Electrical Machines
The synchronous reactance( Xs) is the

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

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Electrical Machines
When the supply terminal of a DC shunt motor are interchanged

 The motor will stop.
  The motor will run at its normal speed in the same direction as before.
 The motor will run much faster in the same direction.
 The direction of rotation will reverse.

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