D.C. Machines

31. In a DC motor, unidirectional torque is produced with the help of

1. Brushes
2. Commutator
3. End-plates
4. Both a and b

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32. In a DC series motor increasing the load current will

1. Decrease the speed
2. Increase the speed
3. Better commutation
4. Increase the back emf

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33. In a stepper motor the angular displacement

1. can be precisely controlled.
2. it cannot be readily interfaced with micro computer based controller.
3. the angular displacement cannot be precisely controlled.
4. it cannot be used for positioning of work tables and tools in NC machines.

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34. In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop

1. synchronous motor torque
2. dc motor torque
3. induction motor torque
4. induction generator torque

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35. In a synchronous motor, damper winding is provided to

1. Stabilize rotor motion
2. Suppress rotor oscillations
3. Develop necessary starting torque
4. Both B and C

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36. In a transformer the voltage regulation will be zero when it operates at

1. unity p.f.
2. leading p.f.
3. lagging p.f.
4. zero p.f. leading.

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37. In a transformer, zero voltage regulation at full load is

1. Not possible
2. Possible at unity Power factor load
3. Possible at leading Power factor load
4. Possible at lagging Power factor load

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38. In case of a universal motor, torque pulsation is minimized by _________.

1. load inertia
2. rotor inertia
3. both rotor and load inertia
4. none of the above

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39. In order to reduce the harmful effects of harmonics on the A.C. side of a high voltage D.C. transmission system _________ are provideD.

1. synchronous condensers
2. shunt capacitors
3. shunt filters
4. static compensators

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40. In small DC machines, armature slots are sometimes not made axial but are skewed, results in

1. quieter operation
2. slight decrease in losses
3. saving of copper
4. both a and b

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