DC Machines MCQs | DC Machines Multiple Choice Questions and Answers

Questions
1 A shunt motor is fitted with a field regulator for speed control. For constant torque load, the speed will be minimum when the resistance of the regulator is
A 0Ω
B Infinite
C About 10Ω
D About 100Ω

Answer: 0Ω
2 The resistance of the field regulator of a dc shunt motor is of order of
A 0.1Ω
B 1Ω
C 10Ω
D 100Ω

Answer: 100Ω
3 The air gap between stator and armature of an electric motor is kept as small as possible
A To get a stronger magnetic field
B To improve the air circulation
C To reach a higher speed of rotation
D To make the rotation easier

Answer: To get a stronger magnetic field
4 In a dc motor, unidirectional torque is produced with the help of
A Brushes
B Commutator
C End plates
D Both (a) and (b)

Answer: Both (a) and (b)
5 In dc motor, the rotor is
A Welded to the shaft
B Keyed to the shaft
C Soldered to the shaft
D Both to the shaft

Answer: Keyed to the shaft
6 The armature of a dc motor is laminated
A To reduce hysteresis loss
B To reduce eddy current loss
C To reduce the cost of core
D To reduce the mass of the armature

Answer: To reduce eddy current loss
7 As the load on a dc shunt motor is increased, its speed
A Increases proportionately
B Remains constant
C Increases slightly
D Reduces slightly

Answer: Reduces slightly
8 The speed of a series wound dc motor
A Can be controlled by shunt field regulator
B Can not be controlled by diverter
C Increases as flux decreases
D Increases as armature circuit resistance increases

Answer: Increases as flux decreases
9 If the applied voltage to a dc shunt motor is halved and the load torque doubled, the armature current will be
A Unaltered
B Zero
C Doubled
D Halved

Answer: Doubled
10 The starting resistance of a dc motor is usually
A Infinitely large
B Large
C About 100Ω
D Small

Answer: Small
11 The back emf of a dc motor depends on
A Field flux
B Shape of conductors
C Type of slip rings
D Brush material

Answer: Field flux
12 In a dc machine prominent and noisy blue and green sparks indicate
A Excessive brush wear
B Winding fault
C Worn out insulation
D None of these

Answer: Winding fault
13 The flux is maximum in following unit of a dc motor
A Yoke leading end of pole shoe
B Trailing end of pole shoe
C Yoke
D Any of the above

Answer: Yoke
14 In a dc motor, the flux leakage coefficient is typically
A 0.5 to 0.7
B 0.8 to 1.0
C 1.1 to 1.3
D 1.4 to 1.6

Answer: 1.1 to 1.3
15 The current flowing in conductors of a dc motor is
A Ac
B Dc
C Ac as well as dc
D Transient

Answer: Ac
16 The armature reaction in a dc motor is attributed to
A The effect of magnetic field set up by field current
B The effect of magnetic field set up by armature current
C The effect of magnetic field set up by back emf
D Copper losses in the armature

Answer: The effect of magnetic field set up by armature current
17 In a dc motor if the brushes are given a backward shift, then
A Commutation improves and speed decreases
B Commutation is unaffected and speed increases
C Commutation improves and speed increases
D Commutation worsens and speed decreases

Answer: Commutation improves and speed increases
18 The back emf Vb of a dc motor
A Opposes the applied voltage
B Assists the applied voltage
C Does not influence the applied voltage
D None of these

Answer: Opposes the applied voltage
19 Current normally used to excite synchronous and d.c. generators is
A D.C
B A.C. single phase
C A.C. two phase
D A.C. three phase

Answer: D.C
20 In case of a motor
A Only motor action takes place
B Motor action precedes generator action
C Generator action precedes motor action
D Both take place precedes simultaneously

Answer: Motor action precedes generator action
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