Paul, W9AC wrote:
> Looks like this series of Corcom filter may be worth trying on a washer with
> a VFD motor.
> http://www.cor.com/pdf/FC.pdf
> I just ordered one for my GE Harmony Profile-series washer. It's now out of
> warranty, so I can try some more invasive measures. To date, nothing has
> made a dent in reducing RFI/EMI from its direct-drive motor circuitry. I've
> got a feeling that a total cure will involve attacking the noise at multiple
> points along the VFD circuit.
Remember that a line filter only kills trash coming out the power cord. If the
unit is not well shielded (I believe Chris said some he tested are not), you
WILL need to deal with the VFD circuit.
One very good start is to study the current path that drives that motor and
use twisted pair cable for as much of it as possible. Another is to make sure
that "ground" or the chassis is not part of that path, so that the magnetic
field produced by that current is confined to the small region around that
twisted pair.
In other words, you do NOT want the chassis or "ground" to be part of the
current path, either at the fundamental frequency or at the high harmonics
(RF) that are causing us grief. We want both conductors that carry current
between the controller and the motor to be "above ground." This seems
counterintuitive -- we've been taught to "ground" everything as a cure for RFI
-- but tying one side of the motor winding to ground, or adding capacitive
coupling to "ground" causes current to flow in a big loop rather than a small
one, so it produces a bigger field and does more radiation. When the current
on the "hot" conductor is precisely equal to the current on the "return"
conductor and they are very close to each other, both magnetic fields and
radiation are minimized.
Where we DO want capacitance is very close to the SOURCE of the current, so
that the return path for HF current is very short, and thus confined to a
small loop area. We could also use some inductance in series with that path.
Remember that harmonics are strongest if the square waves have faster rise
(and fall) times, so slowing them down reduces RFI.
73,
Jim Brown K9YC
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|