A)The simple type of generators that can be explained is the synchronous generator,this generator has two parts rotor and stator.In general a rotating magnatic field in one winding will induce voltage in the other winding in the presence of magnatic field so the rotor should have two conditions in order to give induce voltage to the stator :
1) It should have magnatic field
2) This magnatic field should be rotating
the magnatic field on the rotor is provided using small external dc supply in a mean of brushess and comutators for small generators and a huge circuit "Brushless exciter for large generators " so the first condtion is satisfied since dc current that will pass on the rotor will produce a current that will produce the magnatic field and so the first condition is satisfied but the second condition is satisfied using the prime mover that will make the magnatic field rotating when rotating the rotor and so we have rotating magnatic field ......this field will induce votlage to the stator winding that is connected to the load
B)I Think the output voltage can be controlled by the speed of rotation since the famous relation of the stator internal voltage (induced voltage on the stator without any losses ) is :
where k : is constant that is related to the design of the generator
phi : is the total flux (weber )
W: is the speed of rotation (in rad/sec)
so controlling the speed will cause us to control the output voltage
note : we can use a variable resistor before the load and so control the voltage that will go to the load but it's not practical because it's waste of power