Radio Interference

by George Lyle

Michael Benthin wrote:

Caframo [fan manufacturer] went from a quality motor built in Canada that DID NOT cause TV interference that I still use after 8 years to one made in China (got it at Camping World) that has a smaller motor and causes interference and lasted one season before being very noisy (bearing play)....for the electric whizzes here-any idea why/how a DC motor can be built that doesn't cause interference? Isn't it a brushes thing???

George Lyle responds:

Yep, the situation is that brushes tend to make sparks, which creates broadband RF signals. The cheaper, smaller brushes in inexpensive motors have to carry more current and tend to spark more than heftier brushes in quality motors. This interference is most evident in AM reception which is why AM radio and television video seem to suffer more than FM radio and TV audio.

The solution to this is to short out the RF signals before they can get very far from the brushes and into the power leads that can serve as antennas. For this you need something that will short out electrical signals at radio frequencies, but not short out the circuit entirely.

The device you need is a simple capacitor. Go down to Radio Shlock and get a few ceramic disc capacitors. The value should be around .01uf (microfarad) and the voltage rating should be 50 volts or higher. Price should be a buck or two, even at R. S. draconian rates.

You want to install this capacitor inside the fan, across the fan's brushes and as close to the brushes as possible. You might have to play around with the value of the capacitor to find the right one.

Since you complained about the quality of the fans, I'll suggest that you check out truck supply outfits to see if they carry better fans. Over-the-road trucks often use such fans inside their cabs and obviously demand better durability than the Camping World crowd. I'd be prepared to pay a goodly sum for the durability, but this would be advantagous in the long run as their better brushes would probably run quietly!

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