Measuring RPM from brushless motor lead - RC Groups
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May 11, 2007, 08:18 AM
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Ralph Weaver's Avatar

Measuring RPM from brushless motor lead

Several data logger measure rpm from one lead of the brushless motor. I assume that it's just hall effect with a bit of filtering. Anyone have more info?
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May 11, 2007, 10:41 AM
AndyKunz's Avatar
You have to watch for the direction of the current to change, not just for the current to be there or not. The PWM causes the current to flow in one direction, but off and on. The commutation causes the direction of the current to change.

A low-pass filter helps get rid of the PWM pulsing.

Check out the commutation sequence in one of the TI data sheets to get a better understanding.

May 11, 2007, 01:24 PM
Registered User
Ralph Weaver's Avatar
Makes sense.

I have a tach that uses an optical sensor, but would like to be able to just clamp onto a wire and get more reliable readings. Maybe I'll get one from Eagle Tree and take it apart - or just adapt my tach to use it.
May 11, 2007, 02:47 PM
"Simplify, then add lightness"
Just a thought, and haven't tried it, but it might be possible to put an led with a series resistor across two of the motor leads and read the rpm with an optical tach. With a low current led and a high value resistor, it shouldn't upset the controller timing. You would have to scale your tach reading for the number of poles.

Again, don't know if it would work, but would be simple to try.

PS just looked at the Eagle brushless rpm sensor, and it looks like it has connections for two of the motor wires. It is probably just a filter and comparator or maybe an opto-isolator.
Last edited by jeffs555; May 11, 2007 at 03:02 PM.

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