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Electric fans are often controlled by a knob which the user turns to power the fan on or off, and control the fan's speed.

On a lot of fans, the knob goes immediately from the off position to the highest speed, followed by the other speed settings in descending order.

For example on the picture below, you can see the knob going from the off position ('0') to the highest ('3'), and then the other two in descending speed order ('2', '1').

box fan top

Why is that?

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I'm not too sure, but I think it's an electrical reason: when a circuit is first switched on, there is, momentarily, a huge spike in current. I suspect the fan slows down using some form of resistor. There is probably a maximum current that can pass through these resistors, to avoid destroying them. So, the fan cannot initially have a high resistance until the current has settled down. –  Brendon Sep 13 '13 at 0:48
    
Sort of related to this would be the gas knob on cookers/stoves. From what I have seen, they go from "high" to "low" turning clockwise, with the off position anti-clockwise of "high". I suspect this is for safety. –  Brendon Sep 13 '13 at 0:51
    
It's worth noting that the fan setting in your car goes in the order you would expect: Off -> 1 (lowest) -> 2 -> 3 -> 4 (highest). If the decision behind the design is because of a spike in current, how come they manage to work around this in cars? –  cimmanon Sep 13 '13 at 20:01
    
@cimmanon there is a difference between 12V DC (car) and 220/120V AC (regular outlet) for the inrush current –  ratchet freak Feb 7 at 9:03
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3 Answers

And I found some thread discussing on this particular question as well.

http://boards.straightdope.com/sdmb/showthread.php?t=292238

Look like is because of the rheostat and how it works.

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I'm so tempted to suggest that they did analytics and found that the fastest speed is the most popular, and the slow one the least. But that's nothing more than a guess.

Although I'm not a motor expert, from a quick research this is what I understood. I'm pretty sure this is correct, but to get a definite answer, I guess the post is more appropriate in the electronics forum.

I'm really simplifying the explanation, which anyhow requires some understanding of electronics.

Anyhow:

Motor Technology

Inductance

Electric motors' key principle of operation is based on inductance. That is, a current flows through a coil to create a magnetic field, and magnets within this field start spin, being attached to a central pole. (It is also possible to have the coil on the spinning component and the magnets fixed.)

As with other inductive setups, once the moving components start spin they create what is known as Back EMF - a force opposite that of the force that caused the movement. In electrical terms, the more back EMF, the more resistance the inductor presents in the circuit.

Single Coil Designs

While the fan is at rest, there is little resistance in the circuit. Thus, the circuit draws a lot of current, which may 'fry' the fan - various components can overheat.

The sooner the fan starts spin, the quicker the resistance rise, and the current consumption drops.

So quick starting of the rotation is beneficial here.

Multi Coil Designs

Another type of design involves an even number of coils arranged in a circle around the spinning pole.

With this design, a different pair of coils are triggered in concession, depending on the angle of the spinning part. This requires a controller that can sense the angular position of the spinning component. This is done sensing the resistance of each coil. But if the back EMF is too low (due to slow rotation) the controller can't sense the position of the fan, which yields jittered behaviour.

Here again, there's a need for quick start.

As you can see, either designs would benefit from a quick start of the fan. Thus, the fastest mode is the one next to the off position.

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My guess is that...

Imagine you have to operate the device in darkness (no light environment) or someone with visual difficulties, placing the off setting next to the highest allow the user to know they've switched off the device.

See it this way, you are at position 2, your intention is to switch off the fan, you knob in the "0,1,2,3" setting, you will not be able to tell immediately if you are at the 1 position or off position as the fan is turning at a similar pace. (Slower but unable to identify easily).

Whereas, by placing the setting as "1,2,3,0", you will be able to tell immediately if you are at position 3 (faster and louder noise) or at position 0 (significantly slower and lesser noise). Thus, user in dark places can "feel" the setting without looking at the knobs.

Don't shoot me if I'm wrong, this is just my guess. Cheers. =)

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When switching the fan off, you turn the knob all the way until it reaches a physical block; it seems like this haptic feedback in itself would suffice for the user to know with 100% confidence that they've powered on the fan. –  Bitgarden Sep 13 '13 at 6:14
    
Oops typo. Last sentence should read "... that they've powered OFF the fan." –  Bitgarden Sep 13 '13 at 18:25
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