Posted by
Don Ehrlich on
Nov 10, 2010; 6:20pm
URL: http://elecraft.85.s1.nabble.com/OT-K3-vs-an-RFI-noisy-furnace-tp5725235p5725915.html
Don,
My last post was wrong in that I actually have had a serious EMI problem
caused by a large variable speed motor. The motor I have is in my treadmill
and, since I am the only person who uses it, it is never in use while I am
using my K3. It did cause a lot of interference to AM radios in the house,
however, and I tried a quick and dirty fix which consisted of wiring a pair
of two microfarad/400 volt mylar film capacitors directly to an AC plug (one
capacitor from each line to the ground pin) and then plugging it into the
same outlet as the treadmill. This was a convenient way of connecting
capacitors from each wire to ground but it was done at the end of the
pigtail and not at the motor controller where it would be more effective.
The capacitors reduced the noise substantially but did not eliminate it.
One advantage in my treadmill system is that it had only the one connection
to house wiring and the treadmill is a compact and self-contained system.
However, the DC motor in it is rated at 2 horsepower which is likely a lot
bigger than the motor you have in your furnace. If you can get to the
furnace wiring where it connects to your house wiring it may not be too
difficult to connect some capacitors there. It could buy you a few db of
relief so it is worth a shot. If you do it then be sure to use high quality
film capacitors of a microfarad or more. They will be of substantial size
because it is important they have very low ESR (equivalent series
resistance).
Don K7FJ
---------------------------.
> I don't want to depress you .. but controlling EMI from a large appliance
> that is designed to heat efficiently with not a thought for interference
> is
> very problematic. In aerospace it is not easy either but at least most
> equipment is physically packaged from the beginning with
> compartmentalization, robust copper or aluminum ground systems, and
> internal
> shielding where needed in anticipation of EMI needs so that when EMI
> problems do occur (and they always do) they are at least manageable.
>
> Don K7FJ
>
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