eee unit 1 dc machines mcq's
TRANSCRIPT
EEE Unit-1 DC Machines MCQ's
MULTIPLE CHOICE QUESTIONS ON UNIT 1: DC MACHINES
Edit
1.The sole purpose of a Commutator in a dc generator is to-------
((A))increase output voltage
((B)) reduce sparking at brushes
((C)) provide smoother output
((D)) convert the induced ac into dc
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Answer : D
2.In a dc generator, the generated emf is directly proportional to the------
((A)) field current
((B)) pole flux
((C)) number of armature parallel paths
((D)) number of dummy coils
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Answer:B
3.An ideal dc generator has .......... voltage regulation.
((A))low
((B))zero
((C))positive
((D))negative
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Answer:B
4.Which generator may have poorest voltage regulation?
((A))series
((B))shunt
((C))compound
((D))high
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Answer:A
5.Voltage equation of a dc motor is-----
((A)) V = Eb + IaRa
((B)) Eb = V + IaRa
((C))V = Eb /IaRa
((D))V = Eb + Ia2Ra
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Answer: A
6.Which of the following motor has the constant speed?
((A))Series motor
((B))Shunt motor
((C))Cumulatively compound motor
((D))All of the above
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Answer:B
7.A DC series motor is best for driving-----
((A)) Lathes
((B))Cranes and hoists
((C))Shears and punches
((D)) Machine tools
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Answer:B
8. In a DC series motor increasing the load current will-------
((A))Decrease the speed
((B)) Increase the speed
((C))Better commutation
((D)) Increase the back emf
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Answer:A
9. The DC motor, which can provide minimum possible speed regulation at full load without any controller is -------
((A))Series
((B))Shunt
((C))Cumulative Compound
((D)) Differential Compound
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Answer:B
10. In 8 - pole wave connected motor armature, the number of parallel paths are------
((A))8
((B)) 4
((C)) 2
((D))1
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Answer:C
11. The d.c. series motor should always be started with load because______
((A)) at no load, it will rotate at dangerously high speed.
((B)) it will fail to start.
((C)) it will not develop high starting torque.
((D)) all are true.
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Answer:A
12. In a d.c. machine, the armature mmf is--------------
((A))stationary w.r.t. armature.
((B)) rotating w.r.t. field.
((C)) stationary w.r.t. field.
((D)) rotating w.r.t. brushes.
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Answer:C
13. The speed of a d. c series motor at no load is-------------
((A)) zero
((B))dangerously high
((C)) very low
((D))can’t be said
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Answer:B
14. The function of commutator in a d. c machine is------
((A)) to change direction of rotation
((B))to change alternating voltage to direct voltage
((C)) to provide easy control of voltage
((D)) to protect the machine from overcurrents
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Answer:B
15. A sinusoidal voltage of frequency 1 Hz is applied to the field of a d.c generator. The armature voltage will be-----------
((A))1 Hz square wave
((B))1 Hz sine wave
((C)) d. c voltage
((D)) zero
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Answer:B
16. The direction of rotation of a dc series motor can be reversed----------
((A)) by interchanging the supply terminals
((B)) by interchanging the field terminals
((C)) either option A or option B .
((D)) none of the above
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Answer:B
17. A dc shunt generator driven at normal speed in the normal direction fails to build up armature voltage because------
((A)) the armature resistance is high
((B)) the field current is not sufficiently high
((C)) there is no residual magnetism.
((D)) the field resistance is low
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Answer:C
18. If a dc motor is connected across an ac supply, the dc motor will-----------
((A)) burn as the eddy currents in the field produces heat
((B)) run at its normal speed
((C)) run at a lower speed
((D)) can’t be said
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Answer:A
19. What would happen if the field of dc shunt motor is opened?
((A))the speed of the motor will be reduced
((B)) it will run at its normal speed
((C)) the speed will be enormously high , and the motor will destroy itself
((D))the motor will come to hault
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Answer:C
20. Which one of the following motors is used in a locomotive?
((A)) Induction motor
((B))DC series motor
((C)) Synchronous motor
((D))Shaded pole motor
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Answer:B
21.The dummy coil in the dc machine is used to------------
((A))eliminate armature reaction
((B)) bring about mechanical balance of armature
((C)) eliminate reactance voltage
((D))reduce wear and tear
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Answer:B
22.The mechanical power developed by the dc motor is maximum when-------
((A))back emf = applied voltage
((B)) back emf = 0
((C)) back emf = 0.5 times the applied voltage
((D)) at the starting of motor
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Answer:C
23.The armature torque of the dc shunt motor is proportional to------
((A)) armature current only
((B)) field flux only
((C)) armature current and field flux both
((D)) none of the above
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Answer:C
24. The reversal of speed of motors used as electric braking is known as----
((A)) regenerative braking
((B)) plugging
((C)) dynamic braking
((D)) static braking
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Answer:B
25. The output power of any electrical motor is taken from the--------
((A)) Armature
((B))Field
((C))Coupling mounted on the shaft
((D)) Motor frame
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Answer:C
26. If the back emf in a dc motor varnishes suddenly, then the motor------
((A)) Run at very high speeds
((B)) Start hunting
((C)) may burn
((D)) come to stall
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Answer:C
27. The direction of the armature current in a dc motor is--------------
((A))The same as the generated emf
((B)) Opposite to that of generated emf
((C)) Not dependent upon the direction of the armature emf
((D)) None of the above
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Answer:B
28. When a dc machine is connected to the dc supply mains it will produce-----
((A)) EMF in opposition to the applied voltage
((B))EMF in phase with the applied voltage
((C)) EMF decreases with time
((D)) None of the above
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Answer:A
29. The direction of rotation of a dc motor can be determined by-----
((A)) Fleming's right hand rule
((B)) Fleming's left hand rule
((C)) Lenz's law
((D)) Ampere's law
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Answer:B
30. A thick wire is used in dc series motor field winding than that in dc shunt motor------
((A)) To create more flux
((B)) To reduce the resistance
((C)) To carry large load current
((D)) Both (b) and (c)
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31. An electric train employing a dc series motor is running at the fixed speed. When a sudden slight drop in the mains voltage occurs, this would result in------
((A)) Drop in the speed and rise in the current
((B)) Rise in the speed and drop in the current
((C)) Rise in the speed and rise in the current
((D)) Drop in speed with current unaltered
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Answer:A
32. A DC shunt motor is driving a constant torque load. On inserting an additional resistance in the armature circuit, the speed of the motor will-----
((A)) Remains unchanged
((B)) Increases
((C)) Decreases
((D)) Be Zero
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Answer:C
Show Answer
33. A DC series motor is best suited for electrical traction because-----------
((A)) It develops high starting torque
((B)) It runs at very high speed
((C)) It is cheaper in cost
((D)) It is more rugged in construction like induction motor
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Answer:A
34. A DC series motor is used for overhauling load. It can work stable if------
((A))The armature is shunted by a resistor
((B))The field winding is reversed
((C)) A resistor is put in series with the machine
((D)) A divertor is put across the field
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Answer:A
35. Which of the following dc motors has maximum self relieving property?
((A))Shunt
((B)) Series
((C))Cumulative compound
((D)) Differential compound
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Answer:B
36. When the direction of power flow in a differential compounded motor reverses, it will operate as a---------
((A)) Cumulative compound generator
((B)) Differential compound generator
((C)) Series motor
((D)) Shunt motor
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Answer:A
37. A separately excited dc generator is feeding dc shunt motor. If the load torque on the motor is halved approximately, then------
((A)) Armature current of both motor and generator are halved
((B)) Armature current of motor is halved and that of the generator is unaltered
((C)) Armature current of generator is halved and that of the motor is unaltered
((D)) Armature current of both motor and generator are unaltered
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Answer:A
38. A flywheel is employed with a dc compound motor to reduce the peak demand, the compound motor should be--------
((A)) Level compound
((B)) Differential compound
((C)) Cumulative compound
((D)) None of the above
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Answer:C
39. The flux leakage coefficient in dc machine is usually-------
((A)) 0.6 to 0.8
((B)) 1.05
((C)) 1.1 to 1.3
((D)) 2 to 2.5
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Answer:C
40. The d.c. series motor should always be started with load because----
((A)) at no load, it will rotate at dangerously high speed.
((B)) it will fail to start.
((C)) it will not develop high starting torque.
((D)) all are true.
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Answer:A
41. The speed of a dc motor can be controlled by varying_________
((A))its flux per pole
((B))resistance of armature circuit
((C))applied voltage
((D)) all of the above
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42. In a DC motor, unidirectional torque is produced with the help of________
((A))Brushes
((B)) Commutator
((C)) End-plates
((D)) Both a and b
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43. The counter emf of dc motor__________
((A)) Often exceeds the supply voltage
((B))Aids the applied voltage
((C))Helps in energy conversion
((D)) Regulates its armature voltage
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Answer:C
44. The \frac{Eb}{V} ratio of a dc motor is an indication of its------------
((A))Efficiency
((B))Speed regulation
((C))Starting torque
((D)) Running Torque
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Answer:A
45. The induced emf in the armature conductors of a dc motor is---------
((A))Sinusoidal
((B)) Trapezoidal
((C)) Rectangular
((D)) Alternating
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Answer:A
46. A dc motor can be looked upon as dc generator with the power flow__________
((A)) Reduced
((B)) Reversed
((C)) Increased
((D)) Modified
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Answer:B
47. A series motor is best suited for driving------
((A))Lathes
((B)) Cranes and hoists
((C))Shears and punches
((D)) Machine tools
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Answer:B
48. The \frac{Ta}{Ia} graph of a dc series motor is a___________-
((A))Parabola from no load to overload
((B))Straight line throughout
((C))Parabola throughout
((D)) Parabola upto full load and a straight line at overloads
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Answeremoticon_grin
49. When load is removed, .......motor will run at the highest speed.
((A))Shunt
((B)) Cumulative - compound
((C))Differential compound
((D)) Series
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Answeremoticon_grin
50. In a d.c. series motor the electromagnetic torque developed is proportional to________
((A)) Ia
((B)) Ia2
((C))Ia^{3}
((D))\sqrt{Ia}
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Answer:B
51. The armature core of a dc machine is laminated to reduce----
((A)) Eddy current loss
((B)) Hysteresis loss
((C)) copper losses
((D)) friction and windage losses
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Answer:A
52. A drawback of a dc series motor is its---------
((A)) low starting torque
((B))constant speed
((C))speed runaway
((D))high starting torque
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Answer:C
53. If the back emf in a dc motor vanishes suddenly, then the motor__________
((A))run at very high speeds
((B)) start hunting
((C)) will burn
((D))come to stall
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Answer:C
54. The direction of the armature current in a dc motor is___________
((A)) The same as the generated emf
((B))Opposite to that of generated emf
((C)) Not dependent upon the direction of the armature emf
((D)) None of the above
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Answer:B
55. When a dc machine is connected to the dc supply mains, it will produce__________
((A)) EMF in opposition to the applied voltage
((B)) EMF in phase with the applied voltage
((C)) EMF decreases with time
((D)) None of the above
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Answer:A
56. The speed of a d.c. motor may be increased by_____________
((A)) increasing the armature current
((B)) decreasing the field current
((C)) Decreasing the applied voltage
((D)) increasing the field current
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Answer:B
57. The armature resistance of a d.c. shunt motor is 0.5 ohm, the supply voltage is 200V and the armature current at start can be__________
((A)) 4A
((B)) 8 A
((C)) 400 A
((D)) 392 A
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Answer:C
58. In d.c. generators iron losses are made up of__________
((A)) hysteresis and friction losses
((B)) hysteresis, eddy current and brush contact losses
((C)) hysteresis and eddy current losses
((D)) hysteresis, eddy current and copper Losses
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Answer:C
59. The effect of inserting a resistance in series with the field winding of a shunt motor is to__________
((A)) increase the magnetic field
((B)) increase the speed of the motor
((C))to reduce power losses
((D))to increase efficiency of motor
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Answer:B
((Q))_2// In a dc shunt motor; the terminal voltage is halved while the torque is kept constant. The resulting approximate variation in speed 'N' and armature
current ' Ia ' will be
((A)) Both N and Ia are doubled.
((B)) N is constant and Ia is doubled.
((C)) N is doubled while Ia is halved.
((D)) N is constant but Ia is halved.
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Answer:B
60. A dc shunt generator has a speed of 800 rpm when delivering 20 A to the load at the terminal voltage of 220V. If the same machine is run as a motor it takes
a line current of 20A from 220V supply. The speed of the machine as a motor will be___________
((A)) 800 rpm.
((B)) more than 800 rpm.
((C)) less than 800 rpm.
((D)) both higher or lower than 800 rpm.
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Answer:C
61. A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of 0.4 mWb and the total number of conductors are 1000. What is the value
of emf?
((A)) 100 Volts.
((B)) 0.1 Volts.
((C)) 1 Volts.
((D)) 10 Volts.
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Answeremoticon_grin
62. The torque, in a _____________is proportional to the square of the armature current.
((A)) DC shunt motor
((B)) Induction motor
((C)) AC motor
((D)) DC series motor
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Answeremoticon_grin
63. A commutator in a d.c. machine___________
((A)) reduces power loss in armature.
((B)) reduces power loss in field circuit.
((C)) converts the induced a.c armature voltage into direct voltage.
((D)) is not necessary.
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Answer:C
64. The speed of a d.c. shunt motor at no-load is________ higher than its speed at rated load.
((A)) 5 to 10%
((B)) 15 to 20%
((C)) 25 to 30% (
((D)) 35 to 40%
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Answer:A
((Q))A d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load, armature reaction weakens the main flux by 5% whereas the armature circuit voltage
drops by 10%. The motor full load speed in r.p.m. is____________
((A)) 1080
((B)) 1203
((C)) 1000
((D)) 1200
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Answer:A
((Q))The introduction of interpoles in between the main pole improves the performance of d.c. machines, because__________
((A)) The interpole produces additional flux to augment the developed torque.
((B)) The flux waveform is improved with reduction in harmonics.
((C)) The inequality of air flux on the top and bottom halves of armature isremoved.
((D)) A counter e.m.f. is induced in the coil undergoing commutation.
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((Q))In DC generators, armature reaction is produced actually by_________
((A)) Its field current.
((B)) Armature conductors.
((C)) Field pole winding.
((D)) Load current in armature.
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((Q))As compared to shunt and compound DC motors, the series DC motor will have the highest torque because of its comparatively ____________ at the start.
((A)) Lower armature resistance.
((B)) Stronger series field.
((C)) Fewer series turns.
((D)) Larger armature current.
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Answeremoticon_grin
((Q))The armature of a dc machine is laminated to reduce:
((A)) Eddy current loss
((B)) Hysteresis loss
((C)) copper losses
((D)) friction and windage losses
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Answer:A
((Q))Application of DC motors is generally restricted to a few load speed applications because of _____.
((A)) cost of the motor is high
((B)) problems with mechanical commutation
((C)) Maintenance problems
((D)) None of the above
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Answer:B
((Q)) Which of the following information available on name plate of a motor?
((A)) HP rating
(( B)) RPM
((C )) Frame model
((D)) all the above
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(Q)) Machine tools are a typical example of____________.
((A)) Constant power load
((B)) Constant torque load
((C)) Variable torque load
((D)) Both A and B
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((Q)) In Fleming's right-hand rule, the thumb point towards________
((A)) direction of flux
((B)) direction of induced e.m.f.
((C))direction of motion of conductor, if forefinger points along the lines of flux
((D) direction of motion of the conductor if forefinger point in the direction of generated e.m.f.
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Answer:C
((Q)) The resistance of armature winding depends on________
((A)) number of conductors
((B)) number of poles
((C)) cross-sectional area of conductor.
((D)) all of above
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Answer:C
((Q)) The output voltage of a simple d.c. generator is____________
((A)) a.c. square wave.
((B)) a.c. sinusoidal wave
((C)) pure d.c,
((D)) pulsating d.c.
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((Q)) In shunt generator interpole winding carries,_________
((A)) armature current
((B)) shunt field current
((C)) full load current
((D)) none of these
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((Q)) Due to which of the following reasons a separately excited generator as compared to a self-excited generator is better?
((A)) is more stable
((B)) has exciting current independent of load current
((C)) is amenable to better voltage
((D)) has all above features.
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Answeremoticon_grin
((Q)) Due to which of the following reasons, copper brushes in D.C. machine are used?
((A)) when high voltage and small currents are involved
((B)) when low voltage and high currents are involved
((C)) in both of the above cases
((D) in none of the above cases.
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Answer:C
((Q)) To maintain constant d.c. voltage at the consumers' terminals the generator used is______.
((A)) series generator
((B)) over compounded generator
((C)) under compounded generator
((D)) flat compound generator
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Answeremoticon_grin
((Q))A four pole lap wound d.c. shunt generator is supplying 46 A to a load. The field current of the generator is 2A. The current per parallel path is___________
((A)) 12 A
((B)) 24 A
((C)) 24 A
((D)) none of these
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Answer:A
((Q))What would be observed if a d.c. shunt motor is started with an open-circuited field?
((A)) The motor pick up fast and acquires full speed while drawing small current
((B)) The motor picks up fast and acquires full speed while drawing large current
((C)) The motor does not pick up speed but draws is large current
((D)) The motor does not pick up speed but draws a small current
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Answer:B
((Q))When the series field is so connected that its ampere turns act in the same direction as those of shunt field, the generator is said to be__________
((A)) series generator
((B)) Shunt generator
((C)) cumulatively compound generator
((D)) differentially compound generator
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Answer:C
((Q))What is the nature of a d.c. machine?
((A)) pure d.c.
((B)) alternating
((C)) pulsating d.c.
((D)) rectified a.c.
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((Q)) Why is the armature of a d.c. machine made of silicon steel stampings?
((A)) to reduce eddy current loss
((B)) to reduce hysteresis loss.
((C)) for the ease with which the slots can be created
((D)) Both A and B
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((Q)) Due to which of the following reasons a twelve pole, lap wound d.c. generator be preferred to four-pole lap wound d.c. generator?
((A)) low speed.
((B)) high speed
((C)) higher voltage
((D)) higher current output.
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Answeremoticon_grin
((Q))The armature coils of a d.c. machine are not made of aluminium, because________
((A)) aluminium is costly
((B)) aluminium has low resistivity
((C)) the thermal conductivity of aluminium low
((D)) the size of the machine will become more
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Answeremoticon_grin
((Q))Brushes of D.C. machines are usually made of__________.
((A)) hard copper
((B)) soft copper
((C)) carbon
((D)) all of above
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Answer:C
(Q)) In the output voltage of a d.c. generator the ripple effects can be reduced by increasing the__________.
((A)) field current
((B)) number of armature coils .
((C)) number of turns in the field winding
((D)) size of the conductor of the armature coil
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Answer:B
((Q))Which of the following factors does not govern the iron losses in a D.C. machine?
((A)) load.
((B)) voltage
((C)) speed
((D)) speed and voltage
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Answer:A
((Q))The armature core of a d.c. machine is usually made of laminated sheets in order to
((A)) reduce hysteresis loss
((B)) reduce armature copper losses.
((C)) reduce eddy current
((D)) increase its surface area for better dissipation of heat
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Answer:C
((Q))The polarity of the interpoles in d.c. generator_________.
((A)) is neutral as these poles do not play part in generating e.m.f.
((B)) is the same as that of the main pole ahead
((C)) is the same as that of the immediately preceding pole
((D)) is opposite to that of the main pole ahead
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Answeremoticon_grin
((Q)) With given power rating for lower current and higher voltage rating of a d.c. machine, one should prefer_________.
((A)) lap winding
((B)) wave winding
((C)) none of these
((D)) any of these
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Answer:B
((Q)) Compound generator is said to be flat compounded if the full load voltage is
((A)) more than the no-load voltage
((B)) less than the no-load voltage
((C)) exactly half of the no-load voltage,
((D)) equal to no-load voltage
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Answeremoticon_grin
((Q)) The decrease in terminal voltage of a shunt generator from, no load condition is due to_________.
((A)) decrease in field current
((B)) armature reaction
((C)) armature resistance drop
((D)) all of these
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Answeremoticon_grin
((Q)) Commutation in a d.c. generator causes___________.
((A)) d.c. changes to a.c.
((B)) d.c. changes to d.c.
((C)) a.c. changes to d.c.
((D)) changes to high voltage d:c.
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Answer:C
((Q)) In D.C. machines lap winding is used for___________
((A)) low voltage, low current
((B)) low voltage, high current
((C)) high voltage, low current
((D)) high voltage, high current
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Answer:B
((Q)) The yoke of a d.c. machine is made up of____________
((A)) copper-zinc alloy
((B)) insulating material
((C)) comparatively poor magnetic material
((D)) very high permeability magnetic material
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Answeremoticon_grin
((Q)) The commutator segments of a d.c. machine are usually made of________
((A)) carbon
((B)) iron
((C)) hard drawn copper
((D)) stainless steel
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Answer:C
((Q)) The e.m.f. generated in the armature of d.c. generator is directly proportional to
((A)) number of poles
((B)) speed of armature
((C)) flux/pole
((D)) all of the above
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Answeremoticon_grin
((Q)) In the armature, D.C. generator induces__________.
((A)) oscillating e.m.f.
((B)) a.c. voltage,
((C)) a.c. superimposed over d.c.
((D)) d.c. voltage
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Answer:B
((Q)) Which type of d.c. generator is used to charge the batteries?
((A)) shunt generator
((B)) series generator
((C)) long shunt compound generator
((D)) any of the-above
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Answer:A
((Q)) If the the number of poles in a wave wound generator is doubled then the generated e.m.f. will__________.
((A)) become half
((B)) become double
((C)) increase to four times
((D)) remain constant
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Answer:B
((Q)) If the number of poles in a lap wound generator be doubled, then the generated e.m.f will_________.
((A)) become half
((B)) become double
((C)) increase to four times
((D)) remain constant
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Answeremoticon_grin
((Q)) Which of the following d.c. generator can build up without any residual magnetism in the places ?
((A)) compound generator
((B)) separately excited generator
((C)) series generator
((D)) shunt generator
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Answer:B
((Q)) The polarity of a D.C. generator can be reversed by___________
((A)) increase field current
((B)) reversing the field current
((C)) reversing field current as well as direction of rotation
((D)) any of the above
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Answer:B
((Q)) In d.c. machine, interpoles are used to_________
((A)) generate more e.m.f. in the armature
((B)) avoid interference of the armature flux with the main flux
((C)) increase the demagnetising effect of armature reaction
((D)) neutralise the effect of armature reaction in the interpole region
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Answer:A
((Q)) In D.C. generators, current to the external circuit from armature comes out from
((A)) slip rings
((B)) commutator
((C)) brush connection
((D)) none of above
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Answer:C
((Q)) The material for commutator is generally__________
((A)) carbon
((B)) mica
((C)) copper
((D)) cast iron
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Answer:C
((Q)) The sparking at the brushes of a d.c. generator is due to________.
((A)) reactance voltage
((B)) light load
((C)) armature reaction
((D)) high resistance of the brushes
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Answer:A
((Q)) While pole flux remains constant, if the speed of the generator is doubled, the e.m.f. generated will be_________.
((A)) half
((B)) twice
((C)) nominal value
((D)) slightly less than nominal
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Answer:B
((Q)) Which would have the highest percentage of voltage regulation?
((A)) series generator
((B)) shunt generator
((C)) compound generator
((D)) separately excited generator
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Answer:A
((Q)) The efficiency of a d.c. machine is maximum when__________.
((A)) variable losses are equal to constant losses
((B)) stray losses are equal to copper losses
((C)) field copper losses are equal to armature copper losses
((D)) magnetic losses are equal to windage loss
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Answer:A
((Q)) The efficiency of a d.c. shunt generator becomes maximum when the__________
((A)) stray losses are equal to constant losses
((B)) armature copper losses are equal to constant losses
((C)) magnetic losses are equal to mechanical losses
((D)) field copper losses are equal to constant losses
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Answer:B
((Q)) Series field resistance of a d.c. series generator is about__________
((A)) 0.1 Ω
((B)) 2.5 Ω
((C)) 10 Ω
((D)) 200 Ω
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Answer:A
((Q)) Which of the following d.c. motor has approximately constant speed? ,
((A)) shunt motor
((B)) series motor
((C)) cumulatively compound motor
((D)) all of the above
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Answer:A
((Q)) If the supply voltage in a shunt motor is increased, which of the following will decreased?
((A)) full load current
((B)) full load speed
((C)) starting torque
((D)) none of the above
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((Q)) Which device changes the alternating e.rn.f. generated by the d.c. generator in its armature coil, to d.c. ?
((A)) slip ring
((B)) rectifier
((C)) rotary conventor
((D)) commutator
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((Q)) If the d.c. series motor is started at no load, it will_________.
((A)) run at very high r.p.m.
((B)) run with.excessive noise
((C)) burn out
((D)) not start
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Answer:A
((Q)) A shunt generator supplies 9A of load current at 400 V. If its armature resistance is 1 Ω and field current is 1 A, then the generated e.m.f. will
be________
((A)) 400 Volts.
((B)) 409 Volts
((C)) 410 Volts
((D)) none of these
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Answer:C
((Q)) Stray losses in d.c. machine are_________
((A)) windage loss
((B)) magnetic losses
((C)) mechanical losses.
((D)) all of these
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((Q)) Which of the following type of d.c. motor is least used?
((A)) series motor
((B)) differentially compound motor
((C)) permanent magnet motor
((D)) cumulative compound motor
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Answer:B
((Q)) Which of the following parts of a d.c. motor can sustain the maximum temperature rise?
((A)) slip rings
((B)) commutator
((C)) field windings
((D)) armature windings
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Answer:B
((Q)) In a d.c. machine constant losses composed of
((A)) mechanical losses
((B)) commutator losses
((C)) iron losses and mechanical losses
((D)) total copper losses of the machine
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Answer:C
((Q)) In a d.c. machine variable losses are composed of
((A)) iron losses
((B)) mechanical losses
((C)) shunt field copper losses
((D)) armature copper losses
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((Q)) If the resistance of the field winding of a D.C. generator is increased, the output voltage will____________
((A)) fluctuate heavily
((B)) remain unaffected
((C)) increase
((D)) decrease.
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((Q)) Brushes in d.c. machines are always placed along___________.
((A)) geometrical neutral axis (GNA)
((B)) magnetic netural axis (MNA)
((C)) (a) and (b) both
((D)) none of the above
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Answer:B
((Q)) In the commutation process___________
((A)) voltage is revised
((B)) current is reversed in the field coil
((C)) current is reversed in the armature coil undergoing commutation
((D)) both Armature current and voltage are reversed
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Answer:C
((Q)) The load current of a d.c. series motor is 50 A, when supplying of a full load torque. If the current is reduced to 25 A, the torque will be_________
((A)) 25% of full load torque
((B)) 50% of full load torque
((C)) 15% of full load torque
((D)) same as full load torque
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Answer:A
((Q)) What is the function of commutator of the d.c. motor?
((A)) to reduce friction
((B)) to convert a.c. into d.c.
((C)) to convert d.c. into a.c
((D)) to feed the current to the armature
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((Q)) Which of the following loads normally start at rated torque?
((A)) crane
((B)) centrifugal pumps
((C)) industrial blowers
((D)) conveyors and printing presses
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Answer:C
((Q)) A punching machine, having intermittent light and heavy loads, should have
((A)) shunt motor
((B)) series motor.
((C)) differentially compound motor
((D)) cumulatively compound motor
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((Q)) The Voltage between commutator segments should not exceed than
((A)) 2 volts
((B)) 15 volts
((C)) 50 volts
((D)) 200 volts
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Answer:A
((Q)) On increasing the load on d.c. series generator its terminal voltage will________
((A)) increase
((B)) decrease slightly
((C)) decrease sharply
((D)) remain same.
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Answer:C
((Q)) What is the flux in the armature core section of a dc machine if the air gap flux be ø Wb?
((A)) ø
((B)) 0.1 ø
((C)) 1.5 ø
((D)) ø /2
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Answer:A
((Q)) Full load speed of a d.c. motor is 1000 rpm and speed regulation being 90%, no load speed will be_________
((A)) 900 r.p.m.
((B)) 1000 r.p.m.
((C)) 1100 r.p.m.
((D)) none
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((Q)) Yoke of a d.c. generator is made of______.
((A)) wood
((B)) copper
((C)) cast iron
((D)) aluminium
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Answer:C
((Q)) A d.,c. shunt motor, running at light load, if the field winding gets opened, what will happen ?
((A)) motor will stop instantly
((B)) motor will take heavy input current
((C)) motor will make mechanical noise
((D)) motor will tend to pick up high speed
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((Q)) A d.c. series motor__________
((A)) a ways runs at constant speed
((B)) should always be started on load
((C)) may stop if the field gets opened
((D)) is not suitable for high starting torque
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Answer:B
((Q)) Which of the following components is a must for d.c. motor?
((A)) armature
((B)) commutator
((C)) slipping
((D)) type of winding
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Answer:B
((Q)) Which type of d.c. motor is preferred for cranes end hoists?
((A)) series motor
((B)) shunt motor
((C)) cumulatively compound motor
((D)) any of the above
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Answer:A
((Q)) Which of the following regulation is preferred for d.c. generator?
((A)) 50% regulation
((B)) 100% regulation
((C)) very low regulation
((D)) infinite regulation
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Answer:C
((Q)) Armature resistance of a d.c. machine is about
((A)) 0.1 Ω
((B)) 25 Ω
((C)) 200 Ω
((D)) none of these
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Answer:A
((Q)) The resistance of Shunt winding of a d.c. machine may be about________
((A)) 0.1 Ω
((B)) 1 Ω
((C)) 250 Ω
((D)) none of these
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Answer:C
((Q)) In shunt motor, armature current is equal to__________
((A)) load current
((B)) the sum of motor current and field current
((C)) the difference of motor current and field current
((D)) both motor and field current
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Answer:C
((Q)) A series motor has________
((A)) widely varying speed
((B)) approximately constant speed at all loads
((C)) low speed at high loads and high speed at low loads
((D)) high speed at 'high loads add low speed at low loads
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Answer:A
((Q)) What is the current drawn by a 220 V d.c. motor of armature resistance 0.5 Ohm and back e.m.f. 200 V?
((A)) 4 A
((B)) 20 A
((C)) 40 A
((D)) 110 A
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Answer:C
Answer:C
((Q)) On which of the following principle does the d.c. motor work?
((A)) corkscrew rule
((B)) right hand thumb rule
((C)) Fleming's left hand rule
((D)) Fleming’s right hand rule
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Answer:C
((Q)) If residual magnetism is not present in a d.c. generator, the induced e.m.f. at zero speed_________
((A)) zero
((B)) unpredictable
((C)) 10% rated voltage
((D)) the same as rated voltage
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Answer:A
((Q)) The armataure resistance of a 200 V d.c. machine is 0.5 ohm. If the full load armature current is 50 A, what will be the induced e.m.f. when, the machine
acts as information generator (ii) motor will be__________
((A)) 230 V, 170 V
((B)) 210 V, 190 V
((C)) 225 V, 175 V
((D)) 202.5 V, 197.5 V
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Answer:C
((Q)) Which motor will have least percentage increase of input current for the same percentage increase in torque?
((A)) shunt motor
((B)) series motor
((C)) cumulatively compound motors
((D)) separately excited motor
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Answer:B
((Q)) Which of the following information available on name plate of a motor?
((A))HP rating
((B)) Frame model
((C)) RPM
((D)) All of the above
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((Q)) The d.c. series motor should always be started with load because
((A)) at no load, it will rotate at dangerously high speed.
((B)) it will fail to start.
((C)) it will not develop high starting torque.
((D)) all are true.
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Answer:A
((Q)) In a d.c. machine, the armature mmf is
((A)) stationary w.r.t. armature.
((B)) rotating w.r.t. field.
((C)) stationary w.r.t. field.
((D)) rotating w.r.t. brushes.
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Answer:C
((Q)) A dc shunt generator has a speed of 800 rpm when delivering 20 A to the load at the
terminal voltage of 220V. If the same machine is run as a motor it takes a line current of 20A from 220V supply. The speed of the machine as a motor will
be_________.
((A)) 800 rpm.
((B)) more than 800 rpm.
((C)) less than 800 rpm.
((D)) both higher or lower than 800 rpm.
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Answer:C
((Q)) A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of 0.4mWb and the total number of conductors are 1000. What is the value
of emf?
((A)) 100 Volts.
((B)) 0.1 Volts.
((C)) 1 Volts.
((D)) 10 Volts.
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((Q)) The torque, in a _____________is proportional to the square of the armature current.
((A)) DC shunt motor
((B)) DC series motor
((C)) Separately excited motor
((D)) Differential compound motor
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Answer:B
((Q)) A commutator in a d.c. machine___________.
((A)) Reduces power loss in armature.
((B)) Reduces power loss in field circuit.
((C)) Converts the induced a.c armature voltage into direct voltage.
((D)) Is not necessary.
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Answer:C
((Q)) The speed of a d.c. shunt motor at no-load is__________ higher than its speed at rated load.
(A) 5 to 10%
(B) 15 to 20%
(C) 25 to 30%
(D) 35 to 40%
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Answer:A
((Q)) A d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load, armature reaction weakens the main flux by 5% whereas the armature circuit voltage
drops by 10%. The motor full load speed in r.p.m. is__________.
((A)) 1080
((B)) 1203
((C)) 1000
((D)) 1200
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Answer:A
((Q)) In DC generators, armature reaction is produced actually by_________.
(A) Its field current.
(B) Armature conductors.
(C) Field pole winding.
(D) Load current in armature.
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((Q)) As compared to shunt and compound DC motors, the series DC motor will have the
highest torque because of its comparatively ____________ at the start.
((A)) Lower armature resistance.
((B)) Stronger series field.
((C)) Fewer series turns.
((D)) Larger armature current.
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