Can I use a dimmer switch to control the speed of my fan?
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.
Do yourself a favor, buy the fan with the speed you want.
Then, by adjusting the flapper's position, you could control the flow to your circuit, and dump the excess airflow.
3 types of fan speed controllers:
(best) Multi pole for multi-pole wound motor.
Capacitive, essentially limiting motor current. Nearly all ceiling fans use this type.
Thyristor controlled (Dimmer type) Note! Standard light dimmers don't work. Thyristor type motor speed controllers are very similar to light dimmers, but with two important differences.
1. Better noise suppression.
2. rheostat only allows output between 60% and 99% of full output. Light dimmer allows 10%-99% output. Light dimmer will allow the motor to stall on low settings and motor will overheat.
"There is currently only one way to control the speed of a single speed AC motor. Using a variable frequency drive." While VF drive is optimum, It's by far not the only way.
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