Re: AC motor control via "dimmers"
- From: Tom2000 <abuse@xxxxxxxxxxxxx>
- Date: Wed, 05 Sep 2007 01:00:21 -0700
On Tue, 04 Sep 2007 20:47:05 -0700, Chris <cfoley1064@xxxxxxxxx>
wrote:
Ceiling fans usually don't work very well with the standard triac-
based lamp dimmers, Les. But if you want to try, there are many
dimmers on the Home Depot website that can handle 600W (about 5A) at
120VAC. I'm sure some of them are in the store. You might want to
try one of those if you've got some spare time and don't mind taking
it back if it doesn't work well (probably).
If you want to get something that will work better, try the NuTone
Ventilation control switch Model 72W (Home Depot Internet/Catalog #
100490891). It's a little pricey at $53.18 ea., but has a different
type of circuit that will work better with the inductive load of a fan
motor. Call ahead to see if the store has it in stock.
Good luck
Chris
I've been doing some reading about motor speed control lately.
I've found that it's important to find out exactly what type of motor
you have before choosing a speed control.
Most common small devices such as fans, aquarium pumps, etc, are
powered by single phase shaded-pole induction motors. At 2.3 amps,
you might have one of these, but at that power level, you're in the
area where a more complex motor might do the job better. You need to
open the fan and take a look at the motor before you proceed.
If it's a shaded-pole motor, you need some sort of Volts/Hz speed
control, with a variable output frequency and the voltage reduced as
the frequency is reduced.
If the Nutone controller Chris mentioned will do the job, the off the
shelf solution is cheaper in terms of money and time than you'd invest
if you tried to build one. I couldn't find any specs on the device
with a quick Google search, so I don't know if it's suitable for your
app or not. You might email Nutone with the details of the fan you're
trying to control. (Their site is http://www.nutone.com )
I roughed out a Volts/Hz controller circuit as a discussion point on
this topic in another forum. I'm not sure it will work as designed,
but it will give you an idea of the sort of thing you'd have to build
if you decided to do the job yourself. You can find the schematic
here: http://www.mindspring.com/~tom2000/picaxe/VFContr.html
The microcontroller would set the switching frequency of the inverter,
from the motor's design frequency down to some lower level, as the
speed control pot is adjusted. As the frequency is reduced, the
voltage is also reduced linearly (V = freq * design V / design freq)
using the micro's PWM output and monitored by the micro's ADC input.
Note that this schematic is simply a talking point. I haven't
breadboarded it or, for that matter, even given it much thought. It
probably needs debugging.
If your fan doesn't use a single phased shaded-pole motor, your speed
control device might actually be simpler. When you know for sure what
sort of motor you're controlling, you might do some Googling.
Good luck with your project. When you've solved your problem, please
report your solution so the rest of us can learn from your experience.
Have fun!
Tom
.
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