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| How to measure RPM of DC fan? |
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| cdev:
Thats a great idea, I definitely will (check for a sensor connection being there). |
| den:
--- Quote from: Fludo on November 27, 2018, 01:02:08 pm ---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. --- End quote --- 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. |
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