for exciteing the alternator field

the field of the alternator consists of strong 
electromagnets which create a magnetic flux linkage around 
it.to make this magnetic field more stronger so that this 
gets locked magneticaly with revolving magnetic field thru 
stator armature when ac supply is given thru stator 
armature core wid ease, dc supply or dc excitation is given 
to it.

To begin with if ac is given then the problem would be 
that the field flux linking with the armature , i.e.imagine 
the arm. cond. looking into the field flux , as the field 
flux passes the arm. conductor (field = consider ac 
excitn.) the arm would see two zeros, one being the natural 
zero of ac excitation and the other being the zero produced 
due to the field's continous forward motion . Hence the 
nett emf produced would however reach one zero in halfcycle 
but would have intermediate sags in its waveform (imagine 
this concept from the view of the arm . cond.). This is the 
reason why the arm. is not given ac excitn.

in order to give the field excitation

the reason why ac is not given to the excitation of field 
of alternator lies behind the concept of mutual 
induction.if ac field'll be given again the problem starts 
when field current will rise and leads to deexcite 
itself.hence it will not give uniform excitation like dc 
current.