DC Generator MCQ Questions

1. In a DC generator, the ripples in the generated DC are reduced by employing

(a) commutator with large number of segments.
(b) equalizer rings.
(c) carbon brushes.
(d) graphite brushes.

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2. In order to ascertain whether a DC machine is lap wound or wave wound on the basis of visual observations of the armature, one should observe the

(a) connections to the brushes
(b) connections to the commutator.
(c) connections to the field winding
(d) direction of end connections.

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3. The coil span

(a) must be exactly equal to pole pitch.
(b) can never be equal to pole pitch.
(c) may or may not be equal to pole pitch.
(d) none of the above.

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4. The commutator pitch for simplex lap winding is

(a) +1
(b) -1
(c) +1 or -1
(d) average pitch

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5. The commutator pitch for simplex wave winding is

(a) +1 or -1
(b) twice average pitch.
(c) average pitch.
(d) half average pitch.

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6. In a DC machine armature winding, the number of commutator segments is equal to the number of armature

(a) coils.
(b) coil sides.
(c) conductors.
(d) turns.

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7. In a drum type DC armature winding the back pitch and front pitch in terms of the coil-sides must be

(a) even and odd respectively.
(b) odd and even respectively.
(c) both even.
(d) both odd.

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8. In a DC machine, fractional pitch is employed to

(a) increase generated voltage.
(b) reduce sparking.
(c) save copper.
(d) improve cooling.
(e) both (b) and (c).

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9. In a DC machine, the armature winding is made up of number of coils distributed in large number of armature slots instead of placing all the coils into two slots to

(a) have maximum EMF at the output terminals.
(b) have maximum EMF generated in the armature.
(c) have minimum heat dissipation from the armature.
(d) make the armature dynamically balanced.

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10. Aluminum is not used as winding wire in DC machine armature as it

(a) is costlier.
(b) is of low resistivity.
(c) is of lower thermal conductivity.
(d) requires large winding space.

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11. Short circuiting is caused in the armature winding due to

(a) failure of insulation between two turns of a coil.
(b) failure of insulation between two commutator bars.
(c) grounding of two or more turns of the same coil.
(d) any of the above.

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12. In a lap wound generator, the equalizer rings are provided to

(a) neutralize the armature reaction effect.
(b) avoid short circuiting.
(c) avoid unequal distribution of currents at brushes.
(d) avoid harmonics generated in the EMF.

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13. In a lap wound DC generator having P poles and Z conductors the maximum number of equalizer rings will be

(a) Z/P
(b) 2Z/P
(c) Z/2P
(d) Z/4P

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14. A frog-leg winding is used in a DC machine to eliminate

(a) the use of equalizers in lap winding.
(b) the use of compensating winding.
(c) the use of compensating winding.
(d) reactance voltage produced due to commutation.

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15. A 250 V DC generator is run at rated speed with no excitation. The open-circuit voltage will be

(a) zero
(b) very small, say about 2 or 3 V.
(c) about 100 V.
(d) 250 V.

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16. At a certain speed and flux, the voltage generated by a DC generator is 230 volts. If the speed in increased by 20% and the flux is simultaneously reduced by 10%, the voltage will be

(a) increased by 10%.
(b) reduced by 20%.
(c) increased by 8%.
(d) decreased by 8%.

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17. In DC machines, the air gap flux distribution in space at no load is

(a) sinusoidal.
(b) triangular.
(c) flat topped.
(d) pulsating.

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18. The armature MMF waveform of a DC machine is

(a) pulsating.
(b) rectangular.
(c) triangular.
(d) sinusoidal.

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19. The armature MMF of a DC machine has

(a) triangular space distribution and rotates at the speed of armature.
(b) trapezoidal space distribution and is stationary in space.
(c) stepped space distribution and rotates at the speed of armature.
(d) triangular space distribution and is stationary in space.

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20. In a DC machine, the actual flux distribution depends upon

(a) size of air gap.
(b) shape of pole shoes.
(c) clearance between tips of the adjacent pole shoes.
(d) all of the above.

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21. In a DC machine, the space wave form of the air-gap flux distribution affects

(a) torque only.
(b) voltage only.
(c) both the torque and voltage.
(d) neither the torque nor the voltage.

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22. In a DC machine, the armature MMF is

(a) stationary with respect to the poles but rotating with respect to the armature.
(b) rotating with respect to field poles as well as armature.
(c) rotating with respect to field poles.
(d) stationary with respect to armature.

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23. Armature reaction in a DC shunt generator, running at full load with the brushes not shifted from the geometrical neutral plane and saturation neglected is

(a) absent.
(b) cross-magnetizing.
(c) demagnetizing.
(d) magnetizing.

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24. In a DC machine without any brush shift, the shift of magnetic neutral axis owing to armature reaction is

(a) in the direction of rotation for both the generator and the motor.
(b) against the direction of rotation for both the generator and the motor.
(c) in the direction of rotation for the generator and against the direction of rotation for the motor.
(d) against the direction of rotation for the generator and in the direction of rotation for the motor.

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25. In a DC machine, on no load the magnetic neutral axis

(a) moves from geometric neutral axis in the direction of rotation.
(b) moves from geometric neutral axis in the opposite direction of rotation.
(c) coincides with the geometrical neutral axis.
(d) none of the above.

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26. Due to magnetic saturation, the flux per pole in a DC machine without brush shift

(a) increases in both the generators and the motors with load.
(b) decreases in both the generators and the motors with load.
(c)[/expander_maker] increases in generators but decreases in motors with load.
(d)[/expander_maker] decreases in generators but increases in motors with load.

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27. If the brushes are given a small amount of forward shift in case of DC generator, the armature reaction effect will be

(a) totally demagnetizing.
(b) totally cross-magnetizing.
(c) partly demagnetizing and partly cross-magnetizing.
(d) totally magnetizing.

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28. In a DC machine, the number of mechanical and electrical degrees will be the same when the number of poles on the machine is

(a) 4
(b) 2
(c) 8
(d) 1

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29. Commutation in a DC machine may be explained as a process

(a) in which magnetic field is set up.
(b) by virtue of which EMF is induced in the armature conductors.
(c) by which current in a coil is reversed during the period it is short circuited by the brush.
(d) how armature flux affects the main field flux.

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30. In a DC shunt generator working on load, the brushes are moved forward in the direction of rotation, as a result of this, commutation will

(a) improve but terminal voltage will fall.
(b) worsen and terminal voltage will fall.
(c) improve and terminal voltage will rise.
(d) worsen and terminal voltage will rise.

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31. In a DC machine, the sparking between brushes and commutator surface may be due to

(a) under commutation.
(b) over commutation.
(c) too rapid reversal of current.
(d) any of the above.

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32. The self induced EMF in the coil undergoing commutation is called the

(a) reactance voltage.
(b) statically induced voltage.
(c) dynamically induced voltage.
(d) none of the above.

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33. The sparking at the brushes, in a DC machine, is due to

(a) armature reaction.
(b) reactance voltage.
(c) presence of commutator.
(d) high resistance of carbon brushes.

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34. The reactance voltage is

(a) directly proportional to armature current and commutation period.
(b) directly proportional to commutation period and inversely proportional to armature current.
(c) directly proportional to armature current and inversely proportional to commutation period.
(d) none of the above.

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35. Sparkless commutation can be achieved by employing

(a) interpoles.
(b) compensating winding.
(c) high resistance carbon brushes.
(d) any one or more of the above methods.

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36. To have sparkless commutation, the armature reaction effect in a DC a machine is neutralized by

(a) compensating winding and commutating poles.
(b) shifting the brush axis from geometrical neutral axis to magnetic neutral axis.
(c) fixing the brush axis in line with the pole axis.
(d) increasing the field excitation.

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