41 + Interview Questions in Dc Motors in Electrical Machines Page 1 InterviewSolution

1.	A PMMDC motor has an armature resistance of 4.2 ohms. When 6 V supply is applied to motor it runs at 1215 rpm drawing 14.5 mA on no-load. The rotational losses is?(a) 86.1 mW(b) 86.1 W(c) 8.6 W(d) 8.6 mWI had been asked this question in examination.Asked question is from Permanent Magnet DC (PMDC) Motors topic in division DC Motors of Electrical Machines
Answer» Correct ANSWER is (a) 86.1 MW To EXPLAIN I would say: Rotational LOSSES = EIa = 5.939 14.5/1000 = 86.1 mW.

Discussion

2.	PMMDC can only be armature controlled.(a) True(b) FalseI have been asked this question in an internship interview.This interesting question is from Permanent Magnet DC (PMDC) Motors in division DC Motors of Electrical Machines
Answer» Correct OPTION is (a) True Best explanation: DUE to ABSENCE of the FIELD winding there is no option of field CONTROL.

Discussion

3.	How to manage run-away issue for dc shunt motor in industry?(a) By using PMDCmotor(b) By using constant field motors(c) This can not be avoided(d) None of the mentionedI had been asked this question by my school teacher while I was bunking the class.The question is from Permanent Magnet DC (PMDC) Motors in division DC Motors of Electrical Machines
Answer» Right answer is (a) By USING PMDCmotor For EXPLANATION: A PMDC MOTOR can be used to replace the dc shunt motor in industries as they have QUITE identical characteristics.

Discussion

4.	Why does DC motor sometimes run too fast when under-loaded?(a) Due to weak field(b) Due to high line voltage(c) Due to brush-shifted to neutral(d) All of the mentionedThe question was posed to me in a job interview.This intriguing question originated from Speed Control of DC Motors topic in portion DC Motors of Electrical Machines
Answer» The correct choice is (a) DUE to weak field To explain I WOULD SAY: All of the reasons cause the dc MOTOR to increase in SPEED when under loaded.

Discussion

5.	PMDC had no ________ excited machine.(a) electrically(b) magnetically(c) none of the mentioned(d) leastI have been asked this question during an online interview.This interesting question is from Permanent Magnet DC (PMDC) Motors in portion DC Motors of Electrical Machines
Answer» Correct CHOICE is (a) electrically To EXPLAIN I WOULD SAY: It has no electrical excitation.

Discussion

6.	PMMDC offers ______ characteristics.(a) shunt(b) series(c) armature(d) cumulativeI had been asked this question in an interview.The question is from Permanent Magnet DC (PMDC) Motors topic in section DC Motors of Electrical Machines
Answer» CORRECT option is (a) SHUNT For explanation I WOULD say: As the field REMAINS constant due to the constant field, it has shunt CHARACTERISTIC.

Discussion

7.	PMMDC are smaller in size due to __________(a) ansence of field winding(b) presence of smaller field winding(c) present of magnets(d) Any of the mentionedI got this question by my college professor while I was bunking the class.My doubt is from Permanent Magnet DC (PMDC) Motors in chapter DC Motors of Electrical Machines
Answer» Correct answer is (a) ansence of field winding To ELABORATE: There is no field winding used in PMMDC to create the FLUX. So it is lighter.

Discussion

8.	Which type of field winding required in PMMDC?(a) series winding(b) shunt winding(c) cumulative winding(d) none of the mentionedThis question was posed to me in class test.The above asked question is from Permanent Magnet DC (PMDC) Motors in portion DC Motors of Electrical Machines
Answer» CORRECT choice is (d) none of the mentioned Easy explanation: There is no field winding used in PMMDC to CREATE the FLUX.

Discussion

9.	Run-away for large DC machine is avoided by using _________(a) a turn or two of cumulative compounding(b) compensating winding(c) stablizer winding(d) all of the mentionedI have been asked this question in class test.This is a very interesting question from Speed Control of DC Motors topic in chapter DC Motors of Electrical Machines
Answer» The correct CHOICE is (b) compensating WINDING The BEST I can explain: The runaway in large MACHINE is avoided by using compensating winding.

Discussion

10.	The qualities aspired to obtain a good permanent magnet is/are ____________(a) high residual flux(b) low coercivity(c) high coercivity(d) high residual flux and high coercivityThis question was posed to me in examination.My question comes from Permanent Magnet DC (PMDC) Motors topic in portion DC Motors of Electrical Machines
Answer» Right choice is (d) HIGH residual flux and high coercivity The BEST explanation: Its both high residual flux as well as coercivity is DESIRED for a GOOD permanent magnet.

Discussion

11.	A laboratory group was working with a set of 3-hp shunt DC motor. But there was a mistake that it was fused with a 0.3A fuse instead 3A fuse. Then it was started _________(a) a flash occurred instantly(b) it ran for 3s and fuse was blown(c) it ran normal(d) none of the mentionedI got this question by my college professor while I was bunking the class.Query is from Speed Control of DC Motors in chapter DC Motors of Electrical Machines
Answer» The CORRECT OPTION is (b) it ran for 3s and fuse was BLOWN To explain I would say: Due to the wrong USAGE of the fuse wire, it could not sustain the current and in few SECONDS it was blown off.

Discussion

12.	The torque limit of speed for a shunt motor _________(a) remains constant till base speed(b) remains constant after base speed(c) varies linearly after base speed(d) varies inversely till base speedThe question was asked in quiz.My question is from Speed Control of DC Motors in division DC Motors of Electrical Machines
Answer» The correct OPTION is (a) REMAINS constant till BASE speed Easiest EXPLANATION: The TORQUE limit of the shunt motor remains constant till the base speed.

Discussion

13.	To implement armature voltage control, it must be ensured that _________(a) it is used on separately excited machine(b) it is used on shunt machine(c) it is used on series machine(d) any of the mentioned machineI have been asked this question by my school principal while I was bunking the class.Question is taken from Speed Control of DC Motors in section DC Motors of Electrical Machines
Answer» The correct OPTION is (a) it is USED on SEPARATELY excited machine The explanation is: It must be separately excited mahcine to implement the armature VOLTAGE control.

Discussion

14.	Armature voltage control works for speeds _____ base speed and field resistance control works well for speed _____ base speed.(a) below, above(b) above, below(c) above, above(d) below, belowI had been asked this question in a national level competition.This question is from Speed Control of DC Motors topic in division DC Motors of Electrical Machines
Answer» Right answer is (a) below, above The explanation: For ARMATURE voltage CONTROL, armature CURRENT is increased so as to reduce speed below base speed. In field control flux is REDUCED so the speed WOULD be above base speed.

Discussion

15.	Small DC motors have best speed control by _________(a) armature voltage control(b) field resistance control(c) any of the methods(d) none of the mentionedI got this question in a job interview.Query is from Speed Control of DC Motors in division DC Motors of Electrical Machines
Answer» Right OPTION is (a) armature voltage control For EXPLANATION: For the small dc motors it is difficult to vary the SPEED using their field control.

Discussion

16.	At a very low speed, increase in field resistance will _________(a) decrease the speed of motor(b) increase the speed of motor(c) not have signicant effect on speed(d) no effectThe question was asked in semester exam.My question is based upon Speed Control of DC Motors topic in portion DC Motors of Electrical Machines
Answer» The correct option is (a) decrease the speed of motor To explain: At low speed increase in ARMATURE CURRENT caused by decrease in emf, will not be enough to compensate for decrease in flux in induced TORQUE EQUATION.

Discussion

17.	Mark the wrong option. Which of the following cause and effect behaviour in speed control is correct when field resistance is increased.(a) Decrease in flux(b) Increase in armature current(c) Increase in EMF(d) Decrease in speedThe question was asked in unit test.My enquiry is from Speed Control of DC Motors in chapter DC Motors of Electrical Machines
Answer» Right choice is (c) INCREASE in EMF To explain: When field resistance is INCREASED, emf ACTUALLY decreases, not increases DUE to fall in FLUX.

Discussion

18.	Ward-Leonard system of system of speed control is not recommended for _________(a) constant speed operation(b) Wide speed(c) frequent-motor reversed(d) very slow speedThis question was posed to me in a job interview.This intriguing question originated from Speed Control of DC Motors topic in section DC Motors of Electrical Machines
Answer» Right choice is (a) constant SPEED operation Explanation: Ward-Leonard SYSTEM of system of speed control is not RECOMMENDED for constant speed operation.

Discussion

19.	For a dc machine, its commutator has a diameter of 50 c rotating at 1000 rpm. For a brush width of 1 cm, the time commutation taken by the machine will be _________(a) 0.382 ms(b) 0.456 ms(c) 0.573 ms(d) 0.312 msI have been asked this question by my college director while I was bunking the class.This question is from DC Motor Starting topic in portion DC Motors of Electrical Machines
Answer» Right answer is (a) 0.382 ms The best explanation: Time of commutation = brush width/peripheral velocity = 160/(PI50*1000) = 0.382 ms.

Discussion

20.	The PWM control of DC motor varies _________(a) linearly with speed(b) inversely with speed(c) parabolically with speed(d) exponentially with speedI have been asked this question in an online quiz.This key question is from Speed Control of DC Motors in division DC Motors of Electrical Machines
Answer» Right CHOICE is (a) linearly with speed The best explanation: In PWM TECHNIQUE duty ratio is a linear FUNCTION with RESPECT to speed.

Discussion

21.	A dc machine is run at rated speed in forward direction and then in backward direction. It is observed that, speeds of the rotation are different, then it leads to the conclusion of _________(a) incorrect brush placement(b) incorrect pole and core alignment(c) incorrect field supply(d) all of the mentionedThis question was addressed to me in a national level competition.I need to ask this question from DC Motor Starting in portion DC Motors of Electrical Machines
Answer» RIGHT ANSWER is (a) incorrect brush placement Easiest explanation: Given at the same speed of OPERATION, if the speeds are DIFFERENT then the brushes are placed in not aligned MANNER.

Discussion

22.	If the students give a forward shift of 10° to the dc generator, then it _________(a) reduces flux per pole(b) improves flux per pole(c) increases the flux density in core(d) none of the mentionedThis question was posed to me in homework.My question is taken from DC Motor Starting topic in division DC Motors of Electrical Machines
Answer» Right choice is (a) reduces flux per pole For explanation: A generator given a FORWARD BRUSH shift will get its flux density REDUCED at the pole as the flux will be in the opposite DIRECTION.

Discussion

23.	The post effects of the armature reaction is _________(a) main field distortion(b) shift in MNA(c) reduction in main field(d) none of the mentionedThe question was asked in final exam.Query is from DC Motor Starting in chapter DC Motors of Electrical Machines
Answer» The correct option is (d) none of the mentioned The best I can explain: There is no such POST effect of armature reaction. The armature reaction PHENOMENA AFFECT the WORKING condition only.

Discussion

24.	The effect of fringing increases as we _________(a) increase air gap(b) decrease air gap(c) increase in flux density(d) introduce more ferric core materialI have been asked this question in an internship interview.My question is taken from DC Motor Starting topic in section DC Motors of Electrical Machines
Answer» The CORRECT option is (a) increase air gap For EXPLANATION: Fringing is the EFFECT in which the magnetic flux lines BULGE out of the flux. and it is introduced at the air GAPS.

Discussion

25.	For a 7.46 kW, 200 V dc shunt motor with full load efficiency of 85% has armature resistance of 0.25 ohms. Calculate the value of starting resistance in ohms for a current 1.5 times of the full load current.(a) 2.788(b) 3.038(c) 2.688(d) 2.588I got this question in an interview for job.This key question is from DC Motor Starting topic in section DC Motors of Electrical Machines
Answer» Right OPTION is (a) 2.788 Explanation: FULL load CURRENT = 7460/(2000.85) = 43.88 A Starting current = 1.543.88 = 65.883 A R = V/I = 200/65.883 = 3.038 OHMS Starting resistance = 3.038 – 0.25 = 2.788 ohms.

Discussion

26.	The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit.(a) True(b) FalseThis question was posed to me in an internship interview.Question is from DC Motor Starting topic in division DC Motors of Electrical Machines
Answer» The CORRECT choice is (b) False Best EXPLANATION: The direct-on-line starter can not LIMIT the SPEED by LIMITING the current.

Discussion

27.	Thyristor controlled starter is preferred over DOL starter due to _________(a) lesser losses(b) controlled direction(c) least resistance offered(d) all of the mentionedThe question was asked during an interview.This key question is from DC Motor Starting in division DC Motors of Electrical Machines
Answer» RIGHT answer is (a) LESSER losses Explanation: A thyristor CONTROLLED METHOD is more efficient as it has lesser resistance and losses while OPERATING.

Discussion

28.	The shunt motor starters that can be used is/are ___________(a) 3-point and 4-point starter(b) 5-point starter(c) 4-point starter(d) 5-point and 3-point starterThis question was addressed to me in final exam.I need to ask this question from DC Motor Starting in portion DC Motors of Electrical Machines
Answer» The CORRECT CHOICE is (a) 3-point and 4-point starter Explanation: Both 3-point and 4-point STARTERS can be USED.

Discussion

29.	A 100 hp, 250 V, 350 A shunt dc motor with an armature resistance of 0.05 ohms. To limit maximum starting current to twice the rated of its value, what will be the number of stages of starting resistances?(a) 3(b) 2(c) 4(d) 5I had been asked this question in exam.Origin of the question is DC Motor Starting in section DC Motors of Electrical Machines
Answer» Right ANSWER is (a) 3 For explanation: n = LOG(Ra/Rt)/log(Imin/Imax); Rt = Vt/Imax = 250/700 = 0.357 ohms = log(0.05/0.357)/log(350/700) = 2.84 = 3 (APPROX).

Discussion

30.	Considering a human handed control system for the dc motor speed control, if the resistance wire cut out too slowly, then the _____________(a) starting resistance would burn(b) field winding would burn(c) speed will rise steeply(d) any of the mentionedI had been asked this question in final exam.Question is taken from DC Motor Starting in section DC Motors of Electrical Machines
Answer» Correct ANSWER is (a) starting resistance would burn To EXPLAIN I would SAY: Due to the slow cut off the resistance the current will be high for a LARGE amount of TIME and it will cause the machine to burn.

Discussion

31.	For a 100 hp 250 V, compound dc motor with compensating winding has a field current of 5 A to produce a voltage of 250 V at 1200 rpm. What will be the shunt field current of this machine at no load?(a) 5 A(b) 5.6 A(c) 4 A(d) 0 AI had been asked this question in an interview.The above asked question is from Operating Characteristics of DC Motors in chapter DC Motors of Electrical Machines
Answer»

Discussion

32.	A starting resistance is inserted at the starting in an induction motor as well as dc motor.(a) Induction motor has to control starting torque whereas in dc motor, it is done to avoid large current(b) To limit starting current in both the machines(c) To limit starting speed(d) All of the mentionedThe question was asked in an interview for job.I would like to ask this question from DC Motor Starting in division DC Motors of Electrical Machines
Answer» CORRECT answer is (a) Induction motor has to control STARTING torque whereas in dc motor, it is DONE to avoid LARGE current Explanation: For both the machines the purpose is different.

Discussion

33.	A dc shunt motor is connected to the source through 3-point starter. Suddenly if we starter handle is moved fastly from off to on position, then the _________(a) motor will draw large current(b) motor will not start(c) motor will burn(d) all of the mentionedI have been asked this question during an online exam.Question is taken from Operating Characteristics of DC Motors in division DC Motors of Electrical Machines
Answer» RIGHT CHOICE is (d) all of the mentioned The best I can explain: As the starter is placed very RAPIDLY from off to on the resistance SEEN by the MACHINE will be very less and it will draw a large starting current.

Discussion

34.	A student is given a differential compound motor and he has been asked to make it start. How will he try?(a) By shorting series field at start(b) To run as shunt motor at start(c) By making rated current at start(d) All of the mentionedI have been asked this question in exam.The query is from Operating Characteristics of DC Motors in chapter DC Motors of Electrical Machines
Answer» CORRECT ANSWER is (d) All of the mentioned To EXPLAIN I would say: At starting, the flux should be rated so that there is no abnormal SITUATION arise.

Discussion

35.	It is impossible to start a differential compounded dc motor.(a) True(b) FalseThe question was asked in an interview.I'm obligated to ask this question of Operating Characteristics of DC Motors topic in chapter DC Motors of Electrical Machines
Answer» The correct answer is (a) True The explanation is: It is so because of the unstable speed and torque CHARACTERISTICS. At starting, armature current and series field current are high. Since the series flux SUBTRACTS from shunt flux, series flux actually reverses the magnetic polarity. This MOTOR will typically be STILL or turn slowly in wrong direction while burning up because of excessive current.

Discussion

36.	The speed torque of the differential compound dc motor is shown below. What conclusions can be made?(a) This is an unstable machine(b) There is regenerative increment in the speed(c) This is impractical to be used(d) All of the mentionedThe question was posed to me at a job interview.Asked question is from Operating Characteristics of DC Motors topic in chapter DC Motors of Electrical Machines
Answer» The correct OPTION is (d) All of the mentioned To ELABORATE: This is basically an unstable scenario which MAKES the machine UNABLE to be USED.

Discussion

37.	Choose the best option which identifies about the below characteristics.(a) (1)series motor (2) Cumulative compound motor (3) Shunt motor(b) (1)Cumulative compound motor (2) series motor (3) Shunt motor(c) (1)Cumulative compound motor (2) Shunt motor (3) series motor(d) (1)Shunt motor (2) series motor (3) Cumulative compound motorI had been asked this question by my school principal while I was bunking the class.The above asked question is from Operating Characteristics of DC Motors topic in section DC Motors of Electrical Machines
Answer» Right ANSWER is (a) (1)series motor (2) Cumulative compound motor (3) Shunt motor For explanation I WOULD say: Shunt MACHINE has ALMOST constant SPEED while the series varies as inverse relation with torque.

Discussion

38.	The flux and the internally generated voltage of a dc machine is a ________ function of its magneto-motive force.(a) non-linear(b) linear(c) constant(d) inverseI had been asked this question in exam.My question is based upon Operating Characteristics of DC Motors in chapter DC Motors of Electrical Machines
Answer» Correct choice is (a) non-LINEAR Best EXPLANATION: The flux and INDUCED emf are non linear FUNCTION of its mmf.

Discussion

39.	For a dc shunt motor of 5 kW, running at 1000 rpm, the induced torque will be ____________(a) 47.76 N(b) 57.76 N(c) 35.76 N(d) 37.76 NI got this question during an interview.I'd like to ask this question from Operating Characteristics of DC Motors topic in division DC Motors of Electrical Machines
Answer» CORRECT OPTION is (a) 47.76 N Easiest explanation: Torque = Power/(speed in rev/s) = 5000/(2PI1000/60) = 47.76 N.

Discussion

40.	It is advised not to run dc series motor with no load. Why?(a) Because zero torque at no load will make speed infinite(b) Because zero torque as no load will not let machine start(c) Because infinite torque will be produced(d) None of the mentionedThe question was posed to me during an online exam.I would like to ask this question from Operating Characteristics of DC Motors in portion DC Motors of Electrical Machines
Answer» The correct OPTION is (a) Because zero torque at no load will MAKE SPEED INFINITE Best explanation: If the torque is zero in dc series motor then the speed will be infinite causing the damage of the machine permanently.

Discussion

41.	Mark the correct option which affects the terminal voltage of a dc shunt motor.(a) Armature reaction(b) Source voltage variations(c) Compensating winding(d) Any of the mentionedThe question was posed to me in a job interview.I want to ask this question from Operating Characteristics of DC Motors in portion DC Motors of Electrical Machines
Answer» The CORRECT CHOICE is (c) Compensating winding To explain I would say: If the DC motor has compensating winding, flux will REMAIN constant regardless of LOAD.

Discussion

Explore topic-wise InterviewSolutions in .

PMMDC can only be armature controlled.(a) True(b) FalseI have been asked this question in an internship interview.This interesting question is from Permanent Magnet DC (PMDC) Motors in division DC Motors of Electrical Machines

PMDC had no ________ excited machine.(a) electrically(b) magnetically(c) none of the mentioned(d) leastI have been asked this question during an online interview.This interesting question is from Permanent Magnet DC (PMDC) Motors in portion DC Motors of Electrical Machines

PMMDC offers ______ characteristics.(a) shunt(b) series(c) armature(d) cumulativeI had been asked this question in an interview.The question is from Permanent Magnet DC (PMDC) Motors topic in section DC Motors of Electrical Machines

Which type of field winding required in PMMDC?(a) series winding(b) shunt winding(c) cumulative winding(d) none of the mentionedThis question was posed to me in class test.The above asked question is from Permanent Magnet DC (PMDC) Motors in portion DC Motors of Electrical Machines

Small DC motors have best speed control by _________(a) armature voltage control(b) field resistance control(c) any of the methods(d) none of the mentionedI got this question in a job interview.Query is from Speed Control of DC Motors in division DC Motors of Electrical Machines

The PWM control of DC motor varies _________(a) linearly with speed(b) inversely with speed(c) parabolically with speed(d) exponentially with speedI have been asked this question in an online quiz.This key question is from Speed Control of DC Motors in division DC Motors of Electrical Machines

The post effects of the armature reaction is _________(a) main field distortion(b) shift in MNA(c) reduction in main field(d) none of the mentionedThe question was asked in final exam.Query is from DC Motor Starting in chapter DC Motors of Electrical Machines

The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit.(a) True(b) FalseThis question was posed to me in an internship interview.Question is from DC Motor Starting topic in division DC Motors of Electrical Machines

Thyristor controlled starter is preferred over DOL starter due to _________(a) lesser losses(b) controlled direction(c) least resistance offered(d) all of the mentionedThe question was asked during an interview.This key question is from DC Motor Starting in division DC Motors of Electrical Machines

It is impossible to start a differential compounded dc motor.(a) True(b) FalseThe question was asked in an interview.I'm obligated to ask this question of Operating Characteristics of DC Motors topic in chapter DC Motors of Electrical Machines

For a dc shunt motor of 5 kW, running at 1000 rpm, the induced torque will be ____________(a) 47.76 N(b) 57.76 N(c) 35.76 N(d) 37.76 NI got this question during an interview.I'd like to ask this question from Operating Characteristics of DC Motors topic in division DC Motors of Electrical Machines