# An idea for a tachometer



## solver (Jun 14, 2012)

One day, I played with a motor, that I extracted from defunct hard drive. 

It has four leads, so plenty of choises. I just picked two, and attached them to multimeter(measuring frequency(Hz),
while the motor's axle was attached to lathe's chuck. 

I managed to get the lathe to run, and multimeter says something, like '~80'

I have also a non-contact tach, and, to my surprise, it showed some ~800 rpm.

So the hdd motor shows the rpm, although 1/10 of it. Just move the decimal point.


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## tvoght (Jun 14, 2012)

A permanent magnet motor will generally have a pretty linear voltage/RPM output voltage when used as a generator. The relationship may not be as straightforward as moving the decimal point as you observed, but should be a simple multiplier.

--Tim


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## tomrux (Jun 15, 2012)

frequency will be straight linear. voltage will increase with speed to a point then plateu.
Frequency ratio will depend on the number of poles in the particular motor.

Tom R


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## raggle (Jun 15, 2012)

This page from Tony Jeffree's site should be of interest

http://www.jeffree.co.uk/pages/speedmeasurement.html

Use of old motors and meters are towards the bottom, but reading the whole page is recommended.

Ray


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## Hopefuldave (Jun 15, 2012)

Yep, they usually have 12 stator pole-pieces wound in alternate pairs (two leads per set of six) and the rotor has a ceramic magnet with as many alternating poles, so 6 pulses /rev which gives the 1/10th RPM - if they had 120 poles-pieces they'd be direct-reading! They're "brushless DC" motors, so are driven with what amounts to a tiny VFD on a chip 

Most DMM frequency inputs have an amplifier driven into limiting so a small signal or large will give the same(ish) voltage change for the counter - as you've found out!

Dave H. (the other one)


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## gunboatbay (Jun 23, 2012)

An even simpler method for a tachometer is to use a digital bicycle speedometer. About $4.00 U.S. from the internet. Mine's been on the lathe for about 2 years and still going great on the original button battery. If anyone wants instructions, I'll be glad to email them.

Art


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