[RFI] Getting a Gas Furnace, and Electric AC unit... (Dave Cole)

AE4CW ae4cw at att.net
Mon Aug 13 12:45:34 PDT 2012

Jim, K9YC is certainly correct that the proliferation of variable speed
motors in various household appliances is a big problem for us.  

Let me relate my recent experience with a new basement level Trane heating
and cooling system.  First, I presumed the new system would be fine since I
had the same model system in the attic that included a variable speed motor
on the furnace blower; it was totally RF quite. However, that turned out not
to be true.  The new system produced very significant RF pulse groups spaced
18KHz apart from about 10MHz past 30MHz. Some frequencies in that range were
worst than others. These pulses peaked at -78dBm and the groups consisted of
up to 50 individual pulses across 9KHz of spectrum. Most interestingly, the
pulse group amplitude was much worse with the AC compressor running.  BTW, I
observed these pulses using a panadapter based on LP-Pan hardware and NaP3
software. This combination is able to resolve individual pulses down to less
than 10 Hz apart.

After receiving absolutely no useful help from Trane (though the local
installed tried hard to help), I discovered that the furnace blower motor
manufacturer has a web site complete with telephone numbers for the
engineering staff.  Fortunately the design engineer for the motor turned out
to be a Ham.  To make a long story shorter, the 1HP version of the motor is
much noisier than the 3/4 and lower HP versions (which was in the attic
furnace).  Further, to get a higher SEER rating Trane had used a variable
speed motor for the AC condenser fan (outside unit)even though the fan runs
at a constant speed!  So, I had two big RF generators!!  The Ham motor
engineer supplied filters for both motors.  Those in combination with
judicious use of multi-turn #31 ferrites and an additional line filter took
the noise down to a tolerable level though not zero.  During all this, I
began to notice identical 18KHz noise signatures all around my neighborhood
presumably from similar variable speed motors.

The moral here is be very suspicious of any variable speed motor, and don't
be surprised to find them in fixed speed applications.  There is real
benefit in using variable speed motors where less than full speed can be
used for a significant period of on time. That's because fan motor power
consumption goes us as the cube of the speed. But, poor designs of the motor
drives can produce terrible RFI! If anyone is interested in more specifics,
please contact me off list at ae4cw at att dot net.
Chuck, AE4CW

-----Original Message-----

   4. Getting a Gas Furnace, and Electric AC unit... (Dave Cole)
RFI mailing list
RFI at contesting.com

End of RFI Digest, Vol 115, Issue 15

More information about the RFI mailing list