[TowerTalk] filter for rotator DC motor PWM amplifier

[Grant Saviers]

I'm seeking some guidance for the design of a PWM amplifier noise filter 
for a 180v 1/2 hp DC motor that drives a ring rotator.  My googling 
turns up a lot of information for 3 phase VFD filters but little for DC 
motor applications, so I'm hoping there is some experience/expertise 
among towertalkians.

The PWM amp is a commercial Minarik 16Khz switch and drives the motor 
via twisted pair.  The amp is in a RFI gasketed Hoffman steel enclosure 
and I have a Corcom 20a/240v line filter on the AC source.  However, I 
have hash everywhere and S9 noise on a separate tower 20m antenna.  The 
overwhelming source is the PWM amp, not motor brush hash. The box is 
grounded to the tower master ground plate.

I think a DC filter design is similar that of AC line filters, a equal 
current forcing/common mode balun followed by capacitors followed by an 
L-C filter in each leg.   My major question is about the practical 
parameters of a balun in this application.  A high perm toroid seems a 
likely choice for a parallel L1-L2 balanced winding.  I have some 
FT-193-JA cores, u=5000 and the question is what assumptions/parameters 
govern the balun winding design?  For the L in the L-C section, high 
current toroid inductors are widely available (e.g 50uH @ 10amps).

Any links to references would be appreciated.

Grant KZ1W