D.C. Generators

31. For the parallel operation of two or more D.C. compound generators, we should ensure that

1. voltage of the incoming generator should be same as that of bus bar
2. polarity of incoming generator should be same as that of bus bar
3. all the series fields should be run in parallel by means of equilizer connection
4. series fields of all generators should be either on positive side or negative side of the armature

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32. If a D.C. generator fails to build up the probable cause could not be

1. imperfect brush contact
2. field resistance less than the critical resistance
3. no residual magnetism in the generator
4. faulty shunt connections tending to reduce the residual magnetism

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33. If a self excited D.C. generator after being installed, fails to build up on its first trial run, the first thing to do is to

1. reverse the field connections
2. increase the field resistance
3. increase the speed of primemover
4. check armature insulation resistance

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34. If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by

1. Blv
2. Blv2
3. Bl2v
4. Bl2v2

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35. If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be

1. demagnetisation only
2. cross magnetisation as well as mag¬netisation
3. crossmagnetisation as well as demagnetising
4. cross magnetisation only

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36. If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be

1. zero
2. small
3. the same as rated voltage
4. high

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37. In a commutator

1. copper is harder than mica
2. mica and copper are equally hard
3. mica is harder than copper
4. none of the above

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38. In a D.C. generator

1. external resistance = internal char-acteristic – armature reaction
2. internal characteristic = magnetisation characteristic – ohmic drop
3. external characteristic = magnetisation characteristic – ohmic drop – armature reaction
4. magnetisation characteristic = external characteristic

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39. In a D.C. generator in case the resistance of the field winding is increased, then output voltage will

1. increase
2. decrease
3. remain unaffected
4. fluctuate heavily

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40. In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralised by all of the following except

1. interpoles
2. dummy coils
3. compensating winding
4. shifting of axis of brushes

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