Correct calculation of fan rpm in relation to pwm%

Hi everyone,

given a fan specs:

min rpm = 300

max rpm = 1500

what is the correct way to translate pwm% to rpm?

here are two methods:

1. The most simple:

20% = 1500*0.2 = 300 rpm

30% = 1500*0.3 = 450 rpm

...

...

2. considering 300 rpm is the minimum speed, you take the delta between 300 and 1500 = 1200. Than adding the percentage to 300

For example:

20% = 1200*0.2 = 240. 300+240 = 540

30% = 1200*0.3 = 360. 300+360 = 660

...

...

What do you think? which is more correct?

Unfortunately neither. The only accurate way to turn a PWM % into RPM is to run the scale it for that specific fan. Set 25% PWM = xxx rpm. Then 30%, 35%, 50%, 75% or whatever numbers you need. As I mentioned earlier, most PWM % on fans are not perfectly linear. It's fine to calculate a 50% PWM curve for a 1500 rpm fan to equal 750 rpm. Just don't be surprised if it is off to one side or the other. In my experience, you will see more deviation at the upper and lower limits (>85% and <30%), while the results match the math better in the middle.

That is just strictly motor signal response. Additional physical restrictions may further alter the end speed. Two identical fans at 50% PWM are not likely to run the same speed if one is in free air and the other against a dense radiator or heavy dust filter.

ok.. so if I want to "know" what is the 25% pwm in rpm value for a specific fan I should just run that fan at 25% in the software and see what is the rpm?

But it will vary slightly, let's say 1400..1420..1415.. So I should just go for the middle or something?

Yes, set it and take the real result. If you can't for whatever reason, just go with the straight calculation (PWM %) x (max rpm).

When I say variance, I don't mean tiny 10-20 rpm fluctuations you are going to see no matter what. Some fans will have very steep %/rpm curves in at the upper end or abnormally flat at the lower end. I have some that might go something like 80% (of 1500 rpm) = 1200, 85%=1275, 90%=1450, 95%=1460, 99%=1470. It also is not uncommon to see a really flat low end like 25%=400, 30%=435, 35%=475, then a jump up at 40%=600.

Keep in mind small differences in speed have virtually no effect on cooling. 100 rpm difference is about the minimum interval for observation. The only reason to fuss about smaller number is if you find your self hovering around specific motor behavior on a fan. Some will shift into a different tone at rpm xxx in response to a motor phase pole change or something similar. You can hear it sounds distinctly different at 925 rpm vs 885 rpm. In those cases, always go with the under speed. For cooling purposes, you can get away with low, medium high. We tend to be far more discriminatory about noise and that is where the granular control comes in.