[RFI] HVAC Variable Speed Motors

Johnson, Karin Karin.Johnson at nielsen.com
Mon Apr 12 09:58:30 PDT 2010

Before you go condemning all HVAC systems using variable speed motors,
please understand 
the total system as an RFI generator.  Most of the variable speed motors
used in home
residential HVAC systems are made by one company.  It used to be GE, but
is now Regal-Beloit,
who purchased the ECM (Electronically Commutated Motor) line from GE a
few years back.
Before I get flamed too badly here, I do not work for GE or Regal
Beloit.  But I did solve
a messy RFI problem in my home when I had a new HVAC system installed
that used ECM devices
both in the air handler (inside unit) and the compressor (outside unit).

Most of these ECM devices are used only for the fans and blower
assembly, not the compressor
motor in the outside units.  If you do a bit of research about ECM you
will find that the entire
electronic assembly that controls the PWM is entirely contained inside
of the motor housing which
is generally a metallic enclosure around the motor windings.  What I
found with my particular installation was
that the internal wiring inside of the air handler is a virtual rat's
nest of wires.  There is no particular care
taken with routing the wiring in a neat fashion.  The control wiring
(low voltage) is very close and many times
intermixed with the 220 volt wiring going to the motor.  The cure in my
case was not what I call simple
but was very effective.  It involved a CORCOM line filter for the motor
line voltage plus a couple of
type 31 toroid cores and some shielding braid to make a shielded cable
for the thermostat wiring.
I won't go into the gory details of what the remedy process was but
suffice to say I did make a very 
substantial improvement in radiated noise from the air handler motor.
My 40 meter dipole is located 
within 5 feet of the air handler motor, and the RFI level was reduced
from very noticable to unheard.
The major culprit was the thermostat wiring, which I ultimately
shielded, plus added the toroids to 
the control cable going to the outside compressor unit.  I believe that
most of the radiated junk comes
from creating a loop out of the control wiring which has to run from the
inside air handler to the outside
compressor unit.  And given the distances that these two devices must be
located from each other can be
very efficient radiators in themselves. The trick is to stomp on the RFI
at the source. 
The point I am trying make here is that the controller for the ECM
device is not located remotely to the 
motor, it is internal and very well shielded to start with.  Again this
is for the HOME type HVAC systems
not the larger industrial types.  
Karin Anne Johnson  P.E.
Palm Harbor, Florida

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