A generator can be regulated for either - constant voltage or constant current. CV is preferred so utility power generators use that, as well as car alternators. To regulate voltage, the speed must be held constant as well as mag field strength. But when load current occurs, the mag field surrounding the stator winding tends to oppose the mag field from the rotor (field winding). A generator regulated for CV operation has a series inductive reactance on paar with load resistance.
Example, if load resistance R is 100%, or 1.0 per unit ohm, open circuit voltage is 1.0 pu volt, the inductive or synchronous reactance XS, is between 50% and 250%, 0.50 to 2.50 pu ohm. Let's take 1.0 pu ohm for an average value of XS. If Voc (open circuit voltage) is 1.0 volts per unit (V pu), with load resistance of 1.0 ohm pu, we must add the j1.0 ohm pu of reactance to the load. The total impedance is 1.0 + j1.0 = sqrt(2) ohms, with a 45 degree angle, or 1.414 ohm pu magnitude.
The current would then be 0.7071 amp pu, not 1.0 A pu. The terminal voltage at the load is 0.7071 Vpu. If you wish to maintain 1.0 V pu at the terminals, then the field current must be changed to compensate for XS. As load current increases, field current is increased so as to increase field strength to make up for voltage dropped across inductive reactance.
Of course the stator windings have resistance as well, but the voltage drop due to this resistance is but a few percent. The drop due to inductive reactance can be 29% as above, 40%, 50%, or so since inductive reactance is quite high.
A good CVS (constant voltage source) ideally has a very low internal impedance, low resistance and low inductive reactance. But generators, including car units) have a lot of turns wrapped around high permeability ferromagnetic material to get the strong magnetic field, as well as a small air gap between stator and rotor. This means that inductance is very large.
A CVS w/ a large series reactance will not maintain a steady terminal voltage as current changes, so regulation is needed. In addition, a car unit has speed changes as motor speed changes. So the regulator must correct for this as well.
A CCS (constant current source) can be achieved as well. Instead of constant speed, we use constant torque to get constant current. Again, winding resistance, inductance, and for long transmission lines, capacitance, can affect regulation. So regulation is needed with a CCS as well.
CVS is used by power companies for several reasons, as well as in cars. I won't elaborate in order to stay focused on original question. CVS or CCS can be had with genrators, but CVS is intentionally realized for practical reasons.Claude