Synchronous Motors
The net armature voltage of a synchronous motor is equal to the

arithmetic difference of Eb and V
arithmetic sum of Eb and V
vector difference of Eh and V
vector sum of Eb and V

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Synchronous Motors
A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have

lagging power factor with over-excitation
leading power factor with over-excitation
leading power factor with under-excitation
no change of power factor

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