Posted by
Don Ehrlich on
Nov 10, 2010; 5:59pm
URL: http://elecraft.85.s1.nabble.com/OT-K3-vs-an-RFI-noisy-furnace-tp5725235p5725833.html
I have never had a noisy furnace or an appliance with the type of motor
control you describe. But, as an engineer who fought various EMI problems
on military and other aerospace equipment I can guess what you are facing.
It is broadband conducted noise (inherent in sharp risetime pulses of
current) and there is no kind of shielding that will cure it. The noise is
conducted (not radiated) from the motor to your house wiring and the wiring
in your house is the 'antenna' that radiates the noise everywhere.
The only hope you have is to keep those sharp risetime pulses out of the
wiring coming from the furnace. To do that you can connect capacitors from
each wire to ground AS CLOSE TO THE ACTUAL MOTOR LOAD as possible .. which
means taking the furnace apart to some extent, possibly voiding your
warranty, etc. Ferrite inductors in each lead including the ground coming
from the furnace will help as well. All of that requires some knowledge of
capacitor and inductor ratings and characteristics and in the end, if my
experience is any guide, may not work very well anyway because of other
circuits (thermostat wiring, etc) as well as the fact that the ground system
to which the capacitors are attached may not be robust enough to effectively
return the interfering currents to their source in the motor controller.
The current spikes going to a variable speed furnace motor are likely to be
many amps in amplitude (much higher than the steady AC current) and it takes
only milliamps traveling through your house wiring to cause significant EMI.
That gives you a feel for the scale of the problem.
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
---------------
Hi all,
I misspoke earlier this year when I said our new Lennox high efficiency
furnace was not generating any noticeable RFI. Quite to the contrary,
the new Lennox furnace is very rich in RFI energy. One observation is
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