Can I use a dimmer switch to control the speed of my
fan?
Cannabis Fact Contributed by: FRIEDELECTRICIAN
Cannabis Tip Submitted: 07-10-2009
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 contraption?
NO. Thermal contraptions 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 contraption operates when current runs thru
it. When to much current goes thru it will burn up the
contraption, wiring, etc... All contraptions 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 contraptions. 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.
Cannabis Fact 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.
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