Question CPU fan almost stops constantly

[Spacekeleton]

Hi, I was removing GPU from my build today and after that I was looking at how my system works and noticed that once every few seconds CPU fan almost stops itself or goes at speeds so low I can clearly see fan's blades for a brief moment.
I checked bios and it said CPU fan is at 0 RPM though it was spinning. Curve start is set to 13% at 40C degrees but my CPU is usually at lower temps like 35-38. Cooler is Noctua NH-D15.
Is it normal?

 

[Satan-IR]

What is the CPU? Anyway if at idle it's sitting at 35-38 C I'd say the cooling is doing it's job.

That cooler has large heatsink with quite a few pipes running through it and does a lot of passive siddipation of heat through the pipes and blades. Fan would only need to work hard under heavy load.

However I'm not sure about it stopping or going that slow. Has the sound of the fans changed at all copared to before?

 

[DRagor]

Curve start is set to 13% at 40C degrees

According to fan specification its operating range is 300-1500 RPM. So you should not set the curve below 20% as it can indeed make the fan go below its minimum speed and stop (which to the eye will look like you described).

 

[Spacekeleton]

What is the CPU? Anyway if at idle it's sitting at 35-38 C I'd say the cooling is doing it's job.

That cooler has large heatsink with quite a few pipes running through it and does a lot of passive siddipation of heat through the pipes and blades. Fan would only need to work hard under heavy load.

However I'm not sure about it stopping or going that slow. Has the sound of the fans changed at all copared to before?

Idle temps are 29-30 degrees, 35-38 are at some load.
CPU is i7 10700K
And I think I am going crazy, I noticed that it's not stopping, I just can sometimes see fan blades when I try to look at different parts of fan. Like when you look at watch and try to follow their minute/second hand.
Though I am not sure if it is okay that CPU fan is showing 0 RPM.

 

[Nine Layer Nige]

I try to get my Arctic Freezer i35 to turn as slow as possible when idling, it very almost stops for absolute quite running. All my other case fans go to zero rpm at idle too, using my custom fan profile in the BIOS.
Just as long as your fan ramps up under load, that would seem normal.

 

[Paperdoc]

One feature of most fan headers is monitoring the fan for failure - that is, for a speed reading that is zero, or less than some trigger value. SOME mobos actually allow a fan to stop at low temps, but I think that's not a good idea. Anyway, assuming that stopping is NOT considered normal, all mobos take corrective action when a fan stops. When that failure is detected, the header raises the fan signals to call for full speed so it DOES start up reliably. If the fan does re-start immediately, the header then reduces the speed signal to where it was before. If the fan fails to re-start, you get a fan failure warning on screen.

If that process were not in place, the alternative would be this. The fan would stall when run too slow, but there would still be a small current flowing through it because there IS a voltage supplied, just not enough to re-start the fan. That heats up the motor, but it is not turning so it cannot cool itself. Moreover, the item it is supposed to cool (in this case, the CPU) would be getting NO cooling air flow, so it would heat up. As it does that, the mobo header would respond by raising the speed signal to the fan in response to higher temps. Eventually the signal would be enough to re-start the stalled fan, which would restore cooling air flow until it cooled down again. And repeat. In the meantime, each cycle like this would produce a heat-up period for both the fan and the target heat source, and then a cool-down period. Thermal cycling like that is to be avoided; hence the built-in process to detect fan stalling and try to re-start immediately.

Probably you are observing this process repeatedly. The fan is re-started and returned to its slow speed at a setting that it too slow, so it stalls again, and repeats the re-start-slow-down-stall cycle. As DRagor said, change your minimum speed setting to a higher value.

 

[Satan-IR]

Idle temps are 29-30 degrees, 35-38 are at some load.
CPU is i7 10700K
And I think I am going crazy, I noticed that it's not stopping, I just can sometimes see fan blades when I try to look at different parts of fan. Like when you look at watch and try to follow their minute/second hand.
Though I am not sure if it is okay that CPU fan is showing 0 RPM.

That's even better temps than I thought were at idle. As said already if the fan ramps up under load and you don't get high temps all is good.

Also what app/suite/software shows fan RPM as zero?

Increasing minimum fan speed is good advice too.

 

[Spacekeleton]

That's even better temps than I thought were at idle. As said already if the fan ramps up under load and you don't get high temps all is good.

Also what app/suite/software shows fan RPM as zero?

Increasing minimum fan speed is good advice too.

It's shown in mobo BIOS

 

[Satan-IR]

It's shown in mobo BIOS

Well increasing the minimums would be a good idea.