85 + Interview Questions in Characteristics Of Dc & Ac Motors in Electric Drives Page 1 InterviewSolution

1.	The slope of the V-I curve is 19°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) .3254 Ω(b) .3608 Ω(c) .3543 Ω(d) .3443 ΩI got this question in an online interview.My query is from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT choice is (d) .3443 Ω To EXPLAIN: The SLOPE of the V-I curve is resistance. The slope given is 19° so R=tan(19°)=.3443 Ω. The slope of the V-I curve is resistance.

Discussion

2.	Calculate the phase angle of the sinusoidal waveform z(t)=78sin(456πt+2π÷78).(a) π÷39(b) 2π÷5(c) π÷74(d) 2π÷4This question was posed to me by my college professor while I was bunking the class.Question is from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT option is (a) π÷39 For explanation I would say: Sinusoidal WAVEFORM is generally expressed in the form of V=Vmsin(ωt+α) where Vm REPRESENTS PEAK value, ω represents angular frequency, α represents a PHASE difference.

Discussion

3.	Calculate the active power in an 8 Ω resistor with 8 A current flowing through it.(a) 512 W(b) 514 W(c) 512 W(d) 518 WThe question was asked in semester exam.This question is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Right choice is (a) 512 W For explanation: The resistor is a linear element. It only absorbs REAL POWER and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the ANGLE between V & I is 90°. P = I^2R = 8×8×8 = 512 W.

Discussion

4.	A 3-phase induction motor runs at almost 1500 rpm at no load and 900 rpm at full load when supplied with power from a 50 Hz, 3-phase supply. What is the corresponding speed of the rotor field with respect to the rotor?(a) 300 revolution per minute(b) 400 revolution per minute(c) 600 revolution per minute(d) 500 revolution per minuteThe question was posed to me in semester exam.The above asked question is from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives
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Discussion

5.	For a practical synchronous motor, the pull-out torque will occur when the torque angle is nearly equal to ________(a) 0°(b) 30°(c) 45°(d) 75°I have been asked this question during an interview.Asked question is from DC Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» Right answer is (d) 75° Best explanation: In a practical synchronous MOTOR, the armature RESISTANCE cannot be neglected and hence the pull-out occurs at delta=beta which is the IMPEDANCE ANGLE and is practically 75°.

Discussion

6.	In an induction motor, when the number of stator slots is not equal to an integral number of rotor slots _________(a) There may be a discontinuity in torque slip characteristics(b) A high starting torque will be available(c) The machine performs better(d) The machine may fail to startI had been asked this question in an international level competition.This is a very interesting question from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT CHOICE is (C) The machine performs better The explanation is: When the NUMBER of stator slots is not an integral multiple of a number of rotor slots the machine will not fail to start. It does not CAUSE the cogging phenomenon.

Discussion

7.	The slope of the V-I curve is 7°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) .122 Ω(b) .360 Ω(c) .377 Ω(d) .578 ΩI had been asked this question in quiz.The origin of the question is DC Motors topic in division Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT CHOICE is (a) .122 Ω Best explanation: The slope of the V-I CURVE is resistance. The slope given is 7° so R=tan(7°)=.122 Ω. The slope of the I-V curve is reciprocal of resistance.

Discussion

8.	In a synchronous machine, the phase sequence can be reversed by reversing the _________(a) Rotor direction(b) Field polarities(c) Armature terminal(d) Rotor direction and armature terminalThe question was asked in final exam.My doubt stems from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct OPTION is (a) ROTOR DIRECTION To elaborate: In SYNCHRONOUS generator, the phase sequence is governed by the direction of rotation of the rotor and in a synchronous motor, the phase sequence governs the direction of rotation of the rotor.

Discussion

9.	Which of the following are used in preventing the hunting phenomenon in synchronous generators?(a) Damper bars(b) Short pitch chords(c) Distributed winding(d) Damper bars and short pitch chordsThis question was addressed to me in class test.This intriguing question comes from DC Motors topic in division Characteristics of DC & AC Motors of Electric Drives
Answer» The correct option is (a) Damper bars To EXPLAIN: Damper bars try to maintain synchronism between the ROTATING MAGNETIC field and the rotor so they help in preventing hunting. It PRODUCES surges in the MACHINE.

Discussion

10.	The hunting phenomenon in a synchronous motor is also referred to as _________(a) Surging(b) Phase swinging(c) Cogging(d) Surging and phase swingingThe question was asked in quiz.My doubt is from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
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Discussion

11.	The leakage flux paths are ________ on the angular position of the rotor.(a) Dependent(b) Proportional(c) Independent(d) Dependent and independentI have been asked this question during an internship interview.My doubt stems from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» Correct answer is (C) Independent For explanation I would say: LEAKAGE FLUX and leakage reactance are constant IRRESPECTIVE of rotor angular position. Armature reaction though is dependent on the angular position of the rotor in salient POLE machine but in cylindrical rotor machine, both the quantities are independent of rotor position.

Discussion

12.	Calculate the phase angle of the sinusoidal waveform z(t)=8cos(45t+2π÷15).(a) 2π÷39(b) 2π÷15(c) π÷4(d) 2π÷44I have been asked this question in quiz.I'd like to ask this question from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» Right choice is (B) 2π÷15 Easy explanation: Sinusoidal waveform is generally EXPRESSED in the form of V=Vmsin(ωt+α) where Vm REPRESENTS PEAK VALUE, ω represents angular frequency, α represents a phase difference.

Discussion

13.	A 4-pole, 3-phase, 60 Hz induction motor is operating at a speed of 1500 rpm. The frequency of the rotor current of the motor in Hz is __________(a) 5(b) 4(c) 2(d) 7This question was addressed to me during an interview.My question is taken from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» Correct choice is (b) 4 To explain I would say: Given a NUMBER of POLES = 4. SUPPLY frequency is 60 Hz. Rotor SPEED is 1500 rpm. Ns = 120×f÷P = 120×60÷4 = 1800 rpm. S=Ns-Nr÷Ns = 1800-1500÷1800 = .166. F2=sf=.166×60=4 Hz.

Discussion

14.	The direct axis is taken along ________(a) Inter-polar axis(b) Rotor pole axis(c) In between inter polar and rotor axis(d) Parallel to interpolar axisThis question was posed to me in an online quiz.Asked question is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Correct OPTION is (B) Rotor pole axis For EXPLANATION: The direct axis is oriented along the rotor pole axis and the QUADRATURE axis is 90° electrical to rotor pole axis. The direct axis is not oriented along the inter-polar axis.

Discussion

15.	Calculate the active power in a 788 ω resistor with 178 A current flowing through it.(a) 24.96 MW(b) 24.44 MW(c) 24.12 MW(d) 26.18 MWThe question was posed to me in final exam.My question comes from DC Motors in section Characteristics of DC & AC Motors of Electric Drives
Answer» The correct option is (a) 24.96 MW The explanation: The resistor is a linear element. It only absorbs real POWER and dissipates it in the FORM of heat. The voltage and current are in the same phase in CASE of the resistor so the angle between V & I is 90°. P=I^2R=178×178×788=24.96 MW.

Discussion

16.	Calculate the active power in a 487 H inductor.(a) 2482 W(b) 1545 W(c) 4565 W(d) 0 WThe question was asked in a national level competition.This key question is from DC Motors topic in division Characteristics of DC & AC Motors of Electric Drives
Answer» The correct CHOICE is (d) 0 W Easy explanation: The inductor is a linear element. It only absorbs reactive power and stores it in the FORM of oscillating energy. The voltage and CURRENT are 90° in phase in case of the inductor so the angle between V & I is 90°. P = VIcos90 = 0 W.

Discussion

17.	A 3-phase induction motor runs at almost 1000 rpm at no load and 950 rpm at full load when supplied with power from a 50 Hz, 3-phase supply. What is the corresponding speed of the rotor field with respect to the rotor?(a) 30 revolution per minute(b) 40 revolution per minute(c) 60 revolution per minute(d) 50 revolution per minuteThe question was posed to me in final exam.My question is based upon DC Motors topic in division Characteristics of DC & AC Motors of Electric Drives
Answer» The correct answer is (d) 50 revolution PER minute The explanation: Supply frequency=50 HZ. No-load speed of motor = 1000 rpm. The full load speed of motor=950 rpm. Since the no-load speed of the motor is almost 1000 rpm, hence SYNCHRONOUS speed near to 1000 rpm. Speed of ROTOR field=1000 rpm. Speed of rotor field with respect to rotor = 1000-950 = 50 rpm.

Discussion

18.	In an induction motor, when the number of stator slots is equal to an integral number of rotor slots _________(a) There may be a discontinuity in torque slip characteristics(b) A high starting torque will be available(c) The maximum torque will be high(d) The machine may fail to startI have been asked this question in examination.My question is from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT choice is (d) The machine may fail to start The best explanation: When the number of stator slots is an integral MULTIPLE of a number of ROTOR slots the machine fails to start and this phenomenon is called cogging.

Discussion

19.	The slope of the V-I curve is 5°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) .3254 Ω(b) .3608 Ω(c) .3543 Ω(d) .3443 ΩThe question was asked during an interview.The origin of the question is DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» RIGHT option is (d) .3443 Ω Explanation: The SLOPE of the V-I curve is resistance. The slope GIVEN is 5° so R=tan(5°)=.3443 ω. The slope of the I-V curve is reciprocal of resistance.

Discussion

20.	A 50 Hz, 4poles, a single phase induction motor is rotating in the clockwise direction at a speed of 1425 rpm. The slip of motor in the direction of rotation & opposite direction of the motor will be respectively.(a) 0.05, 0.95(b) 0.04, 1.96(c) 0.05, 1.95(d) 0.05, 0.02I got this question during an online exam.The origin of the question is DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct answer is (C) 0.05, 1.95 To explain I would SAY: Synchronous speed, Ns=120×50÷4=1500 rpm. GIVEN a number of poles = 4. Supply frequency is 50 HZ. ROTOR speed is 1425 rpm. S=Ns-Nr÷Ns=1500-1425÷1500=.05. Sb=2-s=1.95.

Discussion

21.	The frame of an induction motor is made of _________(a) Aluminum(b) Silicon steel(c) Cast iron(d) Stainless steelI have been asked this question by my school principal while I was bunking the class.This intriguing question originated from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
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Discussion

22.	Calculate the moment of inertia of the thin spherical shell having a mass of 73 kg and diameter of 36 cm.(a) 1.56 kgm^2(b) 1.47 kgm^2(c) 1.38 kgm^2(d) 1.48 kgm^2This question was addressed to me in a national level competition.The query is from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» The correct CHOICE is (a) 1.56 kgm^2 Easy explanation: The moment of inertia of the thin spherical shell can be calculated using the FORMULA I=mr^2×.66. The mass of the thin spherical shell and diameter is given. I=(73)×.66×(.18)^2=1.56 kgm^2. It DEPENDS UPON the orientation of the rotational axis.

Discussion

23.	Calculate the moment of inertia of the disc having a mass of 54 kg and diameter of 91 cm.(a) 5.512 kgm^2(b) 5.589 kgm^2(c) 5.487 kgm^2(d) 5.018 kgm^2I have been asked this question in an online quiz.My query is from DC Motors in section Characteristics of DC & AC Motors of Electric Drives
Answer» Correct option is (b) 5.589 kgm^2 The explanation: The moment of INERTIA of the disc can be calculated using the formula I=mr^2×.5. The mass of the disc and diameter is GIVEN. I=(54)×.5×(.455)^2=5.589 kgm^2. It depends upon the orientation of the ROTATIONAL AXIS.

Discussion

24.	An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 720 rpm. The frequency of the rotor current of the motor in Hz is __________(a) 2(b) 4(c) 3(d) 1I had been asked this question in exam.My doubt stems from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
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Discussion

25.	A three-phase slip ring induction motor is fed from the rotor side with the stator winding short-circuited. The frequency of the current flowing in the short-circuited stator is ____________(a) Slip frequency(b) Supply frequency(c) The frequency corresponding to rotor speed(d) ZeroI have been asked this question during an online interview.This interesting question is from DC Motors in section Characteristics of DC & AC Motors of Electric Drives
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Discussion

26.	Calculate the active power in a 17 ω resistor with 18 A current flowing through it.(a) 5508 W(b) 5104 W(c) 5554 W(d) 5558 WI got this question during an interview for a job.The query is from DC Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» Correct ANSWER is (a) 5508 W For explanation I would SAY: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the ANGLE between V & I is 0°. P=I^2R=18×18×17=5508 W.

Discussion

27.	Calculate the active power in a 181 H inductor.(a) 2448 W(b) 1789 W(c) 4879 W(d) 0 WThe question was asked in an online interview.This intriguing question comes from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT choice is (d) 0 W Easy explanation: The inductor is a linear ELEMENT. It only absorbs reactive power and STORES it in the form of oscillating energy. The VOLTAGE and CURRENT are 90° in phase in case of the inductor so the angle between V & I is 90°. P = VIcos90 = 0 W.

Discussion

28.	Calculate the active power in a 457 F capacitor.(a) 715 W(b) 565 W(c) 545 W(d) 0 WI had been asked this question at a job interview.This is a very interesting question from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (d) 0 W Easy explanation: The capacitor is a linear element. It only absorbs reactive power and stores it in the FORM of OSCILLATING energy. The voltage and current are 90° in phase in case of the capacitor so the angle between V & I is 90°. P = VIcos90 = 0 W. Current leads the voltage in case of the capacitor.

Discussion

29.	Calculate the active power in a 7481 H inductor.(a) 1562 W(b) 4651 W(c) 0 W(d) 4654 WThis question was posed to me during an interview.My question is taken from DC Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» Correct answer is (C) 0 W Explanation: The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating ENERGY. The voltage and current are 90° in phase in case of the inductor so the ANGLE between V & I is 90°. P=VIcos90 = 0 W. Voltage leads the current in case of the inductor.

Discussion

30.	The slope of the V-I curve is 27°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.(a) .384 Ω(b) .509 Ω(c) .354 Ω(d) .343 ΩI have been asked this question in an online quiz.I'd like to ask this question from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» Right CHOICE is (B) .509 Ω To ELABORATE: The slope of the V-I curve is resistance. The slope GIVEN is 27° so R=tan(27°)=.509 ω. The slope of the I-V curve is reciprocal of resistance.

Discussion

31.	Calculate the value of the frequency if the capacitive reactance is 13 Ω and the value of the capacitor is 71 F.(a) .0001725 Hz(b) .0001825 Hz(c) .0001975 Hz(d) .0001679 HzThe question was asked during an interview for a job.This key question is from DC Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT answer is (a) .0001725 Hz For explanation: The frequency is defined as the number of OSCILLATIONS per second. The frequency can be CALCULATED USING the RELATION Xc=1÷2×3.14×f×C. F=1÷Xc×2×3.14×C = 1÷13×2×3.14×71 = .0001725 Hz.

Discussion

32.	Calculate the shaft power developed by a motor using the given data: Eb = 404V and I = 25 A. Assume frictional losses are 444 W and windage losses are 777 W.(a) 8879 W(b) 2177 W(c) 8911 W(d) 8897 WThis question was posed to me in examination.Origin of the question is DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Right choice is (a) 8879 W To explain: Shaft power developed by the MOTOR can be calculated using the FORMULA P = EBI-(rotational LOSSES) = 40425- (444+777) = 8879 W. If rotational losses are neglected, the power developed becomes EQUAL to the shaft power of the motor.

Discussion

33.	Calculate the moment of inertia of the rod about its center having a mass of 11 kg and length of 29 cm.(a) .091 kgm^2(b) .072 kgm^2(c) .076 kgm^2(d) .077 kgm^2I had been asked this question during a job interview.I'm obligated to ask this question of DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (d) .077 kgm^2 The explanation: The moment of inertia of the ROD about its center can be CALCULATED using the FORMULA I=ML^2÷12. The mass of the rod about its center and length is given. I=(11)×.0833×(.29)^2=.077 kgm^2. It depends upon the orientation of the rotational axis.

Discussion

34.	Calculate the moment of inertia of the rod about its end having a mass of 39 kg and length of 88 cm.(a) 9.91 kgm^2(b) 9.92 kgm^2(c) 9.96 kgm^2(d) 9.97 kgm^2The question was posed to me in an online interview.The above asked question is from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT option is (c) 9.96 kgm^2 The explanation: The MOMENT of inertia of the rod about its end can be calculated using the formula I=ML^2÷3. The mass of the rod about its end and length is given. I=(39)×.33×(.88)^2=9.96 kgm^2. It depends upon the orientation of the ROTATIONAL axis.

Discussion

35.	Calculate the phase angle of the sinusoidal waveform x(t)=42sin(4700πt+2π÷3).(a) 2π÷9(b) 2π÷5(c) 2π÷7(d) 2π÷3I got this question in homework.My doubt stems from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The CORRECT choice is (d) 2π÷3 Explanation: SINUSOIDAL waveform is generally expressed in the FORM of V=Vmsin(ωt+α) where Vm represents peak value, ω represents angular frequency, α represents a phase difference.

Discussion

36.	The generated e.m.f from 16-pole armature having 57 turns driven at 78 rev/sec having flux per pole as 5 mWb, with lap winding is ___________(a) 44.16 V(b) 44.15 V(c) 44.46 V(d) 44.49 VThe question was asked in quiz.The doubt is from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives
Answer» The correct option is (c) 44.46 V To explain I would say: 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. In lap winding, the number of parallel paths is equal to a number of poles. Eb = .005×16×57×2×4680÷60×16=44.46 V.

Discussion

37.	Calculate the frequency of the waveform x(t)=45sin(40πt+5π).(a) 24 Hz(b) 27 Hz(c) 23 Hz(d) 20 HzThis question was addressed to me in an interview.Origin of the question is DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives
Answer» The correct OPTION is (d) 20 Hz Best EXPLANATION: The fundamental time period of the sine wave is 2π. The frequency of X(t) is 40π÷2π=20 Hz. The frequency is independent of PHASE shifting and time shifting.

Discussion

38.	Calculate the active power in a 5 Ω resistor with 5 A current flowing through it.(a) 125 W(b) 110 W(c) 115 W(d) 126 WI got this question by my college director while I was bunking the class.My question is based upon DC Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (a) 125 W The explanation: The resistor is a linear ELEMENT. It only ABSORBS real power and dissipates it in the form of heat. The voltage and CURRENT are in the same PHASE in case of the resistor so the angle between V & I is 90°. P=I^2R=5×5×5=125 W.

Discussion

39.	Calculate the active power in a 241 H inductor.(a) 21 W(b) 11 W(c) 0 W(d) .51 WThis question was addressed to me by my college director while I was bunking the class.My enquiry is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (c) 0 W Best EXPLANATION: The inductor is a linear ELEMENT. It only absorbs reactive power and stores it in the FORM of oscillating energy. The voltage and current are 90o in phase in case of the inductor so the angle between V & I is 90°. P = VIcos90 = 0 W.

Discussion

40.	Calculate the active power in a 19 F capacitor.(a) 7.8 W(b) 0 W(c) 5.4 W(d) 1.5 WThe question was posed to me at a job interview.The question is from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Correct choice is (b) 0 W For explanation I would SAY: The CAPACITOR is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90° in phase in case of the capacitor so the ANGLE between V & I is 90°. P = VIcos90 = 0 W. Current leads the voltage in case of the capacitor.

Discussion

41.	Calculate the active power in a 41 H inductor.(a) 2 W(b) 1 W(c) 0 W(d) .5 WThis question was posed to me during an interview.My question is from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives
Answer» The correct answer is (C) 0 W The best I can explain: The inductor is a linear element. It only absorbs reactive power and stores it in the FORM of oscillating energy. The voltage and current are 90° in phase in case of the inductor so the angle between V & I is 90°. P = VIcos90 = 0 W. Voltage leads the current in case of the inductor.

Discussion

42.	Calculate the value of the frequency if the inductive reactance is 45 Ω and the value of the inductor is 15 H.(a) 0.477 Hz(b) 0.544 Hz(c) 0.465 Hz(d) 0.412 HzThis question was posed to me during an interview.Asked question is from DC Motors in section Characteristics of DC & AC Motors of Electric Drives
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Discussion

43.	What is the unit of the admittance?(a) ohm(b) ohm^-1(c) ohm^2(d) ohm^.5This question was posed to me by my college professor while I was bunking the class.My question is taken from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
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Discussion

44.	R.M.S value of the trapezoidal waveform V=Vmsin(Ωt+α).(a) Vm÷2^½(b) Vm÷2^¼(c) Vm÷2^¾(d) Vm÷3^½This question was posed to me at a job interview.My question is from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT CHOICE is (d) Vm÷3^½ The best I can EXPLAIN: R.M.S value of the SINUSOIDAL waveform is Vm÷2^½ and r.m.s value of the trapezoidal waveform is Vm÷3^½. The peak value of the sinusoidal waveform is Vm.

Discussion

45.	Calculate the phase angle of the sinusoidal waveform x(t)=20sin(9πt+π÷7).(a) π÷9(b) π÷5(c) π÷7(d) π÷4I had been asked this question in class test.I'd like to ask this question from DC Motors in section Characteristics of DC & AC Motors of Electric Drives
Answer» Right answer is (c) π÷7 To elaborate: Sinusoidal waveform is generally EXPRESSED in the FORM of V=Vmsin(ωt+α) where Vm represents peak VALUE, ω represents angular FREQUENCY, α represents a phase DIFFERENCE.

Discussion

46.	Calculate the moment of inertia of the solid sphere having a mass of 28 kg and diameter of 15 cm.(a) 0.01575 kgm^2(b) 0.01875 kgm^2(c) 0.01787 kgm^2(d) 0.01568 kgm^2I had been asked this question by my college director while I was bunking the class.This interesting question is from DC Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» CORRECT answer is (a) 0.01575 kgm^2 The explanation: The moment of inertia of the solid sphere can be calculated using the FORMULA I=2×miri^2÷5. The mass of the solid sphere and DIAMETER is GIVEN. I =(28)×.4×(.0375)^2=.01575 kgm^2. It DEPENDS upon the orientation of the rotational axis.

Discussion

47.	The generated e.m.f from 42-pole armature having 74 turns driven at 64 rev/sec having flux per pole as 21 mWb, with wave winding is ___________(a) 4177.171 V(b) 4177.152 V(c) 4100.189 V(d) 4190.454 VThis question was addressed to me in exam.The query is from DC Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer»

Discussion

48.	Calculate the time period of the waveform y(t)=74cos(81πt+π).(a) .024 sec(b) .027 sec(c) .023 sec(d) .025 secThe question was asked in my homework.Asked question is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» The correct option is (a) .024 sec The explanation is: The fundamental time PERIOD of the cosine WAVE is 2π. The time period of y(t) is 2π÷81π=.024 sec. The time period is independent of PHASE SHIFTING and time shifting.

Discussion

49.	780 V, 97 A, 1360 rpm separately excited dc motor with armature resistance (Ra) equal to 9 ohms.Calculate back emf developed in the motor when it operates on one-fourth of the full load. (Assume rotational losses are neglected)(a) 564.75 V(b) 561.75 V(c) 562.45 V(d) 565.12 VI got this question in a job interview.The query is from Modified Speed Torque Characteristics of DC Series Motors in division Characteristics of DC & AC Motors of Electric Drives
Answer» The correct choice is (b) 561.75 V To explain: Back EMF developed in the motor can be CALCULATED using the relation Eb = Vt-I×Ra. In question, it is asking for one-fourth load, but the DATA is given for full load so current becomes one-fourth of the full load current = 97÷4 = 24.25 A. 250 V is TERMINAL voltage it is fixed so Eb = 780-24.25×9 = 561.75 V.

Discussion

50.	Calculate the power developed by a motor using the given data: Eb= 48 V and I= 86 A (Assume rotational losses are neglected.)(a) 4128 W(b) 4150 W(c) 4140 W(d) 4170 WI have been asked this question during an interview for a job.My question is from Modified Speed Torque Characteristics of DC Series Motors in chapter Characteristics of DC & AC Motors of Electric Drives
Answer» Correct choice is (a) 4128 W To explain I WOULD say: Power developed by the motor can be calculated using the FORMULA P = Eb×I = 48×86 = 4128 W. If rotational LOSSES are neglected, the power developed becomes EQUAL to the shaft power of the motor.

Discussion

Explore topic-wise InterviewSolutions in .

Calculate the active power in an 8 Ω resistor with 8 A current flowing through it.(a) 512 W(b) 514 W(c) 512 W(d) 518 WThe question was asked in semester exam.This question is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives

For a practical synchronous motor, the pull-out torque will occur when the torque angle is nearly equal to ________(a) 0°(b) 30°(c) 45°(d) 75°I have been asked this question during an interview.Asked question is from DC Motors in division Characteristics of DC & AC Motors of Electric Drives

The hunting phenomenon in a synchronous motor is also referred to as _________(a) Surging(b) Phase swinging(c) Cogging(d) Surging and phase swingingThe question was asked in quiz.My doubt is from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives

Calculate the phase angle of the sinusoidal waveform z(t)=8cos(45t+2π÷15).(a) 2π÷39(b) 2π÷15(c) π÷4(d) 2π÷44I have been asked this question in quiz.I'd like to ask this question from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives

The direct axis is taken along ________(a) Inter-polar axis(b) Rotor pole axis(c) In between inter polar and rotor axis(d) Parallel to interpolar axisThis question was posed to me in an online quiz.Asked question is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 788 ω resistor with 178 A current flowing through it.(a) 24.96 MW(b) 24.44 MW(c) 24.12 MW(d) 26.18 MWThe question was posed to me in final exam.My question comes from DC Motors in section Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 487 H inductor.(a) 2482 W(b) 1545 W(c) 4565 W(d) 0 WThe question was asked in a national level competition.This key question is from DC Motors topic in division Characteristics of DC & AC Motors of Electric Drives

Calculate the moment of inertia of the disc having a mass of 54 kg and diameter of 91 cm.(a) 5.512 kgm^2(b) 5.589 kgm^2(c) 5.487 kgm^2(d) 5.018 kgm^2I have been asked this question in an online quiz.My query is from DC Motors in section Characteristics of DC & AC Motors of Electric Drives

An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 720 rpm. The frequency of the rotor current of the motor in Hz is __________(a) 2(b) 4(c) 3(d) 1I had been asked this question in exam.My doubt stems from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 17 ω resistor with 18 A current flowing through it.(a) 5508 W(b) 5104 W(c) 5554 W(d) 5558 WI got this question during an interview for a job.The query is from DC Motors in division Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 181 H inductor.(a) 2448 W(b) 1789 W(c) 4879 W(d) 0 WThe question was asked in an online interview.This intriguing question comes from DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 457 F capacitor.(a) 715 W(b) 565 W(c) 545 W(d) 0 WI had been asked this question at a job interview.This is a very interesting question from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 7481 H inductor.(a) 1562 W(b) 4651 W(c) 0 W(d) 4654 WThis question was posed to me during an interview.My question is taken from DC Motors in division Characteristics of DC & AC Motors of Electric Drives

Calculate the phase angle of the sinusoidal waveform x(t)=42sin(4700πt+2π÷3).(a) 2π÷9(b) 2π÷5(c) 2π÷7(d) 2π÷3I got this question in homework.My doubt stems from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives

Calculate the frequency of the waveform x(t)=45sin(40πt+5π).(a) 24 Hz(b) 27 Hz(c) 23 Hz(d) 20 HzThis question was addressed to me in an interview.Origin of the question is DC Motors topic in section Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 5 Ω resistor with 5 A current flowing through it.(a) 125 W(b) 110 W(c) 115 W(d) 126 WI got this question by my college director while I was bunking the class.My question is based upon DC Motors in division Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 241 H inductor.(a) 21 W(b) 11 W(c) 0 W(d) .51 WThis question was addressed to me by my college director while I was bunking the class.My enquiry is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 19 F capacitor.(a) 7.8 W(b) 0 W(c) 5.4 W(d) 1.5 WThe question was posed to me at a job interview.The question is from DC Motors topic in chapter Characteristics of DC & AC Motors of Electric Drives

Calculate the active power in a 41 H inductor.(a) 2 W(b) 1 W(c) 0 W(d) .5 WThis question was posed to me during an interview.My question is from DC Motors topic in portion Characteristics of DC & AC Motors of Electric Drives

What is the unit of the admittance?(a) ohm(b) ohm^-1(c) ohm^2(d) ohm^.5This question was posed to me by my college professor while I was bunking the class.My question is taken from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives

R.M.S value of the trapezoidal waveform V=Vmsin(Ωt+α).(a) Vm÷2^½(b) Vm÷2^¼(c) Vm÷2^¾(d) Vm÷3^½This question was posed to me at a job interview.My question is from DC Motors in portion Characteristics of DC & AC Motors of Electric Drives

Calculate the phase angle of the sinusoidal waveform x(t)=20sin(9πt+π÷7).(a) π÷9(b) π÷5(c) π÷7(d) π÷4I had been asked this question in class test.I'd like to ask this question from DC Motors in section Characteristics of DC & AC Motors of Electric Drives

Calculate the time period of the waveform y(t)=74cos(81πt+π).(a) .024 sec(b) .027 sec(c) .023 sec(d) .025 secThe question was asked in my homework.Asked question is from DC Motors in chapter Characteristics of DC & AC Motors of Electric Drives