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Free download in PDF DC Machines Multiple Choice Questions and Answers for competitive exams. These short objective type questions with answers are very important for competitive exams like IIT-JEE, NEET, AIIMS, JIPMER etc. These short solved questions or quizzes are provided by Gkseries.
(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
(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
(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)
(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
(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
(10)
The starting resistance of a dc motor is usually
[A]
Infinitely large
[B]
Large
[C]
About 100Ω
[D]
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
(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
(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
(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
(15)
The current flowing in conductors of a dc motor is
[A]
Ac
[B]
Dc
[C]
Ac as well as dc
[D]
Transient
(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
(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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