D.C. Generators

71. Open circuited armature coil of a D.C. machine is

1. identified by the scarring of the commutator segment to which open circuited coil is connected
2. indicated by a spark completely around the commutator
3. both (a) and (b)
4. none of the above

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72. Permeance is the reciprocal of

1. flux density
2. reluctance
3. ampere-turns
4. resistance

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73. Satisfactory commutation of D.C. machines requires

1. brushes should be of proper grade and size
2. brushes should smoothly run in the holders
3. smooth, concentric commutator properly undercut
4. all of the above

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74. Shunt generators are most suited for stable parallel operation because of their

1. rising voltage characteristics
2. identical voltage characteristics
3. drooping voltage characteristics
4. linear voltage characteristics

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75. The armature core of a D.C. generator is usually made of

1. silicon steel
2. copper
3. non-ferrous material
4. cast-iron

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76. The armature of D.C. generator is laminated to

1. reduce the bulk
2. provide the bulk
3. insulate the core
4. reduce eddy current loss

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77. The bearings used to support the rotor shafts are generally

1. ball bearings
2. bush bearings
3. magnetic bearmgs
4. needle bearings

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78. The commutator segments are connected to the armature conductors by means of

1. copper lugs
2. resistance wires
3. insulation pads
4. brazing

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79. The critical resistance of the D.C. generator is the resistance of

1. field
2. brushes
3. armature
4. load

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80. The demagnetising component of armature reaction in a D.C. generator

1. reduces generator e.m.f.
2. increases armature speed
3. reduces interpoles flux density
4. results in sparking trouble

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