Hello Group:
Given the recent discussion about HVAC systems and RFI, I am writing this to
chime in on an issue that I haven't seen on this reflector. I have had a
brief discussion with another ham that has the same type of system,
manufactured by the same vendor. His experience is similar to mine. He
went a step further than I did and placed many type 31 toroids on the 240
volt power feeding the compressor, but did not see any decrease in RFI.
Now to the details.
About two years ago I had to replace my HVAC system. I wound up having a
Rheem model RP1748AJVCA unit installed. This system uses a 3 phase motor in
the outside compressor unit. The 3 phase motor is driven by an inverter
that is housed in the outside unit enclosure. It is bonded to the enclosure
via its mounting mechanism. The control interface from the thermostat and
air handler is via RS-485 two wire communication which is routed along with
the 24VAC power voltage for the outside electronics. Suffice to say this is
an evil system with regard to RFI. The initial installation did not have
the thermostat/control wiring replaced with shielded type. I had the
installer come back and replace the wiring with shielded type wiring. This
helped a bit with the RFI. I also installed type 31 toroids on the control
wiring at the compressor and also at the air handler. This simple solution
knocked the RFI down about only 6 dB. Note the RFI is most pronounced from
about 4 MHz to 18 MHz. The remaining RFI is caused by the inverter running
the 3 phase compressor. The noise floor increase due to the RFI is about 20
dB, between the compressor being off and the compressor being on.
Some time ago I believe I had posted a Google Drive link to some
documentation I developed showing the spectrum analyzer plots taken with the
system off and the system on. I might note that the power feeding the
compressor inverter control board does have some internal filtering, by
using toroids of unknown characteristics to me. Along with a substantial
filter board, which appears to be a common mode filter mounted inside the
metal enclosure of the outside compressor unit.
My intent of this post is to simply inform folks to be aware of newer HVAC
systems that use 3 phase inverters to drive the compressor motor.
I have done a bit of design work to implement a possible solution to the RFI
but its cost is prohibitive at this point in time. I would also have to
dis-assemble the filtering boards and components on the outside compressor
unit. Probably voiding the warranty.
So in conclusion, folks, please do your homework and due diligence to
investigate what type of technology the new HVAC system is going to use.
Regards,
Karin Anne Johnson P.E. K3UU
Palm Harbor, Florida
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