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Many AC fans, including inexpensive floor models, have a rotary speed control which rotates from OFF to HIGH to MED to LOW. Since the switch does not rotate 360°, you have to rotate it backwards to shut it off. This has always slightly annoyed me, because it appears to make no logical sense to have to speed the fan up to turn it off.

Why are the speed settings from fastest to slowest? This design seems counter-intuitive, but I assume there is a good reason because the design is so common.

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    \$\begingroup\$ Just to note, ceiling fans with a pull cord seem to follow the paradigm too, only it cycles from low to off and then back to high again. \$\endgroup\$ – Michael Nov 3 '18 at 17:18
  • \$\begingroup\$ @Michael Ceiling fans with a rotary switch though often have a circular option. Go from 0 > 1 > 2 > 3 > 4 > 5 > 0. But those take a lot longer to accelerate to the set speed than small floor models, so they have a different design altogether. \$\endgroup\$ – Mast Nov 4 '18 at 13:22
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    \$\begingroup\$ I have five ceiling fans at home and one of them is opposite this - it goes off to low to medium to high - and it drives everyone crazy. Whether there is currently a reason for the common design or not, there is certainly a precedent created by fans having this design that makes it logical to keep it that way. \$\endgroup\$ – Moshe Katz Nov 4 '18 at 23:30
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    \$\begingroup\$ @MosheKatz I've seen a few floor fans now that use the more intuitive off-low-med-high which indeed is simultaneously counterintuitive if you have done it the other way your whole life. It may be that there is a switchover to Brushless DC motors in some models, which have embedded electronics that knows how to start the motor, and for which the low-med-high fan speed settings are really just targets for the circuit to achieve. \$\endgroup\$ – uhoh Nov 6 '18 at 5:05
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Fans of that type have induction motors with two windings with a capacitor in series with one of the windings. For every individual motor design there is a certain capacitor value that allows the motor to develop maximum torque and operate at the maximum corresponding speed. Smaller capacitor values are used to reduce the torque so that the load overcomes the motor's torque and slows the motor down.

When the motor is turned on, the motor must overcome the static friction of the bearings and get the fan moving. It must then accelerate the motor inertia. That means that using the highest torque setting is desirable for starting the motor. Once the fan is moving, the torque can be reduced for lower speed operation.

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    \$\begingroup\$ Starting on high is also preferred for non-induction motors, when the application allows. \$\endgroup\$ – Brock Adams Nov 3 '18 at 17:43
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    \$\begingroup\$ Seems like this could be tested by unplugging the fan, turning the switch to low, and plugging the fan back in, couldn’t it? \$\endgroup\$ – dudeman Nov 4 '18 at 4:55
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    \$\begingroup\$ @dudeman I have a cheap fan blowing on me at this very moment where I have to turn it to high for a few seconds before switching to low or else it never gets enough speed to keep spinning. \$\endgroup\$ – fluffy Nov 4 '18 at 5:04
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    \$\begingroup\$ So it's effectively an abstraction leak, albeit a fair one for a simple mechanical process. \$\endgroup\$ – Lightness Races in Orbit Nov 5 '18 at 12:00
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    \$\begingroup\$ I have several cheap box fans throughout my house, all with High as the first setting, and all perfectly able to start on the Low setting. Static friction certainly doesn't seem to be a concern here. Maybe they're designing for 10 years from now when the motor is filled with dust, but I'd like to see confirmation of this from one of the actual designers. \$\endgroup\$ – James Nov 5 '18 at 19:38
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It may be intentionally designed to force the fan on "high" for it to spin up before allowing it to drop back down to "low"

The amount of power needed to overcome the static rolling resistance can be higher than what is output to maintain the "low" speed, thus it could end up stalling and not moving or only wiggling back and forth on a "low" setting

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Not just fans

A lot of devices are designed this way. It almost seems standard, even though I agree it is counter-intuitive.

For example, in my kitchen I have:

  • Cooktop hood fan/light - fan goes from Off to High to Low (continuous except for switch Off)
  • Cooktop hood fan/light - light goes from Off to High to Low (discrete settings)
  • Cooktop gas burners - Off to High to Low (continuous except for switch Off)

In the case of the fan, the other answers regarding motor startup make sense and would apply to this fan as well.

In the case of the light, I suspect it is to match the fan since it is part of the same device.

In the case of the burners, I think it is because ignition needs to be done when first turning on a burner and ignition is most reliable on the High setting, so starting with High makes sense.

The end result is all using the counter-intuitive Off->High->Low

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    \$\begingroup\$ The gas burners are because ignition is more reliable at high power (conceptually similar to the fans), and the flame-out sensor needs a bit of warming up, which can take a long time on low. My cooker hood extractor has discrete settings and can be left on low with a separate on switch - just as well as high is deafening \$\endgroup\$ – Chris H Nov 5 '18 at 8:54
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    \$\begingroup\$ The gas burner settings are made so that you can't accidentally turn the cooker off when trying to achieve a low setting. The difference between just-on and just-off is harder to distinguish than full-on and full-off. I would not be surprised if the same reasoning applies to fans. \$\endgroup\$ – Sanchises Nov 5 '18 at 9:46
  • \$\begingroup\$ My cooktop hood fan has four separate buttons for off, low, medium, high and you can start it by pressing any one of the three "not-off" buttons. \$\endgroup\$ – David Richerby Nov 5 '18 at 19:21

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