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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
View Answer
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
View Answer
23
What is the standard direction of a motor?
A
Clockwise
B
Anti-clockwise
C
None of these
D
Either (a) or (b)
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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%
View Answer
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
View Answer
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
View Answer
