80 + Interview Questions in Polyphase Circuits in Network Theory Page 2 InterviewSolution

51.	A symmetrical three-phase, three-wire 440V supply is connected to star-connected load. The impedances in each branch are ZR = (2+j3) Ω, ZY = (1-j2) Ω, ZB = (3+j4) Ω. Find ZBR.(a) (5.22-j0.82) Ω(b) (-3.02+j8) Ω(c) (-5.22+j0.82) Ω(d) (3.8-j0.38) ΩI had been asked this question in a job interview.My question is based upon Star to Delta and Delta to Star Transformation topic in chapter Polyphase Circuits of Network Theory
Answer» RIGHT OPTION is (B) (-3.02+j8) Ω The explanation: ZBR = (ZRZY + ZYZB + ZBZR)/ZY = (-3.02+j8) Ω.

Discussion

52.	A symmetrical three-phase, three-wire 440V supply is connected to star-connected load. The impedances in each branch are ZR = (2+j3) Ω, ZY = (1-j2) Ω, ZB = (3+j4) Ω. Find ZBY.(a) (-5.22+j0.82) Ω(b) (5.22-j0.82) Ω(c) (3.8-j0.38) Ω(d) (-3.02+j8) ΩThis question was posed to me in an online interview.My query is from Star to Delta and Delta to Star Transformation in division Polyphase Circuits of Network Theory
Answer» The CORRECT choice is (b) (5.22-j0.82) Ω Easiest EXPLANATION: ZBY = (ZRZY + ZYZB + ZBZR)/ZR = (5.22-j0.82)Ω.

Discussion

53.	In star connected system, VRY is equal to?(a) VYR(b) -VYR(c) 2VYR(d) 3VYRThis question was posed to me in class test.My question is from Voltage, Current and Power in a Star Connected System topic in section Polyphase Circuits of Network Theory
Answer» Right option is (b) -VYR Easy explanation: The voltage AVAILABLE between any pair of terminals is called the LINE voltage. The double script notation is purposefully USED to represent voltages and currents in poly phase circuits. In star CONNECTED SYSTEM, VRY = – VYR.

Discussion

54.	If the resistors of star connected system are ZR, ZY, ZB then the impedance ZBR in delta connected system will be?(a) (ZRZY + ZYZB + ZBZR)/ZY(b) (ZRZY + ZYZB + ZBZR)/R(c) (ZRZY + ZYZB + ZBZR)/ZB(d) (ZRZY + ZYZB + ZBZR)/(ZB+ZR)I had been asked this question in semester exam.The above asked question is from Star to Delta and Delta to Star Transformation topic in chapter Polyphase Circuits of Network Theory
Answer» Correct answer is (a) (ZRZY + ZYZB + ZBZR)/ZY To explain I WOULD say: After TRANSFORMATION to DELTA, the impedance ZBR in delta connected system will be (ZRZY + ZYZB + ZBZR)/ZY.

Discussion

55.	A symmetrical three-phase, three-wire 440V supply is connected to star-connected load. The impedances in each branch are ZR = (2+j3) Ω, ZY = (1-j2) Ω, ZB = (3+j4) Ω. Find ZRY.(a) (5.22-j0.82) Ω(b) (-3.02+j8) Ω(c) (3.8-j0.38) Ω(d) (-5.22+j0.82) ΩThis question was addressed to me during an online interview.This interesting question is from Star to Delta and Delta to Star Transformation topic in section Polyphase Circuits of Network Theory
Answer» Correct OPTION is (C) (3.8-j0.38) Ω Explanation: ZRY = (ZRZY + ZYZB + ZBZR)/ZB = (3.8-j0.38) Ω.

Discussion

56.	If the resistors of star connected system are ZR, ZY, ZB then the impedanceZRY in delta connected system will be?(a) (ZRZY + ZYZB + ZBZR)/ZB(b) (ZRZY + ZYZB + ZBZR)/ZY(c) (ZRZY + ZYZB + ZBZR)/ZR(d) (ZRZY + ZYZB + ZBZR)/(ZR+ZY)This question was addressed to me during a job interview.The query is from Star to Delta and Delta to Star Transformation in division Polyphase Circuits of Network Theory
Answer» RIGHT OPTION is (a) (ZRZY + ZYZB + ZBZR)/ZB The explanation is: After transformation to DELTA, the impedance ZRY in delta connected system will be (ZRZY + ZYZB + ZBZR)/ZB.

Discussion

57.	If the resistors of star connected system are ZR, ZY, ZB then the impedance ZBY in delta connected system will be?(a) (ZRZY + ZYZB + ZBZR)/(ZB+ZY)(b) (ZRZY + ZYZB + ZBZR)/ZB(c) (ZRZY + ZYZB + ZBZR)/ZY(d) (ZRZY + ZYZB + ZBZR)/ZRI have been asked this question in an international level competition.My question comes from Star to Delta and Delta to Star Transformation in division Polyphase Circuits of Network Theory
Answer» Right choice is (d) (ZRZY + ZYZB + ZBZR)/ZR The BEST EXPLANATION: After transformation to delta, the impedance ZBY in delta connected SYSTEM will be(ZRZY + ZYZB + ZBZR)/ZR.

Discussion

58.	If a resistor ZR is connected between R and N, ZBR between R and B, ZRY between R and Y and ZBY between B and Y form a delta connection, then after transformation to star, the impedance at B is?(a) (ZBRZBY)/(ZRY+ZBY+ZBR)(b) (ZRYZBY)/(ZRY+ZBY+ZBR)(c) (ZBY)/(ZRY+ZBY+ZBR)(d) (ZBR)/(ZRY+ZBY+ZBR)The question was posed to me during an online interview.Asked question is from Star to Delta and Delta to Star Transformation in section Polyphase Circuits of Network Theory
Answer» Right answer is (a) (ZBRZBY)/(ZRY+ZBY+ZBR) Easy explanation: After TRANSFORMATION to STAR, the impedance at Y is (ZBRZBY)/(ZRY+ZBY+ZBR).

Discussion

59.	If a resistor ZR is connected between R and N, ZBR between R and B, ZRY between R and Y and ZBY between B and Y form a delta connection, then after transformation to star, the impedance at Y is?(a) (ZRY)/(ZRY+ZBY+ZBR)(b) (ZBY)/(ZRY+ZBY+ZBR)(c) (ZRYZBY)/(ZRY+ZBY+ZBR)(d) (ZRYZBR)/(ZRY+ZBY+ZBR)I had been asked this question in an internship interview.Question is taken from Star to Delta and Delta to Star Transformation in portion Polyphase Circuits of Network Theory
Answer» Right choice is (c) (ZRYZBY)/(ZRY+ZBY+ZBR) To EXPLAIN I WOULD say: After transformation to STAR, the impedance at Y is (ZRYZBY)/(ZRY+ZBY+ZBR).

Discussion

60.	If a resistor ZR is connected between R and N, ZBR between R and B, ZRY between R and Y and ZBY between B and Y form a delta connection, then after transformation to star, the impedance at R is?(a) (ZBRZBY)/(ZRY+ZBY+ZBR)(b) (ZRYZBR)/(ZRY+ZBY+ZBR)(c) (ZRYZBY)/(ZRY+ZBY+ZBR)(d) (ZRY)/(ZRY+ZBY+ZBR)This question was posed to me during an interview.The origin of the question is Star to Delta and Delta to Star Transformation topic in chapter Polyphase Circuits of Network Theory
Answer» Right choice is (B) (ZRYZBR)/(ZRY+ZBY+ZBR) To explain: After transformation to STAR, the IMPEDANCE at R is (ZRYZBR)/(ZRY+ZBY+ZBR).

Discussion

61.	Which of the following voltage is a phase voltage in the delta connection?(a) VRN(b) VBR(c) VYN(d) VBNI have been asked this question in an internship interview.The above asked question is from Inter Connection of Three-Phase Sources and Loads topic in portion Polyphase Circuits of Network Theory
Answer» Right choice is (B) VBR To elaborate: A balanced three phase SOURCE is ONE in which the three INDIVIDUAL sources have equal magnitude with 120^ phase DIFFERENCES. VBR is a phase voltage in delta connection. And VRN, VYN and VBNare not phase voltages.

Discussion

62.	The relation between line voltage and phase voltage in Delta or Mesh connection is?(a) Vphase > Vline(b) Vphase < Vline(c) Vphase = Vline(d) Vphase >= VlineI got this question in homework.My enquiry is from Inter Connection of Three-Phase Sources and Loads in portion Polyphase Circuits of Network Theory
Answer» Correct option is (c) Vphase = Vline The explanation is: When the sources are connected in DELTA, loads can be connected only ACROSS the THREE line terminal. The relation between line voltage and phase voltage in Delta or Mesh CONNECTION is Vphase = Vline.

Discussion

63.	In the Delta or Mesh connection, there will be __________ number of common terminals.(a) 1(b) 2(c) 3(d) 0I had been asked this question by my college professor while I was bunking the class.The query is from Inter Connection of Three-Phase Sources and Loads in chapter Polyphase Circuits of Network Theory
Answer» The CORRECT option is (d) 0 The explanation is: The three line CONDUCTORS are taken from the three junctions of the mesh or delta connection to feed the three phase load. This CONSTITUTES a three phase, three wire, delta CONNECTED system. In the Delta or Mesh connection, there will be zero number of COMMON terminals. Number of common terminals = 0.

Discussion

64.	Figure below represents three phases of an alternator. The phase voltage for the star connection among the options given below is?(a) VRY(b) VRN(c) VYB(d) VBRThe question was posed to me during an online exam.My enquiry is from Inter Connection of Three-Phase Sources and Loads in division Polyphase Circuits of Network Theory
Answer» Correct option is (b) VRN The BEST I can explain: If the neutral WIRE is not available for external CONNECTION, the system is called a three phase,three wire STAR CONNECTED system. Phase voltage = VRN. And VRY, VYB and VBR are not phase voltages.

Discussion

65.	In the figure shown below, what will be the line voltage?(a) VBR(b) VBN(c) VRN(d) VYNThe question was posed to me in an online interview.My query is from Inter Connection of Three-Phase Sources and Loads in chapter Polyphase Circuits of Network Theory
Answer» Right answer is (a) VBR For explanation I WOULD SAY: The star connected system formed will supply equal line voltages displaced 120^ from ONE another and acting simultaneously in the cicruit like three INDEPENDENT single phase SOURCES in the same frame of a three phase alternator. Line voltage = VBR. And VRN , VYN and VBN are not line voltages.

Discussion

66.	The voltage between ______________ is called line voltage.(a) line and neutral point(b) line and reference(c) line and line(d) neutral point and referenceThis question was addressed to me by my college director while I was bunking the class.This interesting question is from Inter Connection of Three-Phase Sources and Loads topic in portion Polyphase Circuits of Network Theory
Answer» Correct option is (c) line and line Easiest EXPLANATION: The voltage between line and line is called line voltage. And the voltage between line and neutral point is called PHASE voltage. The currents flowing through the PHASES are called the phase currents, while those flowing in the LINES are called the line currents.

Discussion

67.	The voltage between __________ and ___________ is called phase voltage.(a) line and line(b) line and reference(c) neutral point and reference(d) line and neutral pointI have been asked this question in homework.My question comes from Inter Connection of Three-Phase Sources and Loads topic in division Polyphase Circuits of Network Theory
Answer» Right option is (d) LINE and NEUTRAL point The best I can explain: In a three PHASE four WIRE star connected system, the terminals R, Y and B are CALLED the line terminals of the source. The voltage between line and neutral point is called phase voltage. And the voltage between line and line is called line voltage.

Discussion

68.	Each coil in three phase alternator has ________________ number of terminals.(a) 2(b) 4(c) 6(d) 8I have been asked this question in an online quiz.This key question is from Inter Connection of Three-Phase Sources and Loads in portion Polyphase Circuits of Network Theory
Answer» The correct answer is (a) 2 Easy explanation: Each COIL in three phase alternator has 2 NUMBER of terminals, viz. start and FINISH. So, 2 number of terminals. the COILS are inter connected to form a wye or delta connected three phase system to achieve economy and REDUCE the number of conductors and thereby the complexity of the circuit.

Discussion

69.	In wye or star connection _____________ of the three phases are joined together within the alternator.(a) similar ends(b) opposite ends(c) one similar end, two opposite ends(d) one opposite end, two opposite endsThis question was posed to me in my homework.My question is taken from Inter Connection of Three-Phase Sources and Loads in chapter Polyphase Circuits of Network Theory
Answer» Correct CHOICE is (a) similar ends Explanation: In WYE or star connection, similar ends of the three phases are joined TOGETHER within the alternator. The common terminal so formed is referred to as the NEUTRAL point or neutral terminal.

Discussion

70.	If RR^‘, YY^‘ and BB^‘ constitutes three phase sequence if V^‘RR = Vmsinωt its corresponding field magnets are in clockwise direction, then the value of V^‘BB is?(a) Vmsin(ωt-240⁰)(b) Vmsin(ωt-120⁰)(c) Vmsin(ωt+240⁰)(d) VmsinωtI got this question during an online exam.I would like to ask this question from Polyphase System in division Polyphase Circuits of Network Theory
Answer» Correct choice is (a) Vmsin(ωt-240⁰) Best explanation: The VALUE of V^‘BB is V^‘BB = Vmsin(ωt-240⁰). There are only two POSSIBLE phase sequences; they are RBY and RYB.

Discussion

71.	In a three phase alternator, there are __________ independent phase windings or coils.(a) 1(b) 2(c) 3(d) 4The question was posed to me in homework.My question is taken from Inter Connection of Three-Phase Sources and Loads topic in portion Polyphase Circuits of Network Theory
Answer» Right option is (c) 3 For explanation I would say: In a three phase ALTERNATOR, there are 3 independent phase windings or coils. So, 3 independent phase windings or coils. The END connections of the three sets of the coils MAY be brought out of the MACHINE to form three SEPARATE single phase sources to feed three individual circuits.

Discussion

72.	If RR^‘, YY^‘ and BB^‘ constitutes three phase sequence if V^‘RR = Vmsinωt its corresponding field magnets are in clockwise direction, then V^‘YY =?(a) Vmsinωt(b) Vmsin(ωt+120⁰)(c) Vmsin(ωt-120⁰)(d) Vmsin(ωt-240⁰)I have been asked this question in an online quiz.Enquiry is from Polyphase System topic in section Polyphase Circuits of Network Theory
Answer» Correct choice is (c) Vmsin(ωt-120⁰) For explanation: If the field system is ROTATED in the ANTICLOCKWISE direction, then the sequence of voltages in the THREE phases are in order RBY. The value of V^‘YY isV^‘YY = Vmsin(ωt-120⁰).

Discussion

73.	Phase sequence depends on the _________________(a) field(b) rotation of the field(c) armature(d) rotation of the armatureI have been asked this question by my college professor while I was bunking the class.I'm obligated to ask this question of Polyphase System topic in division Polyphase Circuits of Network Theory
Answer» CORRECT answer is (b) rotation of the FIELD Easiest explanation: The SEQUENCE of voltages in the three phases UNDERGO changes ONE after the other abd this is called phase sequence. Phase sequence depends on the rotation of the field not on rotation of armature or on field or on armature.

Discussion

74.	In three phase system at any given instant, the algebraic sum of three voltages must be?(a) 0(b) 1(c) 2(d) 3The question was posed to me in an international level competition.I want to ask this question from Polyphase System in chapter Polyphase Circuits of Network Theory
Answer» RIGHT answer is (a) 0 To elaborate: For three PHASE alternator the three voltages are of same magnitude and FREQUENCY. In three phase system at any GIVEN instant, the algebraic sum of three voltages must be zero. Algebraic sum of three voltages = 0.

Discussion

75.	In a two phase generator, the armature has two distinct windings that are displaced __________ apart.(a) 45⁰(b) 90⁰(c) 135⁰(d) 180⁰This question was addressed to me by my college professor while I was bunking the class.This intriguing question comes from Polyphase System topic in portion Polyphase Circuits of Network Theory
Answer» Correct answer is (b) 90⁰ Best explanation: SINGLE phase VOLTAGES and currents are generated by single phase GENERATORS and the armature of such generator has only one winding or one set of coils. In a two phase generator, the armature has two distinct windings that are DISPLACED 90⁰ APART. θ= 90⁰.

Discussion

76.	In three phase system, the three voltages (currents) differ in phase by __________electrical degrees from each other in a particular sequence.(a) 30(b) 60(c) 90(d) 120The question was asked in an internship interview.My question is taken from Polyphase System topic in section Polyphase Circuits of Network Theory
Answer» Right choice is (d) 120 The best explanation: In general a three phase system of voltages is MERELY a COMBINATION of three SINGLE phase systems of voltages. In three phase system, the three voltages (currents) DIFFER in phase by 120⁰ from each other in a particular sequence.

Discussion

77.	Which motors are called self-starting motors?(a) single phase(b) two phase(c) three phase(d) four phaseI got this question in a job interview.I would like to ask this question from Polyphase System topic in section Polyphase Circuits of Network Theory
Answer» The correct OPTION is (c) three phase The best I can EXPLAIN: Three phase motors are more easily started than single phase motors. Three phase motors are CALLED self-starting motors. Single phase or TWO phase or FOUR phase motors are not called self-starting motors.

Discussion

78.	When the power factor is __________ the power becomes zero 100 times a second in a 50Hz supply.(a) 0(b) 1(c) 2(d) 3I had been asked this question during a job interview.My query is from Polyphase System in section Polyphase Circuits of Network Theory
Answer» RIGHT option is (b) 1 The explanation: The power in a single phase CIRCUIT is PULSATING. When the power factor is one, the power becomes zero 100 times a second in a 50Hz supply. THEREFORE single phase motors have a pulsating TORQUE.

Discussion

79.	The power generated by a machine increases _____________ percent from single phase to two phase.(a) 40.4(b) 41.4(c) 42.4(d) 43.4I got this question during an internship interview.The question is from Polyphase System topic in division Polyphase Circuits of Network Theory
Answer» Right ANSWER is (b) 41.4 Best explanation: In an ac system it is possible to connect two or more NUMBER of individual circuits to a common POLY phase source. The power GENERATED by machine increases 41.4 percent from SINGLE phase to two phase. So, percentage increase = 41.4%.

Discussion

80.	The percentage of power increased from single phase to three phase is?(a) 50(b) 100(c) 150(d) 200I have been asked this question in quiz.My question is based upon Polyphase System topic in division Polyphase Circuits of Network Theory
Answer» RIGHT choice is (a) 50 To explain: The percentage of POWER increased from single phase to THREE phase is 50. So, percentage increase = 50%. Beyond three phase the maximum possible increase is only SEVEN PERCENT but the complications are many.

Discussion

Explore topic-wise InterviewSolutions in .

In star connected system, VRY is equal to?(a) VYR(b) -VYR(c) 2VYR(d) 3VYRThis question was posed to me in class test.My question is from Voltage, Current and Power in a Star Connected System topic in section Polyphase Circuits of Network Theory

Which of the following voltage is a phase voltage in the delta connection?(a) VRN(b) VBR(c) VYN(d) VBNI have been asked this question in an internship interview.The above asked question is from Inter Connection of Three-Phase Sources and Loads topic in portion Polyphase Circuits of Network Theory

In the figure shown below, what will be the line voltage?(a) VBR(b) VBN(c) VRN(d) VYNThe question was posed to me in an online interview.My query is from Inter Connection of Three-Phase Sources and Loads in chapter Polyphase Circuits of Network Theory

Each coil in three phase alternator has ________________ number of terminals.(a) 2(b) 4(c) 6(d) 8I have been asked this question in an online quiz.This key question is from Inter Connection of Three-Phase Sources and Loads in portion Polyphase Circuits of Network Theory

In a three phase alternator, there are __________ independent phase windings or coils.(a) 1(b) 2(c) 3(d) 4The question was posed to me in homework.My question is taken from Inter Connection of Three-Phase Sources and Loads topic in portion Polyphase Circuits of Network Theory

In three phase system at any given instant, the algebraic sum of three voltages must be?(a) 0(b) 1(c) 2(d) 3The question was posed to me in an international level competition.I want to ask this question from Polyphase System in chapter Polyphase Circuits of Network Theory

Which motors are called self-starting motors?(a) single phase(b) two phase(c) three phase(d) four phaseI got this question in a job interview.I would like to ask this question from Polyphase System topic in section Polyphase Circuits of Network Theory

When the power factor is __________ the power becomes zero 100 times a second in a 50Hz supply.(a) 0(b) 1(c) 2(d) 3I had been asked this question during a job interview.My query is from Polyphase System in section Polyphase Circuits of Network Theory

The power generated by a machine increases _____________ percent from single phase to two phase.(a) 40.4(b) 41.4(c) 42.4(d) 43.4I got this question during an internship interview.The question is from Polyphase System topic in division Polyphase Circuits of Network Theory

The percentage of power increased from single phase to three phase is?(a) 50(b) 100(c) 150(d) 200I have been asked this question in quiz.My question is based upon Polyphase System topic in division Polyphase Circuits of Network Theory