On 9/7/2012 9:56 AM, jim feldman wrote:
Remember it's the controller not the motor itself,
although the motor may be acting as the radiator, along with the mains
wiring.
Not quite -- the controller generates square waves which drive the
motor, the pulses have a fast rise time and are thus rich in high order
harmonics, and the wiring TO THE MOTOR radiates the trash. The wiring to
the motor also creates a MAGNETIC field that is proportional in strength
to its loop area, and RF bypass capacitors can combine with ground
conductors to create large current loops.
Two important elements of the solution are 1) reducing that loop area by
making the wiring that carries that current twisted pair; 2) slowing
down the rise time of he pulses by adding a ferrite core to one
conductor carrying that current. However -- #2 should be done quite
carefully, because dissipation in the switching transistor(s) increases
as rise time gets slower. But the strength of high order harmonics is
proportional to rise time, so a little bit could go a long way.
Try and keep filters as close to the controller as possible.
Check to make sure the chassis ground is still tight. More turns around
the same core are way better than more cores. Different core materials
tend to be better at different parts of the spectrum.
A serious common mode choke on the AC line is certainly a good move, and
will kill whatever part of the noise is being radiated by the AC line.
#43 and #31 materials made by Fair-Rite are all you need for the HF
bands. Follow the winding guidelines in the link below.
http://audiosystemsgroup.com/RFI-Ham.pdf
73, Jim K9YC
It's unlikely one thing will fix, so be prepared for multiple passes.
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