how to design a circuit to determine the rpm and torque from a fan?
Firstly, welcome to PF.
We need more information. It will be difficult to measure RPM without some form of external detector, but since Power = Torque * RPM, measuring the power, (or the current at a fixed voltage), will make it possible to calculate torque.
What type of fan are you using?
What is it's size and power?
What is it's rated RPM?
What is it used for? Ventilation? or engine cooling?
Is it 12VDC or AC?
Is the fan driven by an induction motor, a universal brush motor or an IC engine?
Another way is to measure the torque that the fan applies to it's mounts. Perhaps mount the fan on an arm that can pivot, use strain gauges or a spring balance etc
You can buy optical devices to measure propeller rpm from model aircraft shops. If the fan uses a brushless motor it might be possible to measure the rpm by tapping off the controller...but if you are able to do that you probably wouldn't be asking the question here.
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