Just bought a cheapo box fan, which we use in a window at night to draw in
cooler outside air. However, even at the slowest of its 3 speeds, the fan
is too noisy. I'd like to make it less noisy, which I think the best way wo
uld be to make it turn slower. I'm wondering if it would be possible to wi
re in a resistor in series with the motor to accomplish that. If so, what
would be the correct specs on the resistor?
You could try a regular light dimmer. It's easy to build a unit for what
you wish to do rather inexpensively. You can use 4" conduit box,
one with the rounded corners, a dimmer, a duplex outlet and a
combination 4" metal box cover with a position for a switch and the
duplex outlet. You can get a cheep three wire extension cord and cur the
outlet end off and use it to supply power to your dimmer. You'll need a
cord grip to attach to the 4" box. Another route is to use a
regular handy box with rounded corners, dimmer, metal switch cover,
cheap extension cord, two cord grips and cut your dimmer into the middle
of the extension cord. It's very easy to make. ^_^
Small AC fans usually have shaded pole motors, RPM of which is frequency
dependent. By cutting off part of the phase with a light dimmer you will just
make it start even harder than it already is for this type of a motor. There may
be *some* RPM control due to torque losses when dimmer is dialed down, but it's
only a small percentage point around the designed RPM, not from 0 to the max.
Since the actual complaint is noise, not RPMs per se, I would take the fan apart
and try to balance the blades to deal with vibration. You know, disconnect the
motor from power and spin the blades by hand. Mark which blade stops at the
bottom, do it again. If the same blade stops at the bottom again, file some
material off that blade, repeat until no single blade stops at the bottom
repeatedly. Hard to say how effective it will be with lightweight plastic blades
coupled to a badly constructed motor, but is worth a try. Not much else to do:
if you have to look at the motor, you might as well just get yourself a new fan
- motor is the bulk of the cost of it, anyway.
Since the actual complaint here is fan-noise and not fan-speed it may help
to move the fan to another room, possibly a room that is far enough from
your bedroom that fan-noise will be less of a problem. Configure the fan
(in the distant room) to exhaust hot air to the outside of the house. Keep
the door and windows in your bedroom open, and also keep the door to the
fan room open. Then, close windows and doors in the remainder of the house
in such a way that the fan draws cool outside air (only) into your bedroom
and exhausts hot air out through the distant room that has the fan. You
could re-adjust the windows and doors in such a way as to draw cooler
outside air into other parts of the house when you aren't using your
replying to pilgrim, JP Huie wrote:
Just want to add that a larger fan (blade diameter) will produce less noise for
the amount of air moved. So a larger fan (I'd shoot for 20" at least) on a
slowish speed might move enough air AND be quieter. Problem may be finding
one... I bought earlier model of this from Amazon, but reviews on current model
arent' to good. You can check Grainger but they don't seem to give the
diameters, so maybe go to mfg's website for that...
On Mon, 04 Jul 2016 19:44:02 +0000, JP Huie
I've been reading this group for maybe 20 years and I don't remember
But I might have missed say, one lone post.
If he's still reading I have some info, but I'm not going to the
trouble of writing it unless someone who is reading wants to know how
to slow a fan.
My suggestion was about a low cost experiment. I built a variable
voltage box using a triac, pot, trigger diode, capacitor, single 120
volt 15 amp outlet, cord, cord grip and sloped project box. I was able
to use it for all sorts of things including shaded pole motors, light
bulbs and universal AC/DC motors. If he uses a triac type dimmer, it
should work. A dimmer using an SCR might not work very well. The link
below is about LED dimming but shows the waveform output of a triac type
If you use a triac dimmer control the motor torque falls off pretty
rapidly. But as you reduce the fan speed the torque required falls off
rapidly also, something like the 3rd power of the RPM. I never used a
dimmer to control a fan, but it might work. From mickey, it often does work.
I believe a triac dimmer will maintain torque better than a resistor.
The box fans I have make noise because of the air being moved, not
because of balance problems.
I should say that every fan I have except the 4" fan (which is a year
old) is over 25 years old. I have a Tintang** (or something like
that, 2-speed fan) that is about 25 years old, but I've never used a
speed control with it, or with the 4" fan. **Co-workers gave me this
when they felt sorry for me when I was using a fan from the '30's.
But I liked the older fan more, until it caught fire last summer (fan
stopped, overheated, and set fire to too much light oil).
The rest are 30 to 80 years old. I don't think newer fans from the
last 10 years are made differently from my older ones, which are all
(except maybe one) brushless, induction motors. I guess I could try
dimmers on those two newer fans if anyone was curious.
I got the old fans, with iron or heavy steel bases, from my father
when he died in 1955 at age 62. They were in his office or our home.
I've put dimmers on fans and all you do it trade off some fan blade
noise for increased motor noise because the solid state dimmers just
chop the current up and make the motor "buzz". The only way to do it
without the buzz is with an old fashioned transformer based
Otherwise know as a variable autotransformer or Variac. The dimmers
using a triac instead of an SCR may be less likely to make the fan motor
buzz. I've seen Variac light dimmers used in recording studios
because they don't make the lights buss or add noise to the electrical
power for the studio. ^_^
Not in my experience. In one case the dimmer was a foot from my ear
when I lie down and the fan was 18 inches. Still heard no humming or
buzzing. And since the reason I use the dimmer is mostly to get rid
of noise, I would notice.
There is probably a lot of variety in motors and fans, and even maybe
replying to The Daring Dufas, Shawn hill wrote:
best thing to do is buy something that will reduce the amps not the volts
because the speed is changed by a tapped winding in the motor which reduces the
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