Electrical Machines
Consider the following statements regarding dc machines : 1. possible resistance of shunt-field winding is 1 Ω 2. possible resistance of series-field winding is 1 Ω 3. electrographite brushes are used for 230 V, 10 A machines 4. Copper-graphite brushes are used for 230 V, 10 A machines 5. Possible armature-circuit resistance is 1 Ω From these, the correct statements are

 2, 4 ,5
 1, 4
 1, 3, 5
 2, 3 , 5

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Electrical Machines
Compensating windings are used in DC generators

  to neutralize the cross magnetizing effect of the armature reaction.
None of these
  mainly to reduce the eddy currents by providing local short circuit.
  to provide path for the circulation of cooling air.

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