On Sat, 02 May 2009 12:50:28 -0400, GEORGE WALLNER wrote:
>From an
>RFI point of view they are weapons of mass destruction!
Correct on all counts. To understand the problem, study the
section on variable frequency drives in the RFI tutorial on my
website. The primary mechanism is the magnetic field generated by
that current, and the fact that the current loop area is
physically large. For any magnetic field, the field strength is
proportional to the current multiplied by the loop area. The loop
area is large because the return path for the HF current (the
harmonics) is through bypbass capacitors to "ground," and because
the well motor is separated from the controller and the stepdown
transformer.
There are several elements to the capacitance. First are the stray
capacitances of the transformer and the motor. Second are bypass
capacitors at the wrong place in the circuit. You WANT bypass
capacitors next to the switching transistors so they create a very
small loop area for HF current, but NOT at the motor.
Two important steps to fix this. They are additive.
1) Slow down the rise time of the current pulses. There should be
an RFI kit from the mfr to do this. Some big ferrite cores may
help with this, but they must be BIG so that saturation doesn't
kill their effectiveness. In this application, they go around a
single conductor to form a differential choke, NOT a common mode
choke.
2) Reduce the loop area. You can use twisted pair for the phase
conductors, but that won't help with the harmonic current that's
returning on ground.
This is completely separate from what the controller may be
putting back onto the AC line. A conventional line filter will
kill that. It should be bonded to the controller.
73,
Jim Brown K9YC
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