Electrical Machines Questions and Answers – DC Machine Applications

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “DC Machine Applications”.

1. Inspite of heavy initial investments, dc motors are used due to _____________
a) flexibility and ease of control
b) lower losses
c) improved power factor of the system
d) all of the mentioned
View Answer

Answer: a
Explanation: The control of dc machines is very much simplified when compared to other machines. So this makes it very useful when compared.
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2. Separately excited dc generators are used in __________
a) Ward leonard system of speed control
b) Hopkinson’s testing
c) Voltage control
d) None of the mentioned
View Answer

Answer: a
Explanation: Separately excited dc generators are used in Ward leonard system of speed control.

3. Maximum torque in dc series motor is limited by __________
a) commutation
b) heating
c) field control
d) all of the mentioned
View Answer

Answer: a
Explanation: Commutation is the process which reduces the induced emf and so the torque.
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4. Hoists, cranes and battery powered vehicles use _________ motors in the locomotive.
a) dc series
b) dc shunt
c) induction
d) reluctance
View Answer

Answer: a
Explanation: Hoists, cranes require large starting torque which can be provided by dc series motor.

5. There is an application which required pulsating loads, punch presses. The most preferred machines would be __________
a) compound dc machine
b) series dc machine
c) shunt dc machine
d) all of the mentioned
View Answer

Answer: a
Explanation: It is compound machine whose flux can varied higher as well as lower in order to have variable torques.
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6. The manufacturer has mentioned a medium starting torque and 15% speed regulation. Which is the most appropriate motor for this requirement?
a) dc shunt motor
b) induction motor
c) differential motor
d) none of the mentioned
View Answer

Answer: a
Explanation: The dc shunt motor provides a medium values of the torque.

7. Centrifugal pumps, fans-blowers use __________
a) shunt as well as induction motor
b) only shunt motors
c) only induction motor
d) none of the mentioned
View Answer

Answer: a
Explanation: Centrifugal pumps, fans-blowers use shunt as well as induction motor.
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8. Most commercial compound dc generator are normally supplied by manufacturers as over compound machines because __________
a) degree of compounding can be adjusted by diverters acrosss series field
b) they have ideally best for HVDC
c) cost effective than shunt
d) zero percent regulation
View Answer

Answer: a
Explanation: It is usually over compounded so that degree of compounding can be adjusted by diverters across series field.

9. Which of the following are applied with differential compound motor?
a) Hoist
b) Cranes
c) Drilling machines
d) None of the mentioned
View Answer

Answer: a
Explanation: Practically it is never used as the speed can rise very high.
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10. Most commercial compound dc generator are normally supplied by manufacturers as ______ compounded machines.
a) over
b) under
c) level
d) none of the mentioned
View Answer

Answer: a
Explanation: Most commercial compound dc generator are normally supplied by manufacturers as over compounded.

11. Long back ago, magnetic casette were in use, having permanent magnet dc motors which had?
a) magnets on stator and armature on rotor
b) magnet on rotor and armature on stator
c) electronic commutation
d) all of the mentioned
View Answer

Answer: a
Explanation: PMDC motors have field fixed on the stator and rotor has coils.

12. If a dc shunt motor is stopped by forcing starter handle back to OFF position, then the __________
a) dangerous sparking occurs at last stud
b) dangerous sparking occurs at all stud
c) it stops normally
d) heavy sparking
View Answer

Answer: a
Explanation: The sparking will occur at the last stud not all the studs of the starter.

13. Interpoles are used to start the dc motor above the base speed.
a) True
b) False
View Answer

Answer: b
Explanation: Interpoles do not reduce the flux across the windings, hence the speed does not rise above base speed.

14. A 200 V dc motor has external resistances of Ra and Rf in armature and field circuits respectively. The starting current is reduced when?
a) Ra is maximum and Rf minimum
b) Rf is maximum and Ra minimum
c) Ra is minimum and Rf minimum
d) Ra is maximum and Rf maximum
View Answer

Answer: a
Explanation: When Ra is maximum and Rf minimum then the current to the armature is reduced. And hence the sparkings are reduced while starting.

15. A d.c. generator has been provided tappings on the armature winding at intervals of 120 degrees from the side of commutator. The connection will be __________
a) delta connected alternator
b) star connected alternator
c) star connected induction motor
d) any of the mentioned
View Answer

Answer: a
Explanation: Dc generator if has displaced by 120 degrees physically then it will act as Ac machine.

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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