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

[Jim Brown]

Yes and no.  Yes, the conduit, if properly installed, will be a complete 
path and bonded to the electrical enclosure of the controller and the 
motor at both ends, so it provides a good shield at RF.  The potential 
problem is the stray current path that results from stray capacitance 
between motor and frame (which is grounded) back through other ground 
paths to the controller.  We don't want return load current on ground, 
we want it on neutral!

73, Jim K9YC

On 4/24/2013 11:24 AM, GARY HUBER wrote:
> Would it not be just as easy to use EMT, IMC, GRC, RMC, or metallic 
> duct to shield the conductor runs between the VFD and motor at least 
> to the well head to provide RF shielding ? Its been a long time (20 
> plus years) since I took a NEC course on power and grounding, but I'd 
> bet there were other grounding and protection violations (NEC) at the 
> installation receiving the FCC citation.
>
>
> 73 ES DX,
> Gary -- AB9M
>
> -----Original Message----- From: Jim Brown
> Sent: Wednesday, April 24, 2013 12:11 PM
> To: rfi at contesting.com
> Subject: Re: [RFI] FCC issues official citation for well pump RFI on 
> Topband
>
> 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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