Generators are DC. Slip rings are used to produce AC. The armature is only a permanent magnet on simple machines. There is a rotating magnetic field, the strength of which can be varied to alter the output on larger and more complex machines, such as on an aircraft.
Rotational speed does not vary to alter the output voltage. This is done by varying the magnetic field in the armature circuit. On an AC machine altering the rotational speed changes the frequency. (Hertz)
In modern machines there are many variations of the above and control systems are complex, but all follow the basic principal.