Why are fan controls setup that way?

[Merijeek]

Or course, now it's become habit in my 29 years of life. But it just doesn't make sense. I guess it means we can get used to everything.

There's the fan sitting on my floor. It's off. It's got three power levels. When I turn it one click from Off, I get power level three. One more click is level two. One more click is level one - the slowest the blades turn.

Why? Isn't that counterintuitive? Thing is, pretty much every fan I've ever dealt with is that way.

So...why?

-Joe, fan of fans

[Algernon]

The same is true for ceiling fans, and it's always puzzled me too.

[vetbridge]

WAG...I always assumed it was because when first turning on the fan, a person would likely need "high" speed. Once comforted by that, turning down to med and then low would be next. However, if it gets overly cool you go back through high to get to off. Oh well.

[gotpasswords]

I'm sure QED or Crafter Man will wander by to flesh this out, but the motors that are set up as off-high-medium-low are that way so the motor gets a kick of "high" to start turning.

I've got a small desk fan here that refuses to run if I have it set for low and turn it on via a power strip. If I leave the power strip on and turn the fan's switch from off, go through high, on the way to low, it starts just fine.

[vetbridge]

...and that's why WAGs are bad, O.K.?

[Revtim]

I'm pretty sure there was thread about this already, and if one can make a useful search without 'fan' (which is under the minimum of 4 letter search terms) they might be able to find it.

[flight]

Here

[Balthisar]

Originally Posted by gotpasswords

I'm sure QED or Crafter Man will wander by to flesh this out, but the motors that are set up as off-high-medium-low are that way so the motor gets a kick of "high" to start turning.

Just how slow do you turn on your fans?

[Crafter_Man]

There are two reasons, both of which are interrelated:

1. From the fan manufacturer’s perspective, it is best to give the motor full voltage at startup to get the motor up to speed as quickly as possible. This is due to rotational inertia at startup; the inertia must be overcome one way or another, and the most reliable way to overcome the inertia (especially with a cheap motor) is to start on the highest power setting.

2. As explained in #1, when you first apply power to an electric motor (with a load attached to it), the motor must overcome the rotational inertia necessary to get it up to speed. During this startup time the motor’s impedance is fairly low, and it draws quite a bit of current. And to make matters worse, there’s not much air flowing across the motor windings during this time. Once it gets up to speed the motor’s impedance increases, and thus draws less current. There’s also a lot more air flowing across the motor windings once it is up to speed. It makes sense, then, that you do not want the motor to spend much time in the startup phase, since the motor draws lots of current, the windings are getting hot, not much cooling air for the motor windings, etc. If you’re the designer, this phase will make you “nervous,” and you will want the fan to spend as little time in it as possible. Starting the fan on high minimizes the amount of time the fan spends in this phase.

[Algernon]

Originally Posted by Crafter_Man

It makes sense, then, that you do not want the motor to spend much time in the startup phase, since the motor draws lots of current, the windings are getting hot, not much cooling air for the motor windings, etc. If you’re the designer, this phase will make you “nervous,” and you will want the fan to spend as little time in it as possible. Starting the fan on high minimizes the amount of time the fan spends in this phase.

This makes sense. While this explanation was alluded to by others, you articulated it well. I was hoping you'd show up.

(... find this via a vanity search?)

[Johnny Bravo]

Sometimes when I turn my ceiling fan on I'll manually give it a quick turn to get it going faster. If I'm reading the above post correctly, this means that this is actually a good thing from a mechanical perspective?

[Crafter_Man]

Originally Posted by Algernon

find this via a vanity search?

Yea. That’s why I’m posting 5 days late.

Originally Posted by Johnny Bravo

Sometimes when I turn my ceiling fan on I'll manually give it a quick turn to get it going faster. If I'm reading the above post correctly, this means that this is actually a good thing from a mechanical perspective?

Yes… giving the fan a manual “push” would help limit the in-rush current as explained in my previous post. (Some people might disagree, claiming the fan would have to overcome initial synchronization problems or whatever. Don’t pay attention to these people. Ever shut off a fan for one or two seconds, then turn it back on while the blades were still spinning? It would be the same thing.)

[Balthisar]

Originally Posted by Crafter_Man

There are two reasons, both of which are interrelated:...

But this assumes you turn on the fan slowly. On a typical fan with a control knob, it only takes a quarter of a second to bypass HIGH and MEDIUM to get to LOW. The inrush current's already there, but it's not doing anything -- the fan has barely started moving.

Now I'm curious enough to dig the fan out of the basement, and do some testing with a switched outlet to see if there's a difference in starting from LOW (via the switched outlet), and very rapidly (i.e., "normal") turning the fan onto LOW by passing through HIGH and MEDIUM first.

Uhhh... but I need a cheap, dependable way to gage fan RPM's accelleration curve without investing in any fancy electronic equipment. Any ideas? Or anything work in a lab that could do this experiment?

[Algernon]

I was thinking about this last night, and like Balthisar, I have a follow-up question.

It regards ceiling fans. While the speed controls on a ceiling fan follow the same pattern as floor or desktop fans -- namely, off to high to medium to low -- it is done via a chain or cord hanging from the fan itself.

I don't know about everyone else, but every ceiling fan I've seen is actually turned on and off via a wall switch. The ceiling fan itself remains on the speed it was last set to. For example, my bedroom ceiling fan is currently set on low. It's been such a cool spring and moderate summer around here that I haven't had a need to increase its speed.

So each night when I turn on the ceiling fan via the wall switch, it starts turning at the low speed. This practice (which I perceive as commonplace) completely invalidates the engineer's concerns about current draw.

Could the original reason for the speed control sequence be no longer applicable, and is only continued out of habit or custom?

[Tevye, from Fiddler On The Roof]
Tradition, tradition!!
[/Tevye]

[Crafter_Man]

Originally Posted by Balthisar

But this assumes you turn on the fan slowly. On a typical fan with a control knob, it only takes a quarter of a second to bypass HIGH and MEDIUM to get to LOW. The inrush current's already there, but it's not doing anything -- the fan has barely started moving.

I don’t understand what you’re trying to say here. What assumption am I making?

[Alessan]

Oddly enough, fan switches have often reminded me of the safety catch for the Kalashnikov series of rifles - starting on safe, it's one click for automatic, two for single-shot. Coincidence? Probably.

[williamweigand]

I have two floor fans upstairs - - One , Holmes brand , goes in the sequence of off,high,medium, low - but the other , Windmere brand , goes in the sequence of off, low,medium,high...With that one , of course , I have to switch through medium to high to get the fan really moving.

[Balthisar]

Originally Posted by Crafter_Man

I don’t understand what you’re trying to say here. What assumption am I making?

That going from high to low in that order helps the fan start.

[Algernon]

Can I try to re-phrase this?

Crafter_Man says: The reason the fan switch goes from Off to High rather than to Low is so that the rotational inertia can be more easily overcome, which puts less strain on the motor.

Balthisar says: But I click from Off right past High and Medium until I get to Low, and this happens in such a short period of time that the whole rationale for going to High rather than to Low is moot.

Using Balthisars example combined with my example of ceiling fans being turned on via a wall switch, leads me to the conclusion that there really is no good reason in the real world why fan switches are engineered this way.

[Agent Cooper]

Just to be contrary ... I have used a giant Port-A-Cool swamp cooler (36" fan diameter I think) which has three speeds. In the unstructions, it very specifically says to turn it from off to low, then med., then high to let it get up to speed. The control knob is set up like that also.

What makes the difference? bigger blades? bigger engine? both? why?

[Agent Cooper]

that would be ... INstructions, not UNstructions...

[Q.E.D.]

Ok, I've been giving this some thought, and I've got a WAG: inductance. Most multi-speed fans have multiple windings. The speed is more or less determined by the current through the windings, which in turn is determined by the inductive reactance of the windings. Now, here's the thing: when you've got a current flowing through an inductor, and you stop that current flow suddenly, you get a large voltage induced across it, which is proportional in strength to the inductance and opposite in direction to the original flow of current. The higher the inductance, therefore, the higher the induced EMF. This back EMF will serve to try to spin the motor in the opposite direction, so the idea is to start the motor turning on the winding with the lowest inductance, and therefore the highest speed, so as to minimize internal heating and prolong motor life when you switch speeds.

[Crafter_Man]

Balthisar & Algernon:

Yes, you can pass through high and med very quickly. But the fan manufacturers are taking an “all else being equal” approach. They figure that, since they can design the sequence to be anything (high-med-low, low-med-high, etc.), they may as well make it hi-med-low. This is because they assume that many (most?) users will click the switch once, wait, and then select the speed. Or they assume most people want to set the initial speed to high anyway, in order to get the air moving quickly.

The bottom line is this: the fan manufacturers want to give the motor a hefty jolt to overcome the rotational inertia at power up. This is because they often use cheap motors (e.g. dishwashing machine motors) that don’t have much torque at low excitation levels. Or they’re using a high-efficiency motor, which (coincidentally) has the same problem. If they use a motor that has plenty of torque at low speed, it will be more expensive and/or less efficient at higher speeds.

Here’s a short article on the subject:

http://www.usatoday.com/news/science...gling-fans.htm

[Algernon]

The linked article certainly confirms your supposition Crafter_Man.

In spite of the way people in the real world can override the engineering preferences by operating the fan controls less than optimally, I guess it still makes sense to design them so that there is at least an opportunity to have a best case scenario.




BYW, I can relate to the fan motor -- as a 50+ year old, I don't have much "torque at low excitation levels" either.