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3.5: Frequency and Alternators

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  • cycle means from the point in a waveform to where the waveform starts to repeat itself. When we discuss the term frequency, we are referring to how many cycles can occur in one second. Frequency is measured in hertz (shout out to Heinrich Hertz) or CPS (cycles per second). Two factors affect the frequency in an alternator: rotation speed and the number of poles.


    Figure 52. Sine wave cycle

    Rotation speed

    RPM. However, we are not concerned with minutes, but rather, with seconds when dealing with frequency. Therefore, RPM must be converted to rotations per second (RPS). As there are 60 seconds in a minute, all we have to do is to divide the RPM by 60 to convert it to RPS.

    Number of Poles


    Figure 53. Two pole alternator


    Figure 54. Four pole alternator

    Formula time!

    \[ f= \dfrac{P}{2} \times \dfrac{N}{60} \tag{Frequency formula}\]

    • \(f\) = frequency in hertz
    • \(P\) = number of poles
    • \(N\) = rotational speed in RPM

    \[f = \dfrac{PN}{120} \tag{Combined frequency formula}\]

    Video! This video will walk you through how frequency is related to the RPM and the number of poles of an alternator.