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
The residual magnetism of a self-excited dc generator is lost. To build up its emf again

 field winding must be excited by low voltage dc supply
 the field winding must be replaced
 the field winding connection must be reversed
 the armature connection must be reversed

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