How to measure RPM of DC fan?

[shai]

I have a fan that is 30x30mm diameter and the hub is about 16mm diameter (when looking from the top). I read that you can use a Tachometer, but after searching for those on Amazon, I see that you need to apply tape to the object you're measuring RPM of and I'm not sure if there's enough space for the tape size required. Is there a better way to measure a small DC fan's RPM? My fan only has 12v/GND leads, no yellow wire for hall effect sensor. Any help appreciated!

Reason for this is because I tore out a regular 30x30x10 fan's DC "magnetic motor" and fitting my own bearing on it for a custom fan. So I want to swap bearings and see how they affect RPM. (but this is a separate topic  )

[ogden]

In case you have oscilloscope, you need straight core (thru-hole vertical) inductor. No need to glue magnet to rotor, because fan have built-in one

http://paulorenato.com/index.php/electronics-diy/113-measuring-rpm-with-an-oscilloscope

Other way of measuring: flashlight+photodiode+scope. Note that LED can be used as photodiode as well.

[David Hess]

The DC ripple current can also be used to measure the RPM of a DC brushless fan.  Areca uses this trick on their RAID controllers to monitor the cooling fan RPM.  Small expansion card sized fans with a tachometer output are not common.

[Sudo_apt-get_install_yum]

Couldn’t you use the current draw of the motor to get some sort of indication of the motor speed? Higher load = higher current draw, since you know the voltage and the max RPM for that voltage.
I don’t know how accurate this is but for rough estimation it should be fine!

[ogden]

Couldn’t you use the current draw of the motor to get some sort of indication of the motor speed? Higher load = higher current draw, since you know the voltage and the max RPM for that voltage.
I don’t know how accurate this is but for rough estimation it should be fine!

Waste of time. Low friction bearings can lead to faster RPM, yet lower consumption. Even audio spectrum application for mobile phone is way better tool. This looks good

[Giaime]

The DC ripple current can also be used to measure the RPM of a DC brushless fan. 

I did this in the past and it works well. Just compare (with a comparator) the average current consumption (smoothed out) with the unfiltered current, and count pulses with a microcontroller or similar.

[Fludo]

I always used the xenon bulb stroboscopes that flicker with an adjustable RPM.  Once the fan blade appears stationary, the frequency of the stroboscope matches the RPM of the motor.  It doesn't need any special reflector, but won't interface to any other devices.

[Gyro]

I always used the xenon bulb stroboscopes that flicker with an adjustable RPM.  Once the fan blade appears stationary, the frequency of the stroboscope matches the RPM of the motor.  It doesn't need any special reflector, but won't interface to any other devices.

Of course, with with the advent of high intensity LEDs these days, it's much simpler to construct one without having to worry about the high voltages.

[cdev]

I have some centrifugal fans I would like to be able to know the RPM of and they are sealed away inside a metal box. Unfortunately, it looks like they don't have a tachometer connection either. So I am also interested in doing this too.

LEDs generate minute amounts of electricity when exposed to light, in a manner thats proportionate to the amount of light shining on them. So it might be possible to measure their speed with two LEDs and two white labels each affixed to one blade of each fan.

[tsman]

Sometimes the sensor is still fitted. They just don't bother soldering on the extra wire. Check under the label where the wires attach?

[cdev]

Thats a great idea, I definitely will (check for a sensor connection being there).

[den]

I always used the xenon bulb stroboscopes that flicker with an adjustable RPM.  Once the fan blade appears stationary, the frequency of the stroboscope matches the RPM of the motor.  It doesn't need any special reflector, but won't interface to any other devices.

Not so simple. It actually can be a divisor of RPM. With the fan even more interesting - since the blades all look the same, the fan appears stationary when actually turned by one blade further, so strobe frequency can actually also be a multiple of fan RPM (the number of possible divisor frequencies also increases). So strobe method is unfortunately not suitable here.

I personally think optical method is the way to go here. As proposed by a fellow countryman - one lit LED on one side, one LED on the other side of the fan as detector and the scope to sample the detector voltage. Photoresistor probably not suitable as detector - too slow.

[cdev]

Hopefully I can derive a signal by means of the magnetic field as suggested. That would be the easiest.

Otherwise I was thinking of using an LED as the sensor as they generate tiny amounts of power when light shines on them. These fans are unlike most others, they are made by ebm and are very quiet. They are side blowing fans. So their turning would only be visible from a few angles. They are very precisely balanced so whatever was attached to them would have to be very light. A very small square of adhesive coated mylar tape, I am thinking might work best, either reflective or white, depending on where the two LEDs were put. So that the LED would last a long time, I am thinking it would be best to run it at a lower power than its rating.

The amounts of power generated by an LED is very small. When I have connected my voltmeters to LEDs under the lights I currently have the values keep changing. This likely is because of my LED lights flickering at 120 or 60 Hz (I'm in the US) . Would need to look at the 'output' under the sun with an oscilloscope or similar (and shielded cables) to find out what happens under a steady source of light.

[floobydust]

I pick off brushless fan motor RPM by looking at commutation currents, with a data-slicer. It works great as long as you are not using PWM speed-control.
Circuit like this for a small0.1-0.2A fan.