Electrical Machines
The synchronous reactance( Xs) is the

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

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Electrical Machines
Commutation in a DC machine may be explained as a process

  in which magnetic field is set up.
 by virtue of which emf is induced in the armature conductors.
 how alternate flux affects the main field flux.
  by which electric current in a coil is reversed.

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