85 + Interview Questions in Characteristics Of Dc & Ac Motors in Electric Drives Page 2 InterviewSolution

51.	Calculate the radius of the circular ring having a moment of inertia 59 kgm^2 and mass of69 kg.(a) .924 m(b) .928 m(c) .934 m(d) .944 mI had been asked this question in a job interview.The doubt is from Modified Speed Torque Characteristics of DC Series Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» Correct ANSWER is (a) .924 m Easiest explanation: The moment of inertia of the circular ring can be calculated using the FORMULA I=∑miri^2. The moment of inertia of a circular ring and MASS is given. R=((59)÷(69))^.5 = .924 m. It depends upon the orientation of the rotational axis.

Discussion

52.	The slope of the V-I curve is 31°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) 0.600 Ω(b) 0.607 Ω(c) 0.543 Ω(d) 0.648 ΩThis question was posed to me by my college professor while I was bunking the class.The doubt is from Modified Speed Torque Characteristics of DC Series Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (a) 0.600 Ω The EXPLANATION: The slope of the V-I CURVE is RESISTANCE. The slope given is 31° so R=tan(31°)=0.600 Ω. The resistance is the ratio of voltage and CURRENT.

Discussion

53.	Calculate the value of the angular acceleration of the motor using the given data: J = 81 kg-m^2, load torque = 74 N-m, motor torque = 89 N-m.(a) .195 rad/s^2(b) .182 rad/s^2(c) .183 rad/s^2(d) .185 rad/s^2I have been asked this question during a job interview.I want to ask this question from Modified Speed Torque Characteristics of DC Series Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Right choice is (d) .185 rad/s^2 To explain: USING the dynamic equation of motor J×(ANGULAR acceleration) = Motor torque – LOAD torque: 81×(angular acceleration) = 89-74=15, angular acceleration=.185 rad/s^2.

Discussion

54.	340 V, 45 A, 1400 rpm DC separately excited motor having a resistance of .7 ohm excited by an external dc voltage source of 90 V. Calculate the torque developed by the motor on full load.(a) 94.73 N-m(b) 94.52 N-m(c) 93.37 N-m(d) 94.42 N-mI had been asked this question at a job interview.This key question is from Modified Speed Torque Characteristics of DC Series Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» The correct answer is (a) 94.73 N-m Easy explanation: BACK emf developed in the motor during the full LOAD can be calculated using equation Eb = Vt-I×Ra = 308.5 V and machine CONSTANT Km = Eb÷Wm which is equal to 2.1053. TORQUE can be calculated by using the relation T = Km × I = 2.1053×45 = 94.73 N-m.

Discussion

55.	Calculate the value of the frequency if the time period of the signal is 99 sec.(a) 0.08 Hz(b) 0.02 Hz(c) 0.01 Hz(d) 0.04 HzI had been asked this question during a job interview.I'm obligated to ask this question of Modified Speed Torque Characteristics of DC Series Motors topic in portion Characteristics of DC & AC Motors of Electric Drives
Answer»

Discussion

56.	Calculate the moment of inertia of the hollow cylinder having a mass of 78 kg and radius of 49 cm.(a) 9.363 kgm^2(b) 9.265 kgm^2(c) 9.787 kgm^2(d) 9.568 kgm^2The question was posed to me in unit test.This interesting question is from Modified Speed Torque Characteristics of DC Series Motors in section Characteristics of DC & AC Motors of Electric Drives
Answer» The correct option is (a) 9.363 kgm^2 The best I can explain: The moment of inertia of the hollow cylinder can be calculated using the formula I=miri^2÷2. The mass of the hollow cylinder and radius is given. I=(78)×.5×(.49)^2=9.363 kgm^2. It DEPENDS UPON the orientation of the rotational AXIS.

Discussion

57.	The unit of Magnetic flux density is Tesla.(a) True(b) FalseThis question was addressed to me in an interview for internship.I need to ask this question from Modified Speed Torque Characteristics of DC Series Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer»

Discussion

58.	The generated e.m.f from 45-pole armature having 400 turns driven at 70 rev/sec having flux per pole as 90 mWb, with 17 parallel paths is ___________(a) 13341.17 V(b) 12370.14 V(c) 14700.89 V(d) 15690.54 VI had been asked this question by my college director while I was bunking the class.This intriguing question comes from Modified Speed Torque Characteristics of DC Series Motors topic in division Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (a) 13341.17 V The explanation is: The generated e.m.f can be calculated USING the formula Eb = Φ×Z×N×P÷60×A, Φ represent flux per pole, Z REPRESENTS the total number of conductors, P represents the number of poles, A represents the number of PARALLEL paths, N represents speed in rpm. One turn is equal to two conductors. Eb = .09×45×400×2×4200÷60×17 = 13341.17 V.

Discussion

59.	Turn-on and turn-off times of transistor depend on _________(a) Static Characteristic(b) Junction Capacitance(c) Current Gain(d) Voltage GainThe question was asked by my college professor while I was bunking the class.My query is from Modified Speed Torque Characteristics of DC Series Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT option is (b) Junction Capacitance The explanation: The depletion LAYER capacitance and diffusion capacitance affects the TURN-on and turn-off BEHAVIOR of transistors. DUE to these internal capacitances, transistors do not turn on instantly.

Discussion

60.	Calculate mark to space ratio if the system is on for 17 sec and time period is 30 sec.(a) 1.307(b) 1.457(c) 1.478(d) 1.146This question was addressed to me in an online interview.This intriguing question originated from Modified Speed Torque Characteristics of DC Shunt Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer»

Discussion

61.	The slope of the V-I curve is 78°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) 4.732 Ω(b) 4.608 Ω(c) 4.543 Ω(d) 4.648 ΩI have been asked this question during an interview.This interesting question is from Modified Speed Torque Characteristics of DC Shunt Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Right OPTION is (a) 4.732 Ω The explanation: The SLOPE of the V-I curve is RESISTANCE. The slope GIVEN is 78° so R=tan(78°)=4.732 Ω. The slope of the I-V curve is RECIPROCAL of resistance.

Discussion

62.	Which of the following device should be used as a switch in a low power switched mode power supply (SMPS)?(a) GTO(b) BJT(c) MOSFET(d) TRIACI have been asked this question during a job interview.This interesting question is from Modified Speed Torque Characteristics of DC Shunt Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» Correct choice is (c) MOSFET Easiest explanation: MOSFET has a low POWER RATING and high-frequency rating and so it can be USED in low power switch mode power supply. MOSFET stands for metal OXIDE silicon field effect transistor.

Discussion

63.	SCR is uni-directional in nature.(a) True(b) FalseThe question was asked in a job interview.My doubt is from Modified Speed Torque Characteristics of DC Shunt Motors topic in portion Characteristics of DC & AC Motors of Electric Drives
Answer»

Discussion

64.	The conduction loss versus device current characteristic of a power MOSFET is best approximated by a ________(a) Straight line(b) Rectangular hyperbola(c) Parabola(d) Exponential decaying functionsI got this question during an online interview.The doubt is from Modified Speed Torque Characteristics of DC Shunt Motors in section Characteristics of DC & AC Motors of Electric Drives
Answer» RIGHT choice is (C) Parabola The explanation is: A MOSFET in ON state BEHAVES as RESISTANCE so the conduction power loss is given by, P=I^2R. Hence, the power vs current curve will be a parabola.

Discussion

65.	Which of the following device is NOT suitable for parallel operation?(a) MOSFET(b) BJT(c) IGBT(d) TRIACThe question was asked in an internship interview.I would like to ask this question from Modified Speed Torque Characteristics of DC Shunt Motors topic in division Characteristics of DC & AC Motors of Electric Drives
Answer» Right choice is (b) BJT Easy explanation: BJT has a negative temperature COEFFICIENT of RESISTANCE. If it is operated in PARALLEL operation THERMAL run-away will take place and the DEVICE will damage.

Discussion

66.	Armature reaction is demagnetizing in nature due to purely lagging load.(a) True(b) FalseThis question was posed to me during an interview.This interesting question is from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer»

Discussion

67.	Calculate the terminal voltage of Permanent Magnet DC motor having a resistance of 2 Ω and a full load current of 5 A with 20 V back e.m.f.(a) 30 V(b) 25 V(c) 20 V(d) 31 VI got this question during a job interview.The above asked question is from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT choice is (a) 30 V The explanation is: PERMANENT Magnet DC MOTOR is a special type of motor in which flux remains constant. The terminal voltage can be calculated using the relation Vt = Eb+IaRa = 20+5×2 = 30 V.

Discussion

68.	A DC chopper feeds an RLE load. If the value of E is increased by 20%, the current ripple ________(a) increases by 20%(b) decreases by 20%(c) increases only 20%(d) remains the sameThis question was posed to me by my school teacher while I was bunking the class.My question comes from Modified Speed Torque Characteristics of DC Shunt Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» Correct CHOICE is (d) remains the same The explanation is: For a BUCK converter current ripple doesn’t DEPEND on the value of E. ΔIL = Vdc×D×(1-D)×T÷L. The expression is independent of the value of E.

Discussion

69.	In DC chopper, the waveform for input and output voltages is respectively __________(a) Discontinuous and Continuous(b) Continuous and Discontinuous(c) Both continuous(d) Both discontinuousThis question was posed to me by my school principal while I was bunking the class.I want to ask this question from Modified Speed Torque Characteristics of DC Shunt Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Correct CHOICE is (b) CONTINUOUS and Discontinuous Easiest explanation: Chopper has a perfect DC at the input which is chopped into pulses which MEANS the OUTPUT voltage is discontinuous and by varying the DUTY cycle we can vary the average output voltage.

Discussion

70.	The most suitable device for high-frequency inversion in switching mode power supply is ______(a) GTO(b) BJT(c) MOSFET(d) IGBTI have been asked this question in an interview for job.Question is from DC Motors in section Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT option is (c) MOSFET To ELABORATE: MOSFET is fastest AMONG all the power semiconductor devices and has the highest frequency range. MOSFET stands for metal oxide silicon field effect TRANSISTOR.

Discussion

71.	The slope of the I-V curve is 87°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) 0.0524 Ω(b) 0.0254 Ω(c) 0.0543 Ω(d) 0.0648 ΩI have been asked this question in an international level competition.This intriguing question originated from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» RIGHT OPTION is (a) 0.0524 Ω Easy explanation: The SLOPE of the I-V curve is reciprocal of RESISTANCE. The slope given is 87° so R=1÷tan(87°)=.0524 Ω.

Discussion

72.	Calculate the value of the frequency if the time period of the signal is 0 sec.(a) infinity(b) 0.4 Hz(c) 0.2 Hz(d) 0.78 HzThis question was addressed to me in an interview for job.My question is taken from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct answer is (a) infinity Easy explanation: The frequency is defined as the number of oscillations per second. It is reciprocal of the TIME period. It is EXPRESSED in Hz. F = 1÷0 = infinity. It signifies signal is APERIODIC.

Discussion

73.	When a UJT is used for triggering an SCR, then the wave shape of voltage obtained from the UJT circuit will be ____________(a) Square(b) Pulse(c) Trapezoidal(d) Saw-toothThis question was posed to me by my college professor while I was bunking the class.My doubt stems from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (b) Pulse The BEST explanation: UJT relaxation oscillator using RC CIRCUIT is used for SCR triggering and the wave SHAPE obtained is an exponentially decaying pulse. So, we can say pulse waveform is obtained.

Discussion

74.	Displacement is a ____________ quantity.(a) Scalar(b) Vector(c) Scalar and Vector(d) TensorThe question was posed to me during an internship interview.My query is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct answer is (b) Vector To explain I would say: DISPLACEMENT is a vector quantity. It DEPENDS on the initial and final position of the BODY. It has both direction and MAGNITUDE. Distance is a scalar quantity.

Discussion

75.	Calculate the velocity of the wheel if the angular speed is 25 rad/s and radius is 10 m.(a) 250 m/s(b) 260 m/s(c) 270 m/s(d) 240 m/sI got this question in a national level competition.I want to ask this question from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives
Answer» Correct choice is (a) 250 m/s For explanation I WOULD say: The velocity of the wheel can be calculated using the relation V=ω×r. The velocity is the vector PRODUCT of angular speed and RADIUS. V=Ω×r = 25×10 = 250 m/s.

Discussion

76.	The unit of Magento-motive force is Ampere-turns.(a) True(b) FalseI got this question in homework.I'd like to ask this question from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Correct option is (a) True The EXPLANATION: The magneto-motive FORCE is defined as the PRODUCT of current and turns. It is mathematically EXPRESSED as F=NI.

Discussion

77.	For large DC machines, the yoke is usually made of which material?(a) Cast steel(b) Cast iron(c) Iron(d) Cast steel or cast ironThe question was posed to me in an online interview.The above asked question is from DC Motors Characteristics in division Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (a) CAST steel Easy EXPLANATION: Yoke in DC MACHINES is made up of cast steel. Yoke PROVIDES structural support and mechanical strength to the machine. It HELPS in carrying the flux from the North pole to South pole.

Discussion

78.	The slope of the V-I curve is 26°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) .487 Ω(b) .482 Ω(c) .483 Ω(d) .448 ΩThis question was addressed to me in an online interview.I'd like to ask this question from DC Motors Characteristics topic in portion Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT OPTION is (a) .487 Ω For explanation: The slope of the V-I CURVE is RESISTANCE. The slope given is 26° so R=tan(26°) = .487 Ω. The slope of the V-I curve is resistance.

Discussion

79.	Calculate the value of the frequency if the signal completes half of the cycle in 70 sec. Assume signal is periodic.(a) 0.00714 Hz(b) 0.00456 Hz(c) 0.00845 Hz(d) 0.00145 HzThe question was posed to me in unit test.This interesting question is from DC Motors Characteristics in division Characteristics of DC & AC Motors of Electric Drives
Answer» Right choice is (a) 0.00714 Hz The explanation: The frequency is DEFINED as the number of oscillations PER second. It is reciprocal of the TIME period. It is expressed in Hz. The given signal completes half of the cycle in 70 seconds then it will complete a full cycle in 140 seconds. F = 1÷T=1÷140=.00714 Hz.

Discussion

80.	What is the unit of the intensity?(a) Watt/m^2(b) Watt/m(c) Watt/m^4(d) Watt/m^3I got this question at a job interview.The above asked question is from DC Motors Characteristics topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» Right option is (a) Watt/m^2 For EXPLANATION: INTENSITY is defined as the amount of POWER incident on a particular area. It is MATHEMATICALLY EXPRESSED as I = Power incident (Watt)÷Area(m^2).

Discussion

81.	The field control method is suitable for constant torque drives.(a) True(b) FalseI had been asked this question during an online interview.My question is based upon DC Motors Characteristics in division Characteristics of DC & AC Motors of Electric Drives
Answer»

Discussion

82.	Calculate the mass of the ball having a moment of inertia 4.5 kgm^2 and radius of 14 cm.(a) 229.59 kg(b) 228.56 kg(c) 228.54 kg(d) 227.52 kgThe question was asked during an interview.My doubt is from DC Motors Characteristics in section Characteristics of DC & AC Motors of Electric Drives
Answer» Correct option is (a) 229.59 kg To EXPLAIN I would say: The moment of inertia of the ball can be calculated USING the formula I=∑miri^2. The moment of inertia of ball and radius is given. M=(4.5)÷(.14)^2 = 229.59 kg. It DEPENDS upon the ORIENTATION of the ROTATIONAL axis.

Discussion

83.	The unit of active power is Watt.(a) True(b) FalseThis question was posed to me during an interview.This is a very interesting question from DC Motors Characteristics in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct option is (a) True Easy EXPLANATION: The active power in the electrical circuits is a USEFUL power. It DETERMINES the power FACTOR of the system. It is expressed in terms of WATT. P=VIcosΦ.

Discussion

84.	The generated e.m.f from 20-pole armature having 800 conductors driven at 30 rev/sec having flux per pole as 60 mWb, with 16 parallel paths is ___________(a) 1900 V(b) 1840 V(c) 1700 V(d) 1800 VThis question was posed to me in an interview.The origin of the question is DC Motors Characteristics in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (d) 1800 V To elaborate: The generated e.m.f can be calculated using the formula Eb = Φ×Z×N×P÷60×A, Φ represent flux per pole, Z represents the TOTAL number of conductors, P represents the number of POLES, A represents the number of parallel PATHS, N represents speed in rpm. Eb = .06×20×1800×800÷60×16 = 1800 V.

Discussion

85.	Swinburne’s test can be conducted on ___________(a) Series motor(b) Shunt motor(c) Compound motor(d) Shunt and compound motorI had been asked this question during an online exam.My question is taken from DC Motors Characteristics in portion Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT answer is (d) SHUNT and compound motor Best explanation: The test is practically applicable for machines which have flux constant LIKE the shunt and compound machines as this is a no-load test and DC SERIES motor should not be run at no-load because of high speed.

Discussion

Explore topic-wise InterviewSolutions in .

The unit of Magnetic flux density is Tesla.(a) True(b) FalseThis question was addressed to me in an interview for internship.I need to ask this question from Modified Speed Torque Characteristics of DC Series Motors topic in section Characteristics of DC & AC Motors of Electric Drives

SCR is uni-directional in nature.(a) True(b) FalseThe question was asked in a job interview.My doubt is from Modified Speed Torque Characteristics of DC Shunt Motors topic in portion Characteristics of DC & AC Motors of Electric Drives

Armature reaction is demagnetizing in nature due to purely lagging load.(a) True(b) FalseThis question was posed to me during an interview.This interesting question is from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives

The most suitable device for high-frequency inversion in switching mode power supply is ______(a) GTO(b) BJT(c) MOSFET(d) IGBTI have been asked this question in an interview for job.Question is from DC Motors in section Characteristics of DC & AC Motors of Electric Drives

Calculate the value of the frequency if the time period of the signal is 0 sec.(a) infinity(b) 0.4 Hz(c) 0.2 Hz(d) 0.78 HzThis question was addressed to me in an interview for job.My question is taken from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives

Displacement is a ____________ quantity.(a) Scalar(b) Vector(c) Scalar and Vector(d) TensorThe question was posed to me during an internship interview.My query is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives

Calculate the velocity of the wheel if the angular speed is 25 rad/s and radius is 10 m.(a) 250 m/s(b) 260 m/s(c) 270 m/s(d) 240 m/sI got this question in a national level competition.I want to ask this question from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives

The unit of Magento-motive force is Ampere-turns.(a) True(b) FalseI got this question in homework.I'd like to ask this question from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives

What is the unit of the intensity?(a) Watt/m^2(b) Watt/m(c) Watt/m^4(d) Watt/m^3I got this question at a job interview.The above asked question is from DC Motors Characteristics topic in section Characteristics of DC & AC Motors of Electric Drives

The field control method is suitable for constant torque drives.(a) True(b) FalseI had been asked this question during an online interview.My question is based upon DC Motors Characteristics in division Characteristics of DC & AC Motors of Electric Drives

Calculate the mass of the ball having a moment of inertia 4.5 kgm^2 and radius of 14 cm.(a) 229.59 kg(b) 228.56 kg(c) 228.54 kg(d) 227.52 kgThe question was asked during an interview.My doubt is from DC Motors Characteristics in section Characteristics of DC & AC Motors of Electric Drives

The unit of active power is Watt.(a) True(b) FalseThis question was posed to me during an interview.This is a very interesting question from DC Motors Characteristics in chapter Characteristics of DC & AC Motors of Electric Drives

Swinburne’s test can be conducted on ___________(a) Series motor(b) Shunt motor(c) Compound motor(d) Shunt and compound motorI had been asked this question during an online exam.My question is taken from DC Motors Characteristics in portion Characteristics of DC & AC Motors of Electric Drives