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
Ward-Leonard system for controlling the speed of a DC motor is used where

 All of above.
 The speed in both the direction is required.
 The accuracy in speed control is required.
 Very fine speed control over the whole range from zero to normal speed is required.

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