That's a good question. One thing I can tell you: IF proper engineering
design is implemented, either motor technology could be used and RFI would
be reduced to at least the allowable levels for FCC Part 15 devices (which
probably do not apply to the HVAC units, as they may be classified as
"appliances"). I have even seen variable speed motors and drive systems
that were able to meet aerospace radiated and conducted emission
requirements, but you would not want to pay for that amount of filtering
and emission control.
I am sure you have seen the nightmare postings about the variable speed
drives in clothes washing machines and the like. The ones that deliver
those high levels of interference were almost totally lacking in emission
control design (as indicated by the postings of those who tried to tame the
racket on their own). I believe that the drivers for DC brushless motors
may be somewhat "more quiet" than the variable frequency AC drives for AC
motors. That factor could be attributed to the fact that most of the noise
is in the output side of the driver and therefore contained within the HVAC
unit itself (making for a reasonably small loop area from which to
radiate). On the other hand, unless extensive line filtering is employed
with variable frequency drives, much of that energy seems to be reflected
back onto the AC power line feeding the system, thus resulting in large
"antennas" to both carry and radiate throughout your home or business.
Good AC line filtering (with "good" meaning 30 to 50 dB of attenuation at
the frequency of interest) is neither low cost nor compact, especially when
currents are on the order of 10 amps or more. One issue you might
consider: ask the seller of the systems if they have any installed units in
your area (you have to decide what constitutes the "local" area - you may
have to think as far as 50 miles or so). If there are units, get the names
and addresses of those installations, pack along a portable or mobile radio
with suitable antennas for the ham bands of interest, and check them out.
You might want to ask permission of the owners before you go snooping
around their yards or buildings, but that might be one useful way to get
some feel for what these systems generate in the way of RFI, if any.
Important: always keep in mind your station setup: how far are your
antennas located from your house and what is their orientation relative to
the house and its wiring? An HVAC unit that makes a lot of hash noise when
you hold a radio and antenna 3 feet from it may not be a problem if your
antennas are 50 feet away from the house.
Good luck. Let us know how it goes.
Sr EMC Engineer
> [Original Message]
> From: firstname.lastname@example.org <email@example.com>
> To: RFI List <firstname.lastname@example.org>
> Date: 2/22/2011 11:32:58
> Subject: [RFI] AC or DC HVAC Motor Preferred?
> Would I be more likely to avoid RFI with an AC or
> a DC motor in my new HVAC, please?
> They are both variable-speed as that seems to be
> part of the way they get higher SEER ratings.
> Thanks! & 73, KD4E
> David Colburn http://kd4e.com
> SE-GA Tailgate - April 9th, 2011
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