DC Machines Questions And Answers|DC Machines MCQ

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D.C. GENERATOR MCQ’s

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Laminations of core are generally made of
(a) cast iron (b) carbon
(c) silicon steel (d) stainless steel.
Which of the following could be approximately the thickness of laminations of a D.C. machine ?
(a) 0.005 mm (b) 0.05 mm
(c) 0.5 mm (d) 5 mm.
The armature of D.C. generator is laminated to
(a) reduce the bulk (b) provide passage for cooling air
(c) insulate the core (d) reduce eddy current loss.
The resistance of armature winding depends on
(a) length of conductor (b) cross-sectional area of the conductor
(c) number of conductors (d) all of the above.
The field coils of D.C. generator are usually made of
(a) mica (b) copper
(c) cast iron (d) carbon.
The commutator segments are connected to the armature conductors by means of
(a) copper lugs (b) resistance wires
(c) insulation pads (d) brazing.
In a commutator
(a) copper is harder than mica (b) mica and copper are equally hard
(c) mica is harder than copper (d) none of the above.
In D.C. generators the pole shoes are fastened to the pole core by
(a) rivets (b) counter sunk screws
(c) brazing (d) welding.
According to Fleming’s right-hand rule for finding the direction of induced e.m.f., when middle finger
points in the direction of induced e.m.f., forefinger will point in the direction of
(a) motion of conductor (b) lines of force
(c) either of the above (d) none of the above.
Fleming’s right-hand rule regarding direction of induced e.m.f., correlates
(a) magnetic flux, direction of current flow and resultant force
(b) magnetic flux, direction of motion and the direction of e.m.f. induced
(c) magnetic field strength, induced voltage and current
(d) magnetic flux, direction of force and direction of motion of conductor.
While applying Fleming’s right-hand rule to find the direction of induced e.m.f., the thumb points towards
(a) direction of induced e.m.f.
(b) direction of flux
(c) direction of motion of the conductor if forefinger points in the direction of generated e.m.f.
(d) direction of motion of conductor, if forefinger points along the lines of flux.
The bearings used to support the rotor shafts are generally
(a) ball bearings (b) bush bearings
(c) magnetic bearings (d) needle bearings.
In D.C. generators, the cause of rapid brush wear may be
(a) severe sparking (b) rough commutator surface
(c) imperfect contact (d) any of the above.
In lap winding, the number of brushes is always
(a) double the number of poles (b) same as the number of poles
(c) half the number of poles (d) two.
For a D.C. generator when the number of poles and the number of armature conductors are fixed, then
which winding will give the higher e.m.f. ?
(a) Lap winding (b) Wave winding
(c) Either of (a) and (b) above (d) Depends on other features of design.
A 4 pole D.C. generator rotates at 1500 r.p.m. How many times in a second will the e.m.f. induced in
each armature conductor change its direction ?
(a) 100 (b) 50
(c) 400 (d) 200.
If B is the flux density, l the length of conductor and v the velocity of conductor, then induced e.m.f. is
given by
(a) Blv (b) Blv2
(c) Bl2v (d) Bl2v2
In case of a wave wound generator, the average pitch
(a) must be even (b) must be odd
(c) may be odd or even (d) is generally fractional.
In case of a 4 pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch
will be
(a) 4 (b) 8
(c) 16 (d) 32.
The material for commutator brushes is generally
(a) mica (b) copper
(c) cast iron (d) carbon.
The insulating material used between the commutator segments is normally
(a) graphite (b) paper
(c) mica (d) insulating varnish.
In a D.C. generator, if p be the number of poles and N be the r.p.m. of rotor, then the frequency of
magnetic reversals will be
(a)Np/2
(b)Np/60
(c)Np/120
(d)Np/3000
.
For generating large currents on D.C. generators which winding is generally preferred ?
(a) Progressive wave winding (b) Lap winding
(c) Retrogressive wave winding (d) Current depends on design.
The purpose of providing dummy coils in a generator is
(a) to enhance flux density (b) to amplify voltage
(c) to provide mechanical balance for the rotor (d) to reduce eddy currents.

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D.C. MOTOR MCQ

No-load speed of which of the following motor will be highest ?
(a) Shunt motor (b) Series motor
(c) Cumulative compound motor (d) Differentially compound motor.
The direction of rotation of a D.C. series motor can be changed by
(a) interchanging supply terminals (b) interchanging field terminals
(c) either of (a) and (b) above (d) none of the above.
Which of the following applications requires high starting torque ?
(a) Lathe machine (b) Centrifugal pump
(c) Locomotive (d) Air blower.
In case a D.C. motor is connected across A.C. supplies
(a) the motor will burn (b) the motor will run at rated speed
(c) the motor will run at corresponding synchronous speed
(d) the motor will run at slow speed.
If a D.C. motor is to be selected for conveyors, which motor would be preferred ?
(a) Series motor (b) Shunt motor
(c) Differentially compound motor (d) Cumulative compound motor.
Which D.C. motor will be preferred for machine tools ?
(a) Series motor (b) Shunt motor
(c) Cumulative compound motor (d) Differentially compound motor.
Which D.C. motor will be preferred for constant speed line shafting ?
(a) Cumulatively compound motor (b) Differentially compound motor
(c) Shunt motor (d) Series motor.
Differentially compound D.C. motors can find applications requiring
(a) high starting torque (b) low starting torque
(c) variable speed (d) frequent on-off cycles.
Which D.C. motor is preferred for elevators ?
(a) Shunt motor (b) Series motor
(c) Differentially compound motor (d) Cumulatively compound motor.
According to Fleming’s left-hand rule, when the forefinger points in the direction of the field or flux, the
middle finger will point in the direction of
(a) current in the conductor (b) movement of conductor
(c) resultant force on conductor (d) none of the above.
If the field of a D.C. shunt motor gets opened while motor is running
(a) the speed of motor will be reduced (b) the armature current will reduce
(c) the motor will attain dangerously high speed
(d) the motor will continue to run at constant speed.
Starters are used with D.C. motors because
(a) these motors have high starting torque (b) these motors are not self-starting
(c) back e.m.f. of these motors is zero initially
(d) to restrict armature current as there is no back e.m.f. while staring.
Which of the following D.C. motors has self-load properties ?
(a) Series motor (b) Shunt motor
(c) Cumulatively compound motor (d) Differentially compound motor.
In D.C. shunt motors as load is reduced
(a) the speed will increase abruptly
(b) the speed will increase in proportion to reduction in load
(c) the speed will remain almost constant
(d) the speed will reduce.
What will happen if the back e.m.f. of a D.C. motor vanishes
(a) the motor will stop (b) the motor will continue to run
(c) the armature may burn (d) the motor will run noisy.
In case of D.C. shunt motors the speed is dependent on back e.m.f. only because
(a) back e.m.f. is equal to armature drop (b) armature drop is negligible
(c) flux is proportional to armature current (d) flux is practically constant in D.C. shunt motors.
In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be
(a) almost negligible (b) rated full-load current
(c) less than full-load current (d) more than full-load current.
Which D.C. motor will have least percentage increase of input current, for the same percentage increase
in the torque ?
(a) Shunt motor (b) Series motor
(c) Cumulatively compound motor (d) Separately excited motor.
These days D.C. motors are widely used in
(a) pumping sets (b) air compressors
(c) electric traction (d) machine shops.
By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor ?
(a) frame (b) shaft
(c) commutator (d) stator.
In which of the following applications D.C. series motor is invariably used ?
(a) Starter for a car (b) Drive for a water pump
(c) Fan motor (d) Motor operation in A.C. or D.C.
In D.C. machines fractional pitch winding is used
(a) to improve cooling (b) to reduce copper losses
(c) to increase the generated e.m.f. (d) to reduce the sparking.
A three point starter is considered as suitable for
(a) shunt motors (b) shunt as well as compound motors
(c) shunt, compound and series motors (d) all D.C. motors.
Small D.C. motors up to 5 H.P. usually have
(a) 2 poles (b) 4 poles
(c) 6 poles (d) 8 poles.
In case the conditions for maximum power for a D.C. motor are established, the efficiency of the motor
will be
(a) 100% (b) around 90%
(c) anywhere between 75% and 90% (d) less than 50%.
A shearing machine has cyclic load consisting of intermittent light and heavy loads. Which of the following
D.C. motors will be suitable for this purpose ?
(a) Series motor (b) Shunt motor
(c) Cumulative compound motor (d) Differentially compound motor.
The ratio of starting torque to full-load torque is least in case of
(a) series motor (b) shunt motor
(c) compound motors (d) none of the above.
In D.C. motor which of the following can sustain the maximum temperature rise ?
(a) Slip rings (b) Commutator
(c) Field winding (d) Armature winding.
Which of the following law/rule can be used to determine the direction of rotation of D.C. motor ?
(a) Lenz’s law (b) Faraday’s law
(c) Coulomb’s law (d) Fleming’s left-hand rule.
Which of the following loads normally needs staring torque more than the rated torque ?
(a) Blowers (b) Conveyors
(c) Air compressors (d) Centrifugal pumps.
The starting resistance of a D.C. motor is generally
(a) low (b) around 500 Ω
(c) 1000 Ω (d) infinitely large.
In case of conductively compensated D.C. series motors, the compensating winding is provided
(a) as separately wound unit (b) in parallel with armature winding
(c) in series with armature winding (d) in parallel with field winding.
Sparking at the commutator of a D.C. motor may result in
(a) damage to commutator segments (b) damage to commutator insulation
(c) increased power consumption (d) all of the above.
Which of the following motors is preferred for operation in highly explosive atmosphere ?
(a) Series motor (b) Shunt motor
(c) Air motor (d) Battery operated motor.
If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
(a) Staring torque (b) Operating speed
(c) Full-load current (d) All of the above.
When the speed of a D.C. motor increases
(a) back e.m.f. increases and current drawn decreases
(b) back e.m.f. as well as current drawn both increase
(c) back e.m.f. as well as current drawn both decrease
(d) back e.m.f. decreases and current drawn increases.
As compared to an induction motor, the air gap in a D.C. motor is
(a) less than 50% (b) between 50% and 90%
(c) same (d) more.
Field winding of a D.C. series motor is usually provided with thick wire
(a) to provide large flux (b) to reduce the use of insulating materials
(c) as it carries large load current (d) in order to reduce eddy current.
Which one of the following is not the function of pole shoes in a D.C. machine ?
(a) To reduce eddy current loss (b) To support the field coils
(c) To spread out flux for better uniformity (d) To reduce the reluctance of the magnetic path.
The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to
applied voltage is
(a) 4.0 (b) 2.0
(c) 1.0 (d) 0.5.
The condition for maximum power in case of D.C. motor is
(a) back e.m.f. = 2 × supply voltage (b) back e.m.f. = 1/2× supply voltage
(c) supply voltage =1/2 × back e.m.f. (d) supply voltage = back e.m.f.
For which of the following applications a D.C. motor is preferred over an A.C. motor ?
(a) Low speed operation (b) High speed operation
(c) Variable speed operation (d) Fixed speed operation.
What will happen in case 220 V D.C. series motor is connected to 220 V A.C. supply ?
(a) The armature winding of motor will burn
(b) The motor will vibrate violently
(c) The motor will run with less efficiency and more sparking
(d) The motor will not run.
In D.C. machines the residual magnetism is of the order of
(a) 2 to 3 per cent (b) 10 to 15 per cent
(c) 20 to 25 per cent (d) 50 to 75 per cent.

If Ta be the torque and Ia the armature current for a D.C. motor, then which of the following relation is
valid before saturation ?
(a) Ta ∝ Ia (b) Ta ∝I/Ia
(c) Ta ∝ Ia
2 (d) Ta ∝I/(Ia )2 .
Which D.C. motor is generally preferred for cranes and hoists?
(a) Series motor (b) Shunt motor
(c) Cumulatively compound motor (d) Differentially compound motor

ANSWERS

(c) 2. (c) 3. (d) 4. (d) 5. (b) 6. (a) 7. (c)
(b) 9. (b) 10. (b) 11. (d) 12. (a) 13. (d) 14. (b)
(b) 16. (a) 17. (a) 18. (c) 19. (b) 20. (d) 21. (c)
(c) 23. (b) 24. (c) 25. (b) 26. (b) 27. (c) 28. (a)
(a) 30. (b) 31. (c) 32. (b) 33. (d) 34. (a) 35. (c)
(d) 37. (d) 38. (c) 39. (c) 40. (d) 41. (d) 42. (b)
(c) 44. (c) 45. (a) 46. (d) 47. (b) 48. (a) 49. (d)
(c) 51. (b) 52. (c) 53. (d) 54. (b) 55. (a) 56. (c)
(d) 58. (c) 59. (c) 60. (a) 61. (d) 62. (c) 63. (a)
(d) 65. (b) 66. (c) 67. (c) 68. (a) 69. (c) 70. (a).

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