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I have a small portable fan, around 15 cm in diameter and lately it is behaving oddly.

Basically I turn it on, the airflow is good but then after 10 minutes or 30 minutes or sometimes hours, the airflow almost stops.

The fan keeps spinning and it sounds the same but the airflow is just minimal.

There are 3 speed settings, I usually keep it at the lowest but it happens on any of these speeds. Once it happens, if I switch the fan to maximum speed, I can hear the change and it sounds like there should be a bunch of air coming but still... very little air.

The only sure way to make it blow well again is to turn it off, wait until the blade completely stops and then turn it on again. Sometimes I've gotten it to work well again by shaking it or knocking on the side of it but the success rate isn't great. Even though turning it off and on as described takes a solid 30s, its normally way faster than using physical coercion.

So I'm just curious.. what could be the reason for this? I've had some hypotheses:

  1. Perhaps a strange turbulence forms that prevents the air from flowing well - doubt it, I'd imagine this sort of dynamic would be fragile and easily broken by changing the fan speed or just moving the fan around.
  2. The fan is rotating way slower than it sounds - can't really rule it out since I have no means of measuring the actual speed but if this were the case, the airflow is so little that I should be able to count the RPM by eye.
  3. The fan is inexplicably turning in the wrong direction - nope, I looked at the direction its going in as it slows down when stopping and its the same direction as when its starting.
  4. The motor somehow spins without turning the fan blades - given the design (seems like a repurposed computer fan) I rather doubt it.

That leaves me with... no clue. Does anybody have an idea?

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    $\begingroup$ Is thew fan slipping on the shaft once it gets warm? $\endgroup$
    – Solar Mike
    Commented Feb 26, 2022 at 12:42
  • $\begingroup$ Monitor the fan speed. Try putting something like a cable tie in place so that it gets hit by the fan blades. Use a guitar tuner or frequency meter app on your phone to measure the frequency. $\endgroup$
    – Transistor
    Commented Feb 26, 2022 at 12:45
  • $\begingroup$ @Transistor that's a great idea, I'll try that the next time it happens. SolarMike - I don't think so, its a similar design to a typical computer fan where the fanblades are attached to the same piece of plastic that holds the permanent magnets that go around the electromagnet core $\endgroup$
    – user81993
    Commented Feb 26, 2022 at 13:05
  • $\begingroup$ You need to do it before it happens so that you have a reference value. $\endgroup$
    – Transistor
    Commented Feb 26, 2022 at 15:15
  • $\begingroup$ @Transistor I used a frequency meter app while sticking a piece of cardboard into the fan blades, measured twice while the lack of airflow was in effect and half a dozen times while it was working normally to get my technique down. Results were quite stable at lowest speed setting: ~3500Hz while the airflow was good, 4700Hz while the airflow was almost absent. $\endgroup$
    – user81993
    Commented Feb 26, 2022 at 16:37

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It is possible that it is stalling.

If the vector angle of the flow it creates, called relative wind, with the rotating edge of blade becomes larger than 16 degrees it may stall.

When you re start the fan you interrupt the vortex and stall it.

Many poorly designed fans stall but they slowly recover, hence you hear the throbbing.

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