[RFI] Washer RFI/EMI

Christopher E. Brown cbrown at woods.net
Mon Sep 7 14:19:29 PDT 2009

On Mon, 7 Sep 2009, Jim Brown wrote:

> On Mon, 7 Sep 2009 04:15:54 -0500 (CDT), Christopher E. Brown wrote:
>> I strongly suggest _NOT_ even thinking about bypassing the motor leads to
>> *anything*, unless you know the exact operating params of the VFD and
>> really, really know what you are doing.  Variable frequency PWM square
>> wave...
> I agree that considerable caution is a good thing, but I don't agree that
> you should not touch it. It is certainly reasonable to begin with a small
> capacitor value and increase it while watching the current waveform on a
> scope. Dissipation in the switching device is minimized with short (fast)
> switching times, and this is the reason for caution.
>> I have never done it myself, but I have seen what happens when a several
>> KW peak motor controller of a similar type is bypassed with too large a
>> cap, cap go boom...  If you know the max frequency of operation, the
>> distribution and total amount of harmonic energy and peak voltages you
>> could select the correct values but this is not in the manual.
> It is NOT related to the frequency of the pulse, but rather to the
> rise/fall times.
>> Any filters, bypassing, whatever needs to be on the input side of the
>> motor controller, not the output.
> That is certainly the safer way, but it may not be the most effective way,
> and it won't be effective at all for radiation directly from the internal
> wiring due to inadequate shielding.
> 73,
> Jim Brown K9YC

I could have stated that better, I was more worried about people slapping
an AC line filter, bypass caps/etc inline and closing the case back up,
leading to a blown VFD, non-working device, overheating filter or
exploding caps.

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