You cannot use a
light dimmer to control a fan. There is a growing
misconception that this is safe to do, but the logic
behind it is flawed.
Let me explain: A fan
that has no speed control when manufactured has copper
windings inside that determine speed and horsepower.
These windings are fixed and unchangeable, and wired to
be operated at a certain voltage, with a fixed amount of
amp draw. I will explain how this works, but first I
want to dispel the reasoning behind this
misunderstanding.
1. Why do they sell fan
controllers at hardware stores?
They are to
replace the controllers that are already on a
multi-speed fan.
2. I have heard of
rheostat's being used to adjust the speed of a fan, why
wont this work?
Rheostats, like
potentiometers are glorified variable resistors. While
they can be used to adjust the speed of a DC motor, its
a big no-no on AC motors. AC motors need to run at
preset voltage, motor speed, and current draw. It is a
balanced system.
3. Can I use a thermostat to
act as a rheostat?
NO. Thermostats are
on/off switches that turn on/off at a desired
temperature.
4. Can I use a rheostat if I
also use some type of thermal protection device?
NO. Thermal devices fail too. Sometimes they
trip for no reason, then your fan would be off when your
on vacation and that can be disastrous for your crop.
5. What's the best way to run my fan at my
desired speed?
There are 2 ways. First, buy
a fan that runs at your desired speed. Second, you could
purchase a variable frequency drive, but these usually
cost more than the fan itself.
6. Why do
ceiling fans have different speeds if you can't control
speeds of an AC motor?
Multi speed motors
have more than one set of windings. The speed knob on a
fan is a switch that switches current to a different set
of windings. Each set of windings are almost like a
separate motor. They would each have their own
parameters. Remember, the knob is a switch, not a speed
controller.
Please do not invite disaster. Best
case scenario, your replacing fans like they are going
out of style. Worse case, burn your house down. It is
not worth it.
Now I will attempt to explain the science behind it
all.
An electrical device operates when current
runs thru it. When to much current goes thru it will
burn up the device, wiring, etc... All devices have a
resistance to current. The filament in a light bulb is a
good example. A bulb has a fixed resistance. You can
lower or raise the voltage but the resistance will stay
the same. You would affect the current running thru it,
which if you lower(as with dimming) there are no bad
side effects. But increased current with shorten the
life of the bulb, or burn it out immediately.
Wire has almost no resistance, which is why we
use it to take our current to our devices. The inside of
a motor is nothing but wire. But when you wind it in a
series of coils (like inside a motor), you create a
dense magnetic field when current is running thru it,
casing the motor to spin, and do work. This is called
inductive impedance, or sort of a magnetic resistance.
If you stopped a motor from spinning (like
holding onto a fan blade), the motor would smoke, then
burn up. Holding the fan blade eliminates the magnetic
field and creates a rush of current. The same can be
said of reducing the voltage across the fan with some
sort of outside variable resistor. You are essentially
weakening the magnetic resistance and allowing a current
rush outside the operating parameters of the motor. This
usually isn't as harsh as holding a fan blade, but it
can be disastrous. At the very least, it would severely
shorten the life of the motor.
There is
currently only one way to control the speed of a single
speed AC motor. Using a variable frequency drive. They
do not change current, voltage, magnetic field, or any
other factor other than frequency. U.S. power runs at
60Hz. Changing the Hz on the power supply to a motor
will change the speed with little or no adverse effects.
These drives, however, are not cost effective outside of
an industrial environment. Fans are usually cheaper.
Do yourself a favor, buy the fan with the speed you
want.
Contributed by: strong_plaid
Bleed-off excessive airflow, using a
mechanical valve
Controlling the fan speed
may not be necessary. The airflow can be diverted
mechanically as an option, in effect reducing the
airflow without changing the fan's speed.
On the
"out" pipe, one could put a y-split, with a flapper
inside that can direct a fraction of the air to one half
of the split (connected to your actual air circuit), and
the remainder to the other (the "waste" air outlet).
Then, by adjusting the flapper's
position, you could control the flow to your circuit,
and dump the excess airflow.
Last modified: 23:54 - Jul 12,
2003
faq:1540 "Can I use a dimmer
switch to control the speed of my
fan?"
