Alternator Pole Count
The pole count is the total number of magnetic poles on the alternator's rotor and it determines the relation between speed and the frequency produced. The frequency is found by multiplying the speed by the pole count and dividing by 120. A 2-pole alternator produces 50 Hz at 3000 rpm, a 4-pole one at 1500 rpm.
The relation is physical, it is not an adjustable parameter. The moment the alternator is chosen, the operating speed is determined as well.
- 2-pole: smaller and lighter, but since it runs at 3000 rpm the noise and wear are higher. It is generally preferred at small power ratings.
- 4-pole: runs at 1500 rpm; in terms of service life, noise and fuel consumption it is the common choice for continuously running sets.
In terms of speed control the conclusion is this: on a generator there is no such thing as holding the speed "wherever you like"; the speed is a single value corresponding to the target frequency. So if an electronic throttle kit is to be used on a generator, the aim is not to set the speed freely but to establish the correct speed and hold it there. If the frequency must not drift as the load changes, speed feedback is essential.
Context in which this term is used: Why is constant speed important on a generator?