Synchronous Motors
In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then

the torque generated is maximum
the excitation is said to be hundred per cent
the excitation is said to be zero per cent
the motor is said to be fully loaded

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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

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

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