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# How do I calculate the RPM of a electric vacuum motor? Answered

I have a Johnson 12amp 120v 60Hz vacuum motor that I scrounged out of an out of commission vacuum cleaner and was curious of its RPM's to see how it would have to be geared to fit my project needs. If anyone out there can help me please speak up.

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## Discussions

If you don't have an oscilloscope that seandogue recommends, find a safe place to touch the spinning motor with something stiff, like cardboard.  The frequency of the sound from it tells the speed.  If you have a good ear, you can guess the tone.  If you have a guitar tuner, you can use that to give you the note.  As you know, the note A is 440 Hz (cycles).  If the motor touches the cardboard more than once in each revolution, you have to divide the sound frequency by however many times the motor strikes the cardboard.

Let's say the buzz that comes from touching the armatures of the the motor sounds like the note A.  If there are 4 armature poles on the motor, its speed is 440 divided by 4, or 110 revolutions per second.

use cardboard and a guitar tuner to test rpm?????????
really dude?
as you know an A note has 440Hz, but my piano has 8 A notes. sooooo

Yes, you really can get rpm that way; you're converting the rotor rpm to sound, then estimating the frequency of the sound. The 440 Hz A on the piano is the A above middle C  (http://en.wikipedia.org/wiki/Piano_key_frequencies)

But since it has been about three years since either of us wrote, you've probably go on to other things. Also, my suggestion isn't really that great--it's just cheaper than a stroboscope. If the motor runs at 10,000 rpm, you probably couldn't get the method to work at all--the cardboard would be chewed up.

What did you end up doing?

yea in the 10K range. those suckers are fast.

It would be fast, could be 10,000 RPM

L