Dim-witted way to wire a ceiling fan

Q. Last year I installed a dimmer switch on the ceiling fan in my living room. This enabled me to control the brightness of the light, as well as the speed of the fan. During the summer, I used the fan nearly every day, but it gradually began to make loud humming sounds, especially at low speeds. This hum has become so annoying that I've stopped using the fan at all. Is there anything I can do to eliminate this troublesome noise?

A. The humming noise in your ceiling fan is a symptom, but it is not the main problem. Your primary concern should be the fire hazard that was created when you wired a dimmer switch to an electric motor.

The use of dimmers for ceiling fans is a common practice among handy homeowners because they do not realize professional expertise is needed to perform the task correctly. The reason for not connecting a dimmer to an electric motor is basic to a qualified electrician but somewhat esoteric to most others. Here is a basic explanation in common language:

Electric current is endowed with a number of measurable characteristics. These traits are commonly expressed in units of measure such as volts, amps, watts, cycles, etc. By varying these specific attributes, we can transform the very nature of a particular flow of electrical current. In this way, we can alter the functional performance of the appliances we use. Common examples would be adjusting the brightness of a light, the speed of a fan or the loudness of a speaker. But knowing which electrical property to adjust is vitally important because we can affect not only performance, but safety.

When we dim a light, we do so by regulating the voltage. The lower the voltage, the dimmer the light. When we regulate the speed of a motor, a different electrical adjustment is needed, involving the frequency of the cycles, rather than the number of volts.

The electricity in our homes is what we call alternating current. With this type of energy, positive and neutral charges alternate back and forth, 60 times per second. To regulate these alternations, or cycles, we use a switch device known as a "varitrol." Varitrol switches are somewhat more expensive than dimmers, but they are the safe and correct type of hardware for controlling the speed of a ceiling fan.

Using a light dimmer to adjust fan speed is unsafe because lowering the voltage in a motor causes a condition known as "hysteresis." When this occurs, the timing of the magnetic field pulse gets to be out of sequence. The result is overheating of the windings and other internal parts of the motor. This can cause the insulation on the windings to melt, resulting in further overheating and possible fire.

Early symptoms of motor damage include loud humming noises. If your fan is producing such sounds, a thorough evaluation by a licensed electrician is advised. In all likelihood, the fan light fixture will need to be replaced, and separate controls for the light and fan speed will be needed.

• To write to Barry Stone, visit him on the web at, or write AMG, 1776 Jami Lee Court, Suite 218, San Luis Obispo, CA 94301.

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