Blocking a fan makes the blades speed up. This is easiest to observe with a vacuum cleaner: the motor increases in pitch when the nozzle gets occluded. Why does making it harder to pump the air actually make the blades easier to spin?

Fan blades, being airfoils, generally have a high lift to drag ratio. This means that they push the air perpendicular to the plane they are in. Since they are tilted slightly, "perpendicular" means the air is pushed mostly forward (out the fan) and a little prograde (i.e. they impart a little bit of swirling motion to the air). The angular momentum in this swirling motion must be replaced by the motor.

When a fan is blocked, the air starts to be pushed forward but "piles up" against the blockage. The trapped air ends up swirling around which makes it harder for the blades to impart further angular momentum to it. Thus there is less torque on the motor and it speeds up. Is this line of reasoning correct?

A similar argument can be made for blowers which throw the air out like a centrifuge: when airflow gets blocked there won't be new air passing through blades and gaining angular momentum from them.

  • 3
    $\begingroup$ Essentially, blocking the air reduces the load on the motor causing it to speed up $\endgroup$
    – Bob D
    Commented Jan 22, 2023 at 13:26

1 Answer 1


When you block the airflow through a spinning fan, you force the fan blades to stall and in this state, the kinetic energy they transfer to the air is reduced. This means that the torque load imposed on the blades by the air goes down, which allows the fan motor to speed up.

In the ultimate case of no torque load at all, imagine the fan is operating in a vacuum. No energy transfer occurs (since there is no air at all) and the fan can rev up to the point where all the motor power is absorbed by bearing friction instead of air movement.

  • $\begingroup$ Could you expand on the meaning of stall in this context? $\endgroup$
    – benjimin
    Commented Jan 22, 2023 at 21:02
  • $\begingroup$ @benjimin, it means the airflow no longer follows the contours of the fan blades, but has separated from the blades, creating turbulence and thrash instead of accelerating the air. $\endgroup$ Commented Jan 23, 2023 at 0:34
  • $\begingroup$ Is it possible to quantify the increase in RPM that the near-vacuum conditions would cause? Or is it highly dependent from the motor itself? I am asking because I am curious why vacuum cleaners do not control the motors in such a way that RPM remain constant under different conditions. The pitch increase is quite noticeable so I would assume the increase in RPM is also quite big, which in turn means the motor is under more stress $\endgroup$
    – Redirectk
    Commented Jan 24, 2023 at 22:12
  • $\begingroup$ You can quantify it closely by running the motor with the impeller removed, and noting the resulting RPM. this is the unloaded RPM case. if the motor is unloaded, it will be performing almost no work and hence will be consuming almost no power- just spinning at the speed where its induced back voltage equals the mains voltage, and current falls to a low value. $\endgroup$ Commented Jan 25, 2023 at 0:03

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