Electrical Machines
The sparking at the the brushes, in a DC machines is due to

 armature reaction.
 high resistance of carbon brushes.
 presence of commutator.
  reactance voltage.

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Electrical Machines
A compound dc generator is delivering full load current at a terminal voltage of 230 V. Its series-field winding gets short circuited. If its terminal voltage 1. becomes more than 230 V, it is overcompounded generator 2. becomes more than 230 V, it is differentially compounded generator 3. becomes less than 230 V, it is overcompounded generator 4. becomes less than 230 V, it is differentially compounded generator 5. becomes less than 230 V, it is level compounded generator From these, the correct answer is

 2, 3, 5
 1, 4, 5
 2,3
 1, 4

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