Electrical Machines
The synchronous reactance( Xs) is the

 combined reactance due to armature reaction and leakage flux
 reactance due to armature reaction of the machine
 reactance due to leakage flux
 reactance either due to armature reaction or leakage flux

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Electrical Machines
Flashing of DC generator means

  increasing flux density by providing extra ampere-turns in field.
  creation of residual magnetism by a DC source.
None of these
 neutralization of residual magnetism.

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