Can I use a dimmer switch to control the speed of my fan?

  Added by: snoofer  Last edited by: snoofer  Viewed: 122 times   Rated by 12 users: 7.00/10

Can I use a dimmer switch to control the speed of my fan?

Submitted: 07-10-2003

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?"