Hi. I’ve been lurking for a while, enjoying the debates/pissing matches. Tons of fun. I was turning on a fan the other day and thought ``Aha! My chance for SDMB immortality!’’ So here goes:
Most (perhaps all) of the household fans I’ve seen go straight from off'' to high’’ and then down to medium'' and low.’’ It makes no sense to me that fans should start off at full power. It’s like starting off your car in fourth gear.
Maybe there’s a sound engineering reason. Or maybe it’s a psychological thing: start the fans on ``high’’ and customers are just wowed by their power.
Most electric-fan motors – which are designed for the light load of just a fan blade moving through air, and in consideration of low power usage, since they may be left on for significant time – can, once having accelerated to a certain speed, run on a fairly low voltage/current/power; but getting them accelerated reliably requires that they initially receive more oomph. Thus, in making the first switch ON position the high-speed one, it is expected that the fan will get up enough speed while in that position to get started – before you are able to switch it to the following low-speed position. It’s sort of like a JATO.
The device you refer to is called a rheostat. Basicly, a switch with resistance. The closer the electrodes are together the more current can pass through. Hence 0 breaks the connection, 1 is “high” and the further away the contacts get from each other, the less current passes through. Result, a speed control that seems backwards compared to a mechanically geared device.
I remember having a ceiling fan a long time ago that specificly said in the instructions to let the fan start off in HIGH and once it was spinning to switch to a lower speed if desired. They made it clear that starting the fan by immediately switching over to the low speed would be harmful to the motor. I remember thinking it was weird, but only until I actually did start the fan in LOW. You could hear the motor groaning and straining to move the fan blades faster with each turn.
I don’t think many fans have rheostats. No one needs a continuous range of speeds and a rheostat costs more than a switch with 3 or 4 positions. But I haven’t checked them all out.
Why, oh why, everytime I post something does someone have to jump in and disagree with me? Are you just trying to insure Clarks maximum enjoyment? (he mentioned likeing pissing matches)
Stephen Quote,
You could put the ‘open’ at either end (or both) of the rheostat.
But wouldn’t this defeat your argument in a rotary switch design?
Nanobyte Quote,
I don’t think many fans have rheostats. No one needs a continuous range of speeds and a rheostat costs more than a switch with 3 or 4 positions.
I had an old fan lying around so I smashed open the little plastic box that housed the switch. What do you think I found under those little buttons? It was a cheapo that cost less than 15 bucks.
Incedentaly the thing was never, ever turned on to it’s low position but it burnt out in under 3 months.
Ok on to the general rebuttals. My A/C (a rather pricey appliance) has 3 fan positions. They are arranged in the typical 3 button array. Nowhere in the instruction manual does it say to start it on high.
Yes, an electric motor needs to draw more current to start. Note the lights dimming when the compressor motor on the refrigerator kicks in.
My conclusion? Some devices have a low setting that is inadequate to start the motor so the manufacturer puts in a little warranty clause. The celing fan in my bedroom is wired to a reostat (or is it called a potentiometer with A/C power?) But the new one in the living room has a solid state switch of some kind (also a remote control, way cool) The new one always starts out on high no matter what speed you choose then settles down once it’s spinning. The old one in my bedroom? Well I start it on low every night have been for over 10 years now. Aside from swinging around like pendulum, it stills runs as well as the day I bought it. I bet it outlasts the high tech one out front as well. (It’s still way cool though)
Rheostats are rarely if ever used today for speed control because they generate a lot of heat. Speed control of most consumer type AC (alternating current) motors is done with an electronic circuit that uses either SCR’s or Triacs. These are devices that can turn on and off fast enough to prevent part of the Ac waveform from reaching the motor thus causing the motor to run at less than full speed.
No. Whether the ‘Stop’ is adjacent to ‘High’ or ‘Low’ depends on where they place the pin that prevents the knob from rotating 360 degrees. In fact, they make knobs that rotate 360. All sorts of combinations are manufactured, so the question is “why do they choose a knob that moves through ‘High’ to get to ‘Low’?” and the answer is, that the motor develops less heat and wears less when the initial impulse is more energetic.
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Stephen,
After responding to your rely, It occured to me my post was full of ______. So I went to my tool box. In there I looked at the 2 dimmer switches and the one fan speed control I had left over from a job. Lo and behold the dimmers do indeed start from a low voltage position and the fan speed starts out on high. The only other obvious difference was the fan control was built to handle a much higher load than the dimmer. I erreoneous assumed it was the ONLY difference.
As to why people are always disagreeing with me. Well I put that there as an invite to hacik at me and I was a little disapointed. Damn I expected much worse. I guess the reason in this case was because I was wrong
Glad to see my question produced a little heat. Can’t say that I really follow the technical explanations (electronics ain’t my strong suit, to put it mildly), but I enjoyed the show. Thanks.
