[RFI] Washing machine RFI question

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Tue Jan 4 14:35:45 EST 2005


Thank you for this posting.  My wife has been looking at this machine, and
similar ones from other makers, for when our present conventional clothes
washer gives out.  I had no clue about the operational characteristics of
the Duet.

I have done some past work with industrial electronic variable speed motor
drives; they are not nice from an EMC viewpoint.  The variable speed and
quick reversals are possible with either technology:  a DC motor and PWM
speed control, or an AC motor and variable frequency control.  (An AC
induction motor rotates at a speed proportional to the applied drive power
frequency.)  If Whirlpool is using the variable frequency approach, this
can be very difficult to reduce.

First, I would follow up on the suggestion to contact the service center.
Yes, I know, you usually get someone with a grapefruit IQ who doesn't know
how to spell "EMC", but sometimes there are pleasant surprises.  It is
possible that there have been incidents of interference to TVs and other
A/V equipment, or to computers.  If so, they may actually have some cure in
place.  It's worth a try.

What did you try for an AC line filter?  If you tried a computer-type of
plug-in filter, it would be about totally useless in this application.  We
are talking industrial strength here.  Whether it is PWM with a DC motor,
or variable freq with an AC motor, you will need a filter with lots (40 to
60 dB) of attenuation starting down as low as 10 to 15 kHz.  That baby will
be be hefty.  The major part of the noise spectrum will be from a few kHz
to at least 10 MHz.  It should not take a lot of filter to knock down the
stuff above 10 MHz, but the noise source can be very strong below that
range and thus must be reduced in order to make any decent progress on
improving HF reception.  This "monster" filter will need to be located as
close to the washer as possible - like right behind or along side of it.
In this situation, I will go on record as predicting that the vast majority
of the RFI is being conducted into your house power wiring and then
radiated from that wiring.  Given that as the scenario, the most effective
cure is a brick wall filter as close to the source as possible.

If you do strike out with Whirlpool service, I would then check out e-bay
or hamfests for heavy duty industrial power line filters.  Remember,
filters that do little below 30 Mhz are useless.  Excellent filters with
the correct characteristics are widely used in the shielded enclosure
industry - "screen rooms" as they are sometimes called.  Be aware of 3
important issues regarding any filter you might try to help the problem:

1.  Be certain that the filter is rated for the applied AC line voltage
(should have at least a 125V AC rating) and can carry the current required
by the machine.

2.  Ground the case of the filter TO THE WASHER CHASSIS so that the size of
the current loop for the stripped-off energy (noise) is short between the
filter and the noise source.

3.  You also need to know that a true "brick wall" sort of filter, while it
should greatly reduce the emissions, will present a very reflective load
(and a series line inductance) to the noise source (the motor drive
electronics).  The drive system might not like that.  You could end up with
either a driven that will not start up correctly (due to the series
inductance) or that could be damaged by back-emf from the reflected energy.
The latter case especially is why you need to contact the service folks.
You probably have a warranty on the machine, and you don't want them to
hang you up to dry when their noise maker croaks because you throttled its

Leigh, I hope you pursue some fix to this problem.  I suspect that I am not
the only subscriber to this list who may be facing a machine like this in
the future.  We sure would like to know how you are able to resolve the
issue.  In particular, will you get any help from Whirlpool?  Oh yes: does
the machine have an FCC ID on its label?  I would think so, because I'll
bet it has a rather sophisticated micro-controller running the show.

Best luck.

73, Dale

             "Leigh L Klotz,                                               
             <Leigh at WA5ZNU.org                                          To 
             >                         rfi at contesting.com                  
             Sent by:                                                   cc 
             rfi-bounces at conte                                             
             sting.com                                             Subject 
                                       [RFI] Washing machine RFI question  
             12/29/2004 01:09                                              

I have a Whirlpool Duet front-loading washing machine.  When operating,
it produces tremendous noise on HF.  The motor rotates at various speeds
and switches directions quickly, so I assume the noise is from the
controller doing PWM at high power.

I contacted Whirlpool and they told me to call a service center, but I
don't have much faith in that process.

I have tried the obvious things such as an AC line filter and a big
toroid core with the power line through it, though I could only get 2
loops through.

I have the opportunity to experiment with shielding the laundry room as
it will be moved during construction, and if putting bonded metal plates
in the wall will help, I would like to try it...

Has anybody managed to quiet such a device?

Leigh WA5ZNU
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