26 + Interview Questions in Starting Of Induction Motors in Electrical Machines Page 1 InterviewSolution

1.	If an emf at slip frequency is injected at an angle α with existing rotor induced emf, then the ___________(a) power factor only improves(b) speed changes(c) power factor improves and speed changes(d) power factor worsens and speed changesI have been asked this question at a job interview.This question is from Starting of Polyphase Induction Motors-II topic in portion Starting of Induction Motors of Electrical Machines
Answer» Correct CHOICE is (a) power FACTOR only improves Explanation: Power factor INCREASES as the net angle DECREASES.

Discussion

2.	If a 3-phase induction motor is fed from the rotor short circuiting the stator terminals, frequency of the current flowing in the short-circuited stator is?(a) slip frequency(b) supply frequency(c) zero(d) frequency corresponding to the rotor speedThis question was posed to me by my school teacher while I was bunking the class.My enquiry is from Starting of Polyphase Induction Motors-II topic in section Starting of Induction Motors of Electrical Machines
Answer» CORRECT choice is (a) slip frequency The explanation: In the STATOR the FLUX developed will REVOLVE with a speed of slip frequency.

Discussion

3.	A constant torque variable power drive of induction machine has ___________(a) constant air gap flux.(b) constant slip(c) variable air gap flux, constant slip(d) constant slip, constant air gap fluxThis question was addressed to me in examination.Question is taken from Starting of Polyphase Induction Motors-II topic in section Starting of Induction Motors of Electrical Machines
Answer» Right answer is (d) constant slip, constant air GAP flux To elaborate: A constant torque DRIVE should have constant flux and HENCE constant air gap flux. Also it should have constant slip.

Discussion

4.	Torque-slip characteristic of an induction motor is linear in the smaller slip values, because ___________(a) effective rotor resistance is very large compared to reactance(b) rotor resistance is equal to stator resistance(c) rotor resistance is equal to rotor reactance(d) rotor reactance almost equal to stator reactanceThis question was addressed to me in an interview for internship.My enquiry is from Starting of Polyphase Induction Motors-II topic in section Starting of Induction Motors of Electrical Machines
Answer»

Discussion

5.	For 3-phase induction motor, as load increases from no load towards the full load, torque _______ slip.(a) increases in proportion to(b) decreases in proportion to(c) remains constant to(d) increases hyperbolically toThe question was asked during an interview.My enquiry is from Starting of Polyphase Induction Motors-II topic in division Starting of Induction Motors of Electrical Machines
Answer» The correct ANSWER is (a) increases in proportion to The explanation: For the lower SLIP, TORQUE will increase proportionally to the slip as verified by the torque slip characteristic.

Discussion

6.	For 3-phase induction motor, as load increases from no load towards the full load then the ___________(a) power factor improves(b) power factor remains same(c) power factor varies linearly(d) power factor increases till 40% load and then decreasesThe question was asked in homework.This intriguing question originated from Starting of Polyphase Induction Motors-II topic in portion Starting of Induction Motors of Electrical Machines
Answer» RIGHT option is (a) POWER factor improves Best EXPLANATION: Overall during the operation power factor improves as the REACTANCE reduces and the power factor ANGLE reduces.

Discussion

7.	Due to the line to ground fault, phase ‘a’ in the induction motor phase supply gets cut, then machine ___________(a) continues to run with excessive supply current(b) continues to run with reduced slip(c) stalls(d) all of the mentionedThis question was addressed to me during an online exam.I need to ask this question from Starting of Polyphase Induction Motors-II topic in section Starting of Induction Motors of Electrical Machines
Answer» Right ANSWER is (a) continues to RUN with excessive SUPPLY CURRENT For explanation I would say: It continues to run at lowered speed but with increased current.

Discussion

8.	If one of the supply fuses blows off and gets open circuited, then for the connected induction machine continues to run with increased slip.(a) True(b) FalseI got this question in exam.I would like to ask this question from Starting of Polyphase Induction Motors-II topic in section Starting of Induction Motors of Electrical Machines
Answer» Right ANSWER is (a) True The best explanation: It is true because as one of the PHASE is missing, it will lead to decrease in speed assuming machine was PROVIDING the RATED load.

Discussion

9.	Choose from the below which can be obtained by the equivalent circuit of an electrical machine?(a) Complete performance characteristics of the machine(b) Temperature coefficients of the machine components(c) Type of protection to be used in the machine(d) Design parameters of the windingsI had been asked this question in a job interview.Question is from Starting of Polyphase Induction Motors-II in section Starting of Induction Motors of Electrical Machines
Answer» The correct choice is (a) COMPLETE PERFORMANCE characteristics of the machine Explanation: EQUIVALENT circuit can help to GIVE the complete performance characteristics of an ELECTRICAL machine.

Discussion

10.	A 3-phase induction motor is fed at the stator at a frequency f. If the rotor speed is Nr and synchronous speed is Ns, then the absolute speed of the resultant flux in space is?(a) Ns(b) (Ns+Nr) if the flux rotates in the same same direction as the rotor(c) (Ns-Nr) if the flux rotates in the same same direction as the rotor(d) NrI had been asked this question by my college director while I was bunking the class.Enquiry is from Starting of Polyphase Induction Motors-II topic in chapter Starting of Induction Motors of Electrical Machines
Answer» Correct ANSWER is (a) Ns Easiest explanation: The FLUX speed in SPACE will be Ns IRRESPECTIVE of any rotor speed.

Discussion

11.	A center ammeter connected to the rotor end circuit of a 6-pole,50 Hz, induction motor makes 45 complete oscillations in a minute. Then the rotor speed and the speed of stator field w.r.t. rotor is?(a) 985, 15(b) 970, 1000(c) 985, 985(d) 985, 970This question was posed to me at a job interview.I want to ask this question from Starting of Polyphase Induction Motors-II topic in section Starting of Induction Motors of Electrical Machines
Answer» RIGHT option is (a) 985, 15 Explanation: Rotor FREQUENCY= s*F1 = 45/60 = 0.75 Hz. s = 0.75/50 = 0.015 rotor SPEED = 1000(1-0.015) = 985 rpm. Speed of stator field w.r.t. rotor = Ns-Nr = 1000-985=15 rpm.

Discussion

12.	A 3-phase slip ring IM is has its copper bars changed by the aluminium bars in the machine. With the above change ___________(a) starting torque remains same(b) starting torque increase(c) starting torque decrease(d) maximum torque increaseThe question was asked during an interview for a job.This intriguing question originated from Starting of Polyphase Induction Motors-II topic in portion Starting of Induction Motors of Electrical Machines
Answer» The correct answer is (a) starting TORQUE remains same To EXPLAIN I would say: Resistance increase, so the starting torque increase.

Discussion

13.	A 3-phase slip ring IM is has its copper bars replaced by the aluminium bars in the machine. With the above change ___________(a) starting torque increases, maximum torque remains same, smT increase(b) starting torque increases, maximum torque remains same, smT remains same(c) starting torque increases, maximum torque remains increases, smT increase(d) starting torque decreases, maximum torque remains same, smT decreaseI have been asked this question in semester exam.My question is based upon Starting of Polyphase Induction Motors-II topic in portion Starting of Induction Motors of Electrical Machines
Answer» Correct choice is (a) starting torque increases, maximum torque REMAINS same, smT increase The explanation is: As the RESISTANCE has been changed as the aluminium has REPLACED copper, it will not affect the maximum torque.

Discussion

14.	Auto transformer method of starting has most acceptable method for the starting of the induction motor due to __________(a) maximum torque(b) maximum reduction of inrush current(c) variable tapping(d) all of he mentionedI have been asked this question in semester exam.My doubt stems from Starting of Polyphase Induction Motors-I topic in section Starting of Induction Motors of Electrical Machines
Answer» CORRECT option is (a) maximum TORQUE To explain I would say: Maximum torque, reduction of inrush current and variable TURNS ratio makes it a popular method of STARTING of IM.

Discussion

15.	For the application requiring high starting torque and minimum starting current, the most suitable starting method will be?(a) Auto transformer starting method(b) Reactance starting method(c) DOL method(d) Star-Delta methodI got this question by my college professor while I was bunking the class.This interesting question is from Starting of Polyphase Induction Motors-I in chapter Starting of Induction Motors of Electrical Machines
Answer» Correct ANSWER is (a) Auto transformer starting method The explanation is: Auto transformer provides the MAXIMUM starting TORQUE.

Discussion

16.	A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the starting torque will be?(a) x Tfl(b) x^2 Tfl(c) 1/x *Tfl(d) no change in the starting currentI got this question in an interview.My query is from Starting of Polyphase Induction Motors-I topic in portion Starting of Induction Motors of Electrical Machines
Answer» The CORRECT answer is (a) X Tfl The BEST explanation: Tst/Tfl = (x^2)(Ist/Ifl)^2SLIP.

Discussion

17.	A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the starting current reduces by?(a) x(b) x^2(c) 1/x(d) no change in the starting currentThis question was addressed to me in an internship interview.This is a very interesting question from Starting of Polyphase Induction Motors-I topic in portion Starting of Induction Motors of Electrical Machines
Answer» The correct option is (B) x^2 The best I can EXPLAIN: The STARTING current reduces by a fraction of x^2.

Discussion

18.	Series reactor method of starting has main disadvantage of lowering the starting power. This can be overcome by __________(a) using series resistance(b) using parallel reactor(c) using parallel resistance(d) using series reactorI got this question in examination.I'm obligated to ask this question of Starting of Polyphase Induction Motors-I topic in section Starting of Induction Motors of Electrical Machines
Answer» The correct option is (a) using series resistance The EXPLANATION: Adding series resistance INSTEAD od REACTORS will help improving the starting POWER.

Discussion

19.	Series reactor method of starting has main disadvantage of __________(a) lowering the starting power(b) increasing the starting current(c) increasing starting torque(d) all of the mentionedI had been asked this question in an interview for job.I'm obligated to ask this question of Starting of Polyphase Induction Motors-I topic in chapter Starting of Induction Motors of Electrical Machines
Answer» CORRECT choice is (a) lowering the STARTING power Best explanation: MAIN disadvantage of using REACTORS is that ot reduces the starting power of the machine and so the efficiency.

Discussion

20.	A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the starting torque reduces by?(a) x(b) x^2(c) 1/x(d) no change in the starting currentI had been asked this question during a job interview.The origin of the question is Starting of Polyphase Induction Motors-I topic in division Starting of Induction Motors of Electrical Machines
Answer»

Discussion

21.	A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the starting current reduces by?(a) x(b) x^2(c) 1/x(d) no change in the starting currentThis question was addressed to me in a job interview.Enquiry is from Starting of Polyphase Induction Motors-I topic in chapter Starting of Induction Motors of Electrical Machines
Answer» Right option is (a) X To elaborate: The CURRENT will be reduced by a fraction of x.

Discussion

22.	External methods of induction motor starting is employed to __________(a) Reduce the voltage(b) Reduce the current(c) Improve efficiency(d) All of the mentionedThis question was addressed to me in class test.Enquiry is from Starting of Polyphase Induction Motors-I topic in section Starting of Induction Motors of Electrical Machines
Answer» The correct OPTION is (a) REDUCE the voltage Explanation: External methods like AUTO transformer is USED to MAINLY reduce voltage.

Discussion

23.	For a 4-pole, 50 Hz induction motor has starting current as 5 times to that of the full load current working at a speed of 1450rpm. The ratio of starting to full load torque will be?(a) 0.83(b) 0.17(c) 0.67(d) 1I have been asked this question in final exam.I want to ask this question from Starting of Polyphase Induction Motors-I in portion Starting of Induction Motors of Electrical Machines
Answer» CORRECT ANSWER is (a) 0.83 Explanation: Tst/Tfl = (Ist/Ifl)^2slip Tst/Tfl = 250.033 = 0.83

Discussion

24.	Full voltage starting of induction motor may cause __________(a) dip in the voltages(b) high inrush current(c) high losses(d) all of the mentionedI have been asked this question in an online interview.Enquiry is from Starting of Polyphase Induction Motors-I in division Starting of Induction Motors of Electrical Machines
Answer» Correct choice is (a) dip in the voltages Best EXPLANATION: Starting induction MOTOR on full load will CAUSE high inrush current which furthe CAUSES dip in the bus voltage.

Discussion

25.	Starters are required in the induction motor because __________(a) of high starting current(b) they are not self starting(c) torque produced is very low at starting to overcome inertia(d) all of the mentionedI had been asked this question by my college director while I was bunking the class.My query is from Starting of Polyphase Induction Motors-I topic in section Starting of Induction Motors of Electrical Machines
Answer» CORRECT answer is (a) of high STARTING CURRENT To EXPLAIN I would say: Starters are NEEDED in induction motors to limit the starting current.

Discussion

26.	Voltage induce in the induction motor is highest at __________(a) starting(b) standstill(c) rated speed(d) any of the mentionedThe question was posed to me in a national level competition.My query is from Starting of Polyphase Induction Motors-I in division Starting of Induction Motors of Electrical Machines
Answer» Correct CHOICE is (a) starting Explanation: At starting when slip is 1, the VOLTAGE INDUCED is MAXIMUM.

Discussion

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