS.

Discussion

Explore topic-wise InterviewSolutions in .

Which among the given harmonics are called belt harmonics?(a) 5, 7, 11, 13(b) 3, 6, 9, 12(c) 5, 6, 11, 12(d) 7, 11, 13, 15The question was posed to me in exam.The origin of the question is EMF Polygon topic in portion Synchronous Machines of Electrical Machines

The chording angle for eliminating 5^th harmonic should be ____________(a) 30°(b) 34°(c) 36°(d) 35°I have been asked this question in an interview for internship.Enquiry is from EMF Polygon topic in section Synchronous Machines of Electrical Machines

In a machine with coil span 9, if a coil has its one coil-side in slot 1, then its other coil-side must be in ____________(a) slot 9(b) slot 8(c) slot 10(d) slot 2I have been asked this question in final exam.Query is from EMF Polygon topic in section Synchronous Machines of Electrical Machines

What is the coil span for a 2 pole 18 slot machine?(a) 9(b) 4.5(c) 18(d) 6This question was posed to me in an international level competition.The doubt is from EMF Polygon in portion Synchronous Machines of Electrical Machines