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Date: Thu, 24 Jun 1999 07:55:10 -0700
From: Brent Regan <brent@regandesigns.com>
To: lancair.list@olsusa.com
Subject: Re: Flap Motor Noise
References: <19990623044245.AAA14480@truman.olsusa.com>
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>>
Rob Wolf asks:

<How should I prevent electrical noise from my LNC2 flap motor from being
picked up by the audio / intercom system?>

Bill's comments about twisted and shielded twisted cable were right on the mark. When
current travels through a wire a magnetic field is produced in a circular pattern around
the wire, like your fingers when you grab a rope. Putting two wires close together lets
the magnetic field of one wire cancel most of the field of the other, assuming that the
two wires are part of the same circuit. Twisting the wires allows most of the field that
leaks out to cancel itself and shielding gets most of the rest.

When the current in a wire changes, the intensity of the magnetic field (AKA flux)
changes. When a wire is subjected to a changing magnetic field a current is produced in
the wire. Early Marconi radios used a spark to create a rapidly changing magnetic field
in a wire and this "noise" was received across the Atlantic. Motor brushes make pretty
good Marconi transmitters. So do strobes

Things to do to reduce electrical noise:
Tightly twist power leads together.
Connect noisy devices to the battery end of the buss.
Have the ground buss close to the main buss.
Route noisy devices together and away from COM antennas and intercom cables.
Add noise filtering at the source.

Don't do what one builder did and run all the power wires down one side of the cabin and
the ground lines down the other.

For filtering, a good place to start would be to put an automotive alternator filter
across the power leads at the flap motor. Another thing to try would be to run the power
leads through a ferrite bead.

Regards
Brent Regan

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