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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
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