Slowing Fans Down

By John McComas

I would like to emphasize a point about dimmers...
Do NOT use a light dimmer for a fan.
PLEASE spend the extra couple of bucks for a real "fan" speed control.

>>>> There are differences! <<<<<


(Solid state phase angle firing of a triac) It can be a rotary switch or a push-off, push-on type, or slider. The push switch type can be turned on at low. (may not run a fan motor properly) The rotary type switches on at low and increases to bright. (same problem, may not trigger properly Does NOT have a minimum level adjustment control. (This is a very important feature)

OK, so what's this "may not run a fan motor properly" hoopla?? Without getting into more theory than you would probably want to know, suffice it to say that solid state controls are real sensitive to line voltage and differences in load variations. If the voltage jumps around because of the air conditioner kicking on, etc. or flipping on or off a lamp on the same branch circuit. It may cause the dimmer to kick off, or worse, half cycle which sends half wave rectified DC to your AC motor....Not Good...Toast. (Ever notice how they say on the package "for incandescent use only"?) (Light bulbs don't care what you feed them they just average the power)

When you start a dimmer on high, then reduce the speed, you can go to a lower setting, than if you started the dimmer at the lowest setting and increased it slowly, it would start running abruptly at a higher speed. It's kind of like the flywheel effect in that the motor windings kick some voltage back to the dimmer that helps to keep "triggering" it. (If that makes any sense.) If the motor is not running initially, it takes more to trigger it.

Bottom line is DON'T use a light dimmer to control a fan.


(Solid state phase angle firing of a triac with additional needed components for inductive load (motor) operation) I've seen them with a rotary switch or a slider control. They always turn on at high speed and then adjust to slow. Good ones have a minimum speed adjustment. (If they don't have a min. speed adjustment they don't go very slow.) (Get one with a user settable minimum speed adjustment) No solid state dimmer operates real well at real low speeds. Fan dimmer type speed controls are usually rated about 5 amps. Did I say to get one with a minimum speed adjustment??? These controls are quite efficient.


These are actually a selector switch with a bank of AC capacitors that are placed in combinations of series and parallel circuits to vary the capacitance in series with the fan motor. This results in slowing the fan down by limiting the current to the motor with different values of capacitance. They usually have about four speed settings. All the capacitor type controls I've seen are rated at 1.5 amps. They seem to work well for ceiling fans, but may only work well for 8, 10, and maybe 12" desk fans. (depends on motor, etc.) The capacitance values in the de-hummer and the motor must be pretty well matched to get the results you desire.... (They were made for ceiling fan motors with lots of poles and momentum) Too small a capacitance and motor might not start turning on its own. Too much cap. and fan runs too fast at the controls lowest setting. Too big, or powerful a motor, and maybe only one or two slower speeds will keep fan running. A too small, efficient motor and maybe the lowest speed is still too fast. A capacitor type control is very efficient if properly matched to the motor.

RHEOSTAT: $20-100

Rheostats need to be fairly large and give off heat and need to be mounted in some sort of enclosure to allow heat to escape. They are made by winding nichrome wire around a round ceramic form. They have a knob adjustment that slides a carbon brush over the nichrome to adjust the resistance. The rheostat consumes the energy not diverted to the motor and gives off the energy in the form of heat. The power used to run the motor at high speed is the same amount used at low speed except now the rheostat gets hot. Rheostats must get sized bigger for more powerful motors. Rheostats are rated in ohms and watts and need to be sized for the motor you want to control. Rheostats are not readily available anymore, and not recommended except for ones that were bought with the fan they were made to control. Rheostats are the least efficient control.


Variacs, Powerstats (brand names) are actually a variable autotransformer. They are made by wrapping copper wire around a laminated steel donut. They have a knob adjustment that slides a carbon brush over the transformer winding that gives an output voltage from 0 to 140 volts AC. Just like the extra 20 volt boost (140 volt) certain models can be wired for 120 volt input and 0 through 280 volt output. (Like a step-up transformer) Variacs are heavy and usually sit on a table or floor. They are sized for the voltages they will be used for and how many amps they can supply. The smallest ones are about the size of your fist and handle 1 amp loads. The larger ones can be as large as a basketball, weigh 80 pounds and can handle 30 amp loads. I would recommend one with a 3 to 10 amp capacity based on what you can get for the price. New Variacs can cost $200+ but used ones average about $50. Variacs are very efficient.


A Variac will give you the best consistent results controlling fan speeds for all fan types, but they are large and heavy, cost about $50 used. The next best thing is a fan dimmer, or speed control meant for use with fans and has a minimum speed adjustment. They cost about $10 Next would be a capacitor type "de-hummer" control and it gives you about 3 usable speeds, if the control & fan are matched. Cost about $10.

I hope this information helps.
John McComas