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.
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.
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.
There are two reasons, both of which are interrelated:
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.
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.
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.)
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?
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]
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.
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.
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?