## Electrical Machines Expected MCQ PDF 3

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Attempt Free Electrical Machines Expected MCQ PDF 3 Here. Read The Important Electrical MCQ From Below.

1) For the construction of the armature of a d.c. machine, the best suited material is

a. Cast iron

b. Silicon steel

c. Carbon

d. All of these

ANSWER: Silicon steel

2) In a d.c. machine, the form of armature reaction mmf is

a. Triangular

b. Sinusoidal

c. Saw tooth

d. Rectangular

ANSWER: Triangular

Electrical Machines Expected MCQ PDF 3

3) For low reluctance path for the flux in armature, the permeability of the material should be

a. High

b. Low

c. Both (a) and (b)

d. None of these

ANSWER: Low

4) In a d.c. machine, the eddy current losses mainly occurs in

a. Armature

b. Windings

c. Yoke

d. At brush contact

ANSWER: Armature

5) The conductors which will cause distortion in the main field flux of d.c. machine are

a. Demagnetizing armature conductors

b. Cross magnetizing armature conductors

c. Both (a) and (b)

d. None of these

ANSWER: Cross magnetizing armature conductors

Electrical Machines Expected MCQ PDF 3

6) If the brush shift angle is given in electrical degrees then for converting it into mechanical degree by using the relation

a. θelectrical / P

b. 2θelectrical / P

c. θelectrical / 2P

d. 2P / θelectrical

ANSWER: 2θelectrical / P

7) A 6 pole wave wound d.c. generator has 460 armature conductors carrying armature current of 180 A. The armature reaction demagnetizing and cross magnetizing ampere turns per pole if brushes are shifted by 6 degree

electrical from G.N.A are

a. 3220, 230

b. 3220, 260

c. 260, 3220

d. 230, 3220

ANSWER: 230, 3220

Electrical Machines Expected MCQ PDF 3

8) For neutralizing the demagnetizing effect of armature reaction, an extra number of winding turns are provided on each pole. To calculate number of extra turns per pole the equation used is

a. (ATd / I) * leakage coefficient

b. (ATd / I) / leakage coefficient

c. Leakage coefficient / (ATd / I)

d. (ATd / I)

ANSWER: (ATd / I) * leakage coefficient

9) Due to the effect of armature reaction in d.c. machine, the value of generated voltage and flux per pole

a. Increases, decreases

b. Decreases, decreases

c. Decreases, increases

d. Increases, increases

ANSWER: Decreases, decreases

10) The armature reaction in d.c. machine causes distortion in the main field flux. This effect of armature reaction can be reduced by

a. Increasing the length of air gap

b. Decreasing the length of air gap

c. Increasing the number of poles

d. Decreasing the number of poles

ANSWER: Increasing the length of air gap

Electrical Machines Expected MCQ PDF 3

11) Two 4 pole d.c. machines of identical armature, one with wave winding and other with lap winding are wound. Then

a. The lap wound machine will have more generated voltage

b. Wave wound machine will have more generated voltage

c. Both will have same generated voltage

d. None of these

ANSWER: Wave wound machine will have more generated voltage

12) The armature in d.c. machines are always placed on rotor

a. Otherwise commutation will not be possible

b. Otherwise there will not be any induced emf

c. Otherwise current will not flow

d. All of these

ANSWER: Otherwise commutation will not be possible

13) In a d.c. machine, the commutator provides

a. Half wave rectification

b. Full wave rectification

c. Semi controlled rectification

d. Uncontrolled rectification

ANSWER: Full wave rectification

Electrical Machines Expected MCQ PDF 3

14) To have perfect neutralization of armature mmf under the pole shoe, the ampere conductors of compensating winding must be

a. Not equal to the total armature ampere conductors under the pole shoe

b. Equal to the total armature ampere conductors under the pole shoe

c. Half of the total armature ampere conductors under the pole shoe

d. Twice of the total armature ampere conductors under the pole shoe

ANSWER: Equal to the total armature ampere conductors under the pole shoe

15) Ampere turns per pole for compensating winding in d.c. machines is equal to

a. (pole arc / pole pitch) * armature ampere turns per pole

b. (pole pitch / pole arc) * armature ampere turns per pole

c. (pole arc / pole pitch) * total ampere conductors per pole

d. None of these

ANSWER: (pole arc / pole pitch) * armature ampere turns per pole

16) The compensating windings are used to neutralize the armature reaction in d.c. machines. These windings are connected

a. Across armature

b. In series with armature

c. Both (a) and (b)

d. None of these

ANSWER: In series with armature

17) The armature reaction effect in d.c. machines can be completely neutralized by using

a. Only compensating winding

b. Only interpoles

c. Both compensating winding and interpoles

d. None of these

ANSWER: Both compensating winding and interpoles

Electrical Machines Expected MCQ PDF 3

18) Practically in d.c. machines, it may happen that the emf induced in different parallel paths may not be same. This will cause inequality in brush arm currents and will give rise to copper losses. These effects can be avoided by using

a. Compensating windings

b. Interpoles

c. Equalizer rings

d. All of these

ANSWER: Equalizer rings

19) In d.c. machines, equalizer connections are provided in

a. Only wave winding

b. Only lap winding

c. Both wave winding and lap winding

d. None of these

ANSWER: Only lap winding

20) If in the d.c machine, the reversal of current in the coil is faster than ideal or linear commutation then the commutation is said to be

a. Retarded commutation

b. Under commutation

c. Accelerated commutation

d. Ideal commutation

ANSWER: Accelerated commutation

Electrical Machines Expected MCQ PDF 3

21) In d.c. machines, the interpoles have tapering shape which results in

a. Simpler design

b. Reduction in the weight

c. Increase in acceleration of commutation

d. All of these

ANSWER: Increase in acceleration of commutation

22) The generating action and motoring action in d.c. motor is determined by

a. Fleming’s left hand rule, Fleming’s right hand rule

b. Both by Fleming’s left hand rule

c. Both by Fleming’s right hand rule

d. Fleming’s right hand rule, Fleming’s left hand rule

ANSWER: Fleming’s right hand rule, Fleming’s left hand rule

23) In a practical motor, to reverse the direction of rotation

a. Reverse the direction of main field produced by the field winding

b. Reverse the direction of current passing through the armature is reversed

c. Either (a) or (b)

d. None of these

ANSWER: Either (a) or (b)

Electrical Machines Expected MCQ PDF 3

24) A 220 V, d.c. motor draws an armature current of 20 A. Its armature resistance is 0.6 ohm. Then the induced emf in the motor will be

a. 195 V

b. 202 V

c. 208 V

d. 215 V

ANSWER: 208 V

25) If the flux is increased by 50% and speed is reduced by 50% of a d.c. motor (keeping the other parameters constant), then its back emf will become

a. 50 % of the original back emf

b. 0.75 % of the original back emf

c. 100 % of the original back emf

d. 150 % of the original back emf

ANSWER: 0.75 % of the original back emf

Electrical Machines Expected MCQ PDF 3

26) If Eb is the back emf of d.c. motor and V is the terminal voltage, then the condition for maximum power is

a. Eb = V

b. Eb = 2V

c. Eb = ( V / 2 )

d. Eb = V ∧ 2

ANSWER: Eb = ( V / 2 )

27) A 4 pole, lap wound d.c. motor drawing an armature current of 20 A has 360 conductors. If the flux per pole is 0.015 Wb then the gross torque developed by the armature of motor is

a. 10.23 N-m

b. 15.56 N-m

c. 17.17 N-m

d. 19.08 N-m

ANSWER: 17.17 N-m

28) If the no load speed of d.c. motor is 1350 rpm and full load speed is 1150 rpm, then its voltage regulation is

a. 11.56 %

b. 15.36 %

c. 17.39 %

d. 19.39 %

ANSWER: 17.39 %

Electrical Machines Expected MCQ PDF 3

29) As the load on d.c. motor increases, the current drawn by motor

a. Increases

b. Decreases

c. Remains same

d. None of these

ANSWER: Increases

30) If the back emf of a d.c. motor is doubled and flux is halved (keeping other parameters constant) then its speed will become

a. Double of the original speed

b. Square of the original speed

c. Four times of the original speed

d. Half of the original speed

ANSWER: Four times of the original speed

31) Keeping the field excitation constant, the constant of proportionality between back emf and armature speed of d.c. motor is

a. Twice of the proportionality constant between developed torque and armature current

b. Half of the proportionality constant between developed torque and armature current

c. Same as that of proportionality constant between developed torque and armature current

d. None of these

ANSWER: Same as that of proportionality constant between developed torque and armature current

Electrical Machines Expected MCQ PDF 3

32) D.C. shunt motor is also called as

a. Constant flux motor

b. Constant voltage motor

c. Variable voltage motor

d. Constant current motor

ANSWER: Constant flux motor

33) The torque developed in d.c. shunt motor is

a. Directly proportional to the armature current

b. Directly proportional to the square of the armature current

c. Inversely proportional to the armature current

d. Inversely proportional to the square of armature current

ANSWER: Directly proportional to the armature current

34) For a constant torque load, if the armature resistance of shunt motor is doubled (keeping the shunt field constant) then the armature current will

a. Double

b. Get halved

c. Remain same

d. None of these

ANSWER: Remain same

Electrical Machines Expected MCQ PDF 3

35) A 4 pole, 220 V, 10 H.P, d.c. shunt motor is rotating at a speed of 320 rpm. Then the developed useful torque is

a. 210.647 N-m

b. 215.547 N-m

c. 219.484 N-m

d. None of these

ANSWER: 219.484 N-m

36) If a shunt motor is started with its field winding open then

a. It will rotate at the same speed as that with its field winding closed

b. It will rotate at less speed as that with its field winding closed

c. It will rotate at dangerously high speed

d. None of these

ANSWER: It will rotate at dangerously high speed

37) D.C. shunt motors are commonly used in

a. Cranes

b. Electric traction

c. Elevators

d. Lathe machines

ANSWER: Lathe machines

Electrical Machines Expected MCQ PDF 3

38) For d.c. series motor, if Ia is the armature current then the torque developed is

a. α Ia

b. α Ia2

c. α (1 / Ia2)

d. α (1 / Ia)

ANSWER: α Ia2

39) Neglecting saturation, if the current drawn from by a d.c. series motor is increased from 15 A to 18 A, then the increase in the torque expressed as percentage of initial torque will be

a. 22 %

b. 33%

c. 44%

d. 56.78%

ANSWER: 44%

40) A 4 pole 240 V, d.c. series motor has a wave wound armature with 180 conductors. The armature resistance and the field winding resistance are 0.10 ohm and 0.2 ohm respectively. If the motor is drawing 40 A current at 0.015 Wb

flux per pole, then the speed under this condition is

a. 2533.3 rpm

b. 2644.4 rpm

c. 2756.9 rpm

d. 2987.6 rpm

ANSWER: 2533.3 rpm

Electrical Machines Expected MCQ PDF 3

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