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DC Machines MCQs | DC Machines Short Questions and Answers

Questions
41 Carbon brushes are used in electric motors to
A Prevent sparking during commutation
B Provide a path for flow of current
C Brush off carbon deposits on the commutator
D None 0f these

Answer: Provide a path for flow of current
42 Interpoles in dc motors are used for
A Increasing the speed of motor
B Reducing sparking at the commutation
C Decreasing the counter emf
D Converting armature current to dc

Answer: Reducing sparking at the commutation
43 Small dc motors upto 5 HP usually have
A 2 poles
B 4 poles
C 6 poles
D 8 poles

Answer: 2 poles
44 The resistance of the starter of a 220 v, 5 HP dc shunt motor is of the order of
A 0.01Ω
B 0.1Ω
C 1Ω
D 10Ω

Answer: 10Ω
45 A dc motor can be easily identified by
A Yoke
B Size of conductor
C Commutator
D Winding

Answer: Commutator
46 The back emf of a dc motor
A Adds to the supply voltage
B Regulates its armature voltage
C Helps in energy conversion
D Usually exceeds the supply voltage

Answer: Helps in energy conversion
47 In a dc motor, pole shoes are fixed to the magnet core by
A Set of screws
B Key
C Soldering
D Welding

Answer: Set of screws
48 In a dc motor, the ratio of back emf to supply emf is an indication of its
A Efficiency
B Speed regulation
C Starting torque
D Running torque

Answer: Efficiency
49 The value of diverter resistance for a series dc motor is of the order of
A 0.1Ω
B 2Ω
C 20Ω
D 400Ω

Answer: 0.1Ω
50 A large series motor is never started without some load on it, otherwise
A It draws very current
B It develops excessive speed and get damaged
C There results heavy sparking at the brushes
D Circuit gets open circuited

Answer: It develops excessive speed and get damaged
51 When load is removed, which of the following dc motors will run at excessively high speed?
A Shunt motor
B Series motor
C Cumulative compound motor
D Differential compound motor

Answer: Series motor
52 Speed of a motor may be varied by varying
A Field current
B Applied voltage
C Resistance in series with armature
D Any of these

Answer: Any of these
53 The speed of a dc motor is
A Always constant
B Directly proportional to back emf
C Directly proportional to flux
D Inversely proportional to the product of back emf and flux

Answer: Directly proportional to back emf
54 The highest speed attained by a dc shunt motor at rated flux is
A Infinity
B Higher than no load speed
C Equal to the no load speed
D Lower than the no load speed

Answer: Equal to the no load speed
55 A dc motor is preferred to an ac motor for the application in
A Low speed operation
B High speed operation
C Variable speed variation
D Fixed speed operation

Answer: Variable speed variation
56 If 220 V dc series motor is connected to a 220 V ac supply, then at will
A Not run
B Burn out
C Run smoothly
D Run with less efficiency and high sparking

Answer: Run with less efficiency and high sparking
57 Which of the following dc motor has approximately constant speed?
A Series motor
B Shunt motor
C Cumulatively compound motor
D Differentially compound motor

Answer: Shunt motor
58 The back emf of dc motor depends on
A Armature speed N
B Field flux ɸ
C Number of armature conductors
D All of these

Answer: All of these
59 When the direction of power flow reverses, a differentially compounded motor becomes
A Differentially compounded generator
B A shunt generator
C Cumulatively compounded generator
D A series generator

Answer: Cumulatively compounded generator
60 When the direction of power flow reverses, a cumulatively compounded motor becomes
A A differentially compounded generator
B A cumulative compounded generator
C A shunt generator
D A series generator

Answer: A differentially compounded generator

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