IN ORDER TO INDUCE THE VOLTAGE AS PER THE FARADAYS LAW WE RECQUIRE FIELD SO WE ARE PROVIDING IT WITH THE DC SUPPLY AND SINCE ROTOR OF THE ALTERNATOR IS ROTATING THEN EMF IS INDUCED.WE HAVE TO NOTICE HERE THAT SINCE THE ROTOR IS ROTATING FIELD IS ALSO ROTATING AND HENCE THERE IS RATE OF CHANGE OF FLUX.YOU CAN ALSO USE A.C SUPPLY BUT IF WE USE SINCE EMF PRODUCED IS ROTATING AND ROTOR IS ALSO ROTATING RATE OF CHANGE OF FLUX LINKAGES IS LESS THAN FORMER ONE AND SO ITS NOT A EFFICIENT ONE.
because the field poles of alternator will get magnetize in one direction only by using dc. if we use ac instead first it will magnetize then demagnetize amounting for huge hysteresis loss
we cant provide ac...look..providing ac means varyinf flux... phi=phi(max) sinwt...here phi is varying ony due to revolving field (sinwt)...but now if u apply ac excitation, then phi will vary due to 2 varying components ie., phi(max) {phi(max) starts varying due to ac excitation} and already present sinwt...which produce vibrations in rotor..this may lead to unbalancing or even rotor may touch the stator..which is absolutely undesirable...so always dc excitation is provided..
