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Re: [RFI] FCC issues official citation for well pump RFI on Topband

To: rfi@contesting.com
Subject: Re: [RFI] FCC issues official citation for well pump RFI on Topband
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 24 Apr 2013 10:11:32 -0700
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On 4/24/2013 4:27 AM, Mike King - KM0T wrote:
A local college is putting in a well
pump, 60HP 480V 3 Phase. (they are on a very high hill, its super deep) It
is controlled by a VFD.
Variable frequency drives are addressed in my RFI tutorial. They are 
essentially square wave generators, the shape and frequency of which are 
varied to control speed. The wiring carrying this current is an antenna, 
and it also produces a strong magnetic field. The long the wires, and 
the more widely separated they are, the greater the magnetic field and 
the EM field.
One of the major design issues with most large VFD installations is that 
the controller is at considerable distance (across a large room, for 
example) from the motor being controlled, so the antennas are long.  A 
contributing factor is that the wiring will usually have bypass 
capacitors between the phase conductors and ground, and there is stray 
capacitance between the phase conductors and ground at the motor end.  
The result is that much of the return current flows in an un-controlled 
ground path rather than on the neutral, which produces both radiation 
(EM field) and a strong magnetic field.  The magnetic field dominates at 
lower frequencies.
The single most critical thing that must be done to contain the trash 
produced by a VFD is to locate both the 3-phase transformer and the 
motor controller AT THE MOTOR, so that the wiring between these three 
elements has "zero length."
73, Jim K9YC
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