How may the voltage output of an alternator be regulated?

The voltage output of an alternator can be regulated by controlling the voltage output of the DC exciter. In an alternator, the exciter provides the initial magnetic field necessary for the generation of electricity. By adjusting the excitation voltage supplied to the rotor, you can change the magnetic field strength, which directly influences the alternator's output voltage.

When the excitation voltage increases, the magnetic field becomes stronger, leading to a higher output voltage. Conversely, reducing the exciter voltage decreases the magnetic field and lowers the output voltage. This method allows for precise voltage regulation, ensuring that the alternator can meet the demand for electrical power while also maintaining a stable voltage level to avoid potential damage to the electrical systems it powers.

Other options relate to factors that do not directly influence the voltage output in the same manner. For instance, changing the engine's RPM affects the frequency and overall output power but is not a primary means of voltage regulation by itself. Similarly, controlling coolant flow and altering the alternator's position do not play a role in voltage regulation in the context of how an alternator operates.