Which of the following factors directly affects the output frequency of an alternator?

The output frequency of an alternator is primarily determined by the speed of rotation and the number of poles. This is due to the fundamental relationship in alternating current (AC) generation, where the frequency (f) is calculated using the formula:

[ f = \frac{P \times N}{120} ]

where P represents the number of poles and N is the speed of rotation in revolutions per minute (RPM). As the speed increases, the output frequency also increases, given a constant number of poles. Conversely, if the number of poles is altered (commonly done in design depending on the application), the frequency will also adjust accordingly.

The load connected to the alternator does influence the output voltage and current, but it does not directly affect the frequency. Similarly, while the type of fuel used to run the alternator or the temperature of the surrounding environment can have indirect effects on performance or efficiency, they do not have a direct relationship with the output frequency. Thus, the most relevant factor in this case is the combination of the speed of rotation and the number of poles.