[RFI] RFI in the HF spectrum from variable speed Mitsubishi HVAC systems

[Karin Johnson]

Just to chime in on some of the issues regarding variable speed motors used
in home air conditioning systems.
I have some personal experience with quieting a new installation of an 18
SEER split system air conditioner in my
home some 5 years ago.  Most, if not all newer split system air conditioners
are using what are called "ECM type" 
motors for the blower on the inside unit and the fan for the outside unit.
At the time of the install I had 
discovered a new source of RFI that was not present with the prior older
type air conditioner system, which used 
the typical capacitor start motors.  When this new source of RFI was
discovered by me, I found that it was most severe
on 30 meters and 20 meters, as my antennas for these two bands were the
closest to the indoor unit in my attic area.
In the process of tracking down the source of the RFI, I had some extensive
email conversations with some of the
technical guru's at Regal-Beloit, who took over the line of General Electric
ECM motors. 
I'm not sure that every installation would benefit from what I found but
here is the story.  
The ECM motor itself is shielded pretty well, as all of the control and
power electronics are contained within the
shell of the motor.  That plus the motor and squirrel cage blower are
contained inside a metal enclosure would provide
quite a bit of shielding to prevent radiation of any RFI.  However there are
control wires that ingress and egress from
the blower enclosure and that is where to look for problems.  There would be
at least three separate wiring bundles 
that would exit from the inside blower enclosure.  The main 240 power, the
control wiring from the inside unit to 
the outside unit, and the thermostat wiring.  I had available to me a very
high end Agilent portable spectrum analyzer
and a Tektronix current probe which I used to measure the spectral energy on
the two control wiring cables.  I have plots
available of the before and after results of my efforts to suppress the RFI.
I suspect most of the problems that 
folks have with hearing RFI from these types of air conditioning units comes
from the loops created with the control
cable wiring external to the air conditioning units themselves.  Although
the source of the RFI comes from the ECM motors
at a very low level, remember we are usually speaking about microvolt or
tens of microvolt signals as opposed to the
internal switching waveforms used inside of the ECM motors, the egress of
wiring will couple RFI out of the enclosure
and if the loops of wiring create resonance in the ham bands we hear it.  
What I did to cure this particular problem is to install a couple of Clamp
on ferrites of type 31 with multiple turns
on the control wiring from the inside unit to the outside unit, and also
made up a shielded cable for the connection to 
the thermostat from the inside unit.  The also installed a clamp on ferrite
on the control cable at the outside unit
as it entered the enclosure.  As far as the 240 volt power cable, I
purchased a very good Corcom line filter to install
inside the enclosure for the inside unit feeding the 240 power to the ECM
motor.  This probably wasn't necessary but 
I felt that overkill wasn't something that was bad. As mentioned before I
don't the ECM motors by themselves are the
entire culprit in creating the levels of RFI that bother us, but it is the
entire system, and as such the entire system
must be analyzed to find solutions.  Every installation is not going to be
the same and expecting the manufacturer's to
design an out of the box system that works without causing RFI to each of
our unique physical locations is not going
to happen.  Unfortunately we then have to at least provide that expertise to
the installers to help fix the RFI dilema.
To close keep in mind these points.  It isn't so much the air conditioner
system itself but the auxillary wiring that needs to be looked at to reduce
the RFI to acceptable levels.  Also I do have some documentation from my
experience that
I would be willing to share but don't have any place to post it.  It is
mostly PDF and Power Point files.  If anyone
would like to host this I would be willing to zip it up and send it along.


Regards,
Karin Anne Johnson  K3UU P.E.
Palm Harbor, Florida