[RFI] Washer RFI/EMI

[Jim Brown]

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.

> The exception would be replacing an 
>over long split run (read antenna) to the motor with twisted pair of the 
>proper length.

73,

Jim Brown K9YC