InterviewSolution

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1.	For which of the following operations a DC motor is preferred over an AC motor?(a) Low speed operation(b) High speed operation(c) Variable speed operation(d) Fixed speed operationThis question was posed to me in an online interview.My question is from Applications of DC Machines in chapter DC Machines of DC Machines
Answer» Correct answer is (c) VARIABLE speed operation The explanation: Due to the various speed control techniques available and with the HELP of electrical BRAKES various speed levels can be obtained in DC motor. While in AC motor for same purpose we REQUIRE frequency change which requires another complex circuitry.

2.	Which motor is preferred in highly explosive atmosphere?(a) Air motor(b) Shunt motor(c) Series motor(d) Battery operated motorThis question was addressed to me in an online quiz.Enquiry is from Applications of DC Machines topic in portion DC Machines of DC Machines
Answer» RIGHT choice is (a) Air motor To explain I WOULD say: A pneumatic motor (Air motor) is a motor which does mechanical work by expanding compressed air. Air motors generally convert the compressed air energy to mechanical work through either linear or ROTARY motion, and are PREFERRED in highly explosive atmosphere.

3.	Which of the following motor can replace DC series motor?(a) DC shunt motor(b) Cumulative compound motor(c) Wound-rotor induction motor(d) Synchronous motorThe question was asked during an interview.I want to ask this question from Applications of DC Machines in section DC Machines of DC Machines
Answer» RIGHT answer is (c) Wound-rotor induction motor The best explanation: DE series motor’s closest rival is the wound-rotor induction motor with a rotor RESISTANCE control. But ultimately the availability and ECONOMICS of a dc POWER is the deciding factor rather than the motor characteristic.

4.	Nowadays, DC motor is widely used only in ____________(a) Pumping sets(b) Air compressors(c) Electric traction(d) Machine shopsI had been asked this question at a job interview.Asked question is from Applications of DC Machines topic in chapter DC Machines of DC Machines
Answer» Right answer is (c) ELECTRIC traction To elaborate: DC machine is used in tractions, because its characteristics suit perfectly with application. ALSO, its smooth BRAKING capacity and ability to transfer from one mode to other is very GOOD.

5.	Which motor has almost replaced DC shunt motor from its applications?(a) Wound-rotor induction motor(b) Differential compound motor(c) Air motor(d) Squirrel caged induction motorI have been asked this question by my college professor while I was bunking the class.My question is based upon Applications of DC Machines topic in division DC Machines of DC Machines
Answer» Correct option is (d) Squirrel caged INDUCTION motor The best explanation: OWING to the relative simplicity, cheapness and ruggedness of the squirrel CAGE induction motor, the shunt motor is less preferred for constant-speed drives except at low speeds. At high or medium speed applications we USE induction motor, mostly squirrel caged.

6.	Maximum torque in a DC machine is limited by ______________(a) Commutation(b) Heating(c) Losses other than heating(d) StabilityI have been asked this question in an interview.The question is from Applications of DC Machines topic in section DC Machines of DC Machines
Answer» Right choice is (a) Commutation To explain: While for all other motors maximum torque is restricted to certain value as various losses in other motors lead to HEATING of the core materials. In DC machines for maximum torque commutation TIME will OBVIOUSLY decrease and beyond some POINT commutation process can’t be FASTENED.

7.	Which of the following load application normally needs starting torque more than the rated torque?(a) Blowers(b) Conveyors(c) Air compressors(d) Centrifugal pumpsI had been asked this question during an internship interview.Asked question is from Applications of DC Machines in section DC Machines of DC Machines
Answer» Right answer is (b) Conveyors Best explanation: Conveyors need high starting TORQUE initially, and constant torque LATER. Thus, DC series motor is USED in conveyors as it provides very high starting torque, which is practically 5 TIMES the rated torque.

8.	Why are the DC motors preferred for traction applications?(a) Torque and speed are inversely proportional to armature current(b) Torque is proportional to armature current(c) Torque is proportional to square root of armature current(d) The speed is inversely proportional to the torque and the torque is proportional to square of armature currentI have been asked this question in an online quiz.Enquiry is from Applications of DC Machines in division DC Machines of DC Machines
Answer» Correct choice is (d) The SPEED is inversely proportional to the torque and the torque is proportional to SQUARE of armature current The explanation is: DC motors are used for traction as, according to the CHARACTERISTICS of DC motors speed is inversely proportional to torque and square of armature current as WELL, if linear magnetization is concerned. Thus, DC motors are perfectly suitable for traction.

9.	Separately excited DC generators are still used in _________________(a) Thermal power plants(b) Ward Leonard speed control system(c) Hydro power plant(d) In all fieldsThis question was addressed to me in final exam.Question is taken from Applications of DC Machines in portion DC Machines of DC Machines
Answer» RIGHT choice is (b) Ward Leonard speed CONTROL system The explanation: Separately excited DC generators are still used in WIDE output VOLTAGE control like in Ward Leonard speed control. In all POWER plants today, generally AC generators are used due to low cost and less maintenance required.

10.	In world today, around 25% of the motors are manufactured are DC motors.(a) True(b) FalseI got this question during an interview.My question is based upon Applications of DC Machines topic in division DC Machines of DC Machines
Answer» Right ANSWER is (a) True The explanation: For a dc machine, of course, the main attraction lies in its flexibility, versatility and ease of control. This EXPLAINS why in SPITE of its RATHER heavy initial investment it still retains its CHARM in strong competitive industrial applications.

11.	Which DC motor is used with flywheel for intermittent light and heavy loads?(a) Series motor(b) Shunt motor(c) Cumulatively compounded motor(d) Differentially compounded motorI had been asked this question in an online quiz.My query is from Applications of DC Machines topic in section DC Machines of DC Machines
Answer» The correct option is (C) Cumulatively compounded motor Explanation: Cumulative compound DC motor is used with flywheel carrying peaks and so to smooth out the LOAD on the motor as well as to reduce peaks on POWER system. Without flywheel the motor CONSTRUCTION will be much larger.

12.	Which of the following motors can be used to drive the rotary compressor?(a) DC shunt motor(b) DC series motor(c) Universal motor(d) Synchronous motorThe question was asked during an interview.Origin of the question is Applications of DC Machines in portion DC Machines of DC Machines
Answer» The correct option is (d) Synchronous motor The BEST explanation: ROTARY compressor generally demand CONSTANT speed operation throughout the load. Sometimes, DC machines are not able to produce constant speed throughout the process HENCE, synchronous machine is used.

13.	When an electric train is moving down a hill, the DC motor will operate as ____________(a) DC series motor(b) DC series generator(c) DC shunt motor(d) DC shunt generatorThis question was addressed to me in an interview.The query is from Applications of DC Machines topic in portion DC Machines of DC Machines
Answer» Correct answer is (b) DC SERIES generator The explanation: Normally in electric TRACTION purposes DC series motors are EMPLOYED. At above condition the back emf is greater than supply VOLTAGE hence, it will OPERATE as series generator which will provide energy back to the supply.

14.	Which type motors are preferred for lathes?(a) DC shunt motors(b) Squirrel Cage induction motor(c) Synchronous motor(d) DC Shunt Motors or Squirrel cage induction motorThis question was posed to me by my school teacher while I was bunking the class.My enquiry is from Applications of DC Machines in portion DC Machines of DC Machines
Answer» RIGHT option is (d) DC Shunt MOTORS or Squirrel CAGE induction MOTOR Best explanation: Lathes machines requires UNIFORM torque which is provided with squirrel cage induction or DC shunt motors. Hence, they are preferred for lathes. DC shunt motor and induction motor of squirrel cage type follow same shunt characteristics.

15.	DC shunt motor is still used instead of synchronous motor in _____________(a) High speed applications(b) Low speed applications(c) Medium and high-speed applications(d) EverywhereThe question was posed to me during an internship interview.Question is from Applications of DC Machines in section DC Machines of DC Machines
Answer» Correct option is (b) Low speed applications The explanation: At low speeds, DC shunt motors are comparable with synchronous motors. The outstanding feature of a DC shunt motor however is its superb WIDE range flexible speed CONTROL above and below the BASE speed using solid-state controlled RECTIFIERS.