Topband: Gas well pump motor

[Jim Brown]

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