Answer / sankaranarayanan

field flux produced by dc supply is constant where as in ac
supply will not give a constant field flux

So, alternator output will not constant.

Answer / kj

When a wire is moved through a magnetic field it produces a
voltage across the wire. To produce an ac output the wire
must move past a north pole and then a south pole in
continuing sequence.
A dc voltage on the field will maintain the poles with the
same polarity. An ac voltage will cause the polarity to
reverse 100 times a second (50Hz) and this will not be
usable for generating the ac output voltage.
Usually an ac voltage is used but it is rectified either as
half wave or full wave before being put into the field.

Answer / mohamed abd el-raouf

so the magnetic field of the rotor can connect with the magnetic field of the stator

Answer / haytham hashim

coz dc can be controlled in awide range but cann't be

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