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[RFI] HVAC Variable Speed Motors

To: doc@kd4e.com
Subject: [RFI] HVAC Variable Speed Motors
From: W2RU - Bud Hippisley <W2RU@frontiernet.net>
Date: Sat, 10 Apr 2010 16:28:36 -0400
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Apr 10, 2010, at 4:06 PM, kd4e wrote:

> Warning:  Many of the new HVAC systems are promoting
> variable-speed motors in their design - it is one way
> that they increase the "magic" SEER.

The York "Affinity 3S" heat pump system has just such a beast in it.  Wherever 
you see reference in their literature to a "variable speed air handler", you 
know you're dealing with a PWM RFI generator.

Recently I had occasion to check the RFI environment around a home with one of 
these in it.  The installation was between two and three years old and had been 
done by a reputable heating/cooling firm.

The RF noise from this system totally wiped out my mobile installation (TS-480 
with a simple Hustler mast / resonator antenna) on at least 80, 40, and 20, 
both in the driveway and for at least a quarter mile in each direction on the 
road in front of the house.  Similarly, it totally "trashed" reception on my 
Radio Shack all-wave portable from the broadcast band up through 160 meters and 
beyond, and it was audible no matter where I carried that receiver on the 
20-acre property.  By far the biggest amplitudes were found at the thermostat 
on an interior wall of the main floor and on the power / control wiring at the 
compressor outdoors, but there was more than enough hash near _all_ the power 
wiring in the house to mess up ham radio reception on any antenna on the 
property and for some distance beyond.  Interestingly, the air handler chassis 
itself, which was located in an attic area one floor above the thermostat and 
not more than 25 feet away, was relatively "quiet".  

We ran four cycles of turning the system on and off to be absolutely certain 
that it was the source of the noise.  As the fan powered down each time, you 
could hear the PWM waveform change on my mobile rig, just before the fan shut 
off each time.

Interestingly, the house had two heat pump systems -- the Affinity 3S, which 
handled the heating / cooling for a 2-year-old addition, and another one, 
pre-PWM, that handled the original (10-year-old) section of the house.  We 
cycled the old heat pump, too, and found it to be absolutely quiet except for a 
single RF "click" when it turned on, which had been my experience in the 1980s 
when I lived in a heat pump home of my own.

We contacted the dealer that installed the Affinity 3S system, and he knew 
nothing about RFI from variable speed motors but he said he would check with 
York.  I received a call back the very next morning; York knows all about the 
RFI problem and has an RFI Kit for their variable speed units.  Installed by 
the local dealer, it would have cost around $250, plus the cost of 8-conductor 
shielded wire (!) to run between the hi-tech thermostat and the air handler 
chassis in the attic.  Of course, we don't know how effective any retrofit kit 
might be, since it's clear to me (who used to design PWM power circuits for a 
living) that no particular effort appears to have been made to isolate the high 
transient currents in the motor drive circuitry from other circuitry and leads 
going in and out of the chassis, or to keep the PWM current loop area small.  

Give York credit for instantly responding with information about their RFI Kit 
(which, by the way, is a stock item in their Oklahoma facility), but wouldn't 
it be nicer if a little more time and pennies had been put into minimizing the 
loop areas and doing some other prudent avoidance engineering in the first 
place.  Locking the barn after the horse is out does not usually lead to a 
fully satisfactory outcome.

Bud, W2RU

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