DC Machines Quiz | DC Machines Objective Type Questions and Answers

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Questions
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

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

Answer: Anti-clockwise
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

Answer: 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

Answer: Full load speed
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

Answer: 2 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

Answer: Field flux alone
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%

Answer: 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

Answer: Series 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

Answer: Stray losses

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