Electrical Machines
The synchronous reactance( Xs) is the

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

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Electrical Machines
In speed control of DC Series motor, speed lower than the rated speed can be obtained by

 Connecting an additional resistance in stator circuit.
 Any of the above.
 Connecting an additional resistance in the armature circuit.
 Connecting an additional resistance in the field circuit.

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