What factor does the frequency of an alternator depend upon?

The frequency of an alternator is fundamentally dependent on the rotational speed, or RPM (revolutions per minute), of its rotor. In essence, as the rotor spins, it creates a magnetic field that induces alternating current in the stator windings. The relationship between the speed of the rotor and the frequency of the generated current is described by the formula:

Frequency (f) = (Number of poles × RPM) / 120

This equation shows that, for a given number of poles in the alternator, the frequency increases as the RPM increases. Therefore, the correct answer directly correlates with the mechanical rotation of the alternator's rotor. In contrast, factors such as voltage output, runtime efficiency, and load resistance do not influence the frequency directly. Voltage output may vary with changes in load or design but doesn't change the frequency of the electrical output. Likewise, runtime efficiency relates to how well the alternator converts mechanical energy to electrical energy, while load resistance affects current but does not change the frequency at which the alternator operates.