[RFI] Motors/controllers and something to think about

Dale svetanoff at earthlink.net
Thu Aug 16 08:13:15 PDT 2012


The issue of rise time may also be inflexible for some systems.  While I have often encountered design situations in which rise time was considerably faster than required (the digit guys wanted "reserve speed"), there are other cases in which the interfacing devices (at either end of a signal feed) require very fast risetimes and the resulting steep "fronts" they present.  You then have an RFI/EMC issue to resolve.

73, Dale

-----Original Message-----
>From: Larry Benko <xxw0qe at comcast.net>
>Sent: Aug 16, 2012 9:48 AM
>To: rfi at contesting.com
>Subject: Re: [RFI] Motors/controllers and something to think about
>Jim and others,
>I have taken several EMC compliance classes over the years and they 
>always stress that there are 3 main issues that contribute to RFI. They 
>are in no particular order:
>1.) Signal rise time
>2.) Signal current
>3.) Loop area of wire carrying the signal
>Signal current is usually the least flexible parameter but rise time and 
>loop area are quite easy to alter.  Components can affect the signal 
>rise time and good construction can reduce loop area.  Loop area 
>problems can be both printed circuit board issues and external wiring.  
>No earth shaking news here.
>Larry, W0QE
>On 8/15/2012 11:40 PM, Jim Brown wrote:
>> On 8/15/2012 10:30 PM, Roger (K8RI) wrote:
>>> The issue has dealt with specific makes and models with and without RFI,
>>> but as PWM is by its nature an RFI generator we need to stop and think
>>> about how companies (any company) acquires the motors and speed controllers.
>> It is critically important to realize that it is EASY to build a PWM
>> system without RFI if you simply pay attention to the fundamental laws
>> of physics  -- keep track of where the current flows, ALL of the
>> current, including DC, baseband AC, and RF -- and design your product so
>> that it flows in loops that have small cross sectional areas, and over
>> wires that don't make good antennas.  It's really as simple as that.
>> The most recent edition of Henry Ott's classic text on EMC even has
>> chapters devoted to switching power supplies and PWM motor controllers.
>> 73, Jim Brown K9YC
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