Slowing an AC fan

[alank2]

I bought a replacement fan for my old compaq portable and it is too noisy.  Probably pushed too much air and I could slow it down without any adverse effects.  The question is - what is the best way to do this?

[james_s]

Easiest way might be to find a quieter fan. A triac light dimmer will often work on those shaded pole motors, particularly if you only need to drop the speed a bit.

[alank2]

Where can you get a triac light dimmer?  Do you mean something premade or a circuit?

[james_s]

Premade devices, any hardware store has them, or you can find kits on a bare PCB on ebay and other places. You could build your own from scratch but the parts will probably cost more than the kit.

[alank2]

Like this?

https://www.ebay.com/itm/Light-Dimmer-500W-220-240VAC-Lamp-Light-Dimmer-Assembled-Kit-DIAC-TRIAC/141024301279?hash=item20d5b410df:g:NUEAAOSwkNZUfJJa

Will it work at 120VAC as well?

[Yannick99]

Try a diode in serie, you will have 50% slower

[tru3533]

Hard to beat this one:

USD 0,73 Free shipping
https://www.aliexpress.com/item/Free-Shipping-220V-2000W-Speed-Controller-SCR-Voltage-Regulator-Dimming-Dimmers-Thermostat/32826505461.html

It will work on 110 VAC by adjusting the other potmeter.

[David Hess]

Try a diode in serie, you will have 50% slower

Whoa, if this is actually an AC fan than a diode will not work at all.

A TRIAC dimmer or transformer to step the voltage down will work with an AC tubeaxial fan however my preferred solution is to place a high voltage film capacitor with a value somewhere between 1 and 10 microfarads in series.  Unlike a TRIAC dimmer, this produces no switching noise.

[ciccio]

If, as I understand, the fan is inside the computer, than it is a DC fan.
No triac dimmer will reduce it's speed. Where would you put the dimmer: inside the PC?
You need : a) a quieter fan or b) a resisttor in  series with the DC+ power wire of the fan (usually it is red, but check before)=.
One 100 ohm/ 1 W resistor will be good enought, but you shoukd test before reassembling the laptop an closing it.

[alank2]

It is inside the computer - but it is a 120VAC fan - connected directly to AC that comes in.

David - any idea how much reduction I will see with the 1uF to 10uF capacitor?  This sounds easy to implement.

[Connoiseur]

Try a diode in serie, you will have 50% slower

  That will surely f*** up any ac motor (other than universal one). The DC component will saturate the stator. I highly doubt the motor will even spin.

It is inside the computer - but it is a 120VAC fan - connected directly to AC that comes in.

David - any idea how much reduction I will see with the 1uF to 10uF capacitor?  This sounds easy to implement.

AC motors don't like that method of speed reduction. If it's an induction motor, sure you can get the speed down, however the rotor will heat up due to increased slip. If it's a synchronous motor it will either run at full speed or stop completely.

[David Hess]

If, as I understand, the fan is inside the computer, than it is a DC fan.

Why is that?  My computer has AC fans in it.  They are inexpensive, powerful, and reliable.

David - any idea how much reduction I will see with the 1uF to 10uF capacitor?  This sounds easy to implement.

The speed can be reduced to at least one-half or one-third.  At some point below that, the starting torque will be too low to start.

The value of capacitance depends on the size of the motor with larger motors drawing more current so requiring greater capacitance least they not start.  Lower values of capacitance yield lower speed of course.  Somewhere between 1 and 10 microfarads is a good place to start for a common AC tubeaxial fan.  If you want to save money experimenting, consider getting a pair of 2.2uF plastic film capacitors which may be placed in series for 1.1uF or parallel for 4.4uF.  The plastic film capacitors do not need to be AC rated but their DC voltage rating should be 500 volts or higher.

AC motors don't like that method of speed reduction. If it's an induction motor, sure you can get the speed down, however the rotor will heat up due to increased slip. If it's a synchronous motor it will either run at full speed or stop completely.

It works fine with the shaded pole motors found in small fans because they are deliberately designed to withstand locked rotor conditions at full power without damage.

[james_s]

This is a very old computer, the insides are quite unlike a modern PC so yes it has an AC fan with a shaded pole motor.

I like the idea of using a capacitor, some experimentation may be required but it won't hurt anything to experiment.

[Seekonk]

I'd be awful careful with that capacitor.  I use a capacitor in series to run 220V fans on 120V. If the value hits near resonance the voltage increases.  I think I was using 3uf for my fans. I've seen transformers go up in smoke when I chose the wrong value capacitor.

[alank2]

Post # 6 -what is the other potmeter for?

What about this:

https://www.ebay.com/itm/DIY-Electronic-Kit-100W-adjustable-triac-dimmer-Switch-Speed-fans-light-diac/292132223083?hash=item44046ff86b:g:ps4AAOSwWntXM~N-

Or this - it says 220-240V - will it work with 120V:

https://www.ebay.com/itm/Light-Dimmer-500W-220-240VAC-Lamp-Light-Dimmer-Assembled-Kit-DIAC-TRIAC/141024301279?hash=item20d5b410df:g:NUEAAOSwkNZUfJJa

[Seekonk]

Just a thought, you might loose fan noise only to have acoustic motor noise from triac.

[schenkzoola]

A series capacitor is a technique that is used with some frequency to slow certain types of fan motors in the HVAC industry (been there, done that).  This usually works well with shaded pole or PSC motors.  It will not work on a motor with a separate start winding that is switched in and out with a centrifugal switch or a 3 phase motor.  Just monitor motor temperature, it should decrease, not increase with the capacitor value you choose.  You can also monitor power consumption with a kill-a-watt or the like to see if you are on the right track.

A series cap has the same end effect as a triac control, it just increases slip.  The capacitor will be quieter.  I have not yet encountered the resonance issue mentioned by Seekonk, but it is something to be aware of.

As someone else mentioned, the cleanest and best solution would be to try to find a different fan with lower noise specs.

[james_s]

Surely using a quieter fan would make more sense than designing and building a VFD? This is a 35 year old PC, no need to get too fancy.

[Armadillo]

Probably build up mainly of discreet TTL logic ICs, the old time "Heat Juicer".
Hence, the size of the fan may be just right for the old time job, though the noise may made it not so sellable.
Anyway I don't suppose this unit is still put to work ??

[james_s]

It's a collectible classic machine so "put to work" is probably not very accurate, but if it's anything like my own collection of vintage gear it's probably powered up for a few hours now and then to play with it or show it off to other geeks.

[View[+]Finder]

AC Infinity AXIAL S1225, Muffin Box Fan with Speed Controller 115V AC 120mm by 25mm

I got the fan above on Amazon (there is an 80mm version too) to replace the fan on an old power supply (HP6623A). The fan is inexpensive, 115V and very quiet. I have the 120mm so I assume the 80 to be similar. Easier to buy than build.

If your Compaq is the early one with the CRT, there might be more heat than a quieter fan can handle.

Donal

[David Hess]

AC Infinity AXIAL S1225, Muffin Box Fan with Speed Controller 115V AC 120mm by 25mm

I got the fan above on Amazon (there is an 80mm version too) to replace the fan on an old power supply (HP6623A). The fan is inexpensive, 115V and very quiet. I have the 120mm so I assume the 80 to be similar. Easier to buy than build.

It uses some type of switching controller and likely a thyristor phase controller making it electrically noisy.  From the documentation:

May not be suitable for noise sensitive applications such as amplifiers and AV cabinets.

A computer should not care of course although the switching noise could get into any audio.