Need help in lowering voltage to A/C fan

[dude]

I have a couple of rack mounted boxes that get too hot for my liking (Delta 1010 recording boxes). I have a nice small a/c fan from a Boeing 707, my dad worked with Boeing.

I'd like to run the fan on say around 24 volts a/c from a 120 v wall outlet. I have a few wall warts running other rack equipment but they're all d/c the fan is a/c, 60cycle, 120v.

Can I build an a/c rectifier using 1N4007 diodes to lower the voltage keeping the "ac" voltage?  The hum and noise from this fan is way too loud.

I have used 12v PC fans with heat sinks changing ac to dc with 1N4007's to cool down a hot VVR just thought maybe I could go the other way...?

Any help appreciated,

al

al

[andrew_k]

Run 5 in series and remove the blades from all but one.



 

[PRR]

> an a/c rectifier using 1N4007 diodes to lower the voltage keeping the "ac" voltage?

This makes no sense. Or rather: it ain't that easy to go the other way.

Also: if a fan is too-much, 20% will be too little. If you squint a bit, you can say that airflow and noise are like the *square* of voltage. 20% voltage is 4% airflow, moth-power.

Anyway: you CAN'T control AC motors with voltage. They "want" to run at line frequency. 60Hz is 3600RPM. If the motor could run this fast, it would draw zero current. The back-kick of the 3,600RPM exactly equals the forward kick of the 60CPS.

Under a fan load it is probably designed to run 3,400RPM. This 200RPM sag sucks just enough current to equal the air resistance.

If you drop the voltage enough that the motor slows down, the current RISES.

Last time I tried this I filled the attic with smoke. (It was a larger fan.)

Making lower speed AC power is possible but will need other adjustments. There's a whole field of variable-speed AC motors. It is advanced engineering.

Use a DC motor. Voltage determines speed, and there is no drastic motor-melting.

PC fans are THE cat-meow for small audio cooling. 5V wall-warts clog my GoodWill store, and will run a 12V case-fan at a big slow quiet breeze.

[dude]

I hear ya, PC processor fans are great a few diodes and you run it off the heater supply with the heat sink over mosfet you'll never over heat again.

Sounds like a PC power supply fan would be the ticket, bigger and I think they run on 12vdc too, a wall wart at 6vdc would do the trick.
 

But:

when ac fans have a "three speed" fan control what's going down there?

So, lowering the speed RPM, increases the draw, current . I can smell smoke.

It's such a nice fan, built solid and the right size but puts out way too much air flow and wining noise.

I have a router control that lowers the RPM of my router, I wonder what's in that little box with a variable speed dial...?

al

[PRR]

I should have said "with exceptions". Which don't include a fan such as you describe.

> when ac fans have a "three speed" fan control what's going down there?

For "high" power, there are 2, 4, or 6 windings so it can sync 60Hz at 3600, 1800, or 1200 RPM. (Many alternate combinations are possible... a large fan may run 6 8 and 12 poles for 1200 900 and 600.)

For low-price ceiling fans, they may waste considerable power in the fairly large motor, and use that wastage to limit the current rise to some tolerable amount. (A Boeing can't afford such tricks.)

> I have a router control that lowers the RPM of my router

Most small power tools have "universal" motors. These are DC motors rigged to also run correctly on AC. Their speed is in fact some function of applied voltage. They can be wound to run much faster than 3600 RPM (router may be 20,000 RPM). They can be fairly compact for large peak power (but overheat on sustained heavy load). They have brushes which spark and wear. Good for router, bad for cooling audio.

I'm sure it is a fab fan. Can't you use it in the window?

[PRR]

me> "universal" motors. ..DC motors rigged to also run correctly on AC.

You object that your router is marked "AC" only.

That's actually an issue with the switch. AC stops 120 times a second, DC never stops. Breaking DC requires much heftier switch-contacts than AC. Since nobody has enough DC to run a router, they only give you an AC-rated switch.

OTOH, your router control is probably using AC principles, and would jam full-speed on DC.

[Merlin]

Perhaps the easiest way to run a small fan (which doesn't tax your heater supply) is to supply it directly from the mains via a X-type capacitor to reduce the current to the desired level. If it's a DC fan then you can also add a bridge rectifier of course.