[RFI] AC Line filters- Direct-Drive Washers
jim at audiosystemsgroup.com
Fri Apr 4 17:43:43 EDT 2008
Dale, WA9ENA, has given you some great advice. Larry, W0QE, made
an excellent observation about this in a conversation we had
offline. If the interfering signal is 50dB over S9, you've got to
do a MASSIVE job of noise suppression to get that down to the
point where you can hear signals of normal strength on the ham
bands. An S-unit is "officially" 6dB, so S3 is 36dB below S9. To
get the noise down to S3, you're looking for 86dB of suppression!
A REAL 50dB over S9 signal is huge. S-meters on ham rigs are
notoriously poor for linearity, so the actual signal level may be
a lot lower. But even if there's a 20 dB error between S3 and 50
over, you're looking for more than 60 dB of suppression. Such an
effort is not for the faint of heart. You'll need to start with at
least that level of suppression on the power line. Once you've
done that, you're VERY likely to hear trash radiated from wiring
within the box.
These offending motors are variable speed drives, which are
essentially driven by DC power supplies followed by circuitry that
chop the waveform into square waves of varying width, and
frequencies on the order of 10-20 kHz. It's the harmonics of those
square waves that we hear.
As it turns out, Larry has done a lot of looking at this. He
observes that a typical machine is built to make it easy to work
on, and the wiring that drives the motor is a big loop. Because
that loop is large, current it carries generates a very strong
magnetic field, which is picked up on our antennas. The field is
proportional to the loop area. To reduce that field we must make
that loop a LOT SMALLER!
Bottom line -- you may do the world's most wonderful job of
filtering the power line and still have a lot of trash radiated by
that wiring inside the box. If that's what's happening, a possible
cure is to go inside the box and re-wire it so that the current
driving the motor is carried by a twisted pair, not a big loop.
But that might not work if part of the current path is capacity
coupling via the motor frame and the chassis. You could also
reduce the strength of those harmonics that we hear by slowing
down the rise time of the square waves.
Like I said -- not for the faint of heart. :)
Jim Brown K9YC
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