[R-390] High Electrical hash noise level

[Drew P.]

Michael Student wrote:

"Good Luck, You're never going to eliminate the RFI from the PWM motor and drive circuitry. The?layout of the contol board and the exhaust motor?was?accomplished?with no concern for ground loops, shielding,?etc. If you have metal duct work in your HVAC system it?serves as?a fantastic?radiator for all of that noise being generated from the furnance. As for me?160 and 80 are?useless when the furnace is running,40 and up is OK. One?amateur suggested I install a remote kill switch for the furnance in the radio room :-).
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PS. I built my own shielded harness, no effect,ferrites, no effect, lifting safety ground from motor, some reduction but the motor housing is now not NEC compliant!"

You COULD eliminate the RFI, but it would take a lot of work. Put the controller board in a metal box having no large openings, complete shielding.  Filter all leads into or out of the box with capacitor/inductor networks such as "brute force" PI networks. Filter components (caps) to ground get grounded to the box. Note that PWM outputs might not take kindly to high peak currents when driving directly into a grounded capacitor.  In that case, the filter network as viewed from the PWM output should be an L network or an L-PI network. 

If any components on the controller board generate appreciable heat, you would either provide many small ventilation openings, or a big opening or two with metal screen well bonded to the box. Alternatively, the heat-generating components might be heat-sunk to the box from the inside.

Your warranty would be null and void.

Tol'ja' it'd be a lot of work.

Drew