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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
