An alternator consists of a rotating magnetic field inside several coils of wire. The rotating field generates electricity in the coils. Even at a constant RPM, if the voltage regulator senses the need for more electrical output, it increases the strength of the magnetic field. Therefore, it takes more torque to turn the field, and hence, more engine power to turn the alternator. The input energy to the alternator is directly related to the output energy that the alternator is making. More energy out requires more energy in, even at a constant RPM.
Good explanation. I need to look up the electrical diagram of an alternator I guess. I admit that I was wrong. I jumped in without making sure of the equipment I was talking about.