As you well known the DC is direct current which doesn't 
change its direction. therfore we always using the DC 
excitation for Generation due to continues induced magnatic 
field for proper excitation.

THE FIELD STRENGTH IS CONSTANT FOR DC SUPPLY, BUT AC 
SUPPLYS FIELD STRENGTH IS VARIABLE.

i think its b'coz we need constant poles at the ends of the
field(i.e. rotor) so as to lock the rotor wid the rotating
magnetic field.hence DC is used.if AC is used it ll create
NORTH n SOUTH poles alternately on each side of the rotor

alternator is sync. machine which not self start,so we need 
dc excitation to get const field

DC Exciation is mostly used, Because We need a constand 
magnetic field for ouy power generation. So DC Voltage or a 
Natural Magnet can Solve this problem. As using a natural 
magnet will be bulky,So insted we use DC windings with 
inverter power which draws power from the same Generator 
during generation.

no ripple factor in DC excitation but there is ripple factor
in AC

DC supply gives constant , steady magneticflux.
but AC supply gives an alternating flux. so in alternators 
field give to dc supply.

The answer is because these alternators are synchronuse 
generators. The philosophy of  electrical synchronuse 
generators is based on DC excitation in the rotor and  
synchronuse vontages in stator.

However,there are some application for induction generators 
such as wind generators that have AC excitation like other 
induction motors.

As a reslut, if alternator is synchronuse generator it has 
to have DC excitation and if alternator is induction 
generator it has to have AV excitation.

The basic requirement of an alternator is considered to be 
as a sine wave generator. for getting pure sine wave, the 
excitation to be of DC with low ripple factor.
in commercial generators, pure sine wave cannot be achived, 
because of the core property of the material.

The excitation can of AC too. If we provide ac excitation, 
the armature reaction will be more, and there by more core 
loss, leads to heating up of core materials.The harmonics 
disturbance also having consider effect in the power loss.

THATS BECAUSE IN GENERATORS THERE IS A TURBINE WICH CAUSE 
ROTATION TO THE ROTOR SO WE ALREADY HAVE ROTATING ROTOR SO 
THERE IS NO NEED FOR A ROTATING FIELD, AND IF SO IT WILL BE 
HARD TO CONTROL THE SPEED OF THE FIELD

AC SUPPLY IS GIVEN TO THE STATOR WHICH PRODUCES ROTATING 
MAGNETIC FIELD ROTATING AT CONSTANT RPM
INORDER FOR THE ROTOR TO CATCH UP WITH THE ROTATING FIELD 
THE ROTOE SHOULD HAVE ONE DIRECTION MAGNETIC FIELD ONLY 
THEN IT CAN ROTATE.
THEREFORE THE EXCITATION IS DC

Actually the answer is like this:
When we are giving DC excitation to the AC machine,we are 
imposing DC on AC.
In this case the Resulting Frequency (Fr) will be the 
difference of the two.
Hence in this case,Fr = 50 - 0 = 50Hz 
and hence we get the excitation at desired frequency,where 
as if we provide AC excitation, then Fr will become 0Hz & 
so no excitating magnetic field will be created...

as we know that alternator work on the fraday's law ie when
a conductor cut the magnetic flux then a emf is induced. If
we give a.c supply than flux will also alternate and we will
get proper cuts of flux. hence we use dc supply.

In theory, there are 3 kinds of generator or alternator 
that are synchronus, induction and DC. 

So the question is why DC excitaion is used  for 
alternators. The answer is becuase this a synchronus 
generator. And that is philosophy of this kind. For example 
if you use AC Excitaion it will not be synchrounus but it 
will be induction generator

Rotor of the alternator is given dc supply so constant 
magnetic poles are formed.These constant magnetic poles 
produce constant flux.The rotor of the alternator is 
rotated by using prime mover,so constant poles are 
rotated,so these rotating magnetic poles produce rotating 
magnetic field.When this rotating magnetic field cuts three 
phase stator conductors produces three phase supply.