Answer Posted / vino kumar

In case of synchronous machines we have to maintain a
constant frequency.to maintain constant frequency the rotor
of syn.machine should be rotate with same speed of stator
rmf.so finally to achieve magnetic locking between stator
rmf and rotor ,the poles on the rotor should not be vary ,it
should be constant .the ac voltage changes the poles for
every half cycle.
so to maintain fixed polarity poles on rotor and to achieve
magnetic locking the rotor should be excited by dc supply only.

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