Download PDF
Free download in PDF DC Machines Objective Type 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.
(21)
All electrical machines have poles with
[A]
A hetro-polar structure
[B]
A horse shoe structure
[C]
(a) above (b)
[D]
None of these
Answer: A hetro-polar structure
(22)
If back emf of d.c. motor is doubled while its speed is also doubled, then torque developed by the machine will
[A]
Remain same
[B]
Become four times
[C]
Double
[D]
Becomes half
(23)
What is the standard direction of a motor?
[A]
Clockwise
[B]
Anti-clockwise
[C]
None of these
[D]
Either (a) or (b)
(24)
A dc shunt motor is driving a mechanical load at rated voltage and rated excitation. If the load torque becomes double, then the speed of the motor
[A]
Increases slightly
[B]
Decreases slightly
[C]
Becomes double
[D]
Becomes half
Answer: Decreases slightly
(25)
In a dc shunt motor, field excitation is kept at maximum value during starting to
[A]
Increase acceleration time
[B]
Reduce armature heating
[C]
Prevent voltage dip in the supply mains
[D]
Decrease starting torque
Answer: Reduce armature heating
(26)
Two dc series motors connected in series are driving the same mechanical load. If now the motors are connected in parallel, the speed becomes
[A]
Slightly more than double
[B]
Slightly less than double
[C]
Slightly more than half
[D]
Slightly less than half
Answer: Slightly more than double
(27)
By putting a variable resistance (diverter) across the series field in a dc series motor, speed above normal can be obtained because
[A]
Armature current decreases
[B]
Flux gets reduced
[C]
Line current gets decreased
[D]
None of these
Answer: Flux gets reduced
(28)
Which of the following dc motors has the least reduction in speed from no load to rated load?
[A]
Shunt motor with commutating poles
[B]
Series motor with commutating poles
[C]
Series motor without commutating poles
[D]
Compound motor without commutating poles
Answer: Shunt motor with commutating poles
(29)
The output power of any electrical motor is taken from
[A]
The armature
[B]
The conductors
[C]
The coupling mounted on the shaft
[D]
The poles
Answer: The coupling mounted on the shaft
(30)
In a dc machine dull yellow spark indicates
[A]
Winding fault
[B]
Excessive brush wear
[C]
Excessive noise
[D]
None of these
31
If the supply voltage to a shunt motor is increased by 25%, which of the following will decrease?
[A]
Starting torque
[B]
Full load speed
[C]
Full load current
[D]
None of these
32
A dc shunt motor has external resistance R1 in the field circuit and resistance R2 in the armature circuit. The starting armature current for the motor will be minimum when
[A]
R1 is minimum and R2 is maximum
[B]
R1 maximum and R2 is minimum
[C]
Both R1 is minimum and R2 is maximum
[D]
Both R1 is minimum and R2 is minimum
Answer: R1 is minimum and R2 is maximum
33
The brush voltage drops in dc in / of the order of
[A]
2 V
[B]
10 V
[C]
20 V
[D]
40 V
34
Under constant load condition, the speed of a dc motor is effected by
[A]
Field flux alone
[B]
Armature current alone
[C]
Back emf
[D]
Both armature current and field flux
35
If the load current and flux of a dc motor are held constant and voltage applied across are held constant and voltage applied across its armature is increased by 10 per cent, its speed will
[A]
Decrease by about 10 per cent
[B]
Remain unchanged
[C]
Increase by about 10 per cent
[D]
Increase by 20 per cent
Answer: Increase by about 10 per cent
36
If the field circuit of a loaded shunt motor s suddenly opened
[A]
It slow down
[B]
It would draw extremely high armature current
[C]
Its speed becomes dangerously high
[D]
Torque developed by the motor would be reduced to zero
Answer: Its speed becomes dangerously high
37
The speed of a dc shunt motor is required be more than full load speed. This may be achieved by
[A]
Increasing the armature current
[B]
Decreasing the armature current
[C]
Increasing the excitation current
[D]
Reducing the field current
Answer: Reducing the field current
38
For a dc motor operating under condition of maximum transfer of power, the efficiency of the motor is
[A]
100%
[B]
About 90%
[C]
75%
[D]
Less than 50%
39
Which of the following dc motors will have least percentage increase in input current for a given percentage increase in torque?
[A]
Series motor
[B]
Shunt motor
[C]
Separately excited
[D]
Cumulatively compound motor
40
In a dc series motor, the shaft torque is less than the armature torque due to
[A]
Eddy current losses
[B]
Stray losses
[C]
Hysteresis losses
[D]
Copper losses
Please share this page