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I understand that there are very expensive meters that do this ; ex: Ono Sokki FT-7200 or FLuke, but I need a cheaper alternative.

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  • $\begingroup$ I do not know enough about it to explain it in full but I think that "Back EMF" would be the right place to look. Essentially you would look at disturbances to the supply voltage. $\endgroup$
    – Emily Conn
    Commented Oct 27, 2022 at 13:07
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    $\begingroup$ use a microphone ... download Audacity ... audacityteam.org $\endgroup$
    – jsotola
    Commented Oct 27, 2022 at 16:30
  • $\begingroup$ @jsotola, brilliant! -NN $\endgroup$ Commented Oct 27, 2022 at 17:02

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If you just need the reading, it may be possible to do this with your mobile phone running the audio spectrum analyzer app. The rotation frequency is likely to be between dominant frequencies of vibration.

If you are building some electronic device, some frequency to voltage converter may work.

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One way is to open the motor casing, and if the armature has a nut on it then magnetize the nut.

That can then be tracked by a hall effect sensor and from that you can calculate the rpm.

Seen it used to measure the speed of a turbo charger shaft.

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