There are three potential locations for variable speed motors in a
heating/AC system:
1) the main blower motor (the squirrel cage that moves air through the
house)
2) the draft inducer motor (small motor used on high efficiency furnaces to
assist combustion air flow)
3) the condenser cooling fan motor (the motor on the outside A/C
compressor/condenser unit)
Please be sure to identify where your variable speed motor(s) are in the
system. I initially missed the one on the A/C condenser!
Next, try to identify where the switching circuitry is located. On the main
blower and condenser motors it is usually inside the motor housing. On
inducer motors it may be external to the motor, e.g. on the control board.
The goal is to stop the noise produced by the switching circuit at or very
close to where the wiring leaves the metal enclosure of the motor (presuming
the switching circuit is inside the motor housing). A brute force filter is
probably the best approach. Corcom makes some good filters that cover the
HF range, and I installed a 20EP6 on the main blower motor power leads:
http://www.cor.com/Series/PowerLine/EPVP/. The filter was enclosed in a
metal box and mounted as close as possible to the motor using heavy braid to
shield the power leads. This helped significantly, but it did not
completely kill the noise. Often these motors also have multi-conductor
control leads going to the control board. Some of the noise will inevitably
escape via this path. I enclosed these leads in braid, but the improvement
was inconclusive. I discovered that if I disconnected all of the misc.
wires leaving the furnace enclosure (thermostat, humidifier, compressor
control, etc.) virtually all the noise was gone. So, clearly, noise is
getting radiated or perhaps conducted onto these leads and escaping.
The draft inducer motor is a common source of noise on the 90/95% efficient
furnaces. Trane has a kit that is supposed to help though mixed results
have been reported. Search the archives for a modification to this kit.
The condenser blower motor was the real sleeper. To gain a higher SEER
rating Trane has several models that contain a variable speed motor even
though the speed never changes...it's fixed! This one was the biggest noise
producer of all. I installed a Corcom filter with shielded cable plus a
large multi-turn #31 ferrite on the power leads. This help quite a bit but
there is still noise escaping. The best fix would be to replace the motor
with an equivalent fixed speed unit. The SEER loss is minimal.
You might be very lucky and get the motor manufacturer to help with a
solution. The HV/AC manufacturer will likely be of little or no help at
all.
A good panadapter is very helpful in identifying the noise and evaluating
the effectiveness of the solutions you try. It's also a curse because now I
can see all the RFI produced by my neighbors' HV/AC units...and it's growing
day-by-day.
Good luck!
---
Chuck, AE4CW
Today's Topics:
1. Fan motor RFI (Matt)
2. Re: Fan motor RFI (K1TTT)
----------------------------------------------------------------------
Message: 1
Date: Thu, 06 Sep 2012 19:41:58 -0500
From: Matt <wedgef5@gmail.com>
To: rfi@contesting.com
Subject: [RFI] Fan motor RFI
Message-ID: <504942D6.6020305@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
I know there are a lot of fan motor emails to this list, but I couldn't find
any that seemed pertinent to my furnace.
I have a Kenmore furnace (made by ICP?), which I believe is somewhere around
10 years old. We just bought the house last year, so I'm not 100% sure of
the age. It has the dreaded variable-speed fan, and the RFI is horrible.
Can someone tell me what my options are with this unit?
Thanks es 73s
Matt, N0EYE
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