3-14
illustrated in figure 3-12; a two-pole generator must rotate at four times the speed of an eight-pole
generator to produce the same frequency of generated voltage. The frequency of any ac generator in hertz
(Hz), which is the number of cycles per second, is related to the number of poles and the speed of
rotation, as expressed by the equation
where P is the number of poles, N is the speed of rotation in revolutions per minute (rpm), and 120 is
a constant to allow for the conversion of minutes to seconds and from poles to pairs of poles. For
example, a 2-pole, 3600-rpm alternator has a frequency of 60 Hz; determined as follows:
A 4-pole, 1800-rpm generator also has a frequency of 60 Hz. A 6-pole, 500-rpm generator has a
frequency of
A 12-pole, 4000-rpm generator has a frequency of
Q17. What two factors determine the frequency of the output voltage of an alternator?
Q18. What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600
rpm?
