Why is DC excitation used in a synchronous alternator instead of AC excitation?

Synchronous machines work on the principle of “magnetic locking”. For magnetic locking the rotor must rotate at synchronous speed and if we give ac to the rotor then polarity of the field in the rotor changes continuously and it fails to synchronize with the stator rmf and hence there is no magnetic locking and the rotor field produces sinusoidal flux distribution with respect to space angle. To keep it short we can conclude that to get steady state torque and to synchronize the rotor we use dc excitation.

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If you use AC excitation, flux will be alternating. Then what you need is a static coil to induce voltage. This is nothing but a transfomer based on statically induced emf.

For dynamically induced emf, you need a constant flux (provided by DC excitation) and a rotating coil. This is the basic principle of DC/AC generator. The voltage induced is AC in both generators. In DC generators voltage is rectified using a commutator.

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