How to Slow Down a Fan?...

[RedbowTies88]

Hey guys, so I have this fan that I would like to use for cooling purposes in my office system. It's a vintage 120v "Whisper Fan" from Rotron Inc. It draws 7 Watts at 115 Volts.

I actually have 2 of these, I forget how exactly I came by them but #1 is doing quite nicely in a modified dresser drawer that holds my PS3. The problem is this, despite their name they do generate some noise. They flow far more then would ever really be needed to cool my little office amp, and at 1/2-3.4 speed (approx. Listening during startup) they are very nearly silent. Can I wire it with an inline resistor to bring the speed down?

Thanks.

 

[petehall347]

you can get a variable speed controller . basically has an scr etc .

 

[petehall347]

or a temp controlled one here ...

 

[RedbowTies88]

Those would certainly work, however I was hoping I could just run a resistor inline. If that would work that is...

If so I also don't know the formulas to determine what the specs on such a resistor would have to be.

 

[Sam Cogley]

http://www.harborfreight.com/router-speed-control-43060.html

 

[RedbowTies88]

Thanks for the link Sam, but that's a little more involved then I was hoping to get. I also would rather not have to find a place to hide another piece of equipment.

Just for the heck of it I hooked the fan to my Variac. It won't even spin until it hits approx. 68-70V. It doesn't really get noisy until you surpass 95-100v.

 

[RedbowTies88]

Also I forgot to mention that Harbor Freight is an hour away but I have a Radishack 5 minutes down the road...

Would I have to use a transformer to accomplish this? I saw some things on line about a voltage divider using resistors but being a noob I don't quite understand how to achieve the desired result.

 

[petehall347]

i imagine a resistor would need to be a rather large dropper resistor .. i suppose if mounted in front of the fan it would help cooling it .
how many amps does the fan draw ?

 

[RedbowTies88]

i imagine a resistor would need to be a rather large dropper resistor .. i suppose if mounted in front of the fan it would help cooling it .
how many amps does the fan draw ?

It's 7watts @ 115VAC.

I looked up the formula for that just now but it says I need something called power factor?

 

[EchoWars]

You can work with purely resistive values and get close enough...

Figure about 61mA running direct from AC. Approximate impedance would be about 1.9K. If we set a target of about 85V (you said that it doesn't get noisy till about 95V), then we're talking about a current of about 45mA. So we're looking to drop about 35V @ 45mA. A 750 ohm 3W resistor ought to get you pretty close.

 

[Toob]

a dimmer switch from the hardware store?

 

[leesonic]

I thought an in-line capacitor was how you slow down an AC motor?

 

[EchoWars]

You can... 3.5uf will give you about 750 ohms@ 60Hz...BUT (and this is serious), if we're operating directly off a 120V AC line, you are required to have a safety cap. I know of no X/Y rated capacitor so large (I'm sure they exist, but they will be hard to source and pretty unwieldy).

a dimmer switch from the hardware store?

Might work...might not. Dimmers work by varying the AC duty cycle. I'd imagine that some fans won't like that.

 

[Guest106]

The real problem with slowing down AC motors is they tend to overheat, no matter what method you use. Speed controls that vary the duty cycle of the AC are the best, but even then it's going to create heat. The stator in an AC motor is made to move at a set speed. In order to really slow the motor down you need to vary the sign wave that's being fed to it. Or vary the windings so the field is wider apart. Think ceiling fans, they have 3 full sets of windings. To change speeds they switch out what windings are used to create the field.

Do some tests. I did a BUNCH of tests with small AC fan motors. Speed controls, resistors, step down transformers... every time I got it to actually start it would at some point either just over heat, or pick up speed. I ended up using computer fans with a wall wart to supply the power. Much quieter and easy to replace when they need it.

Good luck.

 

[SolderIron]

>The real problem with slowing down AC motors is they tend to overheat, no matter what method you use. Speed controls that vary the duty cycle of the AC are the best, but even then it's going to create heat.

I recall most AC motors speed are fixed by the design (Number of poles and line frequency). Can't change either easily...

 

[PioneerGuy75]

a dimmer switch from the hardware store?

Dimmers work by varying the AC duty cycle. I'd imagine that some fans won't like that.

:nono::nono::nono::nono:

MOST fans don't like that!

 

[sKiZo]

https://www.youtube.com/watch?v=czfGiYDXSKo

Plug a computer case fan into a 5v USB wall wart?

 

[RedbowTies88]

Wow there's so much more to this then expected. I think I'll try the resistor option first.

If the fan burns out in not really going to sweat it, then I could get a small laptop cooling pad and toss that on top.

 

[sKiZo]

Like this?

My HTPC, with four fans in the case (not counting the power supply) and it still ran hot ... and loud. Cut some holes in the top and plunked this bad boy down on top. Finally trade the old guts for one of those new 60w APU ITX boards. Laptop cooler is no longer needed, I'm down to just the CPU and PS fans, and I can barely hear those if I'm right up to the box.